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The phylogeny of arachnomorph arthropods and the origin of the Chelicerata Trevor J. Cotton and Simon J. Braddy ABSTRACT: A new hypothesis of the relationships between arachnomorph arthropods, and the origin of chelicerates, is presented based on a cladistic analysis of 34 taxa and 54 characters. The present study provides a detailed discussion of primary hypotheses of homology and includes a more complete range of terminal taxa than previous analyses. The analysis provides the first convincing synapomorphies for the Arachnomorpha, and suggests that the marrellomorphs are not arachno- morphs. The assignment of Cambrian ‘great appendage’ (or megacheiran) arthropods to the Arachnomorpha is confirmed, and potential synapomorphies uniting them with chelicerates are discussed and tested. Principal amongst these are the loss of the first cephalic appendages (the antennae), loss of the exopods of the second cephalic appendages and modification of the endopods of these appendages into spinose grasping organs. The Arachnomorpha consists of two major clades: (1) a ‘chelicerate-allied’ clade, including chelicerates, megacheirans, Emeraldella, Sidneyia, chelonie- llids and aglaspidids, in which chelicerates and a paraphyletic group of megacherian arthropods form the sister group to the remaining taxa; and; (2) a ‘trilobite-allied’ clade, including trilobites, xandarellids, helmetiids, tegopeltids and naraoiids, the relationships of which are more fully resolved than in previous studies. KEY WORDS: Arachnata, Arachnomorpha, Burgess Shale, Cambrian, Cheliceramorpha, cladistics, evolution, Trilobitoidea, Trilobitomorpha The phylogeny of arthropods has been debated for over a century. Whilst five major groups, the extant Chelicerata, Hexapoda, Crustacea and Myriapoda, and the extinct Palaeozoic Trilobita, have more or less consistently been recognised, the relationships between these groups have been highly contentious (see Wheeler et al. 1993; Wills et al. 1995). Recently, however, some consensus has been reached on issues such as the monophyly of the euarthropods, and the sister group relationship between crustaceans and hexapods, form- ing the Pancrustacea (Budd 1996a; Akam 2000). Furthermore, all recent cladistic studies that have included fossil taxa have recognised trilobites and chelicerates as more closely related to each other than either group is to Pancrustacea (see e.g. Ax 1986; Bergstro ¨ m 1992; Weygoldt 1998; Wills et al. 1998a). This clade has variously been given the names Arachnomorpha (Størmer 1944, 1951; Briggs & Fortey 1989; Briggs et al. 1992; Wills et al. 1995; Weygoldt 1998), Lamellipedia (Hou & Bergstro ¨ m 1997) or Arachnata (Lauterbach 1973, 1980, 1983; Chen et al. 1997; Ramsko ¨ld et al. 1997; Edgecombe & Ramsko ¨ ld 1999; although Lauterbach apparently later rejected the term Arachnata and included all the assigned taxa in the Chelicerata – see Mu ¨ ller & Walossek 1987, p. 53). The term Arachnata has recently been most extensively used, but the current concept of the group is closer to that of Størmer than of Lauterbach, and therefore, the earlier name Arachnomorpha, originally proposed by Heider (1913), is preferred here. This group can be defined as the most inclusive clade, including Chelicerata, but not Pancrustacea (following Chen et al. 1997; Ramsko ¨ld et al. 1997). According to this definition, the Arachnomorpha consists of the chelicerates and their stem group (sensu Ax 1986), as illustrated by Figure 1. Therefore, resolving arachnomorph phylogeny will have important implications for understanding early chelicerate evolution (Dunlop 1999). In addition to the Trilobita and Chelicerata, Størmer (1944) included in his Arachnomorpha a seemingly highly disparate (Gould 1989, 1991) assemblage of Palaeozoic fossil arthro- pods, the Trilobitomorpha (Størmer 1944) or Trilobitoidea (Størmer 1959). These included various problematic arthro- pods from the famous Middle Cambrian Burgess Shale of British Columbia, Canada (see Conway Morris 1982; Briggs et al. 1994), and Cheloniellon and Mimetaster from the Devonian Hunsru ¨ ck Slate of Germany (see Bartels et al. 1998). Since then, our knowledge of arachnomorph diversity has been transformed by the discovery of many new taxa from Cambrian Burgess Shale-type faunas, and to a lesser extent, later faunas from around the world. Of primary importance amongst these new Lagersta ¨tte (reviewed by Conway Morris 1989, 1998) is the Chengjiang fauna from the Lower Cambrian of Yunnan, China (see Hou et al. 1991; Chen et al. 1996; Hou & Bergstro ¨ m 1997). These new finds largely consist of taxa similar to those from the Burgess Shale. For example, the Chengjiang fauna includes taxa that are clearly closely related to Helmetia Walcott, 1918, Tegopelte Simonetta & Delle Cave, 1975 and Alalcomenaeus Simonetta, 1970 (Edgecombe & Ramsko ¨ ld 1999). Similarly, the most widespread trilobitomorph group, the Naraoiidae (sensu Fortey & Theron 1994) or Nektaspida (sensu Hou & Bergstro ¨ m 1997), was originally known only from Naraoia Walcott, 1912 from the Burgess Shale. A number of related genera are now recognised from the Early Cambrian of Poland (Liwia Dzik & Lendzion, 1988), China (Misszhouia Chen et al., 1997) and Greenland (Buenaspis Budd, 1999a), and the Ordovician of Sardinia (Tariccoia Hammann et al., 1990) and South Africa (Soomaspis Fortey & Theron, 1994). Naraoia itself has now also been described from the Early Cambrian of Idaho, the Middle Cambrian of Utah (both Robison 1984) and from the Chengjiang fauna (Zhang & Hou 1985). However, some of the probable arachnomorphs from Cambrian excep- tionally preserved faunas have no obvious affinities to others, despite detailed knowledge of their morphology. Notable among these are Emeraldella Walcott, 1912 and Sidneyia Transactions of the Royal Society of Edinburgh: Earth Sciences, 94, 169–193, 2004 (for 2003)

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Page 1: The phylogeny of arachnomorph arthropods and the origin of the

The phylogeny of arachnomorph arthropodsand the origin of the Chelicerata

Trevor J Cotton and Simon J Braddy

ABSTRACT A new hypothesis of the relationships between arachnomorph arthropods and theorigin of chelicerates is presented based on a cladistic analysis of 34 taxa and 54 characters Thepresent study provides a detailed discussion of primary hypotheses of homology and includes a morecomplete range of terminal taxa than previous analyses The analysis provides the first convincingsynapomorphies for the Arachnomorpha and suggests that the marrellomorphs are not arachno-morphs The assignment of Cambrian lsquogreat appendagersquo (or megacheiran) arthropods to theArachnomorpha is confirmed and potential synapomorphies uniting them with chelicerates arediscussed and tested Principal amongst these are the loss of the first cephalic appendages (theantennae) loss of the exopods of the second cephalic appendages and modification of the endopodsof these appendages into spinose grasping organs The Arachnomorpha consists of two major clades(1) a lsquochelicerate-alliedrsquo clade including chelicerates megacheirans Emeraldella Sidneyia chelonie-llids and aglaspidids in which chelicerates and a paraphyletic group of megacherian arthropodsform the sister group to the remaining taxa and (2) a lsquotrilobite-alliedrsquo clade including trilobitesxandarellids helmetiids tegopeltids and naraoiids the relationships of which are more fully resolvedthan in previous studies

KEY WORDS Arachnata Arachnomorpha Burgess Shale Cambrian Cheliceramorphacladistics evolution Trilobitoidea Trilobitomorpha

The phylogeny of arthropods has been debated for over acentury Whilst five major groups the extant ChelicerataHexapoda Crustacea and Myriapoda and the extinctPalaeozoic Trilobita have more or less consistently beenrecognised the relationships between these groups have beenhighly contentious (see Wheeler et al 1993 Wills et al 1995)Recently however some consensus has been reached on issuessuch as the monophyly of the euarthropods and the sistergroup relationship between crustaceans and hexapods form-ing the Pancrustacea (Budd 1996a Akam 2000) Furthermoreall recent cladistic studies that have included fossil taxa haverecognised trilobites and chelicerates as more closely related toeach other than either group is to Pancrustacea (see eg Ax1986 Bergstrom 1992 Weygoldt 1998 Wills et al 1998a) Thisclade has variously been given the names Arachnomorpha(Stoslashrmer 1944 1951 Briggs amp Fortey 1989 Briggs et al 1992Wills et al 1995 Weygoldt 1998) Lamellipedia (Houamp Bergstrom 1997) or Arachnata (Lauterbach 1973 19801983 Chen et al 1997 Ramskold et al 1997 Edgecombe ampRamskold 1999 although Lauterbach apparently later rejectedthe term Arachnata and included all the assigned taxa in theChelicerata ndash see Muller amp Walossek 1987 p 53) The termArachnata has recently been most extensively used but thecurrent concept of the group is closer to that of Stoslashrmerthan of Lauterbach and therefore the earlier nameArachnomorpha originally proposed by Heider (1913) ispreferred here This group can be defined as the most inclusiveclade including Chelicerata but not Pancrustacea (followingChen et al 1997 Ramskold et al 1997) According to thisdefinition the Arachnomorpha consists of the chelicerates andtheir stem group (sensu Ax 1986) as illustrated by Figure 1Therefore resolving arachnomorph phylogeny will haveimportant implications for understanding early chelicerateevolution (Dunlop 1999)

In addition to the Trilobita and Chelicerata Stoslashrmer (1944)included in his Arachnomorpha a seemingly highly disparate

(Gould 1989 1991) assemblage of Palaeozoic fossil arthro-pods the Trilobitomorpha (Stoslashrmer 1944) or Trilobitoidea(Stoslashrmer 1959) These included various problematic arthro-pods from the famous Middle Cambrian Burgess Shale ofBritish Columbia Canada (see Conway Morris 1982 Briggset al 1994) and Cheloniellon and Mimetaster from theDevonian Hunsruck Slate of Germany (see Bartels et al 1998)Since then our knowledge of arachnomorph diversity hasbeen transformed by the discovery of many new taxa fromCambrian Burgess Shale-type faunas and to a lesser extentlater faunas from around the world Of primary importanceamongst these new Lagerstatte (reviewed by Conway Morris1989 1998) is the Chengjiang fauna from the Lower Cambrianof Yunnan China (see Hou et al 1991 Chen et al 1996 Houamp Bergstrom 1997)

These new finds largely consist of taxa similar to those fromthe Burgess Shale For example the Chengjiang fauna includestaxa that are clearly closely related to Helmetia Walcott 1918Tegopelte Simonetta amp Delle Cave 1975 and AlalcomenaeusSimonetta 1970 (Edgecombe amp Ramskold 1999) Similarlythe most widespread trilobitomorph group the Naraoiidae(sensu Fortey amp Theron 1994) or Nektaspida (sensu Hou ampBergstrom 1997) was originally known only from NaraoiaWalcott 1912 from the Burgess Shale A number of relatedgenera are now recognised from the Early Cambrian of Poland(Liwia Dzik amp Lendzion 1988) China (Misszhouia Chen et al1997) and Greenland (Buenaspis Budd 1999a) and theOrdovician of Sardinia (Tariccoia Hammann et al 1990) andSouth Africa (Soomaspis Fortey amp Theron 1994) Naraoiaitself has now also been described from the Early Cambrian ofIdaho the Middle Cambrian of Utah (both Robison 1984) andfrom the Chengjiang fauna (Zhang amp Hou 1985) Howeversome of the probable arachnomorphs from Cambrian excep-tionally preserved faunas have no obvious affinities to othersdespite detailed knowledge of their morphology Notableamong these are Emeraldella Walcott 1912 and Sidneyia

Transactions of the Royal Society of Edinburgh Earth Sciences 94 169ndash193 2004 (for 2003)

Walcott 1911 from the Burgess Shale Retifacies Hou et al1989 from the Chengjiang fauna and Phytophilaspis Ivantsov1999 from the Lower Cambrian Sinsk Formation of Siberia

Despite uniformally supporting an arachnomorph cladeincluding the trilobites and various Cambrian trilobite-like ormerostome-like arthropods recent studies have largely failedto provide convincing synapomorphies for the group (Dunlop1999) Here cladistic methods are used to address this problemto rigorously assess the limits of the Arachnomorpha and todetermine relationships within the arachnomorph group as awhole The present study is based upon a new matrix that ismore comprehensive than previous work in terms of both therange of characters considered and taxonomic sampling

1 Previous studies

Since Stoslashrmer (1944) studies of the phylogeny of theArachnomorpha have disagreed on the taxa that should beincluded in the group and on the relationships between them(Fig 2) In the earliest relevant cladistic studies Lauterbach(1980 1983) and Ax (1986) suggested a particularly close rela-tionship between chelicerates and a paraphyletic Trilobita ahypothesis originally proposed by Raw (1957) Lauterbach andAx argued that olenellid trilobites (reviewed by Palmer ampRepina 1993) were the sister group to chelicerates and thatother trilobites were the sister group to this clade This idea hasbeen extensively criticised In particular Lauterbach ignored allother arachnomorph taxa (including more plesiomorphic trilo-bites) and a wide range of characters that are potential trilobitesynapomorphies (Fortey amp Whittington 1989 Fortey 1990aRamskold amp Edgecombe 1991 Bergstrom amp Hou 1998) To thepresent authorsrsquo knowledge no subsequent author exceptWeygoldt (1998) has accepted Lauterbachrsquos hypothesis

Most hypotheses of arachnomorph phylogeny have beenpresented as part of analyses of the phylogeny of arthropodsas a whole The pioneering cladistic work of Briggs ampFortey (1989) found that the crustaceans (and Cambrianlsquocrustaceanomorphsrsquo) were paraphyletic with respect to arach-nomorphs (Fig 2A) Within the arachnomorphs a stronglypectinate paraphyletic group including Aglaspis and variousBurgess Shale arthropods was primitive with respect to a cladeof all other taxa This consisted of a [Habelia (NaraoiaTrilobita)] group and a clade including the Burgess Shalelsquogreat appendagersquo arthropods Burgessia Sarotrocercus andchelicerates (Fig 2A) This work was subsequently revised(Briggs et al 1992 1993) to include a representative range ofextant arthropods alongside a different selection of Cambrian

taxa coded for a greater number of characters Whereas theearlier work used Marrella as an outgroup the hypothesis ofBriggs et al (1992 1993) was rooted using the lobopodAysheaia This study supported the monophyly of crustaceansand suggested a very different topology within theArachnomorpha (Fig 2B) Whereas Sarotrocercus Burgessiaand Yohoia were still placed close to the chelicerates the otherlsquogreat appendagersquo taxa Alalcomenaeus and Leanchoilia werefound to be basal arachnomorphs Of the taxa placed in abasal paraphyletic assemblage in the earlier study someremained basal (Emeraldella and Sidneyia) whereas otherswere now more closely related to chelicerates than trilobites(Molaria Aglaspis and Sanctacaris)

In a further refinement the work of Wills et al (19951998a) coded the previously analysed taxa for many newcharacters and considered a small number of additional taxanotably from post-Cambrian Palaeozoic Lagerstatte Whilstsupporting the topology of major euarthropod clades found byBriggs et al this analysis again proposed very different rela-tionships within the Arachnomorpha (Fig 2C) Burgessia wasfound to be the sister group to all other arachnomorphs and aclade of trilobites Molaria and Naraoia the sister group to allremaining taxa Cheloniellon and Aglaspis were successivesister groups to chelicerates Notably and unlike in previousanalyses most Burgess Shale arachnomorphs formed a largeclade which was placed in opposition to the clade consisting ofchelicerates Cheloniellion and Aglaspis

In contrast to these studies Bergstrom (1992) Hou ampBergstrom (1997) and Bergstrom amp Hou (1998) rejected parsi-mony as a phylogenetic criterion and developed an arthropodphylogeny (Fig 2D) based on a small sample of charactersThe possibility of any of the selected characters evolvingconvergently was excluded on purely methodological groundsBergstromrsquos interpretations of homology are also oftenunclear For example he used leg posture which can rarely beadequately determined from fossils as an important character(see Edgecombe amp Ramskold 1999 p 281) Bergstromrsquoswork has been extensively criticised (eg Schram 1993 Briggs1998 Cotton 1999 Edgecombe amp Ramskold 1999) and thesearguments are not repeated here

The cladistic analysis of stem group chelicerates presentedby Dunlop amp Selden (1997) included only taxa that werefound to be the most closely related to chelicerates by Willset al (1995 1998a) The results of this analysis were largelyunresolved (the published cladogram fig 173 is only one of9450 most parsimonious trees) but supported the view of Willset al that Cheloniellon is more closely related to cheliceratesthan Aglaspis However Dunlop amp Selden (1997 p 232) notedthat cheloniellids aglaspidids and chelicerates may not form amonophyletic group with respect to all other arachnomorphsEmerson amp Schram (1997) analysed arthropod phylogeny onthe basis of Arthropod Pattern Theory (Schram amp Emerson1991) Their results are also poorly resolved and the topologyof arachnomorph taxa highly unstable across the varioustreatments of their data they present (eg Emerson amp Schram1997 figs 73AndashC) Many of their characters are difficult tointerpret outside the framework of the theory

Edgecombe amp Ramskold (1999) recently presented a cladis-tic study of some arachnomorph taxa in an attempt to resolvethe relationships of the Trilobita Their work was an improve-ment on previous studies in that they included a comprehen-sive sample of both taxa and characters and presented detaileddiscussions of the homology of these characters Their resultssupported the monophyly of the Helmetiida (sensu Hou ampBergstrom 1997) Naraoiidae and Xandarellida (see Ramskoldet al 1997) within an unresolved clade also including theTrilobita Sidneyia Emeraldella and Retifacies were found to

Figure 1 Widely accepted relationships between major arthropodgroups illustrating the Arachnomorpha concept followed here

170 TREVOR J COTTON AND SIMON J BRADDY

be more basal arachnomorphs on the basis of outgrouprooting with Marellomorpha Unfortunately Edgecombe ampRamskold (1999) made no attempt to establish the monophyly

of the group of lsquotrilobite-allied arachnatesrsquo that they includedin their analysis and did not consider the position of thechelicerates

Figure 2 Previous hypotheses of arachnomorph phylogenys (A) after Briggs amp Fortey (1989 fig 1 1992 fig 3)note that Crustacea are paraphyletic in this analysis (B) after Briggs et al (1992 fig 3 1993 fig 1) (C) afterWills et al (1995 fig 1A 1998a fig 2 1) and (D) after Hou amp Bergstrom (1997 fig 88) (for monotypic familiesthe family names used by Hou amp Bergstrom have been replaced with generic names) Taxa important for therecognition of an arachnomorph clade (ie trilobites chelicerates and crustaceans see text) are shown in boldtype

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 171

2 Cladistic analysis

2 1 Taxonomic scopeThirty-four ingroup taxa were considered in the cladisticanalyses Of these the majority were coded as individualspecies-level terminals although higher taxonomic levels wereemployed in some instances A complete list of species andgenus-level terminals along with details of authorship andother important references is given in Table 1 Species-levelterminals are referred to throughout by generic names only(except in Table 1) since the majority are assigned to mono-typic genera In addition to these ingroup taxa a hypotheticaloutgroup was used for rooting as discussed below

Terminals were selected on the basis of the recent cladisticanalyses of Wills et al (1995 1998a) and Edgecombe ampRamskold (1999) and to a lesser extent previous hypothesesof arthropod relationships The majority of taxa that havebeen included in an arachnomorph group by previous authorsare included in this study Exceptions include AgnostusHabelia Molaria Sanctacaris and Sarotrocercus (all of whichwere considered by Wills et al but not Edgecombe ampRamskold) the putative chelicerate Offacolus andPhytophilaspis The Agnostina are regarded as a clade withinthe Trilobita derived from Eodiscina (eg Fortey 1990a

Fortey amp Theron 1994 Wills et al 1998a) rather than stemgroup crustaceans (eg Shergold 1991 Bergstrom 1992 seeCotton 2002 for a detailed discussion and phylogenetic analy-sis) The Burgess Shale taxa Habelia Walcott 1912 MolariaWalcott 1912 and Sarotrocercus Whittington 1981 and theHunsruck Slate Magnoculocaris blindi (Briggs amp Bartels 20012002) are rather poorly known and hypotheses of theirrelationships constrained by too few characters

Sanctacaris Briggs amp Collins 1988 and the probably closelyrelated Offacolus Orr et al 2000 were both originally describedas having chelicerate affinities Sutton et al (2002) addedOffacolus to the matrix of Wills et al (1995 1998a) and foundit to be nested within the Chelicerata despite its biramouscephalic appendages The material of Sanctacaris is in need ofrestudy the homology of the cephalic appendages and ventralcephalic structures remain unclear (see Budd 2002 and Briggsamp Collins 1988 for alternative schemes) The relationships ofthese lsquosanctacaridsrsquo will be considered in detail later by Braddyand co-workers following re-examination of Sanctacaris anddescription of a new Chengjiang taxon (Babcock amp Zhangin press) that is likely to have a profound impact on theinterpretation of both Sanctacaris and Offacolus

Phytophilaspis Ivantsov 1999 shows a combination of fea-tures that may be homologous to those found in naraoiids (the

Table 1 Authorship and important references for species included in cladistic analyses of arachnomorphs Previous analyses by Briggs amp Fortey(1989 1992) Briggs et al (1992 1993) Wills et al (1995 1998a) and Edgecombe amp Ramskold (1999) informed the coding of many taxa and arenot listed

Name and authorship Other references

Alalcomenaeus cambricus Simonetta 1970 Briggs amp Collins (1999)Buenaspis forteyi Budd 1999aBurgessia bella Walcott 1912 Hughes (1975)Cheloniellon calmani Broili 1932 Broili (1933) Sturmer amp Bergstrom (1978)Cindarella eucalla Chen et al 1996 Ramskold et al (1997)Emeraldella brocki Walcott 1912 Bruton amp Whittington (1983) Briggs amp Robison (1984)Eoredlichia intermedia (Lu 1940) Shu et al (1995) Ramskold amp Edgecombe (1996)Fortiforceps foliosa Hou amp Bergstrom 1997Helmetia expansa Walcott 1918 Briggs in Conway Morris (1982) Briggs Erwin amp Collier (1994)Jianfengia multisegmentalis Hou 1987b Bergstrom amp Hou (1991) Chen amp Zhou (1997)Kuamaia lata Hou 1987a Hou amp Bergstrom (1997) Begstrom amp Hou (1998)

Edgecombe amp Ramskold (1999)Leanchoilia superlata Walcott 1912 Bruton amp Whittington (1983) Briggs amp Robison (1984)Lemoneites Flower 1968 Dunlop amp Selden (1997)Liwia plana (Lendzion 1975) Dzik amp Lendzion (1988) Fortey amp Theron (1994)

Chen Edgecombe amp Ramskold (1997)Marrella splendens Walcott 1912 Whittington (1971)Mimetaster hexagonalis (Gurich 1931) Sturmer amp Bergstrom (1976)Misszhouia longicaudata (Zhang amp Hou 1985) Bergstrom amp Hou (1991) Chen et al (1997) Hou amp Bergstrom (1997)Naraoia Walcott 1912 Whittington (1977) Robison (1984) Zhang amp Hou (1985)

Chen et al (1997)Olenoides serratus (Rominger 1887) Whittington (1975a 1980)Paleomerus hamiltoni Stoslashrmer 1956 Bergstrom (1971) Dunlop amp Selden (1997)Retifacies abnormalis Hou et al 1989 Hou amp Bergstrom (1997)Saperion glumaceum Hou et al 1991 Ramskold et al (1996) Hou amp Bergstrom (1997)Sidneyia inexpectans Walcott 1911 Bruton (1981)Sinoburius lunaris Hou et al 1991 Hou amp Bergstrom (1997) Skioldia aldna Hou amp Bergstrom 1997Soomaspis splendida Fortey amp Theron 1994 Chen et al (1997)Tariccoia arrusensis Hammann et al 1990 Fortey amp Theron (1994) Chen et al (1997)Tegopelte gigas Simonetta amp Delle Cave 1975 Whittington (1985) Ramskold et al (1996)Weinbergina opitzi Richter amp Richter 1929 Sturmer amp Bergstrom (1981) Andersen amp Selden (1997) Dunlop amp Selden (1997)Xandarella spectaculum Hou et al 1991 Bergstrom amp Hou (1991) Ramskold et al (1997)

Hou amp Begstrom (1997)Yohoia tenuis Walcott 1912 Whittington (1974)

172 TREVOR J COTTON AND SIMON J BRADDY

form of the pygidium) xandarellids (the eye slits and overlapof anterior thoracic segments by the head shield) and Trilobita(the hypostome with anterior and posterior lateral wings)Therefore it potentially has a pivotal position in the phylogenyof the trilobite-allied Arachnomorpha However the interpre-tation of these features by Ivantsov (1999) requires confir-mation and adequately determining the homology of thesestructures would require restudy of the material

The Xandarellida known only from the Chengjiang faunawere originally described as an arachnate clade (Ramskoldet al 1997) All three valid genera Xandarella Sinoburius andCindarella were included in the analysis of Edgecome ampRamskold (1999) They differ in important respects andtherefore all three genera are included here to test the mono-phyly of this group in the context of a wider analysis

Wills et al (1995 1998a) represented the Trilobita (exceptAgnostida) by Olenoides the appendages of which are knownfrom the Burgess Shale and the Ordovician Triarthrus (seeCisne 1975 1981 Whittington amp Almond 1987) However thepresent authors follow Edgecombe amp Ramskold (1999) inchoosing Olenoides and Eoredlichia to represent the TrilobitaThese are more basal trilobites (see eg Fortey 1990a b) thanTriarthrus and consequently are more likely to reflect theancestral trilobite condition The putative trilobite KleptothuleBudd 1995 from the Early Cambrian Sirius Passet faunaof North Greenland is not included Its appendages areunknown and homologies between exoskeletal features of thistaxon and other arachnomorphs are uncertain

The Naraoiidae or Nektaspida (reviewed above) are widelyconsidered to be closely related to the trilobites either as aparaphyletic assemblage of lsquosoft-bodiedrsquo trilobites (eg Shuet al 1995 fig 20B) or as the sister group of calcified trilobites(Fortey 1997 Whittington 1977) They have been includedin the Trilobita by some authors However Edgecombe ampRamskold (1999) found no particularly close relationshipbetween naraoiids and trilobites (see above) In order to testthe monophyly of the group all described naraoiid genera areincluded except Maritimella and Orientella (both Repina ampOkuneva 1969) which are probably pseudofossils (Robison1984 p 2)

Tegopelte from the Burgess Shale has also been considered asoft-bodied trilobite (Whittington 1985) Ramskold et al(1996) revised the exoskeletal morphology of Tegopelte andnoted similarities to the Chengjiang taxa Saperion andSkioldia This relationship has been confirmed by cladisticanalysis (Edgecombe amp Ramskold 1999) An undescribed largearthropod from the Soom Shale Lagerstatte (Aldridge et al2001 fig 3442) is probably a tegopeltid (Braddy amp Almond1999) The Helmetiidae based on Helmetia Walcott 1918from the Burgess Shale are united with the tegopeltid group inthe Helmetiida (Hou amp Bergstrom 1997 Edgecombe amp Ram-skold 1999) Helmetia is rather poorly known and awaitsredescription but is very similar to Kuamaia lata Hou 1987aKuamaia muricata Hou amp Bergstrom 1997 and Rhombicalva-ria acantha Hou 1987a from Chengjiang There are no signifi-cant differences between these Chinese taxa and theirtaxonomy may be over split (Delle Cave amp Simonetta 1991Hou amp Bergstrom 1997) The morphology of Kuamaia lata isknown in some detail and it is coded here along with Helmetiaand all three tegopeltid genera

Delle Cave amp Simonetta (1991 table 1) compared severaltaxa to Tegopelte and Helmetia Of these only Retifacies isknown in enough detail to make coding worthwhile Tontoiaand Nathorstia (both Walcott 1912) are nomina dubia (seeWhittington 1985 1980 respectively) and only the exoskeletonis known of Urokodia Hou et al 1989 and Mollisonia Walcott1912 Retifacies has been placed near a trilobite-naraoiid-

helmetiid clade (Delle Cave amp Simonetta 1991 Edgecombe ampRamskold 1999) or as sister group to the naraoiids (Hou ampBergstrom 1997)

There is a long history of comparing Emeraldella andSidneyia with chelicerates (following Stoslashrmer 1944) but theposition of these taxa in cladistic studies has been highlyvariable (see Fig 2) According to the hypothesis ofBergstrom these taxa along with the Aglaspidida andCheloniellida form a clade that is the sister group to thechelicerates More usually aglaspidids or cheloniellids havebeen considered sister taxon to the chelicerates and theseBurgess Shale taxa more distantly related

A cheloniellid-chelicerate clade was supported by Wills et al(1995 1998a) and Sturmer amp Bergstrom (1978 also Bergstrom1979) and Simonetta amp Delle Cave (1981) and Delle Cave ampSimonetta 1991) placed the cheloniellid Triopus as ancestral toall chelicerates The Cheloniellida (sensu Dunlop amp Selden1997) are represented by Cheloniellon herein since theappendages of other cheloniellid taxa are unknown

It has also repeatedly been suggested that cheliceratesevolved from aglaspidids (eg Starobogatov 1990) and aglas-pidids have been included in the Chelicerata by some authors(Stoslashrmer 1944 Weygoldt amp Paulus 1979) The coding ofaglaspidids is considered in detail in Section 22 Lemoneites(Flower 1968) and Paleomerus (Stoslashrmer 1956 Bergstrom 1971)have been assigned to the Aglaspidida by some authors (seeHou amp Bergstrom 1997 pp 96ndash97) They are included tofacilitate comparison with the results of Dunlop amp Selden(1997) Lemoneites is particularly problematic (R A Moorepers comm 2004) and both taxa are known from very fewcharacters ndash hence the present authors have investigated theimplications of their exclusion from the analysis (see Section26) The aglaspidid-like arthropod Kodymirus vagans Chlupacamp Havlıcek 1965 from the Lower Cambrian of the CzechRepublic (redescribed and compared to eurypterids byChlupac 1995) is excluded from the present study becausefeatures of its morphology that are well known generally agreewith those of Aglaspis

The phylogeny of crown group chelicerates is a matter ofconsiderable debate but most authors have considered pyc-nogonids xiphosurans and eurypterids as early divergentgroups within Chelicerata (Dunlop 1999) These hypothesesare represented here by coding a generalised pycnogonid andeurypterid and the Devonian synziphosurine Weinbergina(Sturmer amp Bergstrom 1981) Weinbergina is the best-knownsynziphosurine and is more likely to represent the primitivecondition of xiphosurans than modern examples The phylog-eny of the Xiphosura has recently been studied by Anderson ampSelden (1997) Coding for Eurypterida follows the recent workof Dunlop amp Selden (1997) Dunlop (1998) Braddy et al(1999) and Dunlop amp Webster (1999) Coding for pycnogonidsfollows general works on the group (eg King 1973 Fry 1978)the reconstruction of the pycnogonid stem group by Bergstromet al (1980) and recent work on pycnogonid phylogeny(Munilla 1999 Arango 2002) The pycnogonid familyAmmotheidae is generally accepted as the most plesiomorphicextant group (Arango 2002)

Two groups of Palaeozoic fossil taxa have been included inthe Arachnomorpha by some authors but considered lessclosely related by others Marrella from the Burgess Shaleand two Devonian taxa Mimetaster and Vachonisia havegenerally been considered to form the clade Marrellomorpha(Whittington 1971 Sturmer amp Bergstrom 1976 Bergstrom1979 Wills et al 1995) This has been thought to be the sistergroup to other arachnates (Sturmer amp Bergstrom 1976) abasal schizoramian or a basal euarthropod group (see Fig 2)Bergstrom (1979) included the Carboniferous Cycloidea in the

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 173

Marrellomorpha but there is considerable evidence that theseare rather derived crustaceans (see Schram et al 1997) andthey are not considered further The Burgess Shale taxonBurgessia has also been placed in the Marrellomorpha (Hou ampBergstrom 1997) but was not found to belong to this clade inmost analyses Stoslashrmer (1944) placed Burgessia in a compa-rable systematic position to Marrella he included both in theArachnomorpha but excluded them from the MerostomoideaMarrella Mimetaster and Burgessia were included in thepresent study to assess whether the marellomorphs should beincluded in the Arachnomorpha and test the affinities ofBurgessia

The Cambrian lsquogreat appendagersquo arthropods (theMegacheira of Hou amp Bergstrom 1997) have been nestedwithin the Arachnomorpha in most cladistic studies (egBriggs amp Fortey 1989 Wills et al 1995 1998a Emerson ampSchram 1997) Briggs amp Fortey (1989 1992) recognised mega-cheiran taxa as particularly closely related to chelicerates(Fig 2A) while other authors (Bergstrom 1992 Hou ampBergstrom 1997 Budd 2002) have considered megacheirans tobe primitive euarthropods or ancestral to some (but not all)crustaceans (Delle Cave amp Simonetta 1991 and referencestherein) Since the monophyly of the megacheirans hasnot usually been supported the present authors haveincluded nearly all described taxa These include LeanchoiliaAlalcomenaeus and Yohoia from the Burgess Shale andFortiforceps and Jianfengia from the Chengjiang faunaExcluded from this study are Actaeus Simonetta 1970 fromthe Burgess Shale which may be a synonym of Alalcomenaeus(Briggs amp Collins 1999) Alalcomenaeus illecebrosus (Hou1987b see Hou amp Bergstrom 1997) from the ChengjiangFauna which is probably a chimaera (Briggs amp Collins 1999)and the poorly preserved Leanchoilia hanceyi from the MiddleCambrian of Utah (Briggs amp Robison 1984)

According to the definition given above any assessment ofthe limits of the Arachnomorpha needs to consider the phylo-genetic position of these taxa relative to crustaceans To thisend a generalised crustacean intended to represent the plesio-morphic crustacean condition was included as an ingrouptaxon There has been considerable disagreement over themost basal crustacean group (see Wills 1997 pp 194ndash5Schram amp Hof 1998 pp 245ndash8) The coding used here largelyfollows Walossek amp Mullerrsquos (1990 1997 1998) andWalossekrsquos (1993) concept of the crustacean stem group but isintended to be conservative so that coding on the basis of othertheories of crustacean origins would be similar

22 AglaspididaThe morphology of the appendages of aglaspidids is poorlyknown having been described from three species of bodyfossil Aglaspis spinifer Raasch 1939 Flobertia kochi Hesselbo1992 and Khankaspis bazhanovi Repina amp Okuneva 1969 andtrace fossil evidence (Hesselbo 1988) Of these the appendagesof Aglaspis described by Raasch (1939) Briggs et al (1979)and Hesselbo (1992) are by far the best known The append-ages of Flobertia (described by Raasch 1939 as Aglaspisbarrandei and Hesselbo 1992) agree with those of AglaspisHowever the appendages of Khankaspis show a differentmorphology but have only been poorly illustrated anddescribed (Repina amp Okuneva 1969) This material suggeststhe presence of lobate exopods with lamellate setae This taxonis probably correctly assigned to aglaspidids (cf Whittington1979 p 258) on the basis the central position of the dorsaleyes and presence of genal spines although Hou amp Bergstrom(1997 p 96) suggested that its broad tail spine may indicatethat it is a strabopid

It remains unclear whether lamellate exopods are absent insome aglaspidids (Aglaspis) and present in others (Khankaspis)or whether the apparent differences in appendage morphologybetween these taxa are a result of preservational bias alone Apossible preservational analogue is provided by someChengjiang taxa in which the exopods are very poorly pre-served and the endopods of the cephalic appendages arepreserved as impressions in the dorsal head shield in a similarmanner to those of Aglaspis This is presumably because theendopods were more convex and more heavily sclerotised thanthe exopods This mode of appendage preservation is mostclearly seen in Misszhouia longicaudata (see Chen et al 1997fig 2andashd) but is also found in Sinoburius (see Hou ampBergstrom 1997) and possible protaspides of Naraoia (Houet al 1991)

Here a generalised aglaspidid is coded in two different waysto accommodate this uncertainty In both codings they areconsidered to have possessed a pair of antenniform append-ages followed by 11 pairs of pediform endopods on the headand first eight thoracic tergites (Briggs et al 1979 Hesselbo1988 1992) According to one interpretation (coded asAglaspidida 1) all the appendages are uniramous the exopodis lost throughout According to the other (Aglaspidida 2)exopods consisting of a single lobe fringed with lamellate setae(Repina amp Okuneva 1969 pl 15 figs 1 3 amp 4) are presenton at least some appendages but their distribution andattachment are considered unknown

23 Outgroup rootingA hypothetical plesiomorphic euarthropod was included toallow outgroup rooting Many of the characters consideredcould be coded unambiguously on the basis of recent discus-sions of the arthropod stem group (Budd 1996b 1997 1999b)

Figure 3 Majority rule consensus of lsquotrilobite-alliedrsquo arachnate phy-logeny after Edgecombe amp Ramskold (1999 fig 3)

174 TREVOR J COTTON AND SIMON J BRADDY

The ancestral euarthropod is here considered to have possesseda head with antennae only and a series of post-cephalicbiramous limbs with gnathobases The hypothesised pattern ofthe evolution of plesiomorphic euarthropod characters isshown in Figure 4 The use of a hypothetical outgroup issomewhat unsatisfactory but only affects the relationshipbetween the major clades (Arachnomorpha Crustacea andMarrellomorpha) the topology within the arachnomorphs wasunaffected by the use of this outgroup since identical resultswere obtained with unrooted analyses or by rooting withCrustacea Marrella or both

24 Characters and codingMost previous cladistic analyses with the notable exception ofEdgecombe amp Ramskold (1999) have included only briefdiscussions of the primary hypotheses of homology involved incharacter construction and coding Therefore a full discussionof most characters is provided here Descriptions of charactersand character states below are arranged by organ system orbody region in approximate anterior to posterior order Thedistribution of character states across all taxa is shown in thedata matrix (Table 2)

The terminology of morphological features in arachnomor-phs is often confused Terminology developed for cheliceratestrilobites and crustaceans has variously been applied to taxaincluded in this study so some attempt is made to clarifyterminology herein Where appropriate the present authorsrsquoterminology follows Edgecombe et al (2000) and Wheeleret al (1993) but following Scholtz (1997) the protocerebrallsquosegmentrsquo is considered to be acronal and consequently thedeutocerebral segment to be the first true segment Thereforethe antennae (or antennulae of crustaceans) belong to the firstcephalic segment

The present authors have attempted to include as completea set of characters as possible However characters requiringhypotheses of homology between podomeres (eg the number

of podomeres in endopods of thoracic appendages Willset al 1998a character 51) were not considered followingEdgecombe et al (2000 p 157) Secondly some characters usedby Bergstrom (eg Hou amp Bergstrom 1997 p 109) suchas appendage posture and mode of feeding were excludedfollowing Edgecombe amp Ramskold (1999) Finally somecharacters of the ventral surface of the head were not coded asdiscussed below and illustrated in Figure 7 The present authorsdo not include all potential synapomorphies for the Cheli-cerata as the status of many of these is debated (see eg Shultz1990 2001 Dunlop amp Selden 1997 Weygoldt 1998 Wheeler ampHayashi 1998 Dunlop 1999 Edgecombe et al 2000)

Two methods for the coding of inapplicable characters inphylogenetic analysis have been used First inapplicable char-acter states can be coded as missing data A complex structuremay comprise characters lsquoabsentpresentrsquo and lsquostate1state2rsquowith taxa lacking the structure coded as absent for the firstcharacter and as missing data for the second Some authorshave regarded this method as problematic because it may leadto reconstruction of impossible ancestral states and henceunjustified trees (Platnick et al 1991 Strong amp Lipscomb1999) The alternative is to code the second character as a thirdlsquonot applicablersquo state in taxa that lack the structure Thesemethods are equivalent to Pleijelrsquos (1995) coding methods Cand B respectively In the present study inapplicable charac-ters are treated as missing data for most analyses becausecoding them as a distinct character state reduces characterindependence and effectively weights the inapplicable charac-ter and hence could result in them dominating the analysisThe effects of this assumption were investigated by using adistinct character state in some analyses (see Section 26) andinapplicable characters are shown as distinct to lsquotruersquo missingdata (using the symbol lsquo-rsquo) in the matrix (Table 2)

241 Anterior cephalic appendages and head segmentation1 Appendages of the first segment (antennae) (0) present and(1) absent The majority of taxa considered in the present study

Figure 4 Reconstruction of the euarthropod stem group used to inform coding of hypothetical outgroup Grosstopology largely follows Budd (1996b fig 9 1997 fig 1110) Solid boxes indicate unambiguous apomorphiesand shaded boxes character states which are plesiomorphic for the Arthropoda

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 175

possess a single pair of long uniramous multi- and annulatedantennae at the anterior of the head which presumably had asensory function as found in Crustacea Myriapoda and mostmembers of Hexapoda (ie proturans do not have antennae)These appendages are likely to be a synapomorphy of theEuarthropoda (eg Scholtz 1997 Walossek amp Muller 1997)and therefore the outgroup is coded as State 0 The anterior-most head appendages of most Cambrian megacheiran arthro-pods which have been called lsquogreat appendagesrsquo followingWalcott (1912) consist of a small number of robust spinosepodomeres Uniquely Fortiforceps has a pair of short anten-nae anterior to the lsquogreat appendagesrsquo (Hou amp Bergstrom 1997pp 34ndash8) Therefore it appears likely that the lsquogreat append-agesrsquo are the appendages of the second cephalic segment andthe antennae are lost in megacheirans other than FortiforcepsThis of course depends on recognising the lsquogreat appendagesrsquoas homologous in all of these taxa as discussed below(Character 2) Homology of the megacheiran anterior append-ages with the second cephalic appendages of trilobites waspreviously suggested by Stoslashrmer (1944 p 124) on the basis oftheir post-oral position

The classical view of chelicerate head segmentation main-tains that they too have lost the antennae and the cheliceraeare the appendages of the second cephalic segment Recentrevisions of arthropod phylogeny have uncritically acceptedthis homology (Edgecombe et al 2000 Giribet et al 2001)which is based largely on neuroanatomy The lsquochelicero-neuromerrsquo which innervates the chelicerae seems to be ho-mologous with the tritocerebrum associated with the secondcephalic appendage pair of mandibulates (see Weygoldt 1979Winter 1980) Although some uncertainty exists over theposition of pycnogonids within Arthropoda and the homol-ogy of their cephalic segmentation is poorly understood thisview is also supported by the presence of pre-cheliceralappendages in a pycnogonid larva (Larva D of Muller ampWalossek 1986) from the Upper Cambrian of Sweden(Walossek amp Dunlop 2002) Cheloniellon also has a pair ofpre-oral pediform appendages considered homologous to thechelicerae in addition to its antennae Thus despite recentneontological studies that indicate that the chelicerae arehomologous to the antennae of mandibulates based on pat-terns of Hox gene expression (Damen et al 1998 Telford amp

Table 2 Data matrix for cladistic analyses of arachnomorphs () missing data and (ndash) inapplicable characters Letters indicate multistateuncertainty codings as follows (A) 34 (B) 02 (C) 01 (D) 134 (E) 12

1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 3 3 3 4 4 4 4 4 4 4 4 4 4 5 5 5 5 51 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4

OUTGROUP 0 0 0 0 0 0 0 0 0 0 0 0 ndash ndash 0 0 0 0 0 0 0 0 0 0 0 4 0 ndash 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ndash 0 0 0 0 ndash ndash ndash 0 ndash 0Aglaspidida 1 0 0 1 A 0 0 0 0 0 1 0 ndash ndash ndash ndash ndash 0 0 0 1 0 0 0 0 0 0 0 0 ndash 1 1 0 0 1 0 0 0 0 0 0 0 0 1 2 1 1 0 ndash ndash ndash 1 0 1Aglaspidida 2 0 0 A 0 0 0 0 0 0 1 0 ndash ndash 0 1 0 1 0 0 0 0 0 0 0 0 ndash 1 1 0 0 1 0 0 0 0 0 0 0 0 1 2 1 1 0 ndash ndash ndash 1 0 1Alalcomenaeus 1 1 1 3 ndash 2 0 1 0 0 0 1 0 ndash ndash 1 0 0 0 0 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 0 1 0 1 ndash ndash ndash 1 1 0Buenaspis 0 1 B 0 0 1 0 0 ndash 0 1 0 1 0 0 0 0 1 0 0 0 0 ndash 0 1 1 1 0 1 0 ndash 0Burgessia 0 0 0 3 0 0 0 0 0 0 0 1 0 ndash ndash 0 0 0 1 2 ndash 0 0 0 0 0 4 0 ndash 0 0 0 0 0 0 2 0 0 0 0 0 0 ndash 0 0 0 1 ndash ndash ndash 1 0 0Cheloniellon 0 0 1 4 0 0 0 0 0 1 0 1 0 ndash ndash 0 0 1 0 1 0 0 0 0 0 0 1 0 ndash 0 1 0 0 1 0 0 0 0 0 1 0 0 1 0 1 1 0 ndash ndash ndash 0 ndash 1Cindarella 0 0 0 6 0 0 0 0 0 0 0 1 1 0 0 0 0 1 1 0 0 0 0 0 0 0 1 0 ndash 0 1 0 1 1 0 0 2 1 0 0 0 1 0 ndash 1 0 1 1 0 0 0 0 ndash 0Crustacea 0 0 0 3 0 0 0 0 0 0 0 0 0 ndash ndash 0 1 0 0 0 0 0 0 0 0 0 4 0 ndash 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ndash 0 0 0 ndash ndash ndash 1 0 0Emeraldella 0 0 1 5 0 0 0 0 0 0 0 1 1 0 0 0 0 1 0 2 ndash 0 0 0 0 0 0 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 1 1 1 1 0 1 ndash ndash ndash 1 0 1Eoredlichia 0 0 0 3 0 0 0 0 1 0 0 1 1 0 0 0 0 1 0 0 1 0 0 0 0 0 3 1 1 1 1 0 0 2 0 0 0 0 0 0 0 0 0 ndash 1 0 1 1 0 0 1 0 ndash 0Eurypterida 1 1 1 6 ndash 1 1 0 0 1 1 1 0 ndash ndash 1 0 1 0 1 0 0 0 1 0 0 4 1 0 0 1 0 0 1 0 0 0 0 0 0 0 0 1 3 0 1 0 1 ndash ndash ndash 1 0 0Fortiforceps 0 1 1 4 ndash 0 0 0 0 0 0 1 0 ndash ndash 1 0 0 1 0 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 0 1 0 1 ndash ndash ndash 1 1 0Helmetia 3 1 0 1 0 1 0 0 0 0 0 1 2 0 ndash 0 1 0 0 2 0 0 0 0 0 1 1 0 0 ndash 0 0 1 1 0 1 0 0 ndash 0Jianfengia 1 1 1 4 ndash 1 0 0 0 0 0 1 0 ndash ndash 1 0 1 1 0 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 0 1 0 1 ndash ndash ndash 1 0 0Kuamaia 0 0 0 0 0 0 0 0 0 1 1 0 1 0 0 1 0 0 0 1 0 0 0 1 2 0 ndash 0 1 0 0 2 0 0 0 0 0 1 1 0 0 ndash 0 0 1 1 0 1 0 0 ndash 0Leanchoilia 1 1 1 3 ndash 2 0 1 0 0 0 1 0 ndash ndash 1 0 1 1 B 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 1 1 0 1 ndash ndash ndash 1 0 0Lemoneites 0 1 0 0 0 0 1 1 0 1 0 0 0 0 0 0 0 0 1 2 0 0 ndash ndash ndash 1 0 Liwia 0 1 0 0 0 0 1 0 1 0 0 1 0 1 0 0 0 0 ndash 0 1 1 1 0 0 0 ndash 0Marrella 0 0 1 1 0 0 0 0 0 0 0 0 0 ndash ndash 0 1 0 0 2 ndash 0 0 0 0 0 0 ndash 0 0 0 0 0 0 2 0 0 0 0 0 0 ndash 0 0 0 0 ndash ndash ndash 0 ndash 0Mimetaster 0 0 1 2 0 0 0 0 0 0 0 0 0 ndash ndash 0 1 0 1 C 0 0 0 0 0 0 ndash 0 0 0 0 1 0 0 2 0 0 0 0 0 0 ndash 0 0 0 0 ndash ndash ndash 0 ndash 0Misszhouia 0 0 0 3 0 0 0 0 1 0 0 1 1 1 0 0 0 1 0 0 0 0 0 0 0 0 1 0 ndash 0 1 1 ndash ndash 2 0 0 0 1 0 0 0 0 ndash ndash 0 1 1 0 0 1 0 ndash 0Naraoia 0 0 0 3 1 0 0 0 1 0 0 1 1 1 0 0 0 1 0 0 0 0 0 0 0 0 1 0 ndash 0 1 1 ndash ndash 2 0 0 0 1 0 0 0 0 ndash ndash 0 1 1 0 0 0 ndash 0Olenoides 0 0 0 3 0 0 0 0 1 0 0 1 1 0 0 0 0 1 0 1 1 0 0 0 0 0 3 1 1 1 1 0 0 2 0 0 0 0 0 0 0 0 0 ndash 0 1 1 1 0 0 0 0 ndash 0Paleomerus 0 1 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0 0 0 ndash 0 0 ndash ndash ndash 1 0 Pycnogonida 1 1 1 4 ndash 1 1 0 0 1 1 1 ndash ndash ndash ndash 0 0 1 1 0 0 0 1 0 0 4 0 ndash 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 ndash ndash ndash 1 0 0Retifacies 0 0 0 3 0 0 0 0 0 0 0 1 0 ndash ndash 0 0 1 0 0 0 0 0 0 0 0 0 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 1 0 1 1 0 0 0 1 0 0Saperion 0 2 0 1 0 1 0 0 1 1 0 0 1 0 0 0 1 2 0 ndash 0 1 1 ndash ndash 1 1 ndash ndash 0 0 1 0 0 ndash ndash 1 1 0 0 1 0 ndash 0Sidneyia 0 0 1 0 1 0 0 0 0 1 0 1 0 ndash ndash 0 0 1 0 0 0 0 0 0 0 0 0 0 1 0 0 1 0 0 0 0 0 0 0 0 1 2 1 1 0 1 ndash ndash ndash 1 0 1Sinoburius 0 0 0 4 0 0 0 0 0 0 0 1 0 1 0 0 1 1 0 0 0 1 0 ndash 0 1 0 0 1 0 0 2 1 0 0 0 1 0 ndash 0 0 1 1 0 1 0 0 ndash 0Skioldia 0 2 0 0 0 1 0 0 0 1 2 0 ndash 0 1 1 ndash ndash 1 1 ndash ndash 0 0 1 0 0 ndash ndash 1 1 0 0 1 0 ndash 0Soomaspis 0 1 B 0 0 0 0 1 0 0 ndash 0 1 0 1 0 0 1 0 1 0 0 0 0 ndash 0 1 1 1 0 1 0 ndash 0Tariccoia 0 1 B 0 0 0 0 1 0 0 ndash 0 1 0 1 0 0 1 0 1 0 0 0 0 ndash 0 1 1 1 0 1 0 ndash 0Tegopelte 0 0 0 0 0 0 0 0 1 0 1 1 0 0 0 0 1 E 0 ndash 0 1 0 ndash ndash E 1 ndash ndash 0 0 0 0 ndash ndash 0 1 1 0 0 1 0 ndash 0Weinbergina 1 1 1 6 ndash 1 1 0 0 1 1 1 0 ndash ndash 1 0 1 0 1 0 0 0 1 0 0 4 1 0 0 1 0 0 1 0 0 0 0 0 0 0 0 1 2 0 1 0 1 ndash ndash ndash 1 0 0Xandarella 0 0 0 4 0 0 0 0 0 0 0 1 1 1 0 0 0 1 0 0 0 0 1 0 0 0 1 0 ndash 0 1 0 1 1 0 0 2 1 0 0 0 1 0 ndash 1 0 1 1 0 0 0 ndash 0Yohoia 1 1 1 4 ndash 1 0 0 0 1 1 1 0 ndash ndash 1 0 1 0 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 1 2 0 1 0 1 ndash ndash ndash 1 1 0

176 TREVOR J COTTON AND SIMON J BRADDY

Thomas 1998) and developmental data (Mittmann amp Scholtz2003) morphological evidence from fossils supports the viewthat chelicerates have lost the antennae The Hox gene evi-dence suffers from a lack of comparative data from diversemandibulates especially primitive crustaceans (see Akam2000) and the use of Hox gene expression boundaries asmarkers for segmental homology has been criticised (egAbzhanov et al 1999 although relating to posterior Hoxpatterns) Wills et al (1998a pp 43 amp 48) considered thechelicerae to be appendages of the second (deutocerebral)segment but curiously in support of this cited Schram (1978)who unambiguously followed the homology scheme used inthe present study

The anterior-most appendages of Aglaspis were originallydescribed as chelicerae (Raasch 1939) which lead to theirclassification in the Chelicerata (eg Stoslashrmer 1944) Howeverthe morphology of these appendages (see Briggs et al 1979) ismore similar to that of antennae They are narrow relative tothe thoracic endopods of relatively even diameter proximallyand the podomeres are apparently weakly defined Hesselbo(1988 1992) also argued that the anterior appendages ofAglaspis are likely to be antennae The distal parts are un-known (and hence so is their length cf Wills et al 1998ap 58) and there is no evidence that they were chelate contraryto the coding of Wills et al (1998a character 31) Nothingis known about the anterior appendages of BuenaspisLemoneites or Paleomerus and this character is accordinglycoded as missing data in these taxa

2 Form of endopod of appendages of the second segment(0) pediform and (1) anteriorly directed raptorial appendagewith reduced number of podomeres and terminal podomeresbearing spines on distal margins As discussed above cheli-cerae and lsquogreat appendagesrsquo are both likely to represent theappendages of the second segment They also differ from theassumed primitive biramous euarthropod limb in similar waysand therefore are likely to be homologous modifications ofthese appendages The homology of lsquogreat appendagesrsquo andchelicerae was originally proposed by Henriksen (1928) andsupported by Stoslashrmer (1944) but to the present authorsrsquoknowledge no modern author has discussed this possibility

Both lsquogreat appendagesrsquo (Figs 5A D E) and chelicerae(Figs 5B C) are equipped with strong spinose projections onthe outer (dorsal) side of the distal margins of the terminalpodomeres that are lacking on the proximal podomeresSecondly the number of podomeres in both chelicerae andlsquogreat appendagesrsquo is more or less reduced compared to thenumber in the endopods of biramous limbs In the case oflsquogreat appendagesrsquo they are significantly more robust thanthose of posterior endopods a situation that is matched insome chelicerates The homology of the lsquogreat appendagesrsquoof Alalcomenaeus Leanchoilia Yohoia Jiangfengia andFortiforceps is supported by all of these features and is widelyaccepted (eg Wills et al 1998a character 31 Hou ampBergstrom 1997 Bergstrom amp Hou 1998 Hou 1987a) OnlyHou amp Bergstrom (1991 p 183) have suggested albeit inpassing that the lsquogreat appendagesrsquo of all these taxa may beconvergent a view they appear to have changed The endo-pods of the appendages of other taxa are locomotory legswhich either lack strong spines or have spinose extensionsthat are invariably on the medial (ventral) surface and arestronger on the proximal podomeres than the distal ones (egMisszhouia Fig 6A) In the latter case these spines form partof the feeding system along with the gnathobases and are oftenlocated around the middle of the podomere rather than limitedto the distal margin These endopods are directed ventro-laterally as opposed to anteriorly in the case of both cheliceraeand lsquogreat appendagesrsquo The modified second appendages of

Marrella resemble this plesiomorphic condition here referredto as lsquopediformrsquo except in their anterolateral orientation Insome Recent crustaceans this appendage is modified into asecond antenna but in stem-group crustaceans it is pediform(eg Walossek amp Muller 1997 1998) This character is codedas missing data in a number of taxa for which the morphologyof the second segment appendage is unknown

Some authors (Dzik 1993 Chen amp Zhou 1997 Dewel ampDewel 1997 Budd 2002) have suggested that the lsquogreat ap-pendagesrsquo are homologous with the anterior appendages ofanomalocaridids and the anomalocaridid-like Opabinia andKerygmachela (see Budd 1996b 1999b respectively) Theanterior appendages of most anomalocaridids and otherCambrian lobopodians differ from megacheiran lsquogreat append-agesrsquo in that they consist of a large number of segmentsMoreover there is considerable morphological evidence thatanomalocaridids are part of the euarthropod stem group (Willset al 1995 1998a) and not closely related to the lsquogreatappendagersquo taxa (cf Budd 2002) with the possible exception ofParapeytoia lsquoGreat appendagersquo taxa share a suite of derivedcharacters with other crown group euarthropods includingsclerotised dorsal tergites the incorporation of more thanone pair of appendages into the head sclerotisation of theendopods and strong differentiation of the head that wereexcluded from Buddrsquos (2002) analysis The relationships ofanomalocaridids will be considered in detail later by Braddyand co-workers

3 Exopod of appendages of the second segment (0)present and (1) absent or much reduced The raptorial ap-pendages of the second segment (chelicerae and lsquogreat append-agesrsquo) are uniramous as are the corresponding pediformappendages of Marrella Mimetaster Emeraldella Cheloniellonand Sidneyia The plesiomorphic euarthropod state is found inmost other arachnomorphs including trilobites where thebiramous second segment appendages are undifferentiatedfrom those of posterior segments (Edgecombe et al 2000character 78) The exopods of these appendages are alsopresent in basal members of the crustacean crown group(Edgecombe et al 2000 character 79) and in stem groupcrustaceans (Walossek 1993 Walossek amp Muller 1997 1998)

4 Number of head segments (0) 1 (1) 2 (2) 3 (3) 4 (4) 5(5) 6 and (6) 7 The coding of this character by Edgecombe ampRamskold (1999 character 2) explicitly referred to the numberof limb pairs present in addition to the antenna Here thenumber of post-acronal segments present in the head is codedfollowing Wills et al (1998a character 29) In taxa that lack anantenna the number of somites is inferred to be one greaterthan the number of cephalic appendage pairs (cf Wills et al1998a) as described above

Sturmer amp Bergstrom (1978) suggested that the head ofCheloniellon is defined primarily on the basis of appendagetagmosis rather than the fusion of segments under a singlecephalic-shield (cf Hou amp Bergstrom 1997 pp 98ndash9) Accord-ing to this view the head consists of both the cephalic tergiteand the anteriormost free tergite that share uniramousgnathobasic appendages Wills et al (1998a) coded Cheloniel-lon as having six somites incorporated into the head andtherefore presumably accepted this suggestion There is noevidence that the first free tergite of Cheloniellon was incorpo-rated into the head (except perhaps functionally) and thepresent authors prefer to code the number of somites under thecephalic shield In Sidneyia there is a series of uniramousstrongly gnathobasic appendages similar to those of Cheloniel-lon posterior to the head shield All of these appendages wouldpresumably have to be considered part of the head based onappendage differentiation according to the view of Sturmer ampBergstrom (1978) Some autapomorphic states are included

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 177

(none for Sidneyia one for Marrella and six for Emeraldella)since these will become informative if the character is treatedas ordered

Edgecombe amp Ramskold (1999 p 265) described a novelform of cephalic tagmosis The present authors tentativelyaccept their suggestion that in trilobite-like taxa the fourthpair of biramous cephalic appendages was directly under thecephalo-thoracic junction This may explain previous confu-sion about the number of such appendages incorporated intothe trilobite cephalon (eg Cisne 1975 Whittington 1975aBergstrom amp Brassel 1984) However Edgecome amp Ramskoldaccept the view of Chen et al (1977 p 7) that only the firstthree biramous limbs of Misszhouia were structurally and

functionally part of the head Therefore it seems more accept-able to code these taxa as having only four post-acronalsomites than to use the coding scheme of Edgecombe ampRamskold (1999) The partial integration of the first thoracicsomite under the head shield could be considered to be a formof overlap of the trunk by the head shield (ie a distinct stateof character 9 of Edgecombe amp Ramskold 1999 or Character37 herein) Derived states of this character would then poten-tially be synapomorphic for a clade including xandarellidsnaraoiids helmetiids tegopeltids and trilobites Howeversince the degree of overlap in taxa with the fourth biramousappendages under the cephalo-thoracic articulation is identicalto that found between thoracic tergites (State 0 of Character

Figure 5 Diagrammatic reconstructions of raptorial second-segment appendages in lateral view (except whereotherwise stated) and approximate life orientation showing suggested homology of appendage elements (inbrackets) Roman numerals indicate position in the podomere series and do not necessarily imply homology (A)lsquoGreat appendagersquo of Fortiforceps (after Hou amp Bergstrom 1997) (B) Left chelicera of Leiobunum aldrichi(Arachnida Opiliones) (1) in medial perspective and (2) distal parts in anterior perspective (after Shultz 2000)(C) Chelifore of Nymphon (Pycnogonida) (after Child 1997) (D) lsquoGreat appendagersquo of Yohoia (after Whittington1974 fig 2) (E) lsquoGreat appendagersquo of Leanchoilia (modified after Bruton amp Whittington 1983) Not to scaleAbbreviations (apt) apotele (probably homologous with the mandibulate pretarsus) (cx) coxa (dtmrt)deutomerite and (pt) pretarsus

178 TREVOR J COTTON AND SIMON J BRADDY

37) it is here preferred to regard this as a distinct character(Character 9 herein)

The coding of Alalcomenaeus by Wills et al (1998a) ashaving four post-acronal somites is accepted here but fordifferent reasons Briggs amp Collins (1999) have recenty demon-strated that Alalcomenaeus possessed two pairs of biramousappendages on the head in addition to the great appendagesThis would equate to three head somites according to thehomology scheme of Wills et al (1998a) Here the antennaeare inferred to be lost and therefore four segments incorpo-rated into the head

Recently a number of authors have agreed that the plesio-morphic condition for euarthropods is a four-segmented headas found in stem group crustaceans (Scholtz 1997 Walossek1993 Walossek amp Muller 1997 1998 Edgecombe et al 2000)However reconstruction of the euarthropod stem groupclearly indicates that even crownward taxa had a head consist-ing of only the antennal segment and acron as found intardigrades and onychophorans Therefore it seems that thefour-segmented head is pleisomorphic only for crown groupeuarthropods Some fully arthropodised fossil taxa also have ashorter head that may be pleisomorphic such as the two-segmented head of Marella Retifacies is coded following therecent redescription of Hou amp Bergstrom (1997) rather thanon the basis of the coding by Edgecombe amp Ramskold (1999)for which they provide no explanation A partial uncertaintycoding is used for Aglaspis in which the number of post-antennal cephalic appendages is either three or four (Briggset al 1979)

5 Orientation of the antennae (0) directed anterolaterally(1) strongly deflected laterally and (2) placed well inside shieldmargin curing posteriorly from a transverse proximal element(cf Edgecombe amp Ramskold 1999 character 3) The anteriorappendages of Aglaspis (see character 1) are clearly directedanterolaterally and it is presumed that they continued past thehead shield margin The anterior appendages of arthropodstem group taxa such as Aysheaia Kerygmachela and Anoma-locaris are either directed anterolaterally or ventrally but arenot strongly-deflected laterally Therefore the outgroup iscoded as State 0 This character is coded as missing data fortaxa where the presence of antennae is equivocal (those codedas missing data for Character 1) and as inapplicable in taxawhere the antennae are considered absent (coded as State 1 forCharacter 1)

6 Length of distal spines on terminal podomeres of endo-pods of second segment appendages (0) absent or shorer thanpodomeres (1) subsequal to length of podomeres and (2)longer than the entire podomere series The spinose projectionsof the raptorial appendages described above vary in length Inchelicerae (Figs 5BC) and some lsquogreat appendagesrsquo (eg thoseof Yohoia Fig 5D) the most distal spine is equal in length tothe spinose terminal podomere forming a chelate structureIn Fortiforceps (Fig 5A) the spines are short relative tothe lengths of the podomeres but in Alalcomenaeus andLeanchoilia (Fig 5E) they are extremely long so that thespines are much longer than the entire podomere series Asdescribed above in taxa with pediform endopods of the secondsegment appendages spines on the distal podomeres are shortor entirely absent

7 Chelicerae (0) absent and (1) present Despite theirsimilarities to lsquogreat appendagesrsquo the chelicerae of the eucheli-cerates and chelifores of the pycnogonids are clearly distinctand have been widely recognised as a chelicerate synapomor-phy Chelicerae differ from any lsquogreat appendagesrsquo by thecombination of a smaller number of podomeres the presenceof only a single terminal element and only a single spinoseprojection on the dorsal side of the podomere series (Fig 5)

Amongst extant chelicerates only the pycnogonid Pallenopsishas chelicerae of four podomeres comparable to the numberin lsquogreat appendagesrsquo Bergstrom et al (1980) considered thatthe chelifores of the Devonian pycnogonid Palaeoisopus alsoconsist of four segments but this is not well supported by theirfigures

8 Distal spines of second segment endopods terminating inannulated flagellae (0) absent and (1) present It is widelyrecognised (eg Stoslashrmer 1944 Simonetta 1970 Bruton ampWhittington 1993 Briggs amp Collins 1999) that the terminalparts of the extended spines of Alalcomenaeus and Leanchoiliaformed annulated flagellae Nothing similar is known fromhomologous appendages of any other arthropod althoughsimilar processes may have operated in the transformation ofthe endopod as a whole into the second antennae of derivedcrustaceans and in the origin of multiramous antennulae (firstantennae) in malacostracans

242 Posterior appendages 9 Appendages of first thoracicsomite underneath the cephalo-thoracic articulation (0) ab-sent and (1) present (cf Edgecombe amp Ramskold 1999character 2) For a discussion of this character see Character4 herein

10 Exopods of appendages of third to fifth segments (0)present and (1) reduced or absent A number of taxa haveuniramous appendages on the third to fifth segments In mostthese segments are incorporated into the head (Yohoia cheli-cerates) but in others all (eg in Sidneyia) or some (eg inCheloniellon) of these segments are post-cephalic In all casesthe appendage is morphologically similar to the endopods ofbiramous limbs of other taxa andor other segments and it isinterpreted that the exopod is lost

11 Endopods of thoracic appendages (0) present and (1)reduced or absent The opisthosomal appendages of chelicer-ates lack endopods or have the endopods much reduced (egLimulus Siewing 1985 fig 838 Shultz 2001) a situation that isalso found in the uniramous thoracic appendages of Yohoia(Whittington 1974) It has been suggested that Helmetia(Briggs et al 1994) lacks endopods but pending a redescrip-tion of this genus and considering that they have been docu-mented in the otherwise very similar Kuamaia this is coded asuncertain

12 Exopod shaft of numerous podomeres each bearing asingle seta (0) present and (1) absent The exopods ofcrustaceans (at least plesiomorphically see eg Walossek ampMuller 1998 figs 5middot5 amp 5middot6) and of Marrella and Mimetasterhave multi-annulated shafts with each podomere bearing asingle seta In other taxa the exopod shafts consist of one oftwo lobes bearing numerous setae

The polarity of exopod segmentation is uncertain Thelateral flaps of stem group arthropods such as Opabinia(Whittington 1975b Budd 1996b) and anomalocaridids (egHou et al 1995) are certainly unsegmented but as suggestedby Buddrsquos (1996b fig 8) reconstruction this does notnecessarily suggest the form of the primitive euarthropodexopod Rather segmentation may have been a result of thesclerotization of the cuticle of the exopod shaft The outgroupwas coded as uncertain for this character According to thefirst interpretation of aglaspidid appendage morphology(coded as Aglaspidida 1) the exopods are lacking on allappendages and consequently this and all other charactersdescribing exopod morphology are coded as inapplicable

13 Exopod shaft differentiated into proximal and distallobes (0) absent and (1) present Ramskold amp Edgecombe(1996 Edgecombe amp Ramskold 1999 character 26) havediscussed the distribution of the trilobite-type bilobate exo-pods recognised by this character (Fig 6A) Among taxaincluded here that were not considered by Edgecombe amp

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 179

Ramskold (1999) exopods consist either of a single flattenedlobe or a homonomous series of podomeres (Fig 6B C) Nostem group euarthropod has a bilobate exopod and theoutgroup is consequently also coded as State 0

14 Proximal lobe of exopod (0) flattened lobe and (1)slender shaft

15 Distal lobe of exopod (0) small to moderate sized flapwith short to moderately long attachment to proximal lobeand (1) large teardrop-shaped with long attachment to proxi-mal lobe Neither of these characters can be coded for taxathat do not have an exopod differentiated into proximal anddistal lobes (Character 13 State 1) without asserting thehomology of the single-lobed exopod with one of these partsThe coding of Edgecombe amp Ramskold (1999 p 280) implic-itly homologizes the exopods of Sidneyia and Retifacies withthe proximal lobe In these taxa this view is perhaps supportedby the presence of lamellate exopod setae which match thoseof the trilobite-type proximal lobe However in other taxawith a single-lobed exopod such as Alalcomenaeus (see Briggsamp Collins 1999) the exopod is fringed with sharp spines likethe distal lobe of differentiated exopods Alternatively thebilobate exopod may have originated from the division of aprimitively single-lobed structure The single-lobed exopod isnot clearly homologous with either lobe of divided exopodsand both these characters are consequently coded as inappli-cable to taxa without differentiated proximal and distal exopodlobes Consequently (see Character 13 herein) these characterscan only be coded for taxa that were previously considered byEdgecombe amp Ramskold (1999) and their coding is followedhere except in the cases of Sidneyia and Retifacies

16 Exopod shaft is a deep rounded flap (0) absent and (1)present All bilobate (Character 13) and segmented (Character12) exopod shafts are long and relatively narrow structures (seeFig 6) whereas some single-lobed exopod shafts have theform of rounded flaps These flap-like exopod shafts are largecompared to the length of the setae and at least half as deep asthey are long This appendage structure is found in cheliceratesand lsquogreat appendagersquo arthropods

17 Medially-directed exopod setae (0) absent and (1)present Walossek amp Muller (1998 p 194) suggested that thetilting of exopod setae towards the endopod in post-antennularlimbs with a multi-annulated exopod was an autapomorphyuniting crustaceans and all members of the crustacean stemgroup (the crustacean total group sensu Ax 1986 see Fig 6C)This character is shared with the marellomorphs Marrellaand Mimetaster (see Fig 6B Sturmer amp Bergstrom 1976Bergstrom 1979 fig 1middot3A B) In other taxa the setae eithersurround the entire margin of the exopod shaft or are directeddorsally (Fig 6A) The identification of setae on the dorsalsurface of the lateral flaps in Opabinia (Budd 1996b) suggeststhat the condition in marellomorphs and crustaceans isderived

18 Lamellate exopod setae (0) absent and (1) presentLamellate exopod setae originally described from trilobitesare a classic synapomorphy of the Arachnomorpha (see egBergstrom 1992 Hou amp Bergstrom 1997 pp 42ndash3) They areperhaps best known from Misszhouia (see Chen et al 1996Hou amp Bergstrom 1997 Fig 6A) These setae differ from themore spinose setae of other taxa (Fig 6C) in that they are wideand flat and imbricate over the length of the exopod shaft Thesetae of Marrella (Fig 6B) and similar taxa have variouslybeen considered lamellate (Bergstrom 1979) or non-lamellate(Bergstrom 1992) Since they do not imbricate and do notseem to be strongly flattened (Whittington 1971 Sturmer ampBergstrom 1976 fig 9a) Marrella and Mimetaster are codedas State 0 The setae of Helmetia as shown in Briggs et al(1994 fig 141) seem to be of the lamellate type The setae ofSinoburius have been described by Hou amp Bergstrom (1997p 85) as similar to those of Misszhouia Only the setae areknown of the appendages of Buenaspis (Budd 1999a) andthese also appear to be of the trilobite-type

Figure 6 Diagrammatic reconstructions of (A) arachnomorph and(B C) non-arachnomorph biramous appendages (A) The lsquotrilobite-typersquo biramous appendage of Misszhouia longicauda (after Chen et al1997) (B) Appendage of Marrella splendens (after Whittington 1971)(C) Appendage of the stem-lineage crustacean Martinssonia elongata(after Muller amp Walossek 1997 1998) Not to scale Abbreviations(bas) basis (dle) distal lobe of exopod shaft (ls) lamellar exopod setae(pe) proximal endite or pre-coxa (ple) proximal lobe of exopod shaft(ses) segmented exopod shaft and (ss) spinose exopod setae

180 TREVOR J COTTON AND SIMON J BRADDY

The homology of the book-gill lamellae of xiphosurans withlamellar setae was supported by Walossek amp Muller (1997p 149) and Edgecombe et al (2000 p 174) but rejected bySturmer amp Berstrom (1981) on the grounds that Weinberginapossesses Limulus-like gill lamellae and fringing setaeBook-gills have also been described from a eurypterid (Braddyet al 1999) There are certainly major morphologicaldifferences between book-gills and trilobite-type exopodsFollowing most recent opinion and pending further studyWeinbergina and Eurypterida are coded as possessing lamellatesetae

The form of the setae of Yohoia is uncertain (Whittington1974) the spinose setae seen in the most common reconstruc-tion (Gould 1989 Briggs et al 1994) are not justified by thespecimens Amongst other great appendage arthropods theform of the setae appears to vary those of Jianfengia (seeChen amp Zhou 1997 p 74) and Leanchoilia (see Bruton ampWhittington 1983) are lamellate while those of Fortiforceps(Hou amp Bergstrom 1997) and Alalcomenaeus (Briggs amp Collins1999) are spinose and less densely packed

19 Gnathobase on basis andor prominent endites onendopod (0) present and (1) absent (cf Edgecombe ampRamskold 1999 character 29 Wills et al 1998a characters 36amp 59) The presence of gnathobases on the appendages of someanomalocaridids (Hou et al 1995) and their general distribu-tion amongst non-arachnomorph euarthropods suggests thatState 0 is plesiomorphic The homology of the various enditesand gnathobasic structures found in euarthropods is in need ofcareful assessment and therefore a very general coding (fol-lowing Edgecombe amp Ramskold) is used for this character

243 Eyes 20 Position of lateral facetted eyes (0)ventral and stalked (1) dorsal and sessile and (2) absent (cfEdgecombe amp Ramskold 1999 character 4) Partial uncer-tainty coding is used for a number of taxa where the dorsalsurface is known but the ventral morphology is not andtherefore the presence of ventral eyes cannot be discountedFor example there is good evidence that Leanchoilia lackeddorsal sessile eyes but ventral stalked eyes may have beenpresent (as in L hanceyi see Briggs amp Robison 1984) and a(02) partial uncertainty coding is used This is also the case inmany naraoiids such as Buenaspis However only the outlineof the head shield of Liwia is known and the absence of dorsaleyes in the reconstruction of Dzik amp Lendzion (1988) isconjectural It is unclear whether the autapomorphic conditionof stalked dorsal eyes in Mimetaster (see Sturmer amp Bergstrom1976) is homologous with State 0 or State 1 and a partialuncertainty coding is also used

Whittington (1974) was somewhat equivocal about thenature of the lateral lobes anterior to the head shield ofYohoia These more closely resemble the ventral stalked eyes ofAlacomenaeus as recently described by Briggs amp Collins(1999) than either Whittingtonrsquos (1974) or subsequent (egGould 1989 fig 3middot18 Briggs et al 1994 fig 153) reconstruc-tions suggest In a well preserved specimen showing the dorsalaspect (USNM 57696 Whittington 1974 pl 2 figs 1ndash3) and ina laterally compressed specimen (USNM 57694 Whittington1974 pl 1 fig 1) they are clearly seen to be stalked andrelatively small lobate structures That these structures arelikely to represent stalked ventral eyes was reflected in thecoding of Wills et al (1998a characters 26 amp 27)

21 Visual surface with calcified lenses bounded withcircumocular suture (0) absent and (1) present

22 Dorsal bulge in exoskeleton accommodating drop-shaped ventral eyes (0) absent and (1) present

23 Eye slits (0) absent and (1) presentCharacters 21 to 23 are used exactly as described by

Edgecombe amp Ramskold (1999 character 5) Character 21 is

inapplicable to taxa that do not possess eyes (Character 20State 2)

24 Dorsal median eyes (0) absent and (1) present Dorsalmedian eyes on a tubercle are considered a synapomorphy ofthe Chelicerata (Dunlop amp Selden 1997 Dunlop 1999) Thenature and position of the structures interpreted as dorsalmedian eyes in Mimetaster (Sturmer amp Bergstrom 1976p 83ndash4) are regarded as equivocal

244 Ventral cephalic structures Many characters of theventral surface of the euarthropod head (see Fig 7) of poten-tial phylogenetic utility are poorly known in many importanttaxa In particular the presence of pre-hypostomal frontalorgans (Fig 7A CndashE) is not coded herein (see Edgecombe ampRamskold 1999 p 272) Definitive evidence of the absence ofthese structures is available for very few taxa and the homol-ogy of these structures with the maculae of the trilobitehypostome (see Fig 7B) and the frontal organs of the crusta-cean labrum (eg see Muller amp Walossek 1987 p 39) isuncertain Secondly the detailed homology of the trilobitehypostome with that of other putative arachnomorphs isunclear and consequently a very broad definition is usedherein For example various pre-hypostomal sclerites (Fig7A D) in other arachnomorphs may have been incorporatedalong with a primitive hypostome into a single sclerite that isrecognised as the hypostome in trilobites The structure of thehypostome (eg the presence of paired anterior wings dorsal tothe antennae Fig 7B C) may also be a source of additionalcharacters

25 Expanded cephalic doublure (0) absent and (1)present maximum width more than 30 length of head shieldor more than 25 width of pygidium Chen et al (1996)suggested that the wide doublure of Soomaspis and Tariccoia isa synapomorphy uniting these taxa The doublure of Buenaspis(see Budd 1999a) is poorly known but based on the width ofthe heavily crushed region of the cephalic margin and theposition of a possible impression of the edge of the doublure inone specimen (Budd 1999a pl 1 fig 1) it also appears to berather wide (ie greater than 30 of the length of the headshield) Following Edgecombe amp Ramskoldrsquos coding ofSidneyia the present authors do not consider the postero-median expansion of the doublure of for example Emeraldellaor Retifacies (see Bruton amp Whittington 1983 Hou ampBergstrom 1999 respectively) to be homologous with State 1but to represent the hypostome (see Character 26)

26 Anteromedian margin of cephalon notched accomo-dating strongly sclerotised plate (0) notch and plate absentand (1) notch and plate present (cf Edgecombe amp Rasmkold1999 character 10) The apomorphic state (illustrated inFig 7D) is only known in the taxa discussed by Edgecombe ampRasmkold (1999) Reconstructions of Yohoia show a roundedlobe between the eyes (see Character 20) anterior to anddistinct from the head shield which resembles the anteriorsclerite recognised here This seems to be a misinterpretationSpecimens preserved in dorsal aspect (USNM 57696Whittington 1974 pl 2 figs 1ndash3) and lateral aspect (USNM57694 Whittington 1974 pl 1 fig 1 USNM 155616Whittington 1974 pl 5 fig 2) seem to clearly show that thislsquomedian lobersquo is a downward curving pointed extension of thehead shield

27 Hypostomal sclerite (0) median extension of thedoublure with no suture (1) natant sclerite not in contactwith doublure (2) with narrow overlap with pre-hypostomalsclerite (3) narrow attachment to doublure at hypostomalsuture and (4) absent The homology of hypostomes andlabrums has been discussed by Haas et al (2001) and Budd(2002) The euarthropod labrum is likely to represent a pos-teroventral extension of the pre-segmental acron (Scholtz

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 181

Figure 7 Ventral reconstructions of arachnomorph arthropod heads (A) Misszhouia longicaudata (after Chenet al 1997 figs 2ndash4 7) (B) Ceraurinella typa (after Chen et al 1997 fig 7) (C) Agnostus pisiformis (after Mulleramp Walossek 1987) (D) Kuamaia lata (after Edgecombe amp Ramskold 1999 figs 32 5 amp 6) (E) Cindarella eucala(after Ramskold et al 1997) and (F) Emeraldella brocki (after Bruton amp Whittington 1983) Not to scaleAbbreviations (aw) anterior wing beneath antennae (bs) boomerang-shaped sclerite (bl) blade-like process ofposterior wing (d) doublure of head-shield (cs) connective suture (f) fenestra (fs) facial suture (h) hypostome(hl) hypostomal lobe (emerging through fenestrae of Agnostus) (hs) hypostomal suture (if) intervening field ofhypostomal complex (lb) lateral bridge (lfo) lateral frontal organ (mb) median body (mbr) median bridge (mf)median furrow (mfb) mouth field bridge (mfo) median frontal organ (ol) ovate lobe and (phs) pre-hypostomalsclerite Post-antennal appendages and distal parts of antennae omitted

182 TREVOR J COTTON AND SIMON J BRADDY

1997) This structure is often sclerotised and covers the mouthventrally It is this sclerite that is here identified as thehypostome Therefore this character is considered absent intaxa lacking a sclerite covering the mouth irrespective of themorphology and position of the labrum itself which at leastpartly covers the mouth in all euarthropods other thanpycnogonids (see Edgecombe et al 2000 p 167) The lsquofleshylabrumrsquo of crustaceans does not appear to be sclerotised and ahypostome is consequently considered to be absent Althoughmany derived features are associated with the crustaceanlabrum this is not a good reason to consider it non-homologous with the hypostome-bearing structure of otherarthropods as Walossek amp Muller (1990) argued

In many taxa the mouth is covered by a posteromedianextension of the doublure here considered to represent thehypostome (eg Emeraldella Fig 7F Aglaspis see Hesselbo1992 fig 52) In others the hypostome is separated from thedoublure either by a suture a pre-hypostomal sclerite (egKuamaia lata Fig 7D Saperion glumaceum see Edgecombe ampRamskold 1999 figs 3 amp 4) or an intervening region ofunsclerotised cuticle (the natant condition of Fortey 1990beg Cindarella eucala Fig 7E) The homology of pre-hypostomal sclerites and hypostomes in helmetiids trilobitesand naraoiids (see Fig 7) has been discussed by Chen et al(1996) and Edgecombe amp Ramskold (1999) but it is as yetunclear whether any of the character states described here arederived from any other They are treated here as distinct andthe character as unordered pending further study

No hypostome has been identified in Yohoia (see above)Alalcomenaeus Jianfengia Fortiforceps or Leanchoilia andthere is certainly no broad posterior extension of the doublurein any of these taxa Since there is also no pre-hypostomalsclerite they have received a (134) partial uncertainty codingThe attachment of the hypostome of Tegopelte is unclear butit does not seem to be attached to any doublure (Whittington1985) and therefore it is given an uncertainty (12) coding

The lsquorostrum-like structurersquo on the ventral surface ofLemoneites appears to be similar in morphology and positionrelative to the anterior margin of the head shield (Flower 1968pl 8 figs 4 amp 13) to that described from Aglaspis and is codedas State 0 In many taxa the hypostome is unknown but inonly a small number can it be coded reliably as absentHughesrsquo (1975) suggestion that the hypostome of Burgessia isabsent is accepted preliminarily but an extension of thedoublure may be visible in some of Hughesrsquo figures Fortey(pers comm 2000) also doubts Hughesrsquo assertion

28 Visible ecdysial sutures (0) absent and (1) present Themarginal ecdysial sutures of chelicerates and the dorsal suturesof trilobites are almost certainly not homologous but thepresence of sutures and their position (see Character 29) arecoded separately to allow this to be tested Wills et al (1998aCharacter 2) coded marginal sutures as present in Sidneyiafollowing Brutonrsquos (1981) suggestion but this is consideredequivocal here

29 Position of ecdysial sutures (0) marginal and (1) dor-sal This character is inapplicable to taxa lacking ecdysialsutures (Character 28 State 0) Dorsal sutures whilst presentin the representatives of the Trilobita coded here are probablynot a synapomorphy of trilobites as a whole but for a clade oftrilobites excluding the Olenellida (Whittington 1989 Fortey1990a)

245 Exoskeletal tergites and thoracic tagmosis 30 Min-eralised cuticle (0) absent and (1) present Calcification of theexoskeleton is one of the most convincing synapomorphiesof the Trilobita (Fortey amp Whittington 1989 Ramskold ampEdgecombe 1991 Edgecombe amp Ramskold 1999 Character 1)Cuticle mineralisation is also coded as present in aglaspidids

following Briggs amp Fortey (1982) who suggested that theaglaspidid exoskeleton was originally phosphatic Paleomerusprobably also had a mineralised cuticle but Hou amp Bergstrom(1997 p 97) argued that it was likely to be calcareous Anundescribed Silurian aglaspidid may also have had an origi-nally calcitic cuticle (Fortey amp Theron 1994 p 856) Becauseof the uncertainty about the chemical composition of theexoskeleton in these forms cuticle mineralisation is coded aspotentially homologous in all these taxa and no attempt ismade to code separate states for phosphatic and calcareousmineralisation

31 Trunk tergites with expanded lateral pleurae coveringappendages dorsally (0) absent and (1) present Whilst thepresence of paratergal folds may be a synapomorphy at thelevel of the Euarthropoda (Boudreaux 1979 Wagele 1993Edgecombe et al 2000 Character 142) these are at mostsmall reflections of the margin of the tergites in most euarthro-pods In arachnomorph taxa these are expanded to form largelateral pleurae that cover the appendages dorsally (see egLimulus and Triarthus in Boudreaux 1979) This is inferred tobe the case in some taxa where dorsal features and appendagesare poorly known on the basis of their possession of tergiteswith wide pleural regions This feature was suggested as typicalof lamellipedians (=arachnomorphs) by Hou amp Bergstrom(1997 pp 42ndash3)

The arrangement of the thorax of Marrella Mimetaster andBurgessia differs from that of all the other taxa considered herein that the appendages seem to be attached to the lateralmargins of the body The trunk tergites do not cover theappendages and seem to lack pleurae entirely although theyare covered by a posterior extension of the head shield inBurgessia This character has been clearly described inMimetaster (Sturmer amp Bergstrom 1976 pp 87ndash90 fig 8) andBurgessia (Hughes 1975 p 421)

32 Free thoracic tergites (0) present and (1) absentFollowing Edgecombe amp Ramskold (1999 Character 16) anumber of taxa lack functional post-cephalic articulations andconsequently lack free thoracic tergites Despite this theycode these taxa for a number of characters relating to thestructure of thoracic tergites (eg their characters 18 and 19)These characters are treated here as inapplicable to taxawithout free tergites and the definition of this character hasbeen modified from that of Edgecombe amp Ramskold (1999) tomake this more explicit

33 Decoupling of thoracic tergites and segments (0)absent and (1) present Ramskold et al (1997) described thischaracter of Cindarella and Xandarella where the thoracictergites correspond to a variable increasing posteriorly num-ber of appendage pairs This character cannot be coded fortaxa in which free thoracic tergites are absent (Character 32State 1)

34 Tergite articulations (0) tergites non-overlapping (1)extensive overlap of tergites and (2) edge-to-edge pleuralarticulations In most of the taxa considered here the thoracictergites overlap considerably and relatively evenly over theirwidth This contrasts with the primitive euarthropod condi-tion where the tergites do not overlap medially as seen in stemgroup crustaceans In trilobites such as Helmetia and Kuamaia(see Edgecombe amp Ramskold 1999 character 18) overlap ofthoracic tergites is limited to a well-defined axial region andthe lateral pleurae meet edge-to-edge

The thoracic articulations of naraoiids with free thoracictergites are in some ways similar to those of trilobites with astrong overlap medially that is lacking abaxially andanterolateral articulating facets (Budd 1999a Ramskold ampEdgecombe 1999) However the distribution of these features

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 183

in other trilobite-like arachnomorphs is unclear and a restric-tive coding is used here The present authors do not considerthat this character can be applied reliably to the fused thoracictergites of Saperion Tegopelte and Skioldia and it is coded asinapplicable to all taxa lacking free thoracic tergites (Character32 State 1)

35 Trunk effacement (0) trunk with defined (separate orfused) tergite boundaries (1) trunk tergite boundaries effacedlaterally and (2) trunk tergite boundaries completely effaced(cf Edgecombe amp Ramskold 1999 Character 15)] Contrary toEdgecombe amp Ramskold (1999) the present authors considerSkioldia and Saperion to show a distinct form of tergiteeffacement where the fused tergite boundaries are defined byfurrows axially but effaced laterally The boundaries areeffaced at least laterally in Tegopelte but the axial region ofthe exoskeleton is unknown which accordingly is given amultistate uncertainty coding

36 Cephalic articulation fused (0) absent and (1) presentIn Tegopelte Saperion and Skioldia the articulation ofthe head with the thorax is non-functional and the entireexoskeleton forms a single tergite

37 Head shield overlap of thoracic tergites (0) overlapabsent or identical to overlap between thoracic segments (1)head shield covers first thoracic tergite only and (2) headshield covers mutliple anterior trunk tergites

38 Head shield articulates with reduced anterior thoracictergite (0) absent and (1) present Amongst taxa showing aposteriorly expanded head shield ie one that overlaps thethorax Edgecombe amp Ramskold (1999 Character 9) identifiedtwo distinct character states One of these ndash overlap of only theanteriormost thoracic tergite ndash was limited to taxa assigned tothe Liwiinae (sensu Fortey amp Theron 1994) and another ndashoverlap of multiple tergites with attachment to a reducedanterior thoracic tergite ndash to the Xandarellida None of theadditional taxa considered herein (including Buenaspis whichwas assigned to the Liwiinae by Budd 1999a) shows thesedistinctive morphological features However the expandedhead shields of Marrella Mimetaster and Burgessia which donot articulate with a narrow anterior thoracic tergite may behomologous with the expanded head shields of xandarellidsTo recognise this the degree of overlapping of the thorax andthe possession of the reduced anterior tergite are coded sepa-rately These characters are both coded as inapplicable to taxain which the head shield and thoracic tergites are fused(Character 36 State 1) The crustacean carapace which ismost similar to the head shield of Burgessia is seeminglyabsent in stem group crustaceans (Walossek amp Muller1998)

39 Trunk narrowed anteriorly relative to head shieldwidest posteriorly (0) absent and (1) present (cf Edgecombeamp Ramskold 1999 Character 14) The anterior narrowing ofthe trunk caused by the reduction of opisthosomal segment 1in xiphosurids (Weinbergina of the taxa analysed hereAndersen amp Selden 1997 Dunlop amp Selden 1997 Character14) is not considered to be homologous with the unusual shapeof the thorax in naraoiids recognised by their coding

40 Boundaries of anterior trunk segments reflexed antero-laterally (0) absent boundaries transverse or reflexed postero-laterally and (1) present

41 Joints between posterior tergites functional anteriorones variably fused (0) absent and (1) present

42 Posterior tergite bearing axial spine (0) absent and (1)present

Characters 40 to 42 are coded following Edgecombe ampRamskold (1999 characters 17 19 amp 23 respectively) Inaddition to the taxa considered here a thoracic axial spine ispresent in some olenelloid trilobites (see Lieberman 1998

Characters 73 amp 74) and in the aglaspidid Beckwithia (seeHesselbo 1989)

246 Posterior body termination 43 Postabdomen of seg-ments lacking appendages (0) absent and (1) present

44 Length of postabdomen (0) one segment (1) twosegments (2) three segments and (3) five segments In sometaxa a variable number of posterior thoracic segments bearcomplete tubular tergites and lack appendages In Aglaspisautapomorphically it seems that the posterior three thoracicsegments lack appendages (Briggs et al 1979 Hesselbo 1992)but have unmodified tergites These situations are recognisedas potentially homologous by the coding used here Character44 is coded as inapplicable to taxa lacking a postabdomen

45 Posterior tergites strongly curved in dorsal aspect com-pared to anterior tergites (0) absent and (1) present Asrecognised by Wills et al (1998a Character 17) in some taxawhere the tergites are distinct the curvature of thoracic tergitesincreases posteriorly so that the posterior tergites are highlycurved to semicircular in dorsal aspect This situation isnot known from crustaceans (except isopods) or stem grouparthropods

46 Posterior segments reduced and with highly reducedappendages (0) present and (1) absent In some taxa there area large number of posterior segments that are short sagitallyand have appendages that are much reduced in size comparedto anterior trunk appendages These somites are incorporatedinto the pygidium in some taxa but primitively they arecovered by tiny free trunk tergites In the derived state thetrunk somites and limbs are of a relatively constant size Thedistinction between these states can be seen when attempting tocount the number of segments that make up the body In taxashowing State 0 the number of segments is very high anddifficult to count whereas in other taxa the number ofpost-cephalic segments is easily assessed

47 Pygidium (0) absent and (1) present A pygidium isrecognised here as a posterior tagma consisting of a number offused segments under a single tergite which may or may notincorporate the post-segmental telson This is different to theuse of this term by Edgecombe amp Ramskold (1999) whichfollows Ramskold et al (1997) and very different to its use byWills et al (1998a p 53) as a synapomorphy of the TrilobitaThe present authorsrsquo definition recognises the situation inRetifacies where the spinose postsegmental telson is autapo-morphically not fused to the pygidium as homologous toother multisegmented posterior tagma which include thetelson The distinction between the pygidium and an expandedpost-segmental telson can also be seen in the position of theanus which is consistently in the posteriormost pre-telsonicsegment amongst putative arachnomorphs (see Character 48)In taxa with a pygidium the anal segment is fused into thepygidium (see eg Kuamaia Edgecombe amp Ramskold 1999fig 6) whereas in those taxa lacking a pygidium the anus isbetween the final trunk tergite and the telson

Ramskold et al (1997) argued that the posteriormost tergiteof xandarellids (which incorporates the anal segment) is nothomologous to the posterior tagma recognised as pygidiaherein because posterior thoracic tergites also cover multiplesegments in xandarellids (see Character 33) Evidence ofsegmentation of the pygidium in a variety of taxa suggests thatthe number of tergites fused to form the pygidium is greaterthan the number of appendages For example the pygidium ofKuamaia has two pairs of lateral spines but at least fourpairs of appendages (Hou amp Bergstrom 1997 Edgecombe ampRamskold 1999) and that of Triarthrus has only five axialrings posterior to the articulating ring but over 10 pairs ofappendages (Whittington amp Almond 1987) The fact thatdecoupling of tergites and segments is evident in the thorax of

184 TREVOR J COTTON AND SIMON J BRADDY

Xandarella and Cindarella does not necessarily suggest that asimilar more widespread decoupling in pygidia is the result ofa different developmental process and hence that they arenon-homologous Instead the situation in the xandarellidthorax is potentially homologous to that found (more widely)in pygidia and consequently the terminal xandarellid tergite isa pygidium It is unclear if decoupling of tergites and somites isprimitively limited to the terminal tergite or primitively aproperty of the arachnomorph post-cephalon as a whole

It has been suggested that the tiny pygidium of olenelloidtrilobites which probably best reflects the primitive trilobitestate consists only of the post-segmental telson (Harrington inMoore 1959) and therefore is not a true pygidium HoweverWhittington (1989) showed that the olenelloid pygidium con-sists of at least two and possibly as many as five or sixsegments and therefore is likely to be homologous with thepygidium of other trilobites

48 Position of the anus (0) terminal within telson and (1)at base of telson In crustaceans and stem group arthropodsthe anus is terminal or otherwise situated in the telson (egRehbachiella Walossek 1993 fig 15D) In putative arachno-morphs the anus is either at the junction of the posteriormostthoracic segment and the telson or ventral within a fusedpygidium In the case of taxa with a pygidium the anus isanterior to the posterior margin and where known positionedbetween the posteriormost pair of appendages suggesting thatit is anterior to the post-segmental telson (see eg OlenoidesWhittington 1980)

49 Pygidium with median keel (0) absent and (1) presentEdgecombe amp Ramskold (1999 Character 21) considered thepresence of a median keel on the pygidium as a synapomorphyuniting Soomaspis and Tarricoia They did not explain howthis structure could be distinguished from the raised pygidialaxis of trilobites Homology of these structures is not sup-ported here because the axis of more primitive trilobites(including Eoredlichia) is quite distinct morphologically fromthe naraoiid keel Budd (1999a) noted the presence of a keel onthe pygidium of Buenaspis and suggested (Budd 1999a p 102)that it may be an artefact of dorsoventral compaction How-ever contrary to the coding of Edgecombe amp Ramskold(1999) a keel is visible on the pygidium of Liwia convexa (Dzikamp Lendzion 1988 fig 4CD) preserved in full relief Thereforeit seems unlikely that the keel is a taphonomic artefact

50 Pygidium with broad-based median spine (0) absentand (1) present

51 Pygidium with lateral spines (0) present and (1) absentEdgecombe amp Ramskold (1999 Character 22) coded thepresence of a median spine and two pairs of lateral spines aspotentially homologous in Sinoburius Kuamaia and HelmetiaThe distribution of lateral spines is much wider than that ofterminal spines and therefore they are coded separately hereIn trilobites it has widely been recognised that lateral spinesrepresent the original segmentation of the pygidium Thiscannot be the case for median spines Characters 49 to 51are not applicable to taxa lacking a pygidium (Character 47State 0)

52 Expanded post-segmental telson (0) absent and (1)present In a range of taxa the posterior-most tergite is a large(relative to the thoracic tergites) structure that lacks anyevidence of segmentation and is interpreted as representing anexpanded tergite of the post-segmental telson from whichsegments are released anteriorly during ontogeny On the otherhand the posteriormost tergite of Marrella and probablyMimetaster is small compared to the thoracic tergites androunded This element probably represents the plesiomorphiceuarthropod telson The homology of this character in mosttaxa is clear and only doubtful in Retifacies in which it may

be segmented The figures of Hou amp Bergstrom (1997) providelittle unequivocal evidence for a telson in Retifacies but thepresence of this structure is very clear in colour photographs(eg Chen et al 1996 fig 198A) However these figures donot convincingly demonstrate the segmentation of the telsonRetifacies is interpreted here as being unique in possessingboth a pygidium and an expanded post-segmental telson

53 Telson shape (0) spinose and (1) paddle-shaped Theshape of the expanded telsons identified above varies frombroadly spinose to flattened and paddle-like This differencecan be recognised both on the basis of the ratio of length tobasal width (spinose telsons are relatively long) and by thechange in width posteriorly (spinose telsons reduce in widthposteriorly whereas paddle-shaped telsons increase in width)In eurypterids a wide range of telson shapes is found includ-ing both character states described here It is likely that theplesiomorphic condition is a spinose telson This character isinapplicable to taxa lacking an expanded telson

54 Post-ventral furcae (0) absent and (1) present Thischaracter recognises the potential homology of unsegmentedpaired structures that articulate with the segment immediatelyanterior to the telson The potential homology of these struc-tures (the post-ventral plates) in Emeraldella and Aglaspis wasrecognised by Wills et al (1998a Character 70) and inEmeraldella and Sidneyia by Edgecombe amp Ramskold (1999Character 25) The homology of the segmented pygidial caudalfurcae of Olenoides (see Whittington 1975a 1980) with thesestructures is considered doubtful and coded as equivocal

Despite their distinctive morphology the long caudualfurcae of Cheloniellon may be homologous with the furcae ofAglaspis Emeraldella and Sidneyia This is supported by thecaudal furcae of the Ordovician cheloniellid Duslia which aremore similar in morphology to those of Emeraldella than ofCheloniellon Chlupac (1988 pp 614ndash16) considered thesestructures to be on the terminal tergite in Duslia However afaint tergite boundary is apparent posterior to the attachmentof the furcae (see Chlupac 1988 pl 57 fig 4) Therefore theyare considered here to have been attached to the pre-telsonicsegment as in Cheloniellon Chlupac (1988) and Dunlop ampSelden (1997) supported a close relationship between Dusliaand Cheloniellon

25 MethodsAll analyses were carried out using PAUP Version 40b4a(Swofford 1999) and unless otherwise stated used heuristicsearches with one thousand random addition sequencereplicates The software packages MacClade Version 307(Maddison amp Maddison 1997) and RadCon (Thorley amp Page2000) were used for comparing trees and investigating patternsof character evolution Tree length and other statistics (theConsistency Index CI and Retention Index RI) were calcu-lated by PAUP and MacClade with uninformative charactersexcluded Analyses were run separately using each of thedifferent codings for aglaspidids

Characters were treated as unordered and of equal weight inmost analyses To assess the influence of this assumption fourof the eight multistate characters which have states that areintermediate between others (Wilkinson 1992) were treated asordered in some analyses These characters are the number ofcephalic segments (Character 4) the length of raptorialappendage spines (Character 6) the degree of overlap of thethorax by the head-shield (Character 37) and the number ofsegments making up the postabdomen (Character 44) The lastof these is uninformative when not ordered because only onetaxon shows each of states 0 1 and 3

Support for individual nodes was assessed by bootstrapanalysis (Felsenstein 1985) and by calculating Bremer support

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 185

indices (Bremer 1988 1994) These methods measure twodistinct aspects of support for phylogenetic hypotheses Treesor nodes may be considered well supported (1) to the extent towhich alternative topologies are much less parsimonious asmeasured by the support index (Wilkinson 1996) or (2) wherethey are consistent with a large proportion of characters sothat character sampling is unlikely to have had much influenceon topology as assessed by bootstrapping (Page 1996) Boot-strapping was performed with 100 bootstrap replicates eachof 10 addition sequence replicates Bremer support indicesare based on heuristic searches with 100 addition sequencereplicates

Three pairs of terminals are included in Table 2 that apartfrom missing data are coded identically These are Helmetiaand Kuamaia Soomaspis and Tariccoia and Saperion andSkioldia These pairs are lsquotaxonomic equivalentsrsquo removingthe taxon with most missing data will not influence theposition of the other They can be recoded as single terminalsaccording to the principal of safe taxonomic reduction(Wilkinson 1995a b) and were not considered separately in theanalyses presented here Therefore cladistic analyses werecarried out on the basis of 33 terminals Edgecombe ampRamskold (1999) ignored this and coded the same three pairsof taxa and Eoredlichia and Olenoides identically for theirmore limited set of characters

26 ResultsAnalysis with all characters unordered and using the lsquoAglaspi-dida 1rsquo coding found nine most parsimonious trees (MPTs)each 126 steps long (CI=0middot556 RI=0middot763) Using the lsquoAglas-pidida 2rsquo coding resulted in 18 MPTs of length 125 (CI=0middot560RI=0middot765) Nine of these 18 trees were those found with thefirst coding of aglaspidids The strict component consensus(sensu Wilkinson 1994) of the 27 unique trees found acrossboth analyses is shown in Figure 8 All trees supported themonophyly of the Marrellomorpha (Marrella and Mimetaster)and of a clade including all other taxa except Crustacea (Clade1 of Fig 8) The relationships between these two clades and theCrustacea were unresolved equal numbers of trees in bothanalyses supported each of the topologies [Clade 1 (CrustaceaMarrellomorpha)] [Crustacea (Marrellomorpha Clade 1)]and [Marrellomorpha (Crustacea Clade 1)] All trees placedBurgessia as the sister taxon to the rest of Clade 1 andsupported the division of the rest of Clade 1 into two majorsister groups The topology of one of these clades (Clade 2)which included trilobites helmetiids tegopeltids naraoiidsxandarellids and Retifacies was stable with respect to thecoding of aglaspidids The other clade (Clade 3) consists of aclade of megacheirans and chelicerates (Clade 4) and a cladeincluding aglaspidids Lemoneites Paleomerus CheloniellonEmeraldella and Sidneyia (Clade 5) This latter clade is un-resolved in the strict consensus of all 27 trees (Fig 8) and thestrict consensus of the 18 MPTs found using the lsquoAglaspidida2rsquo coding but was fully resolved in the consensus of the ninetrees found with the lsquoAglaspidida 1rsquo coding Within themegacheiran-chelicerate clade all trees found Fortiforceps tobe the sister group to all other taxa Yohoia the sister group tochelicerates and Alalcomenaeus and Leanchoilia to form aclade Jianfengia is equally parsimoniously placed as the sistergroup to the Yohoia-chelicerate clade or as the sister group tothis clade and the Alalcomenaeus-Leanchoilia clade

Treating characters 4 6 37 and 44 as ordered had a minimaleffect on most parsimonious topology Nine MPTs of 136 steps(CI=0middot515 RI=0middot743) were found with the lsquoAglaspidida 1rsquocoding and 12 MPTs 135 steps in length (CI=0middot519 RI=0middot745)with the lsquoAglaspidida 2rsquo coding These trees all supported a(Crustacea Clade 1) clade excluding the marrellomorphs

Apart from this the strict consensus of these 21 trees isidentical to the consensus of the 27 trees found with allcharacters treated as unordered

Across all analyses four different topologies for Clade 5 (ofFig 8) were found (Fig 9) The distribution of these topologiesacross the four analyses described above and across the sameanalyses repeated after reweighting of each character in pro-portion to their Rescaled Consistency Index (see Farris 1989)from the first analysis is shown in Table 3 The instability of

Figure 8 Strict consensus of 27 MPTs from four separate analyses ofarachnomorph phylogeny with both interpretations of aglaspididmorphology and with some characters treated as ordered or allcharacters unordered Clades referred to in the text are numbered

Figure 9 Alternative equally parsimonious resolutions of clade 5 ofFig 8 (see Table 3)

186 TREVOR J COTTON AND SIMON J BRADDY

this clade was to some extent caused by the inclusion ofLemoneites and Paleomerus which could each be coded foronly 27 of the 54 characters used Analysis with these two taxaexcluded and characters treated as unordered found treescompatible only with Topology D of Figure 9 irrespective ofthe coding used for aglaspidids Analysis with some characterstreated as ordered and using either aglaspidid coding foundthree trees compatible only with Topology B of Figure 9 andthree compatible only with Topology D of Figure 9 ThereforeTopology D is the preferred topology for Clade 5 as shown inTable 3

One of the MPTs resulting from the lsquoAglaspidida 2rsquo coding(with all characters treated as unordered) was chosen as thebasis for further discussion because this tree combined thepreferred topology of Clade 5 with the (Crustacea Clade 1)group that was favoured when some characters were treated asordered Bootstrap percentages and Bremer support values forthis tree (using the Aglaspidida 2 coding) are shown in Figure10 Apomorphies for all ingroup nodes of this tree are shownin Figure 10A

3 Discussion

31 Phylogenetic resultsThe results of the cladistic analyses presented above arewell-resolved and robust with respect to different analyticalparameters Most of the taxa considered form a clade (Clade 1of Fig 8) that is more closely related to chelicerates than tocrustaceans in all analyses and therefore is recognised asthe Arachnomorpha as defined by Chen et al (1996) andRamskold et al (1996) The monophyly of the Marrellomor-pha was supported in all analyses The marrellomorphs werefound to be the sister group to a Crustacea+Clade 1 group inmost MPTs but in a minority of trees formed the sister groupto Clade 1 alone According to the latter result they should beincluded within the Arachnomorpha but according to the firstexcluded from it Both of these alternatives have been sup-ported in previous studies (eg Hou amp Bergstrom 1997 Willset al 1998a respectively) In addition to the marrellomorphsthe Fuxianhuiida Bousfield 1995 and Canadaspidida havebeen placed in the Euarthropoda outside the main mandibu-late and arachnomorph clades (Hou amp Bergstrom 1997)Further study of these taxa of the crustacean or mandibulatestem group and of the marrellomorphs is necessary before the

position of the Marrellomorpha can be resolved Provisionallymarrellomorphs are excluded from the Arachnomorphafollowing the majority of analyses above and Wills et al(1994)

The Burgess Shale arthropod Burgessia was placed as thesister group to remaining arachnomorphs in all analyses OtherArachnomorpha form two major clades one consisting of thelsquotrilobite-alliedrsquo arachnomorphs analysed by Edgecombe ampRamskold (1999) here termed lsquoTrilobitomorpharsquo (defined asTrilobita and all taxa more closely related to Trilobita thanChelicerata) and the other (lsquochelicerate-alliedrsquo) clade includingchelicerates lsquogreat appendagersquo arthropods aglaspididsLemoneites Paleomerus Sidneyia and Emeraldella The latteris here termed lsquoCheliceramorpharsquo (defined as Chelicerata andall taxa more closely related to Chelicerata than Trilobita) Thetopology of the lsquotrilobite-alliedrsquo clade was similar to that foundby Edgecombe amp Ramskold (1999) but the present authorsrsquoresults are more completely resolved unambiguously support-ing the sister-group relationship between trilobites andHelmetiida suggested in the previous study Within thelsquochelicerate-alliedrsquo clade Aglaspidids Paleomerus Lemoneitesand the Burgess Shale Emeraldella and Sidneyia form a cladein opposition to megacheirans and chelicerates

The present authorsrsquo analysis supports a new hypothesis ofthe origin of the chelicerates from within a paraphyleticassemblage of megacheiran arthropods and confirms pyc-nogonids as a sister group to the euchelicerates Previoushypotheses of the chelicerate sister group (see Section 1) aremuch less parsimonious The shortest trees supporting aCheloniellon-chelicerate clade are six steps longer than theMPTs and those supporting an aglaspidid-chelicerate cladeseven steps longer All megacheirans and chelicerates areunited by the flap-shaped rounded exopods and modificationof the second cephalic segment endopods into anteriorlydirected spinose raptorial organs which share a number ofdetailed similarities A clade including all of these taxa apartfrom Fortiforceps is supported by the synapomorphic loss ofthe antennae (or antennulae of crustaceans) and the longerlength of spinose projections of the second cephalic appendageendopods Within this clade Yohoia shares the loss of cephalicexopods the loss of thoracic endopods and a postabdomen oftubular tergites with chelicerates

The poor resolution of Clade 5 (of Fig 8) was partly a resultof the poor preservation of and consequent high proportion ofmissing data for Lemoneites and Paleomerus When these two

Table 3 Numbers of most parsimonious trees supporting each of the alternative topologies for clade 4 of Fig 7 under different analyticalconditions (RCI=Revealed Consistency Index)

Aglaspididcoding

Characters 4 6 36and 43 ordered

Charactersreweighted by RCI

Paleomerus andLemoneites excluded

Topology (see Fig 8)

A B C D

1 91 X 3 3 31 X 61 X X 31 X 61 X X 3 32 9 92 X 3 3 3 32 X 62 X X 32 X 62 X X 3 3Total 6 18 12 54

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 187

taxa were excluded from the analyses only two (B and D) ofthe four topologies found in the complete analysis remainedmost parsimonious (see Fig 9 and Table 3) Secondly Bremersupport values throughout much of the lsquochelicerate-alliedrsquoclade (Clade 3 of Fig 8) were improved when these taxa wereexcluded (see Fig 11)

Dunlop (1999 p 258) suggested that in reconstructing thechelicerate stem lineage lsquowe might predict that the two mostsignificant changes towards the chelicerate condition are thereduction of the antennae and the formation of the nextappendages into a clawrsquo According to the hypothesis pre-sented here both of these adaptations were achieved in EarlyCambrian chelicerate ancestors The recognition of the loss ofthe antennae in the stem group of the chelicerates and thephylogenetic hypothesis presented here suggest that chelicer-ates are most closely related to taxa with only a singleadditional segment fused to the crown group eucheliceratehead of four fused somites Previous hypotheses have derivedchelicerates from Palaeozoic arthropods with a longer head(eg Cheloniellon Sturmer amp Bergstrom 1978 SanctacarisBriggs amp Collins 1988 Emeraldella Bruton amp Whittington1983) The plesiomorphic pattern of head segmentation ismatched in pycnogonids Bergstrom et al (1980) suggestedthat only the anterior four pairs of appendages are incorpo-rated into the head This supports the view that pycnogonidsare primitive with respect to other chelicerates which showmore or less complete fusion of this primitive head and threethoracic segments including Solpugida and Pseudoscorpiones

The present study suggests that a shift from gnathobasicbenthic feeding to a more vagrant lifestyle and raptorialappendage feeding occurred very early in the chelicerate lin-eage Appendage feeding was clearly an important adaptation

Figure 10 One of 18 equally parsimonious cladograms with lsquoAglaspidida 2rsquo coding and all characters unorderedthat is most congruent with results from other analyses Non-terminal branch lengths scaled to reflect number ofapomorphies (A) ACCTRAN apomorphy scheme Character numbers shown above boxes and states shownbelow boxes as in Table 2 and the text Character consistency indices are indicated by shading (B) Levels ofsupport for individual nodes Bremer support indices shown in bold type above branches Bootstrap percentagesin italic type below branches for all nodes with relative frequencies greater than 50

Figure 11 Bremer support values for clades within the lsquochelicerate-alliedrsquo arachnomorphs (clade 3 of Fig 8) when all taxa wereincluded (normal weight font above nodes) and with Lemoneites andPaleomerus excluded (bold type below nodes)

188 TREVOR J COTTON AND SIMON J BRADDY

for the terrestrialization of chelicerates (Dunlop 1997 p 69Dunlop amp Webster 1999) but the model proposed heresuggests that this considerably pre-dated terrestrializationGnathobasic feeding in xiphosurans and eurypterids mayrepresent a reversal to a more primitve benthic arachnomorphlifestyle from a more pycnogonid-like ancestor

32 Evolution of the arachnomorph headThe extant arthropod classes show highly conserved patternsof head segmentation (eg Wills et al 1994) In contrast therehas been a near consensus amongst Cambrian arthropodworkers (although rarely based on an explicit phylogenetichypothesis) that patterns of head segmentation are highlyhomoplastic and hence of little systematic significanceSturmer amp Bergstrom (1978 pp 78ndash9) suggested that lsquoevenclosely related forms may have different numbers of headsegments and appendagesrsquo Bruton amp Whittington (1983pp 576ndash7) that lsquodiscussion on fossil arthropod relationshipsbased on head segmentation appears to be largely irrel-evant and at best speculativersquo and Delle Cave amp Simonetta(1991 p 191) that lsquothere are no obvious phyletic affinitiesbetween genera having the same number of cephalized seg-mentsrsquo Most recently Bergstrom amp Hou (1997 p 104) con-cluded that lsquothe segmental length of the head shield seemsto be of no relevance to the discussion and discrimination ofevolutionary lineagesrsquo It has been claimed that this view issupported by cladistic analyses in which patterns of tagmosishave been found to be rather poor at defining major arthropodclades (Briggs et al 1992 Wills et al 1994)

This suggestion is not only of relevance to systematics buthas featured prominently in recent debates surrounding thenature of the lsquoCambrian explosionrsquo (see Budd amp Jensen 2000for a recent review) Gould (1989 1991) argued that theapparent plasticity of head segmentation in Cambrian com-pared to post-Cambrian arthropod evolution suggests thatbody-plan evolution was only constrained after the Cambrianexplosion According to this view the origin of new highertaxa with distinct body-plans was only possible before theonset of these constraints (eg Valentine 1995) Taxa hererecognised as arachnomorphs clearly played a major role in thedevelopment of Gouldrsquos hypothesis This suggestion has pre-viously been assessed in terms of overall morphological diver-sity which is clearly rather distinct from Gouldrsquos concept ofdisparity Wills et al (1998b) showed that there was littlecorrelation between overall morphological disparity and de-gree of limb specialisation and that the latter was phylogeneti-cally highly plastic They remarked that (Wills et al 1998bp 64) lsquothis finding has important implications for models ofarthropod phylogeny and evolution that attribute overidingimportance to head segmentationrsquo but did not explicitlyexamine head segmentation although it is likely to have beena major component of the index of tagmosis employed

The results of the present study suggest that arguments thatarthropod head segmentation was unusually labile during theCambrian are poorly founded Rather only four major pat-terns of euarthropod head segmentation are identified Theplesiomorphic euarthropod state according to both Walossekamp Muller (1997 1998) and Scholtz (1997) which may moreproperly be the plesiomorphic state for the euarthropod crowngroup only (depending on the phylogenetic position of theMarrellomorpha) consists of four post-acronal segmentsbearing the antennae and three pairs of biramous limbs Thepresent authors suggest that the term lsquocephalonrsquo be restrictedto this kind of head which is found in stem group mandibu-lates in addition to many arachnomorphs Crown groupmandibulates share a lsquobimaxillary headrsquo in which an addi-tional pair of appendages the labium or second maxillae are

incorporated into the head but in crustaceans these are notfused to the carapace (Scholtz 1997) Finally two distinctforms of head tagmosis are found in chelicerates A head withfour pairs of appendages (but without antennae giving a totalof five segments incorporated into the head) which has beencalled the lsquocephalosomarsquo is present in pycnogonids (Walossekamp Dunlop 2002 pp 434ndash5) and probably also in some euch-elicerates (J A Dunlop pers comm 2003) The lsquoprosomarsquo ofmost euchelicerates consists of this cephalosoma and twoadditional segments

In general these patterns of head segmentation are highlyphylogenetically conserved (see Fig 12) Only three homo-plastic changes namely reversal to a more primitive conditionin the Leanchoilia-Alalcomenaeus clade and the convergentorigin of a five-segmented head in Clade 3 (of Fig 8) and theXandarellida are necessary to optimise character 4 onto themost parsimonious cladogram There are also transitions toautapomorphic states in Emeraldella and Sidneyia In the caseof Sidneyia however an argument could be made (followingBergstrom amp Sturmer 1978 see discussion of Character 4) thatthe head consists of five segments as in closely related taxabut autapomorphically each segment has a separate tergiteThis would further increase the phylogenetic stability of headsegmentation Also following Bergstrom amp Sturmer (1978) thehead of Cheloniellon could be considered to consist of twotergites making it more similar to the head of Emeraldella(with six pairs of appendages)

Figure 12 Cladogram from Fig 10 showing changes in head segmen-tation Bold type shows abbreviated head segmentation formulaAbbreviations (Ant) antennae (BA) biramous appendage pairs (Ch)chelicerae (GA) great appendages (UA) uniramous appendage pairsand (X) segment without appendages Synapomorphy A is the auta-pomorphic reduction in head length to a single segment in SidneyiaAlternatively this may represent secondary division of the headshield Synapomorphy B is the increase in the length of the head inEmeraldella to six segments

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 189

4 Acknowledgements

We thank D E G Briggs J A Cotton J A Dunlop R AFortey S Harris R A Moore P Selden O E Tetlie andP Van Roy for comments on earlier drafts of this work Themanuscript was reviewed by M Wills F Schram and oneanonymous referee This research was supported by the Geo-logical Society of London Curry Research Studentshipawarded to TJC by the Department of Earth Sciences Uni-versity of Bristol and by the Department of PalaeontologyThe Natural History Museum London TJC would like tothank C Labandeira J Thompson and R Chapman fortheir hospitality at the National Museum of Natural HistoryWashington DC USA and A Olcott S Schellenberg andJ Young for making his time there so enjoyable

5 References

Abzhanov A Popadic A amp Kaufman T C 1999 Chelicerate Hoxgenes and the homology of arthropod segments Evolution andDevelopment 1 77ndash89

Akam M 2000 Arthropods developmental diversity within a (super)phylum Proceedings of the National Academy of Sciences USA97 4438ndash41

Aldridge R J Gabbott S E amp Theron J N 2001 The Soom ShaleIn Briggs D E G amp Crowther P R (eds) Palaeobiology II340ndash2 Oxford Blackwell Science

Anderson L I amp Selden P A 1997 Opisthosomal fusion andphylogeny of Palaeozoic Xiphosura Lethaia 30 19ndash31

Arango C P 2002 Morphological phylogenetics of the sea spiders(Arthropoda Pycnogonida) Organisms Diversity and Evolution2 107ndash25

Ax P 1986 Das phylogenetische System Stuttgart Gustav FischerVerlag

Babcock L E amp Zhang Wentang In press New non-mineralizedarthropods from the Chengjiang Lagerstatte Lower CambrianYunnan China Palaeontologia Cathyana

Bartels C Briggs D E G amp Brassel G 1998 The fossils ofthe Hunsruck Slate Marine life in the Devonian CambridgeCambridge University Press

Bergstrom J 1971 Paleomerus ndash merostome or merostomoid Lethaia4 393ndash401

Bergstrom J 1979 Morphology of fossil arthropods as a guideto phylogenetic relationships In Gupta A P (ed) ArthropodPhylogeny 3ndash56 New York NY Van Nostrand Reinhold Co

Bergstrom J 1992 The oldest arthropods and the origin of theCrustacea Acta Zoologica 73 287ndash91

Bergstrom J Sturmer W amp Winter G 1980 PalaeoisopusPalaeopantopodus and Palaeothea pycnogonid arthropodsfrom the Lower Devonian Hunsruck Slate West GermanyPalaontologische Zeitschrift 54 7ndash54

Bergstrom J amp Brassel G 1984 Legs in the trilobite Rhenops fromthe Lower Devonian Hunsruck Slate Lethaia 17 67ndash72

Bergstrom J amp Hou X 1998 Chengjiang arthropods and theirbearing on early arthropod evolution In Edgecombe G D(ed) Arthropod fossils and phylogeny 151ndash84 New York NYColumbia University Press

Boudreaux H B 1979 Significance of intersegmental tendon systemin arthropod phylogeny and a monophyletic classification ofArthropoda In Gupta A P (ed) Arthropod Phylogeny 551ndash86New York NY Van Nostrand Reinhold Co

Bousfield E L 1995 A contribution to the natural classification ofLower and Middle Cambrian arthropods food-gathering andfeeding mechanisms Amphipacifica 2 3ndash33

Braddy S J Aldridge R J Gabbott S E amp Theron J N 1999Lamellate book-gills in a late Ordovician eurypterid from theSoom Shale South Africa support for a eurypterid-scorpionclade Lethaia 32 72ndash4

Braddy S J amp Almond J E 1999 Eurypterid trackways from theTable Mountain Group (Ordovician) of South Africa Journal ofAfrican Earth Sciences 29 165ndash77

Bremer K 1988 The limits of amino acid sequence data in angio-sperm phylogenetic reconstruction Evolution 42 795ndash803

Bremer K 1994 Branch support and tree stability Cladistics 10295ndash304

Briggs D E G 1998 Review of Arthropods from the LowerCambrian Chengjiang fauna southwest China by HouXianguang and Jan Bergstrom Fossils and Strata 45

Scandinavian University Press Oslo Norway 1997 Palaeo-geography Palaeoclimatology Palaeoecology 143 193ndash4

Briggs D E G Bruton D L amp Whittington H B 1979 Append-ages of the arthropod Aglaspis spinifer (Upper CambrianWisconsin) and their significance Palaeontology 22 167ndash80

Briggs D E G Fortey R A amp Wills M A 1992 Morphologicaldisparity in the Cambrian Science 256 1670ndash3

Briggs D E G Fortey R A amp Wills MA 1993 How big was theCambrian explosion A taxonomic and morphologic comparisonof Cambrian and Recent arthropods In Lees D R amp EdwardsD (eds) Evolutionary patterns and processes 33ndash44 LondonLinnean Society of London Linnean Society Symposium Series

Briggs D E G Erwin D H amp Collier F J 1994 The fossils of theBurgess Shale Washington DC Smithsonian Institution Press

Briggs D E G amp Bartels C 2001 New arthropods from the LowerDevonian Hunsruck Slate (Lower Emsian Rhenish Massifwestern Germany) Palaeontology 44 275ndash304

Briggs D E G amp Bartels C 2002 Magnoculocaris a new name forthe arthropod Magnoculus Briggs and Bartels 2001 from theLower Devonian Hunsruck Slate Germany Palaeontology 45419

Briggs D E G amp Collins D 1988 A Middle Cambrian cheliceratefrom Mount Stephen British Columbia Palaeontology 31 779ndash98

Briggs D E G amp Collins D 1999 The arthropod Alalcomenaeuscambricus Simonetta from the Middle Cambrian Burgess Shale ofBritish Columbia Paleontology 42 953ndash78

Briggs D E G amp Fortey R A 1982 The cuticle of aglaspididarthropods a red-herring in the early history of vertebratesLethaia 15 25ndash9

Briggs D E G amp Fortey R A 1989 The early radiation andrelationships of the major arthropod groups Science 246 241ndash3

Briggs D E G amp Fortey R A 1992 The early Cambrian radiationof arthropods In Lipps J H amp Signor P W (eds) Origin andEarly Evolution of the Metazoa 335ndash373 New York NY PlenumPress

Briggs D E G amp Robison R A 1984 Exceptionally preservednontrilobite arthropods and Anomalocaris from the MiddleCambrian of Utah University of Kansas PaleontologicalContributions Paper 111 1ndash23

Broili F 1932 Ein neuer Crustacee aus dem rheinischen UnterdevonSitzungsberichte der Mathematisch-Naturwissenschaftlichen(Abteilung) Klasse der Bayerischen Akademie der Wissenschaftenzu Munchen 1932 27ndash38

Broili F 1933 Ein zweites Exemplar von Cheloniellon Sitzungsber-ichte der Bayerischen Akademie der Wissenschaften 1933 11ndash32

Bruton D L 1981 The arthropod Sidneyia inexpectans MiddleCambrian Burgess Shale British Columbia Philosophical Trans-actions of the Royal Society of London Series B 295 619ndash56

Bruton D L amp Whittington H B 1983 Emeraldella and Leanchoiliatwo arthropods from the Burgess Shale British Columbia Philo-sophical Transactions of the Royal Society of London Series B 300553ndash85

Budd G E 1995 Kleptothule rasmusseni gen et sp nov anolenellid-like trilobite from the Sirius Passet fauna (BuenFormation Lower Cambrian North Greenland) Transactions ofthe Royal Society of Edinburgh Earth Sciences 86 1ndash12

Budd G E 1996a Progress and problems in arthropod phylogenyTrends in Ecology and Evolution 11 356ndash8

Budd G E 1996b The morphology of Opabinia regalis and thereconstruction of the arthropod stem-group Lethaia 29 1ndash14

Budd G E 1997 Stem-group arthropods from the Lower CambrianSirius Passet fauna of North Greenland In Fortey R A ampThomas R H (eds) Arthropod Relationships 125ndash38 LondonChapman and Hall Systematics Association Special Volume 55

Budd G E 1999a A nektaspid arthropod from the Early CambrianSirius Passet fauna with a description of retrodeformation basedon functional morphology Palaeontology 42 99ndash122

Budd G E 1999b The morphology and phylogenetic significance ofKerygmachela kiergkegaardi Budd (Buen Formation LowerCambrian N Greenland) Transactions of the Royal Society ofEdinburgh Earth Sciences 89 (for 1998) 249ndash290

Budd G E 2002 A palaeontological solution to the arthropod headproblem Nature 417 271ndash5

Budd G E amp Jensen S 2000 A critical reappraisal of the fossilrecord of the bilaterian phyla Biological Reviews 75 253ndash95

Chen J Zhou G Zhu M amp Yeh K 1996 The Chengjiang biota ndasha unique window of the Cambrian explosion Taichung NationalMuseum of Natural Science [In Chinese]

Chen J Edgecombe G D amp Ramskold L 1997 Morphological andecological disparity in naraoiids (Arthropoda) from the Early

190 TREVOR J COTTON AND SIMON J BRADDY

Cambrian Chengjiang fauna China Records of the AustralianMuseum 49 1ndash24

Chen J amp Zhou G 1997 Biology of the Chengjiang fauna In ChenJ Cheng Y amp Iten H V (eds) The Cambrian explosion and thefossil record Bulletin of the National Museum of Natural ScienceTaiwan 10 11ndash106

Child C A 1997 Some deep-sea Pycnogonida from Argentine slopeand basin Proceedings of the Biological Society of Washington110 128ndash42

Chlupac I 1988 The enigmatic arthropod Duslia from the Ordovicianof Czechslovakia Palaeontology 31 611ndash20

Chlupac I 1995 Lower Cambrian arthropods from the PasekyShale (Barrandian area Czech Republic) Journal of the CzechGeological Society 40 9ndash36

Chlupac I amp Havlıcek V 1965 Kodymirus n g a new aglaspidmerostome of the Cambrian of Bohemia Sbornık GeologickechVid Paleontologie 6 7ndash20

Cisne J L 1975 Anatomy of Triarthrus and the relationships of theTrilobita Fossils and Strata 4 45ndash63

Cisne J L 1981 Triarthrus eatoni (Trilobita) anatomy of its exo-skeletal skeletomuscular and digestive systems Palaeonto-graphica Americana 9 95ndash142

Conway Morris S 1982 Atlas of the Burgess Shale London ThePalaeontological Association

Conway Morris S 1989 The persistence of Burgess Shale-type faunasimplications for the evolution of deeper-water faunas Trans-actions of the Royal Society of Edinburgh Earth Sciences 80271ndash83

Conway Morris S 1998 The crucible of creation Oxford OxfordUniversity Press

Cotton T J 1999 Review of Arthropod fossils and phylogenyPalaeogeography Palaeoclimatology Palaeoecology 151 341ndash3

Cotton T J 2002 The phylogeny and morphological evolution ofCambrian trilobites and their relatives Unpublished PhD ThesisUniversity of Bristol

Damen W G M Hausdorf M Seyfarth E A amp Tautz D 1998 Aconserved mode of head segmentation in arthropods revealed bythe expression pattern of Hox genes in a spider Proceedings of theNational Academy of Sciences USA 95 10665ndash70

Delle Cave L amp Simonetta A M 1991 Early Palaeozoic arthropodsand problems of arthropod phylogeny with some notes on taxa ofdoubtful affinities In Simonetta A M amp Conway Morris S(eds) The early evolution of Metazoa and the significance ofproblematic taxa 189ndash244 Cambridge Cambridge UniversityPress

Dewel R A amp Dewel W C 1997 The place of tardigrades inarthropod evolution In Fortey R A amp Thomas R H (eds)Arthropod Relationships 109ndash23 London Chapman and HallSystematics Association Special Volume 55

Dunlop J A 1997 Palaeozoic arachnids and their significance forarachnid phylogeny Proceedings of the 16th European Colloquiumof Arachnology 65ndash82 Siedlce Poland

Dunlop J A 1998 The origins of tetrapulmonate booklungs and theirsignificance for chelicerate phylogeny In Selden P A (ed)Proceedings of the 17th European Colloquium of ArachnologyEdinburgh 1997 pp 9ndash16

Dunlop J A 1999 Pasando revista a la evolucion de los quelicerados[A review of chelicerate evolution] In Melic A DeHaro J JMedez M amp Ribera I (eds) Volumen Monografico Evolucion yFilogenia de Arthropoda Boletin de la Sociedad EntolomologicaAragonesa 26 255ndash272 [In Spanish and English]

Dunlop J A amp Selden P A 1997 The early history and phylogenyof the chelicerates In Fortey R A amp Thomas R H (eds)Arthropod relationships Systematics Association Special Volume55 221ndash35 London Chapman and Hall

Dunlop J A amp Webster M 1999 Fossil evidence terrestrializationand arachnid phylogeny Journal of Arachnology 27 86ndash93

Dzik J 1993 Early metazoan evolution and the meaning of its fossilrecord Evolutionary Biology 27 339ndash86

Dzik J amp Lendzion K 1988 The oldest arthropods of the EastEuropean Platform Lethaia 21 29ndash38

Edgecombe G D Wilson G D F Colgan D J Gray M R ampCassis G 2000 Arthropod cladistics combined analysisof Histone H3 and U2 snRNA sequences and morphologyCladistics 16 155ndash203

Edgecombe G D amp Ramskold L 1999 Relationships of CambrianArachnata and the systematic position of Trilobita Journal ofPaleontology 73 263ndash87

Emerson M J amp Schram F R 1997 Theories patterns and realitygame plan for arthropod phylogeny In Fortey R A amp Thomas

R H (eds) Arthropod Relationships Systematics AssociationSpecial Volume 55 67ndash87 London Chapman and Hall

Farris J S 1989 The Retention Index and the Rescaled ConsistencyIndex Cladistics 5 417ndash19

Felsenstein J 1985 Confidence limits on phylogenies an approachusing the bootstrap Evolution 39 783ndash91

Flower R H 1968 Merostomes from a Cotter Horizon of the El PasoGroup New Mexico Bureau of Mines and Mineral ResourcesMemoir 22 35ndash44

Fortey R A 1990a Trilobite evolution and systematics In MikulicD G (ed) Arthropod paleobiology 44ndash65 Short Courses inPaleontology 3 Knoxville Tenessee The Paleontological Society

Fortey R A 1990b Ontogeny hypostome attachment and trilobiteclassification Palaeontology 33 529ndash76

Fortey R A 1997 Classification In Kaesler R L (ed) Treatise oninvertebrate paleontology Part O Arthropoda 1 (revised) 289ndash302Boulder Colorado amp Lawrence Kansas Geological Society ofAmerica and University of Kansas Press

Fortey R A amp Theron J N 1994 A new Ordovician arthropodSoomaspis and the agnostid problem Palaeontology 37 841ndash61

Fortey R A amp Whittington H B 1989 The Trilobita as a naturalgroup Historical Biology 2 125ndash38

Fry W G 1978 A classification within the pycnogonids ZoologicalJournal of the Linnean Society of London 63 35ndash58

Giribet G Edgecombe G D amp Wheeler W C 2001 Arthropodphylogeny based on eight molecular loci and morphology Nature413 157ndash61

Gould S J 1989 Wonderful life The Burgess Shale and the nature ofhistory New York NY Norton

Gould S J 1991 The disparity of the Burgess Shale arthropod faunaand the limits of cladistic analysis why we must strive to quantifymorphospace Paleobiology 17 411ndash23

Gurich G 1931 Mimetaster hexagonalis ein neuer Kruster aus demunterdevonischen Bundenbacher Dachschiefer PalaontologischeZeitschrift 12 204ndash38

Hammann W Laske R amp Pillola G L 1990 Tariccoia arrusensisngnsp an unusual trilobite-like arthropod Rediscovery of thelsquophylocaridrsquo beds of Taricco (1922) in the Ordovician lsquoPuddingarsquosequence of Sardinia Bolletino della Societa PaleontologicaItaliana 29 163ndash78

Haas M S Brown S J amp Beeman R W 2001 Pondering theprocephalon the segmental origin of the labrum DevelopmentGenes and Evolution 211 89ndash95

Heider K 1913 Entwicklungsgeschichte und Morphologie derWirbellosen Die Kultur der Gegenwart Theil 3 Abt 4 2 176ndash332

Henriksen K 1928 Critical notes upon Cambrian arthropodsdescribed by Charles D Walcott Videnskabelige Meddelelser fraDansk Naturhistorisk Forening 86 1ndash20

Hesselbo S P 1988 Trace fossils of Cambrian aglaspidid arthropodsLethaia 21 139ndash46

Hesselbo S P 1989 The aglaspidid arthropod Beckwithia from theCambrian of Utah and Wisconsin Journal of Paleontology 63636ndash42

Hesselbo S P 1992 Aglaspidida (Arthropoda) from the UpperCambrian of Wisconsin Journal of Paleontology 66 885ndash923

Hou X 1987a Three new large arthropods from Lower CambrianChengjiang fauna eastern Yunnan Acta Palaeontologica Sinica26 272ndash85 [In Chinese with English summary]

Hou X 1987b Two new arthropods from Lower CambrianChengjiang eastern Yunnan Acta Palaeontologica Sinica 26236ndash56 [In Chinese with English summary]

Hou X Chen J amp Lu H 1989 Early Cambrian new arthropodsfrom Chengjiang Yunnan Acta Palaeontologica Sinica 28 42ndash57[In Chinese with English summary]

Hou X Ramskold L amp Bergstrom J 1991 Composition andpreservation of the Chengjiang fauna a Lower Cambrian soft-bodied biota Zoologica Scripta 20 395ndash411

Hou X Bergstrom J amp Ahlberg P 1995 Anomalocaris and otherlarge animals in the Lower Cambrian Chengjiang fauna ofsouthwest China GFF 117 163ndash83

Hou X amp Bergstrom J 1997 Arthropods of the Lower CambrianChengjiang fauna southwest China Fossils and Strata 45 1ndash116

Hughes C P 1975 Redescription of Burgessia bella from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 4415ndash36

Ivantsov A Y 1999 Trilobite-like arthropod from the LowerCambrian of the Siberian Platform Acta PalaeontologicaPolonica 44 455ndash66

King P E 1973 Pycnogonids London St Martin Press

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 191

Lauterbach K E 1973 Schlussereignisse in der Evolution derStammgruppe der Euarthropoda Zoologische Beitrage 19259ndash99

Lauterbach K E 1980 Schlussereignisse in der Evolution derGrundplans der Arachnata (Arthropoda) Abhandlungen desNaturwissenschaftlichen Vereins in Hamburg 23 163ndash327

Lauterbach K E 1983 Synapomorphien zwischen Trilobiten- undCheliceraten-zweig der Arachnata Zoologischer Anzeiger 210213ndash38

Lendzion K 1975 Fauna of the Mobergella Zone in the Polish LowerCambrian Kwartalnik Geologiczny 19 237ndash42

Lieberman B S 1998 Cladistic analysis of the Early Cambrianolenelloid trilobites Journal of Paleontology 72 59ndash78

Lu Y 1940 On the ontogeny and phylogeny of Redlichia intermediaLu (sp nov) Bulletin of the Geological Society of China 20333ndash42 [In Chinese]

Maddison W P amp Maddison D R 1997 MacClade analysis ofphylogeny and character evolution Version 307 Sunderland MASinauer Associates

Mittmann B amp Scholtz G 2003 Development of the nervous systemin the lsquoheadrsquo of Limulus polyphemus (Chelicerata Xiphosura)morphological evidence for the correspondence between the seg-ments of the chelicerae and the (first) antennae of MandibulataDevelopment Genes and Evolution 213 9ndash17

Moore R C (ed) 1959 Treatise on invertebrate paleontology Part OArthropoda 1 Boulder Colorado Geological Society of America

Muller K J amp Walossek D 1986 Arthropod larvae from the UpperCambrian of Sweden Transactions of the Royal Society ofEdinburgh Earth Sciences 77 157ndash79

Muller K J amp Walossek D 1987 Morphology ontogeny and lifehabit of Agnostus pisiformis from the Upper Cambrian of SwedenFossils and Strata 19 1ndash124

Munilla T 1999 Evolucion y filogenia de los Picnogonidos InMelicA DeHaro J J Medez M amp Ribera I (eds) VolumenMonografico Evolucion y Filogenia de Arthropoda Boletin de laSociedad Entolomologica Aragonesa 26 273ndash91 [In Spanish andEnglish]

Orr P J Siveter D J Briggs D E G Siveter D J amp Sutton M D2000 A new arthropod from the Silurian Konservat-Lagerstatteof Herefordshire Proceedings of the Royal Society of LondonSeries B 267 1497ndash504

Page R D M 1996 On consensus confidence and lsquototal evidencersquoCladistics 12 83ndash92

Palmer A R amp Repina L N 1993 Through a glass darklytaxonomy phylogeny and biostratigraphy of the OlenellinaUniversity of Kansas Paleontological Contributions New Series 31ndash35

Platnick N I Griswold C E amp Coddington J A 1991 On missingentries in cladistic analysis Cladistics 7 337ndash43

Pleijel F 1995 On character coding for phylogeny reconstructionCladistics 11 309ndash15

Raasch G O 1939 Cambrian Merostomata Special Papers of theGeological Society of America 19 1ndash146

Ramskold L Chen J Edgecombe G D amp Zhou G 1996Preservational folds simulating tergite junctions in tegopeltid andnaraoiid arthropods Lethaia 29 15ndash20

Ramskold L Chen J Edgecombe G D amp Zhou G 1997Cindarella and the arachnate clade Xandarellida (ArthropodaEarly Cambrian) from China Transactions of the Royal Society ofEdinburgh Earth Sciences 88 19ndash38

Ramskold L amp Edgecombe G D 1991 Trilobite monophylyrevisited Historical Biology 4 267ndash83

Ramskold L amp Edgecombe G D 1996 Trilobite appendagestructure ndash Eoredlichia revisited Alcheringa 20 269ndash76

Raw F 1957 Origin of chelicerates Journal of Paleontology 31139ndash193

Repina L N amp Okuneva O G 1969 Cambrian arthropods of themaritime territory Paleontological Journal 1969 95ndash103

Richter R amp Richter E 1929 Weinbergina opitz ng n sp einSchwerttrager (Merost Xiphos) aus dem Devon (Rheinland)Senkenbergiana 11 193ndash209

Robison R A 1984 New occurences of the unusual trilobite Naraoiafrom the Cambrian of Idaho and Utah University of KansasPaleontological Contributions Paper 112 1ndash8

Rominger C 1887 Description of primordial fossils from MtStephens NW Territory of Canada Proceedings of the Academyof Natural Sciences of Philadelphia 1887 12ndash19

Scholtz G 1997 Cleavage germ band formation and head segmen-tation the ground pattern of the Euarthropoda In Fortey R Aamp Thomas R H (eds) Arthropod Relationships 317ndash32 LondonChapman and Hall Systematics Association Special Volume 55

Schram F R 1978 Arthropods a convergent phenomenonFieldiana Geology 39 61ndash108

Schram F R 1993 Review of Boxshall Stromberg and Dahl TheCrustacea Origin and Evolution Journal of Crustacean Biology13 820ndash2

Schram F R Vonk R amp Hof C H J 1997 Mazon CreekCycloidea Journal of Paleontology 71 261ndash84

Schram F R amp Emerson M J 1991 Arthropod Pattern Theorya new approach to arthropod phylogeny Memoirs of theQueensland Museum 31 1ndash18

Schram F R amp Hof C H J 1998 Fossils and the interrelationshipsof major crustacean groups In Edgecombe G D (ed) Arthropodfossils and phylogeny pp 233ndash302 New York NY ColumbiaUniversity Press

Shergold J H 1991 Protaspis and early meraspis growth stages of theeodiscoid trilobite Pagetia ocellata Jell and their implications forclassification Alcheringa 15 65ndash86

Shu D Geyer G Ling C amp Zhang Z 1995 Redlichiacean trilobiteswith preserved soft parts from the Lower Cambrian Chengjiangfauna (South China) Beringeria (Special Issue) 2 203ndash41

Shultz J W 1990 Evolutionary morphology and phylogeny ofArachnida Cladistics 6 1ndash38

Shultz J W 2000 Skeletomuscular anatomy of the harvestmanLeiobunum aldrichi (Weed 1893) (Arachnida OpilionesPalpatores) and its evolutionary significance Zoological Journalof the Linnean Society 128 401ndash38

Shultz J W 2001 Gross muscular anatomy of Limulus polyphemus(Xiphosura Chelicerata) and its bearing on evolution in the theArachnida Journal of Arachnology 29 283ndash303

Siewing R (ed) 1985 Lehrbuch der Zoologie II Systematik 3rd ednStuttgart Fischer

Simonetta A M 1970 Studies on the non trilobite arthropods ofthe Burgess Shale (Middle Cambrian) the genera LeanchoiliaAlalcomenaeus Opabinia Burgessia Yohoia and ActaeusPalaeontographica Italica 66 35ndash45

Simonetta A M amp Delle Cave L 1975 The Cambrian non trilobitearthropods from the Burgess Shale of British Columbia A studyof their comparative morphology taxonomy and evolutionarysignificance Palaeontographica Italica 69 1ndash37

Simonetta A M amp Delle Cave L 1981 An essay in the compara-tive and evolutionary morphology of Palaeozoic ArthropodsAccademia Nazionale dei Lincei Atti dei Convegni Lincei 49389ndash439

Starobogatov Y I 1990 Sistema i filogeniya nizshikh khelitserovykh(analiz morfologii paleozoysikh grupp) [The systematics andphylogeny of the lower chelicerates (a morphological analysis ofthe Palaeozoic groups)] Paleontologicheskii Zhurnal 1990 4ndash17

Stoslashrmer L 1944 On the relationships and phylogeny of fossil andrecent Arachnomorpha Skrifter utgitt ac Det Norske Videnskaps-Akademi i Olso I Matematisk-naturvitenskaplig Klasse 5 1ndash158

Stoslashrmer L 1951 Studies on trilobite morphology Part III Theventral cephalic structures with remarks on the zoologicalposition of the trilobites Norsk geologisk tidsskrift 29 108ndash58

Stoslashrmer L 1956 A Lower Cambrian merostome from Sweden Arkivfor Zoologi Ser 2 9 507ndash14

Stoslashrmer L 1959 Trilobitoidea In Moore R C (ed) Treatise onInvertebrate Paleontology part O Arthropoda 1 23ndash37 BoulderColorado Geological Society of America

Strong E E amp Lipscomb D 1999 Character coding and inapplicabledata Cladistics 15 363ndash71

Sturmer W amp Bergstrom J 1976 The arthropods Mimetaster andVachonisia from the Devonian Hunsruck Shale PalaontologischeZeitschrift 50 78ndash111

Sturmer W amp Bergstrom J 1978 The arthropod Cheloniellon fromthe Devonian Hunsruck Shale Palaontologische Zeitschrift 5257ndash81

Sturmer W amp Bergstrom J 1981 Weinbergina a xiphosuran fromthe Devonian Hunsruck Slate Palaontologische Zeitschrift 55237ndash55

Sutton M D Briggs D E G Siveter D J Siveter D J amp Orr PJ 2002 The arthropod Offacolus kingi (Chelicerata) from theSilurian of Herefordshire England computer based morphologi-cal reconstructions and phylogenetic affinity Proceedings of theRoyal Society of London Series B 269 1195ndash1203

Swofford D L 1999 PAUP Phylogenetic Analysis Using Parsimony(and other methods) Version 40b2a Sunderland MA SinauerAssociates

Telford M J amp Thomas R H 1998 Expression of homeobox genesshows chelicerate arthropods retain their deutocerebral segmentProceedings of the National Academy of Sciences USA 9510671ndash5

192 TREVOR J COTTON AND SIMON J BRADDY

Thorley J L amp Page R D M 2000 RadCon phylogenetic treecomparison and consensus Bioinformatics 16 486ndash7

Valentine J W 1995 Why no new phyla after the Cambrian ndashgenome and ecospace hypotheses revisited Palaios 10 190ndash194

Wagele J W 1993 Rejection of the lsquoUniramiarsquo hypothesis andimplications of the Mandibulata concept Zoologische JahrbucherAbteilung fur Systematik Geographie und Biologie der Tiere 120253ndash88

Walcott C D 1911 Middle Cambrian Merostomata Cambriangeology and palaeontology II Smithsonian MiscellaneousCollections 57 17ndash40

Walcott C D 1912 Middle Cambrian Branchiopoda MalacostracaTrilobita and Merostomata Cambrian geology and palaeontol-ogy II Smithsonian Miscellaneous Collections 57 145ndash228

Walcott C D 1918 Geological explorations in the CanadianRockies Explorations and fieldwork of the SmithsonianInstitution in 1917 Smithsonian Miscellaneous Collections 684ndash20

Walossek D 1993 The Upper Cambrian Rehbachiella kinnekullensisand the phylogeny of Branchiopoda and Crustacea Fossils andStrata 32 1ndash202

Walossek D amp Dunlop J A 2002 A larval sea spider (ArthropodaPycnogonida) from the Upper Cambrian lsquoOrstenrsquo of Swedenand the phylogenetic position of pycnogonids Palaeontology 45421ndash46

Walossek D amp Muller K J 1990 Upper Cambrian stem-lineagecrustaceans and their bearing upon the monophyletic origin ofCrustacea and the position of Agnostus Lethaia 23 409ndash27

Walossek D amp Muller K J 1997 Cambrian lsquoOrstenrsquo-type arthro-pods and the phylogeny of Crustacea In Fortey R A amp ThomasR H (eds) Arthropod Relationships 139ndash53 SystematicsAssociation Special Volume 55 London Chapman and Hall

Walossek D amp Muller K J 1998 Early arthropod phylogeny in lightof the Cambrian lsquoOrstenrsquo fossils In Edgecombe G D (ed)Arthropod fossils and phylogeny 185ndash231 New York NYColumbia University Press

Weygoldt P 1979 Significance of later embryonic stages and headdevelopment in arthropod phylogeny In Gupta A P (ed)Arthropod Phylogeny 107ndash35 New York NY Van NostrandReinhold Co

Weygoldt P 1998 Evolution and systematics of the ChelicerataExperimental and Applied Acarology 22 63ndash79

Weygoldt P amp Paulus H F 1979 Unterzuchungen aur Morpholo-gie Taxonomie und Phylogenie der Chelicerata Zeitschriftfur Zoologische Systematik und Evolutionsforschung 17 85ndash116177ndash200

Wheeler W C Cartwright P amp Hayashi C Y 1993 Arthropodphylogeny a combined approach Cladistics 9 1ndash39

Wheeler W C amp Hayashi C Y 1998 The phylogeny of extantchelicerate orders Cladistics 14 173ndash92

Whittington H B 1971 Redescription of Marrella splendens(Trilobitoidea) from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 209 1ndash24

Whittington H B 1974 Yohoia Walcott and Plenocaris n genarthropods from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 231 1ndash21

Whittington H B 1975a Trilobites with appendages from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 497ndash136

Whittington H B 1975b The enigmatic animal Opabinia regalisMiddle Cambrian Burgess Shale British Columbia PhilosophicalTransactions of the Royal Society of London Series B 271 1ndash43

Whittington H B 1977 The Middle Cambrian trilobite NaraoiaBurgess Shale British Columbia Philosophical Transactions of theRoyal Society Series B 280 409ndash43

Whittington H B 1979 Early arthropods their appendages andrelationships In House M R (ed) The origin of majorinvertebrate groups 253ndash68 Systematics Association SpecialVolume 12 New York NY Academic Press

Whittington H B 1980 Exoskeleton moult stage appendage mor-phology and habits of the Middle Cambrian trilobite Olenoidesserratus Palaeontology 23 171ndash204

Whittington H B 1981 Rare arthropods from the Burgess ShaleMiddle Cambrian British Columbia Philosophical Transactionsof the Royal Society of London Series B 292 329ndash57

Whittington H B 1985 Tegopelte gigas a second soft-bodied trilo-bite from the Burgess Shale Middle Cambrian British ColumbiaJournal of Palaeontology 23 171ndash204

Whittington H B 1989 Olenelloid trilobites type species functionalmorphology and higher classification Philosophical Transactionsof the Royal Society of London Series B 324 111ndash47

Whittington H B amp Almond J E 1987 Appendages and habits ofthe Upper Ordovician trilobite Triarthus eatoni PhilosophicalTransactions of the Royal Society of London Series B 317 1ndash46

Wilkinson M 1992 Ordered versus unordered characters Cladistics8 375ndash85

Wilkinson M 1994 Common cladistic information and its consensusrepresentation reduced Adams and reduced cladistic consensustrees and profiles Systematic Biology 43 343ndash68

Wilkinson M 1995a Coping with abundant missing entries in phylo-genetic inference using parsimony Systematic Biology 44 501ndash14

Wilkinson M 1995b TAXEQ2 software and documentation BristolSchool of Biological Sciences University of Bristol

Wilkinson M 1996 On the distribution of homoplasy and choosingamong trees Taxon 45 263ndash66

Wills M A 1997 A phylogeny of Recent and fossil Crustacea derivedfrom morphological characters In Fortey R A amp ThomasR H (eds) Arthropod Relationships 189ndash209 SystematicsAssociation Special Volume 55 London Chapman and Hall

Wills M A Briggs D E G amp Fortey R A 1994 Disparity asan evolutionary index a comparison of Cambrian and Recentarthropods Paleobiology 20 93ndash130

Wills M A Briggs D E G Fortey R A amp Wilkinson M 1995The significance of fossils in understanding arthropod evolutionVerhandlungen der deutschen zoologischen Gesselschaft 88 203ndash15

Wills M A Briggs D E G Fortey R A Wilkinson M amp SneathP H A 1998a An arthropod phylogeny based on fossil andrecent taxa In Edgecombe G D (ed) Arthropod fossils andphylogeny 33ndash105 New York NY Columbia University Press

Wills M A Briggs D E G amp Fortey R A 1998b Evolutionarycorrelates of arthropod tagmosis scrambled legs In ForteyR A amp Thomas R H (eds) Arthropod Relationships 57ndash65Systematics Association Special Volume 55 London Chapmanand Hall

Winter G 1980 Beitrage zur Morphologie und Embryologie desvorderen Korperabschnitts (Cephalosoma) der PantopodaGerstaecker 1863 I Entstehung und Struktur des Zentralnerven-systems Zeitschrift fur Zoologische Systematik und Evolutionsfor-schung 18 27ndash61

Zhang W amp Hou X 1985 Preliminary notes on the occurrence of theunusual trilobite Naraoia in Asia Acta Palaeontologica Sinica 24591ndash5 [In Chinese with English summary]

TREVOR J COTTON Department of Palaeontology The Natural History Museum CromwellRoad London SW7 5BD UKe-mail trevor_cottonhotmailcom

SIMON J BRADDY Department of Earth Sciences University of Bristol Wills MemorialBuilding Queenrsquos Road Bristol BS8 1RJ UKe-mail S J Braddybrisacuk

MS received 28 July 2003 Accepted for publication 15 March 2004

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 193

Page 2: The phylogeny of arachnomorph arthropods and the origin of the

Walcott 1911 from the Burgess Shale Retifacies Hou et al1989 from the Chengjiang fauna and Phytophilaspis Ivantsov1999 from the Lower Cambrian Sinsk Formation of Siberia

Despite uniformally supporting an arachnomorph cladeincluding the trilobites and various Cambrian trilobite-like ormerostome-like arthropods recent studies have largely failedto provide convincing synapomorphies for the group (Dunlop1999) Here cladistic methods are used to address this problemto rigorously assess the limits of the Arachnomorpha and todetermine relationships within the arachnomorph group as awhole The present study is based upon a new matrix that ismore comprehensive than previous work in terms of both therange of characters considered and taxonomic sampling

1 Previous studies

Since Stoslashrmer (1944) studies of the phylogeny of theArachnomorpha have disagreed on the taxa that should beincluded in the group and on the relationships between them(Fig 2) In the earliest relevant cladistic studies Lauterbach(1980 1983) and Ax (1986) suggested a particularly close rela-tionship between chelicerates and a paraphyletic Trilobita ahypothesis originally proposed by Raw (1957) Lauterbach andAx argued that olenellid trilobites (reviewed by Palmer ampRepina 1993) were the sister group to chelicerates and thatother trilobites were the sister group to this clade This idea hasbeen extensively criticised In particular Lauterbach ignored allother arachnomorph taxa (including more plesiomorphic trilo-bites) and a wide range of characters that are potential trilobitesynapomorphies (Fortey amp Whittington 1989 Fortey 1990aRamskold amp Edgecombe 1991 Bergstrom amp Hou 1998) To thepresent authorsrsquo knowledge no subsequent author exceptWeygoldt (1998) has accepted Lauterbachrsquos hypothesis

Most hypotheses of arachnomorph phylogeny have beenpresented as part of analyses of the phylogeny of arthropodsas a whole The pioneering cladistic work of Briggs ampFortey (1989) found that the crustaceans (and Cambrianlsquocrustaceanomorphsrsquo) were paraphyletic with respect to arach-nomorphs (Fig 2A) Within the arachnomorphs a stronglypectinate paraphyletic group including Aglaspis and variousBurgess Shale arthropods was primitive with respect to a cladeof all other taxa This consisted of a [Habelia (NaraoiaTrilobita)] group and a clade including the Burgess Shalelsquogreat appendagersquo arthropods Burgessia Sarotrocercus andchelicerates (Fig 2A) This work was subsequently revised(Briggs et al 1992 1993) to include a representative range ofextant arthropods alongside a different selection of Cambrian

taxa coded for a greater number of characters Whereas theearlier work used Marrella as an outgroup the hypothesis ofBriggs et al (1992 1993) was rooted using the lobopodAysheaia This study supported the monophyly of crustaceansand suggested a very different topology within theArachnomorpha (Fig 2B) Whereas Sarotrocercus Burgessiaand Yohoia were still placed close to the chelicerates the otherlsquogreat appendagersquo taxa Alalcomenaeus and Leanchoilia werefound to be basal arachnomorphs Of the taxa placed in abasal paraphyletic assemblage in the earlier study someremained basal (Emeraldella and Sidneyia) whereas otherswere now more closely related to chelicerates than trilobites(Molaria Aglaspis and Sanctacaris)

In a further refinement the work of Wills et al (19951998a) coded the previously analysed taxa for many newcharacters and considered a small number of additional taxanotably from post-Cambrian Palaeozoic Lagerstatte Whilstsupporting the topology of major euarthropod clades found byBriggs et al this analysis again proposed very different rela-tionships within the Arachnomorpha (Fig 2C) Burgessia wasfound to be the sister group to all other arachnomorphs and aclade of trilobites Molaria and Naraoia the sister group to allremaining taxa Cheloniellon and Aglaspis were successivesister groups to chelicerates Notably and unlike in previousanalyses most Burgess Shale arachnomorphs formed a largeclade which was placed in opposition to the clade consisting ofchelicerates Cheloniellion and Aglaspis

In contrast to these studies Bergstrom (1992) Hou ampBergstrom (1997) and Bergstrom amp Hou (1998) rejected parsi-mony as a phylogenetic criterion and developed an arthropodphylogeny (Fig 2D) based on a small sample of charactersThe possibility of any of the selected characters evolvingconvergently was excluded on purely methodological groundsBergstromrsquos interpretations of homology are also oftenunclear For example he used leg posture which can rarely beadequately determined from fossils as an important character(see Edgecombe amp Ramskold 1999 p 281) Bergstromrsquoswork has been extensively criticised (eg Schram 1993 Briggs1998 Cotton 1999 Edgecombe amp Ramskold 1999) and thesearguments are not repeated here

The cladistic analysis of stem group chelicerates presentedby Dunlop amp Selden (1997) included only taxa that werefound to be the most closely related to chelicerates by Willset al (1995 1998a) The results of this analysis were largelyunresolved (the published cladogram fig 173 is only one of9450 most parsimonious trees) but supported the view of Willset al that Cheloniellon is more closely related to cheliceratesthan Aglaspis However Dunlop amp Selden (1997 p 232) notedthat cheloniellids aglaspidids and chelicerates may not form amonophyletic group with respect to all other arachnomorphsEmerson amp Schram (1997) analysed arthropod phylogeny onthe basis of Arthropod Pattern Theory (Schram amp Emerson1991) Their results are also poorly resolved and the topologyof arachnomorph taxa highly unstable across the varioustreatments of their data they present (eg Emerson amp Schram1997 figs 73AndashC) Many of their characters are difficult tointerpret outside the framework of the theory

Edgecombe amp Ramskold (1999) recently presented a cladis-tic study of some arachnomorph taxa in an attempt to resolvethe relationships of the Trilobita Their work was an improve-ment on previous studies in that they included a comprehen-sive sample of both taxa and characters and presented detaileddiscussions of the homology of these characters Their resultssupported the monophyly of the Helmetiida (sensu Hou ampBergstrom 1997) Naraoiidae and Xandarellida (see Ramskoldet al 1997) within an unresolved clade also including theTrilobita Sidneyia Emeraldella and Retifacies were found to

Figure 1 Widely accepted relationships between major arthropodgroups illustrating the Arachnomorpha concept followed here

170 TREVOR J COTTON AND SIMON J BRADDY

be more basal arachnomorphs on the basis of outgrouprooting with Marellomorpha Unfortunately Edgecombe ampRamskold (1999) made no attempt to establish the monophyly

of the group of lsquotrilobite-allied arachnatesrsquo that they includedin their analysis and did not consider the position of thechelicerates

Figure 2 Previous hypotheses of arachnomorph phylogenys (A) after Briggs amp Fortey (1989 fig 1 1992 fig 3)note that Crustacea are paraphyletic in this analysis (B) after Briggs et al (1992 fig 3 1993 fig 1) (C) afterWills et al (1995 fig 1A 1998a fig 2 1) and (D) after Hou amp Bergstrom (1997 fig 88) (for monotypic familiesthe family names used by Hou amp Bergstrom have been replaced with generic names) Taxa important for therecognition of an arachnomorph clade (ie trilobites chelicerates and crustaceans see text) are shown in boldtype

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 171

2 Cladistic analysis

2 1 Taxonomic scopeThirty-four ingroup taxa were considered in the cladisticanalyses Of these the majority were coded as individualspecies-level terminals although higher taxonomic levels wereemployed in some instances A complete list of species andgenus-level terminals along with details of authorship andother important references is given in Table 1 Species-levelterminals are referred to throughout by generic names only(except in Table 1) since the majority are assigned to mono-typic genera In addition to these ingroup taxa a hypotheticaloutgroup was used for rooting as discussed below

Terminals were selected on the basis of the recent cladisticanalyses of Wills et al (1995 1998a) and Edgecombe ampRamskold (1999) and to a lesser extent previous hypothesesof arthropod relationships The majority of taxa that havebeen included in an arachnomorph group by previous authorsare included in this study Exceptions include AgnostusHabelia Molaria Sanctacaris and Sarotrocercus (all of whichwere considered by Wills et al but not Edgecombe ampRamskold) the putative chelicerate Offacolus andPhytophilaspis The Agnostina are regarded as a clade withinthe Trilobita derived from Eodiscina (eg Fortey 1990a

Fortey amp Theron 1994 Wills et al 1998a) rather than stemgroup crustaceans (eg Shergold 1991 Bergstrom 1992 seeCotton 2002 for a detailed discussion and phylogenetic analy-sis) The Burgess Shale taxa Habelia Walcott 1912 MolariaWalcott 1912 and Sarotrocercus Whittington 1981 and theHunsruck Slate Magnoculocaris blindi (Briggs amp Bartels 20012002) are rather poorly known and hypotheses of theirrelationships constrained by too few characters

Sanctacaris Briggs amp Collins 1988 and the probably closelyrelated Offacolus Orr et al 2000 were both originally describedas having chelicerate affinities Sutton et al (2002) addedOffacolus to the matrix of Wills et al (1995 1998a) and foundit to be nested within the Chelicerata despite its biramouscephalic appendages The material of Sanctacaris is in need ofrestudy the homology of the cephalic appendages and ventralcephalic structures remain unclear (see Budd 2002 and Briggsamp Collins 1988 for alternative schemes) The relationships ofthese lsquosanctacaridsrsquo will be considered in detail later by Braddyand co-workers following re-examination of Sanctacaris anddescription of a new Chengjiang taxon (Babcock amp Zhangin press) that is likely to have a profound impact on theinterpretation of both Sanctacaris and Offacolus

Phytophilaspis Ivantsov 1999 shows a combination of fea-tures that may be homologous to those found in naraoiids (the

Table 1 Authorship and important references for species included in cladistic analyses of arachnomorphs Previous analyses by Briggs amp Fortey(1989 1992) Briggs et al (1992 1993) Wills et al (1995 1998a) and Edgecombe amp Ramskold (1999) informed the coding of many taxa and arenot listed

Name and authorship Other references

Alalcomenaeus cambricus Simonetta 1970 Briggs amp Collins (1999)Buenaspis forteyi Budd 1999aBurgessia bella Walcott 1912 Hughes (1975)Cheloniellon calmani Broili 1932 Broili (1933) Sturmer amp Bergstrom (1978)Cindarella eucalla Chen et al 1996 Ramskold et al (1997)Emeraldella brocki Walcott 1912 Bruton amp Whittington (1983) Briggs amp Robison (1984)Eoredlichia intermedia (Lu 1940) Shu et al (1995) Ramskold amp Edgecombe (1996)Fortiforceps foliosa Hou amp Bergstrom 1997Helmetia expansa Walcott 1918 Briggs in Conway Morris (1982) Briggs Erwin amp Collier (1994)Jianfengia multisegmentalis Hou 1987b Bergstrom amp Hou (1991) Chen amp Zhou (1997)Kuamaia lata Hou 1987a Hou amp Bergstrom (1997) Begstrom amp Hou (1998)

Edgecombe amp Ramskold (1999)Leanchoilia superlata Walcott 1912 Bruton amp Whittington (1983) Briggs amp Robison (1984)Lemoneites Flower 1968 Dunlop amp Selden (1997)Liwia plana (Lendzion 1975) Dzik amp Lendzion (1988) Fortey amp Theron (1994)

Chen Edgecombe amp Ramskold (1997)Marrella splendens Walcott 1912 Whittington (1971)Mimetaster hexagonalis (Gurich 1931) Sturmer amp Bergstrom (1976)Misszhouia longicaudata (Zhang amp Hou 1985) Bergstrom amp Hou (1991) Chen et al (1997) Hou amp Bergstrom (1997)Naraoia Walcott 1912 Whittington (1977) Robison (1984) Zhang amp Hou (1985)

Chen et al (1997)Olenoides serratus (Rominger 1887) Whittington (1975a 1980)Paleomerus hamiltoni Stoslashrmer 1956 Bergstrom (1971) Dunlop amp Selden (1997)Retifacies abnormalis Hou et al 1989 Hou amp Bergstrom (1997)Saperion glumaceum Hou et al 1991 Ramskold et al (1996) Hou amp Bergstrom (1997)Sidneyia inexpectans Walcott 1911 Bruton (1981)Sinoburius lunaris Hou et al 1991 Hou amp Bergstrom (1997) Skioldia aldna Hou amp Bergstrom 1997Soomaspis splendida Fortey amp Theron 1994 Chen et al (1997)Tariccoia arrusensis Hammann et al 1990 Fortey amp Theron (1994) Chen et al (1997)Tegopelte gigas Simonetta amp Delle Cave 1975 Whittington (1985) Ramskold et al (1996)Weinbergina opitzi Richter amp Richter 1929 Sturmer amp Bergstrom (1981) Andersen amp Selden (1997) Dunlop amp Selden (1997)Xandarella spectaculum Hou et al 1991 Bergstrom amp Hou (1991) Ramskold et al (1997)

Hou amp Begstrom (1997)Yohoia tenuis Walcott 1912 Whittington (1974)

172 TREVOR J COTTON AND SIMON J BRADDY

form of the pygidium) xandarellids (the eye slits and overlapof anterior thoracic segments by the head shield) and Trilobita(the hypostome with anterior and posterior lateral wings)Therefore it potentially has a pivotal position in the phylogenyof the trilobite-allied Arachnomorpha However the interpre-tation of these features by Ivantsov (1999) requires confir-mation and adequately determining the homology of thesestructures would require restudy of the material

The Xandarellida known only from the Chengjiang faunawere originally described as an arachnate clade (Ramskoldet al 1997) All three valid genera Xandarella Sinoburius andCindarella were included in the analysis of Edgecome ampRamskold (1999) They differ in important respects andtherefore all three genera are included here to test the mono-phyly of this group in the context of a wider analysis

Wills et al (1995 1998a) represented the Trilobita (exceptAgnostida) by Olenoides the appendages of which are knownfrom the Burgess Shale and the Ordovician Triarthrus (seeCisne 1975 1981 Whittington amp Almond 1987) However thepresent authors follow Edgecombe amp Ramskold (1999) inchoosing Olenoides and Eoredlichia to represent the TrilobitaThese are more basal trilobites (see eg Fortey 1990a b) thanTriarthrus and consequently are more likely to reflect theancestral trilobite condition The putative trilobite KleptothuleBudd 1995 from the Early Cambrian Sirius Passet faunaof North Greenland is not included Its appendages areunknown and homologies between exoskeletal features of thistaxon and other arachnomorphs are uncertain

The Naraoiidae or Nektaspida (reviewed above) are widelyconsidered to be closely related to the trilobites either as aparaphyletic assemblage of lsquosoft-bodiedrsquo trilobites (eg Shuet al 1995 fig 20B) or as the sister group of calcified trilobites(Fortey 1997 Whittington 1977) They have been includedin the Trilobita by some authors However Edgecombe ampRamskold (1999) found no particularly close relationshipbetween naraoiids and trilobites (see above) In order to testthe monophyly of the group all described naraoiid genera areincluded except Maritimella and Orientella (both Repina ampOkuneva 1969) which are probably pseudofossils (Robison1984 p 2)

Tegopelte from the Burgess Shale has also been considered asoft-bodied trilobite (Whittington 1985) Ramskold et al(1996) revised the exoskeletal morphology of Tegopelte andnoted similarities to the Chengjiang taxa Saperion andSkioldia This relationship has been confirmed by cladisticanalysis (Edgecombe amp Ramskold 1999) An undescribed largearthropod from the Soom Shale Lagerstatte (Aldridge et al2001 fig 3442) is probably a tegopeltid (Braddy amp Almond1999) The Helmetiidae based on Helmetia Walcott 1918from the Burgess Shale are united with the tegopeltid group inthe Helmetiida (Hou amp Bergstrom 1997 Edgecombe amp Ram-skold 1999) Helmetia is rather poorly known and awaitsredescription but is very similar to Kuamaia lata Hou 1987aKuamaia muricata Hou amp Bergstrom 1997 and Rhombicalva-ria acantha Hou 1987a from Chengjiang There are no signifi-cant differences between these Chinese taxa and theirtaxonomy may be over split (Delle Cave amp Simonetta 1991Hou amp Bergstrom 1997) The morphology of Kuamaia lata isknown in some detail and it is coded here along with Helmetiaand all three tegopeltid genera

Delle Cave amp Simonetta (1991 table 1) compared severaltaxa to Tegopelte and Helmetia Of these only Retifacies isknown in enough detail to make coding worthwhile Tontoiaand Nathorstia (both Walcott 1912) are nomina dubia (seeWhittington 1985 1980 respectively) and only the exoskeletonis known of Urokodia Hou et al 1989 and Mollisonia Walcott1912 Retifacies has been placed near a trilobite-naraoiid-

helmetiid clade (Delle Cave amp Simonetta 1991 Edgecombe ampRamskold 1999) or as sister group to the naraoiids (Hou ampBergstrom 1997)

There is a long history of comparing Emeraldella andSidneyia with chelicerates (following Stoslashrmer 1944) but theposition of these taxa in cladistic studies has been highlyvariable (see Fig 2) According to the hypothesis ofBergstrom these taxa along with the Aglaspidida andCheloniellida form a clade that is the sister group to thechelicerates More usually aglaspidids or cheloniellids havebeen considered sister taxon to the chelicerates and theseBurgess Shale taxa more distantly related

A cheloniellid-chelicerate clade was supported by Wills et al(1995 1998a) and Sturmer amp Bergstrom (1978 also Bergstrom1979) and Simonetta amp Delle Cave (1981) and Delle Cave ampSimonetta 1991) placed the cheloniellid Triopus as ancestral toall chelicerates The Cheloniellida (sensu Dunlop amp Selden1997) are represented by Cheloniellon herein since theappendages of other cheloniellid taxa are unknown

It has also repeatedly been suggested that cheliceratesevolved from aglaspidids (eg Starobogatov 1990) and aglas-pidids have been included in the Chelicerata by some authors(Stoslashrmer 1944 Weygoldt amp Paulus 1979) The coding ofaglaspidids is considered in detail in Section 22 Lemoneites(Flower 1968) and Paleomerus (Stoslashrmer 1956 Bergstrom 1971)have been assigned to the Aglaspidida by some authors (seeHou amp Bergstrom 1997 pp 96ndash97) They are included tofacilitate comparison with the results of Dunlop amp Selden(1997) Lemoneites is particularly problematic (R A Moorepers comm 2004) and both taxa are known from very fewcharacters ndash hence the present authors have investigated theimplications of their exclusion from the analysis (see Section26) The aglaspidid-like arthropod Kodymirus vagans Chlupacamp Havlıcek 1965 from the Lower Cambrian of the CzechRepublic (redescribed and compared to eurypterids byChlupac 1995) is excluded from the present study becausefeatures of its morphology that are well known generally agreewith those of Aglaspis

The phylogeny of crown group chelicerates is a matter ofconsiderable debate but most authors have considered pyc-nogonids xiphosurans and eurypterids as early divergentgroups within Chelicerata (Dunlop 1999) These hypothesesare represented here by coding a generalised pycnogonid andeurypterid and the Devonian synziphosurine Weinbergina(Sturmer amp Bergstrom 1981) Weinbergina is the best-knownsynziphosurine and is more likely to represent the primitivecondition of xiphosurans than modern examples The phylog-eny of the Xiphosura has recently been studied by Anderson ampSelden (1997) Coding for Eurypterida follows the recent workof Dunlop amp Selden (1997) Dunlop (1998) Braddy et al(1999) and Dunlop amp Webster (1999) Coding for pycnogonidsfollows general works on the group (eg King 1973 Fry 1978)the reconstruction of the pycnogonid stem group by Bergstromet al (1980) and recent work on pycnogonid phylogeny(Munilla 1999 Arango 2002) The pycnogonid familyAmmotheidae is generally accepted as the most plesiomorphicextant group (Arango 2002)

Two groups of Palaeozoic fossil taxa have been included inthe Arachnomorpha by some authors but considered lessclosely related by others Marrella from the Burgess Shaleand two Devonian taxa Mimetaster and Vachonisia havegenerally been considered to form the clade Marrellomorpha(Whittington 1971 Sturmer amp Bergstrom 1976 Bergstrom1979 Wills et al 1995) This has been thought to be the sistergroup to other arachnates (Sturmer amp Bergstrom 1976) abasal schizoramian or a basal euarthropod group (see Fig 2)Bergstrom (1979) included the Carboniferous Cycloidea in the

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 173

Marrellomorpha but there is considerable evidence that theseare rather derived crustaceans (see Schram et al 1997) andthey are not considered further The Burgess Shale taxonBurgessia has also been placed in the Marrellomorpha (Hou ampBergstrom 1997) but was not found to belong to this clade inmost analyses Stoslashrmer (1944) placed Burgessia in a compa-rable systematic position to Marrella he included both in theArachnomorpha but excluded them from the MerostomoideaMarrella Mimetaster and Burgessia were included in thepresent study to assess whether the marellomorphs should beincluded in the Arachnomorpha and test the affinities ofBurgessia

The Cambrian lsquogreat appendagersquo arthropods (theMegacheira of Hou amp Bergstrom 1997) have been nestedwithin the Arachnomorpha in most cladistic studies (egBriggs amp Fortey 1989 Wills et al 1995 1998a Emerson ampSchram 1997) Briggs amp Fortey (1989 1992) recognised mega-cheiran taxa as particularly closely related to chelicerates(Fig 2A) while other authors (Bergstrom 1992 Hou ampBergstrom 1997 Budd 2002) have considered megacheirans tobe primitive euarthropods or ancestral to some (but not all)crustaceans (Delle Cave amp Simonetta 1991 and referencestherein) Since the monophyly of the megacheirans hasnot usually been supported the present authors haveincluded nearly all described taxa These include LeanchoiliaAlalcomenaeus and Yohoia from the Burgess Shale andFortiforceps and Jianfengia from the Chengjiang faunaExcluded from this study are Actaeus Simonetta 1970 fromthe Burgess Shale which may be a synonym of Alalcomenaeus(Briggs amp Collins 1999) Alalcomenaeus illecebrosus (Hou1987b see Hou amp Bergstrom 1997) from the ChengjiangFauna which is probably a chimaera (Briggs amp Collins 1999)and the poorly preserved Leanchoilia hanceyi from the MiddleCambrian of Utah (Briggs amp Robison 1984)

According to the definition given above any assessment ofthe limits of the Arachnomorpha needs to consider the phylo-genetic position of these taxa relative to crustaceans To thisend a generalised crustacean intended to represent the plesio-morphic crustacean condition was included as an ingrouptaxon There has been considerable disagreement over themost basal crustacean group (see Wills 1997 pp 194ndash5Schram amp Hof 1998 pp 245ndash8) The coding used here largelyfollows Walossek amp Mullerrsquos (1990 1997 1998) andWalossekrsquos (1993) concept of the crustacean stem group but isintended to be conservative so that coding on the basis of othertheories of crustacean origins would be similar

22 AglaspididaThe morphology of the appendages of aglaspidids is poorlyknown having been described from three species of bodyfossil Aglaspis spinifer Raasch 1939 Flobertia kochi Hesselbo1992 and Khankaspis bazhanovi Repina amp Okuneva 1969 andtrace fossil evidence (Hesselbo 1988) Of these the appendagesof Aglaspis described by Raasch (1939) Briggs et al (1979)and Hesselbo (1992) are by far the best known The append-ages of Flobertia (described by Raasch 1939 as Aglaspisbarrandei and Hesselbo 1992) agree with those of AglaspisHowever the appendages of Khankaspis show a differentmorphology but have only been poorly illustrated anddescribed (Repina amp Okuneva 1969) This material suggeststhe presence of lobate exopods with lamellate setae This taxonis probably correctly assigned to aglaspidids (cf Whittington1979 p 258) on the basis the central position of the dorsaleyes and presence of genal spines although Hou amp Bergstrom(1997 p 96) suggested that its broad tail spine may indicatethat it is a strabopid

It remains unclear whether lamellate exopods are absent insome aglaspidids (Aglaspis) and present in others (Khankaspis)or whether the apparent differences in appendage morphologybetween these taxa are a result of preservational bias alone Apossible preservational analogue is provided by someChengjiang taxa in which the exopods are very poorly pre-served and the endopods of the cephalic appendages arepreserved as impressions in the dorsal head shield in a similarmanner to those of Aglaspis This is presumably because theendopods were more convex and more heavily sclerotised thanthe exopods This mode of appendage preservation is mostclearly seen in Misszhouia longicaudata (see Chen et al 1997fig 2andashd) but is also found in Sinoburius (see Hou ampBergstrom 1997) and possible protaspides of Naraoia (Houet al 1991)

Here a generalised aglaspidid is coded in two different waysto accommodate this uncertainty In both codings they areconsidered to have possessed a pair of antenniform append-ages followed by 11 pairs of pediform endopods on the headand first eight thoracic tergites (Briggs et al 1979 Hesselbo1988 1992) According to one interpretation (coded asAglaspidida 1) all the appendages are uniramous the exopodis lost throughout According to the other (Aglaspidida 2)exopods consisting of a single lobe fringed with lamellate setae(Repina amp Okuneva 1969 pl 15 figs 1 3 amp 4) are presenton at least some appendages but their distribution andattachment are considered unknown

23 Outgroup rootingA hypothetical plesiomorphic euarthropod was included toallow outgroup rooting Many of the characters consideredcould be coded unambiguously on the basis of recent discus-sions of the arthropod stem group (Budd 1996b 1997 1999b)

Figure 3 Majority rule consensus of lsquotrilobite-alliedrsquo arachnate phy-logeny after Edgecombe amp Ramskold (1999 fig 3)

174 TREVOR J COTTON AND SIMON J BRADDY

The ancestral euarthropod is here considered to have possesseda head with antennae only and a series of post-cephalicbiramous limbs with gnathobases The hypothesised pattern ofthe evolution of plesiomorphic euarthropod characters isshown in Figure 4 The use of a hypothetical outgroup issomewhat unsatisfactory but only affects the relationshipbetween the major clades (Arachnomorpha Crustacea andMarrellomorpha) the topology within the arachnomorphs wasunaffected by the use of this outgroup since identical resultswere obtained with unrooted analyses or by rooting withCrustacea Marrella or both

24 Characters and codingMost previous cladistic analyses with the notable exception ofEdgecombe amp Ramskold (1999) have included only briefdiscussions of the primary hypotheses of homology involved incharacter construction and coding Therefore a full discussionof most characters is provided here Descriptions of charactersand character states below are arranged by organ system orbody region in approximate anterior to posterior order Thedistribution of character states across all taxa is shown in thedata matrix (Table 2)

The terminology of morphological features in arachnomor-phs is often confused Terminology developed for cheliceratestrilobites and crustaceans has variously been applied to taxaincluded in this study so some attempt is made to clarifyterminology herein Where appropriate the present authorsrsquoterminology follows Edgecombe et al (2000) and Wheeleret al (1993) but following Scholtz (1997) the protocerebrallsquosegmentrsquo is considered to be acronal and consequently thedeutocerebral segment to be the first true segment Thereforethe antennae (or antennulae of crustaceans) belong to the firstcephalic segment

The present authors have attempted to include as completea set of characters as possible However characters requiringhypotheses of homology between podomeres (eg the number

of podomeres in endopods of thoracic appendages Willset al 1998a character 51) were not considered followingEdgecombe et al (2000 p 157) Secondly some characters usedby Bergstrom (eg Hou amp Bergstrom 1997 p 109) suchas appendage posture and mode of feeding were excludedfollowing Edgecombe amp Ramskold (1999) Finally somecharacters of the ventral surface of the head were not coded asdiscussed below and illustrated in Figure 7 The present authorsdo not include all potential synapomorphies for the Cheli-cerata as the status of many of these is debated (see eg Shultz1990 2001 Dunlop amp Selden 1997 Weygoldt 1998 Wheeler ampHayashi 1998 Dunlop 1999 Edgecombe et al 2000)

Two methods for the coding of inapplicable characters inphylogenetic analysis have been used First inapplicable char-acter states can be coded as missing data A complex structuremay comprise characters lsquoabsentpresentrsquo and lsquostate1state2rsquowith taxa lacking the structure coded as absent for the firstcharacter and as missing data for the second Some authorshave regarded this method as problematic because it may leadto reconstruction of impossible ancestral states and henceunjustified trees (Platnick et al 1991 Strong amp Lipscomb1999) The alternative is to code the second character as a thirdlsquonot applicablersquo state in taxa that lack the structure Thesemethods are equivalent to Pleijelrsquos (1995) coding methods Cand B respectively In the present study inapplicable charac-ters are treated as missing data for most analyses becausecoding them as a distinct character state reduces characterindependence and effectively weights the inapplicable charac-ter and hence could result in them dominating the analysisThe effects of this assumption were investigated by using adistinct character state in some analyses (see Section 26) andinapplicable characters are shown as distinct to lsquotruersquo missingdata (using the symbol lsquo-rsquo) in the matrix (Table 2)

241 Anterior cephalic appendages and head segmentation1 Appendages of the first segment (antennae) (0) present and(1) absent The majority of taxa considered in the present study

Figure 4 Reconstruction of the euarthropod stem group used to inform coding of hypothetical outgroup Grosstopology largely follows Budd (1996b fig 9 1997 fig 1110) Solid boxes indicate unambiguous apomorphiesand shaded boxes character states which are plesiomorphic for the Arthropoda

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 175

possess a single pair of long uniramous multi- and annulatedantennae at the anterior of the head which presumably had asensory function as found in Crustacea Myriapoda and mostmembers of Hexapoda (ie proturans do not have antennae)These appendages are likely to be a synapomorphy of theEuarthropoda (eg Scholtz 1997 Walossek amp Muller 1997)and therefore the outgroup is coded as State 0 The anterior-most head appendages of most Cambrian megacheiran arthro-pods which have been called lsquogreat appendagesrsquo followingWalcott (1912) consist of a small number of robust spinosepodomeres Uniquely Fortiforceps has a pair of short anten-nae anterior to the lsquogreat appendagesrsquo (Hou amp Bergstrom 1997pp 34ndash8) Therefore it appears likely that the lsquogreat append-agesrsquo are the appendages of the second cephalic segment andthe antennae are lost in megacheirans other than FortiforcepsThis of course depends on recognising the lsquogreat appendagesrsquoas homologous in all of these taxa as discussed below(Character 2) Homology of the megacheiran anterior append-ages with the second cephalic appendages of trilobites waspreviously suggested by Stoslashrmer (1944 p 124) on the basis oftheir post-oral position

The classical view of chelicerate head segmentation main-tains that they too have lost the antennae and the cheliceraeare the appendages of the second cephalic segment Recentrevisions of arthropod phylogeny have uncritically acceptedthis homology (Edgecombe et al 2000 Giribet et al 2001)which is based largely on neuroanatomy The lsquochelicero-neuromerrsquo which innervates the chelicerae seems to be ho-mologous with the tritocerebrum associated with the secondcephalic appendage pair of mandibulates (see Weygoldt 1979Winter 1980) Although some uncertainty exists over theposition of pycnogonids within Arthropoda and the homol-ogy of their cephalic segmentation is poorly understood thisview is also supported by the presence of pre-cheliceralappendages in a pycnogonid larva (Larva D of Muller ampWalossek 1986) from the Upper Cambrian of Sweden(Walossek amp Dunlop 2002) Cheloniellon also has a pair ofpre-oral pediform appendages considered homologous to thechelicerae in addition to its antennae Thus despite recentneontological studies that indicate that the chelicerae arehomologous to the antennae of mandibulates based on pat-terns of Hox gene expression (Damen et al 1998 Telford amp

Table 2 Data matrix for cladistic analyses of arachnomorphs () missing data and (ndash) inapplicable characters Letters indicate multistateuncertainty codings as follows (A) 34 (B) 02 (C) 01 (D) 134 (E) 12

1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 3 3 3 4 4 4 4 4 4 4 4 4 4 5 5 5 5 51 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4

OUTGROUP 0 0 0 0 0 0 0 0 0 0 0 0 ndash ndash 0 0 0 0 0 0 0 0 0 0 0 4 0 ndash 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ndash 0 0 0 0 ndash ndash ndash 0 ndash 0Aglaspidida 1 0 0 1 A 0 0 0 0 0 1 0 ndash ndash ndash ndash ndash 0 0 0 1 0 0 0 0 0 0 0 0 ndash 1 1 0 0 1 0 0 0 0 0 0 0 0 1 2 1 1 0 ndash ndash ndash 1 0 1Aglaspidida 2 0 0 A 0 0 0 0 0 0 1 0 ndash ndash 0 1 0 1 0 0 0 0 0 0 0 0 ndash 1 1 0 0 1 0 0 0 0 0 0 0 0 1 2 1 1 0 ndash ndash ndash 1 0 1Alalcomenaeus 1 1 1 3 ndash 2 0 1 0 0 0 1 0 ndash ndash 1 0 0 0 0 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 0 1 0 1 ndash ndash ndash 1 1 0Buenaspis 0 1 B 0 0 1 0 0 ndash 0 1 0 1 0 0 0 0 1 0 0 0 0 ndash 0 1 1 1 0 1 0 ndash 0Burgessia 0 0 0 3 0 0 0 0 0 0 0 1 0 ndash ndash 0 0 0 1 2 ndash 0 0 0 0 0 4 0 ndash 0 0 0 0 0 0 2 0 0 0 0 0 0 ndash 0 0 0 1 ndash ndash ndash 1 0 0Cheloniellon 0 0 1 4 0 0 0 0 0 1 0 1 0 ndash ndash 0 0 1 0 1 0 0 0 0 0 0 1 0 ndash 0 1 0 0 1 0 0 0 0 0 1 0 0 1 0 1 1 0 ndash ndash ndash 0 ndash 1Cindarella 0 0 0 6 0 0 0 0 0 0 0 1 1 0 0 0 0 1 1 0 0 0 0 0 0 0 1 0 ndash 0 1 0 1 1 0 0 2 1 0 0 0 1 0 ndash 1 0 1 1 0 0 0 0 ndash 0Crustacea 0 0 0 3 0 0 0 0 0 0 0 0 0 ndash ndash 0 1 0 0 0 0 0 0 0 0 0 4 0 ndash 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ndash 0 0 0 ndash ndash ndash 1 0 0Emeraldella 0 0 1 5 0 0 0 0 0 0 0 1 1 0 0 0 0 1 0 2 ndash 0 0 0 0 0 0 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 1 1 1 1 0 1 ndash ndash ndash 1 0 1Eoredlichia 0 0 0 3 0 0 0 0 1 0 0 1 1 0 0 0 0 1 0 0 1 0 0 0 0 0 3 1 1 1 1 0 0 2 0 0 0 0 0 0 0 0 0 ndash 1 0 1 1 0 0 1 0 ndash 0Eurypterida 1 1 1 6 ndash 1 1 0 0 1 1 1 0 ndash ndash 1 0 1 0 1 0 0 0 1 0 0 4 1 0 0 1 0 0 1 0 0 0 0 0 0 0 0 1 3 0 1 0 1 ndash ndash ndash 1 0 0Fortiforceps 0 1 1 4 ndash 0 0 0 0 0 0 1 0 ndash ndash 1 0 0 1 0 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 0 1 0 1 ndash ndash ndash 1 1 0Helmetia 3 1 0 1 0 1 0 0 0 0 0 1 2 0 ndash 0 1 0 0 2 0 0 0 0 0 1 1 0 0 ndash 0 0 1 1 0 1 0 0 ndash 0Jianfengia 1 1 1 4 ndash 1 0 0 0 0 0 1 0 ndash ndash 1 0 1 1 0 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 0 1 0 1 ndash ndash ndash 1 0 0Kuamaia 0 0 0 0 0 0 0 0 0 1 1 0 1 0 0 1 0 0 0 1 0 0 0 1 2 0 ndash 0 1 0 0 2 0 0 0 0 0 1 1 0 0 ndash 0 0 1 1 0 1 0 0 ndash 0Leanchoilia 1 1 1 3 ndash 2 0 1 0 0 0 1 0 ndash ndash 1 0 1 1 B 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 1 1 0 1 ndash ndash ndash 1 0 0Lemoneites 0 1 0 0 0 0 1 1 0 1 0 0 0 0 0 0 0 0 1 2 0 0 ndash ndash ndash 1 0 Liwia 0 1 0 0 0 0 1 0 1 0 0 1 0 1 0 0 0 0 ndash 0 1 1 1 0 0 0 ndash 0Marrella 0 0 1 1 0 0 0 0 0 0 0 0 0 ndash ndash 0 1 0 0 2 ndash 0 0 0 0 0 0 ndash 0 0 0 0 0 0 2 0 0 0 0 0 0 ndash 0 0 0 0 ndash ndash ndash 0 ndash 0Mimetaster 0 0 1 2 0 0 0 0 0 0 0 0 0 ndash ndash 0 1 0 1 C 0 0 0 0 0 0 ndash 0 0 0 0 1 0 0 2 0 0 0 0 0 0 ndash 0 0 0 0 ndash ndash ndash 0 ndash 0Misszhouia 0 0 0 3 0 0 0 0 1 0 0 1 1 1 0 0 0 1 0 0 0 0 0 0 0 0 1 0 ndash 0 1 1 ndash ndash 2 0 0 0 1 0 0 0 0 ndash ndash 0 1 1 0 0 1 0 ndash 0Naraoia 0 0 0 3 1 0 0 0 1 0 0 1 1 1 0 0 0 1 0 0 0 0 0 0 0 0 1 0 ndash 0 1 1 ndash ndash 2 0 0 0 1 0 0 0 0 ndash ndash 0 1 1 0 0 0 ndash 0Olenoides 0 0 0 3 0 0 0 0 1 0 0 1 1 0 0 0 0 1 0 1 1 0 0 0 0 0 3 1 1 1 1 0 0 2 0 0 0 0 0 0 0 0 0 ndash 0 1 1 1 0 0 0 0 ndash 0Paleomerus 0 1 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0 0 0 ndash 0 0 ndash ndash ndash 1 0 Pycnogonida 1 1 1 4 ndash 1 1 0 0 1 1 1 ndash ndash ndash ndash 0 0 1 1 0 0 0 1 0 0 4 0 ndash 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 ndash ndash ndash 1 0 0Retifacies 0 0 0 3 0 0 0 0 0 0 0 1 0 ndash ndash 0 0 1 0 0 0 0 0 0 0 0 0 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 1 0 1 1 0 0 0 1 0 0Saperion 0 2 0 1 0 1 0 0 1 1 0 0 1 0 0 0 1 2 0 ndash 0 1 1 ndash ndash 1 1 ndash ndash 0 0 1 0 0 ndash ndash 1 1 0 0 1 0 ndash 0Sidneyia 0 0 1 0 1 0 0 0 0 1 0 1 0 ndash ndash 0 0 1 0 0 0 0 0 0 0 0 0 0 1 0 0 1 0 0 0 0 0 0 0 0 1 2 1 1 0 1 ndash ndash ndash 1 0 1Sinoburius 0 0 0 4 0 0 0 0 0 0 0 1 0 1 0 0 1 1 0 0 0 1 0 ndash 0 1 0 0 1 0 0 2 1 0 0 0 1 0 ndash 0 0 1 1 0 1 0 0 ndash 0Skioldia 0 2 0 0 0 1 0 0 0 1 2 0 ndash 0 1 1 ndash ndash 1 1 ndash ndash 0 0 1 0 0 ndash ndash 1 1 0 0 1 0 ndash 0Soomaspis 0 1 B 0 0 0 0 1 0 0 ndash 0 1 0 1 0 0 1 0 1 0 0 0 0 ndash 0 1 1 1 0 1 0 ndash 0Tariccoia 0 1 B 0 0 0 0 1 0 0 ndash 0 1 0 1 0 0 1 0 1 0 0 0 0 ndash 0 1 1 1 0 1 0 ndash 0Tegopelte 0 0 0 0 0 0 0 0 1 0 1 1 0 0 0 0 1 E 0 ndash 0 1 0 ndash ndash E 1 ndash ndash 0 0 0 0 ndash ndash 0 1 1 0 0 1 0 ndash 0Weinbergina 1 1 1 6 ndash 1 1 0 0 1 1 1 0 ndash ndash 1 0 1 0 1 0 0 0 1 0 0 4 1 0 0 1 0 0 1 0 0 0 0 0 0 0 0 1 2 0 1 0 1 ndash ndash ndash 1 0 0Xandarella 0 0 0 4 0 0 0 0 0 0 0 1 1 1 0 0 0 1 0 0 0 0 1 0 0 0 1 0 ndash 0 1 0 1 1 0 0 2 1 0 0 0 1 0 ndash 1 0 1 1 0 0 0 ndash 0Yohoia 1 1 1 4 ndash 1 0 0 0 1 1 1 0 ndash ndash 1 0 1 0 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 1 2 0 1 0 1 ndash ndash ndash 1 1 0

176 TREVOR J COTTON AND SIMON J BRADDY

Thomas 1998) and developmental data (Mittmann amp Scholtz2003) morphological evidence from fossils supports the viewthat chelicerates have lost the antennae The Hox gene evi-dence suffers from a lack of comparative data from diversemandibulates especially primitive crustaceans (see Akam2000) and the use of Hox gene expression boundaries asmarkers for segmental homology has been criticised (egAbzhanov et al 1999 although relating to posterior Hoxpatterns) Wills et al (1998a pp 43 amp 48) considered thechelicerae to be appendages of the second (deutocerebral)segment but curiously in support of this cited Schram (1978)who unambiguously followed the homology scheme used inthe present study

The anterior-most appendages of Aglaspis were originallydescribed as chelicerae (Raasch 1939) which lead to theirclassification in the Chelicerata (eg Stoslashrmer 1944) Howeverthe morphology of these appendages (see Briggs et al 1979) ismore similar to that of antennae They are narrow relative tothe thoracic endopods of relatively even diameter proximallyand the podomeres are apparently weakly defined Hesselbo(1988 1992) also argued that the anterior appendages ofAglaspis are likely to be antennae The distal parts are un-known (and hence so is their length cf Wills et al 1998ap 58) and there is no evidence that they were chelate contraryto the coding of Wills et al (1998a character 31) Nothingis known about the anterior appendages of BuenaspisLemoneites or Paleomerus and this character is accordinglycoded as missing data in these taxa

2 Form of endopod of appendages of the second segment(0) pediform and (1) anteriorly directed raptorial appendagewith reduced number of podomeres and terminal podomeresbearing spines on distal margins As discussed above cheli-cerae and lsquogreat appendagesrsquo are both likely to represent theappendages of the second segment They also differ from theassumed primitive biramous euarthropod limb in similar waysand therefore are likely to be homologous modifications ofthese appendages The homology of lsquogreat appendagesrsquo andchelicerae was originally proposed by Henriksen (1928) andsupported by Stoslashrmer (1944) but to the present authorsrsquoknowledge no modern author has discussed this possibility

Both lsquogreat appendagesrsquo (Figs 5A D E) and chelicerae(Figs 5B C) are equipped with strong spinose projections onthe outer (dorsal) side of the distal margins of the terminalpodomeres that are lacking on the proximal podomeresSecondly the number of podomeres in both chelicerae andlsquogreat appendagesrsquo is more or less reduced compared to thenumber in the endopods of biramous limbs In the case oflsquogreat appendagesrsquo they are significantly more robust thanthose of posterior endopods a situation that is matched insome chelicerates The homology of the lsquogreat appendagesrsquoof Alalcomenaeus Leanchoilia Yohoia Jiangfengia andFortiforceps is supported by all of these features and is widelyaccepted (eg Wills et al 1998a character 31 Hou ampBergstrom 1997 Bergstrom amp Hou 1998 Hou 1987a) OnlyHou amp Bergstrom (1991 p 183) have suggested albeit inpassing that the lsquogreat appendagesrsquo of all these taxa may beconvergent a view they appear to have changed The endo-pods of the appendages of other taxa are locomotory legswhich either lack strong spines or have spinose extensionsthat are invariably on the medial (ventral) surface and arestronger on the proximal podomeres than the distal ones (egMisszhouia Fig 6A) In the latter case these spines form partof the feeding system along with the gnathobases and are oftenlocated around the middle of the podomere rather than limitedto the distal margin These endopods are directed ventro-laterally as opposed to anteriorly in the case of both cheliceraeand lsquogreat appendagesrsquo The modified second appendages of

Marrella resemble this plesiomorphic condition here referredto as lsquopediformrsquo except in their anterolateral orientation Insome Recent crustaceans this appendage is modified into asecond antenna but in stem-group crustaceans it is pediform(eg Walossek amp Muller 1997 1998) This character is codedas missing data in a number of taxa for which the morphologyof the second segment appendage is unknown

Some authors (Dzik 1993 Chen amp Zhou 1997 Dewel ampDewel 1997 Budd 2002) have suggested that the lsquogreat ap-pendagesrsquo are homologous with the anterior appendages ofanomalocaridids and the anomalocaridid-like Opabinia andKerygmachela (see Budd 1996b 1999b respectively) Theanterior appendages of most anomalocaridids and otherCambrian lobopodians differ from megacheiran lsquogreat append-agesrsquo in that they consist of a large number of segmentsMoreover there is considerable morphological evidence thatanomalocaridids are part of the euarthropod stem group (Willset al 1995 1998a) and not closely related to the lsquogreatappendagersquo taxa (cf Budd 2002) with the possible exception ofParapeytoia lsquoGreat appendagersquo taxa share a suite of derivedcharacters with other crown group euarthropods includingsclerotised dorsal tergites the incorporation of more thanone pair of appendages into the head sclerotisation of theendopods and strong differentiation of the head that wereexcluded from Buddrsquos (2002) analysis The relationships ofanomalocaridids will be considered in detail later by Braddyand co-workers

3 Exopod of appendages of the second segment (0)present and (1) absent or much reduced The raptorial ap-pendages of the second segment (chelicerae and lsquogreat append-agesrsquo) are uniramous as are the corresponding pediformappendages of Marrella Mimetaster Emeraldella Cheloniellonand Sidneyia The plesiomorphic euarthropod state is found inmost other arachnomorphs including trilobites where thebiramous second segment appendages are undifferentiatedfrom those of posterior segments (Edgecombe et al 2000character 78) The exopods of these appendages are alsopresent in basal members of the crustacean crown group(Edgecombe et al 2000 character 79) and in stem groupcrustaceans (Walossek 1993 Walossek amp Muller 1997 1998)

4 Number of head segments (0) 1 (1) 2 (2) 3 (3) 4 (4) 5(5) 6 and (6) 7 The coding of this character by Edgecombe ampRamskold (1999 character 2) explicitly referred to the numberof limb pairs present in addition to the antenna Here thenumber of post-acronal segments present in the head is codedfollowing Wills et al (1998a character 29) In taxa that lack anantenna the number of somites is inferred to be one greaterthan the number of cephalic appendage pairs (cf Wills et al1998a) as described above

Sturmer amp Bergstrom (1978) suggested that the head ofCheloniellon is defined primarily on the basis of appendagetagmosis rather than the fusion of segments under a singlecephalic-shield (cf Hou amp Bergstrom 1997 pp 98ndash9) Accord-ing to this view the head consists of both the cephalic tergiteand the anteriormost free tergite that share uniramousgnathobasic appendages Wills et al (1998a) coded Cheloniel-lon as having six somites incorporated into the head andtherefore presumably accepted this suggestion There is noevidence that the first free tergite of Cheloniellon was incorpo-rated into the head (except perhaps functionally) and thepresent authors prefer to code the number of somites under thecephalic shield In Sidneyia there is a series of uniramousstrongly gnathobasic appendages similar to those of Cheloniel-lon posterior to the head shield All of these appendages wouldpresumably have to be considered part of the head based onappendage differentiation according to the view of Sturmer ampBergstrom (1978) Some autapomorphic states are included

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 177

(none for Sidneyia one for Marrella and six for Emeraldella)since these will become informative if the character is treatedas ordered

Edgecombe amp Ramskold (1999 p 265) described a novelform of cephalic tagmosis The present authors tentativelyaccept their suggestion that in trilobite-like taxa the fourthpair of biramous cephalic appendages was directly under thecephalo-thoracic junction This may explain previous confu-sion about the number of such appendages incorporated intothe trilobite cephalon (eg Cisne 1975 Whittington 1975aBergstrom amp Brassel 1984) However Edgecome amp Ramskoldaccept the view of Chen et al (1977 p 7) that only the firstthree biramous limbs of Misszhouia were structurally and

functionally part of the head Therefore it seems more accept-able to code these taxa as having only four post-acronalsomites than to use the coding scheme of Edgecombe ampRamskold (1999) The partial integration of the first thoracicsomite under the head shield could be considered to be a formof overlap of the trunk by the head shield (ie a distinct stateof character 9 of Edgecombe amp Ramskold 1999 or Character37 herein) Derived states of this character would then poten-tially be synapomorphic for a clade including xandarellidsnaraoiids helmetiids tegopeltids and trilobites Howeversince the degree of overlap in taxa with the fourth biramousappendages under the cephalo-thoracic articulation is identicalto that found between thoracic tergites (State 0 of Character

Figure 5 Diagrammatic reconstructions of raptorial second-segment appendages in lateral view (except whereotherwise stated) and approximate life orientation showing suggested homology of appendage elements (inbrackets) Roman numerals indicate position in the podomere series and do not necessarily imply homology (A)lsquoGreat appendagersquo of Fortiforceps (after Hou amp Bergstrom 1997) (B) Left chelicera of Leiobunum aldrichi(Arachnida Opiliones) (1) in medial perspective and (2) distal parts in anterior perspective (after Shultz 2000)(C) Chelifore of Nymphon (Pycnogonida) (after Child 1997) (D) lsquoGreat appendagersquo of Yohoia (after Whittington1974 fig 2) (E) lsquoGreat appendagersquo of Leanchoilia (modified after Bruton amp Whittington 1983) Not to scaleAbbreviations (apt) apotele (probably homologous with the mandibulate pretarsus) (cx) coxa (dtmrt)deutomerite and (pt) pretarsus

178 TREVOR J COTTON AND SIMON J BRADDY

37) it is here preferred to regard this as a distinct character(Character 9 herein)

The coding of Alalcomenaeus by Wills et al (1998a) ashaving four post-acronal somites is accepted here but fordifferent reasons Briggs amp Collins (1999) have recenty demon-strated that Alalcomenaeus possessed two pairs of biramousappendages on the head in addition to the great appendagesThis would equate to three head somites according to thehomology scheme of Wills et al (1998a) Here the antennaeare inferred to be lost and therefore four segments incorpo-rated into the head

Recently a number of authors have agreed that the plesio-morphic condition for euarthropods is a four-segmented headas found in stem group crustaceans (Scholtz 1997 Walossek1993 Walossek amp Muller 1997 1998 Edgecombe et al 2000)However reconstruction of the euarthropod stem groupclearly indicates that even crownward taxa had a head consist-ing of only the antennal segment and acron as found intardigrades and onychophorans Therefore it seems that thefour-segmented head is pleisomorphic only for crown groupeuarthropods Some fully arthropodised fossil taxa also have ashorter head that may be pleisomorphic such as the two-segmented head of Marella Retifacies is coded following therecent redescription of Hou amp Bergstrom (1997) rather thanon the basis of the coding by Edgecombe amp Ramskold (1999)for which they provide no explanation A partial uncertaintycoding is used for Aglaspis in which the number of post-antennal cephalic appendages is either three or four (Briggset al 1979)

5 Orientation of the antennae (0) directed anterolaterally(1) strongly deflected laterally and (2) placed well inside shieldmargin curing posteriorly from a transverse proximal element(cf Edgecombe amp Ramskold 1999 character 3) The anteriorappendages of Aglaspis (see character 1) are clearly directedanterolaterally and it is presumed that they continued past thehead shield margin The anterior appendages of arthropodstem group taxa such as Aysheaia Kerygmachela and Anoma-locaris are either directed anterolaterally or ventrally but arenot strongly-deflected laterally Therefore the outgroup iscoded as State 0 This character is coded as missing data fortaxa where the presence of antennae is equivocal (those codedas missing data for Character 1) and as inapplicable in taxawhere the antennae are considered absent (coded as State 1 forCharacter 1)

6 Length of distal spines on terminal podomeres of endo-pods of second segment appendages (0) absent or shorer thanpodomeres (1) subsequal to length of podomeres and (2)longer than the entire podomere series The spinose projectionsof the raptorial appendages described above vary in length Inchelicerae (Figs 5BC) and some lsquogreat appendagesrsquo (eg thoseof Yohoia Fig 5D) the most distal spine is equal in length tothe spinose terminal podomere forming a chelate structureIn Fortiforceps (Fig 5A) the spines are short relative tothe lengths of the podomeres but in Alalcomenaeus andLeanchoilia (Fig 5E) they are extremely long so that thespines are much longer than the entire podomere series Asdescribed above in taxa with pediform endopods of the secondsegment appendages spines on the distal podomeres are shortor entirely absent

7 Chelicerae (0) absent and (1) present Despite theirsimilarities to lsquogreat appendagesrsquo the chelicerae of the eucheli-cerates and chelifores of the pycnogonids are clearly distinctand have been widely recognised as a chelicerate synapomor-phy Chelicerae differ from any lsquogreat appendagesrsquo by thecombination of a smaller number of podomeres the presenceof only a single terminal element and only a single spinoseprojection on the dorsal side of the podomere series (Fig 5)

Amongst extant chelicerates only the pycnogonid Pallenopsishas chelicerae of four podomeres comparable to the numberin lsquogreat appendagesrsquo Bergstrom et al (1980) considered thatthe chelifores of the Devonian pycnogonid Palaeoisopus alsoconsist of four segments but this is not well supported by theirfigures

8 Distal spines of second segment endopods terminating inannulated flagellae (0) absent and (1) present It is widelyrecognised (eg Stoslashrmer 1944 Simonetta 1970 Bruton ampWhittington 1993 Briggs amp Collins 1999) that the terminalparts of the extended spines of Alalcomenaeus and Leanchoiliaformed annulated flagellae Nothing similar is known fromhomologous appendages of any other arthropod althoughsimilar processes may have operated in the transformation ofthe endopod as a whole into the second antennae of derivedcrustaceans and in the origin of multiramous antennulae (firstantennae) in malacostracans

242 Posterior appendages 9 Appendages of first thoracicsomite underneath the cephalo-thoracic articulation (0) ab-sent and (1) present (cf Edgecombe amp Ramskold 1999character 2) For a discussion of this character see Character4 herein

10 Exopods of appendages of third to fifth segments (0)present and (1) reduced or absent A number of taxa haveuniramous appendages on the third to fifth segments In mostthese segments are incorporated into the head (Yohoia cheli-cerates) but in others all (eg in Sidneyia) or some (eg inCheloniellon) of these segments are post-cephalic In all casesthe appendage is morphologically similar to the endopods ofbiramous limbs of other taxa andor other segments and it isinterpreted that the exopod is lost

11 Endopods of thoracic appendages (0) present and (1)reduced or absent The opisthosomal appendages of chelicer-ates lack endopods or have the endopods much reduced (egLimulus Siewing 1985 fig 838 Shultz 2001) a situation that isalso found in the uniramous thoracic appendages of Yohoia(Whittington 1974) It has been suggested that Helmetia(Briggs et al 1994) lacks endopods but pending a redescrip-tion of this genus and considering that they have been docu-mented in the otherwise very similar Kuamaia this is coded asuncertain

12 Exopod shaft of numerous podomeres each bearing asingle seta (0) present and (1) absent The exopods ofcrustaceans (at least plesiomorphically see eg Walossek ampMuller 1998 figs 5middot5 amp 5middot6) and of Marrella and Mimetasterhave multi-annulated shafts with each podomere bearing asingle seta In other taxa the exopod shafts consist of one oftwo lobes bearing numerous setae

The polarity of exopod segmentation is uncertain Thelateral flaps of stem group arthropods such as Opabinia(Whittington 1975b Budd 1996b) and anomalocaridids (egHou et al 1995) are certainly unsegmented but as suggestedby Buddrsquos (1996b fig 8) reconstruction this does notnecessarily suggest the form of the primitive euarthropodexopod Rather segmentation may have been a result of thesclerotization of the cuticle of the exopod shaft The outgroupwas coded as uncertain for this character According to thefirst interpretation of aglaspidid appendage morphology(coded as Aglaspidida 1) the exopods are lacking on allappendages and consequently this and all other charactersdescribing exopod morphology are coded as inapplicable

13 Exopod shaft differentiated into proximal and distallobes (0) absent and (1) present Ramskold amp Edgecombe(1996 Edgecombe amp Ramskold 1999 character 26) havediscussed the distribution of the trilobite-type bilobate exo-pods recognised by this character (Fig 6A) Among taxaincluded here that were not considered by Edgecombe amp

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 179

Ramskold (1999) exopods consist either of a single flattenedlobe or a homonomous series of podomeres (Fig 6B C) Nostem group euarthropod has a bilobate exopod and theoutgroup is consequently also coded as State 0

14 Proximal lobe of exopod (0) flattened lobe and (1)slender shaft

15 Distal lobe of exopod (0) small to moderate sized flapwith short to moderately long attachment to proximal lobeand (1) large teardrop-shaped with long attachment to proxi-mal lobe Neither of these characters can be coded for taxathat do not have an exopod differentiated into proximal anddistal lobes (Character 13 State 1) without asserting thehomology of the single-lobed exopod with one of these partsThe coding of Edgecombe amp Ramskold (1999 p 280) implic-itly homologizes the exopods of Sidneyia and Retifacies withthe proximal lobe In these taxa this view is perhaps supportedby the presence of lamellate exopod setae which match thoseof the trilobite-type proximal lobe However in other taxawith a single-lobed exopod such as Alalcomenaeus (see Briggsamp Collins 1999) the exopod is fringed with sharp spines likethe distal lobe of differentiated exopods Alternatively thebilobate exopod may have originated from the division of aprimitively single-lobed structure The single-lobed exopod isnot clearly homologous with either lobe of divided exopodsand both these characters are consequently coded as inappli-cable to taxa without differentiated proximal and distal exopodlobes Consequently (see Character 13 herein) these characterscan only be coded for taxa that were previously considered byEdgecombe amp Ramskold (1999) and their coding is followedhere except in the cases of Sidneyia and Retifacies

16 Exopod shaft is a deep rounded flap (0) absent and (1)present All bilobate (Character 13) and segmented (Character12) exopod shafts are long and relatively narrow structures (seeFig 6) whereas some single-lobed exopod shafts have theform of rounded flaps These flap-like exopod shafts are largecompared to the length of the setae and at least half as deep asthey are long This appendage structure is found in cheliceratesand lsquogreat appendagersquo arthropods

17 Medially-directed exopod setae (0) absent and (1)present Walossek amp Muller (1998 p 194) suggested that thetilting of exopod setae towards the endopod in post-antennularlimbs with a multi-annulated exopod was an autapomorphyuniting crustaceans and all members of the crustacean stemgroup (the crustacean total group sensu Ax 1986 see Fig 6C)This character is shared with the marellomorphs Marrellaand Mimetaster (see Fig 6B Sturmer amp Bergstrom 1976Bergstrom 1979 fig 1middot3A B) In other taxa the setae eithersurround the entire margin of the exopod shaft or are directeddorsally (Fig 6A) The identification of setae on the dorsalsurface of the lateral flaps in Opabinia (Budd 1996b) suggeststhat the condition in marellomorphs and crustaceans isderived

18 Lamellate exopod setae (0) absent and (1) presentLamellate exopod setae originally described from trilobitesare a classic synapomorphy of the Arachnomorpha (see egBergstrom 1992 Hou amp Bergstrom 1997 pp 42ndash3) They areperhaps best known from Misszhouia (see Chen et al 1996Hou amp Bergstrom 1997 Fig 6A) These setae differ from themore spinose setae of other taxa (Fig 6C) in that they are wideand flat and imbricate over the length of the exopod shaft Thesetae of Marrella (Fig 6B) and similar taxa have variouslybeen considered lamellate (Bergstrom 1979) or non-lamellate(Bergstrom 1992) Since they do not imbricate and do notseem to be strongly flattened (Whittington 1971 Sturmer ampBergstrom 1976 fig 9a) Marrella and Mimetaster are codedas State 0 The setae of Helmetia as shown in Briggs et al(1994 fig 141) seem to be of the lamellate type The setae ofSinoburius have been described by Hou amp Bergstrom (1997p 85) as similar to those of Misszhouia Only the setae areknown of the appendages of Buenaspis (Budd 1999a) andthese also appear to be of the trilobite-type

Figure 6 Diagrammatic reconstructions of (A) arachnomorph and(B C) non-arachnomorph biramous appendages (A) The lsquotrilobite-typersquo biramous appendage of Misszhouia longicauda (after Chen et al1997) (B) Appendage of Marrella splendens (after Whittington 1971)(C) Appendage of the stem-lineage crustacean Martinssonia elongata(after Muller amp Walossek 1997 1998) Not to scale Abbreviations(bas) basis (dle) distal lobe of exopod shaft (ls) lamellar exopod setae(pe) proximal endite or pre-coxa (ple) proximal lobe of exopod shaft(ses) segmented exopod shaft and (ss) spinose exopod setae

180 TREVOR J COTTON AND SIMON J BRADDY

The homology of the book-gill lamellae of xiphosurans withlamellar setae was supported by Walossek amp Muller (1997p 149) and Edgecombe et al (2000 p 174) but rejected bySturmer amp Berstrom (1981) on the grounds that Weinberginapossesses Limulus-like gill lamellae and fringing setaeBook-gills have also been described from a eurypterid (Braddyet al 1999) There are certainly major morphologicaldifferences between book-gills and trilobite-type exopodsFollowing most recent opinion and pending further studyWeinbergina and Eurypterida are coded as possessing lamellatesetae

The form of the setae of Yohoia is uncertain (Whittington1974) the spinose setae seen in the most common reconstruc-tion (Gould 1989 Briggs et al 1994) are not justified by thespecimens Amongst other great appendage arthropods theform of the setae appears to vary those of Jianfengia (seeChen amp Zhou 1997 p 74) and Leanchoilia (see Bruton ampWhittington 1983) are lamellate while those of Fortiforceps(Hou amp Bergstrom 1997) and Alalcomenaeus (Briggs amp Collins1999) are spinose and less densely packed

19 Gnathobase on basis andor prominent endites onendopod (0) present and (1) absent (cf Edgecombe ampRamskold 1999 character 29 Wills et al 1998a characters 36amp 59) The presence of gnathobases on the appendages of someanomalocaridids (Hou et al 1995) and their general distribu-tion amongst non-arachnomorph euarthropods suggests thatState 0 is plesiomorphic The homology of the various enditesand gnathobasic structures found in euarthropods is in need ofcareful assessment and therefore a very general coding (fol-lowing Edgecombe amp Ramskold) is used for this character

243 Eyes 20 Position of lateral facetted eyes (0)ventral and stalked (1) dorsal and sessile and (2) absent (cfEdgecombe amp Ramskold 1999 character 4) Partial uncer-tainty coding is used for a number of taxa where the dorsalsurface is known but the ventral morphology is not andtherefore the presence of ventral eyes cannot be discountedFor example there is good evidence that Leanchoilia lackeddorsal sessile eyes but ventral stalked eyes may have beenpresent (as in L hanceyi see Briggs amp Robison 1984) and a(02) partial uncertainty coding is used This is also the case inmany naraoiids such as Buenaspis However only the outlineof the head shield of Liwia is known and the absence of dorsaleyes in the reconstruction of Dzik amp Lendzion (1988) isconjectural It is unclear whether the autapomorphic conditionof stalked dorsal eyes in Mimetaster (see Sturmer amp Bergstrom1976) is homologous with State 0 or State 1 and a partialuncertainty coding is also used

Whittington (1974) was somewhat equivocal about thenature of the lateral lobes anterior to the head shield ofYohoia These more closely resemble the ventral stalked eyes ofAlacomenaeus as recently described by Briggs amp Collins(1999) than either Whittingtonrsquos (1974) or subsequent (egGould 1989 fig 3middot18 Briggs et al 1994 fig 153) reconstruc-tions suggest In a well preserved specimen showing the dorsalaspect (USNM 57696 Whittington 1974 pl 2 figs 1ndash3) and ina laterally compressed specimen (USNM 57694 Whittington1974 pl 1 fig 1) they are clearly seen to be stalked andrelatively small lobate structures That these structures arelikely to represent stalked ventral eyes was reflected in thecoding of Wills et al (1998a characters 26 amp 27)

21 Visual surface with calcified lenses bounded withcircumocular suture (0) absent and (1) present

22 Dorsal bulge in exoskeleton accommodating drop-shaped ventral eyes (0) absent and (1) present

23 Eye slits (0) absent and (1) presentCharacters 21 to 23 are used exactly as described by

Edgecombe amp Ramskold (1999 character 5) Character 21 is

inapplicable to taxa that do not possess eyes (Character 20State 2)

24 Dorsal median eyes (0) absent and (1) present Dorsalmedian eyes on a tubercle are considered a synapomorphy ofthe Chelicerata (Dunlop amp Selden 1997 Dunlop 1999) Thenature and position of the structures interpreted as dorsalmedian eyes in Mimetaster (Sturmer amp Bergstrom 1976p 83ndash4) are regarded as equivocal

244 Ventral cephalic structures Many characters of theventral surface of the euarthropod head (see Fig 7) of poten-tial phylogenetic utility are poorly known in many importanttaxa In particular the presence of pre-hypostomal frontalorgans (Fig 7A CndashE) is not coded herein (see Edgecombe ampRamskold 1999 p 272) Definitive evidence of the absence ofthese structures is available for very few taxa and the homol-ogy of these structures with the maculae of the trilobitehypostome (see Fig 7B) and the frontal organs of the crusta-cean labrum (eg see Muller amp Walossek 1987 p 39) isuncertain Secondly the detailed homology of the trilobitehypostome with that of other putative arachnomorphs isunclear and consequently a very broad definition is usedherein For example various pre-hypostomal sclerites (Fig7A D) in other arachnomorphs may have been incorporatedalong with a primitive hypostome into a single sclerite that isrecognised as the hypostome in trilobites The structure of thehypostome (eg the presence of paired anterior wings dorsal tothe antennae Fig 7B C) may also be a source of additionalcharacters

25 Expanded cephalic doublure (0) absent and (1)present maximum width more than 30 length of head shieldor more than 25 width of pygidium Chen et al (1996)suggested that the wide doublure of Soomaspis and Tariccoia isa synapomorphy uniting these taxa The doublure of Buenaspis(see Budd 1999a) is poorly known but based on the width ofthe heavily crushed region of the cephalic margin and theposition of a possible impression of the edge of the doublure inone specimen (Budd 1999a pl 1 fig 1) it also appears to berather wide (ie greater than 30 of the length of the headshield) Following Edgecombe amp Ramskoldrsquos coding ofSidneyia the present authors do not consider the postero-median expansion of the doublure of for example Emeraldellaor Retifacies (see Bruton amp Whittington 1983 Hou ampBergstrom 1999 respectively) to be homologous with State 1but to represent the hypostome (see Character 26)

26 Anteromedian margin of cephalon notched accomo-dating strongly sclerotised plate (0) notch and plate absentand (1) notch and plate present (cf Edgecombe amp Rasmkold1999 character 10) The apomorphic state (illustrated inFig 7D) is only known in the taxa discussed by Edgecombe ampRasmkold (1999) Reconstructions of Yohoia show a roundedlobe between the eyes (see Character 20) anterior to anddistinct from the head shield which resembles the anteriorsclerite recognised here This seems to be a misinterpretationSpecimens preserved in dorsal aspect (USNM 57696Whittington 1974 pl 2 figs 1ndash3) and lateral aspect (USNM57694 Whittington 1974 pl 1 fig 1 USNM 155616Whittington 1974 pl 5 fig 2) seem to clearly show that thislsquomedian lobersquo is a downward curving pointed extension of thehead shield

27 Hypostomal sclerite (0) median extension of thedoublure with no suture (1) natant sclerite not in contactwith doublure (2) with narrow overlap with pre-hypostomalsclerite (3) narrow attachment to doublure at hypostomalsuture and (4) absent The homology of hypostomes andlabrums has been discussed by Haas et al (2001) and Budd(2002) The euarthropod labrum is likely to represent a pos-teroventral extension of the pre-segmental acron (Scholtz

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 181

Figure 7 Ventral reconstructions of arachnomorph arthropod heads (A) Misszhouia longicaudata (after Chenet al 1997 figs 2ndash4 7) (B) Ceraurinella typa (after Chen et al 1997 fig 7) (C) Agnostus pisiformis (after Mulleramp Walossek 1987) (D) Kuamaia lata (after Edgecombe amp Ramskold 1999 figs 32 5 amp 6) (E) Cindarella eucala(after Ramskold et al 1997) and (F) Emeraldella brocki (after Bruton amp Whittington 1983) Not to scaleAbbreviations (aw) anterior wing beneath antennae (bs) boomerang-shaped sclerite (bl) blade-like process ofposterior wing (d) doublure of head-shield (cs) connective suture (f) fenestra (fs) facial suture (h) hypostome(hl) hypostomal lobe (emerging through fenestrae of Agnostus) (hs) hypostomal suture (if) intervening field ofhypostomal complex (lb) lateral bridge (lfo) lateral frontal organ (mb) median body (mbr) median bridge (mf)median furrow (mfb) mouth field bridge (mfo) median frontal organ (ol) ovate lobe and (phs) pre-hypostomalsclerite Post-antennal appendages and distal parts of antennae omitted

182 TREVOR J COTTON AND SIMON J BRADDY

1997) This structure is often sclerotised and covers the mouthventrally It is this sclerite that is here identified as thehypostome Therefore this character is considered absent intaxa lacking a sclerite covering the mouth irrespective of themorphology and position of the labrum itself which at leastpartly covers the mouth in all euarthropods other thanpycnogonids (see Edgecombe et al 2000 p 167) The lsquofleshylabrumrsquo of crustaceans does not appear to be sclerotised and ahypostome is consequently considered to be absent Althoughmany derived features are associated with the crustaceanlabrum this is not a good reason to consider it non-homologous with the hypostome-bearing structure of otherarthropods as Walossek amp Muller (1990) argued

In many taxa the mouth is covered by a posteromedianextension of the doublure here considered to represent thehypostome (eg Emeraldella Fig 7F Aglaspis see Hesselbo1992 fig 52) In others the hypostome is separated from thedoublure either by a suture a pre-hypostomal sclerite (egKuamaia lata Fig 7D Saperion glumaceum see Edgecombe ampRamskold 1999 figs 3 amp 4) or an intervening region ofunsclerotised cuticle (the natant condition of Fortey 1990beg Cindarella eucala Fig 7E) The homology of pre-hypostomal sclerites and hypostomes in helmetiids trilobitesand naraoiids (see Fig 7) has been discussed by Chen et al(1996) and Edgecombe amp Ramskold (1999) but it is as yetunclear whether any of the character states described here arederived from any other They are treated here as distinct andthe character as unordered pending further study

No hypostome has been identified in Yohoia (see above)Alalcomenaeus Jianfengia Fortiforceps or Leanchoilia andthere is certainly no broad posterior extension of the doublurein any of these taxa Since there is also no pre-hypostomalsclerite they have received a (134) partial uncertainty codingThe attachment of the hypostome of Tegopelte is unclear butit does not seem to be attached to any doublure (Whittington1985) and therefore it is given an uncertainty (12) coding

The lsquorostrum-like structurersquo on the ventral surface ofLemoneites appears to be similar in morphology and positionrelative to the anterior margin of the head shield (Flower 1968pl 8 figs 4 amp 13) to that described from Aglaspis and is codedas State 0 In many taxa the hypostome is unknown but inonly a small number can it be coded reliably as absentHughesrsquo (1975) suggestion that the hypostome of Burgessia isabsent is accepted preliminarily but an extension of thedoublure may be visible in some of Hughesrsquo figures Fortey(pers comm 2000) also doubts Hughesrsquo assertion

28 Visible ecdysial sutures (0) absent and (1) present Themarginal ecdysial sutures of chelicerates and the dorsal suturesof trilobites are almost certainly not homologous but thepresence of sutures and their position (see Character 29) arecoded separately to allow this to be tested Wills et al (1998aCharacter 2) coded marginal sutures as present in Sidneyiafollowing Brutonrsquos (1981) suggestion but this is consideredequivocal here

29 Position of ecdysial sutures (0) marginal and (1) dor-sal This character is inapplicable to taxa lacking ecdysialsutures (Character 28 State 0) Dorsal sutures whilst presentin the representatives of the Trilobita coded here are probablynot a synapomorphy of trilobites as a whole but for a clade oftrilobites excluding the Olenellida (Whittington 1989 Fortey1990a)

245 Exoskeletal tergites and thoracic tagmosis 30 Min-eralised cuticle (0) absent and (1) present Calcification of theexoskeleton is one of the most convincing synapomorphiesof the Trilobita (Fortey amp Whittington 1989 Ramskold ampEdgecombe 1991 Edgecombe amp Ramskold 1999 Character 1)Cuticle mineralisation is also coded as present in aglaspidids

following Briggs amp Fortey (1982) who suggested that theaglaspidid exoskeleton was originally phosphatic Paleomerusprobably also had a mineralised cuticle but Hou amp Bergstrom(1997 p 97) argued that it was likely to be calcareous Anundescribed Silurian aglaspidid may also have had an origi-nally calcitic cuticle (Fortey amp Theron 1994 p 856) Becauseof the uncertainty about the chemical composition of theexoskeleton in these forms cuticle mineralisation is coded aspotentially homologous in all these taxa and no attempt ismade to code separate states for phosphatic and calcareousmineralisation

31 Trunk tergites with expanded lateral pleurae coveringappendages dorsally (0) absent and (1) present Whilst thepresence of paratergal folds may be a synapomorphy at thelevel of the Euarthropoda (Boudreaux 1979 Wagele 1993Edgecombe et al 2000 Character 142) these are at mostsmall reflections of the margin of the tergites in most euarthro-pods In arachnomorph taxa these are expanded to form largelateral pleurae that cover the appendages dorsally (see egLimulus and Triarthus in Boudreaux 1979) This is inferred tobe the case in some taxa where dorsal features and appendagesare poorly known on the basis of their possession of tergiteswith wide pleural regions This feature was suggested as typicalof lamellipedians (=arachnomorphs) by Hou amp Bergstrom(1997 pp 42ndash3)

The arrangement of the thorax of Marrella Mimetaster andBurgessia differs from that of all the other taxa considered herein that the appendages seem to be attached to the lateralmargins of the body The trunk tergites do not cover theappendages and seem to lack pleurae entirely although theyare covered by a posterior extension of the head shield inBurgessia This character has been clearly described inMimetaster (Sturmer amp Bergstrom 1976 pp 87ndash90 fig 8) andBurgessia (Hughes 1975 p 421)

32 Free thoracic tergites (0) present and (1) absentFollowing Edgecombe amp Ramskold (1999 Character 16) anumber of taxa lack functional post-cephalic articulations andconsequently lack free thoracic tergites Despite this theycode these taxa for a number of characters relating to thestructure of thoracic tergites (eg their characters 18 and 19)These characters are treated here as inapplicable to taxawithout free tergites and the definition of this character hasbeen modified from that of Edgecombe amp Ramskold (1999) tomake this more explicit

33 Decoupling of thoracic tergites and segments (0)absent and (1) present Ramskold et al (1997) described thischaracter of Cindarella and Xandarella where the thoracictergites correspond to a variable increasing posteriorly num-ber of appendage pairs This character cannot be coded fortaxa in which free thoracic tergites are absent (Character 32State 1)

34 Tergite articulations (0) tergites non-overlapping (1)extensive overlap of tergites and (2) edge-to-edge pleuralarticulations In most of the taxa considered here the thoracictergites overlap considerably and relatively evenly over theirwidth This contrasts with the primitive euarthropod condi-tion where the tergites do not overlap medially as seen in stemgroup crustaceans In trilobites such as Helmetia and Kuamaia(see Edgecombe amp Ramskold 1999 character 18) overlap ofthoracic tergites is limited to a well-defined axial region andthe lateral pleurae meet edge-to-edge

The thoracic articulations of naraoiids with free thoracictergites are in some ways similar to those of trilobites with astrong overlap medially that is lacking abaxially andanterolateral articulating facets (Budd 1999a Ramskold ampEdgecombe 1999) However the distribution of these features

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 183

in other trilobite-like arachnomorphs is unclear and a restric-tive coding is used here The present authors do not considerthat this character can be applied reliably to the fused thoracictergites of Saperion Tegopelte and Skioldia and it is coded asinapplicable to all taxa lacking free thoracic tergites (Character32 State 1)

35 Trunk effacement (0) trunk with defined (separate orfused) tergite boundaries (1) trunk tergite boundaries effacedlaterally and (2) trunk tergite boundaries completely effaced(cf Edgecombe amp Ramskold 1999 Character 15)] Contrary toEdgecombe amp Ramskold (1999) the present authors considerSkioldia and Saperion to show a distinct form of tergiteeffacement where the fused tergite boundaries are defined byfurrows axially but effaced laterally The boundaries areeffaced at least laterally in Tegopelte but the axial region ofthe exoskeleton is unknown which accordingly is given amultistate uncertainty coding

36 Cephalic articulation fused (0) absent and (1) presentIn Tegopelte Saperion and Skioldia the articulation ofthe head with the thorax is non-functional and the entireexoskeleton forms a single tergite

37 Head shield overlap of thoracic tergites (0) overlapabsent or identical to overlap between thoracic segments (1)head shield covers first thoracic tergite only and (2) headshield covers mutliple anterior trunk tergites

38 Head shield articulates with reduced anterior thoracictergite (0) absent and (1) present Amongst taxa showing aposteriorly expanded head shield ie one that overlaps thethorax Edgecombe amp Ramskold (1999 Character 9) identifiedtwo distinct character states One of these ndash overlap of only theanteriormost thoracic tergite ndash was limited to taxa assigned tothe Liwiinae (sensu Fortey amp Theron 1994) and another ndashoverlap of multiple tergites with attachment to a reducedanterior thoracic tergite ndash to the Xandarellida None of theadditional taxa considered herein (including Buenaspis whichwas assigned to the Liwiinae by Budd 1999a) shows thesedistinctive morphological features However the expandedhead shields of Marrella Mimetaster and Burgessia which donot articulate with a narrow anterior thoracic tergite may behomologous with the expanded head shields of xandarellidsTo recognise this the degree of overlapping of the thorax andthe possession of the reduced anterior tergite are coded sepa-rately These characters are both coded as inapplicable to taxain which the head shield and thoracic tergites are fused(Character 36 State 1) The crustacean carapace which ismost similar to the head shield of Burgessia is seeminglyabsent in stem group crustaceans (Walossek amp Muller1998)

39 Trunk narrowed anteriorly relative to head shieldwidest posteriorly (0) absent and (1) present (cf Edgecombeamp Ramskold 1999 Character 14) The anterior narrowing ofthe trunk caused by the reduction of opisthosomal segment 1in xiphosurids (Weinbergina of the taxa analysed hereAndersen amp Selden 1997 Dunlop amp Selden 1997 Character14) is not considered to be homologous with the unusual shapeof the thorax in naraoiids recognised by their coding

40 Boundaries of anterior trunk segments reflexed antero-laterally (0) absent boundaries transverse or reflexed postero-laterally and (1) present

41 Joints between posterior tergites functional anteriorones variably fused (0) absent and (1) present

42 Posterior tergite bearing axial spine (0) absent and (1)present

Characters 40 to 42 are coded following Edgecombe ampRamskold (1999 characters 17 19 amp 23 respectively) Inaddition to the taxa considered here a thoracic axial spine ispresent in some olenelloid trilobites (see Lieberman 1998

Characters 73 amp 74) and in the aglaspidid Beckwithia (seeHesselbo 1989)

246 Posterior body termination 43 Postabdomen of seg-ments lacking appendages (0) absent and (1) present

44 Length of postabdomen (0) one segment (1) twosegments (2) three segments and (3) five segments In sometaxa a variable number of posterior thoracic segments bearcomplete tubular tergites and lack appendages In Aglaspisautapomorphically it seems that the posterior three thoracicsegments lack appendages (Briggs et al 1979 Hesselbo 1992)but have unmodified tergites These situations are recognisedas potentially homologous by the coding used here Character44 is coded as inapplicable to taxa lacking a postabdomen

45 Posterior tergites strongly curved in dorsal aspect com-pared to anterior tergites (0) absent and (1) present Asrecognised by Wills et al (1998a Character 17) in some taxawhere the tergites are distinct the curvature of thoracic tergitesincreases posteriorly so that the posterior tergites are highlycurved to semicircular in dorsal aspect This situation isnot known from crustaceans (except isopods) or stem grouparthropods

46 Posterior segments reduced and with highly reducedappendages (0) present and (1) absent In some taxa there area large number of posterior segments that are short sagitallyand have appendages that are much reduced in size comparedto anterior trunk appendages These somites are incorporatedinto the pygidium in some taxa but primitively they arecovered by tiny free trunk tergites In the derived state thetrunk somites and limbs are of a relatively constant size Thedistinction between these states can be seen when attempting tocount the number of segments that make up the body In taxashowing State 0 the number of segments is very high anddifficult to count whereas in other taxa the number ofpost-cephalic segments is easily assessed

47 Pygidium (0) absent and (1) present A pygidium isrecognised here as a posterior tagma consisting of a number offused segments under a single tergite which may or may notincorporate the post-segmental telson This is different to theuse of this term by Edgecombe amp Ramskold (1999) whichfollows Ramskold et al (1997) and very different to its use byWills et al (1998a p 53) as a synapomorphy of the TrilobitaThe present authorsrsquo definition recognises the situation inRetifacies where the spinose postsegmental telson is autapo-morphically not fused to the pygidium as homologous toother multisegmented posterior tagma which include thetelson The distinction between the pygidium and an expandedpost-segmental telson can also be seen in the position of theanus which is consistently in the posteriormost pre-telsonicsegment amongst putative arachnomorphs (see Character 48)In taxa with a pygidium the anal segment is fused into thepygidium (see eg Kuamaia Edgecombe amp Ramskold 1999fig 6) whereas in those taxa lacking a pygidium the anus isbetween the final trunk tergite and the telson

Ramskold et al (1997) argued that the posteriormost tergiteof xandarellids (which incorporates the anal segment) is nothomologous to the posterior tagma recognised as pygidiaherein because posterior thoracic tergites also cover multiplesegments in xandarellids (see Character 33) Evidence ofsegmentation of the pygidium in a variety of taxa suggests thatthe number of tergites fused to form the pygidium is greaterthan the number of appendages For example the pygidium ofKuamaia has two pairs of lateral spines but at least fourpairs of appendages (Hou amp Bergstrom 1997 Edgecombe ampRamskold 1999) and that of Triarthrus has only five axialrings posterior to the articulating ring but over 10 pairs ofappendages (Whittington amp Almond 1987) The fact thatdecoupling of tergites and segments is evident in the thorax of

184 TREVOR J COTTON AND SIMON J BRADDY

Xandarella and Cindarella does not necessarily suggest that asimilar more widespread decoupling in pygidia is the result ofa different developmental process and hence that they arenon-homologous Instead the situation in the xandarellidthorax is potentially homologous to that found (more widely)in pygidia and consequently the terminal xandarellid tergite isa pygidium It is unclear if decoupling of tergites and somites isprimitively limited to the terminal tergite or primitively aproperty of the arachnomorph post-cephalon as a whole

It has been suggested that the tiny pygidium of olenelloidtrilobites which probably best reflects the primitive trilobitestate consists only of the post-segmental telson (Harrington inMoore 1959) and therefore is not a true pygidium HoweverWhittington (1989) showed that the olenelloid pygidium con-sists of at least two and possibly as many as five or sixsegments and therefore is likely to be homologous with thepygidium of other trilobites

48 Position of the anus (0) terminal within telson and (1)at base of telson In crustaceans and stem group arthropodsthe anus is terminal or otherwise situated in the telson (egRehbachiella Walossek 1993 fig 15D) In putative arachno-morphs the anus is either at the junction of the posteriormostthoracic segment and the telson or ventral within a fusedpygidium In the case of taxa with a pygidium the anus isanterior to the posterior margin and where known positionedbetween the posteriormost pair of appendages suggesting thatit is anterior to the post-segmental telson (see eg OlenoidesWhittington 1980)

49 Pygidium with median keel (0) absent and (1) presentEdgecombe amp Ramskold (1999 Character 21) considered thepresence of a median keel on the pygidium as a synapomorphyuniting Soomaspis and Tarricoia They did not explain howthis structure could be distinguished from the raised pygidialaxis of trilobites Homology of these structures is not sup-ported here because the axis of more primitive trilobites(including Eoredlichia) is quite distinct morphologically fromthe naraoiid keel Budd (1999a) noted the presence of a keel onthe pygidium of Buenaspis and suggested (Budd 1999a p 102)that it may be an artefact of dorsoventral compaction How-ever contrary to the coding of Edgecombe amp Ramskold(1999) a keel is visible on the pygidium of Liwia convexa (Dzikamp Lendzion 1988 fig 4CD) preserved in full relief Thereforeit seems unlikely that the keel is a taphonomic artefact

50 Pygidium with broad-based median spine (0) absentand (1) present

51 Pygidium with lateral spines (0) present and (1) absentEdgecombe amp Ramskold (1999 Character 22) coded thepresence of a median spine and two pairs of lateral spines aspotentially homologous in Sinoburius Kuamaia and HelmetiaThe distribution of lateral spines is much wider than that ofterminal spines and therefore they are coded separately hereIn trilobites it has widely been recognised that lateral spinesrepresent the original segmentation of the pygidium Thiscannot be the case for median spines Characters 49 to 51are not applicable to taxa lacking a pygidium (Character 47State 0)

52 Expanded post-segmental telson (0) absent and (1)present In a range of taxa the posterior-most tergite is a large(relative to the thoracic tergites) structure that lacks anyevidence of segmentation and is interpreted as representing anexpanded tergite of the post-segmental telson from whichsegments are released anteriorly during ontogeny On the otherhand the posteriormost tergite of Marrella and probablyMimetaster is small compared to the thoracic tergites androunded This element probably represents the plesiomorphiceuarthropod telson The homology of this character in mosttaxa is clear and only doubtful in Retifacies in which it may

be segmented The figures of Hou amp Bergstrom (1997) providelittle unequivocal evidence for a telson in Retifacies but thepresence of this structure is very clear in colour photographs(eg Chen et al 1996 fig 198A) However these figures donot convincingly demonstrate the segmentation of the telsonRetifacies is interpreted here as being unique in possessingboth a pygidium and an expanded post-segmental telson

53 Telson shape (0) spinose and (1) paddle-shaped Theshape of the expanded telsons identified above varies frombroadly spinose to flattened and paddle-like This differencecan be recognised both on the basis of the ratio of length tobasal width (spinose telsons are relatively long) and by thechange in width posteriorly (spinose telsons reduce in widthposteriorly whereas paddle-shaped telsons increase in width)In eurypterids a wide range of telson shapes is found includ-ing both character states described here It is likely that theplesiomorphic condition is a spinose telson This character isinapplicable to taxa lacking an expanded telson

54 Post-ventral furcae (0) absent and (1) present Thischaracter recognises the potential homology of unsegmentedpaired structures that articulate with the segment immediatelyanterior to the telson The potential homology of these struc-tures (the post-ventral plates) in Emeraldella and Aglaspis wasrecognised by Wills et al (1998a Character 70) and inEmeraldella and Sidneyia by Edgecombe amp Ramskold (1999Character 25) The homology of the segmented pygidial caudalfurcae of Olenoides (see Whittington 1975a 1980) with thesestructures is considered doubtful and coded as equivocal

Despite their distinctive morphology the long caudualfurcae of Cheloniellon may be homologous with the furcae ofAglaspis Emeraldella and Sidneyia This is supported by thecaudal furcae of the Ordovician cheloniellid Duslia which aremore similar in morphology to those of Emeraldella than ofCheloniellon Chlupac (1988 pp 614ndash16) considered thesestructures to be on the terminal tergite in Duslia However afaint tergite boundary is apparent posterior to the attachmentof the furcae (see Chlupac 1988 pl 57 fig 4) Therefore theyare considered here to have been attached to the pre-telsonicsegment as in Cheloniellon Chlupac (1988) and Dunlop ampSelden (1997) supported a close relationship between Dusliaand Cheloniellon

25 MethodsAll analyses were carried out using PAUP Version 40b4a(Swofford 1999) and unless otherwise stated used heuristicsearches with one thousand random addition sequencereplicates The software packages MacClade Version 307(Maddison amp Maddison 1997) and RadCon (Thorley amp Page2000) were used for comparing trees and investigating patternsof character evolution Tree length and other statistics (theConsistency Index CI and Retention Index RI) were calcu-lated by PAUP and MacClade with uninformative charactersexcluded Analyses were run separately using each of thedifferent codings for aglaspidids

Characters were treated as unordered and of equal weight inmost analyses To assess the influence of this assumption fourof the eight multistate characters which have states that areintermediate between others (Wilkinson 1992) were treated asordered in some analyses These characters are the number ofcephalic segments (Character 4) the length of raptorialappendage spines (Character 6) the degree of overlap of thethorax by the head-shield (Character 37) and the number ofsegments making up the postabdomen (Character 44) The lastof these is uninformative when not ordered because only onetaxon shows each of states 0 1 and 3

Support for individual nodes was assessed by bootstrapanalysis (Felsenstein 1985) and by calculating Bremer support

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 185

indices (Bremer 1988 1994) These methods measure twodistinct aspects of support for phylogenetic hypotheses Treesor nodes may be considered well supported (1) to the extent towhich alternative topologies are much less parsimonious asmeasured by the support index (Wilkinson 1996) or (2) wherethey are consistent with a large proportion of characters sothat character sampling is unlikely to have had much influenceon topology as assessed by bootstrapping (Page 1996) Boot-strapping was performed with 100 bootstrap replicates eachof 10 addition sequence replicates Bremer support indicesare based on heuristic searches with 100 addition sequencereplicates

Three pairs of terminals are included in Table 2 that apartfrom missing data are coded identically These are Helmetiaand Kuamaia Soomaspis and Tariccoia and Saperion andSkioldia These pairs are lsquotaxonomic equivalentsrsquo removingthe taxon with most missing data will not influence theposition of the other They can be recoded as single terminalsaccording to the principal of safe taxonomic reduction(Wilkinson 1995a b) and were not considered separately in theanalyses presented here Therefore cladistic analyses werecarried out on the basis of 33 terminals Edgecombe ampRamskold (1999) ignored this and coded the same three pairsof taxa and Eoredlichia and Olenoides identically for theirmore limited set of characters

26 ResultsAnalysis with all characters unordered and using the lsquoAglaspi-dida 1rsquo coding found nine most parsimonious trees (MPTs)each 126 steps long (CI=0middot556 RI=0middot763) Using the lsquoAglas-pidida 2rsquo coding resulted in 18 MPTs of length 125 (CI=0middot560RI=0middot765) Nine of these 18 trees were those found with thefirst coding of aglaspidids The strict component consensus(sensu Wilkinson 1994) of the 27 unique trees found acrossboth analyses is shown in Figure 8 All trees supported themonophyly of the Marrellomorpha (Marrella and Mimetaster)and of a clade including all other taxa except Crustacea (Clade1 of Fig 8) The relationships between these two clades and theCrustacea were unresolved equal numbers of trees in bothanalyses supported each of the topologies [Clade 1 (CrustaceaMarrellomorpha)] [Crustacea (Marrellomorpha Clade 1)]and [Marrellomorpha (Crustacea Clade 1)] All trees placedBurgessia as the sister taxon to the rest of Clade 1 andsupported the division of the rest of Clade 1 into two majorsister groups The topology of one of these clades (Clade 2)which included trilobites helmetiids tegopeltids naraoiidsxandarellids and Retifacies was stable with respect to thecoding of aglaspidids The other clade (Clade 3) consists of aclade of megacheirans and chelicerates (Clade 4) and a cladeincluding aglaspidids Lemoneites Paleomerus CheloniellonEmeraldella and Sidneyia (Clade 5) This latter clade is un-resolved in the strict consensus of all 27 trees (Fig 8) and thestrict consensus of the 18 MPTs found using the lsquoAglaspidida2rsquo coding but was fully resolved in the consensus of the ninetrees found with the lsquoAglaspidida 1rsquo coding Within themegacheiran-chelicerate clade all trees found Fortiforceps tobe the sister group to all other taxa Yohoia the sister group tochelicerates and Alalcomenaeus and Leanchoilia to form aclade Jianfengia is equally parsimoniously placed as the sistergroup to the Yohoia-chelicerate clade or as the sister group tothis clade and the Alalcomenaeus-Leanchoilia clade

Treating characters 4 6 37 and 44 as ordered had a minimaleffect on most parsimonious topology Nine MPTs of 136 steps(CI=0middot515 RI=0middot743) were found with the lsquoAglaspidida 1rsquocoding and 12 MPTs 135 steps in length (CI=0middot519 RI=0middot745)with the lsquoAglaspidida 2rsquo coding These trees all supported a(Crustacea Clade 1) clade excluding the marrellomorphs

Apart from this the strict consensus of these 21 trees isidentical to the consensus of the 27 trees found with allcharacters treated as unordered

Across all analyses four different topologies for Clade 5 (ofFig 8) were found (Fig 9) The distribution of these topologiesacross the four analyses described above and across the sameanalyses repeated after reweighting of each character in pro-portion to their Rescaled Consistency Index (see Farris 1989)from the first analysis is shown in Table 3 The instability of

Figure 8 Strict consensus of 27 MPTs from four separate analyses ofarachnomorph phylogeny with both interpretations of aglaspididmorphology and with some characters treated as ordered or allcharacters unordered Clades referred to in the text are numbered

Figure 9 Alternative equally parsimonious resolutions of clade 5 ofFig 8 (see Table 3)

186 TREVOR J COTTON AND SIMON J BRADDY

this clade was to some extent caused by the inclusion ofLemoneites and Paleomerus which could each be coded foronly 27 of the 54 characters used Analysis with these two taxaexcluded and characters treated as unordered found treescompatible only with Topology D of Figure 9 irrespective ofthe coding used for aglaspidids Analysis with some characterstreated as ordered and using either aglaspidid coding foundthree trees compatible only with Topology B of Figure 9 andthree compatible only with Topology D of Figure 9 ThereforeTopology D is the preferred topology for Clade 5 as shown inTable 3

One of the MPTs resulting from the lsquoAglaspidida 2rsquo coding(with all characters treated as unordered) was chosen as thebasis for further discussion because this tree combined thepreferred topology of Clade 5 with the (Crustacea Clade 1)group that was favoured when some characters were treated asordered Bootstrap percentages and Bremer support values forthis tree (using the Aglaspidida 2 coding) are shown in Figure10 Apomorphies for all ingroup nodes of this tree are shownin Figure 10A

3 Discussion

31 Phylogenetic resultsThe results of the cladistic analyses presented above arewell-resolved and robust with respect to different analyticalparameters Most of the taxa considered form a clade (Clade 1of Fig 8) that is more closely related to chelicerates than tocrustaceans in all analyses and therefore is recognised asthe Arachnomorpha as defined by Chen et al (1996) andRamskold et al (1996) The monophyly of the Marrellomor-pha was supported in all analyses The marrellomorphs werefound to be the sister group to a Crustacea+Clade 1 group inmost MPTs but in a minority of trees formed the sister groupto Clade 1 alone According to the latter result they should beincluded within the Arachnomorpha but according to the firstexcluded from it Both of these alternatives have been sup-ported in previous studies (eg Hou amp Bergstrom 1997 Willset al 1998a respectively) In addition to the marrellomorphsthe Fuxianhuiida Bousfield 1995 and Canadaspidida havebeen placed in the Euarthropoda outside the main mandibu-late and arachnomorph clades (Hou amp Bergstrom 1997)Further study of these taxa of the crustacean or mandibulatestem group and of the marrellomorphs is necessary before the

position of the Marrellomorpha can be resolved Provisionallymarrellomorphs are excluded from the Arachnomorphafollowing the majority of analyses above and Wills et al(1994)

The Burgess Shale arthropod Burgessia was placed as thesister group to remaining arachnomorphs in all analyses OtherArachnomorpha form two major clades one consisting of thelsquotrilobite-alliedrsquo arachnomorphs analysed by Edgecombe ampRamskold (1999) here termed lsquoTrilobitomorpharsquo (defined asTrilobita and all taxa more closely related to Trilobita thanChelicerata) and the other (lsquochelicerate-alliedrsquo) clade includingchelicerates lsquogreat appendagersquo arthropods aglaspididsLemoneites Paleomerus Sidneyia and Emeraldella The latteris here termed lsquoCheliceramorpharsquo (defined as Chelicerata andall taxa more closely related to Chelicerata than Trilobita) Thetopology of the lsquotrilobite-alliedrsquo clade was similar to that foundby Edgecombe amp Ramskold (1999) but the present authorsrsquoresults are more completely resolved unambiguously support-ing the sister-group relationship between trilobites andHelmetiida suggested in the previous study Within thelsquochelicerate-alliedrsquo clade Aglaspidids Paleomerus Lemoneitesand the Burgess Shale Emeraldella and Sidneyia form a cladein opposition to megacheirans and chelicerates

The present authorsrsquo analysis supports a new hypothesis ofthe origin of the chelicerates from within a paraphyleticassemblage of megacheiran arthropods and confirms pyc-nogonids as a sister group to the euchelicerates Previoushypotheses of the chelicerate sister group (see Section 1) aremuch less parsimonious The shortest trees supporting aCheloniellon-chelicerate clade are six steps longer than theMPTs and those supporting an aglaspidid-chelicerate cladeseven steps longer All megacheirans and chelicerates areunited by the flap-shaped rounded exopods and modificationof the second cephalic segment endopods into anteriorlydirected spinose raptorial organs which share a number ofdetailed similarities A clade including all of these taxa apartfrom Fortiforceps is supported by the synapomorphic loss ofthe antennae (or antennulae of crustaceans) and the longerlength of spinose projections of the second cephalic appendageendopods Within this clade Yohoia shares the loss of cephalicexopods the loss of thoracic endopods and a postabdomen oftubular tergites with chelicerates

The poor resolution of Clade 5 (of Fig 8) was partly a resultof the poor preservation of and consequent high proportion ofmissing data for Lemoneites and Paleomerus When these two

Table 3 Numbers of most parsimonious trees supporting each of the alternative topologies for clade 4 of Fig 7 under different analyticalconditions (RCI=Revealed Consistency Index)

Aglaspididcoding

Characters 4 6 36and 43 ordered

Charactersreweighted by RCI

Paleomerus andLemoneites excluded

Topology (see Fig 8)

A B C D

1 91 X 3 3 31 X 61 X X 31 X 61 X X 3 32 9 92 X 3 3 3 32 X 62 X X 32 X 62 X X 3 3Total 6 18 12 54

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 187

taxa were excluded from the analyses only two (B and D) ofthe four topologies found in the complete analysis remainedmost parsimonious (see Fig 9 and Table 3) Secondly Bremersupport values throughout much of the lsquochelicerate-alliedrsquoclade (Clade 3 of Fig 8) were improved when these taxa wereexcluded (see Fig 11)

Dunlop (1999 p 258) suggested that in reconstructing thechelicerate stem lineage lsquowe might predict that the two mostsignificant changes towards the chelicerate condition are thereduction of the antennae and the formation of the nextappendages into a clawrsquo According to the hypothesis pre-sented here both of these adaptations were achieved in EarlyCambrian chelicerate ancestors The recognition of the loss ofthe antennae in the stem group of the chelicerates and thephylogenetic hypothesis presented here suggest that chelicer-ates are most closely related to taxa with only a singleadditional segment fused to the crown group eucheliceratehead of four fused somites Previous hypotheses have derivedchelicerates from Palaeozoic arthropods with a longer head(eg Cheloniellon Sturmer amp Bergstrom 1978 SanctacarisBriggs amp Collins 1988 Emeraldella Bruton amp Whittington1983) The plesiomorphic pattern of head segmentation ismatched in pycnogonids Bergstrom et al (1980) suggestedthat only the anterior four pairs of appendages are incorpo-rated into the head This supports the view that pycnogonidsare primitive with respect to other chelicerates which showmore or less complete fusion of this primitive head and threethoracic segments including Solpugida and Pseudoscorpiones

The present study suggests that a shift from gnathobasicbenthic feeding to a more vagrant lifestyle and raptorialappendage feeding occurred very early in the chelicerate lin-eage Appendage feeding was clearly an important adaptation

Figure 10 One of 18 equally parsimonious cladograms with lsquoAglaspidida 2rsquo coding and all characters unorderedthat is most congruent with results from other analyses Non-terminal branch lengths scaled to reflect number ofapomorphies (A) ACCTRAN apomorphy scheme Character numbers shown above boxes and states shownbelow boxes as in Table 2 and the text Character consistency indices are indicated by shading (B) Levels ofsupport for individual nodes Bremer support indices shown in bold type above branches Bootstrap percentagesin italic type below branches for all nodes with relative frequencies greater than 50

Figure 11 Bremer support values for clades within the lsquochelicerate-alliedrsquo arachnomorphs (clade 3 of Fig 8) when all taxa wereincluded (normal weight font above nodes) and with Lemoneites andPaleomerus excluded (bold type below nodes)

188 TREVOR J COTTON AND SIMON J BRADDY

for the terrestrialization of chelicerates (Dunlop 1997 p 69Dunlop amp Webster 1999) but the model proposed heresuggests that this considerably pre-dated terrestrializationGnathobasic feeding in xiphosurans and eurypterids mayrepresent a reversal to a more primitve benthic arachnomorphlifestyle from a more pycnogonid-like ancestor

32 Evolution of the arachnomorph headThe extant arthropod classes show highly conserved patternsof head segmentation (eg Wills et al 1994) In contrast therehas been a near consensus amongst Cambrian arthropodworkers (although rarely based on an explicit phylogenetichypothesis) that patterns of head segmentation are highlyhomoplastic and hence of little systematic significanceSturmer amp Bergstrom (1978 pp 78ndash9) suggested that lsquoevenclosely related forms may have different numbers of headsegments and appendagesrsquo Bruton amp Whittington (1983pp 576ndash7) that lsquodiscussion on fossil arthropod relationshipsbased on head segmentation appears to be largely irrel-evant and at best speculativersquo and Delle Cave amp Simonetta(1991 p 191) that lsquothere are no obvious phyletic affinitiesbetween genera having the same number of cephalized seg-mentsrsquo Most recently Bergstrom amp Hou (1997 p 104) con-cluded that lsquothe segmental length of the head shield seemsto be of no relevance to the discussion and discrimination ofevolutionary lineagesrsquo It has been claimed that this view issupported by cladistic analyses in which patterns of tagmosishave been found to be rather poor at defining major arthropodclades (Briggs et al 1992 Wills et al 1994)

This suggestion is not only of relevance to systematics buthas featured prominently in recent debates surrounding thenature of the lsquoCambrian explosionrsquo (see Budd amp Jensen 2000for a recent review) Gould (1989 1991) argued that theapparent plasticity of head segmentation in Cambrian com-pared to post-Cambrian arthropod evolution suggests thatbody-plan evolution was only constrained after the Cambrianexplosion According to this view the origin of new highertaxa with distinct body-plans was only possible before theonset of these constraints (eg Valentine 1995) Taxa hererecognised as arachnomorphs clearly played a major role in thedevelopment of Gouldrsquos hypothesis This suggestion has pre-viously been assessed in terms of overall morphological diver-sity which is clearly rather distinct from Gouldrsquos concept ofdisparity Wills et al (1998b) showed that there was littlecorrelation between overall morphological disparity and de-gree of limb specialisation and that the latter was phylogeneti-cally highly plastic They remarked that (Wills et al 1998bp 64) lsquothis finding has important implications for models ofarthropod phylogeny and evolution that attribute overidingimportance to head segmentationrsquo but did not explicitlyexamine head segmentation although it is likely to have beena major component of the index of tagmosis employed

The results of the present study suggest that arguments thatarthropod head segmentation was unusually labile during theCambrian are poorly founded Rather only four major pat-terns of euarthropod head segmentation are identified Theplesiomorphic euarthropod state according to both Walossekamp Muller (1997 1998) and Scholtz (1997) which may moreproperly be the plesiomorphic state for the euarthropod crowngroup only (depending on the phylogenetic position of theMarrellomorpha) consists of four post-acronal segmentsbearing the antennae and three pairs of biramous limbs Thepresent authors suggest that the term lsquocephalonrsquo be restrictedto this kind of head which is found in stem group mandibu-lates in addition to many arachnomorphs Crown groupmandibulates share a lsquobimaxillary headrsquo in which an addi-tional pair of appendages the labium or second maxillae are

incorporated into the head but in crustaceans these are notfused to the carapace (Scholtz 1997) Finally two distinctforms of head tagmosis are found in chelicerates A head withfour pairs of appendages (but without antennae giving a totalof five segments incorporated into the head) which has beencalled the lsquocephalosomarsquo is present in pycnogonids (Walossekamp Dunlop 2002 pp 434ndash5) and probably also in some euch-elicerates (J A Dunlop pers comm 2003) The lsquoprosomarsquo ofmost euchelicerates consists of this cephalosoma and twoadditional segments

In general these patterns of head segmentation are highlyphylogenetically conserved (see Fig 12) Only three homo-plastic changes namely reversal to a more primitive conditionin the Leanchoilia-Alalcomenaeus clade and the convergentorigin of a five-segmented head in Clade 3 (of Fig 8) and theXandarellida are necessary to optimise character 4 onto themost parsimonious cladogram There are also transitions toautapomorphic states in Emeraldella and Sidneyia In the caseof Sidneyia however an argument could be made (followingBergstrom amp Sturmer 1978 see discussion of Character 4) thatthe head consists of five segments as in closely related taxabut autapomorphically each segment has a separate tergiteThis would further increase the phylogenetic stability of headsegmentation Also following Bergstrom amp Sturmer (1978) thehead of Cheloniellon could be considered to consist of twotergites making it more similar to the head of Emeraldella(with six pairs of appendages)

Figure 12 Cladogram from Fig 10 showing changes in head segmen-tation Bold type shows abbreviated head segmentation formulaAbbreviations (Ant) antennae (BA) biramous appendage pairs (Ch)chelicerae (GA) great appendages (UA) uniramous appendage pairsand (X) segment without appendages Synapomorphy A is the auta-pomorphic reduction in head length to a single segment in SidneyiaAlternatively this may represent secondary division of the headshield Synapomorphy B is the increase in the length of the head inEmeraldella to six segments

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 189

4 Acknowledgements

We thank D E G Briggs J A Cotton J A Dunlop R AFortey S Harris R A Moore P Selden O E Tetlie andP Van Roy for comments on earlier drafts of this work Themanuscript was reviewed by M Wills F Schram and oneanonymous referee This research was supported by the Geo-logical Society of London Curry Research Studentshipawarded to TJC by the Department of Earth Sciences Uni-versity of Bristol and by the Department of PalaeontologyThe Natural History Museum London TJC would like tothank C Labandeira J Thompson and R Chapman fortheir hospitality at the National Museum of Natural HistoryWashington DC USA and A Olcott S Schellenberg andJ Young for making his time there so enjoyable

5 References

Abzhanov A Popadic A amp Kaufman T C 1999 Chelicerate Hoxgenes and the homology of arthropod segments Evolution andDevelopment 1 77ndash89

Akam M 2000 Arthropods developmental diversity within a (super)phylum Proceedings of the National Academy of Sciences USA97 4438ndash41

Aldridge R J Gabbott S E amp Theron J N 2001 The Soom ShaleIn Briggs D E G amp Crowther P R (eds) Palaeobiology II340ndash2 Oxford Blackwell Science

Anderson L I amp Selden P A 1997 Opisthosomal fusion andphylogeny of Palaeozoic Xiphosura Lethaia 30 19ndash31

Arango C P 2002 Morphological phylogenetics of the sea spiders(Arthropoda Pycnogonida) Organisms Diversity and Evolution2 107ndash25

Ax P 1986 Das phylogenetische System Stuttgart Gustav FischerVerlag

Babcock L E amp Zhang Wentang In press New non-mineralizedarthropods from the Chengjiang Lagerstatte Lower CambrianYunnan China Palaeontologia Cathyana

Bartels C Briggs D E G amp Brassel G 1998 The fossils ofthe Hunsruck Slate Marine life in the Devonian CambridgeCambridge University Press

Bergstrom J 1971 Paleomerus ndash merostome or merostomoid Lethaia4 393ndash401

Bergstrom J 1979 Morphology of fossil arthropods as a guideto phylogenetic relationships In Gupta A P (ed) ArthropodPhylogeny 3ndash56 New York NY Van Nostrand Reinhold Co

Bergstrom J 1992 The oldest arthropods and the origin of theCrustacea Acta Zoologica 73 287ndash91

Bergstrom J Sturmer W amp Winter G 1980 PalaeoisopusPalaeopantopodus and Palaeothea pycnogonid arthropodsfrom the Lower Devonian Hunsruck Slate West GermanyPalaontologische Zeitschrift 54 7ndash54

Bergstrom J amp Brassel G 1984 Legs in the trilobite Rhenops fromthe Lower Devonian Hunsruck Slate Lethaia 17 67ndash72

Bergstrom J amp Hou X 1998 Chengjiang arthropods and theirbearing on early arthropod evolution In Edgecombe G D(ed) Arthropod fossils and phylogeny 151ndash84 New York NYColumbia University Press

Boudreaux H B 1979 Significance of intersegmental tendon systemin arthropod phylogeny and a monophyletic classification ofArthropoda In Gupta A P (ed) Arthropod Phylogeny 551ndash86New York NY Van Nostrand Reinhold Co

Bousfield E L 1995 A contribution to the natural classification ofLower and Middle Cambrian arthropods food-gathering andfeeding mechanisms Amphipacifica 2 3ndash33

Braddy S J Aldridge R J Gabbott S E amp Theron J N 1999Lamellate book-gills in a late Ordovician eurypterid from theSoom Shale South Africa support for a eurypterid-scorpionclade Lethaia 32 72ndash4

Braddy S J amp Almond J E 1999 Eurypterid trackways from theTable Mountain Group (Ordovician) of South Africa Journal ofAfrican Earth Sciences 29 165ndash77

Bremer K 1988 The limits of amino acid sequence data in angio-sperm phylogenetic reconstruction Evolution 42 795ndash803

Bremer K 1994 Branch support and tree stability Cladistics 10295ndash304

Briggs D E G 1998 Review of Arthropods from the LowerCambrian Chengjiang fauna southwest China by HouXianguang and Jan Bergstrom Fossils and Strata 45

Scandinavian University Press Oslo Norway 1997 Palaeo-geography Palaeoclimatology Palaeoecology 143 193ndash4

Briggs D E G Bruton D L amp Whittington H B 1979 Append-ages of the arthropod Aglaspis spinifer (Upper CambrianWisconsin) and their significance Palaeontology 22 167ndash80

Briggs D E G Fortey R A amp Wills M A 1992 Morphologicaldisparity in the Cambrian Science 256 1670ndash3

Briggs D E G Fortey R A amp Wills MA 1993 How big was theCambrian explosion A taxonomic and morphologic comparisonof Cambrian and Recent arthropods In Lees D R amp EdwardsD (eds) Evolutionary patterns and processes 33ndash44 LondonLinnean Society of London Linnean Society Symposium Series

Briggs D E G Erwin D H amp Collier F J 1994 The fossils of theBurgess Shale Washington DC Smithsonian Institution Press

Briggs D E G amp Bartels C 2001 New arthropods from the LowerDevonian Hunsruck Slate (Lower Emsian Rhenish Massifwestern Germany) Palaeontology 44 275ndash304

Briggs D E G amp Bartels C 2002 Magnoculocaris a new name forthe arthropod Magnoculus Briggs and Bartels 2001 from theLower Devonian Hunsruck Slate Germany Palaeontology 45419

Briggs D E G amp Collins D 1988 A Middle Cambrian cheliceratefrom Mount Stephen British Columbia Palaeontology 31 779ndash98

Briggs D E G amp Collins D 1999 The arthropod Alalcomenaeuscambricus Simonetta from the Middle Cambrian Burgess Shale ofBritish Columbia Paleontology 42 953ndash78

Briggs D E G amp Fortey R A 1982 The cuticle of aglaspididarthropods a red-herring in the early history of vertebratesLethaia 15 25ndash9

Briggs D E G amp Fortey R A 1989 The early radiation andrelationships of the major arthropod groups Science 246 241ndash3

Briggs D E G amp Fortey R A 1992 The early Cambrian radiationof arthropods In Lipps J H amp Signor P W (eds) Origin andEarly Evolution of the Metazoa 335ndash373 New York NY PlenumPress

Briggs D E G amp Robison R A 1984 Exceptionally preservednontrilobite arthropods and Anomalocaris from the MiddleCambrian of Utah University of Kansas PaleontologicalContributions Paper 111 1ndash23

Broili F 1932 Ein neuer Crustacee aus dem rheinischen UnterdevonSitzungsberichte der Mathematisch-Naturwissenschaftlichen(Abteilung) Klasse der Bayerischen Akademie der Wissenschaftenzu Munchen 1932 27ndash38

Broili F 1933 Ein zweites Exemplar von Cheloniellon Sitzungsber-ichte der Bayerischen Akademie der Wissenschaften 1933 11ndash32

Bruton D L 1981 The arthropod Sidneyia inexpectans MiddleCambrian Burgess Shale British Columbia Philosophical Trans-actions of the Royal Society of London Series B 295 619ndash56

Bruton D L amp Whittington H B 1983 Emeraldella and Leanchoiliatwo arthropods from the Burgess Shale British Columbia Philo-sophical Transactions of the Royal Society of London Series B 300553ndash85

Budd G E 1995 Kleptothule rasmusseni gen et sp nov anolenellid-like trilobite from the Sirius Passet fauna (BuenFormation Lower Cambrian North Greenland) Transactions ofthe Royal Society of Edinburgh Earth Sciences 86 1ndash12

Budd G E 1996a Progress and problems in arthropod phylogenyTrends in Ecology and Evolution 11 356ndash8

Budd G E 1996b The morphology of Opabinia regalis and thereconstruction of the arthropod stem-group Lethaia 29 1ndash14

Budd G E 1997 Stem-group arthropods from the Lower CambrianSirius Passet fauna of North Greenland In Fortey R A ampThomas R H (eds) Arthropod Relationships 125ndash38 LondonChapman and Hall Systematics Association Special Volume 55

Budd G E 1999a A nektaspid arthropod from the Early CambrianSirius Passet fauna with a description of retrodeformation basedon functional morphology Palaeontology 42 99ndash122

Budd G E 1999b The morphology and phylogenetic significance ofKerygmachela kiergkegaardi Budd (Buen Formation LowerCambrian N Greenland) Transactions of the Royal Society ofEdinburgh Earth Sciences 89 (for 1998) 249ndash290

Budd G E 2002 A palaeontological solution to the arthropod headproblem Nature 417 271ndash5

Budd G E amp Jensen S 2000 A critical reappraisal of the fossilrecord of the bilaterian phyla Biological Reviews 75 253ndash95

Chen J Zhou G Zhu M amp Yeh K 1996 The Chengjiang biota ndasha unique window of the Cambrian explosion Taichung NationalMuseum of Natural Science [In Chinese]

Chen J Edgecombe G D amp Ramskold L 1997 Morphological andecological disparity in naraoiids (Arthropoda) from the Early

190 TREVOR J COTTON AND SIMON J BRADDY

Cambrian Chengjiang fauna China Records of the AustralianMuseum 49 1ndash24

Chen J amp Zhou G 1997 Biology of the Chengjiang fauna In ChenJ Cheng Y amp Iten H V (eds) The Cambrian explosion and thefossil record Bulletin of the National Museum of Natural ScienceTaiwan 10 11ndash106

Child C A 1997 Some deep-sea Pycnogonida from Argentine slopeand basin Proceedings of the Biological Society of Washington110 128ndash42

Chlupac I 1988 The enigmatic arthropod Duslia from the Ordovicianof Czechslovakia Palaeontology 31 611ndash20

Chlupac I 1995 Lower Cambrian arthropods from the PasekyShale (Barrandian area Czech Republic) Journal of the CzechGeological Society 40 9ndash36

Chlupac I amp Havlıcek V 1965 Kodymirus n g a new aglaspidmerostome of the Cambrian of Bohemia Sbornık GeologickechVid Paleontologie 6 7ndash20

Cisne J L 1975 Anatomy of Triarthrus and the relationships of theTrilobita Fossils and Strata 4 45ndash63

Cisne J L 1981 Triarthrus eatoni (Trilobita) anatomy of its exo-skeletal skeletomuscular and digestive systems Palaeonto-graphica Americana 9 95ndash142

Conway Morris S 1982 Atlas of the Burgess Shale London ThePalaeontological Association

Conway Morris S 1989 The persistence of Burgess Shale-type faunasimplications for the evolution of deeper-water faunas Trans-actions of the Royal Society of Edinburgh Earth Sciences 80271ndash83

Conway Morris S 1998 The crucible of creation Oxford OxfordUniversity Press

Cotton T J 1999 Review of Arthropod fossils and phylogenyPalaeogeography Palaeoclimatology Palaeoecology 151 341ndash3

Cotton T J 2002 The phylogeny and morphological evolution ofCambrian trilobites and their relatives Unpublished PhD ThesisUniversity of Bristol

Damen W G M Hausdorf M Seyfarth E A amp Tautz D 1998 Aconserved mode of head segmentation in arthropods revealed bythe expression pattern of Hox genes in a spider Proceedings of theNational Academy of Sciences USA 95 10665ndash70

Delle Cave L amp Simonetta A M 1991 Early Palaeozoic arthropodsand problems of arthropod phylogeny with some notes on taxa ofdoubtful affinities In Simonetta A M amp Conway Morris S(eds) The early evolution of Metazoa and the significance ofproblematic taxa 189ndash244 Cambridge Cambridge UniversityPress

Dewel R A amp Dewel W C 1997 The place of tardigrades inarthropod evolution In Fortey R A amp Thomas R H (eds)Arthropod Relationships 109ndash23 London Chapman and HallSystematics Association Special Volume 55

Dunlop J A 1997 Palaeozoic arachnids and their significance forarachnid phylogeny Proceedings of the 16th European Colloquiumof Arachnology 65ndash82 Siedlce Poland

Dunlop J A 1998 The origins of tetrapulmonate booklungs and theirsignificance for chelicerate phylogeny In Selden P A (ed)Proceedings of the 17th European Colloquium of ArachnologyEdinburgh 1997 pp 9ndash16

Dunlop J A 1999 Pasando revista a la evolucion de los quelicerados[A review of chelicerate evolution] In Melic A DeHaro J JMedez M amp Ribera I (eds) Volumen Monografico Evolucion yFilogenia de Arthropoda Boletin de la Sociedad EntolomologicaAragonesa 26 255ndash272 [In Spanish and English]

Dunlop J A amp Selden P A 1997 The early history and phylogenyof the chelicerates In Fortey R A amp Thomas R H (eds)Arthropod relationships Systematics Association Special Volume55 221ndash35 London Chapman and Hall

Dunlop J A amp Webster M 1999 Fossil evidence terrestrializationand arachnid phylogeny Journal of Arachnology 27 86ndash93

Dzik J 1993 Early metazoan evolution and the meaning of its fossilrecord Evolutionary Biology 27 339ndash86

Dzik J amp Lendzion K 1988 The oldest arthropods of the EastEuropean Platform Lethaia 21 29ndash38

Edgecombe G D Wilson G D F Colgan D J Gray M R ampCassis G 2000 Arthropod cladistics combined analysisof Histone H3 and U2 snRNA sequences and morphologyCladistics 16 155ndash203

Edgecombe G D amp Ramskold L 1999 Relationships of CambrianArachnata and the systematic position of Trilobita Journal ofPaleontology 73 263ndash87

Emerson M J amp Schram F R 1997 Theories patterns and realitygame plan for arthropod phylogeny In Fortey R A amp Thomas

R H (eds) Arthropod Relationships Systematics AssociationSpecial Volume 55 67ndash87 London Chapman and Hall

Farris J S 1989 The Retention Index and the Rescaled ConsistencyIndex Cladistics 5 417ndash19

Felsenstein J 1985 Confidence limits on phylogenies an approachusing the bootstrap Evolution 39 783ndash91

Flower R H 1968 Merostomes from a Cotter Horizon of the El PasoGroup New Mexico Bureau of Mines and Mineral ResourcesMemoir 22 35ndash44

Fortey R A 1990a Trilobite evolution and systematics In MikulicD G (ed) Arthropod paleobiology 44ndash65 Short Courses inPaleontology 3 Knoxville Tenessee The Paleontological Society

Fortey R A 1990b Ontogeny hypostome attachment and trilobiteclassification Palaeontology 33 529ndash76

Fortey R A 1997 Classification In Kaesler R L (ed) Treatise oninvertebrate paleontology Part O Arthropoda 1 (revised) 289ndash302Boulder Colorado amp Lawrence Kansas Geological Society ofAmerica and University of Kansas Press

Fortey R A amp Theron J N 1994 A new Ordovician arthropodSoomaspis and the agnostid problem Palaeontology 37 841ndash61

Fortey R A amp Whittington H B 1989 The Trilobita as a naturalgroup Historical Biology 2 125ndash38

Fry W G 1978 A classification within the pycnogonids ZoologicalJournal of the Linnean Society of London 63 35ndash58

Giribet G Edgecombe G D amp Wheeler W C 2001 Arthropodphylogeny based on eight molecular loci and morphology Nature413 157ndash61

Gould S J 1989 Wonderful life The Burgess Shale and the nature ofhistory New York NY Norton

Gould S J 1991 The disparity of the Burgess Shale arthropod faunaand the limits of cladistic analysis why we must strive to quantifymorphospace Paleobiology 17 411ndash23

Gurich G 1931 Mimetaster hexagonalis ein neuer Kruster aus demunterdevonischen Bundenbacher Dachschiefer PalaontologischeZeitschrift 12 204ndash38

Hammann W Laske R amp Pillola G L 1990 Tariccoia arrusensisngnsp an unusual trilobite-like arthropod Rediscovery of thelsquophylocaridrsquo beds of Taricco (1922) in the Ordovician lsquoPuddingarsquosequence of Sardinia Bolletino della Societa PaleontologicaItaliana 29 163ndash78

Haas M S Brown S J amp Beeman R W 2001 Pondering theprocephalon the segmental origin of the labrum DevelopmentGenes and Evolution 211 89ndash95

Heider K 1913 Entwicklungsgeschichte und Morphologie derWirbellosen Die Kultur der Gegenwart Theil 3 Abt 4 2 176ndash332

Henriksen K 1928 Critical notes upon Cambrian arthropodsdescribed by Charles D Walcott Videnskabelige Meddelelser fraDansk Naturhistorisk Forening 86 1ndash20

Hesselbo S P 1988 Trace fossils of Cambrian aglaspidid arthropodsLethaia 21 139ndash46

Hesselbo S P 1989 The aglaspidid arthropod Beckwithia from theCambrian of Utah and Wisconsin Journal of Paleontology 63636ndash42

Hesselbo S P 1992 Aglaspidida (Arthropoda) from the UpperCambrian of Wisconsin Journal of Paleontology 66 885ndash923

Hou X 1987a Three new large arthropods from Lower CambrianChengjiang fauna eastern Yunnan Acta Palaeontologica Sinica26 272ndash85 [In Chinese with English summary]

Hou X 1987b Two new arthropods from Lower CambrianChengjiang eastern Yunnan Acta Palaeontologica Sinica 26236ndash56 [In Chinese with English summary]

Hou X Chen J amp Lu H 1989 Early Cambrian new arthropodsfrom Chengjiang Yunnan Acta Palaeontologica Sinica 28 42ndash57[In Chinese with English summary]

Hou X Ramskold L amp Bergstrom J 1991 Composition andpreservation of the Chengjiang fauna a Lower Cambrian soft-bodied biota Zoologica Scripta 20 395ndash411

Hou X Bergstrom J amp Ahlberg P 1995 Anomalocaris and otherlarge animals in the Lower Cambrian Chengjiang fauna ofsouthwest China GFF 117 163ndash83

Hou X amp Bergstrom J 1997 Arthropods of the Lower CambrianChengjiang fauna southwest China Fossils and Strata 45 1ndash116

Hughes C P 1975 Redescription of Burgessia bella from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 4415ndash36

Ivantsov A Y 1999 Trilobite-like arthropod from the LowerCambrian of the Siberian Platform Acta PalaeontologicaPolonica 44 455ndash66

King P E 1973 Pycnogonids London St Martin Press

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 191

Lauterbach K E 1973 Schlussereignisse in der Evolution derStammgruppe der Euarthropoda Zoologische Beitrage 19259ndash99

Lauterbach K E 1980 Schlussereignisse in der Evolution derGrundplans der Arachnata (Arthropoda) Abhandlungen desNaturwissenschaftlichen Vereins in Hamburg 23 163ndash327

Lauterbach K E 1983 Synapomorphien zwischen Trilobiten- undCheliceraten-zweig der Arachnata Zoologischer Anzeiger 210213ndash38

Lendzion K 1975 Fauna of the Mobergella Zone in the Polish LowerCambrian Kwartalnik Geologiczny 19 237ndash42

Lieberman B S 1998 Cladistic analysis of the Early Cambrianolenelloid trilobites Journal of Paleontology 72 59ndash78

Lu Y 1940 On the ontogeny and phylogeny of Redlichia intermediaLu (sp nov) Bulletin of the Geological Society of China 20333ndash42 [In Chinese]

Maddison W P amp Maddison D R 1997 MacClade analysis ofphylogeny and character evolution Version 307 Sunderland MASinauer Associates

Mittmann B amp Scholtz G 2003 Development of the nervous systemin the lsquoheadrsquo of Limulus polyphemus (Chelicerata Xiphosura)morphological evidence for the correspondence between the seg-ments of the chelicerae and the (first) antennae of MandibulataDevelopment Genes and Evolution 213 9ndash17

Moore R C (ed) 1959 Treatise on invertebrate paleontology Part OArthropoda 1 Boulder Colorado Geological Society of America

Muller K J amp Walossek D 1986 Arthropod larvae from the UpperCambrian of Sweden Transactions of the Royal Society ofEdinburgh Earth Sciences 77 157ndash79

Muller K J amp Walossek D 1987 Morphology ontogeny and lifehabit of Agnostus pisiformis from the Upper Cambrian of SwedenFossils and Strata 19 1ndash124

Munilla T 1999 Evolucion y filogenia de los Picnogonidos InMelicA DeHaro J J Medez M amp Ribera I (eds) VolumenMonografico Evolucion y Filogenia de Arthropoda Boletin de laSociedad Entolomologica Aragonesa 26 273ndash91 [In Spanish andEnglish]

Orr P J Siveter D J Briggs D E G Siveter D J amp Sutton M D2000 A new arthropod from the Silurian Konservat-Lagerstatteof Herefordshire Proceedings of the Royal Society of LondonSeries B 267 1497ndash504

Page R D M 1996 On consensus confidence and lsquototal evidencersquoCladistics 12 83ndash92

Palmer A R amp Repina L N 1993 Through a glass darklytaxonomy phylogeny and biostratigraphy of the OlenellinaUniversity of Kansas Paleontological Contributions New Series 31ndash35

Platnick N I Griswold C E amp Coddington J A 1991 On missingentries in cladistic analysis Cladistics 7 337ndash43

Pleijel F 1995 On character coding for phylogeny reconstructionCladistics 11 309ndash15

Raasch G O 1939 Cambrian Merostomata Special Papers of theGeological Society of America 19 1ndash146

Ramskold L Chen J Edgecombe G D amp Zhou G 1996Preservational folds simulating tergite junctions in tegopeltid andnaraoiid arthropods Lethaia 29 15ndash20

Ramskold L Chen J Edgecombe G D amp Zhou G 1997Cindarella and the arachnate clade Xandarellida (ArthropodaEarly Cambrian) from China Transactions of the Royal Society ofEdinburgh Earth Sciences 88 19ndash38

Ramskold L amp Edgecombe G D 1991 Trilobite monophylyrevisited Historical Biology 4 267ndash83

Ramskold L amp Edgecombe G D 1996 Trilobite appendagestructure ndash Eoredlichia revisited Alcheringa 20 269ndash76

Raw F 1957 Origin of chelicerates Journal of Paleontology 31139ndash193

Repina L N amp Okuneva O G 1969 Cambrian arthropods of themaritime territory Paleontological Journal 1969 95ndash103

Richter R amp Richter E 1929 Weinbergina opitz ng n sp einSchwerttrager (Merost Xiphos) aus dem Devon (Rheinland)Senkenbergiana 11 193ndash209

Robison R A 1984 New occurences of the unusual trilobite Naraoiafrom the Cambrian of Idaho and Utah University of KansasPaleontological Contributions Paper 112 1ndash8

Rominger C 1887 Description of primordial fossils from MtStephens NW Territory of Canada Proceedings of the Academyof Natural Sciences of Philadelphia 1887 12ndash19

Scholtz G 1997 Cleavage germ band formation and head segmen-tation the ground pattern of the Euarthropoda In Fortey R Aamp Thomas R H (eds) Arthropod Relationships 317ndash32 LondonChapman and Hall Systematics Association Special Volume 55

Schram F R 1978 Arthropods a convergent phenomenonFieldiana Geology 39 61ndash108

Schram F R 1993 Review of Boxshall Stromberg and Dahl TheCrustacea Origin and Evolution Journal of Crustacean Biology13 820ndash2

Schram F R Vonk R amp Hof C H J 1997 Mazon CreekCycloidea Journal of Paleontology 71 261ndash84

Schram F R amp Emerson M J 1991 Arthropod Pattern Theorya new approach to arthropod phylogeny Memoirs of theQueensland Museum 31 1ndash18

Schram F R amp Hof C H J 1998 Fossils and the interrelationshipsof major crustacean groups In Edgecombe G D (ed) Arthropodfossils and phylogeny pp 233ndash302 New York NY ColumbiaUniversity Press

Shergold J H 1991 Protaspis and early meraspis growth stages of theeodiscoid trilobite Pagetia ocellata Jell and their implications forclassification Alcheringa 15 65ndash86

Shu D Geyer G Ling C amp Zhang Z 1995 Redlichiacean trilobiteswith preserved soft parts from the Lower Cambrian Chengjiangfauna (South China) Beringeria (Special Issue) 2 203ndash41

Shultz J W 1990 Evolutionary morphology and phylogeny ofArachnida Cladistics 6 1ndash38

Shultz J W 2000 Skeletomuscular anatomy of the harvestmanLeiobunum aldrichi (Weed 1893) (Arachnida OpilionesPalpatores) and its evolutionary significance Zoological Journalof the Linnean Society 128 401ndash38

Shultz J W 2001 Gross muscular anatomy of Limulus polyphemus(Xiphosura Chelicerata) and its bearing on evolution in the theArachnida Journal of Arachnology 29 283ndash303

Siewing R (ed) 1985 Lehrbuch der Zoologie II Systematik 3rd ednStuttgart Fischer

Simonetta A M 1970 Studies on the non trilobite arthropods ofthe Burgess Shale (Middle Cambrian) the genera LeanchoiliaAlalcomenaeus Opabinia Burgessia Yohoia and ActaeusPalaeontographica Italica 66 35ndash45

Simonetta A M amp Delle Cave L 1975 The Cambrian non trilobitearthropods from the Burgess Shale of British Columbia A studyof their comparative morphology taxonomy and evolutionarysignificance Palaeontographica Italica 69 1ndash37

Simonetta A M amp Delle Cave L 1981 An essay in the compara-tive and evolutionary morphology of Palaeozoic ArthropodsAccademia Nazionale dei Lincei Atti dei Convegni Lincei 49389ndash439

Starobogatov Y I 1990 Sistema i filogeniya nizshikh khelitserovykh(analiz morfologii paleozoysikh grupp) [The systematics andphylogeny of the lower chelicerates (a morphological analysis ofthe Palaeozoic groups)] Paleontologicheskii Zhurnal 1990 4ndash17

Stoslashrmer L 1944 On the relationships and phylogeny of fossil andrecent Arachnomorpha Skrifter utgitt ac Det Norske Videnskaps-Akademi i Olso I Matematisk-naturvitenskaplig Klasse 5 1ndash158

Stoslashrmer L 1951 Studies on trilobite morphology Part III Theventral cephalic structures with remarks on the zoologicalposition of the trilobites Norsk geologisk tidsskrift 29 108ndash58

Stoslashrmer L 1956 A Lower Cambrian merostome from Sweden Arkivfor Zoologi Ser 2 9 507ndash14

Stoslashrmer L 1959 Trilobitoidea In Moore R C (ed) Treatise onInvertebrate Paleontology part O Arthropoda 1 23ndash37 BoulderColorado Geological Society of America

Strong E E amp Lipscomb D 1999 Character coding and inapplicabledata Cladistics 15 363ndash71

Sturmer W amp Bergstrom J 1976 The arthropods Mimetaster andVachonisia from the Devonian Hunsruck Shale PalaontologischeZeitschrift 50 78ndash111

Sturmer W amp Bergstrom J 1978 The arthropod Cheloniellon fromthe Devonian Hunsruck Shale Palaontologische Zeitschrift 5257ndash81

Sturmer W amp Bergstrom J 1981 Weinbergina a xiphosuran fromthe Devonian Hunsruck Slate Palaontologische Zeitschrift 55237ndash55

Sutton M D Briggs D E G Siveter D J Siveter D J amp Orr PJ 2002 The arthropod Offacolus kingi (Chelicerata) from theSilurian of Herefordshire England computer based morphologi-cal reconstructions and phylogenetic affinity Proceedings of theRoyal Society of London Series B 269 1195ndash1203

Swofford D L 1999 PAUP Phylogenetic Analysis Using Parsimony(and other methods) Version 40b2a Sunderland MA SinauerAssociates

Telford M J amp Thomas R H 1998 Expression of homeobox genesshows chelicerate arthropods retain their deutocerebral segmentProceedings of the National Academy of Sciences USA 9510671ndash5

192 TREVOR J COTTON AND SIMON J BRADDY

Thorley J L amp Page R D M 2000 RadCon phylogenetic treecomparison and consensus Bioinformatics 16 486ndash7

Valentine J W 1995 Why no new phyla after the Cambrian ndashgenome and ecospace hypotheses revisited Palaios 10 190ndash194

Wagele J W 1993 Rejection of the lsquoUniramiarsquo hypothesis andimplications of the Mandibulata concept Zoologische JahrbucherAbteilung fur Systematik Geographie und Biologie der Tiere 120253ndash88

Walcott C D 1911 Middle Cambrian Merostomata Cambriangeology and palaeontology II Smithsonian MiscellaneousCollections 57 17ndash40

Walcott C D 1912 Middle Cambrian Branchiopoda MalacostracaTrilobita and Merostomata Cambrian geology and palaeontol-ogy II Smithsonian Miscellaneous Collections 57 145ndash228

Walcott C D 1918 Geological explorations in the CanadianRockies Explorations and fieldwork of the SmithsonianInstitution in 1917 Smithsonian Miscellaneous Collections 684ndash20

Walossek D 1993 The Upper Cambrian Rehbachiella kinnekullensisand the phylogeny of Branchiopoda and Crustacea Fossils andStrata 32 1ndash202

Walossek D amp Dunlop J A 2002 A larval sea spider (ArthropodaPycnogonida) from the Upper Cambrian lsquoOrstenrsquo of Swedenand the phylogenetic position of pycnogonids Palaeontology 45421ndash46

Walossek D amp Muller K J 1990 Upper Cambrian stem-lineagecrustaceans and their bearing upon the monophyletic origin ofCrustacea and the position of Agnostus Lethaia 23 409ndash27

Walossek D amp Muller K J 1997 Cambrian lsquoOrstenrsquo-type arthro-pods and the phylogeny of Crustacea In Fortey R A amp ThomasR H (eds) Arthropod Relationships 139ndash53 SystematicsAssociation Special Volume 55 London Chapman and Hall

Walossek D amp Muller K J 1998 Early arthropod phylogeny in lightof the Cambrian lsquoOrstenrsquo fossils In Edgecombe G D (ed)Arthropod fossils and phylogeny 185ndash231 New York NYColumbia University Press

Weygoldt P 1979 Significance of later embryonic stages and headdevelopment in arthropod phylogeny In Gupta A P (ed)Arthropod Phylogeny 107ndash35 New York NY Van NostrandReinhold Co

Weygoldt P 1998 Evolution and systematics of the ChelicerataExperimental and Applied Acarology 22 63ndash79

Weygoldt P amp Paulus H F 1979 Unterzuchungen aur Morpholo-gie Taxonomie und Phylogenie der Chelicerata Zeitschriftfur Zoologische Systematik und Evolutionsforschung 17 85ndash116177ndash200

Wheeler W C Cartwright P amp Hayashi C Y 1993 Arthropodphylogeny a combined approach Cladistics 9 1ndash39

Wheeler W C amp Hayashi C Y 1998 The phylogeny of extantchelicerate orders Cladistics 14 173ndash92

Whittington H B 1971 Redescription of Marrella splendens(Trilobitoidea) from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 209 1ndash24

Whittington H B 1974 Yohoia Walcott and Plenocaris n genarthropods from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 231 1ndash21

Whittington H B 1975a Trilobites with appendages from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 497ndash136

Whittington H B 1975b The enigmatic animal Opabinia regalisMiddle Cambrian Burgess Shale British Columbia PhilosophicalTransactions of the Royal Society of London Series B 271 1ndash43

Whittington H B 1977 The Middle Cambrian trilobite NaraoiaBurgess Shale British Columbia Philosophical Transactions of theRoyal Society Series B 280 409ndash43

Whittington H B 1979 Early arthropods their appendages andrelationships In House M R (ed) The origin of majorinvertebrate groups 253ndash68 Systematics Association SpecialVolume 12 New York NY Academic Press

Whittington H B 1980 Exoskeleton moult stage appendage mor-phology and habits of the Middle Cambrian trilobite Olenoidesserratus Palaeontology 23 171ndash204

Whittington H B 1981 Rare arthropods from the Burgess ShaleMiddle Cambrian British Columbia Philosophical Transactionsof the Royal Society of London Series B 292 329ndash57

Whittington H B 1985 Tegopelte gigas a second soft-bodied trilo-bite from the Burgess Shale Middle Cambrian British ColumbiaJournal of Palaeontology 23 171ndash204

Whittington H B 1989 Olenelloid trilobites type species functionalmorphology and higher classification Philosophical Transactionsof the Royal Society of London Series B 324 111ndash47

Whittington H B amp Almond J E 1987 Appendages and habits ofthe Upper Ordovician trilobite Triarthus eatoni PhilosophicalTransactions of the Royal Society of London Series B 317 1ndash46

Wilkinson M 1992 Ordered versus unordered characters Cladistics8 375ndash85

Wilkinson M 1994 Common cladistic information and its consensusrepresentation reduced Adams and reduced cladistic consensustrees and profiles Systematic Biology 43 343ndash68

Wilkinson M 1995a Coping with abundant missing entries in phylo-genetic inference using parsimony Systematic Biology 44 501ndash14

Wilkinson M 1995b TAXEQ2 software and documentation BristolSchool of Biological Sciences University of Bristol

Wilkinson M 1996 On the distribution of homoplasy and choosingamong trees Taxon 45 263ndash66

Wills M A 1997 A phylogeny of Recent and fossil Crustacea derivedfrom morphological characters In Fortey R A amp ThomasR H (eds) Arthropod Relationships 189ndash209 SystematicsAssociation Special Volume 55 London Chapman and Hall

Wills M A Briggs D E G amp Fortey R A 1994 Disparity asan evolutionary index a comparison of Cambrian and Recentarthropods Paleobiology 20 93ndash130

Wills M A Briggs D E G Fortey R A amp Wilkinson M 1995The significance of fossils in understanding arthropod evolutionVerhandlungen der deutschen zoologischen Gesselschaft 88 203ndash15

Wills M A Briggs D E G Fortey R A Wilkinson M amp SneathP H A 1998a An arthropod phylogeny based on fossil andrecent taxa In Edgecombe G D (ed) Arthropod fossils andphylogeny 33ndash105 New York NY Columbia University Press

Wills M A Briggs D E G amp Fortey R A 1998b Evolutionarycorrelates of arthropod tagmosis scrambled legs In ForteyR A amp Thomas R H (eds) Arthropod Relationships 57ndash65Systematics Association Special Volume 55 London Chapmanand Hall

Winter G 1980 Beitrage zur Morphologie und Embryologie desvorderen Korperabschnitts (Cephalosoma) der PantopodaGerstaecker 1863 I Entstehung und Struktur des Zentralnerven-systems Zeitschrift fur Zoologische Systematik und Evolutionsfor-schung 18 27ndash61

Zhang W amp Hou X 1985 Preliminary notes on the occurrence of theunusual trilobite Naraoia in Asia Acta Palaeontologica Sinica 24591ndash5 [In Chinese with English summary]

TREVOR J COTTON Department of Palaeontology The Natural History Museum CromwellRoad London SW7 5BD UKe-mail trevor_cottonhotmailcom

SIMON J BRADDY Department of Earth Sciences University of Bristol Wills MemorialBuilding Queenrsquos Road Bristol BS8 1RJ UKe-mail S J Braddybrisacuk

MS received 28 July 2003 Accepted for publication 15 March 2004

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 193

Page 3: The phylogeny of arachnomorph arthropods and the origin of the

be more basal arachnomorphs on the basis of outgrouprooting with Marellomorpha Unfortunately Edgecombe ampRamskold (1999) made no attempt to establish the monophyly

of the group of lsquotrilobite-allied arachnatesrsquo that they includedin their analysis and did not consider the position of thechelicerates

Figure 2 Previous hypotheses of arachnomorph phylogenys (A) after Briggs amp Fortey (1989 fig 1 1992 fig 3)note that Crustacea are paraphyletic in this analysis (B) after Briggs et al (1992 fig 3 1993 fig 1) (C) afterWills et al (1995 fig 1A 1998a fig 2 1) and (D) after Hou amp Bergstrom (1997 fig 88) (for monotypic familiesthe family names used by Hou amp Bergstrom have been replaced with generic names) Taxa important for therecognition of an arachnomorph clade (ie trilobites chelicerates and crustaceans see text) are shown in boldtype

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 171

2 Cladistic analysis

2 1 Taxonomic scopeThirty-four ingroup taxa were considered in the cladisticanalyses Of these the majority were coded as individualspecies-level terminals although higher taxonomic levels wereemployed in some instances A complete list of species andgenus-level terminals along with details of authorship andother important references is given in Table 1 Species-levelterminals are referred to throughout by generic names only(except in Table 1) since the majority are assigned to mono-typic genera In addition to these ingroup taxa a hypotheticaloutgroup was used for rooting as discussed below

Terminals were selected on the basis of the recent cladisticanalyses of Wills et al (1995 1998a) and Edgecombe ampRamskold (1999) and to a lesser extent previous hypothesesof arthropod relationships The majority of taxa that havebeen included in an arachnomorph group by previous authorsare included in this study Exceptions include AgnostusHabelia Molaria Sanctacaris and Sarotrocercus (all of whichwere considered by Wills et al but not Edgecombe ampRamskold) the putative chelicerate Offacolus andPhytophilaspis The Agnostina are regarded as a clade withinthe Trilobita derived from Eodiscina (eg Fortey 1990a

Fortey amp Theron 1994 Wills et al 1998a) rather than stemgroup crustaceans (eg Shergold 1991 Bergstrom 1992 seeCotton 2002 for a detailed discussion and phylogenetic analy-sis) The Burgess Shale taxa Habelia Walcott 1912 MolariaWalcott 1912 and Sarotrocercus Whittington 1981 and theHunsruck Slate Magnoculocaris blindi (Briggs amp Bartels 20012002) are rather poorly known and hypotheses of theirrelationships constrained by too few characters

Sanctacaris Briggs amp Collins 1988 and the probably closelyrelated Offacolus Orr et al 2000 were both originally describedas having chelicerate affinities Sutton et al (2002) addedOffacolus to the matrix of Wills et al (1995 1998a) and foundit to be nested within the Chelicerata despite its biramouscephalic appendages The material of Sanctacaris is in need ofrestudy the homology of the cephalic appendages and ventralcephalic structures remain unclear (see Budd 2002 and Briggsamp Collins 1988 for alternative schemes) The relationships ofthese lsquosanctacaridsrsquo will be considered in detail later by Braddyand co-workers following re-examination of Sanctacaris anddescription of a new Chengjiang taxon (Babcock amp Zhangin press) that is likely to have a profound impact on theinterpretation of both Sanctacaris and Offacolus

Phytophilaspis Ivantsov 1999 shows a combination of fea-tures that may be homologous to those found in naraoiids (the

Table 1 Authorship and important references for species included in cladistic analyses of arachnomorphs Previous analyses by Briggs amp Fortey(1989 1992) Briggs et al (1992 1993) Wills et al (1995 1998a) and Edgecombe amp Ramskold (1999) informed the coding of many taxa and arenot listed

Name and authorship Other references

Alalcomenaeus cambricus Simonetta 1970 Briggs amp Collins (1999)Buenaspis forteyi Budd 1999aBurgessia bella Walcott 1912 Hughes (1975)Cheloniellon calmani Broili 1932 Broili (1933) Sturmer amp Bergstrom (1978)Cindarella eucalla Chen et al 1996 Ramskold et al (1997)Emeraldella brocki Walcott 1912 Bruton amp Whittington (1983) Briggs amp Robison (1984)Eoredlichia intermedia (Lu 1940) Shu et al (1995) Ramskold amp Edgecombe (1996)Fortiforceps foliosa Hou amp Bergstrom 1997Helmetia expansa Walcott 1918 Briggs in Conway Morris (1982) Briggs Erwin amp Collier (1994)Jianfengia multisegmentalis Hou 1987b Bergstrom amp Hou (1991) Chen amp Zhou (1997)Kuamaia lata Hou 1987a Hou amp Bergstrom (1997) Begstrom amp Hou (1998)

Edgecombe amp Ramskold (1999)Leanchoilia superlata Walcott 1912 Bruton amp Whittington (1983) Briggs amp Robison (1984)Lemoneites Flower 1968 Dunlop amp Selden (1997)Liwia plana (Lendzion 1975) Dzik amp Lendzion (1988) Fortey amp Theron (1994)

Chen Edgecombe amp Ramskold (1997)Marrella splendens Walcott 1912 Whittington (1971)Mimetaster hexagonalis (Gurich 1931) Sturmer amp Bergstrom (1976)Misszhouia longicaudata (Zhang amp Hou 1985) Bergstrom amp Hou (1991) Chen et al (1997) Hou amp Bergstrom (1997)Naraoia Walcott 1912 Whittington (1977) Robison (1984) Zhang amp Hou (1985)

Chen et al (1997)Olenoides serratus (Rominger 1887) Whittington (1975a 1980)Paleomerus hamiltoni Stoslashrmer 1956 Bergstrom (1971) Dunlop amp Selden (1997)Retifacies abnormalis Hou et al 1989 Hou amp Bergstrom (1997)Saperion glumaceum Hou et al 1991 Ramskold et al (1996) Hou amp Bergstrom (1997)Sidneyia inexpectans Walcott 1911 Bruton (1981)Sinoburius lunaris Hou et al 1991 Hou amp Bergstrom (1997) Skioldia aldna Hou amp Bergstrom 1997Soomaspis splendida Fortey amp Theron 1994 Chen et al (1997)Tariccoia arrusensis Hammann et al 1990 Fortey amp Theron (1994) Chen et al (1997)Tegopelte gigas Simonetta amp Delle Cave 1975 Whittington (1985) Ramskold et al (1996)Weinbergina opitzi Richter amp Richter 1929 Sturmer amp Bergstrom (1981) Andersen amp Selden (1997) Dunlop amp Selden (1997)Xandarella spectaculum Hou et al 1991 Bergstrom amp Hou (1991) Ramskold et al (1997)

Hou amp Begstrom (1997)Yohoia tenuis Walcott 1912 Whittington (1974)

172 TREVOR J COTTON AND SIMON J BRADDY

form of the pygidium) xandarellids (the eye slits and overlapof anterior thoracic segments by the head shield) and Trilobita(the hypostome with anterior and posterior lateral wings)Therefore it potentially has a pivotal position in the phylogenyof the trilobite-allied Arachnomorpha However the interpre-tation of these features by Ivantsov (1999) requires confir-mation and adequately determining the homology of thesestructures would require restudy of the material

The Xandarellida known only from the Chengjiang faunawere originally described as an arachnate clade (Ramskoldet al 1997) All three valid genera Xandarella Sinoburius andCindarella were included in the analysis of Edgecome ampRamskold (1999) They differ in important respects andtherefore all three genera are included here to test the mono-phyly of this group in the context of a wider analysis

Wills et al (1995 1998a) represented the Trilobita (exceptAgnostida) by Olenoides the appendages of which are knownfrom the Burgess Shale and the Ordovician Triarthrus (seeCisne 1975 1981 Whittington amp Almond 1987) However thepresent authors follow Edgecombe amp Ramskold (1999) inchoosing Olenoides and Eoredlichia to represent the TrilobitaThese are more basal trilobites (see eg Fortey 1990a b) thanTriarthrus and consequently are more likely to reflect theancestral trilobite condition The putative trilobite KleptothuleBudd 1995 from the Early Cambrian Sirius Passet faunaof North Greenland is not included Its appendages areunknown and homologies between exoskeletal features of thistaxon and other arachnomorphs are uncertain

The Naraoiidae or Nektaspida (reviewed above) are widelyconsidered to be closely related to the trilobites either as aparaphyletic assemblage of lsquosoft-bodiedrsquo trilobites (eg Shuet al 1995 fig 20B) or as the sister group of calcified trilobites(Fortey 1997 Whittington 1977) They have been includedin the Trilobita by some authors However Edgecombe ampRamskold (1999) found no particularly close relationshipbetween naraoiids and trilobites (see above) In order to testthe monophyly of the group all described naraoiid genera areincluded except Maritimella and Orientella (both Repina ampOkuneva 1969) which are probably pseudofossils (Robison1984 p 2)

Tegopelte from the Burgess Shale has also been considered asoft-bodied trilobite (Whittington 1985) Ramskold et al(1996) revised the exoskeletal morphology of Tegopelte andnoted similarities to the Chengjiang taxa Saperion andSkioldia This relationship has been confirmed by cladisticanalysis (Edgecombe amp Ramskold 1999) An undescribed largearthropod from the Soom Shale Lagerstatte (Aldridge et al2001 fig 3442) is probably a tegopeltid (Braddy amp Almond1999) The Helmetiidae based on Helmetia Walcott 1918from the Burgess Shale are united with the tegopeltid group inthe Helmetiida (Hou amp Bergstrom 1997 Edgecombe amp Ram-skold 1999) Helmetia is rather poorly known and awaitsredescription but is very similar to Kuamaia lata Hou 1987aKuamaia muricata Hou amp Bergstrom 1997 and Rhombicalva-ria acantha Hou 1987a from Chengjiang There are no signifi-cant differences between these Chinese taxa and theirtaxonomy may be over split (Delle Cave amp Simonetta 1991Hou amp Bergstrom 1997) The morphology of Kuamaia lata isknown in some detail and it is coded here along with Helmetiaand all three tegopeltid genera

Delle Cave amp Simonetta (1991 table 1) compared severaltaxa to Tegopelte and Helmetia Of these only Retifacies isknown in enough detail to make coding worthwhile Tontoiaand Nathorstia (both Walcott 1912) are nomina dubia (seeWhittington 1985 1980 respectively) and only the exoskeletonis known of Urokodia Hou et al 1989 and Mollisonia Walcott1912 Retifacies has been placed near a trilobite-naraoiid-

helmetiid clade (Delle Cave amp Simonetta 1991 Edgecombe ampRamskold 1999) or as sister group to the naraoiids (Hou ampBergstrom 1997)

There is a long history of comparing Emeraldella andSidneyia with chelicerates (following Stoslashrmer 1944) but theposition of these taxa in cladistic studies has been highlyvariable (see Fig 2) According to the hypothesis ofBergstrom these taxa along with the Aglaspidida andCheloniellida form a clade that is the sister group to thechelicerates More usually aglaspidids or cheloniellids havebeen considered sister taxon to the chelicerates and theseBurgess Shale taxa more distantly related

A cheloniellid-chelicerate clade was supported by Wills et al(1995 1998a) and Sturmer amp Bergstrom (1978 also Bergstrom1979) and Simonetta amp Delle Cave (1981) and Delle Cave ampSimonetta 1991) placed the cheloniellid Triopus as ancestral toall chelicerates The Cheloniellida (sensu Dunlop amp Selden1997) are represented by Cheloniellon herein since theappendages of other cheloniellid taxa are unknown

It has also repeatedly been suggested that cheliceratesevolved from aglaspidids (eg Starobogatov 1990) and aglas-pidids have been included in the Chelicerata by some authors(Stoslashrmer 1944 Weygoldt amp Paulus 1979) The coding ofaglaspidids is considered in detail in Section 22 Lemoneites(Flower 1968) and Paleomerus (Stoslashrmer 1956 Bergstrom 1971)have been assigned to the Aglaspidida by some authors (seeHou amp Bergstrom 1997 pp 96ndash97) They are included tofacilitate comparison with the results of Dunlop amp Selden(1997) Lemoneites is particularly problematic (R A Moorepers comm 2004) and both taxa are known from very fewcharacters ndash hence the present authors have investigated theimplications of their exclusion from the analysis (see Section26) The aglaspidid-like arthropod Kodymirus vagans Chlupacamp Havlıcek 1965 from the Lower Cambrian of the CzechRepublic (redescribed and compared to eurypterids byChlupac 1995) is excluded from the present study becausefeatures of its morphology that are well known generally agreewith those of Aglaspis

The phylogeny of crown group chelicerates is a matter ofconsiderable debate but most authors have considered pyc-nogonids xiphosurans and eurypterids as early divergentgroups within Chelicerata (Dunlop 1999) These hypothesesare represented here by coding a generalised pycnogonid andeurypterid and the Devonian synziphosurine Weinbergina(Sturmer amp Bergstrom 1981) Weinbergina is the best-knownsynziphosurine and is more likely to represent the primitivecondition of xiphosurans than modern examples The phylog-eny of the Xiphosura has recently been studied by Anderson ampSelden (1997) Coding for Eurypterida follows the recent workof Dunlop amp Selden (1997) Dunlop (1998) Braddy et al(1999) and Dunlop amp Webster (1999) Coding for pycnogonidsfollows general works on the group (eg King 1973 Fry 1978)the reconstruction of the pycnogonid stem group by Bergstromet al (1980) and recent work on pycnogonid phylogeny(Munilla 1999 Arango 2002) The pycnogonid familyAmmotheidae is generally accepted as the most plesiomorphicextant group (Arango 2002)

Two groups of Palaeozoic fossil taxa have been included inthe Arachnomorpha by some authors but considered lessclosely related by others Marrella from the Burgess Shaleand two Devonian taxa Mimetaster and Vachonisia havegenerally been considered to form the clade Marrellomorpha(Whittington 1971 Sturmer amp Bergstrom 1976 Bergstrom1979 Wills et al 1995) This has been thought to be the sistergroup to other arachnates (Sturmer amp Bergstrom 1976) abasal schizoramian or a basal euarthropod group (see Fig 2)Bergstrom (1979) included the Carboniferous Cycloidea in the

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 173

Marrellomorpha but there is considerable evidence that theseare rather derived crustaceans (see Schram et al 1997) andthey are not considered further The Burgess Shale taxonBurgessia has also been placed in the Marrellomorpha (Hou ampBergstrom 1997) but was not found to belong to this clade inmost analyses Stoslashrmer (1944) placed Burgessia in a compa-rable systematic position to Marrella he included both in theArachnomorpha but excluded them from the MerostomoideaMarrella Mimetaster and Burgessia were included in thepresent study to assess whether the marellomorphs should beincluded in the Arachnomorpha and test the affinities ofBurgessia

The Cambrian lsquogreat appendagersquo arthropods (theMegacheira of Hou amp Bergstrom 1997) have been nestedwithin the Arachnomorpha in most cladistic studies (egBriggs amp Fortey 1989 Wills et al 1995 1998a Emerson ampSchram 1997) Briggs amp Fortey (1989 1992) recognised mega-cheiran taxa as particularly closely related to chelicerates(Fig 2A) while other authors (Bergstrom 1992 Hou ampBergstrom 1997 Budd 2002) have considered megacheirans tobe primitive euarthropods or ancestral to some (but not all)crustaceans (Delle Cave amp Simonetta 1991 and referencestherein) Since the monophyly of the megacheirans hasnot usually been supported the present authors haveincluded nearly all described taxa These include LeanchoiliaAlalcomenaeus and Yohoia from the Burgess Shale andFortiforceps and Jianfengia from the Chengjiang faunaExcluded from this study are Actaeus Simonetta 1970 fromthe Burgess Shale which may be a synonym of Alalcomenaeus(Briggs amp Collins 1999) Alalcomenaeus illecebrosus (Hou1987b see Hou amp Bergstrom 1997) from the ChengjiangFauna which is probably a chimaera (Briggs amp Collins 1999)and the poorly preserved Leanchoilia hanceyi from the MiddleCambrian of Utah (Briggs amp Robison 1984)

According to the definition given above any assessment ofthe limits of the Arachnomorpha needs to consider the phylo-genetic position of these taxa relative to crustaceans To thisend a generalised crustacean intended to represent the plesio-morphic crustacean condition was included as an ingrouptaxon There has been considerable disagreement over themost basal crustacean group (see Wills 1997 pp 194ndash5Schram amp Hof 1998 pp 245ndash8) The coding used here largelyfollows Walossek amp Mullerrsquos (1990 1997 1998) andWalossekrsquos (1993) concept of the crustacean stem group but isintended to be conservative so that coding on the basis of othertheories of crustacean origins would be similar

22 AglaspididaThe morphology of the appendages of aglaspidids is poorlyknown having been described from three species of bodyfossil Aglaspis spinifer Raasch 1939 Flobertia kochi Hesselbo1992 and Khankaspis bazhanovi Repina amp Okuneva 1969 andtrace fossil evidence (Hesselbo 1988) Of these the appendagesof Aglaspis described by Raasch (1939) Briggs et al (1979)and Hesselbo (1992) are by far the best known The append-ages of Flobertia (described by Raasch 1939 as Aglaspisbarrandei and Hesselbo 1992) agree with those of AglaspisHowever the appendages of Khankaspis show a differentmorphology but have only been poorly illustrated anddescribed (Repina amp Okuneva 1969) This material suggeststhe presence of lobate exopods with lamellate setae This taxonis probably correctly assigned to aglaspidids (cf Whittington1979 p 258) on the basis the central position of the dorsaleyes and presence of genal spines although Hou amp Bergstrom(1997 p 96) suggested that its broad tail spine may indicatethat it is a strabopid

It remains unclear whether lamellate exopods are absent insome aglaspidids (Aglaspis) and present in others (Khankaspis)or whether the apparent differences in appendage morphologybetween these taxa are a result of preservational bias alone Apossible preservational analogue is provided by someChengjiang taxa in which the exopods are very poorly pre-served and the endopods of the cephalic appendages arepreserved as impressions in the dorsal head shield in a similarmanner to those of Aglaspis This is presumably because theendopods were more convex and more heavily sclerotised thanthe exopods This mode of appendage preservation is mostclearly seen in Misszhouia longicaudata (see Chen et al 1997fig 2andashd) but is also found in Sinoburius (see Hou ampBergstrom 1997) and possible protaspides of Naraoia (Houet al 1991)

Here a generalised aglaspidid is coded in two different waysto accommodate this uncertainty In both codings they areconsidered to have possessed a pair of antenniform append-ages followed by 11 pairs of pediform endopods on the headand first eight thoracic tergites (Briggs et al 1979 Hesselbo1988 1992) According to one interpretation (coded asAglaspidida 1) all the appendages are uniramous the exopodis lost throughout According to the other (Aglaspidida 2)exopods consisting of a single lobe fringed with lamellate setae(Repina amp Okuneva 1969 pl 15 figs 1 3 amp 4) are presenton at least some appendages but their distribution andattachment are considered unknown

23 Outgroup rootingA hypothetical plesiomorphic euarthropod was included toallow outgroup rooting Many of the characters consideredcould be coded unambiguously on the basis of recent discus-sions of the arthropod stem group (Budd 1996b 1997 1999b)

Figure 3 Majority rule consensus of lsquotrilobite-alliedrsquo arachnate phy-logeny after Edgecombe amp Ramskold (1999 fig 3)

174 TREVOR J COTTON AND SIMON J BRADDY

The ancestral euarthropod is here considered to have possesseda head with antennae only and a series of post-cephalicbiramous limbs with gnathobases The hypothesised pattern ofthe evolution of plesiomorphic euarthropod characters isshown in Figure 4 The use of a hypothetical outgroup issomewhat unsatisfactory but only affects the relationshipbetween the major clades (Arachnomorpha Crustacea andMarrellomorpha) the topology within the arachnomorphs wasunaffected by the use of this outgroup since identical resultswere obtained with unrooted analyses or by rooting withCrustacea Marrella or both

24 Characters and codingMost previous cladistic analyses with the notable exception ofEdgecombe amp Ramskold (1999) have included only briefdiscussions of the primary hypotheses of homology involved incharacter construction and coding Therefore a full discussionof most characters is provided here Descriptions of charactersand character states below are arranged by organ system orbody region in approximate anterior to posterior order Thedistribution of character states across all taxa is shown in thedata matrix (Table 2)

The terminology of morphological features in arachnomor-phs is often confused Terminology developed for cheliceratestrilobites and crustaceans has variously been applied to taxaincluded in this study so some attempt is made to clarifyterminology herein Where appropriate the present authorsrsquoterminology follows Edgecombe et al (2000) and Wheeleret al (1993) but following Scholtz (1997) the protocerebrallsquosegmentrsquo is considered to be acronal and consequently thedeutocerebral segment to be the first true segment Thereforethe antennae (or antennulae of crustaceans) belong to the firstcephalic segment

The present authors have attempted to include as completea set of characters as possible However characters requiringhypotheses of homology between podomeres (eg the number

of podomeres in endopods of thoracic appendages Willset al 1998a character 51) were not considered followingEdgecombe et al (2000 p 157) Secondly some characters usedby Bergstrom (eg Hou amp Bergstrom 1997 p 109) suchas appendage posture and mode of feeding were excludedfollowing Edgecombe amp Ramskold (1999) Finally somecharacters of the ventral surface of the head were not coded asdiscussed below and illustrated in Figure 7 The present authorsdo not include all potential synapomorphies for the Cheli-cerata as the status of many of these is debated (see eg Shultz1990 2001 Dunlop amp Selden 1997 Weygoldt 1998 Wheeler ampHayashi 1998 Dunlop 1999 Edgecombe et al 2000)

Two methods for the coding of inapplicable characters inphylogenetic analysis have been used First inapplicable char-acter states can be coded as missing data A complex structuremay comprise characters lsquoabsentpresentrsquo and lsquostate1state2rsquowith taxa lacking the structure coded as absent for the firstcharacter and as missing data for the second Some authorshave regarded this method as problematic because it may leadto reconstruction of impossible ancestral states and henceunjustified trees (Platnick et al 1991 Strong amp Lipscomb1999) The alternative is to code the second character as a thirdlsquonot applicablersquo state in taxa that lack the structure Thesemethods are equivalent to Pleijelrsquos (1995) coding methods Cand B respectively In the present study inapplicable charac-ters are treated as missing data for most analyses becausecoding them as a distinct character state reduces characterindependence and effectively weights the inapplicable charac-ter and hence could result in them dominating the analysisThe effects of this assumption were investigated by using adistinct character state in some analyses (see Section 26) andinapplicable characters are shown as distinct to lsquotruersquo missingdata (using the symbol lsquo-rsquo) in the matrix (Table 2)

241 Anterior cephalic appendages and head segmentation1 Appendages of the first segment (antennae) (0) present and(1) absent The majority of taxa considered in the present study

Figure 4 Reconstruction of the euarthropod stem group used to inform coding of hypothetical outgroup Grosstopology largely follows Budd (1996b fig 9 1997 fig 1110) Solid boxes indicate unambiguous apomorphiesand shaded boxes character states which are plesiomorphic for the Arthropoda

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 175

possess a single pair of long uniramous multi- and annulatedantennae at the anterior of the head which presumably had asensory function as found in Crustacea Myriapoda and mostmembers of Hexapoda (ie proturans do not have antennae)These appendages are likely to be a synapomorphy of theEuarthropoda (eg Scholtz 1997 Walossek amp Muller 1997)and therefore the outgroup is coded as State 0 The anterior-most head appendages of most Cambrian megacheiran arthro-pods which have been called lsquogreat appendagesrsquo followingWalcott (1912) consist of a small number of robust spinosepodomeres Uniquely Fortiforceps has a pair of short anten-nae anterior to the lsquogreat appendagesrsquo (Hou amp Bergstrom 1997pp 34ndash8) Therefore it appears likely that the lsquogreat append-agesrsquo are the appendages of the second cephalic segment andthe antennae are lost in megacheirans other than FortiforcepsThis of course depends on recognising the lsquogreat appendagesrsquoas homologous in all of these taxa as discussed below(Character 2) Homology of the megacheiran anterior append-ages with the second cephalic appendages of trilobites waspreviously suggested by Stoslashrmer (1944 p 124) on the basis oftheir post-oral position

The classical view of chelicerate head segmentation main-tains that they too have lost the antennae and the cheliceraeare the appendages of the second cephalic segment Recentrevisions of arthropod phylogeny have uncritically acceptedthis homology (Edgecombe et al 2000 Giribet et al 2001)which is based largely on neuroanatomy The lsquochelicero-neuromerrsquo which innervates the chelicerae seems to be ho-mologous with the tritocerebrum associated with the secondcephalic appendage pair of mandibulates (see Weygoldt 1979Winter 1980) Although some uncertainty exists over theposition of pycnogonids within Arthropoda and the homol-ogy of their cephalic segmentation is poorly understood thisview is also supported by the presence of pre-cheliceralappendages in a pycnogonid larva (Larva D of Muller ampWalossek 1986) from the Upper Cambrian of Sweden(Walossek amp Dunlop 2002) Cheloniellon also has a pair ofpre-oral pediform appendages considered homologous to thechelicerae in addition to its antennae Thus despite recentneontological studies that indicate that the chelicerae arehomologous to the antennae of mandibulates based on pat-terns of Hox gene expression (Damen et al 1998 Telford amp

Table 2 Data matrix for cladistic analyses of arachnomorphs () missing data and (ndash) inapplicable characters Letters indicate multistateuncertainty codings as follows (A) 34 (B) 02 (C) 01 (D) 134 (E) 12

1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 3 3 3 4 4 4 4 4 4 4 4 4 4 5 5 5 5 51 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4

OUTGROUP 0 0 0 0 0 0 0 0 0 0 0 0 ndash ndash 0 0 0 0 0 0 0 0 0 0 0 4 0 ndash 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ndash 0 0 0 0 ndash ndash ndash 0 ndash 0Aglaspidida 1 0 0 1 A 0 0 0 0 0 1 0 ndash ndash ndash ndash ndash 0 0 0 1 0 0 0 0 0 0 0 0 ndash 1 1 0 0 1 0 0 0 0 0 0 0 0 1 2 1 1 0 ndash ndash ndash 1 0 1Aglaspidida 2 0 0 A 0 0 0 0 0 0 1 0 ndash ndash 0 1 0 1 0 0 0 0 0 0 0 0 ndash 1 1 0 0 1 0 0 0 0 0 0 0 0 1 2 1 1 0 ndash ndash ndash 1 0 1Alalcomenaeus 1 1 1 3 ndash 2 0 1 0 0 0 1 0 ndash ndash 1 0 0 0 0 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 0 1 0 1 ndash ndash ndash 1 1 0Buenaspis 0 1 B 0 0 1 0 0 ndash 0 1 0 1 0 0 0 0 1 0 0 0 0 ndash 0 1 1 1 0 1 0 ndash 0Burgessia 0 0 0 3 0 0 0 0 0 0 0 1 0 ndash ndash 0 0 0 1 2 ndash 0 0 0 0 0 4 0 ndash 0 0 0 0 0 0 2 0 0 0 0 0 0 ndash 0 0 0 1 ndash ndash ndash 1 0 0Cheloniellon 0 0 1 4 0 0 0 0 0 1 0 1 0 ndash ndash 0 0 1 0 1 0 0 0 0 0 0 1 0 ndash 0 1 0 0 1 0 0 0 0 0 1 0 0 1 0 1 1 0 ndash ndash ndash 0 ndash 1Cindarella 0 0 0 6 0 0 0 0 0 0 0 1 1 0 0 0 0 1 1 0 0 0 0 0 0 0 1 0 ndash 0 1 0 1 1 0 0 2 1 0 0 0 1 0 ndash 1 0 1 1 0 0 0 0 ndash 0Crustacea 0 0 0 3 0 0 0 0 0 0 0 0 0 ndash ndash 0 1 0 0 0 0 0 0 0 0 0 4 0 ndash 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ndash 0 0 0 ndash ndash ndash 1 0 0Emeraldella 0 0 1 5 0 0 0 0 0 0 0 1 1 0 0 0 0 1 0 2 ndash 0 0 0 0 0 0 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 1 1 1 1 0 1 ndash ndash ndash 1 0 1Eoredlichia 0 0 0 3 0 0 0 0 1 0 0 1 1 0 0 0 0 1 0 0 1 0 0 0 0 0 3 1 1 1 1 0 0 2 0 0 0 0 0 0 0 0 0 ndash 1 0 1 1 0 0 1 0 ndash 0Eurypterida 1 1 1 6 ndash 1 1 0 0 1 1 1 0 ndash ndash 1 0 1 0 1 0 0 0 1 0 0 4 1 0 0 1 0 0 1 0 0 0 0 0 0 0 0 1 3 0 1 0 1 ndash ndash ndash 1 0 0Fortiforceps 0 1 1 4 ndash 0 0 0 0 0 0 1 0 ndash ndash 1 0 0 1 0 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 0 1 0 1 ndash ndash ndash 1 1 0Helmetia 3 1 0 1 0 1 0 0 0 0 0 1 2 0 ndash 0 1 0 0 2 0 0 0 0 0 1 1 0 0 ndash 0 0 1 1 0 1 0 0 ndash 0Jianfengia 1 1 1 4 ndash 1 0 0 0 0 0 1 0 ndash ndash 1 0 1 1 0 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 0 1 0 1 ndash ndash ndash 1 0 0Kuamaia 0 0 0 0 0 0 0 0 0 1 1 0 1 0 0 1 0 0 0 1 0 0 0 1 2 0 ndash 0 1 0 0 2 0 0 0 0 0 1 1 0 0 ndash 0 0 1 1 0 1 0 0 ndash 0Leanchoilia 1 1 1 3 ndash 2 0 1 0 0 0 1 0 ndash ndash 1 0 1 1 B 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 1 1 0 1 ndash ndash ndash 1 0 0Lemoneites 0 1 0 0 0 0 1 1 0 1 0 0 0 0 0 0 0 0 1 2 0 0 ndash ndash ndash 1 0 Liwia 0 1 0 0 0 0 1 0 1 0 0 1 0 1 0 0 0 0 ndash 0 1 1 1 0 0 0 ndash 0Marrella 0 0 1 1 0 0 0 0 0 0 0 0 0 ndash ndash 0 1 0 0 2 ndash 0 0 0 0 0 0 ndash 0 0 0 0 0 0 2 0 0 0 0 0 0 ndash 0 0 0 0 ndash ndash ndash 0 ndash 0Mimetaster 0 0 1 2 0 0 0 0 0 0 0 0 0 ndash ndash 0 1 0 1 C 0 0 0 0 0 0 ndash 0 0 0 0 1 0 0 2 0 0 0 0 0 0 ndash 0 0 0 0 ndash ndash ndash 0 ndash 0Misszhouia 0 0 0 3 0 0 0 0 1 0 0 1 1 1 0 0 0 1 0 0 0 0 0 0 0 0 1 0 ndash 0 1 1 ndash ndash 2 0 0 0 1 0 0 0 0 ndash ndash 0 1 1 0 0 1 0 ndash 0Naraoia 0 0 0 3 1 0 0 0 1 0 0 1 1 1 0 0 0 1 0 0 0 0 0 0 0 0 1 0 ndash 0 1 1 ndash ndash 2 0 0 0 1 0 0 0 0 ndash ndash 0 1 1 0 0 0 ndash 0Olenoides 0 0 0 3 0 0 0 0 1 0 0 1 1 0 0 0 0 1 0 1 1 0 0 0 0 0 3 1 1 1 1 0 0 2 0 0 0 0 0 0 0 0 0 ndash 0 1 1 1 0 0 0 0 ndash 0Paleomerus 0 1 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0 0 0 ndash 0 0 ndash ndash ndash 1 0 Pycnogonida 1 1 1 4 ndash 1 1 0 0 1 1 1 ndash ndash ndash ndash 0 0 1 1 0 0 0 1 0 0 4 0 ndash 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 ndash ndash ndash 1 0 0Retifacies 0 0 0 3 0 0 0 0 0 0 0 1 0 ndash ndash 0 0 1 0 0 0 0 0 0 0 0 0 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 1 0 1 1 0 0 0 1 0 0Saperion 0 2 0 1 0 1 0 0 1 1 0 0 1 0 0 0 1 2 0 ndash 0 1 1 ndash ndash 1 1 ndash ndash 0 0 1 0 0 ndash ndash 1 1 0 0 1 0 ndash 0Sidneyia 0 0 1 0 1 0 0 0 0 1 0 1 0 ndash ndash 0 0 1 0 0 0 0 0 0 0 0 0 0 1 0 0 1 0 0 0 0 0 0 0 0 1 2 1 1 0 1 ndash ndash ndash 1 0 1Sinoburius 0 0 0 4 0 0 0 0 0 0 0 1 0 1 0 0 1 1 0 0 0 1 0 ndash 0 1 0 0 1 0 0 2 1 0 0 0 1 0 ndash 0 0 1 1 0 1 0 0 ndash 0Skioldia 0 2 0 0 0 1 0 0 0 1 2 0 ndash 0 1 1 ndash ndash 1 1 ndash ndash 0 0 1 0 0 ndash ndash 1 1 0 0 1 0 ndash 0Soomaspis 0 1 B 0 0 0 0 1 0 0 ndash 0 1 0 1 0 0 1 0 1 0 0 0 0 ndash 0 1 1 1 0 1 0 ndash 0Tariccoia 0 1 B 0 0 0 0 1 0 0 ndash 0 1 0 1 0 0 1 0 1 0 0 0 0 ndash 0 1 1 1 0 1 0 ndash 0Tegopelte 0 0 0 0 0 0 0 0 1 0 1 1 0 0 0 0 1 E 0 ndash 0 1 0 ndash ndash E 1 ndash ndash 0 0 0 0 ndash ndash 0 1 1 0 0 1 0 ndash 0Weinbergina 1 1 1 6 ndash 1 1 0 0 1 1 1 0 ndash ndash 1 0 1 0 1 0 0 0 1 0 0 4 1 0 0 1 0 0 1 0 0 0 0 0 0 0 0 1 2 0 1 0 1 ndash ndash ndash 1 0 0Xandarella 0 0 0 4 0 0 0 0 0 0 0 1 1 1 0 0 0 1 0 0 0 0 1 0 0 0 1 0 ndash 0 1 0 1 1 0 0 2 1 0 0 0 1 0 ndash 1 0 1 1 0 0 0 ndash 0Yohoia 1 1 1 4 ndash 1 0 0 0 1 1 1 0 ndash ndash 1 0 1 0 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 1 2 0 1 0 1 ndash ndash ndash 1 1 0

176 TREVOR J COTTON AND SIMON J BRADDY

Thomas 1998) and developmental data (Mittmann amp Scholtz2003) morphological evidence from fossils supports the viewthat chelicerates have lost the antennae The Hox gene evi-dence suffers from a lack of comparative data from diversemandibulates especially primitive crustaceans (see Akam2000) and the use of Hox gene expression boundaries asmarkers for segmental homology has been criticised (egAbzhanov et al 1999 although relating to posterior Hoxpatterns) Wills et al (1998a pp 43 amp 48) considered thechelicerae to be appendages of the second (deutocerebral)segment but curiously in support of this cited Schram (1978)who unambiguously followed the homology scheme used inthe present study

The anterior-most appendages of Aglaspis were originallydescribed as chelicerae (Raasch 1939) which lead to theirclassification in the Chelicerata (eg Stoslashrmer 1944) Howeverthe morphology of these appendages (see Briggs et al 1979) ismore similar to that of antennae They are narrow relative tothe thoracic endopods of relatively even diameter proximallyand the podomeres are apparently weakly defined Hesselbo(1988 1992) also argued that the anterior appendages ofAglaspis are likely to be antennae The distal parts are un-known (and hence so is their length cf Wills et al 1998ap 58) and there is no evidence that they were chelate contraryto the coding of Wills et al (1998a character 31) Nothingis known about the anterior appendages of BuenaspisLemoneites or Paleomerus and this character is accordinglycoded as missing data in these taxa

2 Form of endopod of appendages of the second segment(0) pediform and (1) anteriorly directed raptorial appendagewith reduced number of podomeres and terminal podomeresbearing spines on distal margins As discussed above cheli-cerae and lsquogreat appendagesrsquo are both likely to represent theappendages of the second segment They also differ from theassumed primitive biramous euarthropod limb in similar waysand therefore are likely to be homologous modifications ofthese appendages The homology of lsquogreat appendagesrsquo andchelicerae was originally proposed by Henriksen (1928) andsupported by Stoslashrmer (1944) but to the present authorsrsquoknowledge no modern author has discussed this possibility

Both lsquogreat appendagesrsquo (Figs 5A D E) and chelicerae(Figs 5B C) are equipped with strong spinose projections onthe outer (dorsal) side of the distal margins of the terminalpodomeres that are lacking on the proximal podomeresSecondly the number of podomeres in both chelicerae andlsquogreat appendagesrsquo is more or less reduced compared to thenumber in the endopods of biramous limbs In the case oflsquogreat appendagesrsquo they are significantly more robust thanthose of posterior endopods a situation that is matched insome chelicerates The homology of the lsquogreat appendagesrsquoof Alalcomenaeus Leanchoilia Yohoia Jiangfengia andFortiforceps is supported by all of these features and is widelyaccepted (eg Wills et al 1998a character 31 Hou ampBergstrom 1997 Bergstrom amp Hou 1998 Hou 1987a) OnlyHou amp Bergstrom (1991 p 183) have suggested albeit inpassing that the lsquogreat appendagesrsquo of all these taxa may beconvergent a view they appear to have changed The endo-pods of the appendages of other taxa are locomotory legswhich either lack strong spines or have spinose extensionsthat are invariably on the medial (ventral) surface and arestronger on the proximal podomeres than the distal ones (egMisszhouia Fig 6A) In the latter case these spines form partof the feeding system along with the gnathobases and are oftenlocated around the middle of the podomere rather than limitedto the distal margin These endopods are directed ventro-laterally as opposed to anteriorly in the case of both cheliceraeand lsquogreat appendagesrsquo The modified second appendages of

Marrella resemble this plesiomorphic condition here referredto as lsquopediformrsquo except in their anterolateral orientation Insome Recent crustaceans this appendage is modified into asecond antenna but in stem-group crustaceans it is pediform(eg Walossek amp Muller 1997 1998) This character is codedas missing data in a number of taxa for which the morphologyof the second segment appendage is unknown

Some authors (Dzik 1993 Chen amp Zhou 1997 Dewel ampDewel 1997 Budd 2002) have suggested that the lsquogreat ap-pendagesrsquo are homologous with the anterior appendages ofanomalocaridids and the anomalocaridid-like Opabinia andKerygmachela (see Budd 1996b 1999b respectively) Theanterior appendages of most anomalocaridids and otherCambrian lobopodians differ from megacheiran lsquogreat append-agesrsquo in that they consist of a large number of segmentsMoreover there is considerable morphological evidence thatanomalocaridids are part of the euarthropod stem group (Willset al 1995 1998a) and not closely related to the lsquogreatappendagersquo taxa (cf Budd 2002) with the possible exception ofParapeytoia lsquoGreat appendagersquo taxa share a suite of derivedcharacters with other crown group euarthropods includingsclerotised dorsal tergites the incorporation of more thanone pair of appendages into the head sclerotisation of theendopods and strong differentiation of the head that wereexcluded from Buddrsquos (2002) analysis The relationships ofanomalocaridids will be considered in detail later by Braddyand co-workers

3 Exopod of appendages of the second segment (0)present and (1) absent or much reduced The raptorial ap-pendages of the second segment (chelicerae and lsquogreat append-agesrsquo) are uniramous as are the corresponding pediformappendages of Marrella Mimetaster Emeraldella Cheloniellonand Sidneyia The plesiomorphic euarthropod state is found inmost other arachnomorphs including trilobites where thebiramous second segment appendages are undifferentiatedfrom those of posterior segments (Edgecombe et al 2000character 78) The exopods of these appendages are alsopresent in basal members of the crustacean crown group(Edgecombe et al 2000 character 79) and in stem groupcrustaceans (Walossek 1993 Walossek amp Muller 1997 1998)

4 Number of head segments (0) 1 (1) 2 (2) 3 (3) 4 (4) 5(5) 6 and (6) 7 The coding of this character by Edgecombe ampRamskold (1999 character 2) explicitly referred to the numberof limb pairs present in addition to the antenna Here thenumber of post-acronal segments present in the head is codedfollowing Wills et al (1998a character 29) In taxa that lack anantenna the number of somites is inferred to be one greaterthan the number of cephalic appendage pairs (cf Wills et al1998a) as described above

Sturmer amp Bergstrom (1978) suggested that the head ofCheloniellon is defined primarily on the basis of appendagetagmosis rather than the fusion of segments under a singlecephalic-shield (cf Hou amp Bergstrom 1997 pp 98ndash9) Accord-ing to this view the head consists of both the cephalic tergiteand the anteriormost free tergite that share uniramousgnathobasic appendages Wills et al (1998a) coded Cheloniel-lon as having six somites incorporated into the head andtherefore presumably accepted this suggestion There is noevidence that the first free tergite of Cheloniellon was incorpo-rated into the head (except perhaps functionally) and thepresent authors prefer to code the number of somites under thecephalic shield In Sidneyia there is a series of uniramousstrongly gnathobasic appendages similar to those of Cheloniel-lon posterior to the head shield All of these appendages wouldpresumably have to be considered part of the head based onappendage differentiation according to the view of Sturmer ampBergstrom (1978) Some autapomorphic states are included

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 177

(none for Sidneyia one for Marrella and six for Emeraldella)since these will become informative if the character is treatedas ordered

Edgecombe amp Ramskold (1999 p 265) described a novelform of cephalic tagmosis The present authors tentativelyaccept their suggestion that in trilobite-like taxa the fourthpair of biramous cephalic appendages was directly under thecephalo-thoracic junction This may explain previous confu-sion about the number of such appendages incorporated intothe trilobite cephalon (eg Cisne 1975 Whittington 1975aBergstrom amp Brassel 1984) However Edgecome amp Ramskoldaccept the view of Chen et al (1977 p 7) that only the firstthree biramous limbs of Misszhouia were structurally and

functionally part of the head Therefore it seems more accept-able to code these taxa as having only four post-acronalsomites than to use the coding scheme of Edgecombe ampRamskold (1999) The partial integration of the first thoracicsomite under the head shield could be considered to be a formof overlap of the trunk by the head shield (ie a distinct stateof character 9 of Edgecombe amp Ramskold 1999 or Character37 herein) Derived states of this character would then poten-tially be synapomorphic for a clade including xandarellidsnaraoiids helmetiids tegopeltids and trilobites Howeversince the degree of overlap in taxa with the fourth biramousappendages under the cephalo-thoracic articulation is identicalto that found between thoracic tergites (State 0 of Character

Figure 5 Diagrammatic reconstructions of raptorial second-segment appendages in lateral view (except whereotherwise stated) and approximate life orientation showing suggested homology of appendage elements (inbrackets) Roman numerals indicate position in the podomere series and do not necessarily imply homology (A)lsquoGreat appendagersquo of Fortiforceps (after Hou amp Bergstrom 1997) (B) Left chelicera of Leiobunum aldrichi(Arachnida Opiliones) (1) in medial perspective and (2) distal parts in anterior perspective (after Shultz 2000)(C) Chelifore of Nymphon (Pycnogonida) (after Child 1997) (D) lsquoGreat appendagersquo of Yohoia (after Whittington1974 fig 2) (E) lsquoGreat appendagersquo of Leanchoilia (modified after Bruton amp Whittington 1983) Not to scaleAbbreviations (apt) apotele (probably homologous with the mandibulate pretarsus) (cx) coxa (dtmrt)deutomerite and (pt) pretarsus

178 TREVOR J COTTON AND SIMON J BRADDY

37) it is here preferred to regard this as a distinct character(Character 9 herein)

The coding of Alalcomenaeus by Wills et al (1998a) ashaving four post-acronal somites is accepted here but fordifferent reasons Briggs amp Collins (1999) have recenty demon-strated that Alalcomenaeus possessed two pairs of biramousappendages on the head in addition to the great appendagesThis would equate to three head somites according to thehomology scheme of Wills et al (1998a) Here the antennaeare inferred to be lost and therefore four segments incorpo-rated into the head

Recently a number of authors have agreed that the plesio-morphic condition for euarthropods is a four-segmented headas found in stem group crustaceans (Scholtz 1997 Walossek1993 Walossek amp Muller 1997 1998 Edgecombe et al 2000)However reconstruction of the euarthropod stem groupclearly indicates that even crownward taxa had a head consist-ing of only the antennal segment and acron as found intardigrades and onychophorans Therefore it seems that thefour-segmented head is pleisomorphic only for crown groupeuarthropods Some fully arthropodised fossil taxa also have ashorter head that may be pleisomorphic such as the two-segmented head of Marella Retifacies is coded following therecent redescription of Hou amp Bergstrom (1997) rather thanon the basis of the coding by Edgecombe amp Ramskold (1999)for which they provide no explanation A partial uncertaintycoding is used for Aglaspis in which the number of post-antennal cephalic appendages is either three or four (Briggset al 1979)

5 Orientation of the antennae (0) directed anterolaterally(1) strongly deflected laterally and (2) placed well inside shieldmargin curing posteriorly from a transverse proximal element(cf Edgecombe amp Ramskold 1999 character 3) The anteriorappendages of Aglaspis (see character 1) are clearly directedanterolaterally and it is presumed that they continued past thehead shield margin The anterior appendages of arthropodstem group taxa such as Aysheaia Kerygmachela and Anoma-locaris are either directed anterolaterally or ventrally but arenot strongly-deflected laterally Therefore the outgroup iscoded as State 0 This character is coded as missing data fortaxa where the presence of antennae is equivocal (those codedas missing data for Character 1) and as inapplicable in taxawhere the antennae are considered absent (coded as State 1 forCharacter 1)

6 Length of distal spines on terminal podomeres of endo-pods of second segment appendages (0) absent or shorer thanpodomeres (1) subsequal to length of podomeres and (2)longer than the entire podomere series The spinose projectionsof the raptorial appendages described above vary in length Inchelicerae (Figs 5BC) and some lsquogreat appendagesrsquo (eg thoseof Yohoia Fig 5D) the most distal spine is equal in length tothe spinose terminal podomere forming a chelate structureIn Fortiforceps (Fig 5A) the spines are short relative tothe lengths of the podomeres but in Alalcomenaeus andLeanchoilia (Fig 5E) they are extremely long so that thespines are much longer than the entire podomere series Asdescribed above in taxa with pediform endopods of the secondsegment appendages spines on the distal podomeres are shortor entirely absent

7 Chelicerae (0) absent and (1) present Despite theirsimilarities to lsquogreat appendagesrsquo the chelicerae of the eucheli-cerates and chelifores of the pycnogonids are clearly distinctand have been widely recognised as a chelicerate synapomor-phy Chelicerae differ from any lsquogreat appendagesrsquo by thecombination of a smaller number of podomeres the presenceof only a single terminal element and only a single spinoseprojection on the dorsal side of the podomere series (Fig 5)

Amongst extant chelicerates only the pycnogonid Pallenopsishas chelicerae of four podomeres comparable to the numberin lsquogreat appendagesrsquo Bergstrom et al (1980) considered thatthe chelifores of the Devonian pycnogonid Palaeoisopus alsoconsist of four segments but this is not well supported by theirfigures

8 Distal spines of second segment endopods terminating inannulated flagellae (0) absent and (1) present It is widelyrecognised (eg Stoslashrmer 1944 Simonetta 1970 Bruton ampWhittington 1993 Briggs amp Collins 1999) that the terminalparts of the extended spines of Alalcomenaeus and Leanchoiliaformed annulated flagellae Nothing similar is known fromhomologous appendages of any other arthropod althoughsimilar processes may have operated in the transformation ofthe endopod as a whole into the second antennae of derivedcrustaceans and in the origin of multiramous antennulae (firstantennae) in malacostracans

242 Posterior appendages 9 Appendages of first thoracicsomite underneath the cephalo-thoracic articulation (0) ab-sent and (1) present (cf Edgecombe amp Ramskold 1999character 2) For a discussion of this character see Character4 herein

10 Exopods of appendages of third to fifth segments (0)present and (1) reduced or absent A number of taxa haveuniramous appendages on the third to fifth segments In mostthese segments are incorporated into the head (Yohoia cheli-cerates) but in others all (eg in Sidneyia) or some (eg inCheloniellon) of these segments are post-cephalic In all casesthe appendage is morphologically similar to the endopods ofbiramous limbs of other taxa andor other segments and it isinterpreted that the exopod is lost

11 Endopods of thoracic appendages (0) present and (1)reduced or absent The opisthosomal appendages of chelicer-ates lack endopods or have the endopods much reduced (egLimulus Siewing 1985 fig 838 Shultz 2001) a situation that isalso found in the uniramous thoracic appendages of Yohoia(Whittington 1974) It has been suggested that Helmetia(Briggs et al 1994) lacks endopods but pending a redescrip-tion of this genus and considering that they have been docu-mented in the otherwise very similar Kuamaia this is coded asuncertain

12 Exopod shaft of numerous podomeres each bearing asingle seta (0) present and (1) absent The exopods ofcrustaceans (at least plesiomorphically see eg Walossek ampMuller 1998 figs 5middot5 amp 5middot6) and of Marrella and Mimetasterhave multi-annulated shafts with each podomere bearing asingle seta In other taxa the exopod shafts consist of one oftwo lobes bearing numerous setae

The polarity of exopod segmentation is uncertain Thelateral flaps of stem group arthropods such as Opabinia(Whittington 1975b Budd 1996b) and anomalocaridids (egHou et al 1995) are certainly unsegmented but as suggestedby Buddrsquos (1996b fig 8) reconstruction this does notnecessarily suggest the form of the primitive euarthropodexopod Rather segmentation may have been a result of thesclerotization of the cuticle of the exopod shaft The outgroupwas coded as uncertain for this character According to thefirst interpretation of aglaspidid appendage morphology(coded as Aglaspidida 1) the exopods are lacking on allappendages and consequently this and all other charactersdescribing exopod morphology are coded as inapplicable

13 Exopod shaft differentiated into proximal and distallobes (0) absent and (1) present Ramskold amp Edgecombe(1996 Edgecombe amp Ramskold 1999 character 26) havediscussed the distribution of the trilobite-type bilobate exo-pods recognised by this character (Fig 6A) Among taxaincluded here that were not considered by Edgecombe amp

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 179

Ramskold (1999) exopods consist either of a single flattenedlobe or a homonomous series of podomeres (Fig 6B C) Nostem group euarthropod has a bilobate exopod and theoutgroup is consequently also coded as State 0

14 Proximal lobe of exopod (0) flattened lobe and (1)slender shaft

15 Distal lobe of exopod (0) small to moderate sized flapwith short to moderately long attachment to proximal lobeand (1) large teardrop-shaped with long attachment to proxi-mal lobe Neither of these characters can be coded for taxathat do not have an exopod differentiated into proximal anddistal lobes (Character 13 State 1) without asserting thehomology of the single-lobed exopod with one of these partsThe coding of Edgecombe amp Ramskold (1999 p 280) implic-itly homologizes the exopods of Sidneyia and Retifacies withthe proximal lobe In these taxa this view is perhaps supportedby the presence of lamellate exopod setae which match thoseof the trilobite-type proximal lobe However in other taxawith a single-lobed exopod such as Alalcomenaeus (see Briggsamp Collins 1999) the exopod is fringed with sharp spines likethe distal lobe of differentiated exopods Alternatively thebilobate exopod may have originated from the division of aprimitively single-lobed structure The single-lobed exopod isnot clearly homologous with either lobe of divided exopodsand both these characters are consequently coded as inappli-cable to taxa without differentiated proximal and distal exopodlobes Consequently (see Character 13 herein) these characterscan only be coded for taxa that were previously considered byEdgecombe amp Ramskold (1999) and their coding is followedhere except in the cases of Sidneyia and Retifacies

16 Exopod shaft is a deep rounded flap (0) absent and (1)present All bilobate (Character 13) and segmented (Character12) exopod shafts are long and relatively narrow structures (seeFig 6) whereas some single-lobed exopod shafts have theform of rounded flaps These flap-like exopod shafts are largecompared to the length of the setae and at least half as deep asthey are long This appendage structure is found in cheliceratesand lsquogreat appendagersquo arthropods

17 Medially-directed exopod setae (0) absent and (1)present Walossek amp Muller (1998 p 194) suggested that thetilting of exopod setae towards the endopod in post-antennularlimbs with a multi-annulated exopod was an autapomorphyuniting crustaceans and all members of the crustacean stemgroup (the crustacean total group sensu Ax 1986 see Fig 6C)This character is shared with the marellomorphs Marrellaand Mimetaster (see Fig 6B Sturmer amp Bergstrom 1976Bergstrom 1979 fig 1middot3A B) In other taxa the setae eithersurround the entire margin of the exopod shaft or are directeddorsally (Fig 6A) The identification of setae on the dorsalsurface of the lateral flaps in Opabinia (Budd 1996b) suggeststhat the condition in marellomorphs and crustaceans isderived

18 Lamellate exopod setae (0) absent and (1) presentLamellate exopod setae originally described from trilobitesare a classic synapomorphy of the Arachnomorpha (see egBergstrom 1992 Hou amp Bergstrom 1997 pp 42ndash3) They areperhaps best known from Misszhouia (see Chen et al 1996Hou amp Bergstrom 1997 Fig 6A) These setae differ from themore spinose setae of other taxa (Fig 6C) in that they are wideand flat and imbricate over the length of the exopod shaft Thesetae of Marrella (Fig 6B) and similar taxa have variouslybeen considered lamellate (Bergstrom 1979) or non-lamellate(Bergstrom 1992) Since they do not imbricate and do notseem to be strongly flattened (Whittington 1971 Sturmer ampBergstrom 1976 fig 9a) Marrella and Mimetaster are codedas State 0 The setae of Helmetia as shown in Briggs et al(1994 fig 141) seem to be of the lamellate type The setae ofSinoburius have been described by Hou amp Bergstrom (1997p 85) as similar to those of Misszhouia Only the setae areknown of the appendages of Buenaspis (Budd 1999a) andthese also appear to be of the trilobite-type

Figure 6 Diagrammatic reconstructions of (A) arachnomorph and(B C) non-arachnomorph biramous appendages (A) The lsquotrilobite-typersquo biramous appendage of Misszhouia longicauda (after Chen et al1997) (B) Appendage of Marrella splendens (after Whittington 1971)(C) Appendage of the stem-lineage crustacean Martinssonia elongata(after Muller amp Walossek 1997 1998) Not to scale Abbreviations(bas) basis (dle) distal lobe of exopod shaft (ls) lamellar exopod setae(pe) proximal endite or pre-coxa (ple) proximal lobe of exopod shaft(ses) segmented exopod shaft and (ss) spinose exopod setae

180 TREVOR J COTTON AND SIMON J BRADDY

The homology of the book-gill lamellae of xiphosurans withlamellar setae was supported by Walossek amp Muller (1997p 149) and Edgecombe et al (2000 p 174) but rejected bySturmer amp Berstrom (1981) on the grounds that Weinberginapossesses Limulus-like gill lamellae and fringing setaeBook-gills have also been described from a eurypterid (Braddyet al 1999) There are certainly major morphologicaldifferences between book-gills and trilobite-type exopodsFollowing most recent opinion and pending further studyWeinbergina and Eurypterida are coded as possessing lamellatesetae

The form of the setae of Yohoia is uncertain (Whittington1974) the spinose setae seen in the most common reconstruc-tion (Gould 1989 Briggs et al 1994) are not justified by thespecimens Amongst other great appendage arthropods theform of the setae appears to vary those of Jianfengia (seeChen amp Zhou 1997 p 74) and Leanchoilia (see Bruton ampWhittington 1983) are lamellate while those of Fortiforceps(Hou amp Bergstrom 1997) and Alalcomenaeus (Briggs amp Collins1999) are spinose and less densely packed

19 Gnathobase on basis andor prominent endites onendopod (0) present and (1) absent (cf Edgecombe ampRamskold 1999 character 29 Wills et al 1998a characters 36amp 59) The presence of gnathobases on the appendages of someanomalocaridids (Hou et al 1995) and their general distribu-tion amongst non-arachnomorph euarthropods suggests thatState 0 is plesiomorphic The homology of the various enditesand gnathobasic structures found in euarthropods is in need ofcareful assessment and therefore a very general coding (fol-lowing Edgecombe amp Ramskold) is used for this character

243 Eyes 20 Position of lateral facetted eyes (0)ventral and stalked (1) dorsal and sessile and (2) absent (cfEdgecombe amp Ramskold 1999 character 4) Partial uncer-tainty coding is used for a number of taxa where the dorsalsurface is known but the ventral morphology is not andtherefore the presence of ventral eyes cannot be discountedFor example there is good evidence that Leanchoilia lackeddorsal sessile eyes but ventral stalked eyes may have beenpresent (as in L hanceyi see Briggs amp Robison 1984) and a(02) partial uncertainty coding is used This is also the case inmany naraoiids such as Buenaspis However only the outlineof the head shield of Liwia is known and the absence of dorsaleyes in the reconstruction of Dzik amp Lendzion (1988) isconjectural It is unclear whether the autapomorphic conditionof stalked dorsal eyes in Mimetaster (see Sturmer amp Bergstrom1976) is homologous with State 0 or State 1 and a partialuncertainty coding is also used

Whittington (1974) was somewhat equivocal about thenature of the lateral lobes anterior to the head shield ofYohoia These more closely resemble the ventral stalked eyes ofAlacomenaeus as recently described by Briggs amp Collins(1999) than either Whittingtonrsquos (1974) or subsequent (egGould 1989 fig 3middot18 Briggs et al 1994 fig 153) reconstruc-tions suggest In a well preserved specimen showing the dorsalaspect (USNM 57696 Whittington 1974 pl 2 figs 1ndash3) and ina laterally compressed specimen (USNM 57694 Whittington1974 pl 1 fig 1) they are clearly seen to be stalked andrelatively small lobate structures That these structures arelikely to represent stalked ventral eyes was reflected in thecoding of Wills et al (1998a characters 26 amp 27)

21 Visual surface with calcified lenses bounded withcircumocular suture (0) absent and (1) present

22 Dorsal bulge in exoskeleton accommodating drop-shaped ventral eyes (0) absent and (1) present

23 Eye slits (0) absent and (1) presentCharacters 21 to 23 are used exactly as described by

Edgecombe amp Ramskold (1999 character 5) Character 21 is

inapplicable to taxa that do not possess eyes (Character 20State 2)

24 Dorsal median eyes (0) absent and (1) present Dorsalmedian eyes on a tubercle are considered a synapomorphy ofthe Chelicerata (Dunlop amp Selden 1997 Dunlop 1999) Thenature and position of the structures interpreted as dorsalmedian eyes in Mimetaster (Sturmer amp Bergstrom 1976p 83ndash4) are regarded as equivocal

244 Ventral cephalic structures Many characters of theventral surface of the euarthropod head (see Fig 7) of poten-tial phylogenetic utility are poorly known in many importanttaxa In particular the presence of pre-hypostomal frontalorgans (Fig 7A CndashE) is not coded herein (see Edgecombe ampRamskold 1999 p 272) Definitive evidence of the absence ofthese structures is available for very few taxa and the homol-ogy of these structures with the maculae of the trilobitehypostome (see Fig 7B) and the frontal organs of the crusta-cean labrum (eg see Muller amp Walossek 1987 p 39) isuncertain Secondly the detailed homology of the trilobitehypostome with that of other putative arachnomorphs isunclear and consequently a very broad definition is usedherein For example various pre-hypostomal sclerites (Fig7A D) in other arachnomorphs may have been incorporatedalong with a primitive hypostome into a single sclerite that isrecognised as the hypostome in trilobites The structure of thehypostome (eg the presence of paired anterior wings dorsal tothe antennae Fig 7B C) may also be a source of additionalcharacters

25 Expanded cephalic doublure (0) absent and (1)present maximum width more than 30 length of head shieldor more than 25 width of pygidium Chen et al (1996)suggested that the wide doublure of Soomaspis and Tariccoia isa synapomorphy uniting these taxa The doublure of Buenaspis(see Budd 1999a) is poorly known but based on the width ofthe heavily crushed region of the cephalic margin and theposition of a possible impression of the edge of the doublure inone specimen (Budd 1999a pl 1 fig 1) it also appears to berather wide (ie greater than 30 of the length of the headshield) Following Edgecombe amp Ramskoldrsquos coding ofSidneyia the present authors do not consider the postero-median expansion of the doublure of for example Emeraldellaor Retifacies (see Bruton amp Whittington 1983 Hou ampBergstrom 1999 respectively) to be homologous with State 1but to represent the hypostome (see Character 26)

26 Anteromedian margin of cephalon notched accomo-dating strongly sclerotised plate (0) notch and plate absentand (1) notch and plate present (cf Edgecombe amp Rasmkold1999 character 10) The apomorphic state (illustrated inFig 7D) is only known in the taxa discussed by Edgecombe ampRasmkold (1999) Reconstructions of Yohoia show a roundedlobe between the eyes (see Character 20) anterior to anddistinct from the head shield which resembles the anteriorsclerite recognised here This seems to be a misinterpretationSpecimens preserved in dorsal aspect (USNM 57696Whittington 1974 pl 2 figs 1ndash3) and lateral aspect (USNM57694 Whittington 1974 pl 1 fig 1 USNM 155616Whittington 1974 pl 5 fig 2) seem to clearly show that thislsquomedian lobersquo is a downward curving pointed extension of thehead shield

27 Hypostomal sclerite (0) median extension of thedoublure with no suture (1) natant sclerite not in contactwith doublure (2) with narrow overlap with pre-hypostomalsclerite (3) narrow attachment to doublure at hypostomalsuture and (4) absent The homology of hypostomes andlabrums has been discussed by Haas et al (2001) and Budd(2002) The euarthropod labrum is likely to represent a pos-teroventral extension of the pre-segmental acron (Scholtz

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 181

Figure 7 Ventral reconstructions of arachnomorph arthropod heads (A) Misszhouia longicaudata (after Chenet al 1997 figs 2ndash4 7) (B) Ceraurinella typa (after Chen et al 1997 fig 7) (C) Agnostus pisiformis (after Mulleramp Walossek 1987) (D) Kuamaia lata (after Edgecombe amp Ramskold 1999 figs 32 5 amp 6) (E) Cindarella eucala(after Ramskold et al 1997) and (F) Emeraldella brocki (after Bruton amp Whittington 1983) Not to scaleAbbreviations (aw) anterior wing beneath antennae (bs) boomerang-shaped sclerite (bl) blade-like process ofposterior wing (d) doublure of head-shield (cs) connective suture (f) fenestra (fs) facial suture (h) hypostome(hl) hypostomal lobe (emerging through fenestrae of Agnostus) (hs) hypostomal suture (if) intervening field ofhypostomal complex (lb) lateral bridge (lfo) lateral frontal organ (mb) median body (mbr) median bridge (mf)median furrow (mfb) mouth field bridge (mfo) median frontal organ (ol) ovate lobe and (phs) pre-hypostomalsclerite Post-antennal appendages and distal parts of antennae omitted

182 TREVOR J COTTON AND SIMON J BRADDY

1997) This structure is often sclerotised and covers the mouthventrally It is this sclerite that is here identified as thehypostome Therefore this character is considered absent intaxa lacking a sclerite covering the mouth irrespective of themorphology and position of the labrum itself which at leastpartly covers the mouth in all euarthropods other thanpycnogonids (see Edgecombe et al 2000 p 167) The lsquofleshylabrumrsquo of crustaceans does not appear to be sclerotised and ahypostome is consequently considered to be absent Althoughmany derived features are associated with the crustaceanlabrum this is not a good reason to consider it non-homologous with the hypostome-bearing structure of otherarthropods as Walossek amp Muller (1990) argued

In many taxa the mouth is covered by a posteromedianextension of the doublure here considered to represent thehypostome (eg Emeraldella Fig 7F Aglaspis see Hesselbo1992 fig 52) In others the hypostome is separated from thedoublure either by a suture a pre-hypostomal sclerite (egKuamaia lata Fig 7D Saperion glumaceum see Edgecombe ampRamskold 1999 figs 3 amp 4) or an intervening region ofunsclerotised cuticle (the natant condition of Fortey 1990beg Cindarella eucala Fig 7E) The homology of pre-hypostomal sclerites and hypostomes in helmetiids trilobitesand naraoiids (see Fig 7) has been discussed by Chen et al(1996) and Edgecombe amp Ramskold (1999) but it is as yetunclear whether any of the character states described here arederived from any other They are treated here as distinct andthe character as unordered pending further study

No hypostome has been identified in Yohoia (see above)Alalcomenaeus Jianfengia Fortiforceps or Leanchoilia andthere is certainly no broad posterior extension of the doublurein any of these taxa Since there is also no pre-hypostomalsclerite they have received a (134) partial uncertainty codingThe attachment of the hypostome of Tegopelte is unclear butit does not seem to be attached to any doublure (Whittington1985) and therefore it is given an uncertainty (12) coding

The lsquorostrum-like structurersquo on the ventral surface ofLemoneites appears to be similar in morphology and positionrelative to the anterior margin of the head shield (Flower 1968pl 8 figs 4 amp 13) to that described from Aglaspis and is codedas State 0 In many taxa the hypostome is unknown but inonly a small number can it be coded reliably as absentHughesrsquo (1975) suggestion that the hypostome of Burgessia isabsent is accepted preliminarily but an extension of thedoublure may be visible in some of Hughesrsquo figures Fortey(pers comm 2000) also doubts Hughesrsquo assertion

28 Visible ecdysial sutures (0) absent and (1) present Themarginal ecdysial sutures of chelicerates and the dorsal suturesof trilobites are almost certainly not homologous but thepresence of sutures and their position (see Character 29) arecoded separately to allow this to be tested Wills et al (1998aCharacter 2) coded marginal sutures as present in Sidneyiafollowing Brutonrsquos (1981) suggestion but this is consideredequivocal here

29 Position of ecdysial sutures (0) marginal and (1) dor-sal This character is inapplicable to taxa lacking ecdysialsutures (Character 28 State 0) Dorsal sutures whilst presentin the representatives of the Trilobita coded here are probablynot a synapomorphy of trilobites as a whole but for a clade oftrilobites excluding the Olenellida (Whittington 1989 Fortey1990a)

245 Exoskeletal tergites and thoracic tagmosis 30 Min-eralised cuticle (0) absent and (1) present Calcification of theexoskeleton is one of the most convincing synapomorphiesof the Trilobita (Fortey amp Whittington 1989 Ramskold ampEdgecombe 1991 Edgecombe amp Ramskold 1999 Character 1)Cuticle mineralisation is also coded as present in aglaspidids

following Briggs amp Fortey (1982) who suggested that theaglaspidid exoskeleton was originally phosphatic Paleomerusprobably also had a mineralised cuticle but Hou amp Bergstrom(1997 p 97) argued that it was likely to be calcareous Anundescribed Silurian aglaspidid may also have had an origi-nally calcitic cuticle (Fortey amp Theron 1994 p 856) Becauseof the uncertainty about the chemical composition of theexoskeleton in these forms cuticle mineralisation is coded aspotentially homologous in all these taxa and no attempt ismade to code separate states for phosphatic and calcareousmineralisation

31 Trunk tergites with expanded lateral pleurae coveringappendages dorsally (0) absent and (1) present Whilst thepresence of paratergal folds may be a synapomorphy at thelevel of the Euarthropoda (Boudreaux 1979 Wagele 1993Edgecombe et al 2000 Character 142) these are at mostsmall reflections of the margin of the tergites in most euarthro-pods In arachnomorph taxa these are expanded to form largelateral pleurae that cover the appendages dorsally (see egLimulus and Triarthus in Boudreaux 1979) This is inferred tobe the case in some taxa where dorsal features and appendagesare poorly known on the basis of their possession of tergiteswith wide pleural regions This feature was suggested as typicalof lamellipedians (=arachnomorphs) by Hou amp Bergstrom(1997 pp 42ndash3)

The arrangement of the thorax of Marrella Mimetaster andBurgessia differs from that of all the other taxa considered herein that the appendages seem to be attached to the lateralmargins of the body The trunk tergites do not cover theappendages and seem to lack pleurae entirely although theyare covered by a posterior extension of the head shield inBurgessia This character has been clearly described inMimetaster (Sturmer amp Bergstrom 1976 pp 87ndash90 fig 8) andBurgessia (Hughes 1975 p 421)

32 Free thoracic tergites (0) present and (1) absentFollowing Edgecombe amp Ramskold (1999 Character 16) anumber of taxa lack functional post-cephalic articulations andconsequently lack free thoracic tergites Despite this theycode these taxa for a number of characters relating to thestructure of thoracic tergites (eg their characters 18 and 19)These characters are treated here as inapplicable to taxawithout free tergites and the definition of this character hasbeen modified from that of Edgecombe amp Ramskold (1999) tomake this more explicit

33 Decoupling of thoracic tergites and segments (0)absent and (1) present Ramskold et al (1997) described thischaracter of Cindarella and Xandarella where the thoracictergites correspond to a variable increasing posteriorly num-ber of appendage pairs This character cannot be coded fortaxa in which free thoracic tergites are absent (Character 32State 1)

34 Tergite articulations (0) tergites non-overlapping (1)extensive overlap of tergites and (2) edge-to-edge pleuralarticulations In most of the taxa considered here the thoracictergites overlap considerably and relatively evenly over theirwidth This contrasts with the primitive euarthropod condi-tion where the tergites do not overlap medially as seen in stemgroup crustaceans In trilobites such as Helmetia and Kuamaia(see Edgecombe amp Ramskold 1999 character 18) overlap ofthoracic tergites is limited to a well-defined axial region andthe lateral pleurae meet edge-to-edge

The thoracic articulations of naraoiids with free thoracictergites are in some ways similar to those of trilobites with astrong overlap medially that is lacking abaxially andanterolateral articulating facets (Budd 1999a Ramskold ampEdgecombe 1999) However the distribution of these features

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 183

in other trilobite-like arachnomorphs is unclear and a restric-tive coding is used here The present authors do not considerthat this character can be applied reliably to the fused thoracictergites of Saperion Tegopelte and Skioldia and it is coded asinapplicable to all taxa lacking free thoracic tergites (Character32 State 1)

35 Trunk effacement (0) trunk with defined (separate orfused) tergite boundaries (1) trunk tergite boundaries effacedlaterally and (2) trunk tergite boundaries completely effaced(cf Edgecombe amp Ramskold 1999 Character 15)] Contrary toEdgecombe amp Ramskold (1999) the present authors considerSkioldia and Saperion to show a distinct form of tergiteeffacement where the fused tergite boundaries are defined byfurrows axially but effaced laterally The boundaries areeffaced at least laterally in Tegopelte but the axial region ofthe exoskeleton is unknown which accordingly is given amultistate uncertainty coding

36 Cephalic articulation fused (0) absent and (1) presentIn Tegopelte Saperion and Skioldia the articulation ofthe head with the thorax is non-functional and the entireexoskeleton forms a single tergite

37 Head shield overlap of thoracic tergites (0) overlapabsent or identical to overlap between thoracic segments (1)head shield covers first thoracic tergite only and (2) headshield covers mutliple anterior trunk tergites

38 Head shield articulates with reduced anterior thoracictergite (0) absent and (1) present Amongst taxa showing aposteriorly expanded head shield ie one that overlaps thethorax Edgecombe amp Ramskold (1999 Character 9) identifiedtwo distinct character states One of these ndash overlap of only theanteriormost thoracic tergite ndash was limited to taxa assigned tothe Liwiinae (sensu Fortey amp Theron 1994) and another ndashoverlap of multiple tergites with attachment to a reducedanterior thoracic tergite ndash to the Xandarellida None of theadditional taxa considered herein (including Buenaspis whichwas assigned to the Liwiinae by Budd 1999a) shows thesedistinctive morphological features However the expandedhead shields of Marrella Mimetaster and Burgessia which donot articulate with a narrow anterior thoracic tergite may behomologous with the expanded head shields of xandarellidsTo recognise this the degree of overlapping of the thorax andthe possession of the reduced anterior tergite are coded sepa-rately These characters are both coded as inapplicable to taxain which the head shield and thoracic tergites are fused(Character 36 State 1) The crustacean carapace which ismost similar to the head shield of Burgessia is seeminglyabsent in stem group crustaceans (Walossek amp Muller1998)

39 Trunk narrowed anteriorly relative to head shieldwidest posteriorly (0) absent and (1) present (cf Edgecombeamp Ramskold 1999 Character 14) The anterior narrowing ofthe trunk caused by the reduction of opisthosomal segment 1in xiphosurids (Weinbergina of the taxa analysed hereAndersen amp Selden 1997 Dunlop amp Selden 1997 Character14) is not considered to be homologous with the unusual shapeof the thorax in naraoiids recognised by their coding

40 Boundaries of anterior trunk segments reflexed antero-laterally (0) absent boundaries transverse or reflexed postero-laterally and (1) present

41 Joints between posterior tergites functional anteriorones variably fused (0) absent and (1) present

42 Posterior tergite bearing axial spine (0) absent and (1)present

Characters 40 to 42 are coded following Edgecombe ampRamskold (1999 characters 17 19 amp 23 respectively) Inaddition to the taxa considered here a thoracic axial spine ispresent in some olenelloid trilobites (see Lieberman 1998

Characters 73 amp 74) and in the aglaspidid Beckwithia (seeHesselbo 1989)

246 Posterior body termination 43 Postabdomen of seg-ments lacking appendages (0) absent and (1) present

44 Length of postabdomen (0) one segment (1) twosegments (2) three segments and (3) five segments In sometaxa a variable number of posterior thoracic segments bearcomplete tubular tergites and lack appendages In Aglaspisautapomorphically it seems that the posterior three thoracicsegments lack appendages (Briggs et al 1979 Hesselbo 1992)but have unmodified tergites These situations are recognisedas potentially homologous by the coding used here Character44 is coded as inapplicable to taxa lacking a postabdomen

45 Posterior tergites strongly curved in dorsal aspect com-pared to anterior tergites (0) absent and (1) present Asrecognised by Wills et al (1998a Character 17) in some taxawhere the tergites are distinct the curvature of thoracic tergitesincreases posteriorly so that the posterior tergites are highlycurved to semicircular in dorsal aspect This situation isnot known from crustaceans (except isopods) or stem grouparthropods

46 Posterior segments reduced and with highly reducedappendages (0) present and (1) absent In some taxa there area large number of posterior segments that are short sagitallyand have appendages that are much reduced in size comparedto anterior trunk appendages These somites are incorporatedinto the pygidium in some taxa but primitively they arecovered by tiny free trunk tergites In the derived state thetrunk somites and limbs are of a relatively constant size Thedistinction between these states can be seen when attempting tocount the number of segments that make up the body In taxashowing State 0 the number of segments is very high anddifficult to count whereas in other taxa the number ofpost-cephalic segments is easily assessed

47 Pygidium (0) absent and (1) present A pygidium isrecognised here as a posterior tagma consisting of a number offused segments under a single tergite which may or may notincorporate the post-segmental telson This is different to theuse of this term by Edgecombe amp Ramskold (1999) whichfollows Ramskold et al (1997) and very different to its use byWills et al (1998a p 53) as a synapomorphy of the TrilobitaThe present authorsrsquo definition recognises the situation inRetifacies where the spinose postsegmental telson is autapo-morphically not fused to the pygidium as homologous toother multisegmented posterior tagma which include thetelson The distinction between the pygidium and an expandedpost-segmental telson can also be seen in the position of theanus which is consistently in the posteriormost pre-telsonicsegment amongst putative arachnomorphs (see Character 48)In taxa with a pygidium the anal segment is fused into thepygidium (see eg Kuamaia Edgecombe amp Ramskold 1999fig 6) whereas in those taxa lacking a pygidium the anus isbetween the final trunk tergite and the telson

Ramskold et al (1997) argued that the posteriormost tergiteof xandarellids (which incorporates the anal segment) is nothomologous to the posterior tagma recognised as pygidiaherein because posterior thoracic tergites also cover multiplesegments in xandarellids (see Character 33) Evidence ofsegmentation of the pygidium in a variety of taxa suggests thatthe number of tergites fused to form the pygidium is greaterthan the number of appendages For example the pygidium ofKuamaia has two pairs of lateral spines but at least fourpairs of appendages (Hou amp Bergstrom 1997 Edgecombe ampRamskold 1999) and that of Triarthrus has only five axialrings posterior to the articulating ring but over 10 pairs ofappendages (Whittington amp Almond 1987) The fact thatdecoupling of tergites and segments is evident in the thorax of

184 TREVOR J COTTON AND SIMON J BRADDY

Xandarella and Cindarella does not necessarily suggest that asimilar more widespread decoupling in pygidia is the result ofa different developmental process and hence that they arenon-homologous Instead the situation in the xandarellidthorax is potentially homologous to that found (more widely)in pygidia and consequently the terminal xandarellid tergite isa pygidium It is unclear if decoupling of tergites and somites isprimitively limited to the terminal tergite or primitively aproperty of the arachnomorph post-cephalon as a whole

It has been suggested that the tiny pygidium of olenelloidtrilobites which probably best reflects the primitive trilobitestate consists only of the post-segmental telson (Harrington inMoore 1959) and therefore is not a true pygidium HoweverWhittington (1989) showed that the olenelloid pygidium con-sists of at least two and possibly as many as five or sixsegments and therefore is likely to be homologous with thepygidium of other trilobites

48 Position of the anus (0) terminal within telson and (1)at base of telson In crustaceans and stem group arthropodsthe anus is terminal or otherwise situated in the telson (egRehbachiella Walossek 1993 fig 15D) In putative arachno-morphs the anus is either at the junction of the posteriormostthoracic segment and the telson or ventral within a fusedpygidium In the case of taxa with a pygidium the anus isanterior to the posterior margin and where known positionedbetween the posteriormost pair of appendages suggesting thatit is anterior to the post-segmental telson (see eg OlenoidesWhittington 1980)

49 Pygidium with median keel (0) absent and (1) presentEdgecombe amp Ramskold (1999 Character 21) considered thepresence of a median keel on the pygidium as a synapomorphyuniting Soomaspis and Tarricoia They did not explain howthis structure could be distinguished from the raised pygidialaxis of trilobites Homology of these structures is not sup-ported here because the axis of more primitive trilobites(including Eoredlichia) is quite distinct morphologically fromthe naraoiid keel Budd (1999a) noted the presence of a keel onthe pygidium of Buenaspis and suggested (Budd 1999a p 102)that it may be an artefact of dorsoventral compaction How-ever contrary to the coding of Edgecombe amp Ramskold(1999) a keel is visible on the pygidium of Liwia convexa (Dzikamp Lendzion 1988 fig 4CD) preserved in full relief Thereforeit seems unlikely that the keel is a taphonomic artefact

50 Pygidium with broad-based median spine (0) absentand (1) present

51 Pygidium with lateral spines (0) present and (1) absentEdgecombe amp Ramskold (1999 Character 22) coded thepresence of a median spine and two pairs of lateral spines aspotentially homologous in Sinoburius Kuamaia and HelmetiaThe distribution of lateral spines is much wider than that ofterminal spines and therefore they are coded separately hereIn trilobites it has widely been recognised that lateral spinesrepresent the original segmentation of the pygidium Thiscannot be the case for median spines Characters 49 to 51are not applicable to taxa lacking a pygidium (Character 47State 0)

52 Expanded post-segmental telson (0) absent and (1)present In a range of taxa the posterior-most tergite is a large(relative to the thoracic tergites) structure that lacks anyevidence of segmentation and is interpreted as representing anexpanded tergite of the post-segmental telson from whichsegments are released anteriorly during ontogeny On the otherhand the posteriormost tergite of Marrella and probablyMimetaster is small compared to the thoracic tergites androunded This element probably represents the plesiomorphiceuarthropod telson The homology of this character in mosttaxa is clear and only doubtful in Retifacies in which it may

be segmented The figures of Hou amp Bergstrom (1997) providelittle unequivocal evidence for a telson in Retifacies but thepresence of this structure is very clear in colour photographs(eg Chen et al 1996 fig 198A) However these figures donot convincingly demonstrate the segmentation of the telsonRetifacies is interpreted here as being unique in possessingboth a pygidium and an expanded post-segmental telson

53 Telson shape (0) spinose and (1) paddle-shaped Theshape of the expanded telsons identified above varies frombroadly spinose to flattened and paddle-like This differencecan be recognised both on the basis of the ratio of length tobasal width (spinose telsons are relatively long) and by thechange in width posteriorly (spinose telsons reduce in widthposteriorly whereas paddle-shaped telsons increase in width)In eurypterids a wide range of telson shapes is found includ-ing both character states described here It is likely that theplesiomorphic condition is a spinose telson This character isinapplicable to taxa lacking an expanded telson

54 Post-ventral furcae (0) absent and (1) present Thischaracter recognises the potential homology of unsegmentedpaired structures that articulate with the segment immediatelyanterior to the telson The potential homology of these struc-tures (the post-ventral plates) in Emeraldella and Aglaspis wasrecognised by Wills et al (1998a Character 70) and inEmeraldella and Sidneyia by Edgecombe amp Ramskold (1999Character 25) The homology of the segmented pygidial caudalfurcae of Olenoides (see Whittington 1975a 1980) with thesestructures is considered doubtful and coded as equivocal

Despite their distinctive morphology the long caudualfurcae of Cheloniellon may be homologous with the furcae ofAglaspis Emeraldella and Sidneyia This is supported by thecaudal furcae of the Ordovician cheloniellid Duslia which aremore similar in morphology to those of Emeraldella than ofCheloniellon Chlupac (1988 pp 614ndash16) considered thesestructures to be on the terminal tergite in Duslia However afaint tergite boundary is apparent posterior to the attachmentof the furcae (see Chlupac 1988 pl 57 fig 4) Therefore theyare considered here to have been attached to the pre-telsonicsegment as in Cheloniellon Chlupac (1988) and Dunlop ampSelden (1997) supported a close relationship between Dusliaand Cheloniellon

25 MethodsAll analyses were carried out using PAUP Version 40b4a(Swofford 1999) and unless otherwise stated used heuristicsearches with one thousand random addition sequencereplicates The software packages MacClade Version 307(Maddison amp Maddison 1997) and RadCon (Thorley amp Page2000) were used for comparing trees and investigating patternsof character evolution Tree length and other statistics (theConsistency Index CI and Retention Index RI) were calcu-lated by PAUP and MacClade with uninformative charactersexcluded Analyses were run separately using each of thedifferent codings for aglaspidids

Characters were treated as unordered and of equal weight inmost analyses To assess the influence of this assumption fourof the eight multistate characters which have states that areintermediate between others (Wilkinson 1992) were treated asordered in some analyses These characters are the number ofcephalic segments (Character 4) the length of raptorialappendage spines (Character 6) the degree of overlap of thethorax by the head-shield (Character 37) and the number ofsegments making up the postabdomen (Character 44) The lastof these is uninformative when not ordered because only onetaxon shows each of states 0 1 and 3

Support for individual nodes was assessed by bootstrapanalysis (Felsenstein 1985) and by calculating Bremer support

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 185

indices (Bremer 1988 1994) These methods measure twodistinct aspects of support for phylogenetic hypotheses Treesor nodes may be considered well supported (1) to the extent towhich alternative topologies are much less parsimonious asmeasured by the support index (Wilkinson 1996) or (2) wherethey are consistent with a large proportion of characters sothat character sampling is unlikely to have had much influenceon topology as assessed by bootstrapping (Page 1996) Boot-strapping was performed with 100 bootstrap replicates eachof 10 addition sequence replicates Bremer support indicesare based on heuristic searches with 100 addition sequencereplicates

Three pairs of terminals are included in Table 2 that apartfrom missing data are coded identically These are Helmetiaand Kuamaia Soomaspis and Tariccoia and Saperion andSkioldia These pairs are lsquotaxonomic equivalentsrsquo removingthe taxon with most missing data will not influence theposition of the other They can be recoded as single terminalsaccording to the principal of safe taxonomic reduction(Wilkinson 1995a b) and were not considered separately in theanalyses presented here Therefore cladistic analyses werecarried out on the basis of 33 terminals Edgecombe ampRamskold (1999) ignored this and coded the same three pairsof taxa and Eoredlichia and Olenoides identically for theirmore limited set of characters

26 ResultsAnalysis with all characters unordered and using the lsquoAglaspi-dida 1rsquo coding found nine most parsimonious trees (MPTs)each 126 steps long (CI=0middot556 RI=0middot763) Using the lsquoAglas-pidida 2rsquo coding resulted in 18 MPTs of length 125 (CI=0middot560RI=0middot765) Nine of these 18 trees were those found with thefirst coding of aglaspidids The strict component consensus(sensu Wilkinson 1994) of the 27 unique trees found acrossboth analyses is shown in Figure 8 All trees supported themonophyly of the Marrellomorpha (Marrella and Mimetaster)and of a clade including all other taxa except Crustacea (Clade1 of Fig 8) The relationships between these two clades and theCrustacea were unresolved equal numbers of trees in bothanalyses supported each of the topologies [Clade 1 (CrustaceaMarrellomorpha)] [Crustacea (Marrellomorpha Clade 1)]and [Marrellomorpha (Crustacea Clade 1)] All trees placedBurgessia as the sister taxon to the rest of Clade 1 andsupported the division of the rest of Clade 1 into two majorsister groups The topology of one of these clades (Clade 2)which included trilobites helmetiids tegopeltids naraoiidsxandarellids and Retifacies was stable with respect to thecoding of aglaspidids The other clade (Clade 3) consists of aclade of megacheirans and chelicerates (Clade 4) and a cladeincluding aglaspidids Lemoneites Paleomerus CheloniellonEmeraldella and Sidneyia (Clade 5) This latter clade is un-resolved in the strict consensus of all 27 trees (Fig 8) and thestrict consensus of the 18 MPTs found using the lsquoAglaspidida2rsquo coding but was fully resolved in the consensus of the ninetrees found with the lsquoAglaspidida 1rsquo coding Within themegacheiran-chelicerate clade all trees found Fortiforceps tobe the sister group to all other taxa Yohoia the sister group tochelicerates and Alalcomenaeus and Leanchoilia to form aclade Jianfengia is equally parsimoniously placed as the sistergroup to the Yohoia-chelicerate clade or as the sister group tothis clade and the Alalcomenaeus-Leanchoilia clade

Treating characters 4 6 37 and 44 as ordered had a minimaleffect on most parsimonious topology Nine MPTs of 136 steps(CI=0middot515 RI=0middot743) were found with the lsquoAglaspidida 1rsquocoding and 12 MPTs 135 steps in length (CI=0middot519 RI=0middot745)with the lsquoAglaspidida 2rsquo coding These trees all supported a(Crustacea Clade 1) clade excluding the marrellomorphs

Apart from this the strict consensus of these 21 trees isidentical to the consensus of the 27 trees found with allcharacters treated as unordered

Across all analyses four different topologies for Clade 5 (ofFig 8) were found (Fig 9) The distribution of these topologiesacross the four analyses described above and across the sameanalyses repeated after reweighting of each character in pro-portion to their Rescaled Consistency Index (see Farris 1989)from the first analysis is shown in Table 3 The instability of

Figure 8 Strict consensus of 27 MPTs from four separate analyses ofarachnomorph phylogeny with both interpretations of aglaspididmorphology and with some characters treated as ordered or allcharacters unordered Clades referred to in the text are numbered

Figure 9 Alternative equally parsimonious resolutions of clade 5 ofFig 8 (see Table 3)

186 TREVOR J COTTON AND SIMON J BRADDY

this clade was to some extent caused by the inclusion ofLemoneites and Paleomerus which could each be coded foronly 27 of the 54 characters used Analysis with these two taxaexcluded and characters treated as unordered found treescompatible only with Topology D of Figure 9 irrespective ofthe coding used for aglaspidids Analysis with some characterstreated as ordered and using either aglaspidid coding foundthree trees compatible only with Topology B of Figure 9 andthree compatible only with Topology D of Figure 9 ThereforeTopology D is the preferred topology for Clade 5 as shown inTable 3

One of the MPTs resulting from the lsquoAglaspidida 2rsquo coding(with all characters treated as unordered) was chosen as thebasis for further discussion because this tree combined thepreferred topology of Clade 5 with the (Crustacea Clade 1)group that was favoured when some characters were treated asordered Bootstrap percentages and Bremer support values forthis tree (using the Aglaspidida 2 coding) are shown in Figure10 Apomorphies for all ingroup nodes of this tree are shownin Figure 10A

3 Discussion

31 Phylogenetic resultsThe results of the cladistic analyses presented above arewell-resolved and robust with respect to different analyticalparameters Most of the taxa considered form a clade (Clade 1of Fig 8) that is more closely related to chelicerates than tocrustaceans in all analyses and therefore is recognised asthe Arachnomorpha as defined by Chen et al (1996) andRamskold et al (1996) The monophyly of the Marrellomor-pha was supported in all analyses The marrellomorphs werefound to be the sister group to a Crustacea+Clade 1 group inmost MPTs but in a minority of trees formed the sister groupto Clade 1 alone According to the latter result they should beincluded within the Arachnomorpha but according to the firstexcluded from it Both of these alternatives have been sup-ported in previous studies (eg Hou amp Bergstrom 1997 Willset al 1998a respectively) In addition to the marrellomorphsthe Fuxianhuiida Bousfield 1995 and Canadaspidida havebeen placed in the Euarthropoda outside the main mandibu-late and arachnomorph clades (Hou amp Bergstrom 1997)Further study of these taxa of the crustacean or mandibulatestem group and of the marrellomorphs is necessary before the

position of the Marrellomorpha can be resolved Provisionallymarrellomorphs are excluded from the Arachnomorphafollowing the majority of analyses above and Wills et al(1994)

The Burgess Shale arthropod Burgessia was placed as thesister group to remaining arachnomorphs in all analyses OtherArachnomorpha form two major clades one consisting of thelsquotrilobite-alliedrsquo arachnomorphs analysed by Edgecombe ampRamskold (1999) here termed lsquoTrilobitomorpharsquo (defined asTrilobita and all taxa more closely related to Trilobita thanChelicerata) and the other (lsquochelicerate-alliedrsquo) clade includingchelicerates lsquogreat appendagersquo arthropods aglaspididsLemoneites Paleomerus Sidneyia and Emeraldella The latteris here termed lsquoCheliceramorpharsquo (defined as Chelicerata andall taxa more closely related to Chelicerata than Trilobita) Thetopology of the lsquotrilobite-alliedrsquo clade was similar to that foundby Edgecombe amp Ramskold (1999) but the present authorsrsquoresults are more completely resolved unambiguously support-ing the sister-group relationship between trilobites andHelmetiida suggested in the previous study Within thelsquochelicerate-alliedrsquo clade Aglaspidids Paleomerus Lemoneitesand the Burgess Shale Emeraldella and Sidneyia form a cladein opposition to megacheirans and chelicerates

The present authorsrsquo analysis supports a new hypothesis ofthe origin of the chelicerates from within a paraphyleticassemblage of megacheiran arthropods and confirms pyc-nogonids as a sister group to the euchelicerates Previoushypotheses of the chelicerate sister group (see Section 1) aremuch less parsimonious The shortest trees supporting aCheloniellon-chelicerate clade are six steps longer than theMPTs and those supporting an aglaspidid-chelicerate cladeseven steps longer All megacheirans and chelicerates areunited by the flap-shaped rounded exopods and modificationof the second cephalic segment endopods into anteriorlydirected spinose raptorial organs which share a number ofdetailed similarities A clade including all of these taxa apartfrom Fortiforceps is supported by the synapomorphic loss ofthe antennae (or antennulae of crustaceans) and the longerlength of spinose projections of the second cephalic appendageendopods Within this clade Yohoia shares the loss of cephalicexopods the loss of thoracic endopods and a postabdomen oftubular tergites with chelicerates

The poor resolution of Clade 5 (of Fig 8) was partly a resultof the poor preservation of and consequent high proportion ofmissing data for Lemoneites and Paleomerus When these two

Table 3 Numbers of most parsimonious trees supporting each of the alternative topologies for clade 4 of Fig 7 under different analyticalconditions (RCI=Revealed Consistency Index)

Aglaspididcoding

Characters 4 6 36and 43 ordered

Charactersreweighted by RCI

Paleomerus andLemoneites excluded

Topology (see Fig 8)

A B C D

1 91 X 3 3 31 X 61 X X 31 X 61 X X 3 32 9 92 X 3 3 3 32 X 62 X X 32 X 62 X X 3 3Total 6 18 12 54

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 187

taxa were excluded from the analyses only two (B and D) ofthe four topologies found in the complete analysis remainedmost parsimonious (see Fig 9 and Table 3) Secondly Bremersupport values throughout much of the lsquochelicerate-alliedrsquoclade (Clade 3 of Fig 8) were improved when these taxa wereexcluded (see Fig 11)

Dunlop (1999 p 258) suggested that in reconstructing thechelicerate stem lineage lsquowe might predict that the two mostsignificant changes towards the chelicerate condition are thereduction of the antennae and the formation of the nextappendages into a clawrsquo According to the hypothesis pre-sented here both of these adaptations were achieved in EarlyCambrian chelicerate ancestors The recognition of the loss ofthe antennae in the stem group of the chelicerates and thephylogenetic hypothesis presented here suggest that chelicer-ates are most closely related to taxa with only a singleadditional segment fused to the crown group eucheliceratehead of four fused somites Previous hypotheses have derivedchelicerates from Palaeozoic arthropods with a longer head(eg Cheloniellon Sturmer amp Bergstrom 1978 SanctacarisBriggs amp Collins 1988 Emeraldella Bruton amp Whittington1983) The plesiomorphic pattern of head segmentation ismatched in pycnogonids Bergstrom et al (1980) suggestedthat only the anterior four pairs of appendages are incorpo-rated into the head This supports the view that pycnogonidsare primitive with respect to other chelicerates which showmore or less complete fusion of this primitive head and threethoracic segments including Solpugida and Pseudoscorpiones

The present study suggests that a shift from gnathobasicbenthic feeding to a more vagrant lifestyle and raptorialappendage feeding occurred very early in the chelicerate lin-eage Appendage feeding was clearly an important adaptation

Figure 10 One of 18 equally parsimonious cladograms with lsquoAglaspidida 2rsquo coding and all characters unorderedthat is most congruent with results from other analyses Non-terminal branch lengths scaled to reflect number ofapomorphies (A) ACCTRAN apomorphy scheme Character numbers shown above boxes and states shownbelow boxes as in Table 2 and the text Character consistency indices are indicated by shading (B) Levels ofsupport for individual nodes Bremer support indices shown in bold type above branches Bootstrap percentagesin italic type below branches for all nodes with relative frequencies greater than 50

Figure 11 Bremer support values for clades within the lsquochelicerate-alliedrsquo arachnomorphs (clade 3 of Fig 8) when all taxa wereincluded (normal weight font above nodes) and with Lemoneites andPaleomerus excluded (bold type below nodes)

188 TREVOR J COTTON AND SIMON J BRADDY

for the terrestrialization of chelicerates (Dunlop 1997 p 69Dunlop amp Webster 1999) but the model proposed heresuggests that this considerably pre-dated terrestrializationGnathobasic feeding in xiphosurans and eurypterids mayrepresent a reversal to a more primitve benthic arachnomorphlifestyle from a more pycnogonid-like ancestor

32 Evolution of the arachnomorph headThe extant arthropod classes show highly conserved patternsof head segmentation (eg Wills et al 1994) In contrast therehas been a near consensus amongst Cambrian arthropodworkers (although rarely based on an explicit phylogenetichypothesis) that patterns of head segmentation are highlyhomoplastic and hence of little systematic significanceSturmer amp Bergstrom (1978 pp 78ndash9) suggested that lsquoevenclosely related forms may have different numbers of headsegments and appendagesrsquo Bruton amp Whittington (1983pp 576ndash7) that lsquodiscussion on fossil arthropod relationshipsbased on head segmentation appears to be largely irrel-evant and at best speculativersquo and Delle Cave amp Simonetta(1991 p 191) that lsquothere are no obvious phyletic affinitiesbetween genera having the same number of cephalized seg-mentsrsquo Most recently Bergstrom amp Hou (1997 p 104) con-cluded that lsquothe segmental length of the head shield seemsto be of no relevance to the discussion and discrimination ofevolutionary lineagesrsquo It has been claimed that this view issupported by cladistic analyses in which patterns of tagmosishave been found to be rather poor at defining major arthropodclades (Briggs et al 1992 Wills et al 1994)

This suggestion is not only of relevance to systematics buthas featured prominently in recent debates surrounding thenature of the lsquoCambrian explosionrsquo (see Budd amp Jensen 2000for a recent review) Gould (1989 1991) argued that theapparent plasticity of head segmentation in Cambrian com-pared to post-Cambrian arthropod evolution suggests thatbody-plan evolution was only constrained after the Cambrianexplosion According to this view the origin of new highertaxa with distinct body-plans was only possible before theonset of these constraints (eg Valentine 1995) Taxa hererecognised as arachnomorphs clearly played a major role in thedevelopment of Gouldrsquos hypothesis This suggestion has pre-viously been assessed in terms of overall morphological diver-sity which is clearly rather distinct from Gouldrsquos concept ofdisparity Wills et al (1998b) showed that there was littlecorrelation between overall morphological disparity and de-gree of limb specialisation and that the latter was phylogeneti-cally highly plastic They remarked that (Wills et al 1998bp 64) lsquothis finding has important implications for models ofarthropod phylogeny and evolution that attribute overidingimportance to head segmentationrsquo but did not explicitlyexamine head segmentation although it is likely to have beena major component of the index of tagmosis employed

The results of the present study suggest that arguments thatarthropod head segmentation was unusually labile during theCambrian are poorly founded Rather only four major pat-terns of euarthropod head segmentation are identified Theplesiomorphic euarthropod state according to both Walossekamp Muller (1997 1998) and Scholtz (1997) which may moreproperly be the plesiomorphic state for the euarthropod crowngroup only (depending on the phylogenetic position of theMarrellomorpha) consists of four post-acronal segmentsbearing the antennae and three pairs of biramous limbs Thepresent authors suggest that the term lsquocephalonrsquo be restrictedto this kind of head which is found in stem group mandibu-lates in addition to many arachnomorphs Crown groupmandibulates share a lsquobimaxillary headrsquo in which an addi-tional pair of appendages the labium or second maxillae are

incorporated into the head but in crustaceans these are notfused to the carapace (Scholtz 1997) Finally two distinctforms of head tagmosis are found in chelicerates A head withfour pairs of appendages (but without antennae giving a totalof five segments incorporated into the head) which has beencalled the lsquocephalosomarsquo is present in pycnogonids (Walossekamp Dunlop 2002 pp 434ndash5) and probably also in some euch-elicerates (J A Dunlop pers comm 2003) The lsquoprosomarsquo ofmost euchelicerates consists of this cephalosoma and twoadditional segments

In general these patterns of head segmentation are highlyphylogenetically conserved (see Fig 12) Only three homo-plastic changes namely reversal to a more primitive conditionin the Leanchoilia-Alalcomenaeus clade and the convergentorigin of a five-segmented head in Clade 3 (of Fig 8) and theXandarellida are necessary to optimise character 4 onto themost parsimonious cladogram There are also transitions toautapomorphic states in Emeraldella and Sidneyia In the caseof Sidneyia however an argument could be made (followingBergstrom amp Sturmer 1978 see discussion of Character 4) thatthe head consists of five segments as in closely related taxabut autapomorphically each segment has a separate tergiteThis would further increase the phylogenetic stability of headsegmentation Also following Bergstrom amp Sturmer (1978) thehead of Cheloniellon could be considered to consist of twotergites making it more similar to the head of Emeraldella(with six pairs of appendages)

Figure 12 Cladogram from Fig 10 showing changes in head segmen-tation Bold type shows abbreviated head segmentation formulaAbbreviations (Ant) antennae (BA) biramous appendage pairs (Ch)chelicerae (GA) great appendages (UA) uniramous appendage pairsand (X) segment without appendages Synapomorphy A is the auta-pomorphic reduction in head length to a single segment in SidneyiaAlternatively this may represent secondary division of the headshield Synapomorphy B is the increase in the length of the head inEmeraldella to six segments

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 189

4 Acknowledgements

We thank D E G Briggs J A Cotton J A Dunlop R AFortey S Harris R A Moore P Selden O E Tetlie andP Van Roy for comments on earlier drafts of this work Themanuscript was reviewed by M Wills F Schram and oneanonymous referee This research was supported by the Geo-logical Society of London Curry Research Studentshipawarded to TJC by the Department of Earth Sciences Uni-versity of Bristol and by the Department of PalaeontologyThe Natural History Museum London TJC would like tothank C Labandeira J Thompson and R Chapman fortheir hospitality at the National Museum of Natural HistoryWashington DC USA and A Olcott S Schellenberg andJ Young for making his time there so enjoyable

5 References

Abzhanov A Popadic A amp Kaufman T C 1999 Chelicerate Hoxgenes and the homology of arthropod segments Evolution andDevelopment 1 77ndash89

Akam M 2000 Arthropods developmental diversity within a (super)phylum Proceedings of the National Academy of Sciences USA97 4438ndash41

Aldridge R J Gabbott S E amp Theron J N 2001 The Soom ShaleIn Briggs D E G amp Crowther P R (eds) Palaeobiology II340ndash2 Oxford Blackwell Science

Anderson L I amp Selden P A 1997 Opisthosomal fusion andphylogeny of Palaeozoic Xiphosura Lethaia 30 19ndash31

Arango C P 2002 Morphological phylogenetics of the sea spiders(Arthropoda Pycnogonida) Organisms Diversity and Evolution2 107ndash25

Ax P 1986 Das phylogenetische System Stuttgart Gustav FischerVerlag

Babcock L E amp Zhang Wentang In press New non-mineralizedarthropods from the Chengjiang Lagerstatte Lower CambrianYunnan China Palaeontologia Cathyana

Bartels C Briggs D E G amp Brassel G 1998 The fossils ofthe Hunsruck Slate Marine life in the Devonian CambridgeCambridge University Press

Bergstrom J 1971 Paleomerus ndash merostome or merostomoid Lethaia4 393ndash401

Bergstrom J 1979 Morphology of fossil arthropods as a guideto phylogenetic relationships In Gupta A P (ed) ArthropodPhylogeny 3ndash56 New York NY Van Nostrand Reinhold Co

Bergstrom J 1992 The oldest arthropods and the origin of theCrustacea Acta Zoologica 73 287ndash91

Bergstrom J Sturmer W amp Winter G 1980 PalaeoisopusPalaeopantopodus and Palaeothea pycnogonid arthropodsfrom the Lower Devonian Hunsruck Slate West GermanyPalaontologische Zeitschrift 54 7ndash54

Bergstrom J amp Brassel G 1984 Legs in the trilobite Rhenops fromthe Lower Devonian Hunsruck Slate Lethaia 17 67ndash72

Bergstrom J amp Hou X 1998 Chengjiang arthropods and theirbearing on early arthropod evolution In Edgecombe G D(ed) Arthropod fossils and phylogeny 151ndash84 New York NYColumbia University Press

Boudreaux H B 1979 Significance of intersegmental tendon systemin arthropod phylogeny and a monophyletic classification ofArthropoda In Gupta A P (ed) Arthropod Phylogeny 551ndash86New York NY Van Nostrand Reinhold Co

Bousfield E L 1995 A contribution to the natural classification ofLower and Middle Cambrian arthropods food-gathering andfeeding mechanisms Amphipacifica 2 3ndash33

Braddy S J Aldridge R J Gabbott S E amp Theron J N 1999Lamellate book-gills in a late Ordovician eurypterid from theSoom Shale South Africa support for a eurypterid-scorpionclade Lethaia 32 72ndash4

Braddy S J amp Almond J E 1999 Eurypterid trackways from theTable Mountain Group (Ordovician) of South Africa Journal ofAfrican Earth Sciences 29 165ndash77

Bremer K 1988 The limits of amino acid sequence data in angio-sperm phylogenetic reconstruction Evolution 42 795ndash803

Bremer K 1994 Branch support and tree stability Cladistics 10295ndash304

Briggs D E G 1998 Review of Arthropods from the LowerCambrian Chengjiang fauna southwest China by HouXianguang and Jan Bergstrom Fossils and Strata 45

Scandinavian University Press Oslo Norway 1997 Palaeo-geography Palaeoclimatology Palaeoecology 143 193ndash4

Briggs D E G Bruton D L amp Whittington H B 1979 Append-ages of the arthropod Aglaspis spinifer (Upper CambrianWisconsin) and their significance Palaeontology 22 167ndash80

Briggs D E G Fortey R A amp Wills M A 1992 Morphologicaldisparity in the Cambrian Science 256 1670ndash3

Briggs D E G Fortey R A amp Wills MA 1993 How big was theCambrian explosion A taxonomic and morphologic comparisonof Cambrian and Recent arthropods In Lees D R amp EdwardsD (eds) Evolutionary patterns and processes 33ndash44 LondonLinnean Society of London Linnean Society Symposium Series

Briggs D E G Erwin D H amp Collier F J 1994 The fossils of theBurgess Shale Washington DC Smithsonian Institution Press

Briggs D E G amp Bartels C 2001 New arthropods from the LowerDevonian Hunsruck Slate (Lower Emsian Rhenish Massifwestern Germany) Palaeontology 44 275ndash304

Briggs D E G amp Bartels C 2002 Magnoculocaris a new name forthe arthropod Magnoculus Briggs and Bartels 2001 from theLower Devonian Hunsruck Slate Germany Palaeontology 45419

Briggs D E G amp Collins D 1988 A Middle Cambrian cheliceratefrom Mount Stephen British Columbia Palaeontology 31 779ndash98

Briggs D E G amp Collins D 1999 The arthropod Alalcomenaeuscambricus Simonetta from the Middle Cambrian Burgess Shale ofBritish Columbia Paleontology 42 953ndash78

Briggs D E G amp Fortey R A 1982 The cuticle of aglaspididarthropods a red-herring in the early history of vertebratesLethaia 15 25ndash9

Briggs D E G amp Fortey R A 1989 The early radiation andrelationships of the major arthropod groups Science 246 241ndash3

Briggs D E G amp Fortey R A 1992 The early Cambrian radiationof arthropods In Lipps J H amp Signor P W (eds) Origin andEarly Evolution of the Metazoa 335ndash373 New York NY PlenumPress

Briggs D E G amp Robison R A 1984 Exceptionally preservednontrilobite arthropods and Anomalocaris from the MiddleCambrian of Utah University of Kansas PaleontologicalContributions Paper 111 1ndash23

Broili F 1932 Ein neuer Crustacee aus dem rheinischen UnterdevonSitzungsberichte der Mathematisch-Naturwissenschaftlichen(Abteilung) Klasse der Bayerischen Akademie der Wissenschaftenzu Munchen 1932 27ndash38

Broili F 1933 Ein zweites Exemplar von Cheloniellon Sitzungsber-ichte der Bayerischen Akademie der Wissenschaften 1933 11ndash32

Bruton D L 1981 The arthropod Sidneyia inexpectans MiddleCambrian Burgess Shale British Columbia Philosophical Trans-actions of the Royal Society of London Series B 295 619ndash56

Bruton D L amp Whittington H B 1983 Emeraldella and Leanchoiliatwo arthropods from the Burgess Shale British Columbia Philo-sophical Transactions of the Royal Society of London Series B 300553ndash85

Budd G E 1995 Kleptothule rasmusseni gen et sp nov anolenellid-like trilobite from the Sirius Passet fauna (BuenFormation Lower Cambrian North Greenland) Transactions ofthe Royal Society of Edinburgh Earth Sciences 86 1ndash12

Budd G E 1996a Progress and problems in arthropod phylogenyTrends in Ecology and Evolution 11 356ndash8

Budd G E 1996b The morphology of Opabinia regalis and thereconstruction of the arthropod stem-group Lethaia 29 1ndash14

Budd G E 1997 Stem-group arthropods from the Lower CambrianSirius Passet fauna of North Greenland In Fortey R A ampThomas R H (eds) Arthropod Relationships 125ndash38 LondonChapman and Hall Systematics Association Special Volume 55

Budd G E 1999a A nektaspid arthropod from the Early CambrianSirius Passet fauna with a description of retrodeformation basedon functional morphology Palaeontology 42 99ndash122

Budd G E 1999b The morphology and phylogenetic significance ofKerygmachela kiergkegaardi Budd (Buen Formation LowerCambrian N Greenland) Transactions of the Royal Society ofEdinburgh Earth Sciences 89 (for 1998) 249ndash290

Budd G E 2002 A palaeontological solution to the arthropod headproblem Nature 417 271ndash5

Budd G E amp Jensen S 2000 A critical reappraisal of the fossilrecord of the bilaterian phyla Biological Reviews 75 253ndash95

Chen J Zhou G Zhu M amp Yeh K 1996 The Chengjiang biota ndasha unique window of the Cambrian explosion Taichung NationalMuseum of Natural Science [In Chinese]

Chen J Edgecombe G D amp Ramskold L 1997 Morphological andecological disparity in naraoiids (Arthropoda) from the Early

190 TREVOR J COTTON AND SIMON J BRADDY

Cambrian Chengjiang fauna China Records of the AustralianMuseum 49 1ndash24

Chen J amp Zhou G 1997 Biology of the Chengjiang fauna In ChenJ Cheng Y amp Iten H V (eds) The Cambrian explosion and thefossil record Bulletin of the National Museum of Natural ScienceTaiwan 10 11ndash106

Child C A 1997 Some deep-sea Pycnogonida from Argentine slopeand basin Proceedings of the Biological Society of Washington110 128ndash42

Chlupac I 1988 The enigmatic arthropod Duslia from the Ordovicianof Czechslovakia Palaeontology 31 611ndash20

Chlupac I 1995 Lower Cambrian arthropods from the PasekyShale (Barrandian area Czech Republic) Journal of the CzechGeological Society 40 9ndash36

Chlupac I amp Havlıcek V 1965 Kodymirus n g a new aglaspidmerostome of the Cambrian of Bohemia Sbornık GeologickechVid Paleontologie 6 7ndash20

Cisne J L 1975 Anatomy of Triarthrus and the relationships of theTrilobita Fossils and Strata 4 45ndash63

Cisne J L 1981 Triarthrus eatoni (Trilobita) anatomy of its exo-skeletal skeletomuscular and digestive systems Palaeonto-graphica Americana 9 95ndash142

Conway Morris S 1982 Atlas of the Burgess Shale London ThePalaeontological Association

Conway Morris S 1989 The persistence of Burgess Shale-type faunasimplications for the evolution of deeper-water faunas Trans-actions of the Royal Society of Edinburgh Earth Sciences 80271ndash83

Conway Morris S 1998 The crucible of creation Oxford OxfordUniversity Press

Cotton T J 1999 Review of Arthropod fossils and phylogenyPalaeogeography Palaeoclimatology Palaeoecology 151 341ndash3

Cotton T J 2002 The phylogeny and morphological evolution ofCambrian trilobites and their relatives Unpublished PhD ThesisUniversity of Bristol

Damen W G M Hausdorf M Seyfarth E A amp Tautz D 1998 Aconserved mode of head segmentation in arthropods revealed bythe expression pattern of Hox genes in a spider Proceedings of theNational Academy of Sciences USA 95 10665ndash70

Delle Cave L amp Simonetta A M 1991 Early Palaeozoic arthropodsand problems of arthropod phylogeny with some notes on taxa ofdoubtful affinities In Simonetta A M amp Conway Morris S(eds) The early evolution of Metazoa and the significance ofproblematic taxa 189ndash244 Cambridge Cambridge UniversityPress

Dewel R A amp Dewel W C 1997 The place of tardigrades inarthropod evolution In Fortey R A amp Thomas R H (eds)Arthropod Relationships 109ndash23 London Chapman and HallSystematics Association Special Volume 55

Dunlop J A 1997 Palaeozoic arachnids and their significance forarachnid phylogeny Proceedings of the 16th European Colloquiumof Arachnology 65ndash82 Siedlce Poland

Dunlop J A 1998 The origins of tetrapulmonate booklungs and theirsignificance for chelicerate phylogeny In Selden P A (ed)Proceedings of the 17th European Colloquium of ArachnologyEdinburgh 1997 pp 9ndash16

Dunlop J A 1999 Pasando revista a la evolucion de los quelicerados[A review of chelicerate evolution] In Melic A DeHaro J JMedez M amp Ribera I (eds) Volumen Monografico Evolucion yFilogenia de Arthropoda Boletin de la Sociedad EntolomologicaAragonesa 26 255ndash272 [In Spanish and English]

Dunlop J A amp Selden P A 1997 The early history and phylogenyof the chelicerates In Fortey R A amp Thomas R H (eds)Arthropod relationships Systematics Association Special Volume55 221ndash35 London Chapman and Hall

Dunlop J A amp Webster M 1999 Fossil evidence terrestrializationand arachnid phylogeny Journal of Arachnology 27 86ndash93

Dzik J 1993 Early metazoan evolution and the meaning of its fossilrecord Evolutionary Biology 27 339ndash86

Dzik J amp Lendzion K 1988 The oldest arthropods of the EastEuropean Platform Lethaia 21 29ndash38

Edgecombe G D Wilson G D F Colgan D J Gray M R ampCassis G 2000 Arthropod cladistics combined analysisof Histone H3 and U2 snRNA sequences and morphologyCladistics 16 155ndash203

Edgecombe G D amp Ramskold L 1999 Relationships of CambrianArachnata and the systematic position of Trilobita Journal ofPaleontology 73 263ndash87

Emerson M J amp Schram F R 1997 Theories patterns and realitygame plan for arthropod phylogeny In Fortey R A amp Thomas

R H (eds) Arthropod Relationships Systematics AssociationSpecial Volume 55 67ndash87 London Chapman and Hall

Farris J S 1989 The Retention Index and the Rescaled ConsistencyIndex Cladistics 5 417ndash19

Felsenstein J 1985 Confidence limits on phylogenies an approachusing the bootstrap Evolution 39 783ndash91

Flower R H 1968 Merostomes from a Cotter Horizon of the El PasoGroup New Mexico Bureau of Mines and Mineral ResourcesMemoir 22 35ndash44

Fortey R A 1990a Trilobite evolution and systematics In MikulicD G (ed) Arthropod paleobiology 44ndash65 Short Courses inPaleontology 3 Knoxville Tenessee The Paleontological Society

Fortey R A 1990b Ontogeny hypostome attachment and trilobiteclassification Palaeontology 33 529ndash76

Fortey R A 1997 Classification In Kaesler R L (ed) Treatise oninvertebrate paleontology Part O Arthropoda 1 (revised) 289ndash302Boulder Colorado amp Lawrence Kansas Geological Society ofAmerica and University of Kansas Press

Fortey R A amp Theron J N 1994 A new Ordovician arthropodSoomaspis and the agnostid problem Palaeontology 37 841ndash61

Fortey R A amp Whittington H B 1989 The Trilobita as a naturalgroup Historical Biology 2 125ndash38

Fry W G 1978 A classification within the pycnogonids ZoologicalJournal of the Linnean Society of London 63 35ndash58

Giribet G Edgecombe G D amp Wheeler W C 2001 Arthropodphylogeny based on eight molecular loci and morphology Nature413 157ndash61

Gould S J 1989 Wonderful life The Burgess Shale and the nature ofhistory New York NY Norton

Gould S J 1991 The disparity of the Burgess Shale arthropod faunaand the limits of cladistic analysis why we must strive to quantifymorphospace Paleobiology 17 411ndash23

Gurich G 1931 Mimetaster hexagonalis ein neuer Kruster aus demunterdevonischen Bundenbacher Dachschiefer PalaontologischeZeitschrift 12 204ndash38

Hammann W Laske R amp Pillola G L 1990 Tariccoia arrusensisngnsp an unusual trilobite-like arthropod Rediscovery of thelsquophylocaridrsquo beds of Taricco (1922) in the Ordovician lsquoPuddingarsquosequence of Sardinia Bolletino della Societa PaleontologicaItaliana 29 163ndash78

Haas M S Brown S J amp Beeman R W 2001 Pondering theprocephalon the segmental origin of the labrum DevelopmentGenes and Evolution 211 89ndash95

Heider K 1913 Entwicklungsgeschichte und Morphologie derWirbellosen Die Kultur der Gegenwart Theil 3 Abt 4 2 176ndash332

Henriksen K 1928 Critical notes upon Cambrian arthropodsdescribed by Charles D Walcott Videnskabelige Meddelelser fraDansk Naturhistorisk Forening 86 1ndash20

Hesselbo S P 1988 Trace fossils of Cambrian aglaspidid arthropodsLethaia 21 139ndash46

Hesselbo S P 1989 The aglaspidid arthropod Beckwithia from theCambrian of Utah and Wisconsin Journal of Paleontology 63636ndash42

Hesselbo S P 1992 Aglaspidida (Arthropoda) from the UpperCambrian of Wisconsin Journal of Paleontology 66 885ndash923

Hou X 1987a Three new large arthropods from Lower CambrianChengjiang fauna eastern Yunnan Acta Palaeontologica Sinica26 272ndash85 [In Chinese with English summary]

Hou X 1987b Two new arthropods from Lower CambrianChengjiang eastern Yunnan Acta Palaeontologica Sinica 26236ndash56 [In Chinese with English summary]

Hou X Chen J amp Lu H 1989 Early Cambrian new arthropodsfrom Chengjiang Yunnan Acta Palaeontologica Sinica 28 42ndash57[In Chinese with English summary]

Hou X Ramskold L amp Bergstrom J 1991 Composition andpreservation of the Chengjiang fauna a Lower Cambrian soft-bodied biota Zoologica Scripta 20 395ndash411

Hou X Bergstrom J amp Ahlberg P 1995 Anomalocaris and otherlarge animals in the Lower Cambrian Chengjiang fauna ofsouthwest China GFF 117 163ndash83

Hou X amp Bergstrom J 1997 Arthropods of the Lower CambrianChengjiang fauna southwest China Fossils and Strata 45 1ndash116

Hughes C P 1975 Redescription of Burgessia bella from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 4415ndash36

Ivantsov A Y 1999 Trilobite-like arthropod from the LowerCambrian of the Siberian Platform Acta PalaeontologicaPolonica 44 455ndash66

King P E 1973 Pycnogonids London St Martin Press

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 191

Lauterbach K E 1973 Schlussereignisse in der Evolution derStammgruppe der Euarthropoda Zoologische Beitrage 19259ndash99

Lauterbach K E 1980 Schlussereignisse in der Evolution derGrundplans der Arachnata (Arthropoda) Abhandlungen desNaturwissenschaftlichen Vereins in Hamburg 23 163ndash327

Lauterbach K E 1983 Synapomorphien zwischen Trilobiten- undCheliceraten-zweig der Arachnata Zoologischer Anzeiger 210213ndash38

Lendzion K 1975 Fauna of the Mobergella Zone in the Polish LowerCambrian Kwartalnik Geologiczny 19 237ndash42

Lieberman B S 1998 Cladistic analysis of the Early Cambrianolenelloid trilobites Journal of Paleontology 72 59ndash78

Lu Y 1940 On the ontogeny and phylogeny of Redlichia intermediaLu (sp nov) Bulletin of the Geological Society of China 20333ndash42 [In Chinese]

Maddison W P amp Maddison D R 1997 MacClade analysis ofphylogeny and character evolution Version 307 Sunderland MASinauer Associates

Mittmann B amp Scholtz G 2003 Development of the nervous systemin the lsquoheadrsquo of Limulus polyphemus (Chelicerata Xiphosura)morphological evidence for the correspondence between the seg-ments of the chelicerae and the (first) antennae of MandibulataDevelopment Genes and Evolution 213 9ndash17

Moore R C (ed) 1959 Treatise on invertebrate paleontology Part OArthropoda 1 Boulder Colorado Geological Society of America

Muller K J amp Walossek D 1986 Arthropod larvae from the UpperCambrian of Sweden Transactions of the Royal Society ofEdinburgh Earth Sciences 77 157ndash79

Muller K J amp Walossek D 1987 Morphology ontogeny and lifehabit of Agnostus pisiformis from the Upper Cambrian of SwedenFossils and Strata 19 1ndash124

Munilla T 1999 Evolucion y filogenia de los Picnogonidos InMelicA DeHaro J J Medez M amp Ribera I (eds) VolumenMonografico Evolucion y Filogenia de Arthropoda Boletin de laSociedad Entolomologica Aragonesa 26 273ndash91 [In Spanish andEnglish]

Orr P J Siveter D J Briggs D E G Siveter D J amp Sutton M D2000 A new arthropod from the Silurian Konservat-Lagerstatteof Herefordshire Proceedings of the Royal Society of LondonSeries B 267 1497ndash504

Page R D M 1996 On consensus confidence and lsquototal evidencersquoCladistics 12 83ndash92

Palmer A R amp Repina L N 1993 Through a glass darklytaxonomy phylogeny and biostratigraphy of the OlenellinaUniversity of Kansas Paleontological Contributions New Series 31ndash35

Platnick N I Griswold C E amp Coddington J A 1991 On missingentries in cladistic analysis Cladistics 7 337ndash43

Pleijel F 1995 On character coding for phylogeny reconstructionCladistics 11 309ndash15

Raasch G O 1939 Cambrian Merostomata Special Papers of theGeological Society of America 19 1ndash146

Ramskold L Chen J Edgecombe G D amp Zhou G 1996Preservational folds simulating tergite junctions in tegopeltid andnaraoiid arthropods Lethaia 29 15ndash20

Ramskold L Chen J Edgecombe G D amp Zhou G 1997Cindarella and the arachnate clade Xandarellida (ArthropodaEarly Cambrian) from China Transactions of the Royal Society ofEdinburgh Earth Sciences 88 19ndash38

Ramskold L amp Edgecombe G D 1991 Trilobite monophylyrevisited Historical Biology 4 267ndash83

Ramskold L amp Edgecombe G D 1996 Trilobite appendagestructure ndash Eoredlichia revisited Alcheringa 20 269ndash76

Raw F 1957 Origin of chelicerates Journal of Paleontology 31139ndash193

Repina L N amp Okuneva O G 1969 Cambrian arthropods of themaritime territory Paleontological Journal 1969 95ndash103

Richter R amp Richter E 1929 Weinbergina opitz ng n sp einSchwerttrager (Merost Xiphos) aus dem Devon (Rheinland)Senkenbergiana 11 193ndash209

Robison R A 1984 New occurences of the unusual trilobite Naraoiafrom the Cambrian of Idaho and Utah University of KansasPaleontological Contributions Paper 112 1ndash8

Rominger C 1887 Description of primordial fossils from MtStephens NW Territory of Canada Proceedings of the Academyof Natural Sciences of Philadelphia 1887 12ndash19

Scholtz G 1997 Cleavage germ band formation and head segmen-tation the ground pattern of the Euarthropoda In Fortey R Aamp Thomas R H (eds) Arthropod Relationships 317ndash32 LondonChapman and Hall Systematics Association Special Volume 55

Schram F R 1978 Arthropods a convergent phenomenonFieldiana Geology 39 61ndash108

Schram F R 1993 Review of Boxshall Stromberg and Dahl TheCrustacea Origin and Evolution Journal of Crustacean Biology13 820ndash2

Schram F R Vonk R amp Hof C H J 1997 Mazon CreekCycloidea Journal of Paleontology 71 261ndash84

Schram F R amp Emerson M J 1991 Arthropod Pattern Theorya new approach to arthropod phylogeny Memoirs of theQueensland Museum 31 1ndash18

Schram F R amp Hof C H J 1998 Fossils and the interrelationshipsof major crustacean groups In Edgecombe G D (ed) Arthropodfossils and phylogeny pp 233ndash302 New York NY ColumbiaUniversity Press

Shergold J H 1991 Protaspis and early meraspis growth stages of theeodiscoid trilobite Pagetia ocellata Jell and their implications forclassification Alcheringa 15 65ndash86

Shu D Geyer G Ling C amp Zhang Z 1995 Redlichiacean trilobiteswith preserved soft parts from the Lower Cambrian Chengjiangfauna (South China) Beringeria (Special Issue) 2 203ndash41

Shultz J W 1990 Evolutionary morphology and phylogeny ofArachnida Cladistics 6 1ndash38

Shultz J W 2000 Skeletomuscular anatomy of the harvestmanLeiobunum aldrichi (Weed 1893) (Arachnida OpilionesPalpatores) and its evolutionary significance Zoological Journalof the Linnean Society 128 401ndash38

Shultz J W 2001 Gross muscular anatomy of Limulus polyphemus(Xiphosura Chelicerata) and its bearing on evolution in the theArachnida Journal of Arachnology 29 283ndash303

Siewing R (ed) 1985 Lehrbuch der Zoologie II Systematik 3rd ednStuttgart Fischer

Simonetta A M 1970 Studies on the non trilobite arthropods ofthe Burgess Shale (Middle Cambrian) the genera LeanchoiliaAlalcomenaeus Opabinia Burgessia Yohoia and ActaeusPalaeontographica Italica 66 35ndash45

Simonetta A M amp Delle Cave L 1975 The Cambrian non trilobitearthropods from the Burgess Shale of British Columbia A studyof their comparative morphology taxonomy and evolutionarysignificance Palaeontographica Italica 69 1ndash37

Simonetta A M amp Delle Cave L 1981 An essay in the compara-tive and evolutionary morphology of Palaeozoic ArthropodsAccademia Nazionale dei Lincei Atti dei Convegni Lincei 49389ndash439

Starobogatov Y I 1990 Sistema i filogeniya nizshikh khelitserovykh(analiz morfologii paleozoysikh grupp) [The systematics andphylogeny of the lower chelicerates (a morphological analysis ofthe Palaeozoic groups)] Paleontologicheskii Zhurnal 1990 4ndash17

Stoslashrmer L 1944 On the relationships and phylogeny of fossil andrecent Arachnomorpha Skrifter utgitt ac Det Norske Videnskaps-Akademi i Olso I Matematisk-naturvitenskaplig Klasse 5 1ndash158

Stoslashrmer L 1951 Studies on trilobite morphology Part III Theventral cephalic structures with remarks on the zoologicalposition of the trilobites Norsk geologisk tidsskrift 29 108ndash58

Stoslashrmer L 1956 A Lower Cambrian merostome from Sweden Arkivfor Zoologi Ser 2 9 507ndash14

Stoslashrmer L 1959 Trilobitoidea In Moore R C (ed) Treatise onInvertebrate Paleontology part O Arthropoda 1 23ndash37 BoulderColorado Geological Society of America

Strong E E amp Lipscomb D 1999 Character coding and inapplicabledata Cladistics 15 363ndash71

Sturmer W amp Bergstrom J 1976 The arthropods Mimetaster andVachonisia from the Devonian Hunsruck Shale PalaontologischeZeitschrift 50 78ndash111

Sturmer W amp Bergstrom J 1978 The arthropod Cheloniellon fromthe Devonian Hunsruck Shale Palaontologische Zeitschrift 5257ndash81

Sturmer W amp Bergstrom J 1981 Weinbergina a xiphosuran fromthe Devonian Hunsruck Slate Palaontologische Zeitschrift 55237ndash55

Sutton M D Briggs D E G Siveter D J Siveter D J amp Orr PJ 2002 The arthropod Offacolus kingi (Chelicerata) from theSilurian of Herefordshire England computer based morphologi-cal reconstructions and phylogenetic affinity Proceedings of theRoyal Society of London Series B 269 1195ndash1203

Swofford D L 1999 PAUP Phylogenetic Analysis Using Parsimony(and other methods) Version 40b2a Sunderland MA SinauerAssociates

Telford M J amp Thomas R H 1998 Expression of homeobox genesshows chelicerate arthropods retain their deutocerebral segmentProceedings of the National Academy of Sciences USA 9510671ndash5

192 TREVOR J COTTON AND SIMON J BRADDY

Thorley J L amp Page R D M 2000 RadCon phylogenetic treecomparison and consensus Bioinformatics 16 486ndash7

Valentine J W 1995 Why no new phyla after the Cambrian ndashgenome and ecospace hypotheses revisited Palaios 10 190ndash194

Wagele J W 1993 Rejection of the lsquoUniramiarsquo hypothesis andimplications of the Mandibulata concept Zoologische JahrbucherAbteilung fur Systematik Geographie und Biologie der Tiere 120253ndash88

Walcott C D 1911 Middle Cambrian Merostomata Cambriangeology and palaeontology II Smithsonian MiscellaneousCollections 57 17ndash40

Walcott C D 1912 Middle Cambrian Branchiopoda MalacostracaTrilobita and Merostomata Cambrian geology and palaeontol-ogy II Smithsonian Miscellaneous Collections 57 145ndash228

Walcott C D 1918 Geological explorations in the CanadianRockies Explorations and fieldwork of the SmithsonianInstitution in 1917 Smithsonian Miscellaneous Collections 684ndash20

Walossek D 1993 The Upper Cambrian Rehbachiella kinnekullensisand the phylogeny of Branchiopoda and Crustacea Fossils andStrata 32 1ndash202

Walossek D amp Dunlop J A 2002 A larval sea spider (ArthropodaPycnogonida) from the Upper Cambrian lsquoOrstenrsquo of Swedenand the phylogenetic position of pycnogonids Palaeontology 45421ndash46

Walossek D amp Muller K J 1990 Upper Cambrian stem-lineagecrustaceans and their bearing upon the monophyletic origin ofCrustacea and the position of Agnostus Lethaia 23 409ndash27

Walossek D amp Muller K J 1997 Cambrian lsquoOrstenrsquo-type arthro-pods and the phylogeny of Crustacea In Fortey R A amp ThomasR H (eds) Arthropod Relationships 139ndash53 SystematicsAssociation Special Volume 55 London Chapman and Hall

Walossek D amp Muller K J 1998 Early arthropod phylogeny in lightof the Cambrian lsquoOrstenrsquo fossils In Edgecombe G D (ed)Arthropod fossils and phylogeny 185ndash231 New York NYColumbia University Press

Weygoldt P 1979 Significance of later embryonic stages and headdevelopment in arthropod phylogeny In Gupta A P (ed)Arthropod Phylogeny 107ndash35 New York NY Van NostrandReinhold Co

Weygoldt P 1998 Evolution and systematics of the ChelicerataExperimental and Applied Acarology 22 63ndash79

Weygoldt P amp Paulus H F 1979 Unterzuchungen aur Morpholo-gie Taxonomie und Phylogenie der Chelicerata Zeitschriftfur Zoologische Systematik und Evolutionsforschung 17 85ndash116177ndash200

Wheeler W C Cartwright P amp Hayashi C Y 1993 Arthropodphylogeny a combined approach Cladistics 9 1ndash39

Wheeler W C amp Hayashi C Y 1998 The phylogeny of extantchelicerate orders Cladistics 14 173ndash92

Whittington H B 1971 Redescription of Marrella splendens(Trilobitoidea) from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 209 1ndash24

Whittington H B 1974 Yohoia Walcott and Plenocaris n genarthropods from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 231 1ndash21

Whittington H B 1975a Trilobites with appendages from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 497ndash136

Whittington H B 1975b The enigmatic animal Opabinia regalisMiddle Cambrian Burgess Shale British Columbia PhilosophicalTransactions of the Royal Society of London Series B 271 1ndash43

Whittington H B 1977 The Middle Cambrian trilobite NaraoiaBurgess Shale British Columbia Philosophical Transactions of theRoyal Society Series B 280 409ndash43

Whittington H B 1979 Early arthropods their appendages andrelationships In House M R (ed) The origin of majorinvertebrate groups 253ndash68 Systematics Association SpecialVolume 12 New York NY Academic Press

Whittington H B 1980 Exoskeleton moult stage appendage mor-phology and habits of the Middle Cambrian trilobite Olenoidesserratus Palaeontology 23 171ndash204

Whittington H B 1981 Rare arthropods from the Burgess ShaleMiddle Cambrian British Columbia Philosophical Transactionsof the Royal Society of London Series B 292 329ndash57

Whittington H B 1985 Tegopelte gigas a second soft-bodied trilo-bite from the Burgess Shale Middle Cambrian British ColumbiaJournal of Palaeontology 23 171ndash204

Whittington H B 1989 Olenelloid trilobites type species functionalmorphology and higher classification Philosophical Transactionsof the Royal Society of London Series B 324 111ndash47

Whittington H B amp Almond J E 1987 Appendages and habits ofthe Upper Ordovician trilobite Triarthus eatoni PhilosophicalTransactions of the Royal Society of London Series B 317 1ndash46

Wilkinson M 1992 Ordered versus unordered characters Cladistics8 375ndash85

Wilkinson M 1994 Common cladistic information and its consensusrepresentation reduced Adams and reduced cladistic consensustrees and profiles Systematic Biology 43 343ndash68

Wilkinson M 1995a Coping with abundant missing entries in phylo-genetic inference using parsimony Systematic Biology 44 501ndash14

Wilkinson M 1995b TAXEQ2 software and documentation BristolSchool of Biological Sciences University of Bristol

Wilkinson M 1996 On the distribution of homoplasy and choosingamong trees Taxon 45 263ndash66

Wills M A 1997 A phylogeny of Recent and fossil Crustacea derivedfrom morphological characters In Fortey R A amp ThomasR H (eds) Arthropod Relationships 189ndash209 SystematicsAssociation Special Volume 55 London Chapman and Hall

Wills M A Briggs D E G amp Fortey R A 1994 Disparity asan evolutionary index a comparison of Cambrian and Recentarthropods Paleobiology 20 93ndash130

Wills M A Briggs D E G Fortey R A amp Wilkinson M 1995The significance of fossils in understanding arthropod evolutionVerhandlungen der deutschen zoologischen Gesselschaft 88 203ndash15

Wills M A Briggs D E G Fortey R A Wilkinson M amp SneathP H A 1998a An arthropod phylogeny based on fossil andrecent taxa In Edgecombe G D (ed) Arthropod fossils andphylogeny 33ndash105 New York NY Columbia University Press

Wills M A Briggs D E G amp Fortey R A 1998b Evolutionarycorrelates of arthropod tagmosis scrambled legs In ForteyR A amp Thomas R H (eds) Arthropod Relationships 57ndash65Systematics Association Special Volume 55 London Chapmanand Hall

Winter G 1980 Beitrage zur Morphologie und Embryologie desvorderen Korperabschnitts (Cephalosoma) der PantopodaGerstaecker 1863 I Entstehung und Struktur des Zentralnerven-systems Zeitschrift fur Zoologische Systematik und Evolutionsfor-schung 18 27ndash61

Zhang W amp Hou X 1985 Preliminary notes on the occurrence of theunusual trilobite Naraoia in Asia Acta Palaeontologica Sinica 24591ndash5 [In Chinese with English summary]

TREVOR J COTTON Department of Palaeontology The Natural History Museum CromwellRoad London SW7 5BD UKe-mail trevor_cottonhotmailcom

SIMON J BRADDY Department of Earth Sciences University of Bristol Wills MemorialBuilding Queenrsquos Road Bristol BS8 1RJ UKe-mail S J Braddybrisacuk

MS received 28 July 2003 Accepted for publication 15 March 2004

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 193

Page 4: The phylogeny of arachnomorph arthropods and the origin of the

2 Cladistic analysis

2 1 Taxonomic scopeThirty-four ingroup taxa were considered in the cladisticanalyses Of these the majority were coded as individualspecies-level terminals although higher taxonomic levels wereemployed in some instances A complete list of species andgenus-level terminals along with details of authorship andother important references is given in Table 1 Species-levelterminals are referred to throughout by generic names only(except in Table 1) since the majority are assigned to mono-typic genera In addition to these ingroup taxa a hypotheticaloutgroup was used for rooting as discussed below

Terminals were selected on the basis of the recent cladisticanalyses of Wills et al (1995 1998a) and Edgecombe ampRamskold (1999) and to a lesser extent previous hypothesesof arthropod relationships The majority of taxa that havebeen included in an arachnomorph group by previous authorsare included in this study Exceptions include AgnostusHabelia Molaria Sanctacaris and Sarotrocercus (all of whichwere considered by Wills et al but not Edgecombe ampRamskold) the putative chelicerate Offacolus andPhytophilaspis The Agnostina are regarded as a clade withinthe Trilobita derived from Eodiscina (eg Fortey 1990a

Fortey amp Theron 1994 Wills et al 1998a) rather than stemgroup crustaceans (eg Shergold 1991 Bergstrom 1992 seeCotton 2002 for a detailed discussion and phylogenetic analy-sis) The Burgess Shale taxa Habelia Walcott 1912 MolariaWalcott 1912 and Sarotrocercus Whittington 1981 and theHunsruck Slate Magnoculocaris blindi (Briggs amp Bartels 20012002) are rather poorly known and hypotheses of theirrelationships constrained by too few characters

Sanctacaris Briggs amp Collins 1988 and the probably closelyrelated Offacolus Orr et al 2000 were both originally describedas having chelicerate affinities Sutton et al (2002) addedOffacolus to the matrix of Wills et al (1995 1998a) and foundit to be nested within the Chelicerata despite its biramouscephalic appendages The material of Sanctacaris is in need ofrestudy the homology of the cephalic appendages and ventralcephalic structures remain unclear (see Budd 2002 and Briggsamp Collins 1988 for alternative schemes) The relationships ofthese lsquosanctacaridsrsquo will be considered in detail later by Braddyand co-workers following re-examination of Sanctacaris anddescription of a new Chengjiang taxon (Babcock amp Zhangin press) that is likely to have a profound impact on theinterpretation of both Sanctacaris and Offacolus

Phytophilaspis Ivantsov 1999 shows a combination of fea-tures that may be homologous to those found in naraoiids (the

Table 1 Authorship and important references for species included in cladistic analyses of arachnomorphs Previous analyses by Briggs amp Fortey(1989 1992) Briggs et al (1992 1993) Wills et al (1995 1998a) and Edgecombe amp Ramskold (1999) informed the coding of many taxa and arenot listed

Name and authorship Other references

Alalcomenaeus cambricus Simonetta 1970 Briggs amp Collins (1999)Buenaspis forteyi Budd 1999aBurgessia bella Walcott 1912 Hughes (1975)Cheloniellon calmani Broili 1932 Broili (1933) Sturmer amp Bergstrom (1978)Cindarella eucalla Chen et al 1996 Ramskold et al (1997)Emeraldella brocki Walcott 1912 Bruton amp Whittington (1983) Briggs amp Robison (1984)Eoredlichia intermedia (Lu 1940) Shu et al (1995) Ramskold amp Edgecombe (1996)Fortiforceps foliosa Hou amp Bergstrom 1997Helmetia expansa Walcott 1918 Briggs in Conway Morris (1982) Briggs Erwin amp Collier (1994)Jianfengia multisegmentalis Hou 1987b Bergstrom amp Hou (1991) Chen amp Zhou (1997)Kuamaia lata Hou 1987a Hou amp Bergstrom (1997) Begstrom amp Hou (1998)

Edgecombe amp Ramskold (1999)Leanchoilia superlata Walcott 1912 Bruton amp Whittington (1983) Briggs amp Robison (1984)Lemoneites Flower 1968 Dunlop amp Selden (1997)Liwia plana (Lendzion 1975) Dzik amp Lendzion (1988) Fortey amp Theron (1994)

Chen Edgecombe amp Ramskold (1997)Marrella splendens Walcott 1912 Whittington (1971)Mimetaster hexagonalis (Gurich 1931) Sturmer amp Bergstrom (1976)Misszhouia longicaudata (Zhang amp Hou 1985) Bergstrom amp Hou (1991) Chen et al (1997) Hou amp Bergstrom (1997)Naraoia Walcott 1912 Whittington (1977) Robison (1984) Zhang amp Hou (1985)

Chen et al (1997)Olenoides serratus (Rominger 1887) Whittington (1975a 1980)Paleomerus hamiltoni Stoslashrmer 1956 Bergstrom (1971) Dunlop amp Selden (1997)Retifacies abnormalis Hou et al 1989 Hou amp Bergstrom (1997)Saperion glumaceum Hou et al 1991 Ramskold et al (1996) Hou amp Bergstrom (1997)Sidneyia inexpectans Walcott 1911 Bruton (1981)Sinoburius lunaris Hou et al 1991 Hou amp Bergstrom (1997) Skioldia aldna Hou amp Bergstrom 1997Soomaspis splendida Fortey amp Theron 1994 Chen et al (1997)Tariccoia arrusensis Hammann et al 1990 Fortey amp Theron (1994) Chen et al (1997)Tegopelte gigas Simonetta amp Delle Cave 1975 Whittington (1985) Ramskold et al (1996)Weinbergina opitzi Richter amp Richter 1929 Sturmer amp Bergstrom (1981) Andersen amp Selden (1997) Dunlop amp Selden (1997)Xandarella spectaculum Hou et al 1991 Bergstrom amp Hou (1991) Ramskold et al (1997)

Hou amp Begstrom (1997)Yohoia tenuis Walcott 1912 Whittington (1974)

172 TREVOR J COTTON AND SIMON J BRADDY

form of the pygidium) xandarellids (the eye slits and overlapof anterior thoracic segments by the head shield) and Trilobita(the hypostome with anterior and posterior lateral wings)Therefore it potentially has a pivotal position in the phylogenyof the trilobite-allied Arachnomorpha However the interpre-tation of these features by Ivantsov (1999) requires confir-mation and adequately determining the homology of thesestructures would require restudy of the material

The Xandarellida known only from the Chengjiang faunawere originally described as an arachnate clade (Ramskoldet al 1997) All three valid genera Xandarella Sinoburius andCindarella were included in the analysis of Edgecome ampRamskold (1999) They differ in important respects andtherefore all three genera are included here to test the mono-phyly of this group in the context of a wider analysis

Wills et al (1995 1998a) represented the Trilobita (exceptAgnostida) by Olenoides the appendages of which are knownfrom the Burgess Shale and the Ordovician Triarthrus (seeCisne 1975 1981 Whittington amp Almond 1987) However thepresent authors follow Edgecombe amp Ramskold (1999) inchoosing Olenoides and Eoredlichia to represent the TrilobitaThese are more basal trilobites (see eg Fortey 1990a b) thanTriarthrus and consequently are more likely to reflect theancestral trilobite condition The putative trilobite KleptothuleBudd 1995 from the Early Cambrian Sirius Passet faunaof North Greenland is not included Its appendages areunknown and homologies between exoskeletal features of thistaxon and other arachnomorphs are uncertain

The Naraoiidae or Nektaspida (reviewed above) are widelyconsidered to be closely related to the trilobites either as aparaphyletic assemblage of lsquosoft-bodiedrsquo trilobites (eg Shuet al 1995 fig 20B) or as the sister group of calcified trilobites(Fortey 1997 Whittington 1977) They have been includedin the Trilobita by some authors However Edgecombe ampRamskold (1999) found no particularly close relationshipbetween naraoiids and trilobites (see above) In order to testthe monophyly of the group all described naraoiid genera areincluded except Maritimella and Orientella (both Repina ampOkuneva 1969) which are probably pseudofossils (Robison1984 p 2)

Tegopelte from the Burgess Shale has also been considered asoft-bodied trilobite (Whittington 1985) Ramskold et al(1996) revised the exoskeletal morphology of Tegopelte andnoted similarities to the Chengjiang taxa Saperion andSkioldia This relationship has been confirmed by cladisticanalysis (Edgecombe amp Ramskold 1999) An undescribed largearthropod from the Soom Shale Lagerstatte (Aldridge et al2001 fig 3442) is probably a tegopeltid (Braddy amp Almond1999) The Helmetiidae based on Helmetia Walcott 1918from the Burgess Shale are united with the tegopeltid group inthe Helmetiida (Hou amp Bergstrom 1997 Edgecombe amp Ram-skold 1999) Helmetia is rather poorly known and awaitsredescription but is very similar to Kuamaia lata Hou 1987aKuamaia muricata Hou amp Bergstrom 1997 and Rhombicalva-ria acantha Hou 1987a from Chengjiang There are no signifi-cant differences between these Chinese taxa and theirtaxonomy may be over split (Delle Cave amp Simonetta 1991Hou amp Bergstrom 1997) The morphology of Kuamaia lata isknown in some detail and it is coded here along with Helmetiaand all three tegopeltid genera

Delle Cave amp Simonetta (1991 table 1) compared severaltaxa to Tegopelte and Helmetia Of these only Retifacies isknown in enough detail to make coding worthwhile Tontoiaand Nathorstia (both Walcott 1912) are nomina dubia (seeWhittington 1985 1980 respectively) and only the exoskeletonis known of Urokodia Hou et al 1989 and Mollisonia Walcott1912 Retifacies has been placed near a trilobite-naraoiid-

helmetiid clade (Delle Cave amp Simonetta 1991 Edgecombe ampRamskold 1999) or as sister group to the naraoiids (Hou ampBergstrom 1997)

There is a long history of comparing Emeraldella andSidneyia with chelicerates (following Stoslashrmer 1944) but theposition of these taxa in cladistic studies has been highlyvariable (see Fig 2) According to the hypothesis ofBergstrom these taxa along with the Aglaspidida andCheloniellida form a clade that is the sister group to thechelicerates More usually aglaspidids or cheloniellids havebeen considered sister taxon to the chelicerates and theseBurgess Shale taxa more distantly related

A cheloniellid-chelicerate clade was supported by Wills et al(1995 1998a) and Sturmer amp Bergstrom (1978 also Bergstrom1979) and Simonetta amp Delle Cave (1981) and Delle Cave ampSimonetta 1991) placed the cheloniellid Triopus as ancestral toall chelicerates The Cheloniellida (sensu Dunlop amp Selden1997) are represented by Cheloniellon herein since theappendages of other cheloniellid taxa are unknown

It has also repeatedly been suggested that cheliceratesevolved from aglaspidids (eg Starobogatov 1990) and aglas-pidids have been included in the Chelicerata by some authors(Stoslashrmer 1944 Weygoldt amp Paulus 1979) The coding ofaglaspidids is considered in detail in Section 22 Lemoneites(Flower 1968) and Paleomerus (Stoslashrmer 1956 Bergstrom 1971)have been assigned to the Aglaspidida by some authors (seeHou amp Bergstrom 1997 pp 96ndash97) They are included tofacilitate comparison with the results of Dunlop amp Selden(1997) Lemoneites is particularly problematic (R A Moorepers comm 2004) and both taxa are known from very fewcharacters ndash hence the present authors have investigated theimplications of their exclusion from the analysis (see Section26) The aglaspidid-like arthropod Kodymirus vagans Chlupacamp Havlıcek 1965 from the Lower Cambrian of the CzechRepublic (redescribed and compared to eurypterids byChlupac 1995) is excluded from the present study becausefeatures of its morphology that are well known generally agreewith those of Aglaspis

The phylogeny of crown group chelicerates is a matter ofconsiderable debate but most authors have considered pyc-nogonids xiphosurans and eurypterids as early divergentgroups within Chelicerata (Dunlop 1999) These hypothesesare represented here by coding a generalised pycnogonid andeurypterid and the Devonian synziphosurine Weinbergina(Sturmer amp Bergstrom 1981) Weinbergina is the best-knownsynziphosurine and is more likely to represent the primitivecondition of xiphosurans than modern examples The phylog-eny of the Xiphosura has recently been studied by Anderson ampSelden (1997) Coding for Eurypterida follows the recent workof Dunlop amp Selden (1997) Dunlop (1998) Braddy et al(1999) and Dunlop amp Webster (1999) Coding for pycnogonidsfollows general works on the group (eg King 1973 Fry 1978)the reconstruction of the pycnogonid stem group by Bergstromet al (1980) and recent work on pycnogonid phylogeny(Munilla 1999 Arango 2002) The pycnogonid familyAmmotheidae is generally accepted as the most plesiomorphicextant group (Arango 2002)

Two groups of Palaeozoic fossil taxa have been included inthe Arachnomorpha by some authors but considered lessclosely related by others Marrella from the Burgess Shaleand two Devonian taxa Mimetaster and Vachonisia havegenerally been considered to form the clade Marrellomorpha(Whittington 1971 Sturmer amp Bergstrom 1976 Bergstrom1979 Wills et al 1995) This has been thought to be the sistergroup to other arachnates (Sturmer amp Bergstrom 1976) abasal schizoramian or a basal euarthropod group (see Fig 2)Bergstrom (1979) included the Carboniferous Cycloidea in the

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 173

Marrellomorpha but there is considerable evidence that theseare rather derived crustaceans (see Schram et al 1997) andthey are not considered further The Burgess Shale taxonBurgessia has also been placed in the Marrellomorpha (Hou ampBergstrom 1997) but was not found to belong to this clade inmost analyses Stoslashrmer (1944) placed Burgessia in a compa-rable systematic position to Marrella he included both in theArachnomorpha but excluded them from the MerostomoideaMarrella Mimetaster and Burgessia were included in thepresent study to assess whether the marellomorphs should beincluded in the Arachnomorpha and test the affinities ofBurgessia

The Cambrian lsquogreat appendagersquo arthropods (theMegacheira of Hou amp Bergstrom 1997) have been nestedwithin the Arachnomorpha in most cladistic studies (egBriggs amp Fortey 1989 Wills et al 1995 1998a Emerson ampSchram 1997) Briggs amp Fortey (1989 1992) recognised mega-cheiran taxa as particularly closely related to chelicerates(Fig 2A) while other authors (Bergstrom 1992 Hou ampBergstrom 1997 Budd 2002) have considered megacheirans tobe primitive euarthropods or ancestral to some (but not all)crustaceans (Delle Cave amp Simonetta 1991 and referencestherein) Since the monophyly of the megacheirans hasnot usually been supported the present authors haveincluded nearly all described taxa These include LeanchoiliaAlalcomenaeus and Yohoia from the Burgess Shale andFortiforceps and Jianfengia from the Chengjiang faunaExcluded from this study are Actaeus Simonetta 1970 fromthe Burgess Shale which may be a synonym of Alalcomenaeus(Briggs amp Collins 1999) Alalcomenaeus illecebrosus (Hou1987b see Hou amp Bergstrom 1997) from the ChengjiangFauna which is probably a chimaera (Briggs amp Collins 1999)and the poorly preserved Leanchoilia hanceyi from the MiddleCambrian of Utah (Briggs amp Robison 1984)

According to the definition given above any assessment ofthe limits of the Arachnomorpha needs to consider the phylo-genetic position of these taxa relative to crustaceans To thisend a generalised crustacean intended to represent the plesio-morphic crustacean condition was included as an ingrouptaxon There has been considerable disagreement over themost basal crustacean group (see Wills 1997 pp 194ndash5Schram amp Hof 1998 pp 245ndash8) The coding used here largelyfollows Walossek amp Mullerrsquos (1990 1997 1998) andWalossekrsquos (1993) concept of the crustacean stem group but isintended to be conservative so that coding on the basis of othertheories of crustacean origins would be similar

22 AglaspididaThe morphology of the appendages of aglaspidids is poorlyknown having been described from three species of bodyfossil Aglaspis spinifer Raasch 1939 Flobertia kochi Hesselbo1992 and Khankaspis bazhanovi Repina amp Okuneva 1969 andtrace fossil evidence (Hesselbo 1988) Of these the appendagesof Aglaspis described by Raasch (1939) Briggs et al (1979)and Hesselbo (1992) are by far the best known The append-ages of Flobertia (described by Raasch 1939 as Aglaspisbarrandei and Hesselbo 1992) agree with those of AglaspisHowever the appendages of Khankaspis show a differentmorphology but have only been poorly illustrated anddescribed (Repina amp Okuneva 1969) This material suggeststhe presence of lobate exopods with lamellate setae This taxonis probably correctly assigned to aglaspidids (cf Whittington1979 p 258) on the basis the central position of the dorsaleyes and presence of genal spines although Hou amp Bergstrom(1997 p 96) suggested that its broad tail spine may indicatethat it is a strabopid

It remains unclear whether lamellate exopods are absent insome aglaspidids (Aglaspis) and present in others (Khankaspis)or whether the apparent differences in appendage morphologybetween these taxa are a result of preservational bias alone Apossible preservational analogue is provided by someChengjiang taxa in which the exopods are very poorly pre-served and the endopods of the cephalic appendages arepreserved as impressions in the dorsal head shield in a similarmanner to those of Aglaspis This is presumably because theendopods were more convex and more heavily sclerotised thanthe exopods This mode of appendage preservation is mostclearly seen in Misszhouia longicaudata (see Chen et al 1997fig 2andashd) but is also found in Sinoburius (see Hou ampBergstrom 1997) and possible protaspides of Naraoia (Houet al 1991)

Here a generalised aglaspidid is coded in two different waysto accommodate this uncertainty In both codings they areconsidered to have possessed a pair of antenniform append-ages followed by 11 pairs of pediform endopods on the headand first eight thoracic tergites (Briggs et al 1979 Hesselbo1988 1992) According to one interpretation (coded asAglaspidida 1) all the appendages are uniramous the exopodis lost throughout According to the other (Aglaspidida 2)exopods consisting of a single lobe fringed with lamellate setae(Repina amp Okuneva 1969 pl 15 figs 1 3 amp 4) are presenton at least some appendages but their distribution andattachment are considered unknown

23 Outgroup rootingA hypothetical plesiomorphic euarthropod was included toallow outgroup rooting Many of the characters consideredcould be coded unambiguously on the basis of recent discus-sions of the arthropod stem group (Budd 1996b 1997 1999b)

Figure 3 Majority rule consensus of lsquotrilobite-alliedrsquo arachnate phy-logeny after Edgecombe amp Ramskold (1999 fig 3)

174 TREVOR J COTTON AND SIMON J BRADDY

The ancestral euarthropod is here considered to have possesseda head with antennae only and a series of post-cephalicbiramous limbs with gnathobases The hypothesised pattern ofthe evolution of plesiomorphic euarthropod characters isshown in Figure 4 The use of a hypothetical outgroup issomewhat unsatisfactory but only affects the relationshipbetween the major clades (Arachnomorpha Crustacea andMarrellomorpha) the topology within the arachnomorphs wasunaffected by the use of this outgroup since identical resultswere obtained with unrooted analyses or by rooting withCrustacea Marrella or both

24 Characters and codingMost previous cladistic analyses with the notable exception ofEdgecombe amp Ramskold (1999) have included only briefdiscussions of the primary hypotheses of homology involved incharacter construction and coding Therefore a full discussionof most characters is provided here Descriptions of charactersand character states below are arranged by organ system orbody region in approximate anterior to posterior order Thedistribution of character states across all taxa is shown in thedata matrix (Table 2)

The terminology of morphological features in arachnomor-phs is often confused Terminology developed for cheliceratestrilobites and crustaceans has variously been applied to taxaincluded in this study so some attempt is made to clarifyterminology herein Where appropriate the present authorsrsquoterminology follows Edgecombe et al (2000) and Wheeleret al (1993) but following Scholtz (1997) the protocerebrallsquosegmentrsquo is considered to be acronal and consequently thedeutocerebral segment to be the first true segment Thereforethe antennae (or antennulae of crustaceans) belong to the firstcephalic segment

The present authors have attempted to include as completea set of characters as possible However characters requiringhypotheses of homology between podomeres (eg the number

of podomeres in endopods of thoracic appendages Willset al 1998a character 51) were not considered followingEdgecombe et al (2000 p 157) Secondly some characters usedby Bergstrom (eg Hou amp Bergstrom 1997 p 109) suchas appendage posture and mode of feeding were excludedfollowing Edgecombe amp Ramskold (1999) Finally somecharacters of the ventral surface of the head were not coded asdiscussed below and illustrated in Figure 7 The present authorsdo not include all potential synapomorphies for the Cheli-cerata as the status of many of these is debated (see eg Shultz1990 2001 Dunlop amp Selden 1997 Weygoldt 1998 Wheeler ampHayashi 1998 Dunlop 1999 Edgecombe et al 2000)

Two methods for the coding of inapplicable characters inphylogenetic analysis have been used First inapplicable char-acter states can be coded as missing data A complex structuremay comprise characters lsquoabsentpresentrsquo and lsquostate1state2rsquowith taxa lacking the structure coded as absent for the firstcharacter and as missing data for the second Some authorshave regarded this method as problematic because it may leadto reconstruction of impossible ancestral states and henceunjustified trees (Platnick et al 1991 Strong amp Lipscomb1999) The alternative is to code the second character as a thirdlsquonot applicablersquo state in taxa that lack the structure Thesemethods are equivalent to Pleijelrsquos (1995) coding methods Cand B respectively In the present study inapplicable charac-ters are treated as missing data for most analyses becausecoding them as a distinct character state reduces characterindependence and effectively weights the inapplicable charac-ter and hence could result in them dominating the analysisThe effects of this assumption were investigated by using adistinct character state in some analyses (see Section 26) andinapplicable characters are shown as distinct to lsquotruersquo missingdata (using the symbol lsquo-rsquo) in the matrix (Table 2)

241 Anterior cephalic appendages and head segmentation1 Appendages of the first segment (antennae) (0) present and(1) absent The majority of taxa considered in the present study

Figure 4 Reconstruction of the euarthropod stem group used to inform coding of hypothetical outgroup Grosstopology largely follows Budd (1996b fig 9 1997 fig 1110) Solid boxes indicate unambiguous apomorphiesand shaded boxes character states which are plesiomorphic for the Arthropoda

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 175

possess a single pair of long uniramous multi- and annulatedantennae at the anterior of the head which presumably had asensory function as found in Crustacea Myriapoda and mostmembers of Hexapoda (ie proturans do not have antennae)These appendages are likely to be a synapomorphy of theEuarthropoda (eg Scholtz 1997 Walossek amp Muller 1997)and therefore the outgroup is coded as State 0 The anterior-most head appendages of most Cambrian megacheiran arthro-pods which have been called lsquogreat appendagesrsquo followingWalcott (1912) consist of a small number of robust spinosepodomeres Uniquely Fortiforceps has a pair of short anten-nae anterior to the lsquogreat appendagesrsquo (Hou amp Bergstrom 1997pp 34ndash8) Therefore it appears likely that the lsquogreat append-agesrsquo are the appendages of the second cephalic segment andthe antennae are lost in megacheirans other than FortiforcepsThis of course depends on recognising the lsquogreat appendagesrsquoas homologous in all of these taxa as discussed below(Character 2) Homology of the megacheiran anterior append-ages with the second cephalic appendages of trilobites waspreviously suggested by Stoslashrmer (1944 p 124) on the basis oftheir post-oral position

The classical view of chelicerate head segmentation main-tains that they too have lost the antennae and the cheliceraeare the appendages of the second cephalic segment Recentrevisions of arthropod phylogeny have uncritically acceptedthis homology (Edgecombe et al 2000 Giribet et al 2001)which is based largely on neuroanatomy The lsquochelicero-neuromerrsquo which innervates the chelicerae seems to be ho-mologous with the tritocerebrum associated with the secondcephalic appendage pair of mandibulates (see Weygoldt 1979Winter 1980) Although some uncertainty exists over theposition of pycnogonids within Arthropoda and the homol-ogy of their cephalic segmentation is poorly understood thisview is also supported by the presence of pre-cheliceralappendages in a pycnogonid larva (Larva D of Muller ampWalossek 1986) from the Upper Cambrian of Sweden(Walossek amp Dunlop 2002) Cheloniellon also has a pair ofpre-oral pediform appendages considered homologous to thechelicerae in addition to its antennae Thus despite recentneontological studies that indicate that the chelicerae arehomologous to the antennae of mandibulates based on pat-terns of Hox gene expression (Damen et al 1998 Telford amp

Table 2 Data matrix for cladistic analyses of arachnomorphs () missing data and (ndash) inapplicable characters Letters indicate multistateuncertainty codings as follows (A) 34 (B) 02 (C) 01 (D) 134 (E) 12

1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 3 3 3 4 4 4 4 4 4 4 4 4 4 5 5 5 5 51 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4

OUTGROUP 0 0 0 0 0 0 0 0 0 0 0 0 ndash ndash 0 0 0 0 0 0 0 0 0 0 0 4 0 ndash 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ndash 0 0 0 0 ndash ndash ndash 0 ndash 0Aglaspidida 1 0 0 1 A 0 0 0 0 0 1 0 ndash ndash ndash ndash ndash 0 0 0 1 0 0 0 0 0 0 0 0 ndash 1 1 0 0 1 0 0 0 0 0 0 0 0 1 2 1 1 0 ndash ndash ndash 1 0 1Aglaspidida 2 0 0 A 0 0 0 0 0 0 1 0 ndash ndash 0 1 0 1 0 0 0 0 0 0 0 0 ndash 1 1 0 0 1 0 0 0 0 0 0 0 0 1 2 1 1 0 ndash ndash ndash 1 0 1Alalcomenaeus 1 1 1 3 ndash 2 0 1 0 0 0 1 0 ndash ndash 1 0 0 0 0 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 0 1 0 1 ndash ndash ndash 1 1 0Buenaspis 0 1 B 0 0 1 0 0 ndash 0 1 0 1 0 0 0 0 1 0 0 0 0 ndash 0 1 1 1 0 1 0 ndash 0Burgessia 0 0 0 3 0 0 0 0 0 0 0 1 0 ndash ndash 0 0 0 1 2 ndash 0 0 0 0 0 4 0 ndash 0 0 0 0 0 0 2 0 0 0 0 0 0 ndash 0 0 0 1 ndash ndash ndash 1 0 0Cheloniellon 0 0 1 4 0 0 0 0 0 1 0 1 0 ndash ndash 0 0 1 0 1 0 0 0 0 0 0 1 0 ndash 0 1 0 0 1 0 0 0 0 0 1 0 0 1 0 1 1 0 ndash ndash ndash 0 ndash 1Cindarella 0 0 0 6 0 0 0 0 0 0 0 1 1 0 0 0 0 1 1 0 0 0 0 0 0 0 1 0 ndash 0 1 0 1 1 0 0 2 1 0 0 0 1 0 ndash 1 0 1 1 0 0 0 0 ndash 0Crustacea 0 0 0 3 0 0 0 0 0 0 0 0 0 ndash ndash 0 1 0 0 0 0 0 0 0 0 0 4 0 ndash 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ndash 0 0 0 ndash ndash ndash 1 0 0Emeraldella 0 0 1 5 0 0 0 0 0 0 0 1 1 0 0 0 0 1 0 2 ndash 0 0 0 0 0 0 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 1 1 1 1 0 1 ndash ndash ndash 1 0 1Eoredlichia 0 0 0 3 0 0 0 0 1 0 0 1 1 0 0 0 0 1 0 0 1 0 0 0 0 0 3 1 1 1 1 0 0 2 0 0 0 0 0 0 0 0 0 ndash 1 0 1 1 0 0 1 0 ndash 0Eurypterida 1 1 1 6 ndash 1 1 0 0 1 1 1 0 ndash ndash 1 0 1 0 1 0 0 0 1 0 0 4 1 0 0 1 0 0 1 0 0 0 0 0 0 0 0 1 3 0 1 0 1 ndash ndash ndash 1 0 0Fortiforceps 0 1 1 4 ndash 0 0 0 0 0 0 1 0 ndash ndash 1 0 0 1 0 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 0 1 0 1 ndash ndash ndash 1 1 0Helmetia 3 1 0 1 0 1 0 0 0 0 0 1 2 0 ndash 0 1 0 0 2 0 0 0 0 0 1 1 0 0 ndash 0 0 1 1 0 1 0 0 ndash 0Jianfengia 1 1 1 4 ndash 1 0 0 0 0 0 1 0 ndash ndash 1 0 1 1 0 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 0 1 0 1 ndash ndash ndash 1 0 0Kuamaia 0 0 0 0 0 0 0 0 0 1 1 0 1 0 0 1 0 0 0 1 0 0 0 1 2 0 ndash 0 1 0 0 2 0 0 0 0 0 1 1 0 0 ndash 0 0 1 1 0 1 0 0 ndash 0Leanchoilia 1 1 1 3 ndash 2 0 1 0 0 0 1 0 ndash ndash 1 0 1 1 B 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 1 1 0 1 ndash ndash ndash 1 0 0Lemoneites 0 1 0 0 0 0 1 1 0 1 0 0 0 0 0 0 0 0 1 2 0 0 ndash ndash ndash 1 0 Liwia 0 1 0 0 0 0 1 0 1 0 0 1 0 1 0 0 0 0 ndash 0 1 1 1 0 0 0 ndash 0Marrella 0 0 1 1 0 0 0 0 0 0 0 0 0 ndash ndash 0 1 0 0 2 ndash 0 0 0 0 0 0 ndash 0 0 0 0 0 0 2 0 0 0 0 0 0 ndash 0 0 0 0 ndash ndash ndash 0 ndash 0Mimetaster 0 0 1 2 0 0 0 0 0 0 0 0 0 ndash ndash 0 1 0 1 C 0 0 0 0 0 0 ndash 0 0 0 0 1 0 0 2 0 0 0 0 0 0 ndash 0 0 0 0 ndash ndash ndash 0 ndash 0Misszhouia 0 0 0 3 0 0 0 0 1 0 0 1 1 1 0 0 0 1 0 0 0 0 0 0 0 0 1 0 ndash 0 1 1 ndash ndash 2 0 0 0 1 0 0 0 0 ndash ndash 0 1 1 0 0 1 0 ndash 0Naraoia 0 0 0 3 1 0 0 0 1 0 0 1 1 1 0 0 0 1 0 0 0 0 0 0 0 0 1 0 ndash 0 1 1 ndash ndash 2 0 0 0 1 0 0 0 0 ndash ndash 0 1 1 0 0 0 ndash 0Olenoides 0 0 0 3 0 0 0 0 1 0 0 1 1 0 0 0 0 1 0 1 1 0 0 0 0 0 3 1 1 1 1 0 0 2 0 0 0 0 0 0 0 0 0 ndash 0 1 1 1 0 0 0 0 ndash 0Paleomerus 0 1 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0 0 0 ndash 0 0 ndash ndash ndash 1 0 Pycnogonida 1 1 1 4 ndash 1 1 0 0 1 1 1 ndash ndash ndash ndash 0 0 1 1 0 0 0 1 0 0 4 0 ndash 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 ndash ndash ndash 1 0 0Retifacies 0 0 0 3 0 0 0 0 0 0 0 1 0 ndash ndash 0 0 1 0 0 0 0 0 0 0 0 0 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 1 0 1 1 0 0 0 1 0 0Saperion 0 2 0 1 0 1 0 0 1 1 0 0 1 0 0 0 1 2 0 ndash 0 1 1 ndash ndash 1 1 ndash ndash 0 0 1 0 0 ndash ndash 1 1 0 0 1 0 ndash 0Sidneyia 0 0 1 0 1 0 0 0 0 1 0 1 0 ndash ndash 0 0 1 0 0 0 0 0 0 0 0 0 0 1 0 0 1 0 0 0 0 0 0 0 0 1 2 1 1 0 1 ndash ndash ndash 1 0 1Sinoburius 0 0 0 4 0 0 0 0 0 0 0 1 0 1 0 0 1 1 0 0 0 1 0 ndash 0 1 0 0 1 0 0 2 1 0 0 0 1 0 ndash 0 0 1 1 0 1 0 0 ndash 0Skioldia 0 2 0 0 0 1 0 0 0 1 2 0 ndash 0 1 1 ndash ndash 1 1 ndash ndash 0 0 1 0 0 ndash ndash 1 1 0 0 1 0 ndash 0Soomaspis 0 1 B 0 0 0 0 1 0 0 ndash 0 1 0 1 0 0 1 0 1 0 0 0 0 ndash 0 1 1 1 0 1 0 ndash 0Tariccoia 0 1 B 0 0 0 0 1 0 0 ndash 0 1 0 1 0 0 1 0 1 0 0 0 0 ndash 0 1 1 1 0 1 0 ndash 0Tegopelte 0 0 0 0 0 0 0 0 1 0 1 1 0 0 0 0 1 E 0 ndash 0 1 0 ndash ndash E 1 ndash ndash 0 0 0 0 ndash ndash 0 1 1 0 0 1 0 ndash 0Weinbergina 1 1 1 6 ndash 1 1 0 0 1 1 1 0 ndash ndash 1 0 1 0 1 0 0 0 1 0 0 4 1 0 0 1 0 0 1 0 0 0 0 0 0 0 0 1 2 0 1 0 1 ndash ndash ndash 1 0 0Xandarella 0 0 0 4 0 0 0 0 0 0 0 1 1 1 0 0 0 1 0 0 0 0 1 0 0 0 1 0 ndash 0 1 0 1 1 0 0 2 1 0 0 0 1 0 ndash 1 0 1 1 0 0 0 ndash 0Yohoia 1 1 1 4 ndash 1 0 0 0 1 1 1 0 ndash ndash 1 0 1 0 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 1 2 0 1 0 1 ndash ndash ndash 1 1 0

176 TREVOR J COTTON AND SIMON J BRADDY

Thomas 1998) and developmental data (Mittmann amp Scholtz2003) morphological evidence from fossils supports the viewthat chelicerates have lost the antennae The Hox gene evi-dence suffers from a lack of comparative data from diversemandibulates especially primitive crustaceans (see Akam2000) and the use of Hox gene expression boundaries asmarkers for segmental homology has been criticised (egAbzhanov et al 1999 although relating to posterior Hoxpatterns) Wills et al (1998a pp 43 amp 48) considered thechelicerae to be appendages of the second (deutocerebral)segment but curiously in support of this cited Schram (1978)who unambiguously followed the homology scheme used inthe present study

The anterior-most appendages of Aglaspis were originallydescribed as chelicerae (Raasch 1939) which lead to theirclassification in the Chelicerata (eg Stoslashrmer 1944) Howeverthe morphology of these appendages (see Briggs et al 1979) ismore similar to that of antennae They are narrow relative tothe thoracic endopods of relatively even diameter proximallyand the podomeres are apparently weakly defined Hesselbo(1988 1992) also argued that the anterior appendages ofAglaspis are likely to be antennae The distal parts are un-known (and hence so is their length cf Wills et al 1998ap 58) and there is no evidence that they were chelate contraryto the coding of Wills et al (1998a character 31) Nothingis known about the anterior appendages of BuenaspisLemoneites or Paleomerus and this character is accordinglycoded as missing data in these taxa

2 Form of endopod of appendages of the second segment(0) pediform and (1) anteriorly directed raptorial appendagewith reduced number of podomeres and terminal podomeresbearing spines on distal margins As discussed above cheli-cerae and lsquogreat appendagesrsquo are both likely to represent theappendages of the second segment They also differ from theassumed primitive biramous euarthropod limb in similar waysand therefore are likely to be homologous modifications ofthese appendages The homology of lsquogreat appendagesrsquo andchelicerae was originally proposed by Henriksen (1928) andsupported by Stoslashrmer (1944) but to the present authorsrsquoknowledge no modern author has discussed this possibility

Both lsquogreat appendagesrsquo (Figs 5A D E) and chelicerae(Figs 5B C) are equipped with strong spinose projections onthe outer (dorsal) side of the distal margins of the terminalpodomeres that are lacking on the proximal podomeresSecondly the number of podomeres in both chelicerae andlsquogreat appendagesrsquo is more or less reduced compared to thenumber in the endopods of biramous limbs In the case oflsquogreat appendagesrsquo they are significantly more robust thanthose of posterior endopods a situation that is matched insome chelicerates The homology of the lsquogreat appendagesrsquoof Alalcomenaeus Leanchoilia Yohoia Jiangfengia andFortiforceps is supported by all of these features and is widelyaccepted (eg Wills et al 1998a character 31 Hou ampBergstrom 1997 Bergstrom amp Hou 1998 Hou 1987a) OnlyHou amp Bergstrom (1991 p 183) have suggested albeit inpassing that the lsquogreat appendagesrsquo of all these taxa may beconvergent a view they appear to have changed The endo-pods of the appendages of other taxa are locomotory legswhich either lack strong spines or have spinose extensionsthat are invariably on the medial (ventral) surface and arestronger on the proximal podomeres than the distal ones (egMisszhouia Fig 6A) In the latter case these spines form partof the feeding system along with the gnathobases and are oftenlocated around the middle of the podomere rather than limitedto the distal margin These endopods are directed ventro-laterally as opposed to anteriorly in the case of both cheliceraeand lsquogreat appendagesrsquo The modified second appendages of

Marrella resemble this plesiomorphic condition here referredto as lsquopediformrsquo except in their anterolateral orientation Insome Recent crustaceans this appendage is modified into asecond antenna but in stem-group crustaceans it is pediform(eg Walossek amp Muller 1997 1998) This character is codedas missing data in a number of taxa for which the morphologyof the second segment appendage is unknown

Some authors (Dzik 1993 Chen amp Zhou 1997 Dewel ampDewel 1997 Budd 2002) have suggested that the lsquogreat ap-pendagesrsquo are homologous with the anterior appendages ofanomalocaridids and the anomalocaridid-like Opabinia andKerygmachela (see Budd 1996b 1999b respectively) Theanterior appendages of most anomalocaridids and otherCambrian lobopodians differ from megacheiran lsquogreat append-agesrsquo in that they consist of a large number of segmentsMoreover there is considerable morphological evidence thatanomalocaridids are part of the euarthropod stem group (Willset al 1995 1998a) and not closely related to the lsquogreatappendagersquo taxa (cf Budd 2002) with the possible exception ofParapeytoia lsquoGreat appendagersquo taxa share a suite of derivedcharacters with other crown group euarthropods includingsclerotised dorsal tergites the incorporation of more thanone pair of appendages into the head sclerotisation of theendopods and strong differentiation of the head that wereexcluded from Buddrsquos (2002) analysis The relationships ofanomalocaridids will be considered in detail later by Braddyand co-workers

3 Exopod of appendages of the second segment (0)present and (1) absent or much reduced The raptorial ap-pendages of the second segment (chelicerae and lsquogreat append-agesrsquo) are uniramous as are the corresponding pediformappendages of Marrella Mimetaster Emeraldella Cheloniellonand Sidneyia The plesiomorphic euarthropod state is found inmost other arachnomorphs including trilobites where thebiramous second segment appendages are undifferentiatedfrom those of posterior segments (Edgecombe et al 2000character 78) The exopods of these appendages are alsopresent in basal members of the crustacean crown group(Edgecombe et al 2000 character 79) and in stem groupcrustaceans (Walossek 1993 Walossek amp Muller 1997 1998)

4 Number of head segments (0) 1 (1) 2 (2) 3 (3) 4 (4) 5(5) 6 and (6) 7 The coding of this character by Edgecombe ampRamskold (1999 character 2) explicitly referred to the numberof limb pairs present in addition to the antenna Here thenumber of post-acronal segments present in the head is codedfollowing Wills et al (1998a character 29) In taxa that lack anantenna the number of somites is inferred to be one greaterthan the number of cephalic appendage pairs (cf Wills et al1998a) as described above

Sturmer amp Bergstrom (1978) suggested that the head ofCheloniellon is defined primarily on the basis of appendagetagmosis rather than the fusion of segments under a singlecephalic-shield (cf Hou amp Bergstrom 1997 pp 98ndash9) Accord-ing to this view the head consists of both the cephalic tergiteand the anteriormost free tergite that share uniramousgnathobasic appendages Wills et al (1998a) coded Cheloniel-lon as having six somites incorporated into the head andtherefore presumably accepted this suggestion There is noevidence that the first free tergite of Cheloniellon was incorpo-rated into the head (except perhaps functionally) and thepresent authors prefer to code the number of somites under thecephalic shield In Sidneyia there is a series of uniramousstrongly gnathobasic appendages similar to those of Cheloniel-lon posterior to the head shield All of these appendages wouldpresumably have to be considered part of the head based onappendage differentiation according to the view of Sturmer ampBergstrom (1978) Some autapomorphic states are included

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 177

(none for Sidneyia one for Marrella and six for Emeraldella)since these will become informative if the character is treatedas ordered

Edgecombe amp Ramskold (1999 p 265) described a novelform of cephalic tagmosis The present authors tentativelyaccept their suggestion that in trilobite-like taxa the fourthpair of biramous cephalic appendages was directly under thecephalo-thoracic junction This may explain previous confu-sion about the number of such appendages incorporated intothe trilobite cephalon (eg Cisne 1975 Whittington 1975aBergstrom amp Brassel 1984) However Edgecome amp Ramskoldaccept the view of Chen et al (1977 p 7) that only the firstthree biramous limbs of Misszhouia were structurally and

functionally part of the head Therefore it seems more accept-able to code these taxa as having only four post-acronalsomites than to use the coding scheme of Edgecombe ampRamskold (1999) The partial integration of the first thoracicsomite under the head shield could be considered to be a formof overlap of the trunk by the head shield (ie a distinct stateof character 9 of Edgecombe amp Ramskold 1999 or Character37 herein) Derived states of this character would then poten-tially be synapomorphic for a clade including xandarellidsnaraoiids helmetiids tegopeltids and trilobites Howeversince the degree of overlap in taxa with the fourth biramousappendages under the cephalo-thoracic articulation is identicalto that found between thoracic tergites (State 0 of Character

Figure 5 Diagrammatic reconstructions of raptorial second-segment appendages in lateral view (except whereotherwise stated) and approximate life orientation showing suggested homology of appendage elements (inbrackets) Roman numerals indicate position in the podomere series and do not necessarily imply homology (A)lsquoGreat appendagersquo of Fortiforceps (after Hou amp Bergstrom 1997) (B) Left chelicera of Leiobunum aldrichi(Arachnida Opiliones) (1) in medial perspective and (2) distal parts in anterior perspective (after Shultz 2000)(C) Chelifore of Nymphon (Pycnogonida) (after Child 1997) (D) lsquoGreat appendagersquo of Yohoia (after Whittington1974 fig 2) (E) lsquoGreat appendagersquo of Leanchoilia (modified after Bruton amp Whittington 1983) Not to scaleAbbreviations (apt) apotele (probably homologous with the mandibulate pretarsus) (cx) coxa (dtmrt)deutomerite and (pt) pretarsus

178 TREVOR J COTTON AND SIMON J BRADDY

37) it is here preferred to regard this as a distinct character(Character 9 herein)

The coding of Alalcomenaeus by Wills et al (1998a) ashaving four post-acronal somites is accepted here but fordifferent reasons Briggs amp Collins (1999) have recenty demon-strated that Alalcomenaeus possessed two pairs of biramousappendages on the head in addition to the great appendagesThis would equate to three head somites according to thehomology scheme of Wills et al (1998a) Here the antennaeare inferred to be lost and therefore four segments incorpo-rated into the head

Recently a number of authors have agreed that the plesio-morphic condition for euarthropods is a four-segmented headas found in stem group crustaceans (Scholtz 1997 Walossek1993 Walossek amp Muller 1997 1998 Edgecombe et al 2000)However reconstruction of the euarthropod stem groupclearly indicates that even crownward taxa had a head consist-ing of only the antennal segment and acron as found intardigrades and onychophorans Therefore it seems that thefour-segmented head is pleisomorphic only for crown groupeuarthropods Some fully arthropodised fossil taxa also have ashorter head that may be pleisomorphic such as the two-segmented head of Marella Retifacies is coded following therecent redescription of Hou amp Bergstrom (1997) rather thanon the basis of the coding by Edgecombe amp Ramskold (1999)for which they provide no explanation A partial uncertaintycoding is used for Aglaspis in which the number of post-antennal cephalic appendages is either three or four (Briggset al 1979)

5 Orientation of the antennae (0) directed anterolaterally(1) strongly deflected laterally and (2) placed well inside shieldmargin curing posteriorly from a transverse proximal element(cf Edgecombe amp Ramskold 1999 character 3) The anteriorappendages of Aglaspis (see character 1) are clearly directedanterolaterally and it is presumed that they continued past thehead shield margin The anterior appendages of arthropodstem group taxa such as Aysheaia Kerygmachela and Anoma-locaris are either directed anterolaterally or ventrally but arenot strongly-deflected laterally Therefore the outgroup iscoded as State 0 This character is coded as missing data fortaxa where the presence of antennae is equivocal (those codedas missing data for Character 1) and as inapplicable in taxawhere the antennae are considered absent (coded as State 1 forCharacter 1)

6 Length of distal spines on terminal podomeres of endo-pods of second segment appendages (0) absent or shorer thanpodomeres (1) subsequal to length of podomeres and (2)longer than the entire podomere series The spinose projectionsof the raptorial appendages described above vary in length Inchelicerae (Figs 5BC) and some lsquogreat appendagesrsquo (eg thoseof Yohoia Fig 5D) the most distal spine is equal in length tothe spinose terminal podomere forming a chelate structureIn Fortiforceps (Fig 5A) the spines are short relative tothe lengths of the podomeres but in Alalcomenaeus andLeanchoilia (Fig 5E) they are extremely long so that thespines are much longer than the entire podomere series Asdescribed above in taxa with pediform endopods of the secondsegment appendages spines on the distal podomeres are shortor entirely absent

7 Chelicerae (0) absent and (1) present Despite theirsimilarities to lsquogreat appendagesrsquo the chelicerae of the eucheli-cerates and chelifores of the pycnogonids are clearly distinctand have been widely recognised as a chelicerate synapomor-phy Chelicerae differ from any lsquogreat appendagesrsquo by thecombination of a smaller number of podomeres the presenceof only a single terminal element and only a single spinoseprojection on the dorsal side of the podomere series (Fig 5)

Amongst extant chelicerates only the pycnogonid Pallenopsishas chelicerae of four podomeres comparable to the numberin lsquogreat appendagesrsquo Bergstrom et al (1980) considered thatthe chelifores of the Devonian pycnogonid Palaeoisopus alsoconsist of four segments but this is not well supported by theirfigures

8 Distal spines of second segment endopods terminating inannulated flagellae (0) absent and (1) present It is widelyrecognised (eg Stoslashrmer 1944 Simonetta 1970 Bruton ampWhittington 1993 Briggs amp Collins 1999) that the terminalparts of the extended spines of Alalcomenaeus and Leanchoiliaformed annulated flagellae Nothing similar is known fromhomologous appendages of any other arthropod althoughsimilar processes may have operated in the transformation ofthe endopod as a whole into the second antennae of derivedcrustaceans and in the origin of multiramous antennulae (firstantennae) in malacostracans

242 Posterior appendages 9 Appendages of first thoracicsomite underneath the cephalo-thoracic articulation (0) ab-sent and (1) present (cf Edgecombe amp Ramskold 1999character 2) For a discussion of this character see Character4 herein

10 Exopods of appendages of third to fifth segments (0)present and (1) reduced or absent A number of taxa haveuniramous appendages on the third to fifth segments In mostthese segments are incorporated into the head (Yohoia cheli-cerates) but in others all (eg in Sidneyia) or some (eg inCheloniellon) of these segments are post-cephalic In all casesthe appendage is morphologically similar to the endopods ofbiramous limbs of other taxa andor other segments and it isinterpreted that the exopod is lost

11 Endopods of thoracic appendages (0) present and (1)reduced or absent The opisthosomal appendages of chelicer-ates lack endopods or have the endopods much reduced (egLimulus Siewing 1985 fig 838 Shultz 2001) a situation that isalso found in the uniramous thoracic appendages of Yohoia(Whittington 1974) It has been suggested that Helmetia(Briggs et al 1994) lacks endopods but pending a redescrip-tion of this genus and considering that they have been docu-mented in the otherwise very similar Kuamaia this is coded asuncertain

12 Exopod shaft of numerous podomeres each bearing asingle seta (0) present and (1) absent The exopods ofcrustaceans (at least plesiomorphically see eg Walossek ampMuller 1998 figs 5middot5 amp 5middot6) and of Marrella and Mimetasterhave multi-annulated shafts with each podomere bearing asingle seta In other taxa the exopod shafts consist of one oftwo lobes bearing numerous setae

The polarity of exopod segmentation is uncertain Thelateral flaps of stem group arthropods such as Opabinia(Whittington 1975b Budd 1996b) and anomalocaridids (egHou et al 1995) are certainly unsegmented but as suggestedby Buddrsquos (1996b fig 8) reconstruction this does notnecessarily suggest the form of the primitive euarthropodexopod Rather segmentation may have been a result of thesclerotization of the cuticle of the exopod shaft The outgroupwas coded as uncertain for this character According to thefirst interpretation of aglaspidid appendage morphology(coded as Aglaspidida 1) the exopods are lacking on allappendages and consequently this and all other charactersdescribing exopod morphology are coded as inapplicable

13 Exopod shaft differentiated into proximal and distallobes (0) absent and (1) present Ramskold amp Edgecombe(1996 Edgecombe amp Ramskold 1999 character 26) havediscussed the distribution of the trilobite-type bilobate exo-pods recognised by this character (Fig 6A) Among taxaincluded here that were not considered by Edgecombe amp

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 179

Ramskold (1999) exopods consist either of a single flattenedlobe or a homonomous series of podomeres (Fig 6B C) Nostem group euarthropod has a bilobate exopod and theoutgroup is consequently also coded as State 0

14 Proximal lobe of exopod (0) flattened lobe and (1)slender shaft

15 Distal lobe of exopod (0) small to moderate sized flapwith short to moderately long attachment to proximal lobeand (1) large teardrop-shaped with long attachment to proxi-mal lobe Neither of these characters can be coded for taxathat do not have an exopod differentiated into proximal anddistal lobes (Character 13 State 1) without asserting thehomology of the single-lobed exopod with one of these partsThe coding of Edgecombe amp Ramskold (1999 p 280) implic-itly homologizes the exopods of Sidneyia and Retifacies withthe proximal lobe In these taxa this view is perhaps supportedby the presence of lamellate exopod setae which match thoseof the trilobite-type proximal lobe However in other taxawith a single-lobed exopod such as Alalcomenaeus (see Briggsamp Collins 1999) the exopod is fringed with sharp spines likethe distal lobe of differentiated exopods Alternatively thebilobate exopod may have originated from the division of aprimitively single-lobed structure The single-lobed exopod isnot clearly homologous with either lobe of divided exopodsand both these characters are consequently coded as inappli-cable to taxa without differentiated proximal and distal exopodlobes Consequently (see Character 13 herein) these characterscan only be coded for taxa that were previously considered byEdgecombe amp Ramskold (1999) and their coding is followedhere except in the cases of Sidneyia and Retifacies

16 Exopod shaft is a deep rounded flap (0) absent and (1)present All bilobate (Character 13) and segmented (Character12) exopod shafts are long and relatively narrow structures (seeFig 6) whereas some single-lobed exopod shafts have theform of rounded flaps These flap-like exopod shafts are largecompared to the length of the setae and at least half as deep asthey are long This appendage structure is found in cheliceratesand lsquogreat appendagersquo arthropods

17 Medially-directed exopod setae (0) absent and (1)present Walossek amp Muller (1998 p 194) suggested that thetilting of exopod setae towards the endopod in post-antennularlimbs with a multi-annulated exopod was an autapomorphyuniting crustaceans and all members of the crustacean stemgroup (the crustacean total group sensu Ax 1986 see Fig 6C)This character is shared with the marellomorphs Marrellaand Mimetaster (see Fig 6B Sturmer amp Bergstrom 1976Bergstrom 1979 fig 1middot3A B) In other taxa the setae eithersurround the entire margin of the exopod shaft or are directeddorsally (Fig 6A) The identification of setae on the dorsalsurface of the lateral flaps in Opabinia (Budd 1996b) suggeststhat the condition in marellomorphs and crustaceans isderived

18 Lamellate exopod setae (0) absent and (1) presentLamellate exopod setae originally described from trilobitesare a classic synapomorphy of the Arachnomorpha (see egBergstrom 1992 Hou amp Bergstrom 1997 pp 42ndash3) They areperhaps best known from Misszhouia (see Chen et al 1996Hou amp Bergstrom 1997 Fig 6A) These setae differ from themore spinose setae of other taxa (Fig 6C) in that they are wideand flat and imbricate over the length of the exopod shaft Thesetae of Marrella (Fig 6B) and similar taxa have variouslybeen considered lamellate (Bergstrom 1979) or non-lamellate(Bergstrom 1992) Since they do not imbricate and do notseem to be strongly flattened (Whittington 1971 Sturmer ampBergstrom 1976 fig 9a) Marrella and Mimetaster are codedas State 0 The setae of Helmetia as shown in Briggs et al(1994 fig 141) seem to be of the lamellate type The setae ofSinoburius have been described by Hou amp Bergstrom (1997p 85) as similar to those of Misszhouia Only the setae areknown of the appendages of Buenaspis (Budd 1999a) andthese also appear to be of the trilobite-type

Figure 6 Diagrammatic reconstructions of (A) arachnomorph and(B C) non-arachnomorph biramous appendages (A) The lsquotrilobite-typersquo biramous appendage of Misszhouia longicauda (after Chen et al1997) (B) Appendage of Marrella splendens (after Whittington 1971)(C) Appendage of the stem-lineage crustacean Martinssonia elongata(after Muller amp Walossek 1997 1998) Not to scale Abbreviations(bas) basis (dle) distal lobe of exopod shaft (ls) lamellar exopod setae(pe) proximal endite or pre-coxa (ple) proximal lobe of exopod shaft(ses) segmented exopod shaft and (ss) spinose exopod setae

180 TREVOR J COTTON AND SIMON J BRADDY

The homology of the book-gill lamellae of xiphosurans withlamellar setae was supported by Walossek amp Muller (1997p 149) and Edgecombe et al (2000 p 174) but rejected bySturmer amp Berstrom (1981) on the grounds that Weinberginapossesses Limulus-like gill lamellae and fringing setaeBook-gills have also been described from a eurypterid (Braddyet al 1999) There are certainly major morphologicaldifferences between book-gills and trilobite-type exopodsFollowing most recent opinion and pending further studyWeinbergina and Eurypterida are coded as possessing lamellatesetae

The form of the setae of Yohoia is uncertain (Whittington1974) the spinose setae seen in the most common reconstruc-tion (Gould 1989 Briggs et al 1994) are not justified by thespecimens Amongst other great appendage arthropods theform of the setae appears to vary those of Jianfengia (seeChen amp Zhou 1997 p 74) and Leanchoilia (see Bruton ampWhittington 1983) are lamellate while those of Fortiforceps(Hou amp Bergstrom 1997) and Alalcomenaeus (Briggs amp Collins1999) are spinose and less densely packed

19 Gnathobase on basis andor prominent endites onendopod (0) present and (1) absent (cf Edgecombe ampRamskold 1999 character 29 Wills et al 1998a characters 36amp 59) The presence of gnathobases on the appendages of someanomalocaridids (Hou et al 1995) and their general distribu-tion amongst non-arachnomorph euarthropods suggests thatState 0 is plesiomorphic The homology of the various enditesand gnathobasic structures found in euarthropods is in need ofcareful assessment and therefore a very general coding (fol-lowing Edgecombe amp Ramskold) is used for this character

243 Eyes 20 Position of lateral facetted eyes (0)ventral and stalked (1) dorsal and sessile and (2) absent (cfEdgecombe amp Ramskold 1999 character 4) Partial uncer-tainty coding is used for a number of taxa where the dorsalsurface is known but the ventral morphology is not andtherefore the presence of ventral eyes cannot be discountedFor example there is good evidence that Leanchoilia lackeddorsal sessile eyes but ventral stalked eyes may have beenpresent (as in L hanceyi see Briggs amp Robison 1984) and a(02) partial uncertainty coding is used This is also the case inmany naraoiids such as Buenaspis However only the outlineof the head shield of Liwia is known and the absence of dorsaleyes in the reconstruction of Dzik amp Lendzion (1988) isconjectural It is unclear whether the autapomorphic conditionof stalked dorsal eyes in Mimetaster (see Sturmer amp Bergstrom1976) is homologous with State 0 or State 1 and a partialuncertainty coding is also used

Whittington (1974) was somewhat equivocal about thenature of the lateral lobes anterior to the head shield ofYohoia These more closely resemble the ventral stalked eyes ofAlacomenaeus as recently described by Briggs amp Collins(1999) than either Whittingtonrsquos (1974) or subsequent (egGould 1989 fig 3middot18 Briggs et al 1994 fig 153) reconstruc-tions suggest In a well preserved specimen showing the dorsalaspect (USNM 57696 Whittington 1974 pl 2 figs 1ndash3) and ina laterally compressed specimen (USNM 57694 Whittington1974 pl 1 fig 1) they are clearly seen to be stalked andrelatively small lobate structures That these structures arelikely to represent stalked ventral eyes was reflected in thecoding of Wills et al (1998a characters 26 amp 27)

21 Visual surface with calcified lenses bounded withcircumocular suture (0) absent and (1) present

22 Dorsal bulge in exoskeleton accommodating drop-shaped ventral eyes (0) absent and (1) present

23 Eye slits (0) absent and (1) presentCharacters 21 to 23 are used exactly as described by

Edgecombe amp Ramskold (1999 character 5) Character 21 is

inapplicable to taxa that do not possess eyes (Character 20State 2)

24 Dorsal median eyes (0) absent and (1) present Dorsalmedian eyes on a tubercle are considered a synapomorphy ofthe Chelicerata (Dunlop amp Selden 1997 Dunlop 1999) Thenature and position of the structures interpreted as dorsalmedian eyes in Mimetaster (Sturmer amp Bergstrom 1976p 83ndash4) are regarded as equivocal

244 Ventral cephalic structures Many characters of theventral surface of the euarthropod head (see Fig 7) of poten-tial phylogenetic utility are poorly known in many importanttaxa In particular the presence of pre-hypostomal frontalorgans (Fig 7A CndashE) is not coded herein (see Edgecombe ampRamskold 1999 p 272) Definitive evidence of the absence ofthese structures is available for very few taxa and the homol-ogy of these structures with the maculae of the trilobitehypostome (see Fig 7B) and the frontal organs of the crusta-cean labrum (eg see Muller amp Walossek 1987 p 39) isuncertain Secondly the detailed homology of the trilobitehypostome with that of other putative arachnomorphs isunclear and consequently a very broad definition is usedherein For example various pre-hypostomal sclerites (Fig7A D) in other arachnomorphs may have been incorporatedalong with a primitive hypostome into a single sclerite that isrecognised as the hypostome in trilobites The structure of thehypostome (eg the presence of paired anterior wings dorsal tothe antennae Fig 7B C) may also be a source of additionalcharacters

25 Expanded cephalic doublure (0) absent and (1)present maximum width more than 30 length of head shieldor more than 25 width of pygidium Chen et al (1996)suggested that the wide doublure of Soomaspis and Tariccoia isa synapomorphy uniting these taxa The doublure of Buenaspis(see Budd 1999a) is poorly known but based on the width ofthe heavily crushed region of the cephalic margin and theposition of a possible impression of the edge of the doublure inone specimen (Budd 1999a pl 1 fig 1) it also appears to berather wide (ie greater than 30 of the length of the headshield) Following Edgecombe amp Ramskoldrsquos coding ofSidneyia the present authors do not consider the postero-median expansion of the doublure of for example Emeraldellaor Retifacies (see Bruton amp Whittington 1983 Hou ampBergstrom 1999 respectively) to be homologous with State 1but to represent the hypostome (see Character 26)

26 Anteromedian margin of cephalon notched accomo-dating strongly sclerotised plate (0) notch and plate absentand (1) notch and plate present (cf Edgecombe amp Rasmkold1999 character 10) The apomorphic state (illustrated inFig 7D) is only known in the taxa discussed by Edgecombe ampRasmkold (1999) Reconstructions of Yohoia show a roundedlobe between the eyes (see Character 20) anterior to anddistinct from the head shield which resembles the anteriorsclerite recognised here This seems to be a misinterpretationSpecimens preserved in dorsal aspect (USNM 57696Whittington 1974 pl 2 figs 1ndash3) and lateral aspect (USNM57694 Whittington 1974 pl 1 fig 1 USNM 155616Whittington 1974 pl 5 fig 2) seem to clearly show that thislsquomedian lobersquo is a downward curving pointed extension of thehead shield

27 Hypostomal sclerite (0) median extension of thedoublure with no suture (1) natant sclerite not in contactwith doublure (2) with narrow overlap with pre-hypostomalsclerite (3) narrow attachment to doublure at hypostomalsuture and (4) absent The homology of hypostomes andlabrums has been discussed by Haas et al (2001) and Budd(2002) The euarthropod labrum is likely to represent a pos-teroventral extension of the pre-segmental acron (Scholtz

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 181

Figure 7 Ventral reconstructions of arachnomorph arthropod heads (A) Misszhouia longicaudata (after Chenet al 1997 figs 2ndash4 7) (B) Ceraurinella typa (after Chen et al 1997 fig 7) (C) Agnostus pisiformis (after Mulleramp Walossek 1987) (D) Kuamaia lata (after Edgecombe amp Ramskold 1999 figs 32 5 amp 6) (E) Cindarella eucala(after Ramskold et al 1997) and (F) Emeraldella brocki (after Bruton amp Whittington 1983) Not to scaleAbbreviations (aw) anterior wing beneath antennae (bs) boomerang-shaped sclerite (bl) blade-like process ofposterior wing (d) doublure of head-shield (cs) connective suture (f) fenestra (fs) facial suture (h) hypostome(hl) hypostomal lobe (emerging through fenestrae of Agnostus) (hs) hypostomal suture (if) intervening field ofhypostomal complex (lb) lateral bridge (lfo) lateral frontal organ (mb) median body (mbr) median bridge (mf)median furrow (mfb) mouth field bridge (mfo) median frontal organ (ol) ovate lobe and (phs) pre-hypostomalsclerite Post-antennal appendages and distal parts of antennae omitted

182 TREVOR J COTTON AND SIMON J BRADDY

1997) This structure is often sclerotised and covers the mouthventrally It is this sclerite that is here identified as thehypostome Therefore this character is considered absent intaxa lacking a sclerite covering the mouth irrespective of themorphology and position of the labrum itself which at leastpartly covers the mouth in all euarthropods other thanpycnogonids (see Edgecombe et al 2000 p 167) The lsquofleshylabrumrsquo of crustaceans does not appear to be sclerotised and ahypostome is consequently considered to be absent Althoughmany derived features are associated with the crustaceanlabrum this is not a good reason to consider it non-homologous with the hypostome-bearing structure of otherarthropods as Walossek amp Muller (1990) argued

In many taxa the mouth is covered by a posteromedianextension of the doublure here considered to represent thehypostome (eg Emeraldella Fig 7F Aglaspis see Hesselbo1992 fig 52) In others the hypostome is separated from thedoublure either by a suture a pre-hypostomal sclerite (egKuamaia lata Fig 7D Saperion glumaceum see Edgecombe ampRamskold 1999 figs 3 amp 4) or an intervening region ofunsclerotised cuticle (the natant condition of Fortey 1990beg Cindarella eucala Fig 7E) The homology of pre-hypostomal sclerites and hypostomes in helmetiids trilobitesand naraoiids (see Fig 7) has been discussed by Chen et al(1996) and Edgecombe amp Ramskold (1999) but it is as yetunclear whether any of the character states described here arederived from any other They are treated here as distinct andthe character as unordered pending further study

No hypostome has been identified in Yohoia (see above)Alalcomenaeus Jianfengia Fortiforceps or Leanchoilia andthere is certainly no broad posterior extension of the doublurein any of these taxa Since there is also no pre-hypostomalsclerite they have received a (134) partial uncertainty codingThe attachment of the hypostome of Tegopelte is unclear butit does not seem to be attached to any doublure (Whittington1985) and therefore it is given an uncertainty (12) coding

The lsquorostrum-like structurersquo on the ventral surface ofLemoneites appears to be similar in morphology and positionrelative to the anterior margin of the head shield (Flower 1968pl 8 figs 4 amp 13) to that described from Aglaspis and is codedas State 0 In many taxa the hypostome is unknown but inonly a small number can it be coded reliably as absentHughesrsquo (1975) suggestion that the hypostome of Burgessia isabsent is accepted preliminarily but an extension of thedoublure may be visible in some of Hughesrsquo figures Fortey(pers comm 2000) also doubts Hughesrsquo assertion

28 Visible ecdysial sutures (0) absent and (1) present Themarginal ecdysial sutures of chelicerates and the dorsal suturesof trilobites are almost certainly not homologous but thepresence of sutures and their position (see Character 29) arecoded separately to allow this to be tested Wills et al (1998aCharacter 2) coded marginal sutures as present in Sidneyiafollowing Brutonrsquos (1981) suggestion but this is consideredequivocal here

29 Position of ecdysial sutures (0) marginal and (1) dor-sal This character is inapplicable to taxa lacking ecdysialsutures (Character 28 State 0) Dorsal sutures whilst presentin the representatives of the Trilobita coded here are probablynot a synapomorphy of trilobites as a whole but for a clade oftrilobites excluding the Olenellida (Whittington 1989 Fortey1990a)

245 Exoskeletal tergites and thoracic tagmosis 30 Min-eralised cuticle (0) absent and (1) present Calcification of theexoskeleton is one of the most convincing synapomorphiesof the Trilobita (Fortey amp Whittington 1989 Ramskold ampEdgecombe 1991 Edgecombe amp Ramskold 1999 Character 1)Cuticle mineralisation is also coded as present in aglaspidids

following Briggs amp Fortey (1982) who suggested that theaglaspidid exoskeleton was originally phosphatic Paleomerusprobably also had a mineralised cuticle but Hou amp Bergstrom(1997 p 97) argued that it was likely to be calcareous Anundescribed Silurian aglaspidid may also have had an origi-nally calcitic cuticle (Fortey amp Theron 1994 p 856) Becauseof the uncertainty about the chemical composition of theexoskeleton in these forms cuticle mineralisation is coded aspotentially homologous in all these taxa and no attempt ismade to code separate states for phosphatic and calcareousmineralisation

31 Trunk tergites with expanded lateral pleurae coveringappendages dorsally (0) absent and (1) present Whilst thepresence of paratergal folds may be a synapomorphy at thelevel of the Euarthropoda (Boudreaux 1979 Wagele 1993Edgecombe et al 2000 Character 142) these are at mostsmall reflections of the margin of the tergites in most euarthro-pods In arachnomorph taxa these are expanded to form largelateral pleurae that cover the appendages dorsally (see egLimulus and Triarthus in Boudreaux 1979) This is inferred tobe the case in some taxa where dorsal features and appendagesare poorly known on the basis of their possession of tergiteswith wide pleural regions This feature was suggested as typicalof lamellipedians (=arachnomorphs) by Hou amp Bergstrom(1997 pp 42ndash3)

The arrangement of the thorax of Marrella Mimetaster andBurgessia differs from that of all the other taxa considered herein that the appendages seem to be attached to the lateralmargins of the body The trunk tergites do not cover theappendages and seem to lack pleurae entirely although theyare covered by a posterior extension of the head shield inBurgessia This character has been clearly described inMimetaster (Sturmer amp Bergstrom 1976 pp 87ndash90 fig 8) andBurgessia (Hughes 1975 p 421)

32 Free thoracic tergites (0) present and (1) absentFollowing Edgecombe amp Ramskold (1999 Character 16) anumber of taxa lack functional post-cephalic articulations andconsequently lack free thoracic tergites Despite this theycode these taxa for a number of characters relating to thestructure of thoracic tergites (eg their characters 18 and 19)These characters are treated here as inapplicable to taxawithout free tergites and the definition of this character hasbeen modified from that of Edgecombe amp Ramskold (1999) tomake this more explicit

33 Decoupling of thoracic tergites and segments (0)absent and (1) present Ramskold et al (1997) described thischaracter of Cindarella and Xandarella where the thoracictergites correspond to a variable increasing posteriorly num-ber of appendage pairs This character cannot be coded fortaxa in which free thoracic tergites are absent (Character 32State 1)

34 Tergite articulations (0) tergites non-overlapping (1)extensive overlap of tergites and (2) edge-to-edge pleuralarticulations In most of the taxa considered here the thoracictergites overlap considerably and relatively evenly over theirwidth This contrasts with the primitive euarthropod condi-tion where the tergites do not overlap medially as seen in stemgroup crustaceans In trilobites such as Helmetia and Kuamaia(see Edgecombe amp Ramskold 1999 character 18) overlap ofthoracic tergites is limited to a well-defined axial region andthe lateral pleurae meet edge-to-edge

The thoracic articulations of naraoiids with free thoracictergites are in some ways similar to those of trilobites with astrong overlap medially that is lacking abaxially andanterolateral articulating facets (Budd 1999a Ramskold ampEdgecombe 1999) However the distribution of these features

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 183

in other trilobite-like arachnomorphs is unclear and a restric-tive coding is used here The present authors do not considerthat this character can be applied reliably to the fused thoracictergites of Saperion Tegopelte and Skioldia and it is coded asinapplicable to all taxa lacking free thoracic tergites (Character32 State 1)

35 Trunk effacement (0) trunk with defined (separate orfused) tergite boundaries (1) trunk tergite boundaries effacedlaterally and (2) trunk tergite boundaries completely effaced(cf Edgecombe amp Ramskold 1999 Character 15)] Contrary toEdgecombe amp Ramskold (1999) the present authors considerSkioldia and Saperion to show a distinct form of tergiteeffacement where the fused tergite boundaries are defined byfurrows axially but effaced laterally The boundaries areeffaced at least laterally in Tegopelte but the axial region ofthe exoskeleton is unknown which accordingly is given amultistate uncertainty coding

36 Cephalic articulation fused (0) absent and (1) presentIn Tegopelte Saperion and Skioldia the articulation ofthe head with the thorax is non-functional and the entireexoskeleton forms a single tergite

37 Head shield overlap of thoracic tergites (0) overlapabsent or identical to overlap between thoracic segments (1)head shield covers first thoracic tergite only and (2) headshield covers mutliple anterior trunk tergites

38 Head shield articulates with reduced anterior thoracictergite (0) absent and (1) present Amongst taxa showing aposteriorly expanded head shield ie one that overlaps thethorax Edgecombe amp Ramskold (1999 Character 9) identifiedtwo distinct character states One of these ndash overlap of only theanteriormost thoracic tergite ndash was limited to taxa assigned tothe Liwiinae (sensu Fortey amp Theron 1994) and another ndashoverlap of multiple tergites with attachment to a reducedanterior thoracic tergite ndash to the Xandarellida None of theadditional taxa considered herein (including Buenaspis whichwas assigned to the Liwiinae by Budd 1999a) shows thesedistinctive morphological features However the expandedhead shields of Marrella Mimetaster and Burgessia which donot articulate with a narrow anterior thoracic tergite may behomologous with the expanded head shields of xandarellidsTo recognise this the degree of overlapping of the thorax andthe possession of the reduced anterior tergite are coded sepa-rately These characters are both coded as inapplicable to taxain which the head shield and thoracic tergites are fused(Character 36 State 1) The crustacean carapace which ismost similar to the head shield of Burgessia is seeminglyabsent in stem group crustaceans (Walossek amp Muller1998)

39 Trunk narrowed anteriorly relative to head shieldwidest posteriorly (0) absent and (1) present (cf Edgecombeamp Ramskold 1999 Character 14) The anterior narrowing ofthe trunk caused by the reduction of opisthosomal segment 1in xiphosurids (Weinbergina of the taxa analysed hereAndersen amp Selden 1997 Dunlop amp Selden 1997 Character14) is not considered to be homologous with the unusual shapeof the thorax in naraoiids recognised by their coding

40 Boundaries of anterior trunk segments reflexed antero-laterally (0) absent boundaries transverse or reflexed postero-laterally and (1) present

41 Joints between posterior tergites functional anteriorones variably fused (0) absent and (1) present

42 Posterior tergite bearing axial spine (0) absent and (1)present

Characters 40 to 42 are coded following Edgecombe ampRamskold (1999 characters 17 19 amp 23 respectively) Inaddition to the taxa considered here a thoracic axial spine ispresent in some olenelloid trilobites (see Lieberman 1998

Characters 73 amp 74) and in the aglaspidid Beckwithia (seeHesselbo 1989)

246 Posterior body termination 43 Postabdomen of seg-ments lacking appendages (0) absent and (1) present

44 Length of postabdomen (0) one segment (1) twosegments (2) three segments and (3) five segments In sometaxa a variable number of posterior thoracic segments bearcomplete tubular tergites and lack appendages In Aglaspisautapomorphically it seems that the posterior three thoracicsegments lack appendages (Briggs et al 1979 Hesselbo 1992)but have unmodified tergites These situations are recognisedas potentially homologous by the coding used here Character44 is coded as inapplicable to taxa lacking a postabdomen

45 Posterior tergites strongly curved in dorsal aspect com-pared to anterior tergites (0) absent and (1) present Asrecognised by Wills et al (1998a Character 17) in some taxawhere the tergites are distinct the curvature of thoracic tergitesincreases posteriorly so that the posterior tergites are highlycurved to semicircular in dorsal aspect This situation isnot known from crustaceans (except isopods) or stem grouparthropods

46 Posterior segments reduced and with highly reducedappendages (0) present and (1) absent In some taxa there area large number of posterior segments that are short sagitallyand have appendages that are much reduced in size comparedto anterior trunk appendages These somites are incorporatedinto the pygidium in some taxa but primitively they arecovered by tiny free trunk tergites In the derived state thetrunk somites and limbs are of a relatively constant size Thedistinction between these states can be seen when attempting tocount the number of segments that make up the body In taxashowing State 0 the number of segments is very high anddifficult to count whereas in other taxa the number ofpost-cephalic segments is easily assessed

47 Pygidium (0) absent and (1) present A pygidium isrecognised here as a posterior tagma consisting of a number offused segments under a single tergite which may or may notincorporate the post-segmental telson This is different to theuse of this term by Edgecombe amp Ramskold (1999) whichfollows Ramskold et al (1997) and very different to its use byWills et al (1998a p 53) as a synapomorphy of the TrilobitaThe present authorsrsquo definition recognises the situation inRetifacies where the spinose postsegmental telson is autapo-morphically not fused to the pygidium as homologous toother multisegmented posterior tagma which include thetelson The distinction between the pygidium and an expandedpost-segmental telson can also be seen in the position of theanus which is consistently in the posteriormost pre-telsonicsegment amongst putative arachnomorphs (see Character 48)In taxa with a pygidium the anal segment is fused into thepygidium (see eg Kuamaia Edgecombe amp Ramskold 1999fig 6) whereas in those taxa lacking a pygidium the anus isbetween the final trunk tergite and the telson

Ramskold et al (1997) argued that the posteriormost tergiteof xandarellids (which incorporates the anal segment) is nothomologous to the posterior tagma recognised as pygidiaherein because posterior thoracic tergites also cover multiplesegments in xandarellids (see Character 33) Evidence ofsegmentation of the pygidium in a variety of taxa suggests thatthe number of tergites fused to form the pygidium is greaterthan the number of appendages For example the pygidium ofKuamaia has two pairs of lateral spines but at least fourpairs of appendages (Hou amp Bergstrom 1997 Edgecombe ampRamskold 1999) and that of Triarthrus has only five axialrings posterior to the articulating ring but over 10 pairs ofappendages (Whittington amp Almond 1987) The fact thatdecoupling of tergites and segments is evident in the thorax of

184 TREVOR J COTTON AND SIMON J BRADDY

Xandarella and Cindarella does not necessarily suggest that asimilar more widespread decoupling in pygidia is the result ofa different developmental process and hence that they arenon-homologous Instead the situation in the xandarellidthorax is potentially homologous to that found (more widely)in pygidia and consequently the terminal xandarellid tergite isa pygidium It is unclear if decoupling of tergites and somites isprimitively limited to the terminal tergite or primitively aproperty of the arachnomorph post-cephalon as a whole

It has been suggested that the tiny pygidium of olenelloidtrilobites which probably best reflects the primitive trilobitestate consists only of the post-segmental telson (Harrington inMoore 1959) and therefore is not a true pygidium HoweverWhittington (1989) showed that the olenelloid pygidium con-sists of at least two and possibly as many as five or sixsegments and therefore is likely to be homologous with thepygidium of other trilobites

48 Position of the anus (0) terminal within telson and (1)at base of telson In crustaceans and stem group arthropodsthe anus is terminal or otherwise situated in the telson (egRehbachiella Walossek 1993 fig 15D) In putative arachno-morphs the anus is either at the junction of the posteriormostthoracic segment and the telson or ventral within a fusedpygidium In the case of taxa with a pygidium the anus isanterior to the posterior margin and where known positionedbetween the posteriormost pair of appendages suggesting thatit is anterior to the post-segmental telson (see eg OlenoidesWhittington 1980)

49 Pygidium with median keel (0) absent and (1) presentEdgecombe amp Ramskold (1999 Character 21) considered thepresence of a median keel on the pygidium as a synapomorphyuniting Soomaspis and Tarricoia They did not explain howthis structure could be distinguished from the raised pygidialaxis of trilobites Homology of these structures is not sup-ported here because the axis of more primitive trilobites(including Eoredlichia) is quite distinct morphologically fromthe naraoiid keel Budd (1999a) noted the presence of a keel onthe pygidium of Buenaspis and suggested (Budd 1999a p 102)that it may be an artefact of dorsoventral compaction How-ever contrary to the coding of Edgecombe amp Ramskold(1999) a keel is visible on the pygidium of Liwia convexa (Dzikamp Lendzion 1988 fig 4CD) preserved in full relief Thereforeit seems unlikely that the keel is a taphonomic artefact

50 Pygidium with broad-based median spine (0) absentand (1) present

51 Pygidium with lateral spines (0) present and (1) absentEdgecombe amp Ramskold (1999 Character 22) coded thepresence of a median spine and two pairs of lateral spines aspotentially homologous in Sinoburius Kuamaia and HelmetiaThe distribution of lateral spines is much wider than that ofterminal spines and therefore they are coded separately hereIn trilobites it has widely been recognised that lateral spinesrepresent the original segmentation of the pygidium Thiscannot be the case for median spines Characters 49 to 51are not applicable to taxa lacking a pygidium (Character 47State 0)

52 Expanded post-segmental telson (0) absent and (1)present In a range of taxa the posterior-most tergite is a large(relative to the thoracic tergites) structure that lacks anyevidence of segmentation and is interpreted as representing anexpanded tergite of the post-segmental telson from whichsegments are released anteriorly during ontogeny On the otherhand the posteriormost tergite of Marrella and probablyMimetaster is small compared to the thoracic tergites androunded This element probably represents the plesiomorphiceuarthropod telson The homology of this character in mosttaxa is clear and only doubtful in Retifacies in which it may

be segmented The figures of Hou amp Bergstrom (1997) providelittle unequivocal evidence for a telson in Retifacies but thepresence of this structure is very clear in colour photographs(eg Chen et al 1996 fig 198A) However these figures donot convincingly demonstrate the segmentation of the telsonRetifacies is interpreted here as being unique in possessingboth a pygidium and an expanded post-segmental telson

53 Telson shape (0) spinose and (1) paddle-shaped Theshape of the expanded telsons identified above varies frombroadly spinose to flattened and paddle-like This differencecan be recognised both on the basis of the ratio of length tobasal width (spinose telsons are relatively long) and by thechange in width posteriorly (spinose telsons reduce in widthposteriorly whereas paddle-shaped telsons increase in width)In eurypterids a wide range of telson shapes is found includ-ing both character states described here It is likely that theplesiomorphic condition is a spinose telson This character isinapplicable to taxa lacking an expanded telson

54 Post-ventral furcae (0) absent and (1) present Thischaracter recognises the potential homology of unsegmentedpaired structures that articulate with the segment immediatelyanterior to the telson The potential homology of these struc-tures (the post-ventral plates) in Emeraldella and Aglaspis wasrecognised by Wills et al (1998a Character 70) and inEmeraldella and Sidneyia by Edgecombe amp Ramskold (1999Character 25) The homology of the segmented pygidial caudalfurcae of Olenoides (see Whittington 1975a 1980) with thesestructures is considered doubtful and coded as equivocal

Despite their distinctive morphology the long caudualfurcae of Cheloniellon may be homologous with the furcae ofAglaspis Emeraldella and Sidneyia This is supported by thecaudal furcae of the Ordovician cheloniellid Duslia which aremore similar in morphology to those of Emeraldella than ofCheloniellon Chlupac (1988 pp 614ndash16) considered thesestructures to be on the terminal tergite in Duslia However afaint tergite boundary is apparent posterior to the attachmentof the furcae (see Chlupac 1988 pl 57 fig 4) Therefore theyare considered here to have been attached to the pre-telsonicsegment as in Cheloniellon Chlupac (1988) and Dunlop ampSelden (1997) supported a close relationship between Dusliaand Cheloniellon

25 MethodsAll analyses were carried out using PAUP Version 40b4a(Swofford 1999) and unless otherwise stated used heuristicsearches with one thousand random addition sequencereplicates The software packages MacClade Version 307(Maddison amp Maddison 1997) and RadCon (Thorley amp Page2000) were used for comparing trees and investigating patternsof character evolution Tree length and other statistics (theConsistency Index CI and Retention Index RI) were calcu-lated by PAUP and MacClade with uninformative charactersexcluded Analyses were run separately using each of thedifferent codings for aglaspidids

Characters were treated as unordered and of equal weight inmost analyses To assess the influence of this assumption fourof the eight multistate characters which have states that areintermediate between others (Wilkinson 1992) were treated asordered in some analyses These characters are the number ofcephalic segments (Character 4) the length of raptorialappendage spines (Character 6) the degree of overlap of thethorax by the head-shield (Character 37) and the number ofsegments making up the postabdomen (Character 44) The lastof these is uninformative when not ordered because only onetaxon shows each of states 0 1 and 3

Support for individual nodes was assessed by bootstrapanalysis (Felsenstein 1985) and by calculating Bremer support

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 185

indices (Bremer 1988 1994) These methods measure twodistinct aspects of support for phylogenetic hypotheses Treesor nodes may be considered well supported (1) to the extent towhich alternative topologies are much less parsimonious asmeasured by the support index (Wilkinson 1996) or (2) wherethey are consistent with a large proportion of characters sothat character sampling is unlikely to have had much influenceon topology as assessed by bootstrapping (Page 1996) Boot-strapping was performed with 100 bootstrap replicates eachof 10 addition sequence replicates Bremer support indicesare based on heuristic searches with 100 addition sequencereplicates

Three pairs of terminals are included in Table 2 that apartfrom missing data are coded identically These are Helmetiaand Kuamaia Soomaspis and Tariccoia and Saperion andSkioldia These pairs are lsquotaxonomic equivalentsrsquo removingthe taxon with most missing data will not influence theposition of the other They can be recoded as single terminalsaccording to the principal of safe taxonomic reduction(Wilkinson 1995a b) and were not considered separately in theanalyses presented here Therefore cladistic analyses werecarried out on the basis of 33 terminals Edgecombe ampRamskold (1999) ignored this and coded the same three pairsof taxa and Eoredlichia and Olenoides identically for theirmore limited set of characters

26 ResultsAnalysis with all characters unordered and using the lsquoAglaspi-dida 1rsquo coding found nine most parsimonious trees (MPTs)each 126 steps long (CI=0middot556 RI=0middot763) Using the lsquoAglas-pidida 2rsquo coding resulted in 18 MPTs of length 125 (CI=0middot560RI=0middot765) Nine of these 18 trees were those found with thefirst coding of aglaspidids The strict component consensus(sensu Wilkinson 1994) of the 27 unique trees found acrossboth analyses is shown in Figure 8 All trees supported themonophyly of the Marrellomorpha (Marrella and Mimetaster)and of a clade including all other taxa except Crustacea (Clade1 of Fig 8) The relationships between these two clades and theCrustacea were unresolved equal numbers of trees in bothanalyses supported each of the topologies [Clade 1 (CrustaceaMarrellomorpha)] [Crustacea (Marrellomorpha Clade 1)]and [Marrellomorpha (Crustacea Clade 1)] All trees placedBurgessia as the sister taxon to the rest of Clade 1 andsupported the division of the rest of Clade 1 into two majorsister groups The topology of one of these clades (Clade 2)which included trilobites helmetiids tegopeltids naraoiidsxandarellids and Retifacies was stable with respect to thecoding of aglaspidids The other clade (Clade 3) consists of aclade of megacheirans and chelicerates (Clade 4) and a cladeincluding aglaspidids Lemoneites Paleomerus CheloniellonEmeraldella and Sidneyia (Clade 5) This latter clade is un-resolved in the strict consensus of all 27 trees (Fig 8) and thestrict consensus of the 18 MPTs found using the lsquoAglaspidida2rsquo coding but was fully resolved in the consensus of the ninetrees found with the lsquoAglaspidida 1rsquo coding Within themegacheiran-chelicerate clade all trees found Fortiforceps tobe the sister group to all other taxa Yohoia the sister group tochelicerates and Alalcomenaeus and Leanchoilia to form aclade Jianfengia is equally parsimoniously placed as the sistergroup to the Yohoia-chelicerate clade or as the sister group tothis clade and the Alalcomenaeus-Leanchoilia clade

Treating characters 4 6 37 and 44 as ordered had a minimaleffect on most parsimonious topology Nine MPTs of 136 steps(CI=0middot515 RI=0middot743) were found with the lsquoAglaspidida 1rsquocoding and 12 MPTs 135 steps in length (CI=0middot519 RI=0middot745)with the lsquoAglaspidida 2rsquo coding These trees all supported a(Crustacea Clade 1) clade excluding the marrellomorphs

Apart from this the strict consensus of these 21 trees isidentical to the consensus of the 27 trees found with allcharacters treated as unordered

Across all analyses four different topologies for Clade 5 (ofFig 8) were found (Fig 9) The distribution of these topologiesacross the four analyses described above and across the sameanalyses repeated after reweighting of each character in pro-portion to their Rescaled Consistency Index (see Farris 1989)from the first analysis is shown in Table 3 The instability of

Figure 8 Strict consensus of 27 MPTs from four separate analyses ofarachnomorph phylogeny with both interpretations of aglaspididmorphology and with some characters treated as ordered or allcharacters unordered Clades referred to in the text are numbered

Figure 9 Alternative equally parsimonious resolutions of clade 5 ofFig 8 (see Table 3)

186 TREVOR J COTTON AND SIMON J BRADDY

this clade was to some extent caused by the inclusion ofLemoneites and Paleomerus which could each be coded foronly 27 of the 54 characters used Analysis with these two taxaexcluded and characters treated as unordered found treescompatible only with Topology D of Figure 9 irrespective ofthe coding used for aglaspidids Analysis with some characterstreated as ordered and using either aglaspidid coding foundthree trees compatible only with Topology B of Figure 9 andthree compatible only with Topology D of Figure 9 ThereforeTopology D is the preferred topology for Clade 5 as shown inTable 3

One of the MPTs resulting from the lsquoAglaspidida 2rsquo coding(with all characters treated as unordered) was chosen as thebasis for further discussion because this tree combined thepreferred topology of Clade 5 with the (Crustacea Clade 1)group that was favoured when some characters were treated asordered Bootstrap percentages and Bremer support values forthis tree (using the Aglaspidida 2 coding) are shown in Figure10 Apomorphies for all ingroup nodes of this tree are shownin Figure 10A

3 Discussion

31 Phylogenetic resultsThe results of the cladistic analyses presented above arewell-resolved and robust with respect to different analyticalparameters Most of the taxa considered form a clade (Clade 1of Fig 8) that is more closely related to chelicerates than tocrustaceans in all analyses and therefore is recognised asthe Arachnomorpha as defined by Chen et al (1996) andRamskold et al (1996) The monophyly of the Marrellomor-pha was supported in all analyses The marrellomorphs werefound to be the sister group to a Crustacea+Clade 1 group inmost MPTs but in a minority of trees formed the sister groupto Clade 1 alone According to the latter result they should beincluded within the Arachnomorpha but according to the firstexcluded from it Both of these alternatives have been sup-ported in previous studies (eg Hou amp Bergstrom 1997 Willset al 1998a respectively) In addition to the marrellomorphsthe Fuxianhuiida Bousfield 1995 and Canadaspidida havebeen placed in the Euarthropoda outside the main mandibu-late and arachnomorph clades (Hou amp Bergstrom 1997)Further study of these taxa of the crustacean or mandibulatestem group and of the marrellomorphs is necessary before the

position of the Marrellomorpha can be resolved Provisionallymarrellomorphs are excluded from the Arachnomorphafollowing the majority of analyses above and Wills et al(1994)

The Burgess Shale arthropod Burgessia was placed as thesister group to remaining arachnomorphs in all analyses OtherArachnomorpha form two major clades one consisting of thelsquotrilobite-alliedrsquo arachnomorphs analysed by Edgecombe ampRamskold (1999) here termed lsquoTrilobitomorpharsquo (defined asTrilobita and all taxa more closely related to Trilobita thanChelicerata) and the other (lsquochelicerate-alliedrsquo) clade includingchelicerates lsquogreat appendagersquo arthropods aglaspididsLemoneites Paleomerus Sidneyia and Emeraldella The latteris here termed lsquoCheliceramorpharsquo (defined as Chelicerata andall taxa more closely related to Chelicerata than Trilobita) Thetopology of the lsquotrilobite-alliedrsquo clade was similar to that foundby Edgecombe amp Ramskold (1999) but the present authorsrsquoresults are more completely resolved unambiguously support-ing the sister-group relationship between trilobites andHelmetiida suggested in the previous study Within thelsquochelicerate-alliedrsquo clade Aglaspidids Paleomerus Lemoneitesand the Burgess Shale Emeraldella and Sidneyia form a cladein opposition to megacheirans and chelicerates

The present authorsrsquo analysis supports a new hypothesis ofthe origin of the chelicerates from within a paraphyleticassemblage of megacheiran arthropods and confirms pyc-nogonids as a sister group to the euchelicerates Previoushypotheses of the chelicerate sister group (see Section 1) aremuch less parsimonious The shortest trees supporting aCheloniellon-chelicerate clade are six steps longer than theMPTs and those supporting an aglaspidid-chelicerate cladeseven steps longer All megacheirans and chelicerates areunited by the flap-shaped rounded exopods and modificationof the second cephalic segment endopods into anteriorlydirected spinose raptorial organs which share a number ofdetailed similarities A clade including all of these taxa apartfrom Fortiforceps is supported by the synapomorphic loss ofthe antennae (or antennulae of crustaceans) and the longerlength of spinose projections of the second cephalic appendageendopods Within this clade Yohoia shares the loss of cephalicexopods the loss of thoracic endopods and a postabdomen oftubular tergites with chelicerates

The poor resolution of Clade 5 (of Fig 8) was partly a resultof the poor preservation of and consequent high proportion ofmissing data for Lemoneites and Paleomerus When these two

Table 3 Numbers of most parsimonious trees supporting each of the alternative topologies for clade 4 of Fig 7 under different analyticalconditions (RCI=Revealed Consistency Index)

Aglaspididcoding

Characters 4 6 36and 43 ordered

Charactersreweighted by RCI

Paleomerus andLemoneites excluded

Topology (see Fig 8)

A B C D

1 91 X 3 3 31 X 61 X X 31 X 61 X X 3 32 9 92 X 3 3 3 32 X 62 X X 32 X 62 X X 3 3Total 6 18 12 54

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 187

taxa were excluded from the analyses only two (B and D) ofthe four topologies found in the complete analysis remainedmost parsimonious (see Fig 9 and Table 3) Secondly Bremersupport values throughout much of the lsquochelicerate-alliedrsquoclade (Clade 3 of Fig 8) were improved when these taxa wereexcluded (see Fig 11)

Dunlop (1999 p 258) suggested that in reconstructing thechelicerate stem lineage lsquowe might predict that the two mostsignificant changes towards the chelicerate condition are thereduction of the antennae and the formation of the nextappendages into a clawrsquo According to the hypothesis pre-sented here both of these adaptations were achieved in EarlyCambrian chelicerate ancestors The recognition of the loss ofthe antennae in the stem group of the chelicerates and thephylogenetic hypothesis presented here suggest that chelicer-ates are most closely related to taxa with only a singleadditional segment fused to the crown group eucheliceratehead of four fused somites Previous hypotheses have derivedchelicerates from Palaeozoic arthropods with a longer head(eg Cheloniellon Sturmer amp Bergstrom 1978 SanctacarisBriggs amp Collins 1988 Emeraldella Bruton amp Whittington1983) The plesiomorphic pattern of head segmentation ismatched in pycnogonids Bergstrom et al (1980) suggestedthat only the anterior four pairs of appendages are incorpo-rated into the head This supports the view that pycnogonidsare primitive with respect to other chelicerates which showmore or less complete fusion of this primitive head and threethoracic segments including Solpugida and Pseudoscorpiones

The present study suggests that a shift from gnathobasicbenthic feeding to a more vagrant lifestyle and raptorialappendage feeding occurred very early in the chelicerate lin-eage Appendage feeding was clearly an important adaptation

Figure 10 One of 18 equally parsimonious cladograms with lsquoAglaspidida 2rsquo coding and all characters unorderedthat is most congruent with results from other analyses Non-terminal branch lengths scaled to reflect number ofapomorphies (A) ACCTRAN apomorphy scheme Character numbers shown above boxes and states shownbelow boxes as in Table 2 and the text Character consistency indices are indicated by shading (B) Levels ofsupport for individual nodes Bremer support indices shown in bold type above branches Bootstrap percentagesin italic type below branches for all nodes with relative frequencies greater than 50

Figure 11 Bremer support values for clades within the lsquochelicerate-alliedrsquo arachnomorphs (clade 3 of Fig 8) when all taxa wereincluded (normal weight font above nodes) and with Lemoneites andPaleomerus excluded (bold type below nodes)

188 TREVOR J COTTON AND SIMON J BRADDY

for the terrestrialization of chelicerates (Dunlop 1997 p 69Dunlop amp Webster 1999) but the model proposed heresuggests that this considerably pre-dated terrestrializationGnathobasic feeding in xiphosurans and eurypterids mayrepresent a reversal to a more primitve benthic arachnomorphlifestyle from a more pycnogonid-like ancestor

32 Evolution of the arachnomorph headThe extant arthropod classes show highly conserved patternsof head segmentation (eg Wills et al 1994) In contrast therehas been a near consensus amongst Cambrian arthropodworkers (although rarely based on an explicit phylogenetichypothesis) that patterns of head segmentation are highlyhomoplastic and hence of little systematic significanceSturmer amp Bergstrom (1978 pp 78ndash9) suggested that lsquoevenclosely related forms may have different numbers of headsegments and appendagesrsquo Bruton amp Whittington (1983pp 576ndash7) that lsquodiscussion on fossil arthropod relationshipsbased on head segmentation appears to be largely irrel-evant and at best speculativersquo and Delle Cave amp Simonetta(1991 p 191) that lsquothere are no obvious phyletic affinitiesbetween genera having the same number of cephalized seg-mentsrsquo Most recently Bergstrom amp Hou (1997 p 104) con-cluded that lsquothe segmental length of the head shield seemsto be of no relevance to the discussion and discrimination ofevolutionary lineagesrsquo It has been claimed that this view issupported by cladistic analyses in which patterns of tagmosishave been found to be rather poor at defining major arthropodclades (Briggs et al 1992 Wills et al 1994)

This suggestion is not only of relevance to systematics buthas featured prominently in recent debates surrounding thenature of the lsquoCambrian explosionrsquo (see Budd amp Jensen 2000for a recent review) Gould (1989 1991) argued that theapparent plasticity of head segmentation in Cambrian com-pared to post-Cambrian arthropod evolution suggests thatbody-plan evolution was only constrained after the Cambrianexplosion According to this view the origin of new highertaxa with distinct body-plans was only possible before theonset of these constraints (eg Valentine 1995) Taxa hererecognised as arachnomorphs clearly played a major role in thedevelopment of Gouldrsquos hypothesis This suggestion has pre-viously been assessed in terms of overall morphological diver-sity which is clearly rather distinct from Gouldrsquos concept ofdisparity Wills et al (1998b) showed that there was littlecorrelation between overall morphological disparity and de-gree of limb specialisation and that the latter was phylogeneti-cally highly plastic They remarked that (Wills et al 1998bp 64) lsquothis finding has important implications for models ofarthropod phylogeny and evolution that attribute overidingimportance to head segmentationrsquo but did not explicitlyexamine head segmentation although it is likely to have beena major component of the index of tagmosis employed

The results of the present study suggest that arguments thatarthropod head segmentation was unusually labile during theCambrian are poorly founded Rather only four major pat-terns of euarthropod head segmentation are identified Theplesiomorphic euarthropod state according to both Walossekamp Muller (1997 1998) and Scholtz (1997) which may moreproperly be the plesiomorphic state for the euarthropod crowngroup only (depending on the phylogenetic position of theMarrellomorpha) consists of four post-acronal segmentsbearing the antennae and three pairs of biramous limbs Thepresent authors suggest that the term lsquocephalonrsquo be restrictedto this kind of head which is found in stem group mandibu-lates in addition to many arachnomorphs Crown groupmandibulates share a lsquobimaxillary headrsquo in which an addi-tional pair of appendages the labium or second maxillae are

incorporated into the head but in crustaceans these are notfused to the carapace (Scholtz 1997) Finally two distinctforms of head tagmosis are found in chelicerates A head withfour pairs of appendages (but without antennae giving a totalof five segments incorporated into the head) which has beencalled the lsquocephalosomarsquo is present in pycnogonids (Walossekamp Dunlop 2002 pp 434ndash5) and probably also in some euch-elicerates (J A Dunlop pers comm 2003) The lsquoprosomarsquo ofmost euchelicerates consists of this cephalosoma and twoadditional segments

In general these patterns of head segmentation are highlyphylogenetically conserved (see Fig 12) Only three homo-plastic changes namely reversal to a more primitive conditionin the Leanchoilia-Alalcomenaeus clade and the convergentorigin of a five-segmented head in Clade 3 (of Fig 8) and theXandarellida are necessary to optimise character 4 onto themost parsimonious cladogram There are also transitions toautapomorphic states in Emeraldella and Sidneyia In the caseof Sidneyia however an argument could be made (followingBergstrom amp Sturmer 1978 see discussion of Character 4) thatthe head consists of five segments as in closely related taxabut autapomorphically each segment has a separate tergiteThis would further increase the phylogenetic stability of headsegmentation Also following Bergstrom amp Sturmer (1978) thehead of Cheloniellon could be considered to consist of twotergites making it more similar to the head of Emeraldella(with six pairs of appendages)

Figure 12 Cladogram from Fig 10 showing changes in head segmen-tation Bold type shows abbreviated head segmentation formulaAbbreviations (Ant) antennae (BA) biramous appendage pairs (Ch)chelicerae (GA) great appendages (UA) uniramous appendage pairsand (X) segment without appendages Synapomorphy A is the auta-pomorphic reduction in head length to a single segment in SidneyiaAlternatively this may represent secondary division of the headshield Synapomorphy B is the increase in the length of the head inEmeraldella to six segments

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 189

4 Acknowledgements

We thank D E G Briggs J A Cotton J A Dunlop R AFortey S Harris R A Moore P Selden O E Tetlie andP Van Roy for comments on earlier drafts of this work Themanuscript was reviewed by M Wills F Schram and oneanonymous referee This research was supported by the Geo-logical Society of London Curry Research Studentshipawarded to TJC by the Department of Earth Sciences Uni-versity of Bristol and by the Department of PalaeontologyThe Natural History Museum London TJC would like tothank C Labandeira J Thompson and R Chapman fortheir hospitality at the National Museum of Natural HistoryWashington DC USA and A Olcott S Schellenberg andJ Young for making his time there so enjoyable

5 References

Abzhanov A Popadic A amp Kaufman T C 1999 Chelicerate Hoxgenes and the homology of arthropod segments Evolution andDevelopment 1 77ndash89

Akam M 2000 Arthropods developmental diversity within a (super)phylum Proceedings of the National Academy of Sciences USA97 4438ndash41

Aldridge R J Gabbott S E amp Theron J N 2001 The Soom ShaleIn Briggs D E G amp Crowther P R (eds) Palaeobiology II340ndash2 Oxford Blackwell Science

Anderson L I amp Selden P A 1997 Opisthosomal fusion andphylogeny of Palaeozoic Xiphosura Lethaia 30 19ndash31

Arango C P 2002 Morphological phylogenetics of the sea spiders(Arthropoda Pycnogonida) Organisms Diversity and Evolution2 107ndash25

Ax P 1986 Das phylogenetische System Stuttgart Gustav FischerVerlag

Babcock L E amp Zhang Wentang In press New non-mineralizedarthropods from the Chengjiang Lagerstatte Lower CambrianYunnan China Palaeontologia Cathyana

Bartels C Briggs D E G amp Brassel G 1998 The fossils ofthe Hunsruck Slate Marine life in the Devonian CambridgeCambridge University Press

Bergstrom J 1971 Paleomerus ndash merostome or merostomoid Lethaia4 393ndash401

Bergstrom J 1979 Morphology of fossil arthropods as a guideto phylogenetic relationships In Gupta A P (ed) ArthropodPhylogeny 3ndash56 New York NY Van Nostrand Reinhold Co

Bergstrom J 1992 The oldest arthropods and the origin of theCrustacea Acta Zoologica 73 287ndash91

Bergstrom J Sturmer W amp Winter G 1980 PalaeoisopusPalaeopantopodus and Palaeothea pycnogonid arthropodsfrom the Lower Devonian Hunsruck Slate West GermanyPalaontologische Zeitschrift 54 7ndash54

Bergstrom J amp Brassel G 1984 Legs in the trilobite Rhenops fromthe Lower Devonian Hunsruck Slate Lethaia 17 67ndash72

Bergstrom J amp Hou X 1998 Chengjiang arthropods and theirbearing on early arthropod evolution In Edgecombe G D(ed) Arthropod fossils and phylogeny 151ndash84 New York NYColumbia University Press

Boudreaux H B 1979 Significance of intersegmental tendon systemin arthropod phylogeny and a monophyletic classification ofArthropoda In Gupta A P (ed) Arthropod Phylogeny 551ndash86New York NY Van Nostrand Reinhold Co

Bousfield E L 1995 A contribution to the natural classification ofLower and Middle Cambrian arthropods food-gathering andfeeding mechanisms Amphipacifica 2 3ndash33

Braddy S J Aldridge R J Gabbott S E amp Theron J N 1999Lamellate book-gills in a late Ordovician eurypterid from theSoom Shale South Africa support for a eurypterid-scorpionclade Lethaia 32 72ndash4

Braddy S J amp Almond J E 1999 Eurypterid trackways from theTable Mountain Group (Ordovician) of South Africa Journal ofAfrican Earth Sciences 29 165ndash77

Bremer K 1988 The limits of amino acid sequence data in angio-sperm phylogenetic reconstruction Evolution 42 795ndash803

Bremer K 1994 Branch support and tree stability Cladistics 10295ndash304

Briggs D E G 1998 Review of Arthropods from the LowerCambrian Chengjiang fauna southwest China by HouXianguang and Jan Bergstrom Fossils and Strata 45

Scandinavian University Press Oslo Norway 1997 Palaeo-geography Palaeoclimatology Palaeoecology 143 193ndash4

Briggs D E G Bruton D L amp Whittington H B 1979 Append-ages of the arthropod Aglaspis spinifer (Upper CambrianWisconsin) and their significance Palaeontology 22 167ndash80

Briggs D E G Fortey R A amp Wills M A 1992 Morphologicaldisparity in the Cambrian Science 256 1670ndash3

Briggs D E G Fortey R A amp Wills MA 1993 How big was theCambrian explosion A taxonomic and morphologic comparisonof Cambrian and Recent arthropods In Lees D R amp EdwardsD (eds) Evolutionary patterns and processes 33ndash44 LondonLinnean Society of London Linnean Society Symposium Series

Briggs D E G Erwin D H amp Collier F J 1994 The fossils of theBurgess Shale Washington DC Smithsonian Institution Press

Briggs D E G amp Bartels C 2001 New arthropods from the LowerDevonian Hunsruck Slate (Lower Emsian Rhenish Massifwestern Germany) Palaeontology 44 275ndash304

Briggs D E G amp Bartels C 2002 Magnoculocaris a new name forthe arthropod Magnoculus Briggs and Bartels 2001 from theLower Devonian Hunsruck Slate Germany Palaeontology 45419

Briggs D E G amp Collins D 1988 A Middle Cambrian cheliceratefrom Mount Stephen British Columbia Palaeontology 31 779ndash98

Briggs D E G amp Collins D 1999 The arthropod Alalcomenaeuscambricus Simonetta from the Middle Cambrian Burgess Shale ofBritish Columbia Paleontology 42 953ndash78

Briggs D E G amp Fortey R A 1982 The cuticle of aglaspididarthropods a red-herring in the early history of vertebratesLethaia 15 25ndash9

Briggs D E G amp Fortey R A 1989 The early radiation andrelationships of the major arthropod groups Science 246 241ndash3

Briggs D E G amp Fortey R A 1992 The early Cambrian radiationof arthropods In Lipps J H amp Signor P W (eds) Origin andEarly Evolution of the Metazoa 335ndash373 New York NY PlenumPress

Briggs D E G amp Robison R A 1984 Exceptionally preservednontrilobite arthropods and Anomalocaris from the MiddleCambrian of Utah University of Kansas PaleontologicalContributions Paper 111 1ndash23

Broili F 1932 Ein neuer Crustacee aus dem rheinischen UnterdevonSitzungsberichte der Mathematisch-Naturwissenschaftlichen(Abteilung) Klasse der Bayerischen Akademie der Wissenschaftenzu Munchen 1932 27ndash38

Broili F 1933 Ein zweites Exemplar von Cheloniellon Sitzungsber-ichte der Bayerischen Akademie der Wissenschaften 1933 11ndash32

Bruton D L 1981 The arthropod Sidneyia inexpectans MiddleCambrian Burgess Shale British Columbia Philosophical Trans-actions of the Royal Society of London Series B 295 619ndash56

Bruton D L amp Whittington H B 1983 Emeraldella and Leanchoiliatwo arthropods from the Burgess Shale British Columbia Philo-sophical Transactions of the Royal Society of London Series B 300553ndash85

Budd G E 1995 Kleptothule rasmusseni gen et sp nov anolenellid-like trilobite from the Sirius Passet fauna (BuenFormation Lower Cambrian North Greenland) Transactions ofthe Royal Society of Edinburgh Earth Sciences 86 1ndash12

Budd G E 1996a Progress and problems in arthropod phylogenyTrends in Ecology and Evolution 11 356ndash8

Budd G E 1996b The morphology of Opabinia regalis and thereconstruction of the arthropod stem-group Lethaia 29 1ndash14

Budd G E 1997 Stem-group arthropods from the Lower CambrianSirius Passet fauna of North Greenland In Fortey R A ampThomas R H (eds) Arthropod Relationships 125ndash38 LondonChapman and Hall Systematics Association Special Volume 55

Budd G E 1999a A nektaspid arthropod from the Early CambrianSirius Passet fauna with a description of retrodeformation basedon functional morphology Palaeontology 42 99ndash122

Budd G E 1999b The morphology and phylogenetic significance ofKerygmachela kiergkegaardi Budd (Buen Formation LowerCambrian N Greenland) Transactions of the Royal Society ofEdinburgh Earth Sciences 89 (for 1998) 249ndash290

Budd G E 2002 A palaeontological solution to the arthropod headproblem Nature 417 271ndash5

Budd G E amp Jensen S 2000 A critical reappraisal of the fossilrecord of the bilaterian phyla Biological Reviews 75 253ndash95

Chen J Zhou G Zhu M amp Yeh K 1996 The Chengjiang biota ndasha unique window of the Cambrian explosion Taichung NationalMuseum of Natural Science [In Chinese]

Chen J Edgecombe G D amp Ramskold L 1997 Morphological andecological disparity in naraoiids (Arthropoda) from the Early

190 TREVOR J COTTON AND SIMON J BRADDY

Cambrian Chengjiang fauna China Records of the AustralianMuseum 49 1ndash24

Chen J amp Zhou G 1997 Biology of the Chengjiang fauna In ChenJ Cheng Y amp Iten H V (eds) The Cambrian explosion and thefossil record Bulletin of the National Museum of Natural ScienceTaiwan 10 11ndash106

Child C A 1997 Some deep-sea Pycnogonida from Argentine slopeand basin Proceedings of the Biological Society of Washington110 128ndash42

Chlupac I 1988 The enigmatic arthropod Duslia from the Ordovicianof Czechslovakia Palaeontology 31 611ndash20

Chlupac I 1995 Lower Cambrian arthropods from the PasekyShale (Barrandian area Czech Republic) Journal of the CzechGeological Society 40 9ndash36

Chlupac I amp Havlıcek V 1965 Kodymirus n g a new aglaspidmerostome of the Cambrian of Bohemia Sbornık GeologickechVid Paleontologie 6 7ndash20

Cisne J L 1975 Anatomy of Triarthrus and the relationships of theTrilobita Fossils and Strata 4 45ndash63

Cisne J L 1981 Triarthrus eatoni (Trilobita) anatomy of its exo-skeletal skeletomuscular and digestive systems Palaeonto-graphica Americana 9 95ndash142

Conway Morris S 1982 Atlas of the Burgess Shale London ThePalaeontological Association

Conway Morris S 1989 The persistence of Burgess Shale-type faunasimplications for the evolution of deeper-water faunas Trans-actions of the Royal Society of Edinburgh Earth Sciences 80271ndash83

Conway Morris S 1998 The crucible of creation Oxford OxfordUniversity Press

Cotton T J 1999 Review of Arthropod fossils and phylogenyPalaeogeography Palaeoclimatology Palaeoecology 151 341ndash3

Cotton T J 2002 The phylogeny and morphological evolution ofCambrian trilobites and their relatives Unpublished PhD ThesisUniversity of Bristol

Damen W G M Hausdorf M Seyfarth E A amp Tautz D 1998 Aconserved mode of head segmentation in arthropods revealed bythe expression pattern of Hox genes in a spider Proceedings of theNational Academy of Sciences USA 95 10665ndash70

Delle Cave L amp Simonetta A M 1991 Early Palaeozoic arthropodsand problems of arthropod phylogeny with some notes on taxa ofdoubtful affinities In Simonetta A M amp Conway Morris S(eds) The early evolution of Metazoa and the significance ofproblematic taxa 189ndash244 Cambridge Cambridge UniversityPress

Dewel R A amp Dewel W C 1997 The place of tardigrades inarthropod evolution In Fortey R A amp Thomas R H (eds)Arthropod Relationships 109ndash23 London Chapman and HallSystematics Association Special Volume 55

Dunlop J A 1997 Palaeozoic arachnids and their significance forarachnid phylogeny Proceedings of the 16th European Colloquiumof Arachnology 65ndash82 Siedlce Poland

Dunlop J A 1998 The origins of tetrapulmonate booklungs and theirsignificance for chelicerate phylogeny In Selden P A (ed)Proceedings of the 17th European Colloquium of ArachnologyEdinburgh 1997 pp 9ndash16

Dunlop J A 1999 Pasando revista a la evolucion de los quelicerados[A review of chelicerate evolution] In Melic A DeHaro J JMedez M amp Ribera I (eds) Volumen Monografico Evolucion yFilogenia de Arthropoda Boletin de la Sociedad EntolomologicaAragonesa 26 255ndash272 [In Spanish and English]

Dunlop J A amp Selden P A 1997 The early history and phylogenyof the chelicerates In Fortey R A amp Thomas R H (eds)Arthropod relationships Systematics Association Special Volume55 221ndash35 London Chapman and Hall

Dunlop J A amp Webster M 1999 Fossil evidence terrestrializationand arachnid phylogeny Journal of Arachnology 27 86ndash93

Dzik J 1993 Early metazoan evolution and the meaning of its fossilrecord Evolutionary Biology 27 339ndash86

Dzik J amp Lendzion K 1988 The oldest arthropods of the EastEuropean Platform Lethaia 21 29ndash38

Edgecombe G D Wilson G D F Colgan D J Gray M R ampCassis G 2000 Arthropod cladistics combined analysisof Histone H3 and U2 snRNA sequences and morphologyCladistics 16 155ndash203

Edgecombe G D amp Ramskold L 1999 Relationships of CambrianArachnata and the systematic position of Trilobita Journal ofPaleontology 73 263ndash87

Emerson M J amp Schram F R 1997 Theories patterns and realitygame plan for arthropod phylogeny In Fortey R A amp Thomas

R H (eds) Arthropod Relationships Systematics AssociationSpecial Volume 55 67ndash87 London Chapman and Hall

Farris J S 1989 The Retention Index and the Rescaled ConsistencyIndex Cladistics 5 417ndash19

Felsenstein J 1985 Confidence limits on phylogenies an approachusing the bootstrap Evolution 39 783ndash91

Flower R H 1968 Merostomes from a Cotter Horizon of the El PasoGroup New Mexico Bureau of Mines and Mineral ResourcesMemoir 22 35ndash44

Fortey R A 1990a Trilobite evolution and systematics In MikulicD G (ed) Arthropod paleobiology 44ndash65 Short Courses inPaleontology 3 Knoxville Tenessee The Paleontological Society

Fortey R A 1990b Ontogeny hypostome attachment and trilobiteclassification Palaeontology 33 529ndash76

Fortey R A 1997 Classification In Kaesler R L (ed) Treatise oninvertebrate paleontology Part O Arthropoda 1 (revised) 289ndash302Boulder Colorado amp Lawrence Kansas Geological Society ofAmerica and University of Kansas Press

Fortey R A amp Theron J N 1994 A new Ordovician arthropodSoomaspis and the agnostid problem Palaeontology 37 841ndash61

Fortey R A amp Whittington H B 1989 The Trilobita as a naturalgroup Historical Biology 2 125ndash38

Fry W G 1978 A classification within the pycnogonids ZoologicalJournal of the Linnean Society of London 63 35ndash58

Giribet G Edgecombe G D amp Wheeler W C 2001 Arthropodphylogeny based on eight molecular loci and morphology Nature413 157ndash61

Gould S J 1989 Wonderful life The Burgess Shale and the nature ofhistory New York NY Norton

Gould S J 1991 The disparity of the Burgess Shale arthropod faunaand the limits of cladistic analysis why we must strive to quantifymorphospace Paleobiology 17 411ndash23

Gurich G 1931 Mimetaster hexagonalis ein neuer Kruster aus demunterdevonischen Bundenbacher Dachschiefer PalaontologischeZeitschrift 12 204ndash38

Hammann W Laske R amp Pillola G L 1990 Tariccoia arrusensisngnsp an unusual trilobite-like arthropod Rediscovery of thelsquophylocaridrsquo beds of Taricco (1922) in the Ordovician lsquoPuddingarsquosequence of Sardinia Bolletino della Societa PaleontologicaItaliana 29 163ndash78

Haas M S Brown S J amp Beeman R W 2001 Pondering theprocephalon the segmental origin of the labrum DevelopmentGenes and Evolution 211 89ndash95

Heider K 1913 Entwicklungsgeschichte und Morphologie derWirbellosen Die Kultur der Gegenwart Theil 3 Abt 4 2 176ndash332

Henriksen K 1928 Critical notes upon Cambrian arthropodsdescribed by Charles D Walcott Videnskabelige Meddelelser fraDansk Naturhistorisk Forening 86 1ndash20

Hesselbo S P 1988 Trace fossils of Cambrian aglaspidid arthropodsLethaia 21 139ndash46

Hesselbo S P 1989 The aglaspidid arthropod Beckwithia from theCambrian of Utah and Wisconsin Journal of Paleontology 63636ndash42

Hesselbo S P 1992 Aglaspidida (Arthropoda) from the UpperCambrian of Wisconsin Journal of Paleontology 66 885ndash923

Hou X 1987a Three new large arthropods from Lower CambrianChengjiang fauna eastern Yunnan Acta Palaeontologica Sinica26 272ndash85 [In Chinese with English summary]

Hou X 1987b Two new arthropods from Lower CambrianChengjiang eastern Yunnan Acta Palaeontologica Sinica 26236ndash56 [In Chinese with English summary]

Hou X Chen J amp Lu H 1989 Early Cambrian new arthropodsfrom Chengjiang Yunnan Acta Palaeontologica Sinica 28 42ndash57[In Chinese with English summary]

Hou X Ramskold L amp Bergstrom J 1991 Composition andpreservation of the Chengjiang fauna a Lower Cambrian soft-bodied biota Zoologica Scripta 20 395ndash411

Hou X Bergstrom J amp Ahlberg P 1995 Anomalocaris and otherlarge animals in the Lower Cambrian Chengjiang fauna ofsouthwest China GFF 117 163ndash83

Hou X amp Bergstrom J 1997 Arthropods of the Lower CambrianChengjiang fauna southwest China Fossils and Strata 45 1ndash116

Hughes C P 1975 Redescription of Burgessia bella from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 4415ndash36

Ivantsov A Y 1999 Trilobite-like arthropod from the LowerCambrian of the Siberian Platform Acta PalaeontologicaPolonica 44 455ndash66

King P E 1973 Pycnogonids London St Martin Press

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 191

Lauterbach K E 1973 Schlussereignisse in der Evolution derStammgruppe der Euarthropoda Zoologische Beitrage 19259ndash99

Lauterbach K E 1980 Schlussereignisse in der Evolution derGrundplans der Arachnata (Arthropoda) Abhandlungen desNaturwissenschaftlichen Vereins in Hamburg 23 163ndash327

Lauterbach K E 1983 Synapomorphien zwischen Trilobiten- undCheliceraten-zweig der Arachnata Zoologischer Anzeiger 210213ndash38

Lendzion K 1975 Fauna of the Mobergella Zone in the Polish LowerCambrian Kwartalnik Geologiczny 19 237ndash42

Lieberman B S 1998 Cladistic analysis of the Early Cambrianolenelloid trilobites Journal of Paleontology 72 59ndash78

Lu Y 1940 On the ontogeny and phylogeny of Redlichia intermediaLu (sp nov) Bulletin of the Geological Society of China 20333ndash42 [In Chinese]

Maddison W P amp Maddison D R 1997 MacClade analysis ofphylogeny and character evolution Version 307 Sunderland MASinauer Associates

Mittmann B amp Scholtz G 2003 Development of the nervous systemin the lsquoheadrsquo of Limulus polyphemus (Chelicerata Xiphosura)morphological evidence for the correspondence between the seg-ments of the chelicerae and the (first) antennae of MandibulataDevelopment Genes and Evolution 213 9ndash17

Moore R C (ed) 1959 Treatise on invertebrate paleontology Part OArthropoda 1 Boulder Colorado Geological Society of America

Muller K J amp Walossek D 1986 Arthropod larvae from the UpperCambrian of Sweden Transactions of the Royal Society ofEdinburgh Earth Sciences 77 157ndash79

Muller K J amp Walossek D 1987 Morphology ontogeny and lifehabit of Agnostus pisiformis from the Upper Cambrian of SwedenFossils and Strata 19 1ndash124

Munilla T 1999 Evolucion y filogenia de los Picnogonidos InMelicA DeHaro J J Medez M amp Ribera I (eds) VolumenMonografico Evolucion y Filogenia de Arthropoda Boletin de laSociedad Entolomologica Aragonesa 26 273ndash91 [In Spanish andEnglish]

Orr P J Siveter D J Briggs D E G Siveter D J amp Sutton M D2000 A new arthropod from the Silurian Konservat-Lagerstatteof Herefordshire Proceedings of the Royal Society of LondonSeries B 267 1497ndash504

Page R D M 1996 On consensus confidence and lsquototal evidencersquoCladistics 12 83ndash92

Palmer A R amp Repina L N 1993 Through a glass darklytaxonomy phylogeny and biostratigraphy of the OlenellinaUniversity of Kansas Paleontological Contributions New Series 31ndash35

Platnick N I Griswold C E amp Coddington J A 1991 On missingentries in cladistic analysis Cladistics 7 337ndash43

Pleijel F 1995 On character coding for phylogeny reconstructionCladistics 11 309ndash15

Raasch G O 1939 Cambrian Merostomata Special Papers of theGeological Society of America 19 1ndash146

Ramskold L Chen J Edgecombe G D amp Zhou G 1996Preservational folds simulating tergite junctions in tegopeltid andnaraoiid arthropods Lethaia 29 15ndash20

Ramskold L Chen J Edgecombe G D amp Zhou G 1997Cindarella and the arachnate clade Xandarellida (ArthropodaEarly Cambrian) from China Transactions of the Royal Society ofEdinburgh Earth Sciences 88 19ndash38

Ramskold L amp Edgecombe G D 1991 Trilobite monophylyrevisited Historical Biology 4 267ndash83

Ramskold L amp Edgecombe G D 1996 Trilobite appendagestructure ndash Eoredlichia revisited Alcheringa 20 269ndash76

Raw F 1957 Origin of chelicerates Journal of Paleontology 31139ndash193

Repina L N amp Okuneva O G 1969 Cambrian arthropods of themaritime territory Paleontological Journal 1969 95ndash103

Richter R amp Richter E 1929 Weinbergina opitz ng n sp einSchwerttrager (Merost Xiphos) aus dem Devon (Rheinland)Senkenbergiana 11 193ndash209

Robison R A 1984 New occurences of the unusual trilobite Naraoiafrom the Cambrian of Idaho and Utah University of KansasPaleontological Contributions Paper 112 1ndash8

Rominger C 1887 Description of primordial fossils from MtStephens NW Territory of Canada Proceedings of the Academyof Natural Sciences of Philadelphia 1887 12ndash19

Scholtz G 1997 Cleavage germ band formation and head segmen-tation the ground pattern of the Euarthropoda In Fortey R Aamp Thomas R H (eds) Arthropod Relationships 317ndash32 LondonChapman and Hall Systematics Association Special Volume 55

Schram F R 1978 Arthropods a convergent phenomenonFieldiana Geology 39 61ndash108

Schram F R 1993 Review of Boxshall Stromberg and Dahl TheCrustacea Origin and Evolution Journal of Crustacean Biology13 820ndash2

Schram F R Vonk R amp Hof C H J 1997 Mazon CreekCycloidea Journal of Paleontology 71 261ndash84

Schram F R amp Emerson M J 1991 Arthropod Pattern Theorya new approach to arthropod phylogeny Memoirs of theQueensland Museum 31 1ndash18

Schram F R amp Hof C H J 1998 Fossils and the interrelationshipsof major crustacean groups In Edgecombe G D (ed) Arthropodfossils and phylogeny pp 233ndash302 New York NY ColumbiaUniversity Press

Shergold J H 1991 Protaspis and early meraspis growth stages of theeodiscoid trilobite Pagetia ocellata Jell and their implications forclassification Alcheringa 15 65ndash86

Shu D Geyer G Ling C amp Zhang Z 1995 Redlichiacean trilobiteswith preserved soft parts from the Lower Cambrian Chengjiangfauna (South China) Beringeria (Special Issue) 2 203ndash41

Shultz J W 1990 Evolutionary morphology and phylogeny ofArachnida Cladistics 6 1ndash38

Shultz J W 2000 Skeletomuscular anatomy of the harvestmanLeiobunum aldrichi (Weed 1893) (Arachnida OpilionesPalpatores) and its evolutionary significance Zoological Journalof the Linnean Society 128 401ndash38

Shultz J W 2001 Gross muscular anatomy of Limulus polyphemus(Xiphosura Chelicerata) and its bearing on evolution in the theArachnida Journal of Arachnology 29 283ndash303

Siewing R (ed) 1985 Lehrbuch der Zoologie II Systematik 3rd ednStuttgart Fischer

Simonetta A M 1970 Studies on the non trilobite arthropods ofthe Burgess Shale (Middle Cambrian) the genera LeanchoiliaAlalcomenaeus Opabinia Burgessia Yohoia and ActaeusPalaeontographica Italica 66 35ndash45

Simonetta A M amp Delle Cave L 1975 The Cambrian non trilobitearthropods from the Burgess Shale of British Columbia A studyof their comparative morphology taxonomy and evolutionarysignificance Palaeontographica Italica 69 1ndash37

Simonetta A M amp Delle Cave L 1981 An essay in the compara-tive and evolutionary morphology of Palaeozoic ArthropodsAccademia Nazionale dei Lincei Atti dei Convegni Lincei 49389ndash439

Starobogatov Y I 1990 Sistema i filogeniya nizshikh khelitserovykh(analiz morfologii paleozoysikh grupp) [The systematics andphylogeny of the lower chelicerates (a morphological analysis ofthe Palaeozoic groups)] Paleontologicheskii Zhurnal 1990 4ndash17

Stoslashrmer L 1944 On the relationships and phylogeny of fossil andrecent Arachnomorpha Skrifter utgitt ac Det Norske Videnskaps-Akademi i Olso I Matematisk-naturvitenskaplig Klasse 5 1ndash158

Stoslashrmer L 1951 Studies on trilobite morphology Part III Theventral cephalic structures with remarks on the zoologicalposition of the trilobites Norsk geologisk tidsskrift 29 108ndash58

Stoslashrmer L 1956 A Lower Cambrian merostome from Sweden Arkivfor Zoologi Ser 2 9 507ndash14

Stoslashrmer L 1959 Trilobitoidea In Moore R C (ed) Treatise onInvertebrate Paleontology part O Arthropoda 1 23ndash37 BoulderColorado Geological Society of America

Strong E E amp Lipscomb D 1999 Character coding and inapplicabledata Cladistics 15 363ndash71

Sturmer W amp Bergstrom J 1976 The arthropods Mimetaster andVachonisia from the Devonian Hunsruck Shale PalaontologischeZeitschrift 50 78ndash111

Sturmer W amp Bergstrom J 1978 The arthropod Cheloniellon fromthe Devonian Hunsruck Shale Palaontologische Zeitschrift 5257ndash81

Sturmer W amp Bergstrom J 1981 Weinbergina a xiphosuran fromthe Devonian Hunsruck Slate Palaontologische Zeitschrift 55237ndash55

Sutton M D Briggs D E G Siveter D J Siveter D J amp Orr PJ 2002 The arthropod Offacolus kingi (Chelicerata) from theSilurian of Herefordshire England computer based morphologi-cal reconstructions and phylogenetic affinity Proceedings of theRoyal Society of London Series B 269 1195ndash1203

Swofford D L 1999 PAUP Phylogenetic Analysis Using Parsimony(and other methods) Version 40b2a Sunderland MA SinauerAssociates

Telford M J amp Thomas R H 1998 Expression of homeobox genesshows chelicerate arthropods retain their deutocerebral segmentProceedings of the National Academy of Sciences USA 9510671ndash5

192 TREVOR J COTTON AND SIMON J BRADDY

Thorley J L amp Page R D M 2000 RadCon phylogenetic treecomparison and consensus Bioinformatics 16 486ndash7

Valentine J W 1995 Why no new phyla after the Cambrian ndashgenome and ecospace hypotheses revisited Palaios 10 190ndash194

Wagele J W 1993 Rejection of the lsquoUniramiarsquo hypothesis andimplications of the Mandibulata concept Zoologische JahrbucherAbteilung fur Systematik Geographie und Biologie der Tiere 120253ndash88

Walcott C D 1911 Middle Cambrian Merostomata Cambriangeology and palaeontology II Smithsonian MiscellaneousCollections 57 17ndash40

Walcott C D 1912 Middle Cambrian Branchiopoda MalacostracaTrilobita and Merostomata Cambrian geology and palaeontol-ogy II Smithsonian Miscellaneous Collections 57 145ndash228

Walcott C D 1918 Geological explorations in the CanadianRockies Explorations and fieldwork of the SmithsonianInstitution in 1917 Smithsonian Miscellaneous Collections 684ndash20

Walossek D 1993 The Upper Cambrian Rehbachiella kinnekullensisand the phylogeny of Branchiopoda and Crustacea Fossils andStrata 32 1ndash202

Walossek D amp Dunlop J A 2002 A larval sea spider (ArthropodaPycnogonida) from the Upper Cambrian lsquoOrstenrsquo of Swedenand the phylogenetic position of pycnogonids Palaeontology 45421ndash46

Walossek D amp Muller K J 1990 Upper Cambrian stem-lineagecrustaceans and their bearing upon the monophyletic origin ofCrustacea and the position of Agnostus Lethaia 23 409ndash27

Walossek D amp Muller K J 1997 Cambrian lsquoOrstenrsquo-type arthro-pods and the phylogeny of Crustacea In Fortey R A amp ThomasR H (eds) Arthropod Relationships 139ndash53 SystematicsAssociation Special Volume 55 London Chapman and Hall

Walossek D amp Muller K J 1998 Early arthropod phylogeny in lightof the Cambrian lsquoOrstenrsquo fossils In Edgecombe G D (ed)Arthropod fossils and phylogeny 185ndash231 New York NYColumbia University Press

Weygoldt P 1979 Significance of later embryonic stages and headdevelopment in arthropod phylogeny In Gupta A P (ed)Arthropod Phylogeny 107ndash35 New York NY Van NostrandReinhold Co

Weygoldt P 1998 Evolution and systematics of the ChelicerataExperimental and Applied Acarology 22 63ndash79

Weygoldt P amp Paulus H F 1979 Unterzuchungen aur Morpholo-gie Taxonomie und Phylogenie der Chelicerata Zeitschriftfur Zoologische Systematik und Evolutionsforschung 17 85ndash116177ndash200

Wheeler W C Cartwright P amp Hayashi C Y 1993 Arthropodphylogeny a combined approach Cladistics 9 1ndash39

Wheeler W C amp Hayashi C Y 1998 The phylogeny of extantchelicerate orders Cladistics 14 173ndash92

Whittington H B 1971 Redescription of Marrella splendens(Trilobitoidea) from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 209 1ndash24

Whittington H B 1974 Yohoia Walcott and Plenocaris n genarthropods from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 231 1ndash21

Whittington H B 1975a Trilobites with appendages from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 497ndash136

Whittington H B 1975b The enigmatic animal Opabinia regalisMiddle Cambrian Burgess Shale British Columbia PhilosophicalTransactions of the Royal Society of London Series B 271 1ndash43

Whittington H B 1977 The Middle Cambrian trilobite NaraoiaBurgess Shale British Columbia Philosophical Transactions of theRoyal Society Series B 280 409ndash43

Whittington H B 1979 Early arthropods their appendages andrelationships In House M R (ed) The origin of majorinvertebrate groups 253ndash68 Systematics Association SpecialVolume 12 New York NY Academic Press

Whittington H B 1980 Exoskeleton moult stage appendage mor-phology and habits of the Middle Cambrian trilobite Olenoidesserratus Palaeontology 23 171ndash204

Whittington H B 1981 Rare arthropods from the Burgess ShaleMiddle Cambrian British Columbia Philosophical Transactionsof the Royal Society of London Series B 292 329ndash57

Whittington H B 1985 Tegopelte gigas a second soft-bodied trilo-bite from the Burgess Shale Middle Cambrian British ColumbiaJournal of Palaeontology 23 171ndash204

Whittington H B 1989 Olenelloid trilobites type species functionalmorphology and higher classification Philosophical Transactionsof the Royal Society of London Series B 324 111ndash47

Whittington H B amp Almond J E 1987 Appendages and habits ofthe Upper Ordovician trilobite Triarthus eatoni PhilosophicalTransactions of the Royal Society of London Series B 317 1ndash46

Wilkinson M 1992 Ordered versus unordered characters Cladistics8 375ndash85

Wilkinson M 1994 Common cladistic information and its consensusrepresentation reduced Adams and reduced cladistic consensustrees and profiles Systematic Biology 43 343ndash68

Wilkinson M 1995a Coping with abundant missing entries in phylo-genetic inference using parsimony Systematic Biology 44 501ndash14

Wilkinson M 1995b TAXEQ2 software and documentation BristolSchool of Biological Sciences University of Bristol

Wilkinson M 1996 On the distribution of homoplasy and choosingamong trees Taxon 45 263ndash66

Wills M A 1997 A phylogeny of Recent and fossil Crustacea derivedfrom morphological characters In Fortey R A amp ThomasR H (eds) Arthropod Relationships 189ndash209 SystematicsAssociation Special Volume 55 London Chapman and Hall

Wills M A Briggs D E G amp Fortey R A 1994 Disparity asan evolutionary index a comparison of Cambrian and Recentarthropods Paleobiology 20 93ndash130

Wills M A Briggs D E G Fortey R A amp Wilkinson M 1995The significance of fossils in understanding arthropod evolutionVerhandlungen der deutschen zoologischen Gesselschaft 88 203ndash15

Wills M A Briggs D E G Fortey R A Wilkinson M amp SneathP H A 1998a An arthropod phylogeny based on fossil andrecent taxa In Edgecombe G D (ed) Arthropod fossils andphylogeny 33ndash105 New York NY Columbia University Press

Wills M A Briggs D E G amp Fortey R A 1998b Evolutionarycorrelates of arthropod tagmosis scrambled legs In ForteyR A amp Thomas R H (eds) Arthropod Relationships 57ndash65Systematics Association Special Volume 55 London Chapmanand Hall

Winter G 1980 Beitrage zur Morphologie und Embryologie desvorderen Korperabschnitts (Cephalosoma) der PantopodaGerstaecker 1863 I Entstehung und Struktur des Zentralnerven-systems Zeitschrift fur Zoologische Systematik und Evolutionsfor-schung 18 27ndash61

Zhang W amp Hou X 1985 Preliminary notes on the occurrence of theunusual trilobite Naraoia in Asia Acta Palaeontologica Sinica 24591ndash5 [In Chinese with English summary]

TREVOR J COTTON Department of Palaeontology The Natural History Museum CromwellRoad London SW7 5BD UKe-mail trevor_cottonhotmailcom

SIMON J BRADDY Department of Earth Sciences University of Bristol Wills MemorialBuilding Queenrsquos Road Bristol BS8 1RJ UKe-mail S J Braddybrisacuk

MS received 28 July 2003 Accepted for publication 15 March 2004

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 193

Page 5: The phylogeny of arachnomorph arthropods and the origin of the

form of the pygidium) xandarellids (the eye slits and overlapof anterior thoracic segments by the head shield) and Trilobita(the hypostome with anterior and posterior lateral wings)Therefore it potentially has a pivotal position in the phylogenyof the trilobite-allied Arachnomorpha However the interpre-tation of these features by Ivantsov (1999) requires confir-mation and adequately determining the homology of thesestructures would require restudy of the material

The Xandarellida known only from the Chengjiang faunawere originally described as an arachnate clade (Ramskoldet al 1997) All three valid genera Xandarella Sinoburius andCindarella were included in the analysis of Edgecome ampRamskold (1999) They differ in important respects andtherefore all three genera are included here to test the mono-phyly of this group in the context of a wider analysis

Wills et al (1995 1998a) represented the Trilobita (exceptAgnostida) by Olenoides the appendages of which are knownfrom the Burgess Shale and the Ordovician Triarthrus (seeCisne 1975 1981 Whittington amp Almond 1987) However thepresent authors follow Edgecombe amp Ramskold (1999) inchoosing Olenoides and Eoredlichia to represent the TrilobitaThese are more basal trilobites (see eg Fortey 1990a b) thanTriarthrus and consequently are more likely to reflect theancestral trilobite condition The putative trilobite KleptothuleBudd 1995 from the Early Cambrian Sirius Passet faunaof North Greenland is not included Its appendages areunknown and homologies between exoskeletal features of thistaxon and other arachnomorphs are uncertain

The Naraoiidae or Nektaspida (reviewed above) are widelyconsidered to be closely related to the trilobites either as aparaphyletic assemblage of lsquosoft-bodiedrsquo trilobites (eg Shuet al 1995 fig 20B) or as the sister group of calcified trilobites(Fortey 1997 Whittington 1977) They have been includedin the Trilobita by some authors However Edgecombe ampRamskold (1999) found no particularly close relationshipbetween naraoiids and trilobites (see above) In order to testthe monophyly of the group all described naraoiid genera areincluded except Maritimella and Orientella (both Repina ampOkuneva 1969) which are probably pseudofossils (Robison1984 p 2)

Tegopelte from the Burgess Shale has also been considered asoft-bodied trilobite (Whittington 1985) Ramskold et al(1996) revised the exoskeletal morphology of Tegopelte andnoted similarities to the Chengjiang taxa Saperion andSkioldia This relationship has been confirmed by cladisticanalysis (Edgecombe amp Ramskold 1999) An undescribed largearthropod from the Soom Shale Lagerstatte (Aldridge et al2001 fig 3442) is probably a tegopeltid (Braddy amp Almond1999) The Helmetiidae based on Helmetia Walcott 1918from the Burgess Shale are united with the tegopeltid group inthe Helmetiida (Hou amp Bergstrom 1997 Edgecombe amp Ram-skold 1999) Helmetia is rather poorly known and awaitsredescription but is very similar to Kuamaia lata Hou 1987aKuamaia muricata Hou amp Bergstrom 1997 and Rhombicalva-ria acantha Hou 1987a from Chengjiang There are no signifi-cant differences between these Chinese taxa and theirtaxonomy may be over split (Delle Cave amp Simonetta 1991Hou amp Bergstrom 1997) The morphology of Kuamaia lata isknown in some detail and it is coded here along with Helmetiaand all three tegopeltid genera

Delle Cave amp Simonetta (1991 table 1) compared severaltaxa to Tegopelte and Helmetia Of these only Retifacies isknown in enough detail to make coding worthwhile Tontoiaand Nathorstia (both Walcott 1912) are nomina dubia (seeWhittington 1985 1980 respectively) and only the exoskeletonis known of Urokodia Hou et al 1989 and Mollisonia Walcott1912 Retifacies has been placed near a trilobite-naraoiid-

helmetiid clade (Delle Cave amp Simonetta 1991 Edgecombe ampRamskold 1999) or as sister group to the naraoiids (Hou ampBergstrom 1997)

There is a long history of comparing Emeraldella andSidneyia with chelicerates (following Stoslashrmer 1944) but theposition of these taxa in cladistic studies has been highlyvariable (see Fig 2) According to the hypothesis ofBergstrom these taxa along with the Aglaspidida andCheloniellida form a clade that is the sister group to thechelicerates More usually aglaspidids or cheloniellids havebeen considered sister taxon to the chelicerates and theseBurgess Shale taxa more distantly related

A cheloniellid-chelicerate clade was supported by Wills et al(1995 1998a) and Sturmer amp Bergstrom (1978 also Bergstrom1979) and Simonetta amp Delle Cave (1981) and Delle Cave ampSimonetta 1991) placed the cheloniellid Triopus as ancestral toall chelicerates The Cheloniellida (sensu Dunlop amp Selden1997) are represented by Cheloniellon herein since theappendages of other cheloniellid taxa are unknown

It has also repeatedly been suggested that cheliceratesevolved from aglaspidids (eg Starobogatov 1990) and aglas-pidids have been included in the Chelicerata by some authors(Stoslashrmer 1944 Weygoldt amp Paulus 1979) The coding ofaglaspidids is considered in detail in Section 22 Lemoneites(Flower 1968) and Paleomerus (Stoslashrmer 1956 Bergstrom 1971)have been assigned to the Aglaspidida by some authors (seeHou amp Bergstrom 1997 pp 96ndash97) They are included tofacilitate comparison with the results of Dunlop amp Selden(1997) Lemoneites is particularly problematic (R A Moorepers comm 2004) and both taxa are known from very fewcharacters ndash hence the present authors have investigated theimplications of their exclusion from the analysis (see Section26) The aglaspidid-like arthropod Kodymirus vagans Chlupacamp Havlıcek 1965 from the Lower Cambrian of the CzechRepublic (redescribed and compared to eurypterids byChlupac 1995) is excluded from the present study becausefeatures of its morphology that are well known generally agreewith those of Aglaspis

The phylogeny of crown group chelicerates is a matter ofconsiderable debate but most authors have considered pyc-nogonids xiphosurans and eurypterids as early divergentgroups within Chelicerata (Dunlop 1999) These hypothesesare represented here by coding a generalised pycnogonid andeurypterid and the Devonian synziphosurine Weinbergina(Sturmer amp Bergstrom 1981) Weinbergina is the best-knownsynziphosurine and is more likely to represent the primitivecondition of xiphosurans than modern examples The phylog-eny of the Xiphosura has recently been studied by Anderson ampSelden (1997) Coding for Eurypterida follows the recent workof Dunlop amp Selden (1997) Dunlop (1998) Braddy et al(1999) and Dunlop amp Webster (1999) Coding for pycnogonidsfollows general works on the group (eg King 1973 Fry 1978)the reconstruction of the pycnogonid stem group by Bergstromet al (1980) and recent work on pycnogonid phylogeny(Munilla 1999 Arango 2002) The pycnogonid familyAmmotheidae is generally accepted as the most plesiomorphicextant group (Arango 2002)

Two groups of Palaeozoic fossil taxa have been included inthe Arachnomorpha by some authors but considered lessclosely related by others Marrella from the Burgess Shaleand two Devonian taxa Mimetaster and Vachonisia havegenerally been considered to form the clade Marrellomorpha(Whittington 1971 Sturmer amp Bergstrom 1976 Bergstrom1979 Wills et al 1995) This has been thought to be the sistergroup to other arachnates (Sturmer amp Bergstrom 1976) abasal schizoramian or a basal euarthropod group (see Fig 2)Bergstrom (1979) included the Carboniferous Cycloidea in the

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 173

Marrellomorpha but there is considerable evidence that theseare rather derived crustaceans (see Schram et al 1997) andthey are not considered further The Burgess Shale taxonBurgessia has also been placed in the Marrellomorpha (Hou ampBergstrom 1997) but was not found to belong to this clade inmost analyses Stoslashrmer (1944) placed Burgessia in a compa-rable systematic position to Marrella he included both in theArachnomorpha but excluded them from the MerostomoideaMarrella Mimetaster and Burgessia were included in thepresent study to assess whether the marellomorphs should beincluded in the Arachnomorpha and test the affinities ofBurgessia

The Cambrian lsquogreat appendagersquo arthropods (theMegacheira of Hou amp Bergstrom 1997) have been nestedwithin the Arachnomorpha in most cladistic studies (egBriggs amp Fortey 1989 Wills et al 1995 1998a Emerson ampSchram 1997) Briggs amp Fortey (1989 1992) recognised mega-cheiran taxa as particularly closely related to chelicerates(Fig 2A) while other authors (Bergstrom 1992 Hou ampBergstrom 1997 Budd 2002) have considered megacheirans tobe primitive euarthropods or ancestral to some (but not all)crustaceans (Delle Cave amp Simonetta 1991 and referencestherein) Since the monophyly of the megacheirans hasnot usually been supported the present authors haveincluded nearly all described taxa These include LeanchoiliaAlalcomenaeus and Yohoia from the Burgess Shale andFortiforceps and Jianfengia from the Chengjiang faunaExcluded from this study are Actaeus Simonetta 1970 fromthe Burgess Shale which may be a synonym of Alalcomenaeus(Briggs amp Collins 1999) Alalcomenaeus illecebrosus (Hou1987b see Hou amp Bergstrom 1997) from the ChengjiangFauna which is probably a chimaera (Briggs amp Collins 1999)and the poorly preserved Leanchoilia hanceyi from the MiddleCambrian of Utah (Briggs amp Robison 1984)

According to the definition given above any assessment ofthe limits of the Arachnomorpha needs to consider the phylo-genetic position of these taxa relative to crustaceans To thisend a generalised crustacean intended to represent the plesio-morphic crustacean condition was included as an ingrouptaxon There has been considerable disagreement over themost basal crustacean group (see Wills 1997 pp 194ndash5Schram amp Hof 1998 pp 245ndash8) The coding used here largelyfollows Walossek amp Mullerrsquos (1990 1997 1998) andWalossekrsquos (1993) concept of the crustacean stem group but isintended to be conservative so that coding on the basis of othertheories of crustacean origins would be similar

22 AglaspididaThe morphology of the appendages of aglaspidids is poorlyknown having been described from three species of bodyfossil Aglaspis spinifer Raasch 1939 Flobertia kochi Hesselbo1992 and Khankaspis bazhanovi Repina amp Okuneva 1969 andtrace fossil evidence (Hesselbo 1988) Of these the appendagesof Aglaspis described by Raasch (1939) Briggs et al (1979)and Hesselbo (1992) are by far the best known The append-ages of Flobertia (described by Raasch 1939 as Aglaspisbarrandei and Hesselbo 1992) agree with those of AglaspisHowever the appendages of Khankaspis show a differentmorphology but have only been poorly illustrated anddescribed (Repina amp Okuneva 1969) This material suggeststhe presence of lobate exopods with lamellate setae This taxonis probably correctly assigned to aglaspidids (cf Whittington1979 p 258) on the basis the central position of the dorsaleyes and presence of genal spines although Hou amp Bergstrom(1997 p 96) suggested that its broad tail spine may indicatethat it is a strabopid

It remains unclear whether lamellate exopods are absent insome aglaspidids (Aglaspis) and present in others (Khankaspis)or whether the apparent differences in appendage morphologybetween these taxa are a result of preservational bias alone Apossible preservational analogue is provided by someChengjiang taxa in which the exopods are very poorly pre-served and the endopods of the cephalic appendages arepreserved as impressions in the dorsal head shield in a similarmanner to those of Aglaspis This is presumably because theendopods were more convex and more heavily sclerotised thanthe exopods This mode of appendage preservation is mostclearly seen in Misszhouia longicaudata (see Chen et al 1997fig 2andashd) but is also found in Sinoburius (see Hou ampBergstrom 1997) and possible protaspides of Naraoia (Houet al 1991)

Here a generalised aglaspidid is coded in two different waysto accommodate this uncertainty In both codings they areconsidered to have possessed a pair of antenniform append-ages followed by 11 pairs of pediform endopods on the headand first eight thoracic tergites (Briggs et al 1979 Hesselbo1988 1992) According to one interpretation (coded asAglaspidida 1) all the appendages are uniramous the exopodis lost throughout According to the other (Aglaspidida 2)exopods consisting of a single lobe fringed with lamellate setae(Repina amp Okuneva 1969 pl 15 figs 1 3 amp 4) are presenton at least some appendages but their distribution andattachment are considered unknown

23 Outgroup rootingA hypothetical plesiomorphic euarthropod was included toallow outgroup rooting Many of the characters consideredcould be coded unambiguously on the basis of recent discus-sions of the arthropod stem group (Budd 1996b 1997 1999b)

Figure 3 Majority rule consensus of lsquotrilobite-alliedrsquo arachnate phy-logeny after Edgecombe amp Ramskold (1999 fig 3)

174 TREVOR J COTTON AND SIMON J BRADDY

The ancestral euarthropod is here considered to have possesseda head with antennae only and a series of post-cephalicbiramous limbs with gnathobases The hypothesised pattern ofthe evolution of plesiomorphic euarthropod characters isshown in Figure 4 The use of a hypothetical outgroup issomewhat unsatisfactory but only affects the relationshipbetween the major clades (Arachnomorpha Crustacea andMarrellomorpha) the topology within the arachnomorphs wasunaffected by the use of this outgroup since identical resultswere obtained with unrooted analyses or by rooting withCrustacea Marrella or both

24 Characters and codingMost previous cladistic analyses with the notable exception ofEdgecombe amp Ramskold (1999) have included only briefdiscussions of the primary hypotheses of homology involved incharacter construction and coding Therefore a full discussionof most characters is provided here Descriptions of charactersand character states below are arranged by organ system orbody region in approximate anterior to posterior order Thedistribution of character states across all taxa is shown in thedata matrix (Table 2)

The terminology of morphological features in arachnomor-phs is often confused Terminology developed for cheliceratestrilobites and crustaceans has variously been applied to taxaincluded in this study so some attempt is made to clarifyterminology herein Where appropriate the present authorsrsquoterminology follows Edgecombe et al (2000) and Wheeleret al (1993) but following Scholtz (1997) the protocerebrallsquosegmentrsquo is considered to be acronal and consequently thedeutocerebral segment to be the first true segment Thereforethe antennae (or antennulae of crustaceans) belong to the firstcephalic segment

The present authors have attempted to include as completea set of characters as possible However characters requiringhypotheses of homology between podomeres (eg the number

of podomeres in endopods of thoracic appendages Willset al 1998a character 51) were not considered followingEdgecombe et al (2000 p 157) Secondly some characters usedby Bergstrom (eg Hou amp Bergstrom 1997 p 109) suchas appendage posture and mode of feeding were excludedfollowing Edgecombe amp Ramskold (1999) Finally somecharacters of the ventral surface of the head were not coded asdiscussed below and illustrated in Figure 7 The present authorsdo not include all potential synapomorphies for the Cheli-cerata as the status of many of these is debated (see eg Shultz1990 2001 Dunlop amp Selden 1997 Weygoldt 1998 Wheeler ampHayashi 1998 Dunlop 1999 Edgecombe et al 2000)

Two methods for the coding of inapplicable characters inphylogenetic analysis have been used First inapplicable char-acter states can be coded as missing data A complex structuremay comprise characters lsquoabsentpresentrsquo and lsquostate1state2rsquowith taxa lacking the structure coded as absent for the firstcharacter and as missing data for the second Some authorshave regarded this method as problematic because it may leadto reconstruction of impossible ancestral states and henceunjustified trees (Platnick et al 1991 Strong amp Lipscomb1999) The alternative is to code the second character as a thirdlsquonot applicablersquo state in taxa that lack the structure Thesemethods are equivalent to Pleijelrsquos (1995) coding methods Cand B respectively In the present study inapplicable charac-ters are treated as missing data for most analyses becausecoding them as a distinct character state reduces characterindependence and effectively weights the inapplicable charac-ter and hence could result in them dominating the analysisThe effects of this assumption were investigated by using adistinct character state in some analyses (see Section 26) andinapplicable characters are shown as distinct to lsquotruersquo missingdata (using the symbol lsquo-rsquo) in the matrix (Table 2)

241 Anterior cephalic appendages and head segmentation1 Appendages of the first segment (antennae) (0) present and(1) absent The majority of taxa considered in the present study

Figure 4 Reconstruction of the euarthropod stem group used to inform coding of hypothetical outgroup Grosstopology largely follows Budd (1996b fig 9 1997 fig 1110) Solid boxes indicate unambiguous apomorphiesand shaded boxes character states which are plesiomorphic for the Arthropoda

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 175

possess a single pair of long uniramous multi- and annulatedantennae at the anterior of the head which presumably had asensory function as found in Crustacea Myriapoda and mostmembers of Hexapoda (ie proturans do not have antennae)These appendages are likely to be a synapomorphy of theEuarthropoda (eg Scholtz 1997 Walossek amp Muller 1997)and therefore the outgroup is coded as State 0 The anterior-most head appendages of most Cambrian megacheiran arthro-pods which have been called lsquogreat appendagesrsquo followingWalcott (1912) consist of a small number of robust spinosepodomeres Uniquely Fortiforceps has a pair of short anten-nae anterior to the lsquogreat appendagesrsquo (Hou amp Bergstrom 1997pp 34ndash8) Therefore it appears likely that the lsquogreat append-agesrsquo are the appendages of the second cephalic segment andthe antennae are lost in megacheirans other than FortiforcepsThis of course depends on recognising the lsquogreat appendagesrsquoas homologous in all of these taxa as discussed below(Character 2) Homology of the megacheiran anterior append-ages with the second cephalic appendages of trilobites waspreviously suggested by Stoslashrmer (1944 p 124) on the basis oftheir post-oral position

The classical view of chelicerate head segmentation main-tains that they too have lost the antennae and the cheliceraeare the appendages of the second cephalic segment Recentrevisions of arthropod phylogeny have uncritically acceptedthis homology (Edgecombe et al 2000 Giribet et al 2001)which is based largely on neuroanatomy The lsquochelicero-neuromerrsquo which innervates the chelicerae seems to be ho-mologous with the tritocerebrum associated with the secondcephalic appendage pair of mandibulates (see Weygoldt 1979Winter 1980) Although some uncertainty exists over theposition of pycnogonids within Arthropoda and the homol-ogy of their cephalic segmentation is poorly understood thisview is also supported by the presence of pre-cheliceralappendages in a pycnogonid larva (Larva D of Muller ampWalossek 1986) from the Upper Cambrian of Sweden(Walossek amp Dunlop 2002) Cheloniellon also has a pair ofpre-oral pediform appendages considered homologous to thechelicerae in addition to its antennae Thus despite recentneontological studies that indicate that the chelicerae arehomologous to the antennae of mandibulates based on pat-terns of Hox gene expression (Damen et al 1998 Telford amp

Table 2 Data matrix for cladistic analyses of arachnomorphs () missing data and (ndash) inapplicable characters Letters indicate multistateuncertainty codings as follows (A) 34 (B) 02 (C) 01 (D) 134 (E) 12

1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 3 3 3 4 4 4 4 4 4 4 4 4 4 5 5 5 5 51 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4

OUTGROUP 0 0 0 0 0 0 0 0 0 0 0 0 ndash ndash 0 0 0 0 0 0 0 0 0 0 0 4 0 ndash 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ndash 0 0 0 0 ndash ndash ndash 0 ndash 0Aglaspidida 1 0 0 1 A 0 0 0 0 0 1 0 ndash ndash ndash ndash ndash 0 0 0 1 0 0 0 0 0 0 0 0 ndash 1 1 0 0 1 0 0 0 0 0 0 0 0 1 2 1 1 0 ndash ndash ndash 1 0 1Aglaspidida 2 0 0 A 0 0 0 0 0 0 1 0 ndash ndash 0 1 0 1 0 0 0 0 0 0 0 0 ndash 1 1 0 0 1 0 0 0 0 0 0 0 0 1 2 1 1 0 ndash ndash ndash 1 0 1Alalcomenaeus 1 1 1 3 ndash 2 0 1 0 0 0 1 0 ndash ndash 1 0 0 0 0 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 0 1 0 1 ndash ndash ndash 1 1 0Buenaspis 0 1 B 0 0 1 0 0 ndash 0 1 0 1 0 0 0 0 1 0 0 0 0 ndash 0 1 1 1 0 1 0 ndash 0Burgessia 0 0 0 3 0 0 0 0 0 0 0 1 0 ndash ndash 0 0 0 1 2 ndash 0 0 0 0 0 4 0 ndash 0 0 0 0 0 0 2 0 0 0 0 0 0 ndash 0 0 0 1 ndash ndash ndash 1 0 0Cheloniellon 0 0 1 4 0 0 0 0 0 1 0 1 0 ndash ndash 0 0 1 0 1 0 0 0 0 0 0 1 0 ndash 0 1 0 0 1 0 0 0 0 0 1 0 0 1 0 1 1 0 ndash ndash ndash 0 ndash 1Cindarella 0 0 0 6 0 0 0 0 0 0 0 1 1 0 0 0 0 1 1 0 0 0 0 0 0 0 1 0 ndash 0 1 0 1 1 0 0 2 1 0 0 0 1 0 ndash 1 0 1 1 0 0 0 0 ndash 0Crustacea 0 0 0 3 0 0 0 0 0 0 0 0 0 ndash ndash 0 1 0 0 0 0 0 0 0 0 0 4 0 ndash 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ndash 0 0 0 ndash ndash ndash 1 0 0Emeraldella 0 0 1 5 0 0 0 0 0 0 0 1 1 0 0 0 0 1 0 2 ndash 0 0 0 0 0 0 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 1 1 1 1 0 1 ndash ndash ndash 1 0 1Eoredlichia 0 0 0 3 0 0 0 0 1 0 0 1 1 0 0 0 0 1 0 0 1 0 0 0 0 0 3 1 1 1 1 0 0 2 0 0 0 0 0 0 0 0 0 ndash 1 0 1 1 0 0 1 0 ndash 0Eurypterida 1 1 1 6 ndash 1 1 0 0 1 1 1 0 ndash ndash 1 0 1 0 1 0 0 0 1 0 0 4 1 0 0 1 0 0 1 0 0 0 0 0 0 0 0 1 3 0 1 0 1 ndash ndash ndash 1 0 0Fortiforceps 0 1 1 4 ndash 0 0 0 0 0 0 1 0 ndash ndash 1 0 0 1 0 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 0 1 0 1 ndash ndash ndash 1 1 0Helmetia 3 1 0 1 0 1 0 0 0 0 0 1 2 0 ndash 0 1 0 0 2 0 0 0 0 0 1 1 0 0 ndash 0 0 1 1 0 1 0 0 ndash 0Jianfengia 1 1 1 4 ndash 1 0 0 0 0 0 1 0 ndash ndash 1 0 1 1 0 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 0 1 0 1 ndash ndash ndash 1 0 0Kuamaia 0 0 0 0 0 0 0 0 0 1 1 0 1 0 0 1 0 0 0 1 0 0 0 1 2 0 ndash 0 1 0 0 2 0 0 0 0 0 1 1 0 0 ndash 0 0 1 1 0 1 0 0 ndash 0Leanchoilia 1 1 1 3 ndash 2 0 1 0 0 0 1 0 ndash ndash 1 0 1 1 B 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 1 1 0 1 ndash ndash ndash 1 0 0Lemoneites 0 1 0 0 0 0 1 1 0 1 0 0 0 0 0 0 0 0 1 2 0 0 ndash ndash ndash 1 0 Liwia 0 1 0 0 0 0 1 0 1 0 0 1 0 1 0 0 0 0 ndash 0 1 1 1 0 0 0 ndash 0Marrella 0 0 1 1 0 0 0 0 0 0 0 0 0 ndash ndash 0 1 0 0 2 ndash 0 0 0 0 0 0 ndash 0 0 0 0 0 0 2 0 0 0 0 0 0 ndash 0 0 0 0 ndash ndash ndash 0 ndash 0Mimetaster 0 0 1 2 0 0 0 0 0 0 0 0 0 ndash ndash 0 1 0 1 C 0 0 0 0 0 0 ndash 0 0 0 0 1 0 0 2 0 0 0 0 0 0 ndash 0 0 0 0 ndash ndash ndash 0 ndash 0Misszhouia 0 0 0 3 0 0 0 0 1 0 0 1 1 1 0 0 0 1 0 0 0 0 0 0 0 0 1 0 ndash 0 1 1 ndash ndash 2 0 0 0 1 0 0 0 0 ndash ndash 0 1 1 0 0 1 0 ndash 0Naraoia 0 0 0 3 1 0 0 0 1 0 0 1 1 1 0 0 0 1 0 0 0 0 0 0 0 0 1 0 ndash 0 1 1 ndash ndash 2 0 0 0 1 0 0 0 0 ndash ndash 0 1 1 0 0 0 ndash 0Olenoides 0 0 0 3 0 0 0 0 1 0 0 1 1 0 0 0 0 1 0 1 1 0 0 0 0 0 3 1 1 1 1 0 0 2 0 0 0 0 0 0 0 0 0 ndash 0 1 1 1 0 0 0 0 ndash 0Paleomerus 0 1 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0 0 0 ndash 0 0 ndash ndash ndash 1 0 Pycnogonida 1 1 1 4 ndash 1 1 0 0 1 1 1 ndash ndash ndash ndash 0 0 1 1 0 0 0 1 0 0 4 0 ndash 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 ndash ndash ndash 1 0 0Retifacies 0 0 0 3 0 0 0 0 0 0 0 1 0 ndash ndash 0 0 1 0 0 0 0 0 0 0 0 0 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 1 0 1 1 0 0 0 1 0 0Saperion 0 2 0 1 0 1 0 0 1 1 0 0 1 0 0 0 1 2 0 ndash 0 1 1 ndash ndash 1 1 ndash ndash 0 0 1 0 0 ndash ndash 1 1 0 0 1 0 ndash 0Sidneyia 0 0 1 0 1 0 0 0 0 1 0 1 0 ndash ndash 0 0 1 0 0 0 0 0 0 0 0 0 0 1 0 0 1 0 0 0 0 0 0 0 0 1 2 1 1 0 1 ndash ndash ndash 1 0 1Sinoburius 0 0 0 4 0 0 0 0 0 0 0 1 0 1 0 0 1 1 0 0 0 1 0 ndash 0 1 0 0 1 0 0 2 1 0 0 0 1 0 ndash 0 0 1 1 0 1 0 0 ndash 0Skioldia 0 2 0 0 0 1 0 0 0 1 2 0 ndash 0 1 1 ndash ndash 1 1 ndash ndash 0 0 1 0 0 ndash ndash 1 1 0 0 1 0 ndash 0Soomaspis 0 1 B 0 0 0 0 1 0 0 ndash 0 1 0 1 0 0 1 0 1 0 0 0 0 ndash 0 1 1 1 0 1 0 ndash 0Tariccoia 0 1 B 0 0 0 0 1 0 0 ndash 0 1 0 1 0 0 1 0 1 0 0 0 0 ndash 0 1 1 1 0 1 0 ndash 0Tegopelte 0 0 0 0 0 0 0 0 1 0 1 1 0 0 0 0 1 E 0 ndash 0 1 0 ndash ndash E 1 ndash ndash 0 0 0 0 ndash ndash 0 1 1 0 0 1 0 ndash 0Weinbergina 1 1 1 6 ndash 1 1 0 0 1 1 1 0 ndash ndash 1 0 1 0 1 0 0 0 1 0 0 4 1 0 0 1 0 0 1 0 0 0 0 0 0 0 0 1 2 0 1 0 1 ndash ndash ndash 1 0 0Xandarella 0 0 0 4 0 0 0 0 0 0 0 1 1 1 0 0 0 1 0 0 0 0 1 0 0 0 1 0 ndash 0 1 0 1 1 0 0 2 1 0 0 0 1 0 ndash 1 0 1 1 0 0 0 ndash 0Yohoia 1 1 1 4 ndash 1 0 0 0 1 1 1 0 ndash ndash 1 0 1 0 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 1 2 0 1 0 1 ndash ndash ndash 1 1 0

176 TREVOR J COTTON AND SIMON J BRADDY

Thomas 1998) and developmental data (Mittmann amp Scholtz2003) morphological evidence from fossils supports the viewthat chelicerates have lost the antennae The Hox gene evi-dence suffers from a lack of comparative data from diversemandibulates especially primitive crustaceans (see Akam2000) and the use of Hox gene expression boundaries asmarkers for segmental homology has been criticised (egAbzhanov et al 1999 although relating to posterior Hoxpatterns) Wills et al (1998a pp 43 amp 48) considered thechelicerae to be appendages of the second (deutocerebral)segment but curiously in support of this cited Schram (1978)who unambiguously followed the homology scheme used inthe present study

The anterior-most appendages of Aglaspis were originallydescribed as chelicerae (Raasch 1939) which lead to theirclassification in the Chelicerata (eg Stoslashrmer 1944) Howeverthe morphology of these appendages (see Briggs et al 1979) ismore similar to that of antennae They are narrow relative tothe thoracic endopods of relatively even diameter proximallyand the podomeres are apparently weakly defined Hesselbo(1988 1992) also argued that the anterior appendages ofAglaspis are likely to be antennae The distal parts are un-known (and hence so is their length cf Wills et al 1998ap 58) and there is no evidence that they were chelate contraryto the coding of Wills et al (1998a character 31) Nothingis known about the anterior appendages of BuenaspisLemoneites or Paleomerus and this character is accordinglycoded as missing data in these taxa

2 Form of endopod of appendages of the second segment(0) pediform and (1) anteriorly directed raptorial appendagewith reduced number of podomeres and terminal podomeresbearing spines on distal margins As discussed above cheli-cerae and lsquogreat appendagesrsquo are both likely to represent theappendages of the second segment They also differ from theassumed primitive biramous euarthropod limb in similar waysand therefore are likely to be homologous modifications ofthese appendages The homology of lsquogreat appendagesrsquo andchelicerae was originally proposed by Henriksen (1928) andsupported by Stoslashrmer (1944) but to the present authorsrsquoknowledge no modern author has discussed this possibility

Both lsquogreat appendagesrsquo (Figs 5A D E) and chelicerae(Figs 5B C) are equipped with strong spinose projections onthe outer (dorsal) side of the distal margins of the terminalpodomeres that are lacking on the proximal podomeresSecondly the number of podomeres in both chelicerae andlsquogreat appendagesrsquo is more or less reduced compared to thenumber in the endopods of biramous limbs In the case oflsquogreat appendagesrsquo they are significantly more robust thanthose of posterior endopods a situation that is matched insome chelicerates The homology of the lsquogreat appendagesrsquoof Alalcomenaeus Leanchoilia Yohoia Jiangfengia andFortiforceps is supported by all of these features and is widelyaccepted (eg Wills et al 1998a character 31 Hou ampBergstrom 1997 Bergstrom amp Hou 1998 Hou 1987a) OnlyHou amp Bergstrom (1991 p 183) have suggested albeit inpassing that the lsquogreat appendagesrsquo of all these taxa may beconvergent a view they appear to have changed The endo-pods of the appendages of other taxa are locomotory legswhich either lack strong spines or have spinose extensionsthat are invariably on the medial (ventral) surface and arestronger on the proximal podomeres than the distal ones (egMisszhouia Fig 6A) In the latter case these spines form partof the feeding system along with the gnathobases and are oftenlocated around the middle of the podomere rather than limitedto the distal margin These endopods are directed ventro-laterally as opposed to anteriorly in the case of both cheliceraeand lsquogreat appendagesrsquo The modified second appendages of

Marrella resemble this plesiomorphic condition here referredto as lsquopediformrsquo except in their anterolateral orientation Insome Recent crustaceans this appendage is modified into asecond antenna but in stem-group crustaceans it is pediform(eg Walossek amp Muller 1997 1998) This character is codedas missing data in a number of taxa for which the morphologyof the second segment appendage is unknown

Some authors (Dzik 1993 Chen amp Zhou 1997 Dewel ampDewel 1997 Budd 2002) have suggested that the lsquogreat ap-pendagesrsquo are homologous with the anterior appendages ofanomalocaridids and the anomalocaridid-like Opabinia andKerygmachela (see Budd 1996b 1999b respectively) Theanterior appendages of most anomalocaridids and otherCambrian lobopodians differ from megacheiran lsquogreat append-agesrsquo in that they consist of a large number of segmentsMoreover there is considerable morphological evidence thatanomalocaridids are part of the euarthropod stem group (Willset al 1995 1998a) and not closely related to the lsquogreatappendagersquo taxa (cf Budd 2002) with the possible exception ofParapeytoia lsquoGreat appendagersquo taxa share a suite of derivedcharacters with other crown group euarthropods includingsclerotised dorsal tergites the incorporation of more thanone pair of appendages into the head sclerotisation of theendopods and strong differentiation of the head that wereexcluded from Buddrsquos (2002) analysis The relationships ofanomalocaridids will be considered in detail later by Braddyand co-workers

3 Exopod of appendages of the second segment (0)present and (1) absent or much reduced The raptorial ap-pendages of the second segment (chelicerae and lsquogreat append-agesrsquo) are uniramous as are the corresponding pediformappendages of Marrella Mimetaster Emeraldella Cheloniellonand Sidneyia The plesiomorphic euarthropod state is found inmost other arachnomorphs including trilobites where thebiramous second segment appendages are undifferentiatedfrom those of posterior segments (Edgecombe et al 2000character 78) The exopods of these appendages are alsopresent in basal members of the crustacean crown group(Edgecombe et al 2000 character 79) and in stem groupcrustaceans (Walossek 1993 Walossek amp Muller 1997 1998)

4 Number of head segments (0) 1 (1) 2 (2) 3 (3) 4 (4) 5(5) 6 and (6) 7 The coding of this character by Edgecombe ampRamskold (1999 character 2) explicitly referred to the numberof limb pairs present in addition to the antenna Here thenumber of post-acronal segments present in the head is codedfollowing Wills et al (1998a character 29) In taxa that lack anantenna the number of somites is inferred to be one greaterthan the number of cephalic appendage pairs (cf Wills et al1998a) as described above

Sturmer amp Bergstrom (1978) suggested that the head ofCheloniellon is defined primarily on the basis of appendagetagmosis rather than the fusion of segments under a singlecephalic-shield (cf Hou amp Bergstrom 1997 pp 98ndash9) Accord-ing to this view the head consists of both the cephalic tergiteand the anteriormost free tergite that share uniramousgnathobasic appendages Wills et al (1998a) coded Cheloniel-lon as having six somites incorporated into the head andtherefore presumably accepted this suggestion There is noevidence that the first free tergite of Cheloniellon was incorpo-rated into the head (except perhaps functionally) and thepresent authors prefer to code the number of somites under thecephalic shield In Sidneyia there is a series of uniramousstrongly gnathobasic appendages similar to those of Cheloniel-lon posterior to the head shield All of these appendages wouldpresumably have to be considered part of the head based onappendage differentiation according to the view of Sturmer ampBergstrom (1978) Some autapomorphic states are included

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 177

(none for Sidneyia one for Marrella and six for Emeraldella)since these will become informative if the character is treatedas ordered

Edgecombe amp Ramskold (1999 p 265) described a novelform of cephalic tagmosis The present authors tentativelyaccept their suggestion that in trilobite-like taxa the fourthpair of biramous cephalic appendages was directly under thecephalo-thoracic junction This may explain previous confu-sion about the number of such appendages incorporated intothe trilobite cephalon (eg Cisne 1975 Whittington 1975aBergstrom amp Brassel 1984) However Edgecome amp Ramskoldaccept the view of Chen et al (1977 p 7) that only the firstthree biramous limbs of Misszhouia were structurally and

functionally part of the head Therefore it seems more accept-able to code these taxa as having only four post-acronalsomites than to use the coding scheme of Edgecombe ampRamskold (1999) The partial integration of the first thoracicsomite under the head shield could be considered to be a formof overlap of the trunk by the head shield (ie a distinct stateof character 9 of Edgecombe amp Ramskold 1999 or Character37 herein) Derived states of this character would then poten-tially be synapomorphic for a clade including xandarellidsnaraoiids helmetiids tegopeltids and trilobites Howeversince the degree of overlap in taxa with the fourth biramousappendages under the cephalo-thoracic articulation is identicalto that found between thoracic tergites (State 0 of Character

Figure 5 Diagrammatic reconstructions of raptorial second-segment appendages in lateral view (except whereotherwise stated) and approximate life orientation showing suggested homology of appendage elements (inbrackets) Roman numerals indicate position in the podomere series and do not necessarily imply homology (A)lsquoGreat appendagersquo of Fortiforceps (after Hou amp Bergstrom 1997) (B) Left chelicera of Leiobunum aldrichi(Arachnida Opiliones) (1) in medial perspective and (2) distal parts in anterior perspective (after Shultz 2000)(C) Chelifore of Nymphon (Pycnogonida) (after Child 1997) (D) lsquoGreat appendagersquo of Yohoia (after Whittington1974 fig 2) (E) lsquoGreat appendagersquo of Leanchoilia (modified after Bruton amp Whittington 1983) Not to scaleAbbreviations (apt) apotele (probably homologous with the mandibulate pretarsus) (cx) coxa (dtmrt)deutomerite and (pt) pretarsus

178 TREVOR J COTTON AND SIMON J BRADDY

37) it is here preferred to regard this as a distinct character(Character 9 herein)

The coding of Alalcomenaeus by Wills et al (1998a) ashaving four post-acronal somites is accepted here but fordifferent reasons Briggs amp Collins (1999) have recenty demon-strated that Alalcomenaeus possessed two pairs of biramousappendages on the head in addition to the great appendagesThis would equate to three head somites according to thehomology scheme of Wills et al (1998a) Here the antennaeare inferred to be lost and therefore four segments incorpo-rated into the head

Recently a number of authors have agreed that the plesio-morphic condition for euarthropods is a four-segmented headas found in stem group crustaceans (Scholtz 1997 Walossek1993 Walossek amp Muller 1997 1998 Edgecombe et al 2000)However reconstruction of the euarthropod stem groupclearly indicates that even crownward taxa had a head consist-ing of only the antennal segment and acron as found intardigrades and onychophorans Therefore it seems that thefour-segmented head is pleisomorphic only for crown groupeuarthropods Some fully arthropodised fossil taxa also have ashorter head that may be pleisomorphic such as the two-segmented head of Marella Retifacies is coded following therecent redescription of Hou amp Bergstrom (1997) rather thanon the basis of the coding by Edgecombe amp Ramskold (1999)for which they provide no explanation A partial uncertaintycoding is used for Aglaspis in which the number of post-antennal cephalic appendages is either three or four (Briggset al 1979)

5 Orientation of the antennae (0) directed anterolaterally(1) strongly deflected laterally and (2) placed well inside shieldmargin curing posteriorly from a transverse proximal element(cf Edgecombe amp Ramskold 1999 character 3) The anteriorappendages of Aglaspis (see character 1) are clearly directedanterolaterally and it is presumed that they continued past thehead shield margin The anterior appendages of arthropodstem group taxa such as Aysheaia Kerygmachela and Anoma-locaris are either directed anterolaterally or ventrally but arenot strongly-deflected laterally Therefore the outgroup iscoded as State 0 This character is coded as missing data fortaxa where the presence of antennae is equivocal (those codedas missing data for Character 1) and as inapplicable in taxawhere the antennae are considered absent (coded as State 1 forCharacter 1)

6 Length of distal spines on terminal podomeres of endo-pods of second segment appendages (0) absent or shorer thanpodomeres (1) subsequal to length of podomeres and (2)longer than the entire podomere series The spinose projectionsof the raptorial appendages described above vary in length Inchelicerae (Figs 5BC) and some lsquogreat appendagesrsquo (eg thoseof Yohoia Fig 5D) the most distal spine is equal in length tothe spinose terminal podomere forming a chelate structureIn Fortiforceps (Fig 5A) the spines are short relative tothe lengths of the podomeres but in Alalcomenaeus andLeanchoilia (Fig 5E) they are extremely long so that thespines are much longer than the entire podomere series Asdescribed above in taxa with pediform endopods of the secondsegment appendages spines on the distal podomeres are shortor entirely absent

7 Chelicerae (0) absent and (1) present Despite theirsimilarities to lsquogreat appendagesrsquo the chelicerae of the eucheli-cerates and chelifores of the pycnogonids are clearly distinctand have been widely recognised as a chelicerate synapomor-phy Chelicerae differ from any lsquogreat appendagesrsquo by thecombination of a smaller number of podomeres the presenceof only a single terminal element and only a single spinoseprojection on the dorsal side of the podomere series (Fig 5)

Amongst extant chelicerates only the pycnogonid Pallenopsishas chelicerae of four podomeres comparable to the numberin lsquogreat appendagesrsquo Bergstrom et al (1980) considered thatthe chelifores of the Devonian pycnogonid Palaeoisopus alsoconsist of four segments but this is not well supported by theirfigures

8 Distal spines of second segment endopods terminating inannulated flagellae (0) absent and (1) present It is widelyrecognised (eg Stoslashrmer 1944 Simonetta 1970 Bruton ampWhittington 1993 Briggs amp Collins 1999) that the terminalparts of the extended spines of Alalcomenaeus and Leanchoiliaformed annulated flagellae Nothing similar is known fromhomologous appendages of any other arthropod althoughsimilar processes may have operated in the transformation ofthe endopod as a whole into the second antennae of derivedcrustaceans and in the origin of multiramous antennulae (firstantennae) in malacostracans

242 Posterior appendages 9 Appendages of first thoracicsomite underneath the cephalo-thoracic articulation (0) ab-sent and (1) present (cf Edgecombe amp Ramskold 1999character 2) For a discussion of this character see Character4 herein

10 Exopods of appendages of third to fifth segments (0)present and (1) reduced or absent A number of taxa haveuniramous appendages on the third to fifth segments In mostthese segments are incorporated into the head (Yohoia cheli-cerates) but in others all (eg in Sidneyia) or some (eg inCheloniellon) of these segments are post-cephalic In all casesthe appendage is morphologically similar to the endopods ofbiramous limbs of other taxa andor other segments and it isinterpreted that the exopod is lost

11 Endopods of thoracic appendages (0) present and (1)reduced or absent The opisthosomal appendages of chelicer-ates lack endopods or have the endopods much reduced (egLimulus Siewing 1985 fig 838 Shultz 2001) a situation that isalso found in the uniramous thoracic appendages of Yohoia(Whittington 1974) It has been suggested that Helmetia(Briggs et al 1994) lacks endopods but pending a redescrip-tion of this genus and considering that they have been docu-mented in the otherwise very similar Kuamaia this is coded asuncertain

12 Exopod shaft of numerous podomeres each bearing asingle seta (0) present and (1) absent The exopods ofcrustaceans (at least plesiomorphically see eg Walossek ampMuller 1998 figs 5middot5 amp 5middot6) and of Marrella and Mimetasterhave multi-annulated shafts with each podomere bearing asingle seta In other taxa the exopod shafts consist of one oftwo lobes bearing numerous setae

The polarity of exopod segmentation is uncertain Thelateral flaps of stem group arthropods such as Opabinia(Whittington 1975b Budd 1996b) and anomalocaridids (egHou et al 1995) are certainly unsegmented but as suggestedby Buddrsquos (1996b fig 8) reconstruction this does notnecessarily suggest the form of the primitive euarthropodexopod Rather segmentation may have been a result of thesclerotization of the cuticle of the exopod shaft The outgroupwas coded as uncertain for this character According to thefirst interpretation of aglaspidid appendage morphology(coded as Aglaspidida 1) the exopods are lacking on allappendages and consequently this and all other charactersdescribing exopod morphology are coded as inapplicable

13 Exopod shaft differentiated into proximal and distallobes (0) absent and (1) present Ramskold amp Edgecombe(1996 Edgecombe amp Ramskold 1999 character 26) havediscussed the distribution of the trilobite-type bilobate exo-pods recognised by this character (Fig 6A) Among taxaincluded here that were not considered by Edgecombe amp

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 179

Ramskold (1999) exopods consist either of a single flattenedlobe or a homonomous series of podomeres (Fig 6B C) Nostem group euarthropod has a bilobate exopod and theoutgroup is consequently also coded as State 0

14 Proximal lobe of exopod (0) flattened lobe and (1)slender shaft

15 Distal lobe of exopod (0) small to moderate sized flapwith short to moderately long attachment to proximal lobeand (1) large teardrop-shaped with long attachment to proxi-mal lobe Neither of these characters can be coded for taxathat do not have an exopod differentiated into proximal anddistal lobes (Character 13 State 1) without asserting thehomology of the single-lobed exopod with one of these partsThe coding of Edgecombe amp Ramskold (1999 p 280) implic-itly homologizes the exopods of Sidneyia and Retifacies withthe proximal lobe In these taxa this view is perhaps supportedby the presence of lamellate exopod setae which match thoseof the trilobite-type proximal lobe However in other taxawith a single-lobed exopod such as Alalcomenaeus (see Briggsamp Collins 1999) the exopod is fringed with sharp spines likethe distal lobe of differentiated exopods Alternatively thebilobate exopod may have originated from the division of aprimitively single-lobed structure The single-lobed exopod isnot clearly homologous with either lobe of divided exopodsand both these characters are consequently coded as inappli-cable to taxa without differentiated proximal and distal exopodlobes Consequently (see Character 13 herein) these characterscan only be coded for taxa that were previously considered byEdgecombe amp Ramskold (1999) and their coding is followedhere except in the cases of Sidneyia and Retifacies

16 Exopod shaft is a deep rounded flap (0) absent and (1)present All bilobate (Character 13) and segmented (Character12) exopod shafts are long and relatively narrow structures (seeFig 6) whereas some single-lobed exopod shafts have theform of rounded flaps These flap-like exopod shafts are largecompared to the length of the setae and at least half as deep asthey are long This appendage structure is found in cheliceratesand lsquogreat appendagersquo arthropods

17 Medially-directed exopod setae (0) absent and (1)present Walossek amp Muller (1998 p 194) suggested that thetilting of exopod setae towards the endopod in post-antennularlimbs with a multi-annulated exopod was an autapomorphyuniting crustaceans and all members of the crustacean stemgroup (the crustacean total group sensu Ax 1986 see Fig 6C)This character is shared with the marellomorphs Marrellaand Mimetaster (see Fig 6B Sturmer amp Bergstrom 1976Bergstrom 1979 fig 1middot3A B) In other taxa the setae eithersurround the entire margin of the exopod shaft or are directeddorsally (Fig 6A) The identification of setae on the dorsalsurface of the lateral flaps in Opabinia (Budd 1996b) suggeststhat the condition in marellomorphs and crustaceans isderived

18 Lamellate exopod setae (0) absent and (1) presentLamellate exopod setae originally described from trilobitesare a classic synapomorphy of the Arachnomorpha (see egBergstrom 1992 Hou amp Bergstrom 1997 pp 42ndash3) They areperhaps best known from Misszhouia (see Chen et al 1996Hou amp Bergstrom 1997 Fig 6A) These setae differ from themore spinose setae of other taxa (Fig 6C) in that they are wideand flat and imbricate over the length of the exopod shaft Thesetae of Marrella (Fig 6B) and similar taxa have variouslybeen considered lamellate (Bergstrom 1979) or non-lamellate(Bergstrom 1992) Since they do not imbricate and do notseem to be strongly flattened (Whittington 1971 Sturmer ampBergstrom 1976 fig 9a) Marrella and Mimetaster are codedas State 0 The setae of Helmetia as shown in Briggs et al(1994 fig 141) seem to be of the lamellate type The setae ofSinoburius have been described by Hou amp Bergstrom (1997p 85) as similar to those of Misszhouia Only the setae areknown of the appendages of Buenaspis (Budd 1999a) andthese also appear to be of the trilobite-type

Figure 6 Diagrammatic reconstructions of (A) arachnomorph and(B C) non-arachnomorph biramous appendages (A) The lsquotrilobite-typersquo biramous appendage of Misszhouia longicauda (after Chen et al1997) (B) Appendage of Marrella splendens (after Whittington 1971)(C) Appendage of the stem-lineage crustacean Martinssonia elongata(after Muller amp Walossek 1997 1998) Not to scale Abbreviations(bas) basis (dle) distal lobe of exopod shaft (ls) lamellar exopod setae(pe) proximal endite or pre-coxa (ple) proximal lobe of exopod shaft(ses) segmented exopod shaft and (ss) spinose exopod setae

180 TREVOR J COTTON AND SIMON J BRADDY

The homology of the book-gill lamellae of xiphosurans withlamellar setae was supported by Walossek amp Muller (1997p 149) and Edgecombe et al (2000 p 174) but rejected bySturmer amp Berstrom (1981) on the grounds that Weinberginapossesses Limulus-like gill lamellae and fringing setaeBook-gills have also been described from a eurypterid (Braddyet al 1999) There are certainly major morphologicaldifferences between book-gills and trilobite-type exopodsFollowing most recent opinion and pending further studyWeinbergina and Eurypterida are coded as possessing lamellatesetae

The form of the setae of Yohoia is uncertain (Whittington1974) the spinose setae seen in the most common reconstruc-tion (Gould 1989 Briggs et al 1994) are not justified by thespecimens Amongst other great appendage arthropods theform of the setae appears to vary those of Jianfengia (seeChen amp Zhou 1997 p 74) and Leanchoilia (see Bruton ampWhittington 1983) are lamellate while those of Fortiforceps(Hou amp Bergstrom 1997) and Alalcomenaeus (Briggs amp Collins1999) are spinose and less densely packed

19 Gnathobase on basis andor prominent endites onendopod (0) present and (1) absent (cf Edgecombe ampRamskold 1999 character 29 Wills et al 1998a characters 36amp 59) The presence of gnathobases on the appendages of someanomalocaridids (Hou et al 1995) and their general distribu-tion amongst non-arachnomorph euarthropods suggests thatState 0 is plesiomorphic The homology of the various enditesand gnathobasic structures found in euarthropods is in need ofcareful assessment and therefore a very general coding (fol-lowing Edgecombe amp Ramskold) is used for this character

243 Eyes 20 Position of lateral facetted eyes (0)ventral and stalked (1) dorsal and sessile and (2) absent (cfEdgecombe amp Ramskold 1999 character 4) Partial uncer-tainty coding is used for a number of taxa where the dorsalsurface is known but the ventral morphology is not andtherefore the presence of ventral eyes cannot be discountedFor example there is good evidence that Leanchoilia lackeddorsal sessile eyes but ventral stalked eyes may have beenpresent (as in L hanceyi see Briggs amp Robison 1984) and a(02) partial uncertainty coding is used This is also the case inmany naraoiids such as Buenaspis However only the outlineof the head shield of Liwia is known and the absence of dorsaleyes in the reconstruction of Dzik amp Lendzion (1988) isconjectural It is unclear whether the autapomorphic conditionof stalked dorsal eyes in Mimetaster (see Sturmer amp Bergstrom1976) is homologous with State 0 or State 1 and a partialuncertainty coding is also used

Whittington (1974) was somewhat equivocal about thenature of the lateral lobes anterior to the head shield ofYohoia These more closely resemble the ventral stalked eyes ofAlacomenaeus as recently described by Briggs amp Collins(1999) than either Whittingtonrsquos (1974) or subsequent (egGould 1989 fig 3middot18 Briggs et al 1994 fig 153) reconstruc-tions suggest In a well preserved specimen showing the dorsalaspect (USNM 57696 Whittington 1974 pl 2 figs 1ndash3) and ina laterally compressed specimen (USNM 57694 Whittington1974 pl 1 fig 1) they are clearly seen to be stalked andrelatively small lobate structures That these structures arelikely to represent stalked ventral eyes was reflected in thecoding of Wills et al (1998a characters 26 amp 27)

21 Visual surface with calcified lenses bounded withcircumocular suture (0) absent and (1) present

22 Dorsal bulge in exoskeleton accommodating drop-shaped ventral eyes (0) absent and (1) present

23 Eye slits (0) absent and (1) presentCharacters 21 to 23 are used exactly as described by

Edgecombe amp Ramskold (1999 character 5) Character 21 is

inapplicable to taxa that do not possess eyes (Character 20State 2)

24 Dorsal median eyes (0) absent and (1) present Dorsalmedian eyes on a tubercle are considered a synapomorphy ofthe Chelicerata (Dunlop amp Selden 1997 Dunlop 1999) Thenature and position of the structures interpreted as dorsalmedian eyes in Mimetaster (Sturmer amp Bergstrom 1976p 83ndash4) are regarded as equivocal

244 Ventral cephalic structures Many characters of theventral surface of the euarthropod head (see Fig 7) of poten-tial phylogenetic utility are poorly known in many importanttaxa In particular the presence of pre-hypostomal frontalorgans (Fig 7A CndashE) is not coded herein (see Edgecombe ampRamskold 1999 p 272) Definitive evidence of the absence ofthese structures is available for very few taxa and the homol-ogy of these structures with the maculae of the trilobitehypostome (see Fig 7B) and the frontal organs of the crusta-cean labrum (eg see Muller amp Walossek 1987 p 39) isuncertain Secondly the detailed homology of the trilobitehypostome with that of other putative arachnomorphs isunclear and consequently a very broad definition is usedherein For example various pre-hypostomal sclerites (Fig7A D) in other arachnomorphs may have been incorporatedalong with a primitive hypostome into a single sclerite that isrecognised as the hypostome in trilobites The structure of thehypostome (eg the presence of paired anterior wings dorsal tothe antennae Fig 7B C) may also be a source of additionalcharacters

25 Expanded cephalic doublure (0) absent and (1)present maximum width more than 30 length of head shieldor more than 25 width of pygidium Chen et al (1996)suggested that the wide doublure of Soomaspis and Tariccoia isa synapomorphy uniting these taxa The doublure of Buenaspis(see Budd 1999a) is poorly known but based on the width ofthe heavily crushed region of the cephalic margin and theposition of a possible impression of the edge of the doublure inone specimen (Budd 1999a pl 1 fig 1) it also appears to berather wide (ie greater than 30 of the length of the headshield) Following Edgecombe amp Ramskoldrsquos coding ofSidneyia the present authors do not consider the postero-median expansion of the doublure of for example Emeraldellaor Retifacies (see Bruton amp Whittington 1983 Hou ampBergstrom 1999 respectively) to be homologous with State 1but to represent the hypostome (see Character 26)

26 Anteromedian margin of cephalon notched accomo-dating strongly sclerotised plate (0) notch and plate absentand (1) notch and plate present (cf Edgecombe amp Rasmkold1999 character 10) The apomorphic state (illustrated inFig 7D) is only known in the taxa discussed by Edgecombe ampRasmkold (1999) Reconstructions of Yohoia show a roundedlobe between the eyes (see Character 20) anterior to anddistinct from the head shield which resembles the anteriorsclerite recognised here This seems to be a misinterpretationSpecimens preserved in dorsal aspect (USNM 57696Whittington 1974 pl 2 figs 1ndash3) and lateral aspect (USNM57694 Whittington 1974 pl 1 fig 1 USNM 155616Whittington 1974 pl 5 fig 2) seem to clearly show that thislsquomedian lobersquo is a downward curving pointed extension of thehead shield

27 Hypostomal sclerite (0) median extension of thedoublure with no suture (1) natant sclerite not in contactwith doublure (2) with narrow overlap with pre-hypostomalsclerite (3) narrow attachment to doublure at hypostomalsuture and (4) absent The homology of hypostomes andlabrums has been discussed by Haas et al (2001) and Budd(2002) The euarthropod labrum is likely to represent a pos-teroventral extension of the pre-segmental acron (Scholtz

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 181

Figure 7 Ventral reconstructions of arachnomorph arthropod heads (A) Misszhouia longicaudata (after Chenet al 1997 figs 2ndash4 7) (B) Ceraurinella typa (after Chen et al 1997 fig 7) (C) Agnostus pisiformis (after Mulleramp Walossek 1987) (D) Kuamaia lata (after Edgecombe amp Ramskold 1999 figs 32 5 amp 6) (E) Cindarella eucala(after Ramskold et al 1997) and (F) Emeraldella brocki (after Bruton amp Whittington 1983) Not to scaleAbbreviations (aw) anterior wing beneath antennae (bs) boomerang-shaped sclerite (bl) blade-like process ofposterior wing (d) doublure of head-shield (cs) connective suture (f) fenestra (fs) facial suture (h) hypostome(hl) hypostomal lobe (emerging through fenestrae of Agnostus) (hs) hypostomal suture (if) intervening field ofhypostomal complex (lb) lateral bridge (lfo) lateral frontal organ (mb) median body (mbr) median bridge (mf)median furrow (mfb) mouth field bridge (mfo) median frontal organ (ol) ovate lobe and (phs) pre-hypostomalsclerite Post-antennal appendages and distal parts of antennae omitted

182 TREVOR J COTTON AND SIMON J BRADDY

1997) This structure is often sclerotised and covers the mouthventrally It is this sclerite that is here identified as thehypostome Therefore this character is considered absent intaxa lacking a sclerite covering the mouth irrespective of themorphology and position of the labrum itself which at leastpartly covers the mouth in all euarthropods other thanpycnogonids (see Edgecombe et al 2000 p 167) The lsquofleshylabrumrsquo of crustaceans does not appear to be sclerotised and ahypostome is consequently considered to be absent Althoughmany derived features are associated with the crustaceanlabrum this is not a good reason to consider it non-homologous with the hypostome-bearing structure of otherarthropods as Walossek amp Muller (1990) argued

In many taxa the mouth is covered by a posteromedianextension of the doublure here considered to represent thehypostome (eg Emeraldella Fig 7F Aglaspis see Hesselbo1992 fig 52) In others the hypostome is separated from thedoublure either by a suture a pre-hypostomal sclerite (egKuamaia lata Fig 7D Saperion glumaceum see Edgecombe ampRamskold 1999 figs 3 amp 4) or an intervening region ofunsclerotised cuticle (the natant condition of Fortey 1990beg Cindarella eucala Fig 7E) The homology of pre-hypostomal sclerites and hypostomes in helmetiids trilobitesand naraoiids (see Fig 7) has been discussed by Chen et al(1996) and Edgecombe amp Ramskold (1999) but it is as yetunclear whether any of the character states described here arederived from any other They are treated here as distinct andthe character as unordered pending further study

No hypostome has been identified in Yohoia (see above)Alalcomenaeus Jianfengia Fortiforceps or Leanchoilia andthere is certainly no broad posterior extension of the doublurein any of these taxa Since there is also no pre-hypostomalsclerite they have received a (134) partial uncertainty codingThe attachment of the hypostome of Tegopelte is unclear butit does not seem to be attached to any doublure (Whittington1985) and therefore it is given an uncertainty (12) coding

The lsquorostrum-like structurersquo on the ventral surface ofLemoneites appears to be similar in morphology and positionrelative to the anterior margin of the head shield (Flower 1968pl 8 figs 4 amp 13) to that described from Aglaspis and is codedas State 0 In many taxa the hypostome is unknown but inonly a small number can it be coded reliably as absentHughesrsquo (1975) suggestion that the hypostome of Burgessia isabsent is accepted preliminarily but an extension of thedoublure may be visible in some of Hughesrsquo figures Fortey(pers comm 2000) also doubts Hughesrsquo assertion

28 Visible ecdysial sutures (0) absent and (1) present Themarginal ecdysial sutures of chelicerates and the dorsal suturesof trilobites are almost certainly not homologous but thepresence of sutures and their position (see Character 29) arecoded separately to allow this to be tested Wills et al (1998aCharacter 2) coded marginal sutures as present in Sidneyiafollowing Brutonrsquos (1981) suggestion but this is consideredequivocal here

29 Position of ecdysial sutures (0) marginal and (1) dor-sal This character is inapplicable to taxa lacking ecdysialsutures (Character 28 State 0) Dorsal sutures whilst presentin the representatives of the Trilobita coded here are probablynot a synapomorphy of trilobites as a whole but for a clade oftrilobites excluding the Olenellida (Whittington 1989 Fortey1990a)

245 Exoskeletal tergites and thoracic tagmosis 30 Min-eralised cuticle (0) absent and (1) present Calcification of theexoskeleton is one of the most convincing synapomorphiesof the Trilobita (Fortey amp Whittington 1989 Ramskold ampEdgecombe 1991 Edgecombe amp Ramskold 1999 Character 1)Cuticle mineralisation is also coded as present in aglaspidids

following Briggs amp Fortey (1982) who suggested that theaglaspidid exoskeleton was originally phosphatic Paleomerusprobably also had a mineralised cuticle but Hou amp Bergstrom(1997 p 97) argued that it was likely to be calcareous Anundescribed Silurian aglaspidid may also have had an origi-nally calcitic cuticle (Fortey amp Theron 1994 p 856) Becauseof the uncertainty about the chemical composition of theexoskeleton in these forms cuticle mineralisation is coded aspotentially homologous in all these taxa and no attempt ismade to code separate states for phosphatic and calcareousmineralisation

31 Trunk tergites with expanded lateral pleurae coveringappendages dorsally (0) absent and (1) present Whilst thepresence of paratergal folds may be a synapomorphy at thelevel of the Euarthropoda (Boudreaux 1979 Wagele 1993Edgecombe et al 2000 Character 142) these are at mostsmall reflections of the margin of the tergites in most euarthro-pods In arachnomorph taxa these are expanded to form largelateral pleurae that cover the appendages dorsally (see egLimulus and Triarthus in Boudreaux 1979) This is inferred tobe the case in some taxa where dorsal features and appendagesare poorly known on the basis of their possession of tergiteswith wide pleural regions This feature was suggested as typicalof lamellipedians (=arachnomorphs) by Hou amp Bergstrom(1997 pp 42ndash3)

The arrangement of the thorax of Marrella Mimetaster andBurgessia differs from that of all the other taxa considered herein that the appendages seem to be attached to the lateralmargins of the body The trunk tergites do not cover theappendages and seem to lack pleurae entirely although theyare covered by a posterior extension of the head shield inBurgessia This character has been clearly described inMimetaster (Sturmer amp Bergstrom 1976 pp 87ndash90 fig 8) andBurgessia (Hughes 1975 p 421)

32 Free thoracic tergites (0) present and (1) absentFollowing Edgecombe amp Ramskold (1999 Character 16) anumber of taxa lack functional post-cephalic articulations andconsequently lack free thoracic tergites Despite this theycode these taxa for a number of characters relating to thestructure of thoracic tergites (eg their characters 18 and 19)These characters are treated here as inapplicable to taxawithout free tergites and the definition of this character hasbeen modified from that of Edgecombe amp Ramskold (1999) tomake this more explicit

33 Decoupling of thoracic tergites and segments (0)absent and (1) present Ramskold et al (1997) described thischaracter of Cindarella and Xandarella where the thoracictergites correspond to a variable increasing posteriorly num-ber of appendage pairs This character cannot be coded fortaxa in which free thoracic tergites are absent (Character 32State 1)

34 Tergite articulations (0) tergites non-overlapping (1)extensive overlap of tergites and (2) edge-to-edge pleuralarticulations In most of the taxa considered here the thoracictergites overlap considerably and relatively evenly over theirwidth This contrasts with the primitive euarthropod condi-tion where the tergites do not overlap medially as seen in stemgroup crustaceans In trilobites such as Helmetia and Kuamaia(see Edgecombe amp Ramskold 1999 character 18) overlap ofthoracic tergites is limited to a well-defined axial region andthe lateral pleurae meet edge-to-edge

The thoracic articulations of naraoiids with free thoracictergites are in some ways similar to those of trilobites with astrong overlap medially that is lacking abaxially andanterolateral articulating facets (Budd 1999a Ramskold ampEdgecombe 1999) However the distribution of these features

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 183

in other trilobite-like arachnomorphs is unclear and a restric-tive coding is used here The present authors do not considerthat this character can be applied reliably to the fused thoracictergites of Saperion Tegopelte and Skioldia and it is coded asinapplicable to all taxa lacking free thoracic tergites (Character32 State 1)

35 Trunk effacement (0) trunk with defined (separate orfused) tergite boundaries (1) trunk tergite boundaries effacedlaterally and (2) trunk tergite boundaries completely effaced(cf Edgecombe amp Ramskold 1999 Character 15)] Contrary toEdgecombe amp Ramskold (1999) the present authors considerSkioldia and Saperion to show a distinct form of tergiteeffacement where the fused tergite boundaries are defined byfurrows axially but effaced laterally The boundaries areeffaced at least laterally in Tegopelte but the axial region ofthe exoskeleton is unknown which accordingly is given amultistate uncertainty coding

36 Cephalic articulation fused (0) absent and (1) presentIn Tegopelte Saperion and Skioldia the articulation ofthe head with the thorax is non-functional and the entireexoskeleton forms a single tergite

37 Head shield overlap of thoracic tergites (0) overlapabsent or identical to overlap between thoracic segments (1)head shield covers first thoracic tergite only and (2) headshield covers mutliple anterior trunk tergites

38 Head shield articulates with reduced anterior thoracictergite (0) absent and (1) present Amongst taxa showing aposteriorly expanded head shield ie one that overlaps thethorax Edgecombe amp Ramskold (1999 Character 9) identifiedtwo distinct character states One of these ndash overlap of only theanteriormost thoracic tergite ndash was limited to taxa assigned tothe Liwiinae (sensu Fortey amp Theron 1994) and another ndashoverlap of multiple tergites with attachment to a reducedanterior thoracic tergite ndash to the Xandarellida None of theadditional taxa considered herein (including Buenaspis whichwas assigned to the Liwiinae by Budd 1999a) shows thesedistinctive morphological features However the expandedhead shields of Marrella Mimetaster and Burgessia which donot articulate with a narrow anterior thoracic tergite may behomologous with the expanded head shields of xandarellidsTo recognise this the degree of overlapping of the thorax andthe possession of the reduced anterior tergite are coded sepa-rately These characters are both coded as inapplicable to taxain which the head shield and thoracic tergites are fused(Character 36 State 1) The crustacean carapace which ismost similar to the head shield of Burgessia is seeminglyabsent in stem group crustaceans (Walossek amp Muller1998)

39 Trunk narrowed anteriorly relative to head shieldwidest posteriorly (0) absent and (1) present (cf Edgecombeamp Ramskold 1999 Character 14) The anterior narrowing ofthe trunk caused by the reduction of opisthosomal segment 1in xiphosurids (Weinbergina of the taxa analysed hereAndersen amp Selden 1997 Dunlop amp Selden 1997 Character14) is not considered to be homologous with the unusual shapeof the thorax in naraoiids recognised by their coding

40 Boundaries of anterior trunk segments reflexed antero-laterally (0) absent boundaries transverse or reflexed postero-laterally and (1) present

41 Joints between posterior tergites functional anteriorones variably fused (0) absent and (1) present

42 Posterior tergite bearing axial spine (0) absent and (1)present

Characters 40 to 42 are coded following Edgecombe ampRamskold (1999 characters 17 19 amp 23 respectively) Inaddition to the taxa considered here a thoracic axial spine ispresent in some olenelloid trilobites (see Lieberman 1998

Characters 73 amp 74) and in the aglaspidid Beckwithia (seeHesselbo 1989)

246 Posterior body termination 43 Postabdomen of seg-ments lacking appendages (0) absent and (1) present

44 Length of postabdomen (0) one segment (1) twosegments (2) three segments and (3) five segments In sometaxa a variable number of posterior thoracic segments bearcomplete tubular tergites and lack appendages In Aglaspisautapomorphically it seems that the posterior three thoracicsegments lack appendages (Briggs et al 1979 Hesselbo 1992)but have unmodified tergites These situations are recognisedas potentially homologous by the coding used here Character44 is coded as inapplicable to taxa lacking a postabdomen

45 Posterior tergites strongly curved in dorsal aspect com-pared to anterior tergites (0) absent and (1) present Asrecognised by Wills et al (1998a Character 17) in some taxawhere the tergites are distinct the curvature of thoracic tergitesincreases posteriorly so that the posterior tergites are highlycurved to semicircular in dorsal aspect This situation isnot known from crustaceans (except isopods) or stem grouparthropods

46 Posterior segments reduced and with highly reducedappendages (0) present and (1) absent In some taxa there area large number of posterior segments that are short sagitallyand have appendages that are much reduced in size comparedto anterior trunk appendages These somites are incorporatedinto the pygidium in some taxa but primitively they arecovered by tiny free trunk tergites In the derived state thetrunk somites and limbs are of a relatively constant size Thedistinction between these states can be seen when attempting tocount the number of segments that make up the body In taxashowing State 0 the number of segments is very high anddifficult to count whereas in other taxa the number ofpost-cephalic segments is easily assessed

47 Pygidium (0) absent and (1) present A pygidium isrecognised here as a posterior tagma consisting of a number offused segments under a single tergite which may or may notincorporate the post-segmental telson This is different to theuse of this term by Edgecombe amp Ramskold (1999) whichfollows Ramskold et al (1997) and very different to its use byWills et al (1998a p 53) as a synapomorphy of the TrilobitaThe present authorsrsquo definition recognises the situation inRetifacies where the spinose postsegmental telson is autapo-morphically not fused to the pygidium as homologous toother multisegmented posterior tagma which include thetelson The distinction between the pygidium and an expandedpost-segmental telson can also be seen in the position of theanus which is consistently in the posteriormost pre-telsonicsegment amongst putative arachnomorphs (see Character 48)In taxa with a pygidium the anal segment is fused into thepygidium (see eg Kuamaia Edgecombe amp Ramskold 1999fig 6) whereas in those taxa lacking a pygidium the anus isbetween the final trunk tergite and the telson

Ramskold et al (1997) argued that the posteriormost tergiteof xandarellids (which incorporates the anal segment) is nothomologous to the posterior tagma recognised as pygidiaherein because posterior thoracic tergites also cover multiplesegments in xandarellids (see Character 33) Evidence ofsegmentation of the pygidium in a variety of taxa suggests thatthe number of tergites fused to form the pygidium is greaterthan the number of appendages For example the pygidium ofKuamaia has two pairs of lateral spines but at least fourpairs of appendages (Hou amp Bergstrom 1997 Edgecombe ampRamskold 1999) and that of Triarthrus has only five axialrings posterior to the articulating ring but over 10 pairs ofappendages (Whittington amp Almond 1987) The fact thatdecoupling of tergites and segments is evident in the thorax of

184 TREVOR J COTTON AND SIMON J BRADDY

Xandarella and Cindarella does not necessarily suggest that asimilar more widespread decoupling in pygidia is the result ofa different developmental process and hence that they arenon-homologous Instead the situation in the xandarellidthorax is potentially homologous to that found (more widely)in pygidia and consequently the terminal xandarellid tergite isa pygidium It is unclear if decoupling of tergites and somites isprimitively limited to the terminal tergite or primitively aproperty of the arachnomorph post-cephalon as a whole

It has been suggested that the tiny pygidium of olenelloidtrilobites which probably best reflects the primitive trilobitestate consists only of the post-segmental telson (Harrington inMoore 1959) and therefore is not a true pygidium HoweverWhittington (1989) showed that the olenelloid pygidium con-sists of at least two and possibly as many as five or sixsegments and therefore is likely to be homologous with thepygidium of other trilobites

48 Position of the anus (0) terminal within telson and (1)at base of telson In crustaceans and stem group arthropodsthe anus is terminal or otherwise situated in the telson (egRehbachiella Walossek 1993 fig 15D) In putative arachno-morphs the anus is either at the junction of the posteriormostthoracic segment and the telson or ventral within a fusedpygidium In the case of taxa with a pygidium the anus isanterior to the posterior margin and where known positionedbetween the posteriormost pair of appendages suggesting thatit is anterior to the post-segmental telson (see eg OlenoidesWhittington 1980)

49 Pygidium with median keel (0) absent and (1) presentEdgecombe amp Ramskold (1999 Character 21) considered thepresence of a median keel on the pygidium as a synapomorphyuniting Soomaspis and Tarricoia They did not explain howthis structure could be distinguished from the raised pygidialaxis of trilobites Homology of these structures is not sup-ported here because the axis of more primitive trilobites(including Eoredlichia) is quite distinct morphologically fromthe naraoiid keel Budd (1999a) noted the presence of a keel onthe pygidium of Buenaspis and suggested (Budd 1999a p 102)that it may be an artefact of dorsoventral compaction How-ever contrary to the coding of Edgecombe amp Ramskold(1999) a keel is visible on the pygidium of Liwia convexa (Dzikamp Lendzion 1988 fig 4CD) preserved in full relief Thereforeit seems unlikely that the keel is a taphonomic artefact

50 Pygidium with broad-based median spine (0) absentand (1) present

51 Pygidium with lateral spines (0) present and (1) absentEdgecombe amp Ramskold (1999 Character 22) coded thepresence of a median spine and two pairs of lateral spines aspotentially homologous in Sinoburius Kuamaia and HelmetiaThe distribution of lateral spines is much wider than that ofterminal spines and therefore they are coded separately hereIn trilobites it has widely been recognised that lateral spinesrepresent the original segmentation of the pygidium Thiscannot be the case for median spines Characters 49 to 51are not applicable to taxa lacking a pygidium (Character 47State 0)

52 Expanded post-segmental telson (0) absent and (1)present In a range of taxa the posterior-most tergite is a large(relative to the thoracic tergites) structure that lacks anyevidence of segmentation and is interpreted as representing anexpanded tergite of the post-segmental telson from whichsegments are released anteriorly during ontogeny On the otherhand the posteriormost tergite of Marrella and probablyMimetaster is small compared to the thoracic tergites androunded This element probably represents the plesiomorphiceuarthropod telson The homology of this character in mosttaxa is clear and only doubtful in Retifacies in which it may

be segmented The figures of Hou amp Bergstrom (1997) providelittle unequivocal evidence for a telson in Retifacies but thepresence of this structure is very clear in colour photographs(eg Chen et al 1996 fig 198A) However these figures donot convincingly demonstrate the segmentation of the telsonRetifacies is interpreted here as being unique in possessingboth a pygidium and an expanded post-segmental telson

53 Telson shape (0) spinose and (1) paddle-shaped Theshape of the expanded telsons identified above varies frombroadly spinose to flattened and paddle-like This differencecan be recognised both on the basis of the ratio of length tobasal width (spinose telsons are relatively long) and by thechange in width posteriorly (spinose telsons reduce in widthposteriorly whereas paddle-shaped telsons increase in width)In eurypterids a wide range of telson shapes is found includ-ing both character states described here It is likely that theplesiomorphic condition is a spinose telson This character isinapplicable to taxa lacking an expanded telson

54 Post-ventral furcae (0) absent and (1) present Thischaracter recognises the potential homology of unsegmentedpaired structures that articulate with the segment immediatelyanterior to the telson The potential homology of these struc-tures (the post-ventral plates) in Emeraldella and Aglaspis wasrecognised by Wills et al (1998a Character 70) and inEmeraldella and Sidneyia by Edgecombe amp Ramskold (1999Character 25) The homology of the segmented pygidial caudalfurcae of Olenoides (see Whittington 1975a 1980) with thesestructures is considered doubtful and coded as equivocal

Despite their distinctive morphology the long caudualfurcae of Cheloniellon may be homologous with the furcae ofAglaspis Emeraldella and Sidneyia This is supported by thecaudal furcae of the Ordovician cheloniellid Duslia which aremore similar in morphology to those of Emeraldella than ofCheloniellon Chlupac (1988 pp 614ndash16) considered thesestructures to be on the terminal tergite in Duslia However afaint tergite boundary is apparent posterior to the attachmentof the furcae (see Chlupac 1988 pl 57 fig 4) Therefore theyare considered here to have been attached to the pre-telsonicsegment as in Cheloniellon Chlupac (1988) and Dunlop ampSelden (1997) supported a close relationship between Dusliaand Cheloniellon

25 MethodsAll analyses were carried out using PAUP Version 40b4a(Swofford 1999) and unless otherwise stated used heuristicsearches with one thousand random addition sequencereplicates The software packages MacClade Version 307(Maddison amp Maddison 1997) and RadCon (Thorley amp Page2000) were used for comparing trees and investigating patternsof character evolution Tree length and other statistics (theConsistency Index CI and Retention Index RI) were calcu-lated by PAUP and MacClade with uninformative charactersexcluded Analyses were run separately using each of thedifferent codings for aglaspidids

Characters were treated as unordered and of equal weight inmost analyses To assess the influence of this assumption fourof the eight multistate characters which have states that areintermediate between others (Wilkinson 1992) were treated asordered in some analyses These characters are the number ofcephalic segments (Character 4) the length of raptorialappendage spines (Character 6) the degree of overlap of thethorax by the head-shield (Character 37) and the number ofsegments making up the postabdomen (Character 44) The lastof these is uninformative when not ordered because only onetaxon shows each of states 0 1 and 3

Support for individual nodes was assessed by bootstrapanalysis (Felsenstein 1985) and by calculating Bremer support

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 185

indices (Bremer 1988 1994) These methods measure twodistinct aspects of support for phylogenetic hypotheses Treesor nodes may be considered well supported (1) to the extent towhich alternative topologies are much less parsimonious asmeasured by the support index (Wilkinson 1996) or (2) wherethey are consistent with a large proportion of characters sothat character sampling is unlikely to have had much influenceon topology as assessed by bootstrapping (Page 1996) Boot-strapping was performed with 100 bootstrap replicates eachof 10 addition sequence replicates Bremer support indicesare based on heuristic searches with 100 addition sequencereplicates

Three pairs of terminals are included in Table 2 that apartfrom missing data are coded identically These are Helmetiaand Kuamaia Soomaspis and Tariccoia and Saperion andSkioldia These pairs are lsquotaxonomic equivalentsrsquo removingthe taxon with most missing data will not influence theposition of the other They can be recoded as single terminalsaccording to the principal of safe taxonomic reduction(Wilkinson 1995a b) and were not considered separately in theanalyses presented here Therefore cladistic analyses werecarried out on the basis of 33 terminals Edgecombe ampRamskold (1999) ignored this and coded the same three pairsof taxa and Eoredlichia and Olenoides identically for theirmore limited set of characters

26 ResultsAnalysis with all characters unordered and using the lsquoAglaspi-dida 1rsquo coding found nine most parsimonious trees (MPTs)each 126 steps long (CI=0middot556 RI=0middot763) Using the lsquoAglas-pidida 2rsquo coding resulted in 18 MPTs of length 125 (CI=0middot560RI=0middot765) Nine of these 18 trees were those found with thefirst coding of aglaspidids The strict component consensus(sensu Wilkinson 1994) of the 27 unique trees found acrossboth analyses is shown in Figure 8 All trees supported themonophyly of the Marrellomorpha (Marrella and Mimetaster)and of a clade including all other taxa except Crustacea (Clade1 of Fig 8) The relationships between these two clades and theCrustacea were unresolved equal numbers of trees in bothanalyses supported each of the topologies [Clade 1 (CrustaceaMarrellomorpha)] [Crustacea (Marrellomorpha Clade 1)]and [Marrellomorpha (Crustacea Clade 1)] All trees placedBurgessia as the sister taxon to the rest of Clade 1 andsupported the division of the rest of Clade 1 into two majorsister groups The topology of one of these clades (Clade 2)which included trilobites helmetiids tegopeltids naraoiidsxandarellids and Retifacies was stable with respect to thecoding of aglaspidids The other clade (Clade 3) consists of aclade of megacheirans and chelicerates (Clade 4) and a cladeincluding aglaspidids Lemoneites Paleomerus CheloniellonEmeraldella and Sidneyia (Clade 5) This latter clade is un-resolved in the strict consensus of all 27 trees (Fig 8) and thestrict consensus of the 18 MPTs found using the lsquoAglaspidida2rsquo coding but was fully resolved in the consensus of the ninetrees found with the lsquoAglaspidida 1rsquo coding Within themegacheiran-chelicerate clade all trees found Fortiforceps tobe the sister group to all other taxa Yohoia the sister group tochelicerates and Alalcomenaeus and Leanchoilia to form aclade Jianfengia is equally parsimoniously placed as the sistergroup to the Yohoia-chelicerate clade or as the sister group tothis clade and the Alalcomenaeus-Leanchoilia clade

Treating characters 4 6 37 and 44 as ordered had a minimaleffect on most parsimonious topology Nine MPTs of 136 steps(CI=0middot515 RI=0middot743) were found with the lsquoAglaspidida 1rsquocoding and 12 MPTs 135 steps in length (CI=0middot519 RI=0middot745)with the lsquoAglaspidida 2rsquo coding These trees all supported a(Crustacea Clade 1) clade excluding the marrellomorphs

Apart from this the strict consensus of these 21 trees isidentical to the consensus of the 27 trees found with allcharacters treated as unordered

Across all analyses four different topologies for Clade 5 (ofFig 8) were found (Fig 9) The distribution of these topologiesacross the four analyses described above and across the sameanalyses repeated after reweighting of each character in pro-portion to their Rescaled Consistency Index (see Farris 1989)from the first analysis is shown in Table 3 The instability of

Figure 8 Strict consensus of 27 MPTs from four separate analyses ofarachnomorph phylogeny with both interpretations of aglaspididmorphology and with some characters treated as ordered or allcharacters unordered Clades referred to in the text are numbered

Figure 9 Alternative equally parsimonious resolutions of clade 5 ofFig 8 (see Table 3)

186 TREVOR J COTTON AND SIMON J BRADDY

this clade was to some extent caused by the inclusion ofLemoneites and Paleomerus which could each be coded foronly 27 of the 54 characters used Analysis with these two taxaexcluded and characters treated as unordered found treescompatible only with Topology D of Figure 9 irrespective ofthe coding used for aglaspidids Analysis with some characterstreated as ordered and using either aglaspidid coding foundthree trees compatible only with Topology B of Figure 9 andthree compatible only with Topology D of Figure 9 ThereforeTopology D is the preferred topology for Clade 5 as shown inTable 3

One of the MPTs resulting from the lsquoAglaspidida 2rsquo coding(with all characters treated as unordered) was chosen as thebasis for further discussion because this tree combined thepreferred topology of Clade 5 with the (Crustacea Clade 1)group that was favoured when some characters were treated asordered Bootstrap percentages and Bremer support values forthis tree (using the Aglaspidida 2 coding) are shown in Figure10 Apomorphies for all ingroup nodes of this tree are shownin Figure 10A

3 Discussion

31 Phylogenetic resultsThe results of the cladistic analyses presented above arewell-resolved and robust with respect to different analyticalparameters Most of the taxa considered form a clade (Clade 1of Fig 8) that is more closely related to chelicerates than tocrustaceans in all analyses and therefore is recognised asthe Arachnomorpha as defined by Chen et al (1996) andRamskold et al (1996) The monophyly of the Marrellomor-pha was supported in all analyses The marrellomorphs werefound to be the sister group to a Crustacea+Clade 1 group inmost MPTs but in a minority of trees formed the sister groupto Clade 1 alone According to the latter result they should beincluded within the Arachnomorpha but according to the firstexcluded from it Both of these alternatives have been sup-ported in previous studies (eg Hou amp Bergstrom 1997 Willset al 1998a respectively) In addition to the marrellomorphsthe Fuxianhuiida Bousfield 1995 and Canadaspidida havebeen placed in the Euarthropoda outside the main mandibu-late and arachnomorph clades (Hou amp Bergstrom 1997)Further study of these taxa of the crustacean or mandibulatestem group and of the marrellomorphs is necessary before the

position of the Marrellomorpha can be resolved Provisionallymarrellomorphs are excluded from the Arachnomorphafollowing the majority of analyses above and Wills et al(1994)

The Burgess Shale arthropod Burgessia was placed as thesister group to remaining arachnomorphs in all analyses OtherArachnomorpha form two major clades one consisting of thelsquotrilobite-alliedrsquo arachnomorphs analysed by Edgecombe ampRamskold (1999) here termed lsquoTrilobitomorpharsquo (defined asTrilobita and all taxa more closely related to Trilobita thanChelicerata) and the other (lsquochelicerate-alliedrsquo) clade includingchelicerates lsquogreat appendagersquo arthropods aglaspididsLemoneites Paleomerus Sidneyia and Emeraldella The latteris here termed lsquoCheliceramorpharsquo (defined as Chelicerata andall taxa more closely related to Chelicerata than Trilobita) Thetopology of the lsquotrilobite-alliedrsquo clade was similar to that foundby Edgecombe amp Ramskold (1999) but the present authorsrsquoresults are more completely resolved unambiguously support-ing the sister-group relationship between trilobites andHelmetiida suggested in the previous study Within thelsquochelicerate-alliedrsquo clade Aglaspidids Paleomerus Lemoneitesand the Burgess Shale Emeraldella and Sidneyia form a cladein opposition to megacheirans and chelicerates

The present authorsrsquo analysis supports a new hypothesis ofthe origin of the chelicerates from within a paraphyleticassemblage of megacheiran arthropods and confirms pyc-nogonids as a sister group to the euchelicerates Previoushypotheses of the chelicerate sister group (see Section 1) aremuch less parsimonious The shortest trees supporting aCheloniellon-chelicerate clade are six steps longer than theMPTs and those supporting an aglaspidid-chelicerate cladeseven steps longer All megacheirans and chelicerates areunited by the flap-shaped rounded exopods and modificationof the second cephalic segment endopods into anteriorlydirected spinose raptorial organs which share a number ofdetailed similarities A clade including all of these taxa apartfrom Fortiforceps is supported by the synapomorphic loss ofthe antennae (or antennulae of crustaceans) and the longerlength of spinose projections of the second cephalic appendageendopods Within this clade Yohoia shares the loss of cephalicexopods the loss of thoracic endopods and a postabdomen oftubular tergites with chelicerates

The poor resolution of Clade 5 (of Fig 8) was partly a resultof the poor preservation of and consequent high proportion ofmissing data for Lemoneites and Paleomerus When these two

Table 3 Numbers of most parsimonious trees supporting each of the alternative topologies for clade 4 of Fig 7 under different analyticalconditions (RCI=Revealed Consistency Index)

Aglaspididcoding

Characters 4 6 36and 43 ordered

Charactersreweighted by RCI

Paleomerus andLemoneites excluded

Topology (see Fig 8)

A B C D

1 91 X 3 3 31 X 61 X X 31 X 61 X X 3 32 9 92 X 3 3 3 32 X 62 X X 32 X 62 X X 3 3Total 6 18 12 54

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 187

taxa were excluded from the analyses only two (B and D) ofthe four topologies found in the complete analysis remainedmost parsimonious (see Fig 9 and Table 3) Secondly Bremersupport values throughout much of the lsquochelicerate-alliedrsquoclade (Clade 3 of Fig 8) were improved when these taxa wereexcluded (see Fig 11)

Dunlop (1999 p 258) suggested that in reconstructing thechelicerate stem lineage lsquowe might predict that the two mostsignificant changes towards the chelicerate condition are thereduction of the antennae and the formation of the nextappendages into a clawrsquo According to the hypothesis pre-sented here both of these adaptations were achieved in EarlyCambrian chelicerate ancestors The recognition of the loss ofthe antennae in the stem group of the chelicerates and thephylogenetic hypothesis presented here suggest that chelicer-ates are most closely related to taxa with only a singleadditional segment fused to the crown group eucheliceratehead of four fused somites Previous hypotheses have derivedchelicerates from Palaeozoic arthropods with a longer head(eg Cheloniellon Sturmer amp Bergstrom 1978 SanctacarisBriggs amp Collins 1988 Emeraldella Bruton amp Whittington1983) The plesiomorphic pattern of head segmentation ismatched in pycnogonids Bergstrom et al (1980) suggestedthat only the anterior four pairs of appendages are incorpo-rated into the head This supports the view that pycnogonidsare primitive with respect to other chelicerates which showmore or less complete fusion of this primitive head and threethoracic segments including Solpugida and Pseudoscorpiones

The present study suggests that a shift from gnathobasicbenthic feeding to a more vagrant lifestyle and raptorialappendage feeding occurred very early in the chelicerate lin-eage Appendage feeding was clearly an important adaptation

Figure 10 One of 18 equally parsimonious cladograms with lsquoAglaspidida 2rsquo coding and all characters unorderedthat is most congruent with results from other analyses Non-terminal branch lengths scaled to reflect number ofapomorphies (A) ACCTRAN apomorphy scheme Character numbers shown above boxes and states shownbelow boxes as in Table 2 and the text Character consistency indices are indicated by shading (B) Levels ofsupport for individual nodes Bremer support indices shown in bold type above branches Bootstrap percentagesin italic type below branches for all nodes with relative frequencies greater than 50

Figure 11 Bremer support values for clades within the lsquochelicerate-alliedrsquo arachnomorphs (clade 3 of Fig 8) when all taxa wereincluded (normal weight font above nodes) and with Lemoneites andPaleomerus excluded (bold type below nodes)

188 TREVOR J COTTON AND SIMON J BRADDY

for the terrestrialization of chelicerates (Dunlop 1997 p 69Dunlop amp Webster 1999) but the model proposed heresuggests that this considerably pre-dated terrestrializationGnathobasic feeding in xiphosurans and eurypterids mayrepresent a reversal to a more primitve benthic arachnomorphlifestyle from a more pycnogonid-like ancestor

32 Evolution of the arachnomorph headThe extant arthropod classes show highly conserved patternsof head segmentation (eg Wills et al 1994) In contrast therehas been a near consensus amongst Cambrian arthropodworkers (although rarely based on an explicit phylogenetichypothesis) that patterns of head segmentation are highlyhomoplastic and hence of little systematic significanceSturmer amp Bergstrom (1978 pp 78ndash9) suggested that lsquoevenclosely related forms may have different numbers of headsegments and appendagesrsquo Bruton amp Whittington (1983pp 576ndash7) that lsquodiscussion on fossil arthropod relationshipsbased on head segmentation appears to be largely irrel-evant and at best speculativersquo and Delle Cave amp Simonetta(1991 p 191) that lsquothere are no obvious phyletic affinitiesbetween genera having the same number of cephalized seg-mentsrsquo Most recently Bergstrom amp Hou (1997 p 104) con-cluded that lsquothe segmental length of the head shield seemsto be of no relevance to the discussion and discrimination ofevolutionary lineagesrsquo It has been claimed that this view issupported by cladistic analyses in which patterns of tagmosishave been found to be rather poor at defining major arthropodclades (Briggs et al 1992 Wills et al 1994)

This suggestion is not only of relevance to systematics buthas featured prominently in recent debates surrounding thenature of the lsquoCambrian explosionrsquo (see Budd amp Jensen 2000for a recent review) Gould (1989 1991) argued that theapparent plasticity of head segmentation in Cambrian com-pared to post-Cambrian arthropod evolution suggests thatbody-plan evolution was only constrained after the Cambrianexplosion According to this view the origin of new highertaxa with distinct body-plans was only possible before theonset of these constraints (eg Valentine 1995) Taxa hererecognised as arachnomorphs clearly played a major role in thedevelopment of Gouldrsquos hypothesis This suggestion has pre-viously been assessed in terms of overall morphological diver-sity which is clearly rather distinct from Gouldrsquos concept ofdisparity Wills et al (1998b) showed that there was littlecorrelation between overall morphological disparity and de-gree of limb specialisation and that the latter was phylogeneti-cally highly plastic They remarked that (Wills et al 1998bp 64) lsquothis finding has important implications for models ofarthropod phylogeny and evolution that attribute overidingimportance to head segmentationrsquo but did not explicitlyexamine head segmentation although it is likely to have beena major component of the index of tagmosis employed

The results of the present study suggest that arguments thatarthropod head segmentation was unusually labile during theCambrian are poorly founded Rather only four major pat-terns of euarthropod head segmentation are identified Theplesiomorphic euarthropod state according to both Walossekamp Muller (1997 1998) and Scholtz (1997) which may moreproperly be the plesiomorphic state for the euarthropod crowngroup only (depending on the phylogenetic position of theMarrellomorpha) consists of four post-acronal segmentsbearing the antennae and three pairs of biramous limbs Thepresent authors suggest that the term lsquocephalonrsquo be restrictedto this kind of head which is found in stem group mandibu-lates in addition to many arachnomorphs Crown groupmandibulates share a lsquobimaxillary headrsquo in which an addi-tional pair of appendages the labium or second maxillae are

incorporated into the head but in crustaceans these are notfused to the carapace (Scholtz 1997) Finally two distinctforms of head tagmosis are found in chelicerates A head withfour pairs of appendages (but without antennae giving a totalof five segments incorporated into the head) which has beencalled the lsquocephalosomarsquo is present in pycnogonids (Walossekamp Dunlop 2002 pp 434ndash5) and probably also in some euch-elicerates (J A Dunlop pers comm 2003) The lsquoprosomarsquo ofmost euchelicerates consists of this cephalosoma and twoadditional segments

In general these patterns of head segmentation are highlyphylogenetically conserved (see Fig 12) Only three homo-plastic changes namely reversal to a more primitive conditionin the Leanchoilia-Alalcomenaeus clade and the convergentorigin of a five-segmented head in Clade 3 (of Fig 8) and theXandarellida are necessary to optimise character 4 onto themost parsimonious cladogram There are also transitions toautapomorphic states in Emeraldella and Sidneyia In the caseof Sidneyia however an argument could be made (followingBergstrom amp Sturmer 1978 see discussion of Character 4) thatthe head consists of five segments as in closely related taxabut autapomorphically each segment has a separate tergiteThis would further increase the phylogenetic stability of headsegmentation Also following Bergstrom amp Sturmer (1978) thehead of Cheloniellon could be considered to consist of twotergites making it more similar to the head of Emeraldella(with six pairs of appendages)

Figure 12 Cladogram from Fig 10 showing changes in head segmen-tation Bold type shows abbreviated head segmentation formulaAbbreviations (Ant) antennae (BA) biramous appendage pairs (Ch)chelicerae (GA) great appendages (UA) uniramous appendage pairsand (X) segment without appendages Synapomorphy A is the auta-pomorphic reduction in head length to a single segment in SidneyiaAlternatively this may represent secondary division of the headshield Synapomorphy B is the increase in the length of the head inEmeraldella to six segments

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 189

4 Acknowledgements

We thank D E G Briggs J A Cotton J A Dunlop R AFortey S Harris R A Moore P Selden O E Tetlie andP Van Roy for comments on earlier drafts of this work Themanuscript was reviewed by M Wills F Schram and oneanonymous referee This research was supported by the Geo-logical Society of London Curry Research Studentshipawarded to TJC by the Department of Earth Sciences Uni-versity of Bristol and by the Department of PalaeontologyThe Natural History Museum London TJC would like tothank C Labandeira J Thompson and R Chapman fortheir hospitality at the National Museum of Natural HistoryWashington DC USA and A Olcott S Schellenberg andJ Young for making his time there so enjoyable

5 References

Abzhanov A Popadic A amp Kaufman T C 1999 Chelicerate Hoxgenes and the homology of arthropod segments Evolution andDevelopment 1 77ndash89

Akam M 2000 Arthropods developmental diversity within a (super)phylum Proceedings of the National Academy of Sciences USA97 4438ndash41

Aldridge R J Gabbott S E amp Theron J N 2001 The Soom ShaleIn Briggs D E G amp Crowther P R (eds) Palaeobiology II340ndash2 Oxford Blackwell Science

Anderson L I amp Selden P A 1997 Opisthosomal fusion andphylogeny of Palaeozoic Xiphosura Lethaia 30 19ndash31

Arango C P 2002 Morphological phylogenetics of the sea spiders(Arthropoda Pycnogonida) Organisms Diversity and Evolution2 107ndash25

Ax P 1986 Das phylogenetische System Stuttgart Gustav FischerVerlag

Babcock L E amp Zhang Wentang In press New non-mineralizedarthropods from the Chengjiang Lagerstatte Lower CambrianYunnan China Palaeontologia Cathyana

Bartels C Briggs D E G amp Brassel G 1998 The fossils ofthe Hunsruck Slate Marine life in the Devonian CambridgeCambridge University Press

Bergstrom J 1971 Paleomerus ndash merostome or merostomoid Lethaia4 393ndash401

Bergstrom J 1979 Morphology of fossil arthropods as a guideto phylogenetic relationships In Gupta A P (ed) ArthropodPhylogeny 3ndash56 New York NY Van Nostrand Reinhold Co

Bergstrom J 1992 The oldest arthropods and the origin of theCrustacea Acta Zoologica 73 287ndash91

Bergstrom J Sturmer W amp Winter G 1980 PalaeoisopusPalaeopantopodus and Palaeothea pycnogonid arthropodsfrom the Lower Devonian Hunsruck Slate West GermanyPalaontologische Zeitschrift 54 7ndash54

Bergstrom J amp Brassel G 1984 Legs in the trilobite Rhenops fromthe Lower Devonian Hunsruck Slate Lethaia 17 67ndash72

Bergstrom J amp Hou X 1998 Chengjiang arthropods and theirbearing on early arthropod evolution In Edgecombe G D(ed) Arthropod fossils and phylogeny 151ndash84 New York NYColumbia University Press

Boudreaux H B 1979 Significance of intersegmental tendon systemin arthropod phylogeny and a monophyletic classification ofArthropoda In Gupta A P (ed) Arthropod Phylogeny 551ndash86New York NY Van Nostrand Reinhold Co

Bousfield E L 1995 A contribution to the natural classification ofLower and Middle Cambrian arthropods food-gathering andfeeding mechanisms Amphipacifica 2 3ndash33

Braddy S J Aldridge R J Gabbott S E amp Theron J N 1999Lamellate book-gills in a late Ordovician eurypterid from theSoom Shale South Africa support for a eurypterid-scorpionclade Lethaia 32 72ndash4

Braddy S J amp Almond J E 1999 Eurypterid trackways from theTable Mountain Group (Ordovician) of South Africa Journal ofAfrican Earth Sciences 29 165ndash77

Bremer K 1988 The limits of amino acid sequence data in angio-sperm phylogenetic reconstruction Evolution 42 795ndash803

Bremer K 1994 Branch support and tree stability Cladistics 10295ndash304

Briggs D E G 1998 Review of Arthropods from the LowerCambrian Chengjiang fauna southwest China by HouXianguang and Jan Bergstrom Fossils and Strata 45

Scandinavian University Press Oslo Norway 1997 Palaeo-geography Palaeoclimatology Palaeoecology 143 193ndash4

Briggs D E G Bruton D L amp Whittington H B 1979 Append-ages of the arthropod Aglaspis spinifer (Upper CambrianWisconsin) and their significance Palaeontology 22 167ndash80

Briggs D E G Fortey R A amp Wills M A 1992 Morphologicaldisparity in the Cambrian Science 256 1670ndash3

Briggs D E G Fortey R A amp Wills MA 1993 How big was theCambrian explosion A taxonomic and morphologic comparisonof Cambrian and Recent arthropods In Lees D R amp EdwardsD (eds) Evolutionary patterns and processes 33ndash44 LondonLinnean Society of London Linnean Society Symposium Series

Briggs D E G Erwin D H amp Collier F J 1994 The fossils of theBurgess Shale Washington DC Smithsonian Institution Press

Briggs D E G amp Bartels C 2001 New arthropods from the LowerDevonian Hunsruck Slate (Lower Emsian Rhenish Massifwestern Germany) Palaeontology 44 275ndash304

Briggs D E G amp Bartels C 2002 Magnoculocaris a new name forthe arthropod Magnoculus Briggs and Bartels 2001 from theLower Devonian Hunsruck Slate Germany Palaeontology 45419

Briggs D E G amp Collins D 1988 A Middle Cambrian cheliceratefrom Mount Stephen British Columbia Palaeontology 31 779ndash98

Briggs D E G amp Collins D 1999 The arthropod Alalcomenaeuscambricus Simonetta from the Middle Cambrian Burgess Shale ofBritish Columbia Paleontology 42 953ndash78

Briggs D E G amp Fortey R A 1982 The cuticle of aglaspididarthropods a red-herring in the early history of vertebratesLethaia 15 25ndash9

Briggs D E G amp Fortey R A 1989 The early radiation andrelationships of the major arthropod groups Science 246 241ndash3

Briggs D E G amp Fortey R A 1992 The early Cambrian radiationof arthropods In Lipps J H amp Signor P W (eds) Origin andEarly Evolution of the Metazoa 335ndash373 New York NY PlenumPress

Briggs D E G amp Robison R A 1984 Exceptionally preservednontrilobite arthropods and Anomalocaris from the MiddleCambrian of Utah University of Kansas PaleontologicalContributions Paper 111 1ndash23

Broili F 1932 Ein neuer Crustacee aus dem rheinischen UnterdevonSitzungsberichte der Mathematisch-Naturwissenschaftlichen(Abteilung) Klasse der Bayerischen Akademie der Wissenschaftenzu Munchen 1932 27ndash38

Broili F 1933 Ein zweites Exemplar von Cheloniellon Sitzungsber-ichte der Bayerischen Akademie der Wissenschaften 1933 11ndash32

Bruton D L 1981 The arthropod Sidneyia inexpectans MiddleCambrian Burgess Shale British Columbia Philosophical Trans-actions of the Royal Society of London Series B 295 619ndash56

Bruton D L amp Whittington H B 1983 Emeraldella and Leanchoiliatwo arthropods from the Burgess Shale British Columbia Philo-sophical Transactions of the Royal Society of London Series B 300553ndash85

Budd G E 1995 Kleptothule rasmusseni gen et sp nov anolenellid-like trilobite from the Sirius Passet fauna (BuenFormation Lower Cambrian North Greenland) Transactions ofthe Royal Society of Edinburgh Earth Sciences 86 1ndash12

Budd G E 1996a Progress and problems in arthropod phylogenyTrends in Ecology and Evolution 11 356ndash8

Budd G E 1996b The morphology of Opabinia regalis and thereconstruction of the arthropod stem-group Lethaia 29 1ndash14

Budd G E 1997 Stem-group arthropods from the Lower CambrianSirius Passet fauna of North Greenland In Fortey R A ampThomas R H (eds) Arthropod Relationships 125ndash38 LondonChapman and Hall Systematics Association Special Volume 55

Budd G E 1999a A nektaspid arthropod from the Early CambrianSirius Passet fauna with a description of retrodeformation basedon functional morphology Palaeontology 42 99ndash122

Budd G E 1999b The morphology and phylogenetic significance ofKerygmachela kiergkegaardi Budd (Buen Formation LowerCambrian N Greenland) Transactions of the Royal Society ofEdinburgh Earth Sciences 89 (for 1998) 249ndash290

Budd G E 2002 A palaeontological solution to the arthropod headproblem Nature 417 271ndash5

Budd G E amp Jensen S 2000 A critical reappraisal of the fossilrecord of the bilaterian phyla Biological Reviews 75 253ndash95

Chen J Zhou G Zhu M amp Yeh K 1996 The Chengjiang biota ndasha unique window of the Cambrian explosion Taichung NationalMuseum of Natural Science [In Chinese]

Chen J Edgecombe G D amp Ramskold L 1997 Morphological andecological disparity in naraoiids (Arthropoda) from the Early

190 TREVOR J COTTON AND SIMON J BRADDY

Cambrian Chengjiang fauna China Records of the AustralianMuseum 49 1ndash24

Chen J amp Zhou G 1997 Biology of the Chengjiang fauna In ChenJ Cheng Y amp Iten H V (eds) The Cambrian explosion and thefossil record Bulletin of the National Museum of Natural ScienceTaiwan 10 11ndash106

Child C A 1997 Some deep-sea Pycnogonida from Argentine slopeand basin Proceedings of the Biological Society of Washington110 128ndash42

Chlupac I 1988 The enigmatic arthropod Duslia from the Ordovicianof Czechslovakia Palaeontology 31 611ndash20

Chlupac I 1995 Lower Cambrian arthropods from the PasekyShale (Barrandian area Czech Republic) Journal of the CzechGeological Society 40 9ndash36

Chlupac I amp Havlıcek V 1965 Kodymirus n g a new aglaspidmerostome of the Cambrian of Bohemia Sbornık GeologickechVid Paleontologie 6 7ndash20

Cisne J L 1975 Anatomy of Triarthrus and the relationships of theTrilobita Fossils and Strata 4 45ndash63

Cisne J L 1981 Triarthrus eatoni (Trilobita) anatomy of its exo-skeletal skeletomuscular and digestive systems Palaeonto-graphica Americana 9 95ndash142

Conway Morris S 1982 Atlas of the Burgess Shale London ThePalaeontological Association

Conway Morris S 1989 The persistence of Burgess Shale-type faunasimplications for the evolution of deeper-water faunas Trans-actions of the Royal Society of Edinburgh Earth Sciences 80271ndash83

Conway Morris S 1998 The crucible of creation Oxford OxfordUniversity Press

Cotton T J 1999 Review of Arthropod fossils and phylogenyPalaeogeography Palaeoclimatology Palaeoecology 151 341ndash3

Cotton T J 2002 The phylogeny and morphological evolution ofCambrian trilobites and their relatives Unpublished PhD ThesisUniversity of Bristol

Damen W G M Hausdorf M Seyfarth E A amp Tautz D 1998 Aconserved mode of head segmentation in arthropods revealed bythe expression pattern of Hox genes in a spider Proceedings of theNational Academy of Sciences USA 95 10665ndash70

Delle Cave L amp Simonetta A M 1991 Early Palaeozoic arthropodsand problems of arthropod phylogeny with some notes on taxa ofdoubtful affinities In Simonetta A M amp Conway Morris S(eds) The early evolution of Metazoa and the significance ofproblematic taxa 189ndash244 Cambridge Cambridge UniversityPress

Dewel R A amp Dewel W C 1997 The place of tardigrades inarthropod evolution In Fortey R A amp Thomas R H (eds)Arthropod Relationships 109ndash23 London Chapman and HallSystematics Association Special Volume 55

Dunlop J A 1997 Palaeozoic arachnids and their significance forarachnid phylogeny Proceedings of the 16th European Colloquiumof Arachnology 65ndash82 Siedlce Poland

Dunlop J A 1998 The origins of tetrapulmonate booklungs and theirsignificance for chelicerate phylogeny In Selden P A (ed)Proceedings of the 17th European Colloquium of ArachnologyEdinburgh 1997 pp 9ndash16

Dunlop J A 1999 Pasando revista a la evolucion de los quelicerados[A review of chelicerate evolution] In Melic A DeHaro J JMedez M amp Ribera I (eds) Volumen Monografico Evolucion yFilogenia de Arthropoda Boletin de la Sociedad EntolomologicaAragonesa 26 255ndash272 [In Spanish and English]

Dunlop J A amp Selden P A 1997 The early history and phylogenyof the chelicerates In Fortey R A amp Thomas R H (eds)Arthropod relationships Systematics Association Special Volume55 221ndash35 London Chapman and Hall

Dunlop J A amp Webster M 1999 Fossil evidence terrestrializationand arachnid phylogeny Journal of Arachnology 27 86ndash93

Dzik J 1993 Early metazoan evolution and the meaning of its fossilrecord Evolutionary Biology 27 339ndash86

Dzik J amp Lendzion K 1988 The oldest arthropods of the EastEuropean Platform Lethaia 21 29ndash38

Edgecombe G D Wilson G D F Colgan D J Gray M R ampCassis G 2000 Arthropod cladistics combined analysisof Histone H3 and U2 snRNA sequences and morphologyCladistics 16 155ndash203

Edgecombe G D amp Ramskold L 1999 Relationships of CambrianArachnata and the systematic position of Trilobita Journal ofPaleontology 73 263ndash87

Emerson M J amp Schram F R 1997 Theories patterns and realitygame plan for arthropod phylogeny In Fortey R A amp Thomas

R H (eds) Arthropod Relationships Systematics AssociationSpecial Volume 55 67ndash87 London Chapman and Hall

Farris J S 1989 The Retention Index and the Rescaled ConsistencyIndex Cladistics 5 417ndash19

Felsenstein J 1985 Confidence limits on phylogenies an approachusing the bootstrap Evolution 39 783ndash91

Flower R H 1968 Merostomes from a Cotter Horizon of the El PasoGroup New Mexico Bureau of Mines and Mineral ResourcesMemoir 22 35ndash44

Fortey R A 1990a Trilobite evolution and systematics In MikulicD G (ed) Arthropod paleobiology 44ndash65 Short Courses inPaleontology 3 Knoxville Tenessee The Paleontological Society

Fortey R A 1990b Ontogeny hypostome attachment and trilobiteclassification Palaeontology 33 529ndash76

Fortey R A 1997 Classification In Kaesler R L (ed) Treatise oninvertebrate paleontology Part O Arthropoda 1 (revised) 289ndash302Boulder Colorado amp Lawrence Kansas Geological Society ofAmerica and University of Kansas Press

Fortey R A amp Theron J N 1994 A new Ordovician arthropodSoomaspis and the agnostid problem Palaeontology 37 841ndash61

Fortey R A amp Whittington H B 1989 The Trilobita as a naturalgroup Historical Biology 2 125ndash38

Fry W G 1978 A classification within the pycnogonids ZoologicalJournal of the Linnean Society of London 63 35ndash58

Giribet G Edgecombe G D amp Wheeler W C 2001 Arthropodphylogeny based on eight molecular loci and morphology Nature413 157ndash61

Gould S J 1989 Wonderful life The Burgess Shale and the nature ofhistory New York NY Norton

Gould S J 1991 The disparity of the Burgess Shale arthropod faunaand the limits of cladistic analysis why we must strive to quantifymorphospace Paleobiology 17 411ndash23

Gurich G 1931 Mimetaster hexagonalis ein neuer Kruster aus demunterdevonischen Bundenbacher Dachschiefer PalaontologischeZeitschrift 12 204ndash38

Hammann W Laske R amp Pillola G L 1990 Tariccoia arrusensisngnsp an unusual trilobite-like arthropod Rediscovery of thelsquophylocaridrsquo beds of Taricco (1922) in the Ordovician lsquoPuddingarsquosequence of Sardinia Bolletino della Societa PaleontologicaItaliana 29 163ndash78

Haas M S Brown S J amp Beeman R W 2001 Pondering theprocephalon the segmental origin of the labrum DevelopmentGenes and Evolution 211 89ndash95

Heider K 1913 Entwicklungsgeschichte und Morphologie derWirbellosen Die Kultur der Gegenwart Theil 3 Abt 4 2 176ndash332

Henriksen K 1928 Critical notes upon Cambrian arthropodsdescribed by Charles D Walcott Videnskabelige Meddelelser fraDansk Naturhistorisk Forening 86 1ndash20

Hesselbo S P 1988 Trace fossils of Cambrian aglaspidid arthropodsLethaia 21 139ndash46

Hesselbo S P 1989 The aglaspidid arthropod Beckwithia from theCambrian of Utah and Wisconsin Journal of Paleontology 63636ndash42

Hesselbo S P 1992 Aglaspidida (Arthropoda) from the UpperCambrian of Wisconsin Journal of Paleontology 66 885ndash923

Hou X 1987a Three new large arthropods from Lower CambrianChengjiang fauna eastern Yunnan Acta Palaeontologica Sinica26 272ndash85 [In Chinese with English summary]

Hou X 1987b Two new arthropods from Lower CambrianChengjiang eastern Yunnan Acta Palaeontologica Sinica 26236ndash56 [In Chinese with English summary]

Hou X Chen J amp Lu H 1989 Early Cambrian new arthropodsfrom Chengjiang Yunnan Acta Palaeontologica Sinica 28 42ndash57[In Chinese with English summary]

Hou X Ramskold L amp Bergstrom J 1991 Composition andpreservation of the Chengjiang fauna a Lower Cambrian soft-bodied biota Zoologica Scripta 20 395ndash411

Hou X Bergstrom J amp Ahlberg P 1995 Anomalocaris and otherlarge animals in the Lower Cambrian Chengjiang fauna ofsouthwest China GFF 117 163ndash83

Hou X amp Bergstrom J 1997 Arthropods of the Lower CambrianChengjiang fauna southwest China Fossils and Strata 45 1ndash116

Hughes C P 1975 Redescription of Burgessia bella from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 4415ndash36

Ivantsov A Y 1999 Trilobite-like arthropod from the LowerCambrian of the Siberian Platform Acta PalaeontologicaPolonica 44 455ndash66

King P E 1973 Pycnogonids London St Martin Press

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 191

Lauterbach K E 1973 Schlussereignisse in der Evolution derStammgruppe der Euarthropoda Zoologische Beitrage 19259ndash99

Lauterbach K E 1980 Schlussereignisse in der Evolution derGrundplans der Arachnata (Arthropoda) Abhandlungen desNaturwissenschaftlichen Vereins in Hamburg 23 163ndash327

Lauterbach K E 1983 Synapomorphien zwischen Trilobiten- undCheliceraten-zweig der Arachnata Zoologischer Anzeiger 210213ndash38

Lendzion K 1975 Fauna of the Mobergella Zone in the Polish LowerCambrian Kwartalnik Geologiczny 19 237ndash42

Lieberman B S 1998 Cladistic analysis of the Early Cambrianolenelloid trilobites Journal of Paleontology 72 59ndash78

Lu Y 1940 On the ontogeny and phylogeny of Redlichia intermediaLu (sp nov) Bulletin of the Geological Society of China 20333ndash42 [In Chinese]

Maddison W P amp Maddison D R 1997 MacClade analysis ofphylogeny and character evolution Version 307 Sunderland MASinauer Associates

Mittmann B amp Scholtz G 2003 Development of the nervous systemin the lsquoheadrsquo of Limulus polyphemus (Chelicerata Xiphosura)morphological evidence for the correspondence between the seg-ments of the chelicerae and the (first) antennae of MandibulataDevelopment Genes and Evolution 213 9ndash17

Moore R C (ed) 1959 Treatise on invertebrate paleontology Part OArthropoda 1 Boulder Colorado Geological Society of America

Muller K J amp Walossek D 1986 Arthropod larvae from the UpperCambrian of Sweden Transactions of the Royal Society ofEdinburgh Earth Sciences 77 157ndash79

Muller K J amp Walossek D 1987 Morphology ontogeny and lifehabit of Agnostus pisiformis from the Upper Cambrian of SwedenFossils and Strata 19 1ndash124

Munilla T 1999 Evolucion y filogenia de los Picnogonidos InMelicA DeHaro J J Medez M amp Ribera I (eds) VolumenMonografico Evolucion y Filogenia de Arthropoda Boletin de laSociedad Entolomologica Aragonesa 26 273ndash91 [In Spanish andEnglish]

Orr P J Siveter D J Briggs D E G Siveter D J amp Sutton M D2000 A new arthropod from the Silurian Konservat-Lagerstatteof Herefordshire Proceedings of the Royal Society of LondonSeries B 267 1497ndash504

Page R D M 1996 On consensus confidence and lsquototal evidencersquoCladistics 12 83ndash92

Palmer A R amp Repina L N 1993 Through a glass darklytaxonomy phylogeny and biostratigraphy of the OlenellinaUniversity of Kansas Paleontological Contributions New Series 31ndash35

Platnick N I Griswold C E amp Coddington J A 1991 On missingentries in cladistic analysis Cladistics 7 337ndash43

Pleijel F 1995 On character coding for phylogeny reconstructionCladistics 11 309ndash15

Raasch G O 1939 Cambrian Merostomata Special Papers of theGeological Society of America 19 1ndash146

Ramskold L Chen J Edgecombe G D amp Zhou G 1996Preservational folds simulating tergite junctions in tegopeltid andnaraoiid arthropods Lethaia 29 15ndash20

Ramskold L Chen J Edgecombe G D amp Zhou G 1997Cindarella and the arachnate clade Xandarellida (ArthropodaEarly Cambrian) from China Transactions of the Royal Society ofEdinburgh Earth Sciences 88 19ndash38

Ramskold L amp Edgecombe G D 1991 Trilobite monophylyrevisited Historical Biology 4 267ndash83

Ramskold L amp Edgecombe G D 1996 Trilobite appendagestructure ndash Eoredlichia revisited Alcheringa 20 269ndash76

Raw F 1957 Origin of chelicerates Journal of Paleontology 31139ndash193

Repina L N amp Okuneva O G 1969 Cambrian arthropods of themaritime territory Paleontological Journal 1969 95ndash103

Richter R amp Richter E 1929 Weinbergina opitz ng n sp einSchwerttrager (Merost Xiphos) aus dem Devon (Rheinland)Senkenbergiana 11 193ndash209

Robison R A 1984 New occurences of the unusual trilobite Naraoiafrom the Cambrian of Idaho and Utah University of KansasPaleontological Contributions Paper 112 1ndash8

Rominger C 1887 Description of primordial fossils from MtStephens NW Territory of Canada Proceedings of the Academyof Natural Sciences of Philadelphia 1887 12ndash19

Scholtz G 1997 Cleavage germ band formation and head segmen-tation the ground pattern of the Euarthropoda In Fortey R Aamp Thomas R H (eds) Arthropod Relationships 317ndash32 LondonChapman and Hall Systematics Association Special Volume 55

Schram F R 1978 Arthropods a convergent phenomenonFieldiana Geology 39 61ndash108

Schram F R 1993 Review of Boxshall Stromberg and Dahl TheCrustacea Origin and Evolution Journal of Crustacean Biology13 820ndash2

Schram F R Vonk R amp Hof C H J 1997 Mazon CreekCycloidea Journal of Paleontology 71 261ndash84

Schram F R amp Emerson M J 1991 Arthropod Pattern Theorya new approach to arthropod phylogeny Memoirs of theQueensland Museum 31 1ndash18

Schram F R amp Hof C H J 1998 Fossils and the interrelationshipsof major crustacean groups In Edgecombe G D (ed) Arthropodfossils and phylogeny pp 233ndash302 New York NY ColumbiaUniversity Press

Shergold J H 1991 Protaspis and early meraspis growth stages of theeodiscoid trilobite Pagetia ocellata Jell and their implications forclassification Alcheringa 15 65ndash86

Shu D Geyer G Ling C amp Zhang Z 1995 Redlichiacean trilobiteswith preserved soft parts from the Lower Cambrian Chengjiangfauna (South China) Beringeria (Special Issue) 2 203ndash41

Shultz J W 1990 Evolutionary morphology and phylogeny ofArachnida Cladistics 6 1ndash38

Shultz J W 2000 Skeletomuscular anatomy of the harvestmanLeiobunum aldrichi (Weed 1893) (Arachnida OpilionesPalpatores) and its evolutionary significance Zoological Journalof the Linnean Society 128 401ndash38

Shultz J W 2001 Gross muscular anatomy of Limulus polyphemus(Xiphosura Chelicerata) and its bearing on evolution in the theArachnida Journal of Arachnology 29 283ndash303

Siewing R (ed) 1985 Lehrbuch der Zoologie II Systematik 3rd ednStuttgart Fischer

Simonetta A M 1970 Studies on the non trilobite arthropods ofthe Burgess Shale (Middle Cambrian) the genera LeanchoiliaAlalcomenaeus Opabinia Burgessia Yohoia and ActaeusPalaeontographica Italica 66 35ndash45

Simonetta A M amp Delle Cave L 1975 The Cambrian non trilobitearthropods from the Burgess Shale of British Columbia A studyof their comparative morphology taxonomy and evolutionarysignificance Palaeontographica Italica 69 1ndash37

Simonetta A M amp Delle Cave L 1981 An essay in the compara-tive and evolutionary morphology of Palaeozoic ArthropodsAccademia Nazionale dei Lincei Atti dei Convegni Lincei 49389ndash439

Starobogatov Y I 1990 Sistema i filogeniya nizshikh khelitserovykh(analiz morfologii paleozoysikh grupp) [The systematics andphylogeny of the lower chelicerates (a morphological analysis ofthe Palaeozoic groups)] Paleontologicheskii Zhurnal 1990 4ndash17

Stoslashrmer L 1944 On the relationships and phylogeny of fossil andrecent Arachnomorpha Skrifter utgitt ac Det Norske Videnskaps-Akademi i Olso I Matematisk-naturvitenskaplig Klasse 5 1ndash158

Stoslashrmer L 1951 Studies on trilobite morphology Part III Theventral cephalic structures with remarks on the zoologicalposition of the trilobites Norsk geologisk tidsskrift 29 108ndash58

Stoslashrmer L 1956 A Lower Cambrian merostome from Sweden Arkivfor Zoologi Ser 2 9 507ndash14

Stoslashrmer L 1959 Trilobitoidea In Moore R C (ed) Treatise onInvertebrate Paleontology part O Arthropoda 1 23ndash37 BoulderColorado Geological Society of America

Strong E E amp Lipscomb D 1999 Character coding and inapplicabledata Cladistics 15 363ndash71

Sturmer W amp Bergstrom J 1976 The arthropods Mimetaster andVachonisia from the Devonian Hunsruck Shale PalaontologischeZeitschrift 50 78ndash111

Sturmer W amp Bergstrom J 1978 The arthropod Cheloniellon fromthe Devonian Hunsruck Shale Palaontologische Zeitschrift 5257ndash81

Sturmer W amp Bergstrom J 1981 Weinbergina a xiphosuran fromthe Devonian Hunsruck Slate Palaontologische Zeitschrift 55237ndash55

Sutton M D Briggs D E G Siveter D J Siveter D J amp Orr PJ 2002 The arthropod Offacolus kingi (Chelicerata) from theSilurian of Herefordshire England computer based morphologi-cal reconstructions and phylogenetic affinity Proceedings of theRoyal Society of London Series B 269 1195ndash1203

Swofford D L 1999 PAUP Phylogenetic Analysis Using Parsimony(and other methods) Version 40b2a Sunderland MA SinauerAssociates

Telford M J amp Thomas R H 1998 Expression of homeobox genesshows chelicerate arthropods retain their deutocerebral segmentProceedings of the National Academy of Sciences USA 9510671ndash5

192 TREVOR J COTTON AND SIMON J BRADDY

Thorley J L amp Page R D M 2000 RadCon phylogenetic treecomparison and consensus Bioinformatics 16 486ndash7

Valentine J W 1995 Why no new phyla after the Cambrian ndashgenome and ecospace hypotheses revisited Palaios 10 190ndash194

Wagele J W 1993 Rejection of the lsquoUniramiarsquo hypothesis andimplications of the Mandibulata concept Zoologische JahrbucherAbteilung fur Systematik Geographie und Biologie der Tiere 120253ndash88

Walcott C D 1911 Middle Cambrian Merostomata Cambriangeology and palaeontology II Smithsonian MiscellaneousCollections 57 17ndash40

Walcott C D 1912 Middle Cambrian Branchiopoda MalacostracaTrilobita and Merostomata Cambrian geology and palaeontol-ogy II Smithsonian Miscellaneous Collections 57 145ndash228

Walcott C D 1918 Geological explorations in the CanadianRockies Explorations and fieldwork of the SmithsonianInstitution in 1917 Smithsonian Miscellaneous Collections 684ndash20

Walossek D 1993 The Upper Cambrian Rehbachiella kinnekullensisand the phylogeny of Branchiopoda and Crustacea Fossils andStrata 32 1ndash202

Walossek D amp Dunlop J A 2002 A larval sea spider (ArthropodaPycnogonida) from the Upper Cambrian lsquoOrstenrsquo of Swedenand the phylogenetic position of pycnogonids Palaeontology 45421ndash46

Walossek D amp Muller K J 1990 Upper Cambrian stem-lineagecrustaceans and their bearing upon the monophyletic origin ofCrustacea and the position of Agnostus Lethaia 23 409ndash27

Walossek D amp Muller K J 1997 Cambrian lsquoOrstenrsquo-type arthro-pods and the phylogeny of Crustacea In Fortey R A amp ThomasR H (eds) Arthropod Relationships 139ndash53 SystematicsAssociation Special Volume 55 London Chapman and Hall

Walossek D amp Muller K J 1998 Early arthropod phylogeny in lightof the Cambrian lsquoOrstenrsquo fossils In Edgecombe G D (ed)Arthropod fossils and phylogeny 185ndash231 New York NYColumbia University Press

Weygoldt P 1979 Significance of later embryonic stages and headdevelopment in arthropod phylogeny In Gupta A P (ed)Arthropod Phylogeny 107ndash35 New York NY Van NostrandReinhold Co

Weygoldt P 1998 Evolution and systematics of the ChelicerataExperimental and Applied Acarology 22 63ndash79

Weygoldt P amp Paulus H F 1979 Unterzuchungen aur Morpholo-gie Taxonomie und Phylogenie der Chelicerata Zeitschriftfur Zoologische Systematik und Evolutionsforschung 17 85ndash116177ndash200

Wheeler W C Cartwright P amp Hayashi C Y 1993 Arthropodphylogeny a combined approach Cladistics 9 1ndash39

Wheeler W C amp Hayashi C Y 1998 The phylogeny of extantchelicerate orders Cladistics 14 173ndash92

Whittington H B 1971 Redescription of Marrella splendens(Trilobitoidea) from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 209 1ndash24

Whittington H B 1974 Yohoia Walcott and Plenocaris n genarthropods from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 231 1ndash21

Whittington H B 1975a Trilobites with appendages from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 497ndash136

Whittington H B 1975b The enigmatic animal Opabinia regalisMiddle Cambrian Burgess Shale British Columbia PhilosophicalTransactions of the Royal Society of London Series B 271 1ndash43

Whittington H B 1977 The Middle Cambrian trilobite NaraoiaBurgess Shale British Columbia Philosophical Transactions of theRoyal Society Series B 280 409ndash43

Whittington H B 1979 Early arthropods their appendages andrelationships In House M R (ed) The origin of majorinvertebrate groups 253ndash68 Systematics Association SpecialVolume 12 New York NY Academic Press

Whittington H B 1980 Exoskeleton moult stage appendage mor-phology and habits of the Middle Cambrian trilobite Olenoidesserratus Palaeontology 23 171ndash204

Whittington H B 1981 Rare arthropods from the Burgess ShaleMiddle Cambrian British Columbia Philosophical Transactionsof the Royal Society of London Series B 292 329ndash57

Whittington H B 1985 Tegopelte gigas a second soft-bodied trilo-bite from the Burgess Shale Middle Cambrian British ColumbiaJournal of Palaeontology 23 171ndash204

Whittington H B 1989 Olenelloid trilobites type species functionalmorphology and higher classification Philosophical Transactionsof the Royal Society of London Series B 324 111ndash47

Whittington H B amp Almond J E 1987 Appendages and habits ofthe Upper Ordovician trilobite Triarthus eatoni PhilosophicalTransactions of the Royal Society of London Series B 317 1ndash46

Wilkinson M 1992 Ordered versus unordered characters Cladistics8 375ndash85

Wilkinson M 1994 Common cladistic information and its consensusrepresentation reduced Adams and reduced cladistic consensustrees and profiles Systematic Biology 43 343ndash68

Wilkinson M 1995a Coping with abundant missing entries in phylo-genetic inference using parsimony Systematic Biology 44 501ndash14

Wilkinson M 1995b TAXEQ2 software and documentation BristolSchool of Biological Sciences University of Bristol

Wilkinson M 1996 On the distribution of homoplasy and choosingamong trees Taxon 45 263ndash66

Wills M A 1997 A phylogeny of Recent and fossil Crustacea derivedfrom morphological characters In Fortey R A amp ThomasR H (eds) Arthropod Relationships 189ndash209 SystematicsAssociation Special Volume 55 London Chapman and Hall

Wills M A Briggs D E G amp Fortey R A 1994 Disparity asan evolutionary index a comparison of Cambrian and Recentarthropods Paleobiology 20 93ndash130

Wills M A Briggs D E G Fortey R A amp Wilkinson M 1995The significance of fossils in understanding arthropod evolutionVerhandlungen der deutschen zoologischen Gesselschaft 88 203ndash15

Wills M A Briggs D E G Fortey R A Wilkinson M amp SneathP H A 1998a An arthropod phylogeny based on fossil andrecent taxa In Edgecombe G D (ed) Arthropod fossils andphylogeny 33ndash105 New York NY Columbia University Press

Wills M A Briggs D E G amp Fortey R A 1998b Evolutionarycorrelates of arthropod tagmosis scrambled legs In ForteyR A amp Thomas R H (eds) Arthropod Relationships 57ndash65Systematics Association Special Volume 55 London Chapmanand Hall

Winter G 1980 Beitrage zur Morphologie und Embryologie desvorderen Korperabschnitts (Cephalosoma) der PantopodaGerstaecker 1863 I Entstehung und Struktur des Zentralnerven-systems Zeitschrift fur Zoologische Systematik und Evolutionsfor-schung 18 27ndash61

Zhang W amp Hou X 1985 Preliminary notes on the occurrence of theunusual trilobite Naraoia in Asia Acta Palaeontologica Sinica 24591ndash5 [In Chinese with English summary]

TREVOR J COTTON Department of Palaeontology The Natural History Museum CromwellRoad London SW7 5BD UKe-mail trevor_cottonhotmailcom

SIMON J BRADDY Department of Earth Sciences University of Bristol Wills MemorialBuilding Queenrsquos Road Bristol BS8 1RJ UKe-mail S J Braddybrisacuk

MS received 28 July 2003 Accepted for publication 15 March 2004

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 193

Page 6: The phylogeny of arachnomorph arthropods and the origin of the

Marrellomorpha but there is considerable evidence that theseare rather derived crustaceans (see Schram et al 1997) andthey are not considered further The Burgess Shale taxonBurgessia has also been placed in the Marrellomorpha (Hou ampBergstrom 1997) but was not found to belong to this clade inmost analyses Stoslashrmer (1944) placed Burgessia in a compa-rable systematic position to Marrella he included both in theArachnomorpha but excluded them from the MerostomoideaMarrella Mimetaster and Burgessia were included in thepresent study to assess whether the marellomorphs should beincluded in the Arachnomorpha and test the affinities ofBurgessia

The Cambrian lsquogreat appendagersquo arthropods (theMegacheira of Hou amp Bergstrom 1997) have been nestedwithin the Arachnomorpha in most cladistic studies (egBriggs amp Fortey 1989 Wills et al 1995 1998a Emerson ampSchram 1997) Briggs amp Fortey (1989 1992) recognised mega-cheiran taxa as particularly closely related to chelicerates(Fig 2A) while other authors (Bergstrom 1992 Hou ampBergstrom 1997 Budd 2002) have considered megacheirans tobe primitive euarthropods or ancestral to some (but not all)crustaceans (Delle Cave amp Simonetta 1991 and referencestherein) Since the monophyly of the megacheirans hasnot usually been supported the present authors haveincluded nearly all described taxa These include LeanchoiliaAlalcomenaeus and Yohoia from the Burgess Shale andFortiforceps and Jianfengia from the Chengjiang faunaExcluded from this study are Actaeus Simonetta 1970 fromthe Burgess Shale which may be a synonym of Alalcomenaeus(Briggs amp Collins 1999) Alalcomenaeus illecebrosus (Hou1987b see Hou amp Bergstrom 1997) from the ChengjiangFauna which is probably a chimaera (Briggs amp Collins 1999)and the poorly preserved Leanchoilia hanceyi from the MiddleCambrian of Utah (Briggs amp Robison 1984)

According to the definition given above any assessment ofthe limits of the Arachnomorpha needs to consider the phylo-genetic position of these taxa relative to crustaceans To thisend a generalised crustacean intended to represent the plesio-morphic crustacean condition was included as an ingrouptaxon There has been considerable disagreement over themost basal crustacean group (see Wills 1997 pp 194ndash5Schram amp Hof 1998 pp 245ndash8) The coding used here largelyfollows Walossek amp Mullerrsquos (1990 1997 1998) andWalossekrsquos (1993) concept of the crustacean stem group but isintended to be conservative so that coding on the basis of othertheories of crustacean origins would be similar

22 AglaspididaThe morphology of the appendages of aglaspidids is poorlyknown having been described from three species of bodyfossil Aglaspis spinifer Raasch 1939 Flobertia kochi Hesselbo1992 and Khankaspis bazhanovi Repina amp Okuneva 1969 andtrace fossil evidence (Hesselbo 1988) Of these the appendagesof Aglaspis described by Raasch (1939) Briggs et al (1979)and Hesselbo (1992) are by far the best known The append-ages of Flobertia (described by Raasch 1939 as Aglaspisbarrandei and Hesselbo 1992) agree with those of AglaspisHowever the appendages of Khankaspis show a differentmorphology but have only been poorly illustrated anddescribed (Repina amp Okuneva 1969) This material suggeststhe presence of lobate exopods with lamellate setae This taxonis probably correctly assigned to aglaspidids (cf Whittington1979 p 258) on the basis the central position of the dorsaleyes and presence of genal spines although Hou amp Bergstrom(1997 p 96) suggested that its broad tail spine may indicatethat it is a strabopid

It remains unclear whether lamellate exopods are absent insome aglaspidids (Aglaspis) and present in others (Khankaspis)or whether the apparent differences in appendage morphologybetween these taxa are a result of preservational bias alone Apossible preservational analogue is provided by someChengjiang taxa in which the exopods are very poorly pre-served and the endopods of the cephalic appendages arepreserved as impressions in the dorsal head shield in a similarmanner to those of Aglaspis This is presumably because theendopods were more convex and more heavily sclerotised thanthe exopods This mode of appendage preservation is mostclearly seen in Misszhouia longicaudata (see Chen et al 1997fig 2andashd) but is also found in Sinoburius (see Hou ampBergstrom 1997) and possible protaspides of Naraoia (Houet al 1991)

Here a generalised aglaspidid is coded in two different waysto accommodate this uncertainty In both codings they areconsidered to have possessed a pair of antenniform append-ages followed by 11 pairs of pediform endopods on the headand first eight thoracic tergites (Briggs et al 1979 Hesselbo1988 1992) According to one interpretation (coded asAglaspidida 1) all the appendages are uniramous the exopodis lost throughout According to the other (Aglaspidida 2)exopods consisting of a single lobe fringed with lamellate setae(Repina amp Okuneva 1969 pl 15 figs 1 3 amp 4) are presenton at least some appendages but their distribution andattachment are considered unknown

23 Outgroup rootingA hypothetical plesiomorphic euarthropod was included toallow outgroup rooting Many of the characters consideredcould be coded unambiguously on the basis of recent discus-sions of the arthropod stem group (Budd 1996b 1997 1999b)

Figure 3 Majority rule consensus of lsquotrilobite-alliedrsquo arachnate phy-logeny after Edgecombe amp Ramskold (1999 fig 3)

174 TREVOR J COTTON AND SIMON J BRADDY

The ancestral euarthropod is here considered to have possesseda head with antennae only and a series of post-cephalicbiramous limbs with gnathobases The hypothesised pattern ofthe evolution of plesiomorphic euarthropod characters isshown in Figure 4 The use of a hypothetical outgroup issomewhat unsatisfactory but only affects the relationshipbetween the major clades (Arachnomorpha Crustacea andMarrellomorpha) the topology within the arachnomorphs wasunaffected by the use of this outgroup since identical resultswere obtained with unrooted analyses or by rooting withCrustacea Marrella or both

24 Characters and codingMost previous cladistic analyses with the notable exception ofEdgecombe amp Ramskold (1999) have included only briefdiscussions of the primary hypotheses of homology involved incharacter construction and coding Therefore a full discussionof most characters is provided here Descriptions of charactersand character states below are arranged by organ system orbody region in approximate anterior to posterior order Thedistribution of character states across all taxa is shown in thedata matrix (Table 2)

The terminology of morphological features in arachnomor-phs is often confused Terminology developed for cheliceratestrilobites and crustaceans has variously been applied to taxaincluded in this study so some attempt is made to clarifyterminology herein Where appropriate the present authorsrsquoterminology follows Edgecombe et al (2000) and Wheeleret al (1993) but following Scholtz (1997) the protocerebrallsquosegmentrsquo is considered to be acronal and consequently thedeutocerebral segment to be the first true segment Thereforethe antennae (or antennulae of crustaceans) belong to the firstcephalic segment

The present authors have attempted to include as completea set of characters as possible However characters requiringhypotheses of homology between podomeres (eg the number

of podomeres in endopods of thoracic appendages Willset al 1998a character 51) were not considered followingEdgecombe et al (2000 p 157) Secondly some characters usedby Bergstrom (eg Hou amp Bergstrom 1997 p 109) suchas appendage posture and mode of feeding were excludedfollowing Edgecombe amp Ramskold (1999) Finally somecharacters of the ventral surface of the head were not coded asdiscussed below and illustrated in Figure 7 The present authorsdo not include all potential synapomorphies for the Cheli-cerata as the status of many of these is debated (see eg Shultz1990 2001 Dunlop amp Selden 1997 Weygoldt 1998 Wheeler ampHayashi 1998 Dunlop 1999 Edgecombe et al 2000)

Two methods for the coding of inapplicable characters inphylogenetic analysis have been used First inapplicable char-acter states can be coded as missing data A complex structuremay comprise characters lsquoabsentpresentrsquo and lsquostate1state2rsquowith taxa lacking the structure coded as absent for the firstcharacter and as missing data for the second Some authorshave regarded this method as problematic because it may leadto reconstruction of impossible ancestral states and henceunjustified trees (Platnick et al 1991 Strong amp Lipscomb1999) The alternative is to code the second character as a thirdlsquonot applicablersquo state in taxa that lack the structure Thesemethods are equivalent to Pleijelrsquos (1995) coding methods Cand B respectively In the present study inapplicable charac-ters are treated as missing data for most analyses becausecoding them as a distinct character state reduces characterindependence and effectively weights the inapplicable charac-ter and hence could result in them dominating the analysisThe effects of this assumption were investigated by using adistinct character state in some analyses (see Section 26) andinapplicable characters are shown as distinct to lsquotruersquo missingdata (using the symbol lsquo-rsquo) in the matrix (Table 2)

241 Anterior cephalic appendages and head segmentation1 Appendages of the first segment (antennae) (0) present and(1) absent The majority of taxa considered in the present study

Figure 4 Reconstruction of the euarthropod stem group used to inform coding of hypothetical outgroup Grosstopology largely follows Budd (1996b fig 9 1997 fig 1110) Solid boxes indicate unambiguous apomorphiesand shaded boxes character states which are plesiomorphic for the Arthropoda

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 175

possess a single pair of long uniramous multi- and annulatedantennae at the anterior of the head which presumably had asensory function as found in Crustacea Myriapoda and mostmembers of Hexapoda (ie proturans do not have antennae)These appendages are likely to be a synapomorphy of theEuarthropoda (eg Scholtz 1997 Walossek amp Muller 1997)and therefore the outgroup is coded as State 0 The anterior-most head appendages of most Cambrian megacheiran arthro-pods which have been called lsquogreat appendagesrsquo followingWalcott (1912) consist of a small number of robust spinosepodomeres Uniquely Fortiforceps has a pair of short anten-nae anterior to the lsquogreat appendagesrsquo (Hou amp Bergstrom 1997pp 34ndash8) Therefore it appears likely that the lsquogreat append-agesrsquo are the appendages of the second cephalic segment andthe antennae are lost in megacheirans other than FortiforcepsThis of course depends on recognising the lsquogreat appendagesrsquoas homologous in all of these taxa as discussed below(Character 2) Homology of the megacheiran anterior append-ages with the second cephalic appendages of trilobites waspreviously suggested by Stoslashrmer (1944 p 124) on the basis oftheir post-oral position

The classical view of chelicerate head segmentation main-tains that they too have lost the antennae and the cheliceraeare the appendages of the second cephalic segment Recentrevisions of arthropod phylogeny have uncritically acceptedthis homology (Edgecombe et al 2000 Giribet et al 2001)which is based largely on neuroanatomy The lsquochelicero-neuromerrsquo which innervates the chelicerae seems to be ho-mologous with the tritocerebrum associated with the secondcephalic appendage pair of mandibulates (see Weygoldt 1979Winter 1980) Although some uncertainty exists over theposition of pycnogonids within Arthropoda and the homol-ogy of their cephalic segmentation is poorly understood thisview is also supported by the presence of pre-cheliceralappendages in a pycnogonid larva (Larva D of Muller ampWalossek 1986) from the Upper Cambrian of Sweden(Walossek amp Dunlop 2002) Cheloniellon also has a pair ofpre-oral pediform appendages considered homologous to thechelicerae in addition to its antennae Thus despite recentneontological studies that indicate that the chelicerae arehomologous to the antennae of mandibulates based on pat-terns of Hox gene expression (Damen et al 1998 Telford amp

Table 2 Data matrix for cladistic analyses of arachnomorphs () missing data and (ndash) inapplicable characters Letters indicate multistateuncertainty codings as follows (A) 34 (B) 02 (C) 01 (D) 134 (E) 12

1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 3 3 3 4 4 4 4 4 4 4 4 4 4 5 5 5 5 51 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4

OUTGROUP 0 0 0 0 0 0 0 0 0 0 0 0 ndash ndash 0 0 0 0 0 0 0 0 0 0 0 4 0 ndash 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ndash 0 0 0 0 ndash ndash ndash 0 ndash 0Aglaspidida 1 0 0 1 A 0 0 0 0 0 1 0 ndash ndash ndash ndash ndash 0 0 0 1 0 0 0 0 0 0 0 0 ndash 1 1 0 0 1 0 0 0 0 0 0 0 0 1 2 1 1 0 ndash ndash ndash 1 0 1Aglaspidida 2 0 0 A 0 0 0 0 0 0 1 0 ndash ndash 0 1 0 1 0 0 0 0 0 0 0 0 ndash 1 1 0 0 1 0 0 0 0 0 0 0 0 1 2 1 1 0 ndash ndash ndash 1 0 1Alalcomenaeus 1 1 1 3 ndash 2 0 1 0 0 0 1 0 ndash ndash 1 0 0 0 0 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 0 1 0 1 ndash ndash ndash 1 1 0Buenaspis 0 1 B 0 0 1 0 0 ndash 0 1 0 1 0 0 0 0 1 0 0 0 0 ndash 0 1 1 1 0 1 0 ndash 0Burgessia 0 0 0 3 0 0 0 0 0 0 0 1 0 ndash ndash 0 0 0 1 2 ndash 0 0 0 0 0 4 0 ndash 0 0 0 0 0 0 2 0 0 0 0 0 0 ndash 0 0 0 1 ndash ndash ndash 1 0 0Cheloniellon 0 0 1 4 0 0 0 0 0 1 0 1 0 ndash ndash 0 0 1 0 1 0 0 0 0 0 0 1 0 ndash 0 1 0 0 1 0 0 0 0 0 1 0 0 1 0 1 1 0 ndash ndash ndash 0 ndash 1Cindarella 0 0 0 6 0 0 0 0 0 0 0 1 1 0 0 0 0 1 1 0 0 0 0 0 0 0 1 0 ndash 0 1 0 1 1 0 0 2 1 0 0 0 1 0 ndash 1 0 1 1 0 0 0 0 ndash 0Crustacea 0 0 0 3 0 0 0 0 0 0 0 0 0 ndash ndash 0 1 0 0 0 0 0 0 0 0 0 4 0 ndash 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ndash 0 0 0 ndash ndash ndash 1 0 0Emeraldella 0 0 1 5 0 0 0 0 0 0 0 1 1 0 0 0 0 1 0 2 ndash 0 0 0 0 0 0 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 1 1 1 1 0 1 ndash ndash ndash 1 0 1Eoredlichia 0 0 0 3 0 0 0 0 1 0 0 1 1 0 0 0 0 1 0 0 1 0 0 0 0 0 3 1 1 1 1 0 0 2 0 0 0 0 0 0 0 0 0 ndash 1 0 1 1 0 0 1 0 ndash 0Eurypterida 1 1 1 6 ndash 1 1 0 0 1 1 1 0 ndash ndash 1 0 1 0 1 0 0 0 1 0 0 4 1 0 0 1 0 0 1 0 0 0 0 0 0 0 0 1 3 0 1 0 1 ndash ndash ndash 1 0 0Fortiforceps 0 1 1 4 ndash 0 0 0 0 0 0 1 0 ndash ndash 1 0 0 1 0 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 0 1 0 1 ndash ndash ndash 1 1 0Helmetia 3 1 0 1 0 1 0 0 0 0 0 1 2 0 ndash 0 1 0 0 2 0 0 0 0 0 1 1 0 0 ndash 0 0 1 1 0 1 0 0 ndash 0Jianfengia 1 1 1 4 ndash 1 0 0 0 0 0 1 0 ndash ndash 1 0 1 1 0 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 0 1 0 1 ndash ndash ndash 1 0 0Kuamaia 0 0 0 0 0 0 0 0 0 1 1 0 1 0 0 1 0 0 0 1 0 0 0 1 2 0 ndash 0 1 0 0 2 0 0 0 0 0 1 1 0 0 ndash 0 0 1 1 0 1 0 0 ndash 0Leanchoilia 1 1 1 3 ndash 2 0 1 0 0 0 1 0 ndash ndash 1 0 1 1 B 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 1 1 0 1 ndash ndash ndash 1 0 0Lemoneites 0 1 0 0 0 0 1 1 0 1 0 0 0 0 0 0 0 0 1 2 0 0 ndash ndash ndash 1 0 Liwia 0 1 0 0 0 0 1 0 1 0 0 1 0 1 0 0 0 0 ndash 0 1 1 1 0 0 0 ndash 0Marrella 0 0 1 1 0 0 0 0 0 0 0 0 0 ndash ndash 0 1 0 0 2 ndash 0 0 0 0 0 0 ndash 0 0 0 0 0 0 2 0 0 0 0 0 0 ndash 0 0 0 0 ndash ndash ndash 0 ndash 0Mimetaster 0 0 1 2 0 0 0 0 0 0 0 0 0 ndash ndash 0 1 0 1 C 0 0 0 0 0 0 ndash 0 0 0 0 1 0 0 2 0 0 0 0 0 0 ndash 0 0 0 0 ndash ndash ndash 0 ndash 0Misszhouia 0 0 0 3 0 0 0 0 1 0 0 1 1 1 0 0 0 1 0 0 0 0 0 0 0 0 1 0 ndash 0 1 1 ndash ndash 2 0 0 0 1 0 0 0 0 ndash ndash 0 1 1 0 0 1 0 ndash 0Naraoia 0 0 0 3 1 0 0 0 1 0 0 1 1 1 0 0 0 1 0 0 0 0 0 0 0 0 1 0 ndash 0 1 1 ndash ndash 2 0 0 0 1 0 0 0 0 ndash ndash 0 1 1 0 0 0 ndash 0Olenoides 0 0 0 3 0 0 0 0 1 0 0 1 1 0 0 0 0 1 0 1 1 0 0 0 0 0 3 1 1 1 1 0 0 2 0 0 0 0 0 0 0 0 0 ndash 0 1 1 1 0 0 0 0 ndash 0Paleomerus 0 1 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0 0 0 ndash 0 0 ndash ndash ndash 1 0 Pycnogonida 1 1 1 4 ndash 1 1 0 0 1 1 1 ndash ndash ndash ndash 0 0 1 1 0 0 0 1 0 0 4 0 ndash 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 ndash ndash ndash 1 0 0Retifacies 0 0 0 3 0 0 0 0 0 0 0 1 0 ndash ndash 0 0 1 0 0 0 0 0 0 0 0 0 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 1 0 1 1 0 0 0 1 0 0Saperion 0 2 0 1 0 1 0 0 1 1 0 0 1 0 0 0 1 2 0 ndash 0 1 1 ndash ndash 1 1 ndash ndash 0 0 1 0 0 ndash ndash 1 1 0 0 1 0 ndash 0Sidneyia 0 0 1 0 1 0 0 0 0 1 0 1 0 ndash ndash 0 0 1 0 0 0 0 0 0 0 0 0 0 1 0 0 1 0 0 0 0 0 0 0 0 1 2 1 1 0 1 ndash ndash ndash 1 0 1Sinoburius 0 0 0 4 0 0 0 0 0 0 0 1 0 1 0 0 1 1 0 0 0 1 0 ndash 0 1 0 0 1 0 0 2 1 0 0 0 1 0 ndash 0 0 1 1 0 1 0 0 ndash 0Skioldia 0 2 0 0 0 1 0 0 0 1 2 0 ndash 0 1 1 ndash ndash 1 1 ndash ndash 0 0 1 0 0 ndash ndash 1 1 0 0 1 0 ndash 0Soomaspis 0 1 B 0 0 0 0 1 0 0 ndash 0 1 0 1 0 0 1 0 1 0 0 0 0 ndash 0 1 1 1 0 1 0 ndash 0Tariccoia 0 1 B 0 0 0 0 1 0 0 ndash 0 1 0 1 0 0 1 0 1 0 0 0 0 ndash 0 1 1 1 0 1 0 ndash 0Tegopelte 0 0 0 0 0 0 0 0 1 0 1 1 0 0 0 0 1 E 0 ndash 0 1 0 ndash ndash E 1 ndash ndash 0 0 0 0 ndash ndash 0 1 1 0 0 1 0 ndash 0Weinbergina 1 1 1 6 ndash 1 1 0 0 1 1 1 0 ndash ndash 1 0 1 0 1 0 0 0 1 0 0 4 1 0 0 1 0 0 1 0 0 0 0 0 0 0 0 1 2 0 1 0 1 ndash ndash ndash 1 0 0Xandarella 0 0 0 4 0 0 0 0 0 0 0 1 1 1 0 0 0 1 0 0 0 0 1 0 0 0 1 0 ndash 0 1 0 1 1 0 0 2 1 0 0 0 1 0 ndash 1 0 1 1 0 0 0 ndash 0Yohoia 1 1 1 4 ndash 1 0 0 0 1 1 1 0 ndash ndash 1 0 1 0 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 1 2 0 1 0 1 ndash ndash ndash 1 1 0

176 TREVOR J COTTON AND SIMON J BRADDY

Thomas 1998) and developmental data (Mittmann amp Scholtz2003) morphological evidence from fossils supports the viewthat chelicerates have lost the antennae The Hox gene evi-dence suffers from a lack of comparative data from diversemandibulates especially primitive crustaceans (see Akam2000) and the use of Hox gene expression boundaries asmarkers for segmental homology has been criticised (egAbzhanov et al 1999 although relating to posterior Hoxpatterns) Wills et al (1998a pp 43 amp 48) considered thechelicerae to be appendages of the second (deutocerebral)segment but curiously in support of this cited Schram (1978)who unambiguously followed the homology scheme used inthe present study

The anterior-most appendages of Aglaspis were originallydescribed as chelicerae (Raasch 1939) which lead to theirclassification in the Chelicerata (eg Stoslashrmer 1944) Howeverthe morphology of these appendages (see Briggs et al 1979) ismore similar to that of antennae They are narrow relative tothe thoracic endopods of relatively even diameter proximallyand the podomeres are apparently weakly defined Hesselbo(1988 1992) also argued that the anterior appendages ofAglaspis are likely to be antennae The distal parts are un-known (and hence so is their length cf Wills et al 1998ap 58) and there is no evidence that they were chelate contraryto the coding of Wills et al (1998a character 31) Nothingis known about the anterior appendages of BuenaspisLemoneites or Paleomerus and this character is accordinglycoded as missing data in these taxa

2 Form of endopod of appendages of the second segment(0) pediform and (1) anteriorly directed raptorial appendagewith reduced number of podomeres and terminal podomeresbearing spines on distal margins As discussed above cheli-cerae and lsquogreat appendagesrsquo are both likely to represent theappendages of the second segment They also differ from theassumed primitive biramous euarthropod limb in similar waysand therefore are likely to be homologous modifications ofthese appendages The homology of lsquogreat appendagesrsquo andchelicerae was originally proposed by Henriksen (1928) andsupported by Stoslashrmer (1944) but to the present authorsrsquoknowledge no modern author has discussed this possibility

Both lsquogreat appendagesrsquo (Figs 5A D E) and chelicerae(Figs 5B C) are equipped with strong spinose projections onthe outer (dorsal) side of the distal margins of the terminalpodomeres that are lacking on the proximal podomeresSecondly the number of podomeres in both chelicerae andlsquogreat appendagesrsquo is more or less reduced compared to thenumber in the endopods of biramous limbs In the case oflsquogreat appendagesrsquo they are significantly more robust thanthose of posterior endopods a situation that is matched insome chelicerates The homology of the lsquogreat appendagesrsquoof Alalcomenaeus Leanchoilia Yohoia Jiangfengia andFortiforceps is supported by all of these features and is widelyaccepted (eg Wills et al 1998a character 31 Hou ampBergstrom 1997 Bergstrom amp Hou 1998 Hou 1987a) OnlyHou amp Bergstrom (1991 p 183) have suggested albeit inpassing that the lsquogreat appendagesrsquo of all these taxa may beconvergent a view they appear to have changed The endo-pods of the appendages of other taxa are locomotory legswhich either lack strong spines or have spinose extensionsthat are invariably on the medial (ventral) surface and arestronger on the proximal podomeres than the distal ones (egMisszhouia Fig 6A) In the latter case these spines form partof the feeding system along with the gnathobases and are oftenlocated around the middle of the podomere rather than limitedto the distal margin These endopods are directed ventro-laterally as opposed to anteriorly in the case of both cheliceraeand lsquogreat appendagesrsquo The modified second appendages of

Marrella resemble this plesiomorphic condition here referredto as lsquopediformrsquo except in their anterolateral orientation Insome Recent crustaceans this appendage is modified into asecond antenna but in stem-group crustaceans it is pediform(eg Walossek amp Muller 1997 1998) This character is codedas missing data in a number of taxa for which the morphologyof the second segment appendage is unknown

Some authors (Dzik 1993 Chen amp Zhou 1997 Dewel ampDewel 1997 Budd 2002) have suggested that the lsquogreat ap-pendagesrsquo are homologous with the anterior appendages ofanomalocaridids and the anomalocaridid-like Opabinia andKerygmachela (see Budd 1996b 1999b respectively) Theanterior appendages of most anomalocaridids and otherCambrian lobopodians differ from megacheiran lsquogreat append-agesrsquo in that they consist of a large number of segmentsMoreover there is considerable morphological evidence thatanomalocaridids are part of the euarthropod stem group (Willset al 1995 1998a) and not closely related to the lsquogreatappendagersquo taxa (cf Budd 2002) with the possible exception ofParapeytoia lsquoGreat appendagersquo taxa share a suite of derivedcharacters with other crown group euarthropods includingsclerotised dorsal tergites the incorporation of more thanone pair of appendages into the head sclerotisation of theendopods and strong differentiation of the head that wereexcluded from Buddrsquos (2002) analysis The relationships ofanomalocaridids will be considered in detail later by Braddyand co-workers

3 Exopod of appendages of the second segment (0)present and (1) absent or much reduced The raptorial ap-pendages of the second segment (chelicerae and lsquogreat append-agesrsquo) are uniramous as are the corresponding pediformappendages of Marrella Mimetaster Emeraldella Cheloniellonand Sidneyia The plesiomorphic euarthropod state is found inmost other arachnomorphs including trilobites where thebiramous second segment appendages are undifferentiatedfrom those of posterior segments (Edgecombe et al 2000character 78) The exopods of these appendages are alsopresent in basal members of the crustacean crown group(Edgecombe et al 2000 character 79) and in stem groupcrustaceans (Walossek 1993 Walossek amp Muller 1997 1998)

4 Number of head segments (0) 1 (1) 2 (2) 3 (3) 4 (4) 5(5) 6 and (6) 7 The coding of this character by Edgecombe ampRamskold (1999 character 2) explicitly referred to the numberof limb pairs present in addition to the antenna Here thenumber of post-acronal segments present in the head is codedfollowing Wills et al (1998a character 29) In taxa that lack anantenna the number of somites is inferred to be one greaterthan the number of cephalic appendage pairs (cf Wills et al1998a) as described above

Sturmer amp Bergstrom (1978) suggested that the head ofCheloniellon is defined primarily on the basis of appendagetagmosis rather than the fusion of segments under a singlecephalic-shield (cf Hou amp Bergstrom 1997 pp 98ndash9) Accord-ing to this view the head consists of both the cephalic tergiteand the anteriormost free tergite that share uniramousgnathobasic appendages Wills et al (1998a) coded Cheloniel-lon as having six somites incorporated into the head andtherefore presumably accepted this suggestion There is noevidence that the first free tergite of Cheloniellon was incorpo-rated into the head (except perhaps functionally) and thepresent authors prefer to code the number of somites under thecephalic shield In Sidneyia there is a series of uniramousstrongly gnathobasic appendages similar to those of Cheloniel-lon posterior to the head shield All of these appendages wouldpresumably have to be considered part of the head based onappendage differentiation according to the view of Sturmer ampBergstrom (1978) Some autapomorphic states are included

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 177

(none for Sidneyia one for Marrella and six for Emeraldella)since these will become informative if the character is treatedas ordered

Edgecombe amp Ramskold (1999 p 265) described a novelform of cephalic tagmosis The present authors tentativelyaccept their suggestion that in trilobite-like taxa the fourthpair of biramous cephalic appendages was directly under thecephalo-thoracic junction This may explain previous confu-sion about the number of such appendages incorporated intothe trilobite cephalon (eg Cisne 1975 Whittington 1975aBergstrom amp Brassel 1984) However Edgecome amp Ramskoldaccept the view of Chen et al (1977 p 7) that only the firstthree biramous limbs of Misszhouia were structurally and

functionally part of the head Therefore it seems more accept-able to code these taxa as having only four post-acronalsomites than to use the coding scheme of Edgecombe ampRamskold (1999) The partial integration of the first thoracicsomite under the head shield could be considered to be a formof overlap of the trunk by the head shield (ie a distinct stateof character 9 of Edgecombe amp Ramskold 1999 or Character37 herein) Derived states of this character would then poten-tially be synapomorphic for a clade including xandarellidsnaraoiids helmetiids tegopeltids and trilobites Howeversince the degree of overlap in taxa with the fourth biramousappendages under the cephalo-thoracic articulation is identicalto that found between thoracic tergites (State 0 of Character

Figure 5 Diagrammatic reconstructions of raptorial second-segment appendages in lateral view (except whereotherwise stated) and approximate life orientation showing suggested homology of appendage elements (inbrackets) Roman numerals indicate position in the podomere series and do not necessarily imply homology (A)lsquoGreat appendagersquo of Fortiforceps (after Hou amp Bergstrom 1997) (B) Left chelicera of Leiobunum aldrichi(Arachnida Opiliones) (1) in medial perspective and (2) distal parts in anterior perspective (after Shultz 2000)(C) Chelifore of Nymphon (Pycnogonida) (after Child 1997) (D) lsquoGreat appendagersquo of Yohoia (after Whittington1974 fig 2) (E) lsquoGreat appendagersquo of Leanchoilia (modified after Bruton amp Whittington 1983) Not to scaleAbbreviations (apt) apotele (probably homologous with the mandibulate pretarsus) (cx) coxa (dtmrt)deutomerite and (pt) pretarsus

178 TREVOR J COTTON AND SIMON J BRADDY

37) it is here preferred to regard this as a distinct character(Character 9 herein)

The coding of Alalcomenaeus by Wills et al (1998a) ashaving four post-acronal somites is accepted here but fordifferent reasons Briggs amp Collins (1999) have recenty demon-strated that Alalcomenaeus possessed two pairs of biramousappendages on the head in addition to the great appendagesThis would equate to three head somites according to thehomology scheme of Wills et al (1998a) Here the antennaeare inferred to be lost and therefore four segments incorpo-rated into the head

Recently a number of authors have agreed that the plesio-morphic condition for euarthropods is a four-segmented headas found in stem group crustaceans (Scholtz 1997 Walossek1993 Walossek amp Muller 1997 1998 Edgecombe et al 2000)However reconstruction of the euarthropod stem groupclearly indicates that even crownward taxa had a head consist-ing of only the antennal segment and acron as found intardigrades and onychophorans Therefore it seems that thefour-segmented head is pleisomorphic only for crown groupeuarthropods Some fully arthropodised fossil taxa also have ashorter head that may be pleisomorphic such as the two-segmented head of Marella Retifacies is coded following therecent redescription of Hou amp Bergstrom (1997) rather thanon the basis of the coding by Edgecombe amp Ramskold (1999)for which they provide no explanation A partial uncertaintycoding is used for Aglaspis in which the number of post-antennal cephalic appendages is either three or four (Briggset al 1979)

5 Orientation of the antennae (0) directed anterolaterally(1) strongly deflected laterally and (2) placed well inside shieldmargin curing posteriorly from a transverse proximal element(cf Edgecombe amp Ramskold 1999 character 3) The anteriorappendages of Aglaspis (see character 1) are clearly directedanterolaterally and it is presumed that they continued past thehead shield margin The anterior appendages of arthropodstem group taxa such as Aysheaia Kerygmachela and Anoma-locaris are either directed anterolaterally or ventrally but arenot strongly-deflected laterally Therefore the outgroup iscoded as State 0 This character is coded as missing data fortaxa where the presence of antennae is equivocal (those codedas missing data for Character 1) and as inapplicable in taxawhere the antennae are considered absent (coded as State 1 forCharacter 1)

6 Length of distal spines on terminal podomeres of endo-pods of second segment appendages (0) absent or shorer thanpodomeres (1) subsequal to length of podomeres and (2)longer than the entire podomere series The spinose projectionsof the raptorial appendages described above vary in length Inchelicerae (Figs 5BC) and some lsquogreat appendagesrsquo (eg thoseof Yohoia Fig 5D) the most distal spine is equal in length tothe spinose terminal podomere forming a chelate structureIn Fortiforceps (Fig 5A) the spines are short relative tothe lengths of the podomeres but in Alalcomenaeus andLeanchoilia (Fig 5E) they are extremely long so that thespines are much longer than the entire podomere series Asdescribed above in taxa with pediform endopods of the secondsegment appendages spines on the distal podomeres are shortor entirely absent

7 Chelicerae (0) absent and (1) present Despite theirsimilarities to lsquogreat appendagesrsquo the chelicerae of the eucheli-cerates and chelifores of the pycnogonids are clearly distinctand have been widely recognised as a chelicerate synapomor-phy Chelicerae differ from any lsquogreat appendagesrsquo by thecombination of a smaller number of podomeres the presenceof only a single terminal element and only a single spinoseprojection on the dorsal side of the podomere series (Fig 5)

Amongst extant chelicerates only the pycnogonid Pallenopsishas chelicerae of four podomeres comparable to the numberin lsquogreat appendagesrsquo Bergstrom et al (1980) considered thatthe chelifores of the Devonian pycnogonid Palaeoisopus alsoconsist of four segments but this is not well supported by theirfigures

8 Distal spines of second segment endopods terminating inannulated flagellae (0) absent and (1) present It is widelyrecognised (eg Stoslashrmer 1944 Simonetta 1970 Bruton ampWhittington 1993 Briggs amp Collins 1999) that the terminalparts of the extended spines of Alalcomenaeus and Leanchoiliaformed annulated flagellae Nothing similar is known fromhomologous appendages of any other arthropod althoughsimilar processes may have operated in the transformation ofthe endopod as a whole into the second antennae of derivedcrustaceans and in the origin of multiramous antennulae (firstantennae) in malacostracans

242 Posterior appendages 9 Appendages of first thoracicsomite underneath the cephalo-thoracic articulation (0) ab-sent and (1) present (cf Edgecombe amp Ramskold 1999character 2) For a discussion of this character see Character4 herein

10 Exopods of appendages of third to fifth segments (0)present and (1) reduced or absent A number of taxa haveuniramous appendages on the third to fifth segments In mostthese segments are incorporated into the head (Yohoia cheli-cerates) but in others all (eg in Sidneyia) or some (eg inCheloniellon) of these segments are post-cephalic In all casesthe appendage is morphologically similar to the endopods ofbiramous limbs of other taxa andor other segments and it isinterpreted that the exopod is lost

11 Endopods of thoracic appendages (0) present and (1)reduced or absent The opisthosomal appendages of chelicer-ates lack endopods or have the endopods much reduced (egLimulus Siewing 1985 fig 838 Shultz 2001) a situation that isalso found in the uniramous thoracic appendages of Yohoia(Whittington 1974) It has been suggested that Helmetia(Briggs et al 1994) lacks endopods but pending a redescrip-tion of this genus and considering that they have been docu-mented in the otherwise very similar Kuamaia this is coded asuncertain

12 Exopod shaft of numerous podomeres each bearing asingle seta (0) present and (1) absent The exopods ofcrustaceans (at least plesiomorphically see eg Walossek ampMuller 1998 figs 5middot5 amp 5middot6) and of Marrella and Mimetasterhave multi-annulated shafts with each podomere bearing asingle seta In other taxa the exopod shafts consist of one oftwo lobes bearing numerous setae

The polarity of exopod segmentation is uncertain Thelateral flaps of stem group arthropods such as Opabinia(Whittington 1975b Budd 1996b) and anomalocaridids (egHou et al 1995) are certainly unsegmented but as suggestedby Buddrsquos (1996b fig 8) reconstruction this does notnecessarily suggest the form of the primitive euarthropodexopod Rather segmentation may have been a result of thesclerotization of the cuticle of the exopod shaft The outgroupwas coded as uncertain for this character According to thefirst interpretation of aglaspidid appendage morphology(coded as Aglaspidida 1) the exopods are lacking on allappendages and consequently this and all other charactersdescribing exopod morphology are coded as inapplicable

13 Exopod shaft differentiated into proximal and distallobes (0) absent and (1) present Ramskold amp Edgecombe(1996 Edgecombe amp Ramskold 1999 character 26) havediscussed the distribution of the trilobite-type bilobate exo-pods recognised by this character (Fig 6A) Among taxaincluded here that were not considered by Edgecombe amp

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 179

Ramskold (1999) exopods consist either of a single flattenedlobe or a homonomous series of podomeres (Fig 6B C) Nostem group euarthropod has a bilobate exopod and theoutgroup is consequently also coded as State 0

14 Proximal lobe of exopod (0) flattened lobe and (1)slender shaft

15 Distal lobe of exopod (0) small to moderate sized flapwith short to moderately long attachment to proximal lobeand (1) large teardrop-shaped with long attachment to proxi-mal lobe Neither of these characters can be coded for taxathat do not have an exopod differentiated into proximal anddistal lobes (Character 13 State 1) without asserting thehomology of the single-lobed exopod with one of these partsThe coding of Edgecombe amp Ramskold (1999 p 280) implic-itly homologizes the exopods of Sidneyia and Retifacies withthe proximal lobe In these taxa this view is perhaps supportedby the presence of lamellate exopod setae which match thoseof the trilobite-type proximal lobe However in other taxawith a single-lobed exopod such as Alalcomenaeus (see Briggsamp Collins 1999) the exopod is fringed with sharp spines likethe distal lobe of differentiated exopods Alternatively thebilobate exopod may have originated from the division of aprimitively single-lobed structure The single-lobed exopod isnot clearly homologous with either lobe of divided exopodsand both these characters are consequently coded as inappli-cable to taxa without differentiated proximal and distal exopodlobes Consequently (see Character 13 herein) these characterscan only be coded for taxa that were previously considered byEdgecombe amp Ramskold (1999) and their coding is followedhere except in the cases of Sidneyia and Retifacies

16 Exopod shaft is a deep rounded flap (0) absent and (1)present All bilobate (Character 13) and segmented (Character12) exopod shafts are long and relatively narrow structures (seeFig 6) whereas some single-lobed exopod shafts have theform of rounded flaps These flap-like exopod shafts are largecompared to the length of the setae and at least half as deep asthey are long This appendage structure is found in cheliceratesand lsquogreat appendagersquo arthropods

17 Medially-directed exopod setae (0) absent and (1)present Walossek amp Muller (1998 p 194) suggested that thetilting of exopod setae towards the endopod in post-antennularlimbs with a multi-annulated exopod was an autapomorphyuniting crustaceans and all members of the crustacean stemgroup (the crustacean total group sensu Ax 1986 see Fig 6C)This character is shared with the marellomorphs Marrellaand Mimetaster (see Fig 6B Sturmer amp Bergstrom 1976Bergstrom 1979 fig 1middot3A B) In other taxa the setae eithersurround the entire margin of the exopod shaft or are directeddorsally (Fig 6A) The identification of setae on the dorsalsurface of the lateral flaps in Opabinia (Budd 1996b) suggeststhat the condition in marellomorphs and crustaceans isderived

18 Lamellate exopod setae (0) absent and (1) presentLamellate exopod setae originally described from trilobitesare a classic synapomorphy of the Arachnomorpha (see egBergstrom 1992 Hou amp Bergstrom 1997 pp 42ndash3) They areperhaps best known from Misszhouia (see Chen et al 1996Hou amp Bergstrom 1997 Fig 6A) These setae differ from themore spinose setae of other taxa (Fig 6C) in that they are wideand flat and imbricate over the length of the exopod shaft Thesetae of Marrella (Fig 6B) and similar taxa have variouslybeen considered lamellate (Bergstrom 1979) or non-lamellate(Bergstrom 1992) Since they do not imbricate and do notseem to be strongly flattened (Whittington 1971 Sturmer ampBergstrom 1976 fig 9a) Marrella and Mimetaster are codedas State 0 The setae of Helmetia as shown in Briggs et al(1994 fig 141) seem to be of the lamellate type The setae ofSinoburius have been described by Hou amp Bergstrom (1997p 85) as similar to those of Misszhouia Only the setae areknown of the appendages of Buenaspis (Budd 1999a) andthese also appear to be of the trilobite-type

Figure 6 Diagrammatic reconstructions of (A) arachnomorph and(B C) non-arachnomorph biramous appendages (A) The lsquotrilobite-typersquo biramous appendage of Misszhouia longicauda (after Chen et al1997) (B) Appendage of Marrella splendens (after Whittington 1971)(C) Appendage of the stem-lineage crustacean Martinssonia elongata(after Muller amp Walossek 1997 1998) Not to scale Abbreviations(bas) basis (dle) distal lobe of exopod shaft (ls) lamellar exopod setae(pe) proximal endite or pre-coxa (ple) proximal lobe of exopod shaft(ses) segmented exopod shaft and (ss) spinose exopod setae

180 TREVOR J COTTON AND SIMON J BRADDY

The homology of the book-gill lamellae of xiphosurans withlamellar setae was supported by Walossek amp Muller (1997p 149) and Edgecombe et al (2000 p 174) but rejected bySturmer amp Berstrom (1981) on the grounds that Weinberginapossesses Limulus-like gill lamellae and fringing setaeBook-gills have also been described from a eurypterid (Braddyet al 1999) There are certainly major morphologicaldifferences between book-gills and trilobite-type exopodsFollowing most recent opinion and pending further studyWeinbergina and Eurypterida are coded as possessing lamellatesetae

The form of the setae of Yohoia is uncertain (Whittington1974) the spinose setae seen in the most common reconstruc-tion (Gould 1989 Briggs et al 1994) are not justified by thespecimens Amongst other great appendage arthropods theform of the setae appears to vary those of Jianfengia (seeChen amp Zhou 1997 p 74) and Leanchoilia (see Bruton ampWhittington 1983) are lamellate while those of Fortiforceps(Hou amp Bergstrom 1997) and Alalcomenaeus (Briggs amp Collins1999) are spinose and less densely packed

19 Gnathobase on basis andor prominent endites onendopod (0) present and (1) absent (cf Edgecombe ampRamskold 1999 character 29 Wills et al 1998a characters 36amp 59) The presence of gnathobases on the appendages of someanomalocaridids (Hou et al 1995) and their general distribu-tion amongst non-arachnomorph euarthropods suggests thatState 0 is plesiomorphic The homology of the various enditesand gnathobasic structures found in euarthropods is in need ofcareful assessment and therefore a very general coding (fol-lowing Edgecombe amp Ramskold) is used for this character

243 Eyes 20 Position of lateral facetted eyes (0)ventral and stalked (1) dorsal and sessile and (2) absent (cfEdgecombe amp Ramskold 1999 character 4) Partial uncer-tainty coding is used for a number of taxa where the dorsalsurface is known but the ventral morphology is not andtherefore the presence of ventral eyes cannot be discountedFor example there is good evidence that Leanchoilia lackeddorsal sessile eyes but ventral stalked eyes may have beenpresent (as in L hanceyi see Briggs amp Robison 1984) and a(02) partial uncertainty coding is used This is also the case inmany naraoiids such as Buenaspis However only the outlineof the head shield of Liwia is known and the absence of dorsaleyes in the reconstruction of Dzik amp Lendzion (1988) isconjectural It is unclear whether the autapomorphic conditionof stalked dorsal eyes in Mimetaster (see Sturmer amp Bergstrom1976) is homologous with State 0 or State 1 and a partialuncertainty coding is also used

Whittington (1974) was somewhat equivocal about thenature of the lateral lobes anterior to the head shield ofYohoia These more closely resemble the ventral stalked eyes ofAlacomenaeus as recently described by Briggs amp Collins(1999) than either Whittingtonrsquos (1974) or subsequent (egGould 1989 fig 3middot18 Briggs et al 1994 fig 153) reconstruc-tions suggest In a well preserved specimen showing the dorsalaspect (USNM 57696 Whittington 1974 pl 2 figs 1ndash3) and ina laterally compressed specimen (USNM 57694 Whittington1974 pl 1 fig 1) they are clearly seen to be stalked andrelatively small lobate structures That these structures arelikely to represent stalked ventral eyes was reflected in thecoding of Wills et al (1998a characters 26 amp 27)

21 Visual surface with calcified lenses bounded withcircumocular suture (0) absent and (1) present

22 Dorsal bulge in exoskeleton accommodating drop-shaped ventral eyes (0) absent and (1) present

23 Eye slits (0) absent and (1) presentCharacters 21 to 23 are used exactly as described by

Edgecombe amp Ramskold (1999 character 5) Character 21 is

inapplicable to taxa that do not possess eyes (Character 20State 2)

24 Dorsal median eyes (0) absent and (1) present Dorsalmedian eyes on a tubercle are considered a synapomorphy ofthe Chelicerata (Dunlop amp Selden 1997 Dunlop 1999) Thenature and position of the structures interpreted as dorsalmedian eyes in Mimetaster (Sturmer amp Bergstrom 1976p 83ndash4) are regarded as equivocal

244 Ventral cephalic structures Many characters of theventral surface of the euarthropod head (see Fig 7) of poten-tial phylogenetic utility are poorly known in many importanttaxa In particular the presence of pre-hypostomal frontalorgans (Fig 7A CndashE) is not coded herein (see Edgecombe ampRamskold 1999 p 272) Definitive evidence of the absence ofthese structures is available for very few taxa and the homol-ogy of these structures with the maculae of the trilobitehypostome (see Fig 7B) and the frontal organs of the crusta-cean labrum (eg see Muller amp Walossek 1987 p 39) isuncertain Secondly the detailed homology of the trilobitehypostome with that of other putative arachnomorphs isunclear and consequently a very broad definition is usedherein For example various pre-hypostomal sclerites (Fig7A D) in other arachnomorphs may have been incorporatedalong with a primitive hypostome into a single sclerite that isrecognised as the hypostome in trilobites The structure of thehypostome (eg the presence of paired anterior wings dorsal tothe antennae Fig 7B C) may also be a source of additionalcharacters

25 Expanded cephalic doublure (0) absent and (1)present maximum width more than 30 length of head shieldor more than 25 width of pygidium Chen et al (1996)suggested that the wide doublure of Soomaspis and Tariccoia isa synapomorphy uniting these taxa The doublure of Buenaspis(see Budd 1999a) is poorly known but based on the width ofthe heavily crushed region of the cephalic margin and theposition of a possible impression of the edge of the doublure inone specimen (Budd 1999a pl 1 fig 1) it also appears to berather wide (ie greater than 30 of the length of the headshield) Following Edgecombe amp Ramskoldrsquos coding ofSidneyia the present authors do not consider the postero-median expansion of the doublure of for example Emeraldellaor Retifacies (see Bruton amp Whittington 1983 Hou ampBergstrom 1999 respectively) to be homologous with State 1but to represent the hypostome (see Character 26)

26 Anteromedian margin of cephalon notched accomo-dating strongly sclerotised plate (0) notch and plate absentand (1) notch and plate present (cf Edgecombe amp Rasmkold1999 character 10) The apomorphic state (illustrated inFig 7D) is only known in the taxa discussed by Edgecombe ampRasmkold (1999) Reconstructions of Yohoia show a roundedlobe between the eyes (see Character 20) anterior to anddistinct from the head shield which resembles the anteriorsclerite recognised here This seems to be a misinterpretationSpecimens preserved in dorsal aspect (USNM 57696Whittington 1974 pl 2 figs 1ndash3) and lateral aspect (USNM57694 Whittington 1974 pl 1 fig 1 USNM 155616Whittington 1974 pl 5 fig 2) seem to clearly show that thislsquomedian lobersquo is a downward curving pointed extension of thehead shield

27 Hypostomal sclerite (0) median extension of thedoublure with no suture (1) natant sclerite not in contactwith doublure (2) with narrow overlap with pre-hypostomalsclerite (3) narrow attachment to doublure at hypostomalsuture and (4) absent The homology of hypostomes andlabrums has been discussed by Haas et al (2001) and Budd(2002) The euarthropod labrum is likely to represent a pos-teroventral extension of the pre-segmental acron (Scholtz

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 181

Figure 7 Ventral reconstructions of arachnomorph arthropod heads (A) Misszhouia longicaudata (after Chenet al 1997 figs 2ndash4 7) (B) Ceraurinella typa (after Chen et al 1997 fig 7) (C) Agnostus pisiformis (after Mulleramp Walossek 1987) (D) Kuamaia lata (after Edgecombe amp Ramskold 1999 figs 32 5 amp 6) (E) Cindarella eucala(after Ramskold et al 1997) and (F) Emeraldella brocki (after Bruton amp Whittington 1983) Not to scaleAbbreviations (aw) anterior wing beneath antennae (bs) boomerang-shaped sclerite (bl) blade-like process ofposterior wing (d) doublure of head-shield (cs) connective suture (f) fenestra (fs) facial suture (h) hypostome(hl) hypostomal lobe (emerging through fenestrae of Agnostus) (hs) hypostomal suture (if) intervening field ofhypostomal complex (lb) lateral bridge (lfo) lateral frontal organ (mb) median body (mbr) median bridge (mf)median furrow (mfb) mouth field bridge (mfo) median frontal organ (ol) ovate lobe and (phs) pre-hypostomalsclerite Post-antennal appendages and distal parts of antennae omitted

182 TREVOR J COTTON AND SIMON J BRADDY

1997) This structure is often sclerotised and covers the mouthventrally It is this sclerite that is here identified as thehypostome Therefore this character is considered absent intaxa lacking a sclerite covering the mouth irrespective of themorphology and position of the labrum itself which at leastpartly covers the mouth in all euarthropods other thanpycnogonids (see Edgecombe et al 2000 p 167) The lsquofleshylabrumrsquo of crustaceans does not appear to be sclerotised and ahypostome is consequently considered to be absent Althoughmany derived features are associated with the crustaceanlabrum this is not a good reason to consider it non-homologous with the hypostome-bearing structure of otherarthropods as Walossek amp Muller (1990) argued

In many taxa the mouth is covered by a posteromedianextension of the doublure here considered to represent thehypostome (eg Emeraldella Fig 7F Aglaspis see Hesselbo1992 fig 52) In others the hypostome is separated from thedoublure either by a suture a pre-hypostomal sclerite (egKuamaia lata Fig 7D Saperion glumaceum see Edgecombe ampRamskold 1999 figs 3 amp 4) or an intervening region ofunsclerotised cuticle (the natant condition of Fortey 1990beg Cindarella eucala Fig 7E) The homology of pre-hypostomal sclerites and hypostomes in helmetiids trilobitesand naraoiids (see Fig 7) has been discussed by Chen et al(1996) and Edgecombe amp Ramskold (1999) but it is as yetunclear whether any of the character states described here arederived from any other They are treated here as distinct andthe character as unordered pending further study

No hypostome has been identified in Yohoia (see above)Alalcomenaeus Jianfengia Fortiforceps or Leanchoilia andthere is certainly no broad posterior extension of the doublurein any of these taxa Since there is also no pre-hypostomalsclerite they have received a (134) partial uncertainty codingThe attachment of the hypostome of Tegopelte is unclear butit does not seem to be attached to any doublure (Whittington1985) and therefore it is given an uncertainty (12) coding

The lsquorostrum-like structurersquo on the ventral surface ofLemoneites appears to be similar in morphology and positionrelative to the anterior margin of the head shield (Flower 1968pl 8 figs 4 amp 13) to that described from Aglaspis and is codedas State 0 In many taxa the hypostome is unknown but inonly a small number can it be coded reliably as absentHughesrsquo (1975) suggestion that the hypostome of Burgessia isabsent is accepted preliminarily but an extension of thedoublure may be visible in some of Hughesrsquo figures Fortey(pers comm 2000) also doubts Hughesrsquo assertion

28 Visible ecdysial sutures (0) absent and (1) present Themarginal ecdysial sutures of chelicerates and the dorsal suturesof trilobites are almost certainly not homologous but thepresence of sutures and their position (see Character 29) arecoded separately to allow this to be tested Wills et al (1998aCharacter 2) coded marginal sutures as present in Sidneyiafollowing Brutonrsquos (1981) suggestion but this is consideredequivocal here

29 Position of ecdysial sutures (0) marginal and (1) dor-sal This character is inapplicable to taxa lacking ecdysialsutures (Character 28 State 0) Dorsal sutures whilst presentin the representatives of the Trilobita coded here are probablynot a synapomorphy of trilobites as a whole but for a clade oftrilobites excluding the Olenellida (Whittington 1989 Fortey1990a)

245 Exoskeletal tergites and thoracic tagmosis 30 Min-eralised cuticle (0) absent and (1) present Calcification of theexoskeleton is one of the most convincing synapomorphiesof the Trilobita (Fortey amp Whittington 1989 Ramskold ampEdgecombe 1991 Edgecombe amp Ramskold 1999 Character 1)Cuticle mineralisation is also coded as present in aglaspidids

following Briggs amp Fortey (1982) who suggested that theaglaspidid exoskeleton was originally phosphatic Paleomerusprobably also had a mineralised cuticle but Hou amp Bergstrom(1997 p 97) argued that it was likely to be calcareous Anundescribed Silurian aglaspidid may also have had an origi-nally calcitic cuticle (Fortey amp Theron 1994 p 856) Becauseof the uncertainty about the chemical composition of theexoskeleton in these forms cuticle mineralisation is coded aspotentially homologous in all these taxa and no attempt ismade to code separate states for phosphatic and calcareousmineralisation

31 Trunk tergites with expanded lateral pleurae coveringappendages dorsally (0) absent and (1) present Whilst thepresence of paratergal folds may be a synapomorphy at thelevel of the Euarthropoda (Boudreaux 1979 Wagele 1993Edgecombe et al 2000 Character 142) these are at mostsmall reflections of the margin of the tergites in most euarthro-pods In arachnomorph taxa these are expanded to form largelateral pleurae that cover the appendages dorsally (see egLimulus and Triarthus in Boudreaux 1979) This is inferred tobe the case in some taxa where dorsal features and appendagesare poorly known on the basis of their possession of tergiteswith wide pleural regions This feature was suggested as typicalof lamellipedians (=arachnomorphs) by Hou amp Bergstrom(1997 pp 42ndash3)

The arrangement of the thorax of Marrella Mimetaster andBurgessia differs from that of all the other taxa considered herein that the appendages seem to be attached to the lateralmargins of the body The trunk tergites do not cover theappendages and seem to lack pleurae entirely although theyare covered by a posterior extension of the head shield inBurgessia This character has been clearly described inMimetaster (Sturmer amp Bergstrom 1976 pp 87ndash90 fig 8) andBurgessia (Hughes 1975 p 421)

32 Free thoracic tergites (0) present and (1) absentFollowing Edgecombe amp Ramskold (1999 Character 16) anumber of taxa lack functional post-cephalic articulations andconsequently lack free thoracic tergites Despite this theycode these taxa for a number of characters relating to thestructure of thoracic tergites (eg their characters 18 and 19)These characters are treated here as inapplicable to taxawithout free tergites and the definition of this character hasbeen modified from that of Edgecombe amp Ramskold (1999) tomake this more explicit

33 Decoupling of thoracic tergites and segments (0)absent and (1) present Ramskold et al (1997) described thischaracter of Cindarella and Xandarella where the thoracictergites correspond to a variable increasing posteriorly num-ber of appendage pairs This character cannot be coded fortaxa in which free thoracic tergites are absent (Character 32State 1)

34 Tergite articulations (0) tergites non-overlapping (1)extensive overlap of tergites and (2) edge-to-edge pleuralarticulations In most of the taxa considered here the thoracictergites overlap considerably and relatively evenly over theirwidth This contrasts with the primitive euarthropod condi-tion where the tergites do not overlap medially as seen in stemgroup crustaceans In trilobites such as Helmetia and Kuamaia(see Edgecombe amp Ramskold 1999 character 18) overlap ofthoracic tergites is limited to a well-defined axial region andthe lateral pleurae meet edge-to-edge

The thoracic articulations of naraoiids with free thoracictergites are in some ways similar to those of trilobites with astrong overlap medially that is lacking abaxially andanterolateral articulating facets (Budd 1999a Ramskold ampEdgecombe 1999) However the distribution of these features

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 183

in other trilobite-like arachnomorphs is unclear and a restric-tive coding is used here The present authors do not considerthat this character can be applied reliably to the fused thoracictergites of Saperion Tegopelte and Skioldia and it is coded asinapplicable to all taxa lacking free thoracic tergites (Character32 State 1)

35 Trunk effacement (0) trunk with defined (separate orfused) tergite boundaries (1) trunk tergite boundaries effacedlaterally and (2) trunk tergite boundaries completely effaced(cf Edgecombe amp Ramskold 1999 Character 15)] Contrary toEdgecombe amp Ramskold (1999) the present authors considerSkioldia and Saperion to show a distinct form of tergiteeffacement where the fused tergite boundaries are defined byfurrows axially but effaced laterally The boundaries areeffaced at least laterally in Tegopelte but the axial region ofthe exoskeleton is unknown which accordingly is given amultistate uncertainty coding

36 Cephalic articulation fused (0) absent and (1) presentIn Tegopelte Saperion and Skioldia the articulation ofthe head with the thorax is non-functional and the entireexoskeleton forms a single tergite

37 Head shield overlap of thoracic tergites (0) overlapabsent or identical to overlap between thoracic segments (1)head shield covers first thoracic tergite only and (2) headshield covers mutliple anterior trunk tergites

38 Head shield articulates with reduced anterior thoracictergite (0) absent and (1) present Amongst taxa showing aposteriorly expanded head shield ie one that overlaps thethorax Edgecombe amp Ramskold (1999 Character 9) identifiedtwo distinct character states One of these ndash overlap of only theanteriormost thoracic tergite ndash was limited to taxa assigned tothe Liwiinae (sensu Fortey amp Theron 1994) and another ndashoverlap of multiple tergites with attachment to a reducedanterior thoracic tergite ndash to the Xandarellida None of theadditional taxa considered herein (including Buenaspis whichwas assigned to the Liwiinae by Budd 1999a) shows thesedistinctive morphological features However the expandedhead shields of Marrella Mimetaster and Burgessia which donot articulate with a narrow anterior thoracic tergite may behomologous with the expanded head shields of xandarellidsTo recognise this the degree of overlapping of the thorax andthe possession of the reduced anterior tergite are coded sepa-rately These characters are both coded as inapplicable to taxain which the head shield and thoracic tergites are fused(Character 36 State 1) The crustacean carapace which ismost similar to the head shield of Burgessia is seeminglyabsent in stem group crustaceans (Walossek amp Muller1998)

39 Trunk narrowed anteriorly relative to head shieldwidest posteriorly (0) absent and (1) present (cf Edgecombeamp Ramskold 1999 Character 14) The anterior narrowing ofthe trunk caused by the reduction of opisthosomal segment 1in xiphosurids (Weinbergina of the taxa analysed hereAndersen amp Selden 1997 Dunlop amp Selden 1997 Character14) is not considered to be homologous with the unusual shapeof the thorax in naraoiids recognised by their coding

40 Boundaries of anterior trunk segments reflexed antero-laterally (0) absent boundaries transverse or reflexed postero-laterally and (1) present

41 Joints between posterior tergites functional anteriorones variably fused (0) absent and (1) present

42 Posterior tergite bearing axial spine (0) absent and (1)present

Characters 40 to 42 are coded following Edgecombe ampRamskold (1999 characters 17 19 amp 23 respectively) Inaddition to the taxa considered here a thoracic axial spine ispresent in some olenelloid trilobites (see Lieberman 1998

Characters 73 amp 74) and in the aglaspidid Beckwithia (seeHesselbo 1989)

246 Posterior body termination 43 Postabdomen of seg-ments lacking appendages (0) absent and (1) present

44 Length of postabdomen (0) one segment (1) twosegments (2) three segments and (3) five segments In sometaxa a variable number of posterior thoracic segments bearcomplete tubular tergites and lack appendages In Aglaspisautapomorphically it seems that the posterior three thoracicsegments lack appendages (Briggs et al 1979 Hesselbo 1992)but have unmodified tergites These situations are recognisedas potentially homologous by the coding used here Character44 is coded as inapplicable to taxa lacking a postabdomen

45 Posterior tergites strongly curved in dorsal aspect com-pared to anterior tergites (0) absent and (1) present Asrecognised by Wills et al (1998a Character 17) in some taxawhere the tergites are distinct the curvature of thoracic tergitesincreases posteriorly so that the posterior tergites are highlycurved to semicircular in dorsal aspect This situation isnot known from crustaceans (except isopods) or stem grouparthropods

46 Posterior segments reduced and with highly reducedappendages (0) present and (1) absent In some taxa there area large number of posterior segments that are short sagitallyand have appendages that are much reduced in size comparedto anterior trunk appendages These somites are incorporatedinto the pygidium in some taxa but primitively they arecovered by tiny free trunk tergites In the derived state thetrunk somites and limbs are of a relatively constant size Thedistinction between these states can be seen when attempting tocount the number of segments that make up the body In taxashowing State 0 the number of segments is very high anddifficult to count whereas in other taxa the number ofpost-cephalic segments is easily assessed

47 Pygidium (0) absent and (1) present A pygidium isrecognised here as a posterior tagma consisting of a number offused segments under a single tergite which may or may notincorporate the post-segmental telson This is different to theuse of this term by Edgecombe amp Ramskold (1999) whichfollows Ramskold et al (1997) and very different to its use byWills et al (1998a p 53) as a synapomorphy of the TrilobitaThe present authorsrsquo definition recognises the situation inRetifacies where the spinose postsegmental telson is autapo-morphically not fused to the pygidium as homologous toother multisegmented posterior tagma which include thetelson The distinction between the pygidium and an expandedpost-segmental telson can also be seen in the position of theanus which is consistently in the posteriormost pre-telsonicsegment amongst putative arachnomorphs (see Character 48)In taxa with a pygidium the anal segment is fused into thepygidium (see eg Kuamaia Edgecombe amp Ramskold 1999fig 6) whereas in those taxa lacking a pygidium the anus isbetween the final trunk tergite and the telson

Ramskold et al (1997) argued that the posteriormost tergiteof xandarellids (which incorporates the anal segment) is nothomologous to the posterior tagma recognised as pygidiaherein because posterior thoracic tergites also cover multiplesegments in xandarellids (see Character 33) Evidence ofsegmentation of the pygidium in a variety of taxa suggests thatthe number of tergites fused to form the pygidium is greaterthan the number of appendages For example the pygidium ofKuamaia has two pairs of lateral spines but at least fourpairs of appendages (Hou amp Bergstrom 1997 Edgecombe ampRamskold 1999) and that of Triarthrus has only five axialrings posterior to the articulating ring but over 10 pairs ofappendages (Whittington amp Almond 1987) The fact thatdecoupling of tergites and segments is evident in the thorax of

184 TREVOR J COTTON AND SIMON J BRADDY

Xandarella and Cindarella does not necessarily suggest that asimilar more widespread decoupling in pygidia is the result ofa different developmental process and hence that they arenon-homologous Instead the situation in the xandarellidthorax is potentially homologous to that found (more widely)in pygidia and consequently the terminal xandarellid tergite isa pygidium It is unclear if decoupling of tergites and somites isprimitively limited to the terminal tergite or primitively aproperty of the arachnomorph post-cephalon as a whole

It has been suggested that the tiny pygidium of olenelloidtrilobites which probably best reflects the primitive trilobitestate consists only of the post-segmental telson (Harrington inMoore 1959) and therefore is not a true pygidium HoweverWhittington (1989) showed that the olenelloid pygidium con-sists of at least two and possibly as many as five or sixsegments and therefore is likely to be homologous with thepygidium of other trilobites

48 Position of the anus (0) terminal within telson and (1)at base of telson In crustaceans and stem group arthropodsthe anus is terminal or otherwise situated in the telson (egRehbachiella Walossek 1993 fig 15D) In putative arachno-morphs the anus is either at the junction of the posteriormostthoracic segment and the telson or ventral within a fusedpygidium In the case of taxa with a pygidium the anus isanterior to the posterior margin and where known positionedbetween the posteriormost pair of appendages suggesting thatit is anterior to the post-segmental telson (see eg OlenoidesWhittington 1980)

49 Pygidium with median keel (0) absent and (1) presentEdgecombe amp Ramskold (1999 Character 21) considered thepresence of a median keel on the pygidium as a synapomorphyuniting Soomaspis and Tarricoia They did not explain howthis structure could be distinguished from the raised pygidialaxis of trilobites Homology of these structures is not sup-ported here because the axis of more primitive trilobites(including Eoredlichia) is quite distinct morphologically fromthe naraoiid keel Budd (1999a) noted the presence of a keel onthe pygidium of Buenaspis and suggested (Budd 1999a p 102)that it may be an artefact of dorsoventral compaction How-ever contrary to the coding of Edgecombe amp Ramskold(1999) a keel is visible on the pygidium of Liwia convexa (Dzikamp Lendzion 1988 fig 4CD) preserved in full relief Thereforeit seems unlikely that the keel is a taphonomic artefact

50 Pygidium with broad-based median spine (0) absentand (1) present

51 Pygidium with lateral spines (0) present and (1) absentEdgecombe amp Ramskold (1999 Character 22) coded thepresence of a median spine and two pairs of lateral spines aspotentially homologous in Sinoburius Kuamaia and HelmetiaThe distribution of lateral spines is much wider than that ofterminal spines and therefore they are coded separately hereIn trilobites it has widely been recognised that lateral spinesrepresent the original segmentation of the pygidium Thiscannot be the case for median spines Characters 49 to 51are not applicable to taxa lacking a pygidium (Character 47State 0)

52 Expanded post-segmental telson (0) absent and (1)present In a range of taxa the posterior-most tergite is a large(relative to the thoracic tergites) structure that lacks anyevidence of segmentation and is interpreted as representing anexpanded tergite of the post-segmental telson from whichsegments are released anteriorly during ontogeny On the otherhand the posteriormost tergite of Marrella and probablyMimetaster is small compared to the thoracic tergites androunded This element probably represents the plesiomorphiceuarthropod telson The homology of this character in mosttaxa is clear and only doubtful in Retifacies in which it may

be segmented The figures of Hou amp Bergstrom (1997) providelittle unequivocal evidence for a telson in Retifacies but thepresence of this structure is very clear in colour photographs(eg Chen et al 1996 fig 198A) However these figures donot convincingly demonstrate the segmentation of the telsonRetifacies is interpreted here as being unique in possessingboth a pygidium and an expanded post-segmental telson

53 Telson shape (0) spinose and (1) paddle-shaped Theshape of the expanded telsons identified above varies frombroadly spinose to flattened and paddle-like This differencecan be recognised both on the basis of the ratio of length tobasal width (spinose telsons are relatively long) and by thechange in width posteriorly (spinose telsons reduce in widthposteriorly whereas paddle-shaped telsons increase in width)In eurypterids a wide range of telson shapes is found includ-ing both character states described here It is likely that theplesiomorphic condition is a spinose telson This character isinapplicable to taxa lacking an expanded telson

54 Post-ventral furcae (0) absent and (1) present Thischaracter recognises the potential homology of unsegmentedpaired structures that articulate with the segment immediatelyanterior to the telson The potential homology of these struc-tures (the post-ventral plates) in Emeraldella and Aglaspis wasrecognised by Wills et al (1998a Character 70) and inEmeraldella and Sidneyia by Edgecombe amp Ramskold (1999Character 25) The homology of the segmented pygidial caudalfurcae of Olenoides (see Whittington 1975a 1980) with thesestructures is considered doubtful and coded as equivocal

Despite their distinctive morphology the long caudualfurcae of Cheloniellon may be homologous with the furcae ofAglaspis Emeraldella and Sidneyia This is supported by thecaudal furcae of the Ordovician cheloniellid Duslia which aremore similar in morphology to those of Emeraldella than ofCheloniellon Chlupac (1988 pp 614ndash16) considered thesestructures to be on the terminal tergite in Duslia However afaint tergite boundary is apparent posterior to the attachmentof the furcae (see Chlupac 1988 pl 57 fig 4) Therefore theyare considered here to have been attached to the pre-telsonicsegment as in Cheloniellon Chlupac (1988) and Dunlop ampSelden (1997) supported a close relationship between Dusliaand Cheloniellon

25 MethodsAll analyses were carried out using PAUP Version 40b4a(Swofford 1999) and unless otherwise stated used heuristicsearches with one thousand random addition sequencereplicates The software packages MacClade Version 307(Maddison amp Maddison 1997) and RadCon (Thorley amp Page2000) were used for comparing trees and investigating patternsof character evolution Tree length and other statistics (theConsistency Index CI and Retention Index RI) were calcu-lated by PAUP and MacClade with uninformative charactersexcluded Analyses were run separately using each of thedifferent codings for aglaspidids

Characters were treated as unordered and of equal weight inmost analyses To assess the influence of this assumption fourof the eight multistate characters which have states that areintermediate between others (Wilkinson 1992) were treated asordered in some analyses These characters are the number ofcephalic segments (Character 4) the length of raptorialappendage spines (Character 6) the degree of overlap of thethorax by the head-shield (Character 37) and the number ofsegments making up the postabdomen (Character 44) The lastof these is uninformative when not ordered because only onetaxon shows each of states 0 1 and 3

Support for individual nodes was assessed by bootstrapanalysis (Felsenstein 1985) and by calculating Bremer support

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 185

indices (Bremer 1988 1994) These methods measure twodistinct aspects of support for phylogenetic hypotheses Treesor nodes may be considered well supported (1) to the extent towhich alternative topologies are much less parsimonious asmeasured by the support index (Wilkinson 1996) or (2) wherethey are consistent with a large proportion of characters sothat character sampling is unlikely to have had much influenceon topology as assessed by bootstrapping (Page 1996) Boot-strapping was performed with 100 bootstrap replicates eachof 10 addition sequence replicates Bremer support indicesare based on heuristic searches with 100 addition sequencereplicates

Three pairs of terminals are included in Table 2 that apartfrom missing data are coded identically These are Helmetiaand Kuamaia Soomaspis and Tariccoia and Saperion andSkioldia These pairs are lsquotaxonomic equivalentsrsquo removingthe taxon with most missing data will not influence theposition of the other They can be recoded as single terminalsaccording to the principal of safe taxonomic reduction(Wilkinson 1995a b) and were not considered separately in theanalyses presented here Therefore cladistic analyses werecarried out on the basis of 33 terminals Edgecombe ampRamskold (1999) ignored this and coded the same three pairsof taxa and Eoredlichia and Olenoides identically for theirmore limited set of characters

26 ResultsAnalysis with all characters unordered and using the lsquoAglaspi-dida 1rsquo coding found nine most parsimonious trees (MPTs)each 126 steps long (CI=0middot556 RI=0middot763) Using the lsquoAglas-pidida 2rsquo coding resulted in 18 MPTs of length 125 (CI=0middot560RI=0middot765) Nine of these 18 trees were those found with thefirst coding of aglaspidids The strict component consensus(sensu Wilkinson 1994) of the 27 unique trees found acrossboth analyses is shown in Figure 8 All trees supported themonophyly of the Marrellomorpha (Marrella and Mimetaster)and of a clade including all other taxa except Crustacea (Clade1 of Fig 8) The relationships between these two clades and theCrustacea were unresolved equal numbers of trees in bothanalyses supported each of the topologies [Clade 1 (CrustaceaMarrellomorpha)] [Crustacea (Marrellomorpha Clade 1)]and [Marrellomorpha (Crustacea Clade 1)] All trees placedBurgessia as the sister taxon to the rest of Clade 1 andsupported the division of the rest of Clade 1 into two majorsister groups The topology of one of these clades (Clade 2)which included trilobites helmetiids tegopeltids naraoiidsxandarellids and Retifacies was stable with respect to thecoding of aglaspidids The other clade (Clade 3) consists of aclade of megacheirans and chelicerates (Clade 4) and a cladeincluding aglaspidids Lemoneites Paleomerus CheloniellonEmeraldella and Sidneyia (Clade 5) This latter clade is un-resolved in the strict consensus of all 27 trees (Fig 8) and thestrict consensus of the 18 MPTs found using the lsquoAglaspidida2rsquo coding but was fully resolved in the consensus of the ninetrees found with the lsquoAglaspidida 1rsquo coding Within themegacheiran-chelicerate clade all trees found Fortiforceps tobe the sister group to all other taxa Yohoia the sister group tochelicerates and Alalcomenaeus and Leanchoilia to form aclade Jianfengia is equally parsimoniously placed as the sistergroup to the Yohoia-chelicerate clade or as the sister group tothis clade and the Alalcomenaeus-Leanchoilia clade

Treating characters 4 6 37 and 44 as ordered had a minimaleffect on most parsimonious topology Nine MPTs of 136 steps(CI=0middot515 RI=0middot743) were found with the lsquoAglaspidida 1rsquocoding and 12 MPTs 135 steps in length (CI=0middot519 RI=0middot745)with the lsquoAglaspidida 2rsquo coding These trees all supported a(Crustacea Clade 1) clade excluding the marrellomorphs

Apart from this the strict consensus of these 21 trees isidentical to the consensus of the 27 trees found with allcharacters treated as unordered

Across all analyses four different topologies for Clade 5 (ofFig 8) were found (Fig 9) The distribution of these topologiesacross the four analyses described above and across the sameanalyses repeated after reweighting of each character in pro-portion to their Rescaled Consistency Index (see Farris 1989)from the first analysis is shown in Table 3 The instability of

Figure 8 Strict consensus of 27 MPTs from four separate analyses ofarachnomorph phylogeny with both interpretations of aglaspididmorphology and with some characters treated as ordered or allcharacters unordered Clades referred to in the text are numbered

Figure 9 Alternative equally parsimonious resolutions of clade 5 ofFig 8 (see Table 3)

186 TREVOR J COTTON AND SIMON J BRADDY

this clade was to some extent caused by the inclusion ofLemoneites and Paleomerus which could each be coded foronly 27 of the 54 characters used Analysis with these two taxaexcluded and characters treated as unordered found treescompatible only with Topology D of Figure 9 irrespective ofthe coding used for aglaspidids Analysis with some characterstreated as ordered and using either aglaspidid coding foundthree trees compatible only with Topology B of Figure 9 andthree compatible only with Topology D of Figure 9 ThereforeTopology D is the preferred topology for Clade 5 as shown inTable 3

One of the MPTs resulting from the lsquoAglaspidida 2rsquo coding(with all characters treated as unordered) was chosen as thebasis for further discussion because this tree combined thepreferred topology of Clade 5 with the (Crustacea Clade 1)group that was favoured when some characters were treated asordered Bootstrap percentages and Bremer support values forthis tree (using the Aglaspidida 2 coding) are shown in Figure10 Apomorphies for all ingroup nodes of this tree are shownin Figure 10A

3 Discussion

31 Phylogenetic resultsThe results of the cladistic analyses presented above arewell-resolved and robust with respect to different analyticalparameters Most of the taxa considered form a clade (Clade 1of Fig 8) that is more closely related to chelicerates than tocrustaceans in all analyses and therefore is recognised asthe Arachnomorpha as defined by Chen et al (1996) andRamskold et al (1996) The monophyly of the Marrellomor-pha was supported in all analyses The marrellomorphs werefound to be the sister group to a Crustacea+Clade 1 group inmost MPTs but in a minority of trees formed the sister groupto Clade 1 alone According to the latter result they should beincluded within the Arachnomorpha but according to the firstexcluded from it Both of these alternatives have been sup-ported in previous studies (eg Hou amp Bergstrom 1997 Willset al 1998a respectively) In addition to the marrellomorphsthe Fuxianhuiida Bousfield 1995 and Canadaspidida havebeen placed in the Euarthropoda outside the main mandibu-late and arachnomorph clades (Hou amp Bergstrom 1997)Further study of these taxa of the crustacean or mandibulatestem group and of the marrellomorphs is necessary before the

position of the Marrellomorpha can be resolved Provisionallymarrellomorphs are excluded from the Arachnomorphafollowing the majority of analyses above and Wills et al(1994)

The Burgess Shale arthropod Burgessia was placed as thesister group to remaining arachnomorphs in all analyses OtherArachnomorpha form two major clades one consisting of thelsquotrilobite-alliedrsquo arachnomorphs analysed by Edgecombe ampRamskold (1999) here termed lsquoTrilobitomorpharsquo (defined asTrilobita and all taxa more closely related to Trilobita thanChelicerata) and the other (lsquochelicerate-alliedrsquo) clade includingchelicerates lsquogreat appendagersquo arthropods aglaspididsLemoneites Paleomerus Sidneyia and Emeraldella The latteris here termed lsquoCheliceramorpharsquo (defined as Chelicerata andall taxa more closely related to Chelicerata than Trilobita) Thetopology of the lsquotrilobite-alliedrsquo clade was similar to that foundby Edgecombe amp Ramskold (1999) but the present authorsrsquoresults are more completely resolved unambiguously support-ing the sister-group relationship between trilobites andHelmetiida suggested in the previous study Within thelsquochelicerate-alliedrsquo clade Aglaspidids Paleomerus Lemoneitesand the Burgess Shale Emeraldella and Sidneyia form a cladein opposition to megacheirans and chelicerates

The present authorsrsquo analysis supports a new hypothesis ofthe origin of the chelicerates from within a paraphyleticassemblage of megacheiran arthropods and confirms pyc-nogonids as a sister group to the euchelicerates Previoushypotheses of the chelicerate sister group (see Section 1) aremuch less parsimonious The shortest trees supporting aCheloniellon-chelicerate clade are six steps longer than theMPTs and those supporting an aglaspidid-chelicerate cladeseven steps longer All megacheirans and chelicerates areunited by the flap-shaped rounded exopods and modificationof the second cephalic segment endopods into anteriorlydirected spinose raptorial organs which share a number ofdetailed similarities A clade including all of these taxa apartfrom Fortiforceps is supported by the synapomorphic loss ofthe antennae (or antennulae of crustaceans) and the longerlength of spinose projections of the second cephalic appendageendopods Within this clade Yohoia shares the loss of cephalicexopods the loss of thoracic endopods and a postabdomen oftubular tergites with chelicerates

The poor resolution of Clade 5 (of Fig 8) was partly a resultof the poor preservation of and consequent high proportion ofmissing data for Lemoneites and Paleomerus When these two

Table 3 Numbers of most parsimonious trees supporting each of the alternative topologies for clade 4 of Fig 7 under different analyticalconditions (RCI=Revealed Consistency Index)

Aglaspididcoding

Characters 4 6 36and 43 ordered

Charactersreweighted by RCI

Paleomerus andLemoneites excluded

Topology (see Fig 8)

A B C D

1 91 X 3 3 31 X 61 X X 31 X 61 X X 3 32 9 92 X 3 3 3 32 X 62 X X 32 X 62 X X 3 3Total 6 18 12 54

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 187

taxa were excluded from the analyses only two (B and D) ofthe four topologies found in the complete analysis remainedmost parsimonious (see Fig 9 and Table 3) Secondly Bremersupport values throughout much of the lsquochelicerate-alliedrsquoclade (Clade 3 of Fig 8) were improved when these taxa wereexcluded (see Fig 11)

Dunlop (1999 p 258) suggested that in reconstructing thechelicerate stem lineage lsquowe might predict that the two mostsignificant changes towards the chelicerate condition are thereduction of the antennae and the formation of the nextappendages into a clawrsquo According to the hypothesis pre-sented here both of these adaptations were achieved in EarlyCambrian chelicerate ancestors The recognition of the loss ofthe antennae in the stem group of the chelicerates and thephylogenetic hypothesis presented here suggest that chelicer-ates are most closely related to taxa with only a singleadditional segment fused to the crown group eucheliceratehead of four fused somites Previous hypotheses have derivedchelicerates from Palaeozoic arthropods with a longer head(eg Cheloniellon Sturmer amp Bergstrom 1978 SanctacarisBriggs amp Collins 1988 Emeraldella Bruton amp Whittington1983) The plesiomorphic pattern of head segmentation ismatched in pycnogonids Bergstrom et al (1980) suggestedthat only the anterior four pairs of appendages are incorpo-rated into the head This supports the view that pycnogonidsare primitive with respect to other chelicerates which showmore or less complete fusion of this primitive head and threethoracic segments including Solpugida and Pseudoscorpiones

The present study suggests that a shift from gnathobasicbenthic feeding to a more vagrant lifestyle and raptorialappendage feeding occurred very early in the chelicerate lin-eage Appendage feeding was clearly an important adaptation

Figure 10 One of 18 equally parsimonious cladograms with lsquoAglaspidida 2rsquo coding and all characters unorderedthat is most congruent with results from other analyses Non-terminal branch lengths scaled to reflect number ofapomorphies (A) ACCTRAN apomorphy scheme Character numbers shown above boxes and states shownbelow boxes as in Table 2 and the text Character consistency indices are indicated by shading (B) Levels ofsupport for individual nodes Bremer support indices shown in bold type above branches Bootstrap percentagesin italic type below branches for all nodes with relative frequencies greater than 50

Figure 11 Bremer support values for clades within the lsquochelicerate-alliedrsquo arachnomorphs (clade 3 of Fig 8) when all taxa wereincluded (normal weight font above nodes) and with Lemoneites andPaleomerus excluded (bold type below nodes)

188 TREVOR J COTTON AND SIMON J BRADDY

for the terrestrialization of chelicerates (Dunlop 1997 p 69Dunlop amp Webster 1999) but the model proposed heresuggests that this considerably pre-dated terrestrializationGnathobasic feeding in xiphosurans and eurypterids mayrepresent a reversal to a more primitve benthic arachnomorphlifestyle from a more pycnogonid-like ancestor

32 Evolution of the arachnomorph headThe extant arthropod classes show highly conserved patternsof head segmentation (eg Wills et al 1994) In contrast therehas been a near consensus amongst Cambrian arthropodworkers (although rarely based on an explicit phylogenetichypothesis) that patterns of head segmentation are highlyhomoplastic and hence of little systematic significanceSturmer amp Bergstrom (1978 pp 78ndash9) suggested that lsquoevenclosely related forms may have different numbers of headsegments and appendagesrsquo Bruton amp Whittington (1983pp 576ndash7) that lsquodiscussion on fossil arthropod relationshipsbased on head segmentation appears to be largely irrel-evant and at best speculativersquo and Delle Cave amp Simonetta(1991 p 191) that lsquothere are no obvious phyletic affinitiesbetween genera having the same number of cephalized seg-mentsrsquo Most recently Bergstrom amp Hou (1997 p 104) con-cluded that lsquothe segmental length of the head shield seemsto be of no relevance to the discussion and discrimination ofevolutionary lineagesrsquo It has been claimed that this view issupported by cladistic analyses in which patterns of tagmosishave been found to be rather poor at defining major arthropodclades (Briggs et al 1992 Wills et al 1994)

This suggestion is not only of relevance to systematics buthas featured prominently in recent debates surrounding thenature of the lsquoCambrian explosionrsquo (see Budd amp Jensen 2000for a recent review) Gould (1989 1991) argued that theapparent plasticity of head segmentation in Cambrian com-pared to post-Cambrian arthropod evolution suggests thatbody-plan evolution was only constrained after the Cambrianexplosion According to this view the origin of new highertaxa with distinct body-plans was only possible before theonset of these constraints (eg Valentine 1995) Taxa hererecognised as arachnomorphs clearly played a major role in thedevelopment of Gouldrsquos hypothesis This suggestion has pre-viously been assessed in terms of overall morphological diver-sity which is clearly rather distinct from Gouldrsquos concept ofdisparity Wills et al (1998b) showed that there was littlecorrelation between overall morphological disparity and de-gree of limb specialisation and that the latter was phylogeneti-cally highly plastic They remarked that (Wills et al 1998bp 64) lsquothis finding has important implications for models ofarthropod phylogeny and evolution that attribute overidingimportance to head segmentationrsquo but did not explicitlyexamine head segmentation although it is likely to have beena major component of the index of tagmosis employed

The results of the present study suggest that arguments thatarthropod head segmentation was unusually labile during theCambrian are poorly founded Rather only four major pat-terns of euarthropod head segmentation are identified Theplesiomorphic euarthropod state according to both Walossekamp Muller (1997 1998) and Scholtz (1997) which may moreproperly be the plesiomorphic state for the euarthropod crowngroup only (depending on the phylogenetic position of theMarrellomorpha) consists of four post-acronal segmentsbearing the antennae and three pairs of biramous limbs Thepresent authors suggest that the term lsquocephalonrsquo be restrictedto this kind of head which is found in stem group mandibu-lates in addition to many arachnomorphs Crown groupmandibulates share a lsquobimaxillary headrsquo in which an addi-tional pair of appendages the labium or second maxillae are

incorporated into the head but in crustaceans these are notfused to the carapace (Scholtz 1997) Finally two distinctforms of head tagmosis are found in chelicerates A head withfour pairs of appendages (but without antennae giving a totalof five segments incorporated into the head) which has beencalled the lsquocephalosomarsquo is present in pycnogonids (Walossekamp Dunlop 2002 pp 434ndash5) and probably also in some euch-elicerates (J A Dunlop pers comm 2003) The lsquoprosomarsquo ofmost euchelicerates consists of this cephalosoma and twoadditional segments

In general these patterns of head segmentation are highlyphylogenetically conserved (see Fig 12) Only three homo-plastic changes namely reversal to a more primitive conditionin the Leanchoilia-Alalcomenaeus clade and the convergentorigin of a five-segmented head in Clade 3 (of Fig 8) and theXandarellida are necessary to optimise character 4 onto themost parsimonious cladogram There are also transitions toautapomorphic states in Emeraldella and Sidneyia In the caseof Sidneyia however an argument could be made (followingBergstrom amp Sturmer 1978 see discussion of Character 4) thatthe head consists of five segments as in closely related taxabut autapomorphically each segment has a separate tergiteThis would further increase the phylogenetic stability of headsegmentation Also following Bergstrom amp Sturmer (1978) thehead of Cheloniellon could be considered to consist of twotergites making it more similar to the head of Emeraldella(with six pairs of appendages)

Figure 12 Cladogram from Fig 10 showing changes in head segmen-tation Bold type shows abbreviated head segmentation formulaAbbreviations (Ant) antennae (BA) biramous appendage pairs (Ch)chelicerae (GA) great appendages (UA) uniramous appendage pairsand (X) segment without appendages Synapomorphy A is the auta-pomorphic reduction in head length to a single segment in SidneyiaAlternatively this may represent secondary division of the headshield Synapomorphy B is the increase in the length of the head inEmeraldella to six segments

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 189

4 Acknowledgements

We thank D E G Briggs J A Cotton J A Dunlop R AFortey S Harris R A Moore P Selden O E Tetlie andP Van Roy for comments on earlier drafts of this work Themanuscript was reviewed by M Wills F Schram and oneanonymous referee This research was supported by the Geo-logical Society of London Curry Research Studentshipawarded to TJC by the Department of Earth Sciences Uni-versity of Bristol and by the Department of PalaeontologyThe Natural History Museum London TJC would like tothank C Labandeira J Thompson and R Chapman fortheir hospitality at the National Museum of Natural HistoryWashington DC USA and A Olcott S Schellenberg andJ Young for making his time there so enjoyable

5 References

Abzhanov A Popadic A amp Kaufman T C 1999 Chelicerate Hoxgenes and the homology of arthropod segments Evolution andDevelopment 1 77ndash89

Akam M 2000 Arthropods developmental diversity within a (super)phylum Proceedings of the National Academy of Sciences USA97 4438ndash41

Aldridge R J Gabbott S E amp Theron J N 2001 The Soom ShaleIn Briggs D E G amp Crowther P R (eds) Palaeobiology II340ndash2 Oxford Blackwell Science

Anderson L I amp Selden P A 1997 Opisthosomal fusion andphylogeny of Palaeozoic Xiphosura Lethaia 30 19ndash31

Arango C P 2002 Morphological phylogenetics of the sea spiders(Arthropoda Pycnogonida) Organisms Diversity and Evolution2 107ndash25

Ax P 1986 Das phylogenetische System Stuttgart Gustav FischerVerlag

Babcock L E amp Zhang Wentang In press New non-mineralizedarthropods from the Chengjiang Lagerstatte Lower CambrianYunnan China Palaeontologia Cathyana

Bartels C Briggs D E G amp Brassel G 1998 The fossils ofthe Hunsruck Slate Marine life in the Devonian CambridgeCambridge University Press

Bergstrom J 1971 Paleomerus ndash merostome or merostomoid Lethaia4 393ndash401

Bergstrom J 1979 Morphology of fossil arthropods as a guideto phylogenetic relationships In Gupta A P (ed) ArthropodPhylogeny 3ndash56 New York NY Van Nostrand Reinhold Co

Bergstrom J 1992 The oldest arthropods and the origin of theCrustacea Acta Zoologica 73 287ndash91

Bergstrom J Sturmer W amp Winter G 1980 PalaeoisopusPalaeopantopodus and Palaeothea pycnogonid arthropodsfrom the Lower Devonian Hunsruck Slate West GermanyPalaontologische Zeitschrift 54 7ndash54

Bergstrom J amp Brassel G 1984 Legs in the trilobite Rhenops fromthe Lower Devonian Hunsruck Slate Lethaia 17 67ndash72

Bergstrom J amp Hou X 1998 Chengjiang arthropods and theirbearing on early arthropod evolution In Edgecombe G D(ed) Arthropod fossils and phylogeny 151ndash84 New York NYColumbia University Press

Boudreaux H B 1979 Significance of intersegmental tendon systemin arthropod phylogeny and a monophyletic classification ofArthropoda In Gupta A P (ed) Arthropod Phylogeny 551ndash86New York NY Van Nostrand Reinhold Co

Bousfield E L 1995 A contribution to the natural classification ofLower and Middle Cambrian arthropods food-gathering andfeeding mechanisms Amphipacifica 2 3ndash33

Braddy S J Aldridge R J Gabbott S E amp Theron J N 1999Lamellate book-gills in a late Ordovician eurypterid from theSoom Shale South Africa support for a eurypterid-scorpionclade Lethaia 32 72ndash4

Braddy S J amp Almond J E 1999 Eurypterid trackways from theTable Mountain Group (Ordovician) of South Africa Journal ofAfrican Earth Sciences 29 165ndash77

Bremer K 1988 The limits of amino acid sequence data in angio-sperm phylogenetic reconstruction Evolution 42 795ndash803

Bremer K 1994 Branch support and tree stability Cladistics 10295ndash304

Briggs D E G 1998 Review of Arthropods from the LowerCambrian Chengjiang fauna southwest China by HouXianguang and Jan Bergstrom Fossils and Strata 45

Scandinavian University Press Oslo Norway 1997 Palaeo-geography Palaeoclimatology Palaeoecology 143 193ndash4

Briggs D E G Bruton D L amp Whittington H B 1979 Append-ages of the arthropod Aglaspis spinifer (Upper CambrianWisconsin) and their significance Palaeontology 22 167ndash80

Briggs D E G Fortey R A amp Wills M A 1992 Morphologicaldisparity in the Cambrian Science 256 1670ndash3

Briggs D E G Fortey R A amp Wills MA 1993 How big was theCambrian explosion A taxonomic and morphologic comparisonof Cambrian and Recent arthropods In Lees D R amp EdwardsD (eds) Evolutionary patterns and processes 33ndash44 LondonLinnean Society of London Linnean Society Symposium Series

Briggs D E G Erwin D H amp Collier F J 1994 The fossils of theBurgess Shale Washington DC Smithsonian Institution Press

Briggs D E G amp Bartels C 2001 New arthropods from the LowerDevonian Hunsruck Slate (Lower Emsian Rhenish Massifwestern Germany) Palaeontology 44 275ndash304

Briggs D E G amp Bartels C 2002 Magnoculocaris a new name forthe arthropod Magnoculus Briggs and Bartels 2001 from theLower Devonian Hunsruck Slate Germany Palaeontology 45419

Briggs D E G amp Collins D 1988 A Middle Cambrian cheliceratefrom Mount Stephen British Columbia Palaeontology 31 779ndash98

Briggs D E G amp Collins D 1999 The arthropod Alalcomenaeuscambricus Simonetta from the Middle Cambrian Burgess Shale ofBritish Columbia Paleontology 42 953ndash78

Briggs D E G amp Fortey R A 1982 The cuticle of aglaspididarthropods a red-herring in the early history of vertebratesLethaia 15 25ndash9

Briggs D E G amp Fortey R A 1989 The early radiation andrelationships of the major arthropod groups Science 246 241ndash3

Briggs D E G amp Fortey R A 1992 The early Cambrian radiationof arthropods In Lipps J H amp Signor P W (eds) Origin andEarly Evolution of the Metazoa 335ndash373 New York NY PlenumPress

Briggs D E G amp Robison R A 1984 Exceptionally preservednontrilobite arthropods and Anomalocaris from the MiddleCambrian of Utah University of Kansas PaleontologicalContributions Paper 111 1ndash23

Broili F 1932 Ein neuer Crustacee aus dem rheinischen UnterdevonSitzungsberichte der Mathematisch-Naturwissenschaftlichen(Abteilung) Klasse der Bayerischen Akademie der Wissenschaftenzu Munchen 1932 27ndash38

Broili F 1933 Ein zweites Exemplar von Cheloniellon Sitzungsber-ichte der Bayerischen Akademie der Wissenschaften 1933 11ndash32

Bruton D L 1981 The arthropod Sidneyia inexpectans MiddleCambrian Burgess Shale British Columbia Philosophical Trans-actions of the Royal Society of London Series B 295 619ndash56

Bruton D L amp Whittington H B 1983 Emeraldella and Leanchoiliatwo arthropods from the Burgess Shale British Columbia Philo-sophical Transactions of the Royal Society of London Series B 300553ndash85

Budd G E 1995 Kleptothule rasmusseni gen et sp nov anolenellid-like trilobite from the Sirius Passet fauna (BuenFormation Lower Cambrian North Greenland) Transactions ofthe Royal Society of Edinburgh Earth Sciences 86 1ndash12

Budd G E 1996a Progress and problems in arthropod phylogenyTrends in Ecology and Evolution 11 356ndash8

Budd G E 1996b The morphology of Opabinia regalis and thereconstruction of the arthropod stem-group Lethaia 29 1ndash14

Budd G E 1997 Stem-group arthropods from the Lower CambrianSirius Passet fauna of North Greenland In Fortey R A ampThomas R H (eds) Arthropod Relationships 125ndash38 LondonChapman and Hall Systematics Association Special Volume 55

Budd G E 1999a A nektaspid arthropod from the Early CambrianSirius Passet fauna with a description of retrodeformation basedon functional morphology Palaeontology 42 99ndash122

Budd G E 1999b The morphology and phylogenetic significance ofKerygmachela kiergkegaardi Budd (Buen Formation LowerCambrian N Greenland) Transactions of the Royal Society ofEdinburgh Earth Sciences 89 (for 1998) 249ndash290

Budd G E 2002 A palaeontological solution to the arthropod headproblem Nature 417 271ndash5

Budd G E amp Jensen S 2000 A critical reappraisal of the fossilrecord of the bilaterian phyla Biological Reviews 75 253ndash95

Chen J Zhou G Zhu M amp Yeh K 1996 The Chengjiang biota ndasha unique window of the Cambrian explosion Taichung NationalMuseum of Natural Science [In Chinese]

Chen J Edgecombe G D amp Ramskold L 1997 Morphological andecological disparity in naraoiids (Arthropoda) from the Early

190 TREVOR J COTTON AND SIMON J BRADDY

Cambrian Chengjiang fauna China Records of the AustralianMuseum 49 1ndash24

Chen J amp Zhou G 1997 Biology of the Chengjiang fauna In ChenJ Cheng Y amp Iten H V (eds) The Cambrian explosion and thefossil record Bulletin of the National Museum of Natural ScienceTaiwan 10 11ndash106

Child C A 1997 Some deep-sea Pycnogonida from Argentine slopeand basin Proceedings of the Biological Society of Washington110 128ndash42

Chlupac I 1988 The enigmatic arthropod Duslia from the Ordovicianof Czechslovakia Palaeontology 31 611ndash20

Chlupac I 1995 Lower Cambrian arthropods from the PasekyShale (Barrandian area Czech Republic) Journal of the CzechGeological Society 40 9ndash36

Chlupac I amp Havlıcek V 1965 Kodymirus n g a new aglaspidmerostome of the Cambrian of Bohemia Sbornık GeologickechVid Paleontologie 6 7ndash20

Cisne J L 1975 Anatomy of Triarthrus and the relationships of theTrilobita Fossils and Strata 4 45ndash63

Cisne J L 1981 Triarthrus eatoni (Trilobita) anatomy of its exo-skeletal skeletomuscular and digestive systems Palaeonto-graphica Americana 9 95ndash142

Conway Morris S 1982 Atlas of the Burgess Shale London ThePalaeontological Association

Conway Morris S 1989 The persistence of Burgess Shale-type faunasimplications for the evolution of deeper-water faunas Trans-actions of the Royal Society of Edinburgh Earth Sciences 80271ndash83

Conway Morris S 1998 The crucible of creation Oxford OxfordUniversity Press

Cotton T J 1999 Review of Arthropod fossils and phylogenyPalaeogeography Palaeoclimatology Palaeoecology 151 341ndash3

Cotton T J 2002 The phylogeny and morphological evolution ofCambrian trilobites and their relatives Unpublished PhD ThesisUniversity of Bristol

Damen W G M Hausdorf M Seyfarth E A amp Tautz D 1998 Aconserved mode of head segmentation in arthropods revealed bythe expression pattern of Hox genes in a spider Proceedings of theNational Academy of Sciences USA 95 10665ndash70

Delle Cave L amp Simonetta A M 1991 Early Palaeozoic arthropodsand problems of arthropod phylogeny with some notes on taxa ofdoubtful affinities In Simonetta A M amp Conway Morris S(eds) The early evolution of Metazoa and the significance ofproblematic taxa 189ndash244 Cambridge Cambridge UniversityPress

Dewel R A amp Dewel W C 1997 The place of tardigrades inarthropod evolution In Fortey R A amp Thomas R H (eds)Arthropod Relationships 109ndash23 London Chapman and HallSystematics Association Special Volume 55

Dunlop J A 1997 Palaeozoic arachnids and their significance forarachnid phylogeny Proceedings of the 16th European Colloquiumof Arachnology 65ndash82 Siedlce Poland

Dunlop J A 1998 The origins of tetrapulmonate booklungs and theirsignificance for chelicerate phylogeny In Selden P A (ed)Proceedings of the 17th European Colloquium of ArachnologyEdinburgh 1997 pp 9ndash16

Dunlop J A 1999 Pasando revista a la evolucion de los quelicerados[A review of chelicerate evolution] In Melic A DeHaro J JMedez M amp Ribera I (eds) Volumen Monografico Evolucion yFilogenia de Arthropoda Boletin de la Sociedad EntolomologicaAragonesa 26 255ndash272 [In Spanish and English]

Dunlop J A amp Selden P A 1997 The early history and phylogenyof the chelicerates In Fortey R A amp Thomas R H (eds)Arthropod relationships Systematics Association Special Volume55 221ndash35 London Chapman and Hall

Dunlop J A amp Webster M 1999 Fossil evidence terrestrializationand arachnid phylogeny Journal of Arachnology 27 86ndash93

Dzik J 1993 Early metazoan evolution and the meaning of its fossilrecord Evolutionary Biology 27 339ndash86

Dzik J amp Lendzion K 1988 The oldest arthropods of the EastEuropean Platform Lethaia 21 29ndash38

Edgecombe G D Wilson G D F Colgan D J Gray M R ampCassis G 2000 Arthropod cladistics combined analysisof Histone H3 and U2 snRNA sequences and morphologyCladistics 16 155ndash203

Edgecombe G D amp Ramskold L 1999 Relationships of CambrianArachnata and the systematic position of Trilobita Journal ofPaleontology 73 263ndash87

Emerson M J amp Schram F R 1997 Theories patterns and realitygame plan for arthropod phylogeny In Fortey R A amp Thomas

R H (eds) Arthropod Relationships Systematics AssociationSpecial Volume 55 67ndash87 London Chapman and Hall

Farris J S 1989 The Retention Index and the Rescaled ConsistencyIndex Cladistics 5 417ndash19

Felsenstein J 1985 Confidence limits on phylogenies an approachusing the bootstrap Evolution 39 783ndash91

Flower R H 1968 Merostomes from a Cotter Horizon of the El PasoGroup New Mexico Bureau of Mines and Mineral ResourcesMemoir 22 35ndash44

Fortey R A 1990a Trilobite evolution and systematics In MikulicD G (ed) Arthropod paleobiology 44ndash65 Short Courses inPaleontology 3 Knoxville Tenessee The Paleontological Society

Fortey R A 1990b Ontogeny hypostome attachment and trilobiteclassification Palaeontology 33 529ndash76

Fortey R A 1997 Classification In Kaesler R L (ed) Treatise oninvertebrate paleontology Part O Arthropoda 1 (revised) 289ndash302Boulder Colorado amp Lawrence Kansas Geological Society ofAmerica and University of Kansas Press

Fortey R A amp Theron J N 1994 A new Ordovician arthropodSoomaspis and the agnostid problem Palaeontology 37 841ndash61

Fortey R A amp Whittington H B 1989 The Trilobita as a naturalgroup Historical Biology 2 125ndash38

Fry W G 1978 A classification within the pycnogonids ZoologicalJournal of the Linnean Society of London 63 35ndash58

Giribet G Edgecombe G D amp Wheeler W C 2001 Arthropodphylogeny based on eight molecular loci and morphology Nature413 157ndash61

Gould S J 1989 Wonderful life The Burgess Shale and the nature ofhistory New York NY Norton

Gould S J 1991 The disparity of the Burgess Shale arthropod faunaand the limits of cladistic analysis why we must strive to quantifymorphospace Paleobiology 17 411ndash23

Gurich G 1931 Mimetaster hexagonalis ein neuer Kruster aus demunterdevonischen Bundenbacher Dachschiefer PalaontologischeZeitschrift 12 204ndash38

Hammann W Laske R amp Pillola G L 1990 Tariccoia arrusensisngnsp an unusual trilobite-like arthropod Rediscovery of thelsquophylocaridrsquo beds of Taricco (1922) in the Ordovician lsquoPuddingarsquosequence of Sardinia Bolletino della Societa PaleontologicaItaliana 29 163ndash78

Haas M S Brown S J amp Beeman R W 2001 Pondering theprocephalon the segmental origin of the labrum DevelopmentGenes and Evolution 211 89ndash95

Heider K 1913 Entwicklungsgeschichte und Morphologie derWirbellosen Die Kultur der Gegenwart Theil 3 Abt 4 2 176ndash332

Henriksen K 1928 Critical notes upon Cambrian arthropodsdescribed by Charles D Walcott Videnskabelige Meddelelser fraDansk Naturhistorisk Forening 86 1ndash20

Hesselbo S P 1988 Trace fossils of Cambrian aglaspidid arthropodsLethaia 21 139ndash46

Hesselbo S P 1989 The aglaspidid arthropod Beckwithia from theCambrian of Utah and Wisconsin Journal of Paleontology 63636ndash42

Hesselbo S P 1992 Aglaspidida (Arthropoda) from the UpperCambrian of Wisconsin Journal of Paleontology 66 885ndash923

Hou X 1987a Three new large arthropods from Lower CambrianChengjiang fauna eastern Yunnan Acta Palaeontologica Sinica26 272ndash85 [In Chinese with English summary]

Hou X 1987b Two new arthropods from Lower CambrianChengjiang eastern Yunnan Acta Palaeontologica Sinica 26236ndash56 [In Chinese with English summary]

Hou X Chen J amp Lu H 1989 Early Cambrian new arthropodsfrom Chengjiang Yunnan Acta Palaeontologica Sinica 28 42ndash57[In Chinese with English summary]

Hou X Ramskold L amp Bergstrom J 1991 Composition andpreservation of the Chengjiang fauna a Lower Cambrian soft-bodied biota Zoologica Scripta 20 395ndash411

Hou X Bergstrom J amp Ahlberg P 1995 Anomalocaris and otherlarge animals in the Lower Cambrian Chengjiang fauna ofsouthwest China GFF 117 163ndash83

Hou X amp Bergstrom J 1997 Arthropods of the Lower CambrianChengjiang fauna southwest China Fossils and Strata 45 1ndash116

Hughes C P 1975 Redescription of Burgessia bella from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 4415ndash36

Ivantsov A Y 1999 Trilobite-like arthropod from the LowerCambrian of the Siberian Platform Acta PalaeontologicaPolonica 44 455ndash66

King P E 1973 Pycnogonids London St Martin Press

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 191

Lauterbach K E 1973 Schlussereignisse in der Evolution derStammgruppe der Euarthropoda Zoologische Beitrage 19259ndash99

Lauterbach K E 1980 Schlussereignisse in der Evolution derGrundplans der Arachnata (Arthropoda) Abhandlungen desNaturwissenschaftlichen Vereins in Hamburg 23 163ndash327

Lauterbach K E 1983 Synapomorphien zwischen Trilobiten- undCheliceraten-zweig der Arachnata Zoologischer Anzeiger 210213ndash38

Lendzion K 1975 Fauna of the Mobergella Zone in the Polish LowerCambrian Kwartalnik Geologiczny 19 237ndash42

Lieberman B S 1998 Cladistic analysis of the Early Cambrianolenelloid trilobites Journal of Paleontology 72 59ndash78

Lu Y 1940 On the ontogeny and phylogeny of Redlichia intermediaLu (sp nov) Bulletin of the Geological Society of China 20333ndash42 [In Chinese]

Maddison W P amp Maddison D R 1997 MacClade analysis ofphylogeny and character evolution Version 307 Sunderland MASinauer Associates

Mittmann B amp Scholtz G 2003 Development of the nervous systemin the lsquoheadrsquo of Limulus polyphemus (Chelicerata Xiphosura)morphological evidence for the correspondence between the seg-ments of the chelicerae and the (first) antennae of MandibulataDevelopment Genes and Evolution 213 9ndash17

Moore R C (ed) 1959 Treatise on invertebrate paleontology Part OArthropoda 1 Boulder Colorado Geological Society of America

Muller K J amp Walossek D 1986 Arthropod larvae from the UpperCambrian of Sweden Transactions of the Royal Society ofEdinburgh Earth Sciences 77 157ndash79

Muller K J amp Walossek D 1987 Morphology ontogeny and lifehabit of Agnostus pisiformis from the Upper Cambrian of SwedenFossils and Strata 19 1ndash124

Munilla T 1999 Evolucion y filogenia de los Picnogonidos InMelicA DeHaro J J Medez M amp Ribera I (eds) VolumenMonografico Evolucion y Filogenia de Arthropoda Boletin de laSociedad Entolomologica Aragonesa 26 273ndash91 [In Spanish andEnglish]

Orr P J Siveter D J Briggs D E G Siveter D J amp Sutton M D2000 A new arthropod from the Silurian Konservat-Lagerstatteof Herefordshire Proceedings of the Royal Society of LondonSeries B 267 1497ndash504

Page R D M 1996 On consensus confidence and lsquototal evidencersquoCladistics 12 83ndash92

Palmer A R amp Repina L N 1993 Through a glass darklytaxonomy phylogeny and biostratigraphy of the OlenellinaUniversity of Kansas Paleontological Contributions New Series 31ndash35

Platnick N I Griswold C E amp Coddington J A 1991 On missingentries in cladistic analysis Cladistics 7 337ndash43

Pleijel F 1995 On character coding for phylogeny reconstructionCladistics 11 309ndash15

Raasch G O 1939 Cambrian Merostomata Special Papers of theGeological Society of America 19 1ndash146

Ramskold L Chen J Edgecombe G D amp Zhou G 1996Preservational folds simulating tergite junctions in tegopeltid andnaraoiid arthropods Lethaia 29 15ndash20

Ramskold L Chen J Edgecombe G D amp Zhou G 1997Cindarella and the arachnate clade Xandarellida (ArthropodaEarly Cambrian) from China Transactions of the Royal Society ofEdinburgh Earth Sciences 88 19ndash38

Ramskold L amp Edgecombe G D 1991 Trilobite monophylyrevisited Historical Biology 4 267ndash83

Ramskold L amp Edgecombe G D 1996 Trilobite appendagestructure ndash Eoredlichia revisited Alcheringa 20 269ndash76

Raw F 1957 Origin of chelicerates Journal of Paleontology 31139ndash193

Repina L N amp Okuneva O G 1969 Cambrian arthropods of themaritime territory Paleontological Journal 1969 95ndash103

Richter R amp Richter E 1929 Weinbergina opitz ng n sp einSchwerttrager (Merost Xiphos) aus dem Devon (Rheinland)Senkenbergiana 11 193ndash209

Robison R A 1984 New occurences of the unusual trilobite Naraoiafrom the Cambrian of Idaho and Utah University of KansasPaleontological Contributions Paper 112 1ndash8

Rominger C 1887 Description of primordial fossils from MtStephens NW Territory of Canada Proceedings of the Academyof Natural Sciences of Philadelphia 1887 12ndash19

Scholtz G 1997 Cleavage germ band formation and head segmen-tation the ground pattern of the Euarthropoda In Fortey R Aamp Thomas R H (eds) Arthropod Relationships 317ndash32 LondonChapman and Hall Systematics Association Special Volume 55

Schram F R 1978 Arthropods a convergent phenomenonFieldiana Geology 39 61ndash108

Schram F R 1993 Review of Boxshall Stromberg and Dahl TheCrustacea Origin and Evolution Journal of Crustacean Biology13 820ndash2

Schram F R Vonk R amp Hof C H J 1997 Mazon CreekCycloidea Journal of Paleontology 71 261ndash84

Schram F R amp Emerson M J 1991 Arthropod Pattern Theorya new approach to arthropod phylogeny Memoirs of theQueensland Museum 31 1ndash18

Schram F R amp Hof C H J 1998 Fossils and the interrelationshipsof major crustacean groups In Edgecombe G D (ed) Arthropodfossils and phylogeny pp 233ndash302 New York NY ColumbiaUniversity Press

Shergold J H 1991 Protaspis and early meraspis growth stages of theeodiscoid trilobite Pagetia ocellata Jell and their implications forclassification Alcheringa 15 65ndash86

Shu D Geyer G Ling C amp Zhang Z 1995 Redlichiacean trilobiteswith preserved soft parts from the Lower Cambrian Chengjiangfauna (South China) Beringeria (Special Issue) 2 203ndash41

Shultz J W 1990 Evolutionary morphology and phylogeny ofArachnida Cladistics 6 1ndash38

Shultz J W 2000 Skeletomuscular anatomy of the harvestmanLeiobunum aldrichi (Weed 1893) (Arachnida OpilionesPalpatores) and its evolutionary significance Zoological Journalof the Linnean Society 128 401ndash38

Shultz J W 2001 Gross muscular anatomy of Limulus polyphemus(Xiphosura Chelicerata) and its bearing on evolution in the theArachnida Journal of Arachnology 29 283ndash303

Siewing R (ed) 1985 Lehrbuch der Zoologie II Systematik 3rd ednStuttgart Fischer

Simonetta A M 1970 Studies on the non trilobite arthropods ofthe Burgess Shale (Middle Cambrian) the genera LeanchoiliaAlalcomenaeus Opabinia Burgessia Yohoia and ActaeusPalaeontographica Italica 66 35ndash45

Simonetta A M amp Delle Cave L 1975 The Cambrian non trilobitearthropods from the Burgess Shale of British Columbia A studyof their comparative morphology taxonomy and evolutionarysignificance Palaeontographica Italica 69 1ndash37

Simonetta A M amp Delle Cave L 1981 An essay in the compara-tive and evolutionary morphology of Palaeozoic ArthropodsAccademia Nazionale dei Lincei Atti dei Convegni Lincei 49389ndash439

Starobogatov Y I 1990 Sistema i filogeniya nizshikh khelitserovykh(analiz morfologii paleozoysikh grupp) [The systematics andphylogeny of the lower chelicerates (a morphological analysis ofthe Palaeozoic groups)] Paleontologicheskii Zhurnal 1990 4ndash17

Stoslashrmer L 1944 On the relationships and phylogeny of fossil andrecent Arachnomorpha Skrifter utgitt ac Det Norske Videnskaps-Akademi i Olso I Matematisk-naturvitenskaplig Klasse 5 1ndash158

Stoslashrmer L 1951 Studies on trilobite morphology Part III Theventral cephalic structures with remarks on the zoologicalposition of the trilobites Norsk geologisk tidsskrift 29 108ndash58

Stoslashrmer L 1956 A Lower Cambrian merostome from Sweden Arkivfor Zoologi Ser 2 9 507ndash14

Stoslashrmer L 1959 Trilobitoidea In Moore R C (ed) Treatise onInvertebrate Paleontology part O Arthropoda 1 23ndash37 BoulderColorado Geological Society of America

Strong E E amp Lipscomb D 1999 Character coding and inapplicabledata Cladistics 15 363ndash71

Sturmer W amp Bergstrom J 1976 The arthropods Mimetaster andVachonisia from the Devonian Hunsruck Shale PalaontologischeZeitschrift 50 78ndash111

Sturmer W amp Bergstrom J 1978 The arthropod Cheloniellon fromthe Devonian Hunsruck Shale Palaontologische Zeitschrift 5257ndash81

Sturmer W amp Bergstrom J 1981 Weinbergina a xiphosuran fromthe Devonian Hunsruck Slate Palaontologische Zeitschrift 55237ndash55

Sutton M D Briggs D E G Siveter D J Siveter D J amp Orr PJ 2002 The arthropod Offacolus kingi (Chelicerata) from theSilurian of Herefordshire England computer based morphologi-cal reconstructions and phylogenetic affinity Proceedings of theRoyal Society of London Series B 269 1195ndash1203

Swofford D L 1999 PAUP Phylogenetic Analysis Using Parsimony(and other methods) Version 40b2a Sunderland MA SinauerAssociates

Telford M J amp Thomas R H 1998 Expression of homeobox genesshows chelicerate arthropods retain their deutocerebral segmentProceedings of the National Academy of Sciences USA 9510671ndash5

192 TREVOR J COTTON AND SIMON J BRADDY

Thorley J L amp Page R D M 2000 RadCon phylogenetic treecomparison and consensus Bioinformatics 16 486ndash7

Valentine J W 1995 Why no new phyla after the Cambrian ndashgenome and ecospace hypotheses revisited Palaios 10 190ndash194

Wagele J W 1993 Rejection of the lsquoUniramiarsquo hypothesis andimplications of the Mandibulata concept Zoologische JahrbucherAbteilung fur Systematik Geographie und Biologie der Tiere 120253ndash88

Walcott C D 1911 Middle Cambrian Merostomata Cambriangeology and palaeontology II Smithsonian MiscellaneousCollections 57 17ndash40

Walcott C D 1912 Middle Cambrian Branchiopoda MalacostracaTrilobita and Merostomata Cambrian geology and palaeontol-ogy II Smithsonian Miscellaneous Collections 57 145ndash228

Walcott C D 1918 Geological explorations in the CanadianRockies Explorations and fieldwork of the SmithsonianInstitution in 1917 Smithsonian Miscellaneous Collections 684ndash20

Walossek D 1993 The Upper Cambrian Rehbachiella kinnekullensisand the phylogeny of Branchiopoda and Crustacea Fossils andStrata 32 1ndash202

Walossek D amp Dunlop J A 2002 A larval sea spider (ArthropodaPycnogonida) from the Upper Cambrian lsquoOrstenrsquo of Swedenand the phylogenetic position of pycnogonids Palaeontology 45421ndash46

Walossek D amp Muller K J 1990 Upper Cambrian stem-lineagecrustaceans and their bearing upon the monophyletic origin ofCrustacea and the position of Agnostus Lethaia 23 409ndash27

Walossek D amp Muller K J 1997 Cambrian lsquoOrstenrsquo-type arthro-pods and the phylogeny of Crustacea In Fortey R A amp ThomasR H (eds) Arthropod Relationships 139ndash53 SystematicsAssociation Special Volume 55 London Chapman and Hall

Walossek D amp Muller K J 1998 Early arthropod phylogeny in lightof the Cambrian lsquoOrstenrsquo fossils In Edgecombe G D (ed)Arthropod fossils and phylogeny 185ndash231 New York NYColumbia University Press

Weygoldt P 1979 Significance of later embryonic stages and headdevelopment in arthropod phylogeny In Gupta A P (ed)Arthropod Phylogeny 107ndash35 New York NY Van NostrandReinhold Co

Weygoldt P 1998 Evolution and systematics of the ChelicerataExperimental and Applied Acarology 22 63ndash79

Weygoldt P amp Paulus H F 1979 Unterzuchungen aur Morpholo-gie Taxonomie und Phylogenie der Chelicerata Zeitschriftfur Zoologische Systematik und Evolutionsforschung 17 85ndash116177ndash200

Wheeler W C Cartwright P amp Hayashi C Y 1993 Arthropodphylogeny a combined approach Cladistics 9 1ndash39

Wheeler W C amp Hayashi C Y 1998 The phylogeny of extantchelicerate orders Cladistics 14 173ndash92

Whittington H B 1971 Redescription of Marrella splendens(Trilobitoidea) from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 209 1ndash24

Whittington H B 1974 Yohoia Walcott and Plenocaris n genarthropods from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 231 1ndash21

Whittington H B 1975a Trilobites with appendages from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 497ndash136

Whittington H B 1975b The enigmatic animal Opabinia regalisMiddle Cambrian Burgess Shale British Columbia PhilosophicalTransactions of the Royal Society of London Series B 271 1ndash43

Whittington H B 1977 The Middle Cambrian trilobite NaraoiaBurgess Shale British Columbia Philosophical Transactions of theRoyal Society Series B 280 409ndash43

Whittington H B 1979 Early arthropods their appendages andrelationships In House M R (ed) The origin of majorinvertebrate groups 253ndash68 Systematics Association SpecialVolume 12 New York NY Academic Press

Whittington H B 1980 Exoskeleton moult stage appendage mor-phology and habits of the Middle Cambrian trilobite Olenoidesserratus Palaeontology 23 171ndash204

Whittington H B 1981 Rare arthropods from the Burgess ShaleMiddle Cambrian British Columbia Philosophical Transactionsof the Royal Society of London Series B 292 329ndash57

Whittington H B 1985 Tegopelte gigas a second soft-bodied trilo-bite from the Burgess Shale Middle Cambrian British ColumbiaJournal of Palaeontology 23 171ndash204

Whittington H B 1989 Olenelloid trilobites type species functionalmorphology and higher classification Philosophical Transactionsof the Royal Society of London Series B 324 111ndash47

Whittington H B amp Almond J E 1987 Appendages and habits ofthe Upper Ordovician trilobite Triarthus eatoni PhilosophicalTransactions of the Royal Society of London Series B 317 1ndash46

Wilkinson M 1992 Ordered versus unordered characters Cladistics8 375ndash85

Wilkinson M 1994 Common cladistic information and its consensusrepresentation reduced Adams and reduced cladistic consensustrees and profiles Systematic Biology 43 343ndash68

Wilkinson M 1995a Coping with abundant missing entries in phylo-genetic inference using parsimony Systematic Biology 44 501ndash14

Wilkinson M 1995b TAXEQ2 software and documentation BristolSchool of Biological Sciences University of Bristol

Wilkinson M 1996 On the distribution of homoplasy and choosingamong trees Taxon 45 263ndash66

Wills M A 1997 A phylogeny of Recent and fossil Crustacea derivedfrom morphological characters In Fortey R A amp ThomasR H (eds) Arthropod Relationships 189ndash209 SystematicsAssociation Special Volume 55 London Chapman and Hall

Wills M A Briggs D E G amp Fortey R A 1994 Disparity asan evolutionary index a comparison of Cambrian and Recentarthropods Paleobiology 20 93ndash130

Wills M A Briggs D E G Fortey R A amp Wilkinson M 1995The significance of fossils in understanding arthropod evolutionVerhandlungen der deutschen zoologischen Gesselschaft 88 203ndash15

Wills M A Briggs D E G Fortey R A Wilkinson M amp SneathP H A 1998a An arthropod phylogeny based on fossil andrecent taxa In Edgecombe G D (ed) Arthropod fossils andphylogeny 33ndash105 New York NY Columbia University Press

Wills M A Briggs D E G amp Fortey R A 1998b Evolutionarycorrelates of arthropod tagmosis scrambled legs In ForteyR A amp Thomas R H (eds) Arthropod Relationships 57ndash65Systematics Association Special Volume 55 London Chapmanand Hall

Winter G 1980 Beitrage zur Morphologie und Embryologie desvorderen Korperabschnitts (Cephalosoma) der PantopodaGerstaecker 1863 I Entstehung und Struktur des Zentralnerven-systems Zeitschrift fur Zoologische Systematik und Evolutionsfor-schung 18 27ndash61

Zhang W amp Hou X 1985 Preliminary notes on the occurrence of theunusual trilobite Naraoia in Asia Acta Palaeontologica Sinica 24591ndash5 [In Chinese with English summary]

TREVOR J COTTON Department of Palaeontology The Natural History Museum CromwellRoad London SW7 5BD UKe-mail trevor_cottonhotmailcom

SIMON J BRADDY Department of Earth Sciences University of Bristol Wills MemorialBuilding Queenrsquos Road Bristol BS8 1RJ UKe-mail S J Braddybrisacuk

MS received 28 July 2003 Accepted for publication 15 March 2004

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 193

Page 7: The phylogeny of arachnomorph arthropods and the origin of the

The ancestral euarthropod is here considered to have possesseda head with antennae only and a series of post-cephalicbiramous limbs with gnathobases The hypothesised pattern ofthe evolution of plesiomorphic euarthropod characters isshown in Figure 4 The use of a hypothetical outgroup issomewhat unsatisfactory but only affects the relationshipbetween the major clades (Arachnomorpha Crustacea andMarrellomorpha) the topology within the arachnomorphs wasunaffected by the use of this outgroup since identical resultswere obtained with unrooted analyses or by rooting withCrustacea Marrella or both

24 Characters and codingMost previous cladistic analyses with the notable exception ofEdgecombe amp Ramskold (1999) have included only briefdiscussions of the primary hypotheses of homology involved incharacter construction and coding Therefore a full discussionof most characters is provided here Descriptions of charactersand character states below are arranged by organ system orbody region in approximate anterior to posterior order Thedistribution of character states across all taxa is shown in thedata matrix (Table 2)

The terminology of morphological features in arachnomor-phs is often confused Terminology developed for cheliceratestrilobites and crustaceans has variously been applied to taxaincluded in this study so some attempt is made to clarifyterminology herein Where appropriate the present authorsrsquoterminology follows Edgecombe et al (2000) and Wheeleret al (1993) but following Scholtz (1997) the protocerebrallsquosegmentrsquo is considered to be acronal and consequently thedeutocerebral segment to be the first true segment Thereforethe antennae (or antennulae of crustaceans) belong to the firstcephalic segment

The present authors have attempted to include as completea set of characters as possible However characters requiringhypotheses of homology between podomeres (eg the number

of podomeres in endopods of thoracic appendages Willset al 1998a character 51) were not considered followingEdgecombe et al (2000 p 157) Secondly some characters usedby Bergstrom (eg Hou amp Bergstrom 1997 p 109) suchas appendage posture and mode of feeding were excludedfollowing Edgecombe amp Ramskold (1999) Finally somecharacters of the ventral surface of the head were not coded asdiscussed below and illustrated in Figure 7 The present authorsdo not include all potential synapomorphies for the Cheli-cerata as the status of many of these is debated (see eg Shultz1990 2001 Dunlop amp Selden 1997 Weygoldt 1998 Wheeler ampHayashi 1998 Dunlop 1999 Edgecombe et al 2000)

Two methods for the coding of inapplicable characters inphylogenetic analysis have been used First inapplicable char-acter states can be coded as missing data A complex structuremay comprise characters lsquoabsentpresentrsquo and lsquostate1state2rsquowith taxa lacking the structure coded as absent for the firstcharacter and as missing data for the second Some authorshave regarded this method as problematic because it may leadto reconstruction of impossible ancestral states and henceunjustified trees (Platnick et al 1991 Strong amp Lipscomb1999) The alternative is to code the second character as a thirdlsquonot applicablersquo state in taxa that lack the structure Thesemethods are equivalent to Pleijelrsquos (1995) coding methods Cand B respectively In the present study inapplicable charac-ters are treated as missing data for most analyses becausecoding them as a distinct character state reduces characterindependence and effectively weights the inapplicable charac-ter and hence could result in them dominating the analysisThe effects of this assumption were investigated by using adistinct character state in some analyses (see Section 26) andinapplicable characters are shown as distinct to lsquotruersquo missingdata (using the symbol lsquo-rsquo) in the matrix (Table 2)

241 Anterior cephalic appendages and head segmentation1 Appendages of the first segment (antennae) (0) present and(1) absent The majority of taxa considered in the present study

Figure 4 Reconstruction of the euarthropod stem group used to inform coding of hypothetical outgroup Grosstopology largely follows Budd (1996b fig 9 1997 fig 1110) Solid boxes indicate unambiguous apomorphiesand shaded boxes character states which are plesiomorphic for the Arthropoda

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 175

possess a single pair of long uniramous multi- and annulatedantennae at the anterior of the head which presumably had asensory function as found in Crustacea Myriapoda and mostmembers of Hexapoda (ie proturans do not have antennae)These appendages are likely to be a synapomorphy of theEuarthropoda (eg Scholtz 1997 Walossek amp Muller 1997)and therefore the outgroup is coded as State 0 The anterior-most head appendages of most Cambrian megacheiran arthro-pods which have been called lsquogreat appendagesrsquo followingWalcott (1912) consist of a small number of robust spinosepodomeres Uniquely Fortiforceps has a pair of short anten-nae anterior to the lsquogreat appendagesrsquo (Hou amp Bergstrom 1997pp 34ndash8) Therefore it appears likely that the lsquogreat append-agesrsquo are the appendages of the second cephalic segment andthe antennae are lost in megacheirans other than FortiforcepsThis of course depends on recognising the lsquogreat appendagesrsquoas homologous in all of these taxa as discussed below(Character 2) Homology of the megacheiran anterior append-ages with the second cephalic appendages of trilobites waspreviously suggested by Stoslashrmer (1944 p 124) on the basis oftheir post-oral position

The classical view of chelicerate head segmentation main-tains that they too have lost the antennae and the cheliceraeare the appendages of the second cephalic segment Recentrevisions of arthropod phylogeny have uncritically acceptedthis homology (Edgecombe et al 2000 Giribet et al 2001)which is based largely on neuroanatomy The lsquochelicero-neuromerrsquo which innervates the chelicerae seems to be ho-mologous with the tritocerebrum associated with the secondcephalic appendage pair of mandibulates (see Weygoldt 1979Winter 1980) Although some uncertainty exists over theposition of pycnogonids within Arthropoda and the homol-ogy of their cephalic segmentation is poorly understood thisview is also supported by the presence of pre-cheliceralappendages in a pycnogonid larva (Larva D of Muller ampWalossek 1986) from the Upper Cambrian of Sweden(Walossek amp Dunlop 2002) Cheloniellon also has a pair ofpre-oral pediform appendages considered homologous to thechelicerae in addition to its antennae Thus despite recentneontological studies that indicate that the chelicerae arehomologous to the antennae of mandibulates based on pat-terns of Hox gene expression (Damen et al 1998 Telford amp

Table 2 Data matrix for cladistic analyses of arachnomorphs () missing data and (ndash) inapplicable characters Letters indicate multistateuncertainty codings as follows (A) 34 (B) 02 (C) 01 (D) 134 (E) 12

1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 3 3 3 4 4 4 4 4 4 4 4 4 4 5 5 5 5 51 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4

OUTGROUP 0 0 0 0 0 0 0 0 0 0 0 0 ndash ndash 0 0 0 0 0 0 0 0 0 0 0 4 0 ndash 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ndash 0 0 0 0 ndash ndash ndash 0 ndash 0Aglaspidida 1 0 0 1 A 0 0 0 0 0 1 0 ndash ndash ndash ndash ndash 0 0 0 1 0 0 0 0 0 0 0 0 ndash 1 1 0 0 1 0 0 0 0 0 0 0 0 1 2 1 1 0 ndash ndash ndash 1 0 1Aglaspidida 2 0 0 A 0 0 0 0 0 0 1 0 ndash ndash 0 1 0 1 0 0 0 0 0 0 0 0 ndash 1 1 0 0 1 0 0 0 0 0 0 0 0 1 2 1 1 0 ndash ndash ndash 1 0 1Alalcomenaeus 1 1 1 3 ndash 2 0 1 0 0 0 1 0 ndash ndash 1 0 0 0 0 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 0 1 0 1 ndash ndash ndash 1 1 0Buenaspis 0 1 B 0 0 1 0 0 ndash 0 1 0 1 0 0 0 0 1 0 0 0 0 ndash 0 1 1 1 0 1 0 ndash 0Burgessia 0 0 0 3 0 0 0 0 0 0 0 1 0 ndash ndash 0 0 0 1 2 ndash 0 0 0 0 0 4 0 ndash 0 0 0 0 0 0 2 0 0 0 0 0 0 ndash 0 0 0 1 ndash ndash ndash 1 0 0Cheloniellon 0 0 1 4 0 0 0 0 0 1 0 1 0 ndash ndash 0 0 1 0 1 0 0 0 0 0 0 1 0 ndash 0 1 0 0 1 0 0 0 0 0 1 0 0 1 0 1 1 0 ndash ndash ndash 0 ndash 1Cindarella 0 0 0 6 0 0 0 0 0 0 0 1 1 0 0 0 0 1 1 0 0 0 0 0 0 0 1 0 ndash 0 1 0 1 1 0 0 2 1 0 0 0 1 0 ndash 1 0 1 1 0 0 0 0 ndash 0Crustacea 0 0 0 3 0 0 0 0 0 0 0 0 0 ndash ndash 0 1 0 0 0 0 0 0 0 0 0 4 0 ndash 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ndash 0 0 0 ndash ndash ndash 1 0 0Emeraldella 0 0 1 5 0 0 0 0 0 0 0 1 1 0 0 0 0 1 0 2 ndash 0 0 0 0 0 0 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 1 1 1 1 0 1 ndash ndash ndash 1 0 1Eoredlichia 0 0 0 3 0 0 0 0 1 0 0 1 1 0 0 0 0 1 0 0 1 0 0 0 0 0 3 1 1 1 1 0 0 2 0 0 0 0 0 0 0 0 0 ndash 1 0 1 1 0 0 1 0 ndash 0Eurypterida 1 1 1 6 ndash 1 1 0 0 1 1 1 0 ndash ndash 1 0 1 0 1 0 0 0 1 0 0 4 1 0 0 1 0 0 1 0 0 0 0 0 0 0 0 1 3 0 1 0 1 ndash ndash ndash 1 0 0Fortiforceps 0 1 1 4 ndash 0 0 0 0 0 0 1 0 ndash ndash 1 0 0 1 0 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 0 1 0 1 ndash ndash ndash 1 1 0Helmetia 3 1 0 1 0 1 0 0 0 0 0 1 2 0 ndash 0 1 0 0 2 0 0 0 0 0 1 1 0 0 ndash 0 0 1 1 0 1 0 0 ndash 0Jianfengia 1 1 1 4 ndash 1 0 0 0 0 0 1 0 ndash ndash 1 0 1 1 0 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 0 1 0 1 ndash ndash ndash 1 0 0Kuamaia 0 0 0 0 0 0 0 0 0 1 1 0 1 0 0 1 0 0 0 1 0 0 0 1 2 0 ndash 0 1 0 0 2 0 0 0 0 0 1 1 0 0 ndash 0 0 1 1 0 1 0 0 ndash 0Leanchoilia 1 1 1 3 ndash 2 0 1 0 0 0 1 0 ndash ndash 1 0 1 1 B 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 1 1 0 1 ndash ndash ndash 1 0 0Lemoneites 0 1 0 0 0 0 1 1 0 1 0 0 0 0 0 0 0 0 1 2 0 0 ndash ndash ndash 1 0 Liwia 0 1 0 0 0 0 1 0 1 0 0 1 0 1 0 0 0 0 ndash 0 1 1 1 0 0 0 ndash 0Marrella 0 0 1 1 0 0 0 0 0 0 0 0 0 ndash ndash 0 1 0 0 2 ndash 0 0 0 0 0 0 ndash 0 0 0 0 0 0 2 0 0 0 0 0 0 ndash 0 0 0 0 ndash ndash ndash 0 ndash 0Mimetaster 0 0 1 2 0 0 0 0 0 0 0 0 0 ndash ndash 0 1 0 1 C 0 0 0 0 0 0 ndash 0 0 0 0 1 0 0 2 0 0 0 0 0 0 ndash 0 0 0 0 ndash ndash ndash 0 ndash 0Misszhouia 0 0 0 3 0 0 0 0 1 0 0 1 1 1 0 0 0 1 0 0 0 0 0 0 0 0 1 0 ndash 0 1 1 ndash ndash 2 0 0 0 1 0 0 0 0 ndash ndash 0 1 1 0 0 1 0 ndash 0Naraoia 0 0 0 3 1 0 0 0 1 0 0 1 1 1 0 0 0 1 0 0 0 0 0 0 0 0 1 0 ndash 0 1 1 ndash ndash 2 0 0 0 1 0 0 0 0 ndash ndash 0 1 1 0 0 0 ndash 0Olenoides 0 0 0 3 0 0 0 0 1 0 0 1 1 0 0 0 0 1 0 1 1 0 0 0 0 0 3 1 1 1 1 0 0 2 0 0 0 0 0 0 0 0 0 ndash 0 1 1 1 0 0 0 0 ndash 0Paleomerus 0 1 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0 0 0 ndash 0 0 ndash ndash ndash 1 0 Pycnogonida 1 1 1 4 ndash 1 1 0 0 1 1 1 ndash ndash ndash ndash 0 0 1 1 0 0 0 1 0 0 4 0 ndash 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 ndash ndash ndash 1 0 0Retifacies 0 0 0 3 0 0 0 0 0 0 0 1 0 ndash ndash 0 0 1 0 0 0 0 0 0 0 0 0 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 1 0 1 1 0 0 0 1 0 0Saperion 0 2 0 1 0 1 0 0 1 1 0 0 1 0 0 0 1 2 0 ndash 0 1 1 ndash ndash 1 1 ndash ndash 0 0 1 0 0 ndash ndash 1 1 0 0 1 0 ndash 0Sidneyia 0 0 1 0 1 0 0 0 0 1 0 1 0 ndash ndash 0 0 1 0 0 0 0 0 0 0 0 0 0 1 0 0 1 0 0 0 0 0 0 0 0 1 2 1 1 0 1 ndash ndash ndash 1 0 1Sinoburius 0 0 0 4 0 0 0 0 0 0 0 1 0 1 0 0 1 1 0 0 0 1 0 ndash 0 1 0 0 1 0 0 2 1 0 0 0 1 0 ndash 0 0 1 1 0 1 0 0 ndash 0Skioldia 0 2 0 0 0 1 0 0 0 1 2 0 ndash 0 1 1 ndash ndash 1 1 ndash ndash 0 0 1 0 0 ndash ndash 1 1 0 0 1 0 ndash 0Soomaspis 0 1 B 0 0 0 0 1 0 0 ndash 0 1 0 1 0 0 1 0 1 0 0 0 0 ndash 0 1 1 1 0 1 0 ndash 0Tariccoia 0 1 B 0 0 0 0 1 0 0 ndash 0 1 0 1 0 0 1 0 1 0 0 0 0 ndash 0 1 1 1 0 1 0 ndash 0Tegopelte 0 0 0 0 0 0 0 0 1 0 1 1 0 0 0 0 1 E 0 ndash 0 1 0 ndash ndash E 1 ndash ndash 0 0 0 0 ndash ndash 0 1 1 0 0 1 0 ndash 0Weinbergina 1 1 1 6 ndash 1 1 0 0 1 1 1 0 ndash ndash 1 0 1 0 1 0 0 0 1 0 0 4 1 0 0 1 0 0 1 0 0 0 0 0 0 0 0 1 2 0 1 0 1 ndash ndash ndash 1 0 0Xandarella 0 0 0 4 0 0 0 0 0 0 0 1 1 1 0 0 0 1 0 0 0 0 1 0 0 0 1 0 ndash 0 1 0 1 1 0 0 2 1 0 0 0 1 0 ndash 1 0 1 1 0 0 0 ndash 0Yohoia 1 1 1 4 ndash 1 0 0 0 1 1 1 0 ndash ndash 1 0 1 0 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 1 2 0 1 0 1 ndash ndash ndash 1 1 0

176 TREVOR J COTTON AND SIMON J BRADDY

Thomas 1998) and developmental data (Mittmann amp Scholtz2003) morphological evidence from fossils supports the viewthat chelicerates have lost the antennae The Hox gene evi-dence suffers from a lack of comparative data from diversemandibulates especially primitive crustaceans (see Akam2000) and the use of Hox gene expression boundaries asmarkers for segmental homology has been criticised (egAbzhanov et al 1999 although relating to posterior Hoxpatterns) Wills et al (1998a pp 43 amp 48) considered thechelicerae to be appendages of the second (deutocerebral)segment but curiously in support of this cited Schram (1978)who unambiguously followed the homology scheme used inthe present study

The anterior-most appendages of Aglaspis were originallydescribed as chelicerae (Raasch 1939) which lead to theirclassification in the Chelicerata (eg Stoslashrmer 1944) Howeverthe morphology of these appendages (see Briggs et al 1979) ismore similar to that of antennae They are narrow relative tothe thoracic endopods of relatively even diameter proximallyand the podomeres are apparently weakly defined Hesselbo(1988 1992) also argued that the anterior appendages ofAglaspis are likely to be antennae The distal parts are un-known (and hence so is their length cf Wills et al 1998ap 58) and there is no evidence that they were chelate contraryto the coding of Wills et al (1998a character 31) Nothingis known about the anterior appendages of BuenaspisLemoneites or Paleomerus and this character is accordinglycoded as missing data in these taxa

2 Form of endopod of appendages of the second segment(0) pediform and (1) anteriorly directed raptorial appendagewith reduced number of podomeres and terminal podomeresbearing spines on distal margins As discussed above cheli-cerae and lsquogreat appendagesrsquo are both likely to represent theappendages of the second segment They also differ from theassumed primitive biramous euarthropod limb in similar waysand therefore are likely to be homologous modifications ofthese appendages The homology of lsquogreat appendagesrsquo andchelicerae was originally proposed by Henriksen (1928) andsupported by Stoslashrmer (1944) but to the present authorsrsquoknowledge no modern author has discussed this possibility

Both lsquogreat appendagesrsquo (Figs 5A D E) and chelicerae(Figs 5B C) are equipped with strong spinose projections onthe outer (dorsal) side of the distal margins of the terminalpodomeres that are lacking on the proximal podomeresSecondly the number of podomeres in both chelicerae andlsquogreat appendagesrsquo is more or less reduced compared to thenumber in the endopods of biramous limbs In the case oflsquogreat appendagesrsquo they are significantly more robust thanthose of posterior endopods a situation that is matched insome chelicerates The homology of the lsquogreat appendagesrsquoof Alalcomenaeus Leanchoilia Yohoia Jiangfengia andFortiforceps is supported by all of these features and is widelyaccepted (eg Wills et al 1998a character 31 Hou ampBergstrom 1997 Bergstrom amp Hou 1998 Hou 1987a) OnlyHou amp Bergstrom (1991 p 183) have suggested albeit inpassing that the lsquogreat appendagesrsquo of all these taxa may beconvergent a view they appear to have changed The endo-pods of the appendages of other taxa are locomotory legswhich either lack strong spines or have spinose extensionsthat are invariably on the medial (ventral) surface and arestronger on the proximal podomeres than the distal ones (egMisszhouia Fig 6A) In the latter case these spines form partof the feeding system along with the gnathobases and are oftenlocated around the middle of the podomere rather than limitedto the distal margin These endopods are directed ventro-laterally as opposed to anteriorly in the case of both cheliceraeand lsquogreat appendagesrsquo The modified second appendages of

Marrella resemble this plesiomorphic condition here referredto as lsquopediformrsquo except in their anterolateral orientation Insome Recent crustaceans this appendage is modified into asecond antenna but in stem-group crustaceans it is pediform(eg Walossek amp Muller 1997 1998) This character is codedas missing data in a number of taxa for which the morphologyof the second segment appendage is unknown

Some authors (Dzik 1993 Chen amp Zhou 1997 Dewel ampDewel 1997 Budd 2002) have suggested that the lsquogreat ap-pendagesrsquo are homologous with the anterior appendages ofanomalocaridids and the anomalocaridid-like Opabinia andKerygmachela (see Budd 1996b 1999b respectively) Theanterior appendages of most anomalocaridids and otherCambrian lobopodians differ from megacheiran lsquogreat append-agesrsquo in that they consist of a large number of segmentsMoreover there is considerable morphological evidence thatanomalocaridids are part of the euarthropod stem group (Willset al 1995 1998a) and not closely related to the lsquogreatappendagersquo taxa (cf Budd 2002) with the possible exception ofParapeytoia lsquoGreat appendagersquo taxa share a suite of derivedcharacters with other crown group euarthropods includingsclerotised dorsal tergites the incorporation of more thanone pair of appendages into the head sclerotisation of theendopods and strong differentiation of the head that wereexcluded from Buddrsquos (2002) analysis The relationships ofanomalocaridids will be considered in detail later by Braddyand co-workers

3 Exopod of appendages of the second segment (0)present and (1) absent or much reduced The raptorial ap-pendages of the second segment (chelicerae and lsquogreat append-agesrsquo) are uniramous as are the corresponding pediformappendages of Marrella Mimetaster Emeraldella Cheloniellonand Sidneyia The plesiomorphic euarthropod state is found inmost other arachnomorphs including trilobites where thebiramous second segment appendages are undifferentiatedfrom those of posterior segments (Edgecombe et al 2000character 78) The exopods of these appendages are alsopresent in basal members of the crustacean crown group(Edgecombe et al 2000 character 79) and in stem groupcrustaceans (Walossek 1993 Walossek amp Muller 1997 1998)

4 Number of head segments (0) 1 (1) 2 (2) 3 (3) 4 (4) 5(5) 6 and (6) 7 The coding of this character by Edgecombe ampRamskold (1999 character 2) explicitly referred to the numberof limb pairs present in addition to the antenna Here thenumber of post-acronal segments present in the head is codedfollowing Wills et al (1998a character 29) In taxa that lack anantenna the number of somites is inferred to be one greaterthan the number of cephalic appendage pairs (cf Wills et al1998a) as described above

Sturmer amp Bergstrom (1978) suggested that the head ofCheloniellon is defined primarily on the basis of appendagetagmosis rather than the fusion of segments under a singlecephalic-shield (cf Hou amp Bergstrom 1997 pp 98ndash9) Accord-ing to this view the head consists of both the cephalic tergiteand the anteriormost free tergite that share uniramousgnathobasic appendages Wills et al (1998a) coded Cheloniel-lon as having six somites incorporated into the head andtherefore presumably accepted this suggestion There is noevidence that the first free tergite of Cheloniellon was incorpo-rated into the head (except perhaps functionally) and thepresent authors prefer to code the number of somites under thecephalic shield In Sidneyia there is a series of uniramousstrongly gnathobasic appendages similar to those of Cheloniel-lon posterior to the head shield All of these appendages wouldpresumably have to be considered part of the head based onappendage differentiation according to the view of Sturmer ampBergstrom (1978) Some autapomorphic states are included

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 177

(none for Sidneyia one for Marrella and six for Emeraldella)since these will become informative if the character is treatedas ordered

Edgecombe amp Ramskold (1999 p 265) described a novelform of cephalic tagmosis The present authors tentativelyaccept their suggestion that in trilobite-like taxa the fourthpair of biramous cephalic appendages was directly under thecephalo-thoracic junction This may explain previous confu-sion about the number of such appendages incorporated intothe trilobite cephalon (eg Cisne 1975 Whittington 1975aBergstrom amp Brassel 1984) However Edgecome amp Ramskoldaccept the view of Chen et al (1977 p 7) that only the firstthree biramous limbs of Misszhouia were structurally and

functionally part of the head Therefore it seems more accept-able to code these taxa as having only four post-acronalsomites than to use the coding scheme of Edgecombe ampRamskold (1999) The partial integration of the first thoracicsomite under the head shield could be considered to be a formof overlap of the trunk by the head shield (ie a distinct stateof character 9 of Edgecombe amp Ramskold 1999 or Character37 herein) Derived states of this character would then poten-tially be synapomorphic for a clade including xandarellidsnaraoiids helmetiids tegopeltids and trilobites Howeversince the degree of overlap in taxa with the fourth biramousappendages under the cephalo-thoracic articulation is identicalto that found between thoracic tergites (State 0 of Character

Figure 5 Diagrammatic reconstructions of raptorial second-segment appendages in lateral view (except whereotherwise stated) and approximate life orientation showing suggested homology of appendage elements (inbrackets) Roman numerals indicate position in the podomere series and do not necessarily imply homology (A)lsquoGreat appendagersquo of Fortiforceps (after Hou amp Bergstrom 1997) (B) Left chelicera of Leiobunum aldrichi(Arachnida Opiliones) (1) in medial perspective and (2) distal parts in anterior perspective (after Shultz 2000)(C) Chelifore of Nymphon (Pycnogonida) (after Child 1997) (D) lsquoGreat appendagersquo of Yohoia (after Whittington1974 fig 2) (E) lsquoGreat appendagersquo of Leanchoilia (modified after Bruton amp Whittington 1983) Not to scaleAbbreviations (apt) apotele (probably homologous with the mandibulate pretarsus) (cx) coxa (dtmrt)deutomerite and (pt) pretarsus

178 TREVOR J COTTON AND SIMON J BRADDY

37) it is here preferred to regard this as a distinct character(Character 9 herein)

The coding of Alalcomenaeus by Wills et al (1998a) ashaving four post-acronal somites is accepted here but fordifferent reasons Briggs amp Collins (1999) have recenty demon-strated that Alalcomenaeus possessed two pairs of biramousappendages on the head in addition to the great appendagesThis would equate to three head somites according to thehomology scheme of Wills et al (1998a) Here the antennaeare inferred to be lost and therefore four segments incorpo-rated into the head

Recently a number of authors have agreed that the plesio-morphic condition for euarthropods is a four-segmented headas found in stem group crustaceans (Scholtz 1997 Walossek1993 Walossek amp Muller 1997 1998 Edgecombe et al 2000)However reconstruction of the euarthropod stem groupclearly indicates that even crownward taxa had a head consist-ing of only the antennal segment and acron as found intardigrades and onychophorans Therefore it seems that thefour-segmented head is pleisomorphic only for crown groupeuarthropods Some fully arthropodised fossil taxa also have ashorter head that may be pleisomorphic such as the two-segmented head of Marella Retifacies is coded following therecent redescription of Hou amp Bergstrom (1997) rather thanon the basis of the coding by Edgecombe amp Ramskold (1999)for which they provide no explanation A partial uncertaintycoding is used for Aglaspis in which the number of post-antennal cephalic appendages is either three or four (Briggset al 1979)

5 Orientation of the antennae (0) directed anterolaterally(1) strongly deflected laterally and (2) placed well inside shieldmargin curing posteriorly from a transverse proximal element(cf Edgecombe amp Ramskold 1999 character 3) The anteriorappendages of Aglaspis (see character 1) are clearly directedanterolaterally and it is presumed that they continued past thehead shield margin The anterior appendages of arthropodstem group taxa such as Aysheaia Kerygmachela and Anoma-locaris are either directed anterolaterally or ventrally but arenot strongly-deflected laterally Therefore the outgroup iscoded as State 0 This character is coded as missing data fortaxa where the presence of antennae is equivocal (those codedas missing data for Character 1) and as inapplicable in taxawhere the antennae are considered absent (coded as State 1 forCharacter 1)

6 Length of distal spines on terminal podomeres of endo-pods of second segment appendages (0) absent or shorer thanpodomeres (1) subsequal to length of podomeres and (2)longer than the entire podomere series The spinose projectionsof the raptorial appendages described above vary in length Inchelicerae (Figs 5BC) and some lsquogreat appendagesrsquo (eg thoseof Yohoia Fig 5D) the most distal spine is equal in length tothe spinose terminal podomere forming a chelate structureIn Fortiforceps (Fig 5A) the spines are short relative tothe lengths of the podomeres but in Alalcomenaeus andLeanchoilia (Fig 5E) they are extremely long so that thespines are much longer than the entire podomere series Asdescribed above in taxa with pediform endopods of the secondsegment appendages spines on the distal podomeres are shortor entirely absent

7 Chelicerae (0) absent and (1) present Despite theirsimilarities to lsquogreat appendagesrsquo the chelicerae of the eucheli-cerates and chelifores of the pycnogonids are clearly distinctand have been widely recognised as a chelicerate synapomor-phy Chelicerae differ from any lsquogreat appendagesrsquo by thecombination of a smaller number of podomeres the presenceof only a single terminal element and only a single spinoseprojection on the dorsal side of the podomere series (Fig 5)

Amongst extant chelicerates only the pycnogonid Pallenopsishas chelicerae of four podomeres comparable to the numberin lsquogreat appendagesrsquo Bergstrom et al (1980) considered thatthe chelifores of the Devonian pycnogonid Palaeoisopus alsoconsist of four segments but this is not well supported by theirfigures

8 Distal spines of second segment endopods terminating inannulated flagellae (0) absent and (1) present It is widelyrecognised (eg Stoslashrmer 1944 Simonetta 1970 Bruton ampWhittington 1993 Briggs amp Collins 1999) that the terminalparts of the extended spines of Alalcomenaeus and Leanchoiliaformed annulated flagellae Nothing similar is known fromhomologous appendages of any other arthropod althoughsimilar processes may have operated in the transformation ofthe endopod as a whole into the second antennae of derivedcrustaceans and in the origin of multiramous antennulae (firstantennae) in malacostracans

242 Posterior appendages 9 Appendages of first thoracicsomite underneath the cephalo-thoracic articulation (0) ab-sent and (1) present (cf Edgecombe amp Ramskold 1999character 2) For a discussion of this character see Character4 herein

10 Exopods of appendages of third to fifth segments (0)present and (1) reduced or absent A number of taxa haveuniramous appendages on the third to fifth segments In mostthese segments are incorporated into the head (Yohoia cheli-cerates) but in others all (eg in Sidneyia) or some (eg inCheloniellon) of these segments are post-cephalic In all casesthe appendage is morphologically similar to the endopods ofbiramous limbs of other taxa andor other segments and it isinterpreted that the exopod is lost

11 Endopods of thoracic appendages (0) present and (1)reduced or absent The opisthosomal appendages of chelicer-ates lack endopods or have the endopods much reduced (egLimulus Siewing 1985 fig 838 Shultz 2001) a situation that isalso found in the uniramous thoracic appendages of Yohoia(Whittington 1974) It has been suggested that Helmetia(Briggs et al 1994) lacks endopods but pending a redescrip-tion of this genus and considering that they have been docu-mented in the otherwise very similar Kuamaia this is coded asuncertain

12 Exopod shaft of numerous podomeres each bearing asingle seta (0) present and (1) absent The exopods ofcrustaceans (at least plesiomorphically see eg Walossek ampMuller 1998 figs 5middot5 amp 5middot6) and of Marrella and Mimetasterhave multi-annulated shafts with each podomere bearing asingle seta In other taxa the exopod shafts consist of one oftwo lobes bearing numerous setae

The polarity of exopod segmentation is uncertain Thelateral flaps of stem group arthropods such as Opabinia(Whittington 1975b Budd 1996b) and anomalocaridids (egHou et al 1995) are certainly unsegmented but as suggestedby Buddrsquos (1996b fig 8) reconstruction this does notnecessarily suggest the form of the primitive euarthropodexopod Rather segmentation may have been a result of thesclerotization of the cuticle of the exopod shaft The outgroupwas coded as uncertain for this character According to thefirst interpretation of aglaspidid appendage morphology(coded as Aglaspidida 1) the exopods are lacking on allappendages and consequently this and all other charactersdescribing exopod morphology are coded as inapplicable

13 Exopod shaft differentiated into proximal and distallobes (0) absent and (1) present Ramskold amp Edgecombe(1996 Edgecombe amp Ramskold 1999 character 26) havediscussed the distribution of the trilobite-type bilobate exo-pods recognised by this character (Fig 6A) Among taxaincluded here that were not considered by Edgecombe amp

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 179

Ramskold (1999) exopods consist either of a single flattenedlobe or a homonomous series of podomeres (Fig 6B C) Nostem group euarthropod has a bilobate exopod and theoutgroup is consequently also coded as State 0

14 Proximal lobe of exopod (0) flattened lobe and (1)slender shaft

15 Distal lobe of exopod (0) small to moderate sized flapwith short to moderately long attachment to proximal lobeand (1) large teardrop-shaped with long attachment to proxi-mal lobe Neither of these characters can be coded for taxathat do not have an exopod differentiated into proximal anddistal lobes (Character 13 State 1) without asserting thehomology of the single-lobed exopod with one of these partsThe coding of Edgecombe amp Ramskold (1999 p 280) implic-itly homologizes the exopods of Sidneyia and Retifacies withthe proximal lobe In these taxa this view is perhaps supportedby the presence of lamellate exopod setae which match thoseof the trilobite-type proximal lobe However in other taxawith a single-lobed exopod such as Alalcomenaeus (see Briggsamp Collins 1999) the exopod is fringed with sharp spines likethe distal lobe of differentiated exopods Alternatively thebilobate exopod may have originated from the division of aprimitively single-lobed structure The single-lobed exopod isnot clearly homologous with either lobe of divided exopodsand both these characters are consequently coded as inappli-cable to taxa without differentiated proximal and distal exopodlobes Consequently (see Character 13 herein) these characterscan only be coded for taxa that were previously considered byEdgecombe amp Ramskold (1999) and their coding is followedhere except in the cases of Sidneyia and Retifacies

16 Exopod shaft is a deep rounded flap (0) absent and (1)present All bilobate (Character 13) and segmented (Character12) exopod shafts are long and relatively narrow structures (seeFig 6) whereas some single-lobed exopod shafts have theform of rounded flaps These flap-like exopod shafts are largecompared to the length of the setae and at least half as deep asthey are long This appendage structure is found in cheliceratesand lsquogreat appendagersquo arthropods

17 Medially-directed exopod setae (0) absent and (1)present Walossek amp Muller (1998 p 194) suggested that thetilting of exopod setae towards the endopod in post-antennularlimbs with a multi-annulated exopod was an autapomorphyuniting crustaceans and all members of the crustacean stemgroup (the crustacean total group sensu Ax 1986 see Fig 6C)This character is shared with the marellomorphs Marrellaand Mimetaster (see Fig 6B Sturmer amp Bergstrom 1976Bergstrom 1979 fig 1middot3A B) In other taxa the setae eithersurround the entire margin of the exopod shaft or are directeddorsally (Fig 6A) The identification of setae on the dorsalsurface of the lateral flaps in Opabinia (Budd 1996b) suggeststhat the condition in marellomorphs and crustaceans isderived

18 Lamellate exopod setae (0) absent and (1) presentLamellate exopod setae originally described from trilobitesare a classic synapomorphy of the Arachnomorpha (see egBergstrom 1992 Hou amp Bergstrom 1997 pp 42ndash3) They areperhaps best known from Misszhouia (see Chen et al 1996Hou amp Bergstrom 1997 Fig 6A) These setae differ from themore spinose setae of other taxa (Fig 6C) in that they are wideand flat and imbricate over the length of the exopod shaft Thesetae of Marrella (Fig 6B) and similar taxa have variouslybeen considered lamellate (Bergstrom 1979) or non-lamellate(Bergstrom 1992) Since they do not imbricate and do notseem to be strongly flattened (Whittington 1971 Sturmer ampBergstrom 1976 fig 9a) Marrella and Mimetaster are codedas State 0 The setae of Helmetia as shown in Briggs et al(1994 fig 141) seem to be of the lamellate type The setae ofSinoburius have been described by Hou amp Bergstrom (1997p 85) as similar to those of Misszhouia Only the setae areknown of the appendages of Buenaspis (Budd 1999a) andthese also appear to be of the trilobite-type

Figure 6 Diagrammatic reconstructions of (A) arachnomorph and(B C) non-arachnomorph biramous appendages (A) The lsquotrilobite-typersquo biramous appendage of Misszhouia longicauda (after Chen et al1997) (B) Appendage of Marrella splendens (after Whittington 1971)(C) Appendage of the stem-lineage crustacean Martinssonia elongata(after Muller amp Walossek 1997 1998) Not to scale Abbreviations(bas) basis (dle) distal lobe of exopod shaft (ls) lamellar exopod setae(pe) proximal endite or pre-coxa (ple) proximal lobe of exopod shaft(ses) segmented exopod shaft and (ss) spinose exopod setae

180 TREVOR J COTTON AND SIMON J BRADDY

The homology of the book-gill lamellae of xiphosurans withlamellar setae was supported by Walossek amp Muller (1997p 149) and Edgecombe et al (2000 p 174) but rejected bySturmer amp Berstrom (1981) on the grounds that Weinberginapossesses Limulus-like gill lamellae and fringing setaeBook-gills have also been described from a eurypterid (Braddyet al 1999) There are certainly major morphologicaldifferences between book-gills and trilobite-type exopodsFollowing most recent opinion and pending further studyWeinbergina and Eurypterida are coded as possessing lamellatesetae

The form of the setae of Yohoia is uncertain (Whittington1974) the spinose setae seen in the most common reconstruc-tion (Gould 1989 Briggs et al 1994) are not justified by thespecimens Amongst other great appendage arthropods theform of the setae appears to vary those of Jianfengia (seeChen amp Zhou 1997 p 74) and Leanchoilia (see Bruton ampWhittington 1983) are lamellate while those of Fortiforceps(Hou amp Bergstrom 1997) and Alalcomenaeus (Briggs amp Collins1999) are spinose and less densely packed

19 Gnathobase on basis andor prominent endites onendopod (0) present and (1) absent (cf Edgecombe ampRamskold 1999 character 29 Wills et al 1998a characters 36amp 59) The presence of gnathobases on the appendages of someanomalocaridids (Hou et al 1995) and their general distribu-tion amongst non-arachnomorph euarthropods suggests thatState 0 is plesiomorphic The homology of the various enditesand gnathobasic structures found in euarthropods is in need ofcareful assessment and therefore a very general coding (fol-lowing Edgecombe amp Ramskold) is used for this character

243 Eyes 20 Position of lateral facetted eyes (0)ventral and stalked (1) dorsal and sessile and (2) absent (cfEdgecombe amp Ramskold 1999 character 4) Partial uncer-tainty coding is used for a number of taxa where the dorsalsurface is known but the ventral morphology is not andtherefore the presence of ventral eyes cannot be discountedFor example there is good evidence that Leanchoilia lackeddorsal sessile eyes but ventral stalked eyes may have beenpresent (as in L hanceyi see Briggs amp Robison 1984) and a(02) partial uncertainty coding is used This is also the case inmany naraoiids such as Buenaspis However only the outlineof the head shield of Liwia is known and the absence of dorsaleyes in the reconstruction of Dzik amp Lendzion (1988) isconjectural It is unclear whether the autapomorphic conditionof stalked dorsal eyes in Mimetaster (see Sturmer amp Bergstrom1976) is homologous with State 0 or State 1 and a partialuncertainty coding is also used

Whittington (1974) was somewhat equivocal about thenature of the lateral lobes anterior to the head shield ofYohoia These more closely resemble the ventral stalked eyes ofAlacomenaeus as recently described by Briggs amp Collins(1999) than either Whittingtonrsquos (1974) or subsequent (egGould 1989 fig 3middot18 Briggs et al 1994 fig 153) reconstruc-tions suggest In a well preserved specimen showing the dorsalaspect (USNM 57696 Whittington 1974 pl 2 figs 1ndash3) and ina laterally compressed specimen (USNM 57694 Whittington1974 pl 1 fig 1) they are clearly seen to be stalked andrelatively small lobate structures That these structures arelikely to represent stalked ventral eyes was reflected in thecoding of Wills et al (1998a characters 26 amp 27)

21 Visual surface with calcified lenses bounded withcircumocular suture (0) absent and (1) present

22 Dorsal bulge in exoskeleton accommodating drop-shaped ventral eyes (0) absent and (1) present

23 Eye slits (0) absent and (1) presentCharacters 21 to 23 are used exactly as described by

Edgecombe amp Ramskold (1999 character 5) Character 21 is

inapplicable to taxa that do not possess eyes (Character 20State 2)

24 Dorsal median eyes (0) absent and (1) present Dorsalmedian eyes on a tubercle are considered a synapomorphy ofthe Chelicerata (Dunlop amp Selden 1997 Dunlop 1999) Thenature and position of the structures interpreted as dorsalmedian eyes in Mimetaster (Sturmer amp Bergstrom 1976p 83ndash4) are regarded as equivocal

244 Ventral cephalic structures Many characters of theventral surface of the euarthropod head (see Fig 7) of poten-tial phylogenetic utility are poorly known in many importanttaxa In particular the presence of pre-hypostomal frontalorgans (Fig 7A CndashE) is not coded herein (see Edgecombe ampRamskold 1999 p 272) Definitive evidence of the absence ofthese structures is available for very few taxa and the homol-ogy of these structures with the maculae of the trilobitehypostome (see Fig 7B) and the frontal organs of the crusta-cean labrum (eg see Muller amp Walossek 1987 p 39) isuncertain Secondly the detailed homology of the trilobitehypostome with that of other putative arachnomorphs isunclear and consequently a very broad definition is usedherein For example various pre-hypostomal sclerites (Fig7A D) in other arachnomorphs may have been incorporatedalong with a primitive hypostome into a single sclerite that isrecognised as the hypostome in trilobites The structure of thehypostome (eg the presence of paired anterior wings dorsal tothe antennae Fig 7B C) may also be a source of additionalcharacters

25 Expanded cephalic doublure (0) absent and (1)present maximum width more than 30 length of head shieldor more than 25 width of pygidium Chen et al (1996)suggested that the wide doublure of Soomaspis and Tariccoia isa synapomorphy uniting these taxa The doublure of Buenaspis(see Budd 1999a) is poorly known but based on the width ofthe heavily crushed region of the cephalic margin and theposition of a possible impression of the edge of the doublure inone specimen (Budd 1999a pl 1 fig 1) it also appears to berather wide (ie greater than 30 of the length of the headshield) Following Edgecombe amp Ramskoldrsquos coding ofSidneyia the present authors do not consider the postero-median expansion of the doublure of for example Emeraldellaor Retifacies (see Bruton amp Whittington 1983 Hou ampBergstrom 1999 respectively) to be homologous with State 1but to represent the hypostome (see Character 26)

26 Anteromedian margin of cephalon notched accomo-dating strongly sclerotised plate (0) notch and plate absentand (1) notch and plate present (cf Edgecombe amp Rasmkold1999 character 10) The apomorphic state (illustrated inFig 7D) is only known in the taxa discussed by Edgecombe ampRasmkold (1999) Reconstructions of Yohoia show a roundedlobe between the eyes (see Character 20) anterior to anddistinct from the head shield which resembles the anteriorsclerite recognised here This seems to be a misinterpretationSpecimens preserved in dorsal aspect (USNM 57696Whittington 1974 pl 2 figs 1ndash3) and lateral aspect (USNM57694 Whittington 1974 pl 1 fig 1 USNM 155616Whittington 1974 pl 5 fig 2) seem to clearly show that thislsquomedian lobersquo is a downward curving pointed extension of thehead shield

27 Hypostomal sclerite (0) median extension of thedoublure with no suture (1) natant sclerite not in contactwith doublure (2) with narrow overlap with pre-hypostomalsclerite (3) narrow attachment to doublure at hypostomalsuture and (4) absent The homology of hypostomes andlabrums has been discussed by Haas et al (2001) and Budd(2002) The euarthropod labrum is likely to represent a pos-teroventral extension of the pre-segmental acron (Scholtz

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 181

Figure 7 Ventral reconstructions of arachnomorph arthropod heads (A) Misszhouia longicaudata (after Chenet al 1997 figs 2ndash4 7) (B) Ceraurinella typa (after Chen et al 1997 fig 7) (C) Agnostus pisiformis (after Mulleramp Walossek 1987) (D) Kuamaia lata (after Edgecombe amp Ramskold 1999 figs 32 5 amp 6) (E) Cindarella eucala(after Ramskold et al 1997) and (F) Emeraldella brocki (after Bruton amp Whittington 1983) Not to scaleAbbreviations (aw) anterior wing beneath antennae (bs) boomerang-shaped sclerite (bl) blade-like process ofposterior wing (d) doublure of head-shield (cs) connective suture (f) fenestra (fs) facial suture (h) hypostome(hl) hypostomal lobe (emerging through fenestrae of Agnostus) (hs) hypostomal suture (if) intervening field ofhypostomal complex (lb) lateral bridge (lfo) lateral frontal organ (mb) median body (mbr) median bridge (mf)median furrow (mfb) mouth field bridge (mfo) median frontal organ (ol) ovate lobe and (phs) pre-hypostomalsclerite Post-antennal appendages and distal parts of antennae omitted

182 TREVOR J COTTON AND SIMON J BRADDY

1997) This structure is often sclerotised and covers the mouthventrally It is this sclerite that is here identified as thehypostome Therefore this character is considered absent intaxa lacking a sclerite covering the mouth irrespective of themorphology and position of the labrum itself which at leastpartly covers the mouth in all euarthropods other thanpycnogonids (see Edgecombe et al 2000 p 167) The lsquofleshylabrumrsquo of crustaceans does not appear to be sclerotised and ahypostome is consequently considered to be absent Althoughmany derived features are associated with the crustaceanlabrum this is not a good reason to consider it non-homologous with the hypostome-bearing structure of otherarthropods as Walossek amp Muller (1990) argued

In many taxa the mouth is covered by a posteromedianextension of the doublure here considered to represent thehypostome (eg Emeraldella Fig 7F Aglaspis see Hesselbo1992 fig 52) In others the hypostome is separated from thedoublure either by a suture a pre-hypostomal sclerite (egKuamaia lata Fig 7D Saperion glumaceum see Edgecombe ampRamskold 1999 figs 3 amp 4) or an intervening region ofunsclerotised cuticle (the natant condition of Fortey 1990beg Cindarella eucala Fig 7E) The homology of pre-hypostomal sclerites and hypostomes in helmetiids trilobitesand naraoiids (see Fig 7) has been discussed by Chen et al(1996) and Edgecombe amp Ramskold (1999) but it is as yetunclear whether any of the character states described here arederived from any other They are treated here as distinct andthe character as unordered pending further study

No hypostome has been identified in Yohoia (see above)Alalcomenaeus Jianfengia Fortiforceps or Leanchoilia andthere is certainly no broad posterior extension of the doublurein any of these taxa Since there is also no pre-hypostomalsclerite they have received a (134) partial uncertainty codingThe attachment of the hypostome of Tegopelte is unclear butit does not seem to be attached to any doublure (Whittington1985) and therefore it is given an uncertainty (12) coding

The lsquorostrum-like structurersquo on the ventral surface ofLemoneites appears to be similar in morphology and positionrelative to the anterior margin of the head shield (Flower 1968pl 8 figs 4 amp 13) to that described from Aglaspis and is codedas State 0 In many taxa the hypostome is unknown but inonly a small number can it be coded reliably as absentHughesrsquo (1975) suggestion that the hypostome of Burgessia isabsent is accepted preliminarily but an extension of thedoublure may be visible in some of Hughesrsquo figures Fortey(pers comm 2000) also doubts Hughesrsquo assertion

28 Visible ecdysial sutures (0) absent and (1) present Themarginal ecdysial sutures of chelicerates and the dorsal suturesof trilobites are almost certainly not homologous but thepresence of sutures and their position (see Character 29) arecoded separately to allow this to be tested Wills et al (1998aCharacter 2) coded marginal sutures as present in Sidneyiafollowing Brutonrsquos (1981) suggestion but this is consideredequivocal here

29 Position of ecdysial sutures (0) marginal and (1) dor-sal This character is inapplicable to taxa lacking ecdysialsutures (Character 28 State 0) Dorsal sutures whilst presentin the representatives of the Trilobita coded here are probablynot a synapomorphy of trilobites as a whole but for a clade oftrilobites excluding the Olenellida (Whittington 1989 Fortey1990a)

245 Exoskeletal tergites and thoracic tagmosis 30 Min-eralised cuticle (0) absent and (1) present Calcification of theexoskeleton is one of the most convincing synapomorphiesof the Trilobita (Fortey amp Whittington 1989 Ramskold ampEdgecombe 1991 Edgecombe amp Ramskold 1999 Character 1)Cuticle mineralisation is also coded as present in aglaspidids

following Briggs amp Fortey (1982) who suggested that theaglaspidid exoskeleton was originally phosphatic Paleomerusprobably also had a mineralised cuticle but Hou amp Bergstrom(1997 p 97) argued that it was likely to be calcareous Anundescribed Silurian aglaspidid may also have had an origi-nally calcitic cuticle (Fortey amp Theron 1994 p 856) Becauseof the uncertainty about the chemical composition of theexoskeleton in these forms cuticle mineralisation is coded aspotentially homologous in all these taxa and no attempt ismade to code separate states for phosphatic and calcareousmineralisation

31 Trunk tergites with expanded lateral pleurae coveringappendages dorsally (0) absent and (1) present Whilst thepresence of paratergal folds may be a synapomorphy at thelevel of the Euarthropoda (Boudreaux 1979 Wagele 1993Edgecombe et al 2000 Character 142) these are at mostsmall reflections of the margin of the tergites in most euarthro-pods In arachnomorph taxa these are expanded to form largelateral pleurae that cover the appendages dorsally (see egLimulus and Triarthus in Boudreaux 1979) This is inferred tobe the case in some taxa where dorsal features and appendagesare poorly known on the basis of their possession of tergiteswith wide pleural regions This feature was suggested as typicalof lamellipedians (=arachnomorphs) by Hou amp Bergstrom(1997 pp 42ndash3)

The arrangement of the thorax of Marrella Mimetaster andBurgessia differs from that of all the other taxa considered herein that the appendages seem to be attached to the lateralmargins of the body The trunk tergites do not cover theappendages and seem to lack pleurae entirely although theyare covered by a posterior extension of the head shield inBurgessia This character has been clearly described inMimetaster (Sturmer amp Bergstrom 1976 pp 87ndash90 fig 8) andBurgessia (Hughes 1975 p 421)

32 Free thoracic tergites (0) present and (1) absentFollowing Edgecombe amp Ramskold (1999 Character 16) anumber of taxa lack functional post-cephalic articulations andconsequently lack free thoracic tergites Despite this theycode these taxa for a number of characters relating to thestructure of thoracic tergites (eg their characters 18 and 19)These characters are treated here as inapplicable to taxawithout free tergites and the definition of this character hasbeen modified from that of Edgecombe amp Ramskold (1999) tomake this more explicit

33 Decoupling of thoracic tergites and segments (0)absent and (1) present Ramskold et al (1997) described thischaracter of Cindarella and Xandarella where the thoracictergites correspond to a variable increasing posteriorly num-ber of appendage pairs This character cannot be coded fortaxa in which free thoracic tergites are absent (Character 32State 1)

34 Tergite articulations (0) tergites non-overlapping (1)extensive overlap of tergites and (2) edge-to-edge pleuralarticulations In most of the taxa considered here the thoracictergites overlap considerably and relatively evenly over theirwidth This contrasts with the primitive euarthropod condi-tion where the tergites do not overlap medially as seen in stemgroup crustaceans In trilobites such as Helmetia and Kuamaia(see Edgecombe amp Ramskold 1999 character 18) overlap ofthoracic tergites is limited to a well-defined axial region andthe lateral pleurae meet edge-to-edge

The thoracic articulations of naraoiids with free thoracictergites are in some ways similar to those of trilobites with astrong overlap medially that is lacking abaxially andanterolateral articulating facets (Budd 1999a Ramskold ampEdgecombe 1999) However the distribution of these features

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 183

in other trilobite-like arachnomorphs is unclear and a restric-tive coding is used here The present authors do not considerthat this character can be applied reliably to the fused thoracictergites of Saperion Tegopelte and Skioldia and it is coded asinapplicable to all taxa lacking free thoracic tergites (Character32 State 1)

35 Trunk effacement (0) trunk with defined (separate orfused) tergite boundaries (1) trunk tergite boundaries effacedlaterally and (2) trunk tergite boundaries completely effaced(cf Edgecombe amp Ramskold 1999 Character 15)] Contrary toEdgecombe amp Ramskold (1999) the present authors considerSkioldia and Saperion to show a distinct form of tergiteeffacement where the fused tergite boundaries are defined byfurrows axially but effaced laterally The boundaries areeffaced at least laterally in Tegopelte but the axial region ofthe exoskeleton is unknown which accordingly is given amultistate uncertainty coding

36 Cephalic articulation fused (0) absent and (1) presentIn Tegopelte Saperion and Skioldia the articulation ofthe head with the thorax is non-functional and the entireexoskeleton forms a single tergite

37 Head shield overlap of thoracic tergites (0) overlapabsent or identical to overlap between thoracic segments (1)head shield covers first thoracic tergite only and (2) headshield covers mutliple anterior trunk tergites

38 Head shield articulates with reduced anterior thoracictergite (0) absent and (1) present Amongst taxa showing aposteriorly expanded head shield ie one that overlaps thethorax Edgecombe amp Ramskold (1999 Character 9) identifiedtwo distinct character states One of these ndash overlap of only theanteriormost thoracic tergite ndash was limited to taxa assigned tothe Liwiinae (sensu Fortey amp Theron 1994) and another ndashoverlap of multiple tergites with attachment to a reducedanterior thoracic tergite ndash to the Xandarellida None of theadditional taxa considered herein (including Buenaspis whichwas assigned to the Liwiinae by Budd 1999a) shows thesedistinctive morphological features However the expandedhead shields of Marrella Mimetaster and Burgessia which donot articulate with a narrow anterior thoracic tergite may behomologous with the expanded head shields of xandarellidsTo recognise this the degree of overlapping of the thorax andthe possession of the reduced anterior tergite are coded sepa-rately These characters are both coded as inapplicable to taxain which the head shield and thoracic tergites are fused(Character 36 State 1) The crustacean carapace which ismost similar to the head shield of Burgessia is seeminglyabsent in stem group crustaceans (Walossek amp Muller1998)

39 Trunk narrowed anteriorly relative to head shieldwidest posteriorly (0) absent and (1) present (cf Edgecombeamp Ramskold 1999 Character 14) The anterior narrowing ofthe trunk caused by the reduction of opisthosomal segment 1in xiphosurids (Weinbergina of the taxa analysed hereAndersen amp Selden 1997 Dunlop amp Selden 1997 Character14) is not considered to be homologous with the unusual shapeof the thorax in naraoiids recognised by their coding

40 Boundaries of anterior trunk segments reflexed antero-laterally (0) absent boundaries transverse or reflexed postero-laterally and (1) present

41 Joints between posterior tergites functional anteriorones variably fused (0) absent and (1) present

42 Posterior tergite bearing axial spine (0) absent and (1)present

Characters 40 to 42 are coded following Edgecombe ampRamskold (1999 characters 17 19 amp 23 respectively) Inaddition to the taxa considered here a thoracic axial spine ispresent in some olenelloid trilobites (see Lieberman 1998

Characters 73 amp 74) and in the aglaspidid Beckwithia (seeHesselbo 1989)

246 Posterior body termination 43 Postabdomen of seg-ments lacking appendages (0) absent and (1) present

44 Length of postabdomen (0) one segment (1) twosegments (2) three segments and (3) five segments In sometaxa a variable number of posterior thoracic segments bearcomplete tubular tergites and lack appendages In Aglaspisautapomorphically it seems that the posterior three thoracicsegments lack appendages (Briggs et al 1979 Hesselbo 1992)but have unmodified tergites These situations are recognisedas potentially homologous by the coding used here Character44 is coded as inapplicable to taxa lacking a postabdomen

45 Posterior tergites strongly curved in dorsal aspect com-pared to anterior tergites (0) absent and (1) present Asrecognised by Wills et al (1998a Character 17) in some taxawhere the tergites are distinct the curvature of thoracic tergitesincreases posteriorly so that the posterior tergites are highlycurved to semicircular in dorsal aspect This situation isnot known from crustaceans (except isopods) or stem grouparthropods

46 Posterior segments reduced and with highly reducedappendages (0) present and (1) absent In some taxa there area large number of posterior segments that are short sagitallyand have appendages that are much reduced in size comparedto anterior trunk appendages These somites are incorporatedinto the pygidium in some taxa but primitively they arecovered by tiny free trunk tergites In the derived state thetrunk somites and limbs are of a relatively constant size Thedistinction between these states can be seen when attempting tocount the number of segments that make up the body In taxashowing State 0 the number of segments is very high anddifficult to count whereas in other taxa the number ofpost-cephalic segments is easily assessed

47 Pygidium (0) absent and (1) present A pygidium isrecognised here as a posterior tagma consisting of a number offused segments under a single tergite which may or may notincorporate the post-segmental telson This is different to theuse of this term by Edgecombe amp Ramskold (1999) whichfollows Ramskold et al (1997) and very different to its use byWills et al (1998a p 53) as a synapomorphy of the TrilobitaThe present authorsrsquo definition recognises the situation inRetifacies where the spinose postsegmental telson is autapo-morphically not fused to the pygidium as homologous toother multisegmented posterior tagma which include thetelson The distinction between the pygidium and an expandedpost-segmental telson can also be seen in the position of theanus which is consistently in the posteriormost pre-telsonicsegment amongst putative arachnomorphs (see Character 48)In taxa with a pygidium the anal segment is fused into thepygidium (see eg Kuamaia Edgecombe amp Ramskold 1999fig 6) whereas in those taxa lacking a pygidium the anus isbetween the final trunk tergite and the telson

Ramskold et al (1997) argued that the posteriormost tergiteof xandarellids (which incorporates the anal segment) is nothomologous to the posterior tagma recognised as pygidiaherein because posterior thoracic tergites also cover multiplesegments in xandarellids (see Character 33) Evidence ofsegmentation of the pygidium in a variety of taxa suggests thatthe number of tergites fused to form the pygidium is greaterthan the number of appendages For example the pygidium ofKuamaia has two pairs of lateral spines but at least fourpairs of appendages (Hou amp Bergstrom 1997 Edgecombe ampRamskold 1999) and that of Triarthrus has only five axialrings posterior to the articulating ring but over 10 pairs ofappendages (Whittington amp Almond 1987) The fact thatdecoupling of tergites and segments is evident in the thorax of

184 TREVOR J COTTON AND SIMON J BRADDY

Xandarella and Cindarella does not necessarily suggest that asimilar more widespread decoupling in pygidia is the result ofa different developmental process and hence that they arenon-homologous Instead the situation in the xandarellidthorax is potentially homologous to that found (more widely)in pygidia and consequently the terminal xandarellid tergite isa pygidium It is unclear if decoupling of tergites and somites isprimitively limited to the terminal tergite or primitively aproperty of the arachnomorph post-cephalon as a whole

It has been suggested that the tiny pygidium of olenelloidtrilobites which probably best reflects the primitive trilobitestate consists only of the post-segmental telson (Harrington inMoore 1959) and therefore is not a true pygidium HoweverWhittington (1989) showed that the olenelloid pygidium con-sists of at least two and possibly as many as five or sixsegments and therefore is likely to be homologous with thepygidium of other trilobites

48 Position of the anus (0) terminal within telson and (1)at base of telson In crustaceans and stem group arthropodsthe anus is terminal or otherwise situated in the telson (egRehbachiella Walossek 1993 fig 15D) In putative arachno-morphs the anus is either at the junction of the posteriormostthoracic segment and the telson or ventral within a fusedpygidium In the case of taxa with a pygidium the anus isanterior to the posterior margin and where known positionedbetween the posteriormost pair of appendages suggesting thatit is anterior to the post-segmental telson (see eg OlenoidesWhittington 1980)

49 Pygidium with median keel (0) absent and (1) presentEdgecombe amp Ramskold (1999 Character 21) considered thepresence of a median keel on the pygidium as a synapomorphyuniting Soomaspis and Tarricoia They did not explain howthis structure could be distinguished from the raised pygidialaxis of trilobites Homology of these structures is not sup-ported here because the axis of more primitive trilobites(including Eoredlichia) is quite distinct morphologically fromthe naraoiid keel Budd (1999a) noted the presence of a keel onthe pygidium of Buenaspis and suggested (Budd 1999a p 102)that it may be an artefact of dorsoventral compaction How-ever contrary to the coding of Edgecombe amp Ramskold(1999) a keel is visible on the pygidium of Liwia convexa (Dzikamp Lendzion 1988 fig 4CD) preserved in full relief Thereforeit seems unlikely that the keel is a taphonomic artefact

50 Pygidium with broad-based median spine (0) absentand (1) present

51 Pygidium with lateral spines (0) present and (1) absentEdgecombe amp Ramskold (1999 Character 22) coded thepresence of a median spine and two pairs of lateral spines aspotentially homologous in Sinoburius Kuamaia and HelmetiaThe distribution of lateral spines is much wider than that ofterminal spines and therefore they are coded separately hereIn trilobites it has widely been recognised that lateral spinesrepresent the original segmentation of the pygidium Thiscannot be the case for median spines Characters 49 to 51are not applicable to taxa lacking a pygidium (Character 47State 0)

52 Expanded post-segmental telson (0) absent and (1)present In a range of taxa the posterior-most tergite is a large(relative to the thoracic tergites) structure that lacks anyevidence of segmentation and is interpreted as representing anexpanded tergite of the post-segmental telson from whichsegments are released anteriorly during ontogeny On the otherhand the posteriormost tergite of Marrella and probablyMimetaster is small compared to the thoracic tergites androunded This element probably represents the plesiomorphiceuarthropod telson The homology of this character in mosttaxa is clear and only doubtful in Retifacies in which it may

be segmented The figures of Hou amp Bergstrom (1997) providelittle unequivocal evidence for a telson in Retifacies but thepresence of this structure is very clear in colour photographs(eg Chen et al 1996 fig 198A) However these figures donot convincingly demonstrate the segmentation of the telsonRetifacies is interpreted here as being unique in possessingboth a pygidium and an expanded post-segmental telson

53 Telson shape (0) spinose and (1) paddle-shaped Theshape of the expanded telsons identified above varies frombroadly spinose to flattened and paddle-like This differencecan be recognised both on the basis of the ratio of length tobasal width (spinose telsons are relatively long) and by thechange in width posteriorly (spinose telsons reduce in widthposteriorly whereas paddle-shaped telsons increase in width)In eurypterids a wide range of telson shapes is found includ-ing both character states described here It is likely that theplesiomorphic condition is a spinose telson This character isinapplicable to taxa lacking an expanded telson

54 Post-ventral furcae (0) absent and (1) present Thischaracter recognises the potential homology of unsegmentedpaired structures that articulate with the segment immediatelyanterior to the telson The potential homology of these struc-tures (the post-ventral plates) in Emeraldella and Aglaspis wasrecognised by Wills et al (1998a Character 70) and inEmeraldella and Sidneyia by Edgecombe amp Ramskold (1999Character 25) The homology of the segmented pygidial caudalfurcae of Olenoides (see Whittington 1975a 1980) with thesestructures is considered doubtful and coded as equivocal

Despite their distinctive morphology the long caudualfurcae of Cheloniellon may be homologous with the furcae ofAglaspis Emeraldella and Sidneyia This is supported by thecaudal furcae of the Ordovician cheloniellid Duslia which aremore similar in morphology to those of Emeraldella than ofCheloniellon Chlupac (1988 pp 614ndash16) considered thesestructures to be on the terminal tergite in Duslia However afaint tergite boundary is apparent posterior to the attachmentof the furcae (see Chlupac 1988 pl 57 fig 4) Therefore theyare considered here to have been attached to the pre-telsonicsegment as in Cheloniellon Chlupac (1988) and Dunlop ampSelden (1997) supported a close relationship between Dusliaand Cheloniellon

25 MethodsAll analyses were carried out using PAUP Version 40b4a(Swofford 1999) and unless otherwise stated used heuristicsearches with one thousand random addition sequencereplicates The software packages MacClade Version 307(Maddison amp Maddison 1997) and RadCon (Thorley amp Page2000) were used for comparing trees and investigating patternsof character evolution Tree length and other statistics (theConsistency Index CI and Retention Index RI) were calcu-lated by PAUP and MacClade with uninformative charactersexcluded Analyses were run separately using each of thedifferent codings for aglaspidids

Characters were treated as unordered and of equal weight inmost analyses To assess the influence of this assumption fourof the eight multistate characters which have states that areintermediate between others (Wilkinson 1992) were treated asordered in some analyses These characters are the number ofcephalic segments (Character 4) the length of raptorialappendage spines (Character 6) the degree of overlap of thethorax by the head-shield (Character 37) and the number ofsegments making up the postabdomen (Character 44) The lastof these is uninformative when not ordered because only onetaxon shows each of states 0 1 and 3

Support for individual nodes was assessed by bootstrapanalysis (Felsenstein 1985) and by calculating Bremer support

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 185

indices (Bremer 1988 1994) These methods measure twodistinct aspects of support for phylogenetic hypotheses Treesor nodes may be considered well supported (1) to the extent towhich alternative topologies are much less parsimonious asmeasured by the support index (Wilkinson 1996) or (2) wherethey are consistent with a large proportion of characters sothat character sampling is unlikely to have had much influenceon topology as assessed by bootstrapping (Page 1996) Boot-strapping was performed with 100 bootstrap replicates eachof 10 addition sequence replicates Bremer support indicesare based on heuristic searches with 100 addition sequencereplicates

Three pairs of terminals are included in Table 2 that apartfrom missing data are coded identically These are Helmetiaand Kuamaia Soomaspis and Tariccoia and Saperion andSkioldia These pairs are lsquotaxonomic equivalentsrsquo removingthe taxon with most missing data will not influence theposition of the other They can be recoded as single terminalsaccording to the principal of safe taxonomic reduction(Wilkinson 1995a b) and were not considered separately in theanalyses presented here Therefore cladistic analyses werecarried out on the basis of 33 terminals Edgecombe ampRamskold (1999) ignored this and coded the same three pairsof taxa and Eoredlichia and Olenoides identically for theirmore limited set of characters

26 ResultsAnalysis with all characters unordered and using the lsquoAglaspi-dida 1rsquo coding found nine most parsimonious trees (MPTs)each 126 steps long (CI=0middot556 RI=0middot763) Using the lsquoAglas-pidida 2rsquo coding resulted in 18 MPTs of length 125 (CI=0middot560RI=0middot765) Nine of these 18 trees were those found with thefirst coding of aglaspidids The strict component consensus(sensu Wilkinson 1994) of the 27 unique trees found acrossboth analyses is shown in Figure 8 All trees supported themonophyly of the Marrellomorpha (Marrella and Mimetaster)and of a clade including all other taxa except Crustacea (Clade1 of Fig 8) The relationships between these two clades and theCrustacea were unresolved equal numbers of trees in bothanalyses supported each of the topologies [Clade 1 (CrustaceaMarrellomorpha)] [Crustacea (Marrellomorpha Clade 1)]and [Marrellomorpha (Crustacea Clade 1)] All trees placedBurgessia as the sister taxon to the rest of Clade 1 andsupported the division of the rest of Clade 1 into two majorsister groups The topology of one of these clades (Clade 2)which included trilobites helmetiids tegopeltids naraoiidsxandarellids and Retifacies was stable with respect to thecoding of aglaspidids The other clade (Clade 3) consists of aclade of megacheirans and chelicerates (Clade 4) and a cladeincluding aglaspidids Lemoneites Paleomerus CheloniellonEmeraldella and Sidneyia (Clade 5) This latter clade is un-resolved in the strict consensus of all 27 trees (Fig 8) and thestrict consensus of the 18 MPTs found using the lsquoAglaspidida2rsquo coding but was fully resolved in the consensus of the ninetrees found with the lsquoAglaspidida 1rsquo coding Within themegacheiran-chelicerate clade all trees found Fortiforceps tobe the sister group to all other taxa Yohoia the sister group tochelicerates and Alalcomenaeus and Leanchoilia to form aclade Jianfengia is equally parsimoniously placed as the sistergroup to the Yohoia-chelicerate clade or as the sister group tothis clade and the Alalcomenaeus-Leanchoilia clade

Treating characters 4 6 37 and 44 as ordered had a minimaleffect on most parsimonious topology Nine MPTs of 136 steps(CI=0middot515 RI=0middot743) were found with the lsquoAglaspidida 1rsquocoding and 12 MPTs 135 steps in length (CI=0middot519 RI=0middot745)with the lsquoAglaspidida 2rsquo coding These trees all supported a(Crustacea Clade 1) clade excluding the marrellomorphs

Apart from this the strict consensus of these 21 trees isidentical to the consensus of the 27 trees found with allcharacters treated as unordered

Across all analyses four different topologies for Clade 5 (ofFig 8) were found (Fig 9) The distribution of these topologiesacross the four analyses described above and across the sameanalyses repeated after reweighting of each character in pro-portion to their Rescaled Consistency Index (see Farris 1989)from the first analysis is shown in Table 3 The instability of

Figure 8 Strict consensus of 27 MPTs from four separate analyses ofarachnomorph phylogeny with both interpretations of aglaspididmorphology and with some characters treated as ordered or allcharacters unordered Clades referred to in the text are numbered

Figure 9 Alternative equally parsimonious resolutions of clade 5 ofFig 8 (see Table 3)

186 TREVOR J COTTON AND SIMON J BRADDY

this clade was to some extent caused by the inclusion ofLemoneites and Paleomerus which could each be coded foronly 27 of the 54 characters used Analysis with these two taxaexcluded and characters treated as unordered found treescompatible only with Topology D of Figure 9 irrespective ofthe coding used for aglaspidids Analysis with some characterstreated as ordered and using either aglaspidid coding foundthree trees compatible only with Topology B of Figure 9 andthree compatible only with Topology D of Figure 9 ThereforeTopology D is the preferred topology for Clade 5 as shown inTable 3

One of the MPTs resulting from the lsquoAglaspidida 2rsquo coding(with all characters treated as unordered) was chosen as thebasis for further discussion because this tree combined thepreferred topology of Clade 5 with the (Crustacea Clade 1)group that was favoured when some characters were treated asordered Bootstrap percentages and Bremer support values forthis tree (using the Aglaspidida 2 coding) are shown in Figure10 Apomorphies for all ingroup nodes of this tree are shownin Figure 10A

3 Discussion

31 Phylogenetic resultsThe results of the cladistic analyses presented above arewell-resolved and robust with respect to different analyticalparameters Most of the taxa considered form a clade (Clade 1of Fig 8) that is more closely related to chelicerates than tocrustaceans in all analyses and therefore is recognised asthe Arachnomorpha as defined by Chen et al (1996) andRamskold et al (1996) The monophyly of the Marrellomor-pha was supported in all analyses The marrellomorphs werefound to be the sister group to a Crustacea+Clade 1 group inmost MPTs but in a minority of trees formed the sister groupto Clade 1 alone According to the latter result they should beincluded within the Arachnomorpha but according to the firstexcluded from it Both of these alternatives have been sup-ported in previous studies (eg Hou amp Bergstrom 1997 Willset al 1998a respectively) In addition to the marrellomorphsthe Fuxianhuiida Bousfield 1995 and Canadaspidida havebeen placed in the Euarthropoda outside the main mandibu-late and arachnomorph clades (Hou amp Bergstrom 1997)Further study of these taxa of the crustacean or mandibulatestem group and of the marrellomorphs is necessary before the

position of the Marrellomorpha can be resolved Provisionallymarrellomorphs are excluded from the Arachnomorphafollowing the majority of analyses above and Wills et al(1994)

The Burgess Shale arthropod Burgessia was placed as thesister group to remaining arachnomorphs in all analyses OtherArachnomorpha form two major clades one consisting of thelsquotrilobite-alliedrsquo arachnomorphs analysed by Edgecombe ampRamskold (1999) here termed lsquoTrilobitomorpharsquo (defined asTrilobita and all taxa more closely related to Trilobita thanChelicerata) and the other (lsquochelicerate-alliedrsquo) clade includingchelicerates lsquogreat appendagersquo arthropods aglaspididsLemoneites Paleomerus Sidneyia and Emeraldella The latteris here termed lsquoCheliceramorpharsquo (defined as Chelicerata andall taxa more closely related to Chelicerata than Trilobita) Thetopology of the lsquotrilobite-alliedrsquo clade was similar to that foundby Edgecombe amp Ramskold (1999) but the present authorsrsquoresults are more completely resolved unambiguously support-ing the sister-group relationship between trilobites andHelmetiida suggested in the previous study Within thelsquochelicerate-alliedrsquo clade Aglaspidids Paleomerus Lemoneitesand the Burgess Shale Emeraldella and Sidneyia form a cladein opposition to megacheirans and chelicerates

The present authorsrsquo analysis supports a new hypothesis ofthe origin of the chelicerates from within a paraphyleticassemblage of megacheiran arthropods and confirms pyc-nogonids as a sister group to the euchelicerates Previoushypotheses of the chelicerate sister group (see Section 1) aremuch less parsimonious The shortest trees supporting aCheloniellon-chelicerate clade are six steps longer than theMPTs and those supporting an aglaspidid-chelicerate cladeseven steps longer All megacheirans and chelicerates areunited by the flap-shaped rounded exopods and modificationof the second cephalic segment endopods into anteriorlydirected spinose raptorial organs which share a number ofdetailed similarities A clade including all of these taxa apartfrom Fortiforceps is supported by the synapomorphic loss ofthe antennae (or antennulae of crustaceans) and the longerlength of spinose projections of the second cephalic appendageendopods Within this clade Yohoia shares the loss of cephalicexopods the loss of thoracic endopods and a postabdomen oftubular tergites with chelicerates

The poor resolution of Clade 5 (of Fig 8) was partly a resultof the poor preservation of and consequent high proportion ofmissing data for Lemoneites and Paleomerus When these two

Table 3 Numbers of most parsimonious trees supporting each of the alternative topologies for clade 4 of Fig 7 under different analyticalconditions (RCI=Revealed Consistency Index)

Aglaspididcoding

Characters 4 6 36and 43 ordered

Charactersreweighted by RCI

Paleomerus andLemoneites excluded

Topology (see Fig 8)

A B C D

1 91 X 3 3 31 X 61 X X 31 X 61 X X 3 32 9 92 X 3 3 3 32 X 62 X X 32 X 62 X X 3 3Total 6 18 12 54

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 187

taxa were excluded from the analyses only two (B and D) ofthe four topologies found in the complete analysis remainedmost parsimonious (see Fig 9 and Table 3) Secondly Bremersupport values throughout much of the lsquochelicerate-alliedrsquoclade (Clade 3 of Fig 8) were improved when these taxa wereexcluded (see Fig 11)

Dunlop (1999 p 258) suggested that in reconstructing thechelicerate stem lineage lsquowe might predict that the two mostsignificant changes towards the chelicerate condition are thereduction of the antennae and the formation of the nextappendages into a clawrsquo According to the hypothesis pre-sented here both of these adaptations were achieved in EarlyCambrian chelicerate ancestors The recognition of the loss ofthe antennae in the stem group of the chelicerates and thephylogenetic hypothesis presented here suggest that chelicer-ates are most closely related to taxa with only a singleadditional segment fused to the crown group eucheliceratehead of four fused somites Previous hypotheses have derivedchelicerates from Palaeozoic arthropods with a longer head(eg Cheloniellon Sturmer amp Bergstrom 1978 SanctacarisBriggs amp Collins 1988 Emeraldella Bruton amp Whittington1983) The plesiomorphic pattern of head segmentation ismatched in pycnogonids Bergstrom et al (1980) suggestedthat only the anterior four pairs of appendages are incorpo-rated into the head This supports the view that pycnogonidsare primitive with respect to other chelicerates which showmore or less complete fusion of this primitive head and threethoracic segments including Solpugida and Pseudoscorpiones

The present study suggests that a shift from gnathobasicbenthic feeding to a more vagrant lifestyle and raptorialappendage feeding occurred very early in the chelicerate lin-eage Appendage feeding was clearly an important adaptation

Figure 10 One of 18 equally parsimonious cladograms with lsquoAglaspidida 2rsquo coding and all characters unorderedthat is most congruent with results from other analyses Non-terminal branch lengths scaled to reflect number ofapomorphies (A) ACCTRAN apomorphy scheme Character numbers shown above boxes and states shownbelow boxes as in Table 2 and the text Character consistency indices are indicated by shading (B) Levels ofsupport for individual nodes Bremer support indices shown in bold type above branches Bootstrap percentagesin italic type below branches for all nodes with relative frequencies greater than 50

Figure 11 Bremer support values for clades within the lsquochelicerate-alliedrsquo arachnomorphs (clade 3 of Fig 8) when all taxa wereincluded (normal weight font above nodes) and with Lemoneites andPaleomerus excluded (bold type below nodes)

188 TREVOR J COTTON AND SIMON J BRADDY

for the terrestrialization of chelicerates (Dunlop 1997 p 69Dunlop amp Webster 1999) but the model proposed heresuggests that this considerably pre-dated terrestrializationGnathobasic feeding in xiphosurans and eurypterids mayrepresent a reversal to a more primitve benthic arachnomorphlifestyle from a more pycnogonid-like ancestor

32 Evolution of the arachnomorph headThe extant arthropod classes show highly conserved patternsof head segmentation (eg Wills et al 1994) In contrast therehas been a near consensus amongst Cambrian arthropodworkers (although rarely based on an explicit phylogenetichypothesis) that patterns of head segmentation are highlyhomoplastic and hence of little systematic significanceSturmer amp Bergstrom (1978 pp 78ndash9) suggested that lsquoevenclosely related forms may have different numbers of headsegments and appendagesrsquo Bruton amp Whittington (1983pp 576ndash7) that lsquodiscussion on fossil arthropod relationshipsbased on head segmentation appears to be largely irrel-evant and at best speculativersquo and Delle Cave amp Simonetta(1991 p 191) that lsquothere are no obvious phyletic affinitiesbetween genera having the same number of cephalized seg-mentsrsquo Most recently Bergstrom amp Hou (1997 p 104) con-cluded that lsquothe segmental length of the head shield seemsto be of no relevance to the discussion and discrimination ofevolutionary lineagesrsquo It has been claimed that this view issupported by cladistic analyses in which patterns of tagmosishave been found to be rather poor at defining major arthropodclades (Briggs et al 1992 Wills et al 1994)

This suggestion is not only of relevance to systematics buthas featured prominently in recent debates surrounding thenature of the lsquoCambrian explosionrsquo (see Budd amp Jensen 2000for a recent review) Gould (1989 1991) argued that theapparent plasticity of head segmentation in Cambrian com-pared to post-Cambrian arthropod evolution suggests thatbody-plan evolution was only constrained after the Cambrianexplosion According to this view the origin of new highertaxa with distinct body-plans was only possible before theonset of these constraints (eg Valentine 1995) Taxa hererecognised as arachnomorphs clearly played a major role in thedevelopment of Gouldrsquos hypothesis This suggestion has pre-viously been assessed in terms of overall morphological diver-sity which is clearly rather distinct from Gouldrsquos concept ofdisparity Wills et al (1998b) showed that there was littlecorrelation between overall morphological disparity and de-gree of limb specialisation and that the latter was phylogeneti-cally highly plastic They remarked that (Wills et al 1998bp 64) lsquothis finding has important implications for models ofarthropod phylogeny and evolution that attribute overidingimportance to head segmentationrsquo but did not explicitlyexamine head segmentation although it is likely to have beena major component of the index of tagmosis employed

The results of the present study suggest that arguments thatarthropod head segmentation was unusually labile during theCambrian are poorly founded Rather only four major pat-terns of euarthropod head segmentation are identified Theplesiomorphic euarthropod state according to both Walossekamp Muller (1997 1998) and Scholtz (1997) which may moreproperly be the plesiomorphic state for the euarthropod crowngroup only (depending on the phylogenetic position of theMarrellomorpha) consists of four post-acronal segmentsbearing the antennae and three pairs of biramous limbs Thepresent authors suggest that the term lsquocephalonrsquo be restrictedto this kind of head which is found in stem group mandibu-lates in addition to many arachnomorphs Crown groupmandibulates share a lsquobimaxillary headrsquo in which an addi-tional pair of appendages the labium or second maxillae are

incorporated into the head but in crustaceans these are notfused to the carapace (Scholtz 1997) Finally two distinctforms of head tagmosis are found in chelicerates A head withfour pairs of appendages (but without antennae giving a totalof five segments incorporated into the head) which has beencalled the lsquocephalosomarsquo is present in pycnogonids (Walossekamp Dunlop 2002 pp 434ndash5) and probably also in some euch-elicerates (J A Dunlop pers comm 2003) The lsquoprosomarsquo ofmost euchelicerates consists of this cephalosoma and twoadditional segments

In general these patterns of head segmentation are highlyphylogenetically conserved (see Fig 12) Only three homo-plastic changes namely reversal to a more primitive conditionin the Leanchoilia-Alalcomenaeus clade and the convergentorigin of a five-segmented head in Clade 3 (of Fig 8) and theXandarellida are necessary to optimise character 4 onto themost parsimonious cladogram There are also transitions toautapomorphic states in Emeraldella and Sidneyia In the caseof Sidneyia however an argument could be made (followingBergstrom amp Sturmer 1978 see discussion of Character 4) thatthe head consists of five segments as in closely related taxabut autapomorphically each segment has a separate tergiteThis would further increase the phylogenetic stability of headsegmentation Also following Bergstrom amp Sturmer (1978) thehead of Cheloniellon could be considered to consist of twotergites making it more similar to the head of Emeraldella(with six pairs of appendages)

Figure 12 Cladogram from Fig 10 showing changes in head segmen-tation Bold type shows abbreviated head segmentation formulaAbbreviations (Ant) antennae (BA) biramous appendage pairs (Ch)chelicerae (GA) great appendages (UA) uniramous appendage pairsand (X) segment without appendages Synapomorphy A is the auta-pomorphic reduction in head length to a single segment in SidneyiaAlternatively this may represent secondary division of the headshield Synapomorphy B is the increase in the length of the head inEmeraldella to six segments

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 189

4 Acknowledgements

We thank D E G Briggs J A Cotton J A Dunlop R AFortey S Harris R A Moore P Selden O E Tetlie andP Van Roy for comments on earlier drafts of this work Themanuscript was reviewed by M Wills F Schram and oneanonymous referee This research was supported by the Geo-logical Society of London Curry Research Studentshipawarded to TJC by the Department of Earth Sciences Uni-versity of Bristol and by the Department of PalaeontologyThe Natural History Museum London TJC would like tothank C Labandeira J Thompson and R Chapman fortheir hospitality at the National Museum of Natural HistoryWashington DC USA and A Olcott S Schellenberg andJ Young for making his time there so enjoyable

5 References

Abzhanov A Popadic A amp Kaufman T C 1999 Chelicerate Hoxgenes and the homology of arthropod segments Evolution andDevelopment 1 77ndash89

Akam M 2000 Arthropods developmental diversity within a (super)phylum Proceedings of the National Academy of Sciences USA97 4438ndash41

Aldridge R J Gabbott S E amp Theron J N 2001 The Soom ShaleIn Briggs D E G amp Crowther P R (eds) Palaeobiology II340ndash2 Oxford Blackwell Science

Anderson L I amp Selden P A 1997 Opisthosomal fusion andphylogeny of Palaeozoic Xiphosura Lethaia 30 19ndash31

Arango C P 2002 Morphological phylogenetics of the sea spiders(Arthropoda Pycnogonida) Organisms Diversity and Evolution2 107ndash25

Ax P 1986 Das phylogenetische System Stuttgart Gustav FischerVerlag

Babcock L E amp Zhang Wentang In press New non-mineralizedarthropods from the Chengjiang Lagerstatte Lower CambrianYunnan China Palaeontologia Cathyana

Bartels C Briggs D E G amp Brassel G 1998 The fossils ofthe Hunsruck Slate Marine life in the Devonian CambridgeCambridge University Press

Bergstrom J 1971 Paleomerus ndash merostome or merostomoid Lethaia4 393ndash401

Bergstrom J 1979 Morphology of fossil arthropods as a guideto phylogenetic relationships In Gupta A P (ed) ArthropodPhylogeny 3ndash56 New York NY Van Nostrand Reinhold Co

Bergstrom J 1992 The oldest arthropods and the origin of theCrustacea Acta Zoologica 73 287ndash91

Bergstrom J Sturmer W amp Winter G 1980 PalaeoisopusPalaeopantopodus and Palaeothea pycnogonid arthropodsfrom the Lower Devonian Hunsruck Slate West GermanyPalaontologische Zeitschrift 54 7ndash54

Bergstrom J amp Brassel G 1984 Legs in the trilobite Rhenops fromthe Lower Devonian Hunsruck Slate Lethaia 17 67ndash72

Bergstrom J amp Hou X 1998 Chengjiang arthropods and theirbearing on early arthropod evolution In Edgecombe G D(ed) Arthropod fossils and phylogeny 151ndash84 New York NYColumbia University Press

Boudreaux H B 1979 Significance of intersegmental tendon systemin arthropod phylogeny and a monophyletic classification ofArthropoda In Gupta A P (ed) Arthropod Phylogeny 551ndash86New York NY Van Nostrand Reinhold Co

Bousfield E L 1995 A contribution to the natural classification ofLower and Middle Cambrian arthropods food-gathering andfeeding mechanisms Amphipacifica 2 3ndash33

Braddy S J Aldridge R J Gabbott S E amp Theron J N 1999Lamellate book-gills in a late Ordovician eurypterid from theSoom Shale South Africa support for a eurypterid-scorpionclade Lethaia 32 72ndash4

Braddy S J amp Almond J E 1999 Eurypterid trackways from theTable Mountain Group (Ordovician) of South Africa Journal ofAfrican Earth Sciences 29 165ndash77

Bremer K 1988 The limits of amino acid sequence data in angio-sperm phylogenetic reconstruction Evolution 42 795ndash803

Bremer K 1994 Branch support and tree stability Cladistics 10295ndash304

Briggs D E G 1998 Review of Arthropods from the LowerCambrian Chengjiang fauna southwest China by HouXianguang and Jan Bergstrom Fossils and Strata 45

Scandinavian University Press Oslo Norway 1997 Palaeo-geography Palaeoclimatology Palaeoecology 143 193ndash4

Briggs D E G Bruton D L amp Whittington H B 1979 Append-ages of the arthropod Aglaspis spinifer (Upper CambrianWisconsin) and their significance Palaeontology 22 167ndash80

Briggs D E G Fortey R A amp Wills M A 1992 Morphologicaldisparity in the Cambrian Science 256 1670ndash3

Briggs D E G Fortey R A amp Wills MA 1993 How big was theCambrian explosion A taxonomic and morphologic comparisonof Cambrian and Recent arthropods In Lees D R amp EdwardsD (eds) Evolutionary patterns and processes 33ndash44 LondonLinnean Society of London Linnean Society Symposium Series

Briggs D E G Erwin D H amp Collier F J 1994 The fossils of theBurgess Shale Washington DC Smithsonian Institution Press

Briggs D E G amp Bartels C 2001 New arthropods from the LowerDevonian Hunsruck Slate (Lower Emsian Rhenish Massifwestern Germany) Palaeontology 44 275ndash304

Briggs D E G amp Bartels C 2002 Magnoculocaris a new name forthe arthropod Magnoculus Briggs and Bartels 2001 from theLower Devonian Hunsruck Slate Germany Palaeontology 45419

Briggs D E G amp Collins D 1988 A Middle Cambrian cheliceratefrom Mount Stephen British Columbia Palaeontology 31 779ndash98

Briggs D E G amp Collins D 1999 The arthropod Alalcomenaeuscambricus Simonetta from the Middle Cambrian Burgess Shale ofBritish Columbia Paleontology 42 953ndash78

Briggs D E G amp Fortey R A 1982 The cuticle of aglaspididarthropods a red-herring in the early history of vertebratesLethaia 15 25ndash9

Briggs D E G amp Fortey R A 1989 The early radiation andrelationships of the major arthropod groups Science 246 241ndash3

Briggs D E G amp Fortey R A 1992 The early Cambrian radiationof arthropods In Lipps J H amp Signor P W (eds) Origin andEarly Evolution of the Metazoa 335ndash373 New York NY PlenumPress

Briggs D E G amp Robison R A 1984 Exceptionally preservednontrilobite arthropods and Anomalocaris from the MiddleCambrian of Utah University of Kansas PaleontologicalContributions Paper 111 1ndash23

Broili F 1932 Ein neuer Crustacee aus dem rheinischen UnterdevonSitzungsberichte der Mathematisch-Naturwissenschaftlichen(Abteilung) Klasse der Bayerischen Akademie der Wissenschaftenzu Munchen 1932 27ndash38

Broili F 1933 Ein zweites Exemplar von Cheloniellon Sitzungsber-ichte der Bayerischen Akademie der Wissenschaften 1933 11ndash32

Bruton D L 1981 The arthropod Sidneyia inexpectans MiddleCambrian Burgess Shale British Columbia Philosophical Trans-actions of the Royal Society of London Series B 295 619ndash56

Bruton D L amp Whittington H B 1983 Emeraldella and Leanchoiliatwo arthropods from the Burgess Shale British Columbia Philo-sophical Transactions of the Royal Society of London Series B 300553ndash85

Budd G E 1995 Kleptothule rasmusseni gen et sp nov anolenellid-like trilobite from the Sirius Passet fauna (BuenFormation Lower Cambrian North Greenland) Transactions ofthe Royal Society of Edinburgh Earth Sciences 86 1ndash12

Budd G E 1996a Progress and problems in arthropod phylogenyTrends in Ecology and Evolution 11 356ndash8

Budd G E 1996b The morphology of Opabinia regalis and thereconstruction of the arthropod stem-group Lethaia 29 1ndash14

Budd G E 1997 Stem-group arthropods from the Lower CambrianSirius Passet fauna of North Greenland In Fortey R A ampThomas R H (eds) Arthropod Relationships 125ndash38 LondonChapman and Hall Systematics Association Special Volume 55

Budd G E 1999a A nektaspid arthropod from the Early CambrianSirius Passet fauna with a description of retrodeformation basedon functional morphology Palaeontology 42 99ndash122

Budd G E 1999b The morphology and phylogenetic significance ofKerygmachela kiergkegaardi Budd (Buen Formation LowerCambrian N Greenland) Transactions of the Royal Society ofEdinburgh Earth Sciences 89 (for 1998) 249ndash290

Budd G E 2002 A palaeontological solution to the arthropod headproblem Nature 417 271ndash5

Budd G E amp Jensen S 2000 A critical reappraisal of the fossilrecord of the bilaterian phyla Biological Reviews 75 253ndash95

Chen J Zhou G Zhu M amp Yeh K 1996 The Chengjiang biota ndasha unique window of the Cambrian explosion Taichung NationalMuseum of Natural Science [In Chinese]

Chen J Edgecombe G D amp Ramskold L 1997 Morphological andecological disparity in naraoiids (Arthropoda) from the Early

190 TREVOR J COTTON AND SIMON J BRADDY

Cambrian Chengjiang fauna China Records of the AustralianMuseum 49 1ndash24

Chen J amp Zhou G 1997 Biology of the Chengjiang fauna In ChenJ Cheng Y amp Iten H V (eds) The Cambrian explosion and thefossil record Bulletin of the National Museum of Natural ScienceTaiwan 10 11ndash106

Child C A 1997 Some deep-sea Pycnogonida from Argentine slopeand basin Proceedings of the Biological Society of Washington110 128ndash42

Chlupac I 1988 The enigmatic arthropod Duslia from the Ordovicianof Czechslovakia Palaeontology 31 611ndash20

Chlupac I 1995 Lower Cambrian arthropods from the PasekyShale (Barrandian area Czech Republic) Journal of the CzechGeological Society 40 9ndash36

Chlupac I amp Havlıcek V 1965 Kodymirus n g a new aglaspidmerostome of the Cambrian of Bohemia Sbornık GeologickechVid Paleontologie 6 7ndash20

Cisne J L 1975 Anatomy of Triarthrus and the relationships of theTrilobita Fossils and Strata 4 45ndash63

Cisne J L 1981 Triarthrus eatoni (Trilobita) anatomy of its exo-skeletal skeletomuscular and digestive systems Palaeonto-graphica Americana 9 95ndash142

Conway Morris S 1982 Atlas of the Burgess Shale London ThePalaeontological Association

Conway Morris S 1989 The persistence of Burgess Shale-type faunasimplications for the evolution of deeper-water faunas Trans-actions of the Royal Society of Edinburgh Earth Sciences 80271ndash83

Conway Morris S 1998 The crucible of creation Oxford OxfordUniversity Press

Cotton T J 1999 Review of Arthropod fossils and phylogenyPalaeogeography Palaeoclimatology Palaeoecology 151 341ndash3

Cotton T J 2002 The phylogeny and morphological evolution ofCambrian trilobites and their relatives Unpublished PhD ThesisUniversity of Bristol

Damen W G M Hausdorf M Seyfarth E A amp Tautz D 1998 Aconserved mode of head segmentation in arthropods revealed bythe expression pattern of Hox genes in a spider Proceedings of theNational Academy of Sciences USA 95 10665ndash70

Delle Cave L amp Simonetta A M 1991 Early Palaeozoic arthropodsand problems of arthropod phylogeny with some notes on taxa ofdoubtful affinities In Simonetta A M amp Conway Morris S(eds) The early evolution of Metazoa and the significance ofproblematic taxa 189ndash244 Cambridge Cambridge UniversityPress

Dewel R A amp Dewel W C 1997 The place of tardigrades inarthropod evolution In Fortey R A amp Thomas R H (eds)Arthropod Relationships 109ndash23 London Chapman and HallSystematics Association Special Volume 55

Dunlop J A 1997 Palaeozoic arachnids and their significance forarachnid phylogeny Proceedings of the 16th European Colloquiumof Arachnology 65ndash82 Siedlce Poland

Dunlop J A 1998 The origins of tetrapulmonate booklungs and theirsignificance for chelicerate phylogeny In Selden P A (ed)Proceedings of the 17th European Colloquium of ArachnologyEdinburgh 1997 pp 9ndash16

Dunlop J A 1999 Pasando revista a la evolucion de los quelicerados[A review of chelicerate evolution] In Melic A DeHaro J JMedez M amp Ribera I (eds) Volumen Monografico Evolucion yFilogenia de Arthropoda Boletin de la Sociedad EntolomologicaAragonesa 26 255ndash272 [In Spanish and English]

Dunlop J A amp Selden P A 1997 The early history and phylogenyof the chelicerates In Fortey R A amp Thomas R H (eds)Arthropod relationships Systematics Association Special Volume55 221ndash35 London Chapman and Hall

Dunlop J A amp Webster M 1999 Fossil evidence terrestrializationand arachnid phylogeny Journal of Arachnology 27 86ndash93

Dzik J 1993 Early metazoan evolution and the meaning of its fossilrecord Evolutionary Biology 27 339ndash86

Dzik J amp Lendzion K 1988 The oldest arthropods of the EastEuropean Platform Lethaia 21 29ndash38

Edgecombe G D Wilson G D F Colgan D J Gray M R ampCassis G 2000 Arthropod cladistics combined analysisof Histone H3 and U2 snRNA sequences and morphologyCladistics 16 155ndash203

Edgecombe G D amp Ramskold L 1999 Relationships of CambrianArachnata and the systematic position of Trilobita Journal ofPaleontology 73 263ndash87

Emerson M J amp Schram F R 1997 Theories patterns and realitygame plan for arthropod phylogeny In Fortey R A amp Thomas

R H (eds) Arthropod Relationships Systematics AssociationSpecial Volume 55 67ndash87 London Chapman and Hall

Farris J S 1989 The Retention Index and the Rescaled ConsistencyIndex Cladistics 5 417ndash19

Felsenstein J 1985 Confidence limits on phylogenies an approachusing the bootstrap Evolution 39 783ndash91

Flower R H 1968 Merostomes from a Cotter Horizon of the El PasoGroup New Mexico Bureau of Mines and Mineral ResourcesMemoir 22 35ndash44

Fortey R A 1990a Trilobite evolution and systematics In MikulicD G (ed) Arthropod paleobiology 44ndash65 Short Courses inPaleontology 3 Knoxville Tenessee The Paleontological Society

Fortey R A 1990b Ontogeny hypostome attachment and trilobiteclassification Palaeontology 33 529ndash76

Fortey R A 1997 Classification In Kaesler R L (ed) Treatise oninvertebrate paleontology Part O Arthropoda 1 (revised) 289ndash302Boulder Colorado amp Lawrence Kansas Geological Society ofAmerica and University of Kansas Press

Fortey R A amp Theron J N 1994 A new Ordovician arthropodSoomaspis and the agnostid problem Palaeontology 37 841ndash61

Fortey R A amp Whittington H B 1989 The Trilobita as a naturalgroup Historical Biology 2 125ndash38

Fry W G 1978 A classification within the pycnogonids ZoologicalJournal of the Linnean Society of London 63 35ndash58

Giribet G Edgecombe G D amp Wheeler W C 2001 Arthropodphylogeny based on eight molecular loci and morphology Nature413 157ndash61

Gould S J 1989 Wonderful life The Burgess Shale and the nature ofhistory New York NY Norton

Gould S J 1991 The disparity of the Burgess Shale arthropod faunaand the limits of cladistic analysis why we must strive to quantifymorphospace Paleobiology 17 411ndash23

Gurich G 1931 Mimetaster hexagonalis ein neuer Kruster aus demunterdevonischen Bundenbacher Dachschiefer PalaontologischeZeitschrift 12 204ndash38

Hammann W Laske R amp Pillola G L 1990 Tariccoia arrusensisngnsp an unusual trilobite-like arthropod Rediscovery of thelsquophylocaridrsquo beds of Taricco (1922) in the Ordovician lsquoPuddingarsquosequence of Sardinia Bolletino della Societa PaleontologicaItaliana 29 163ndash78

Haas M S Brown S J amp Beeman R W 2001 Pondering theprocephalon the segmental origin of the labrum DevelopmentGenes and Evolution 211 89ndash95

Heider K 1913 Entwicklungsgeschichte und Morphologie derWirbellosen Die Kultur der Gegenwart Theil 3 Abt 4 2 176ndash332

Henriksen K 1928 Critical notes upon Cambrian arthropodsdescribed by Charles D Walcott Videnskabelige Meddelelser fraDansk Naturhistorisk Forening 86 1ndash20

Hesselbo S P 1988 Trace fossils of Cambrian aglaspidid arthropodsLethaia 21 139ndash46

Hesselbo S P 1989 The aglaspidid arthropod Beckwithia from theCambrian of Utah and Wisconsin Journal of Paleontology 63636ndash42

Hesselbo S P 1992 Aglaspidida (Arthropoda) from the UpperCambrian of Wisconsin Journal of Paleontology 66 885ndash923

Hou X 1987a Three new large arthropods from Lower CambrianChengjiang fauna eastern Yunnan Acta Palaeontologica Sinica26 272ndash85 [In Chinese with English summary]

Hou X 1987b Two new arthropods from Lower CambrianChengjiang eastern Yunnan Acta Palaeontologica Sinica 26236ndash56 [In Chinese with English summary]

Hou X Chen J amp Lu H 1989 Early Cambrian new arthropodsfrom Chengjiang Yunnan Acta Palaeontologica Sinica 28 42ndash57[In Chinese with English summary]

Hou X Ramskold L amp Bergstrom J 1991 Composition andpreservation of the Chengjiang fauna a Lower Cambrian soft-bodied biota Zoologica Scripta 20 395ndash411

Hou X Bergstrom J amp Ahlberg P 1995 Anomalocaris and otherlarge animals in the Lower Cambrian Chengjiang fauna ofsouthwest China GFF 117 163ndash83

Hou X amp Bergstrom J 1997 Arthropods of the Lower CambrianChengjiang fauna southwest China Fossils and Strata 45 1ndash116

Hughes C P 1975 Redescription of Burgessia bella from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 4415ndash36

Ivantsov A Y 1999 Trilobite-like arthropod from the LowerCambrian of the Siberian Platform Acta PalaeontologicaPolonica 44 455ndash66

King P E 1973 Pycnogonids London St Martin Press

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 191

Lauterbach K E 1973 Schlussereignisse in der Evolution derStammgruppe der Euarthropoda Zoologische Beitrage 19259ndash99

Lauterbach K E 1980 Schlussereignisse in der Evolution derGrundplans der Arachnata (Arthropoda) Abhandlungen desNaturwissenschaftlichen Vereins in Hamburg 23 163ndash327

Lauterbach K E 1983 Synapomorphien zwischen Trilobiten- undCheliceraten-zweig der Arachnata Zoologischer Anzeiger 210213ndash38

Lendzion K 1975 Fauna of the Mobergella Zone in the Polish LowerCambrian Kwartalnik Geologiczny 19 237ndash42

Lieberman B S 1998 Cladistic analysis of the Early Cambrianolenelloid trilobites Journal of Paleontology 72 59ndash78

Lu Y 1940 On the ontogeny and phylogeny of Redlichia intermediaLu (sp nov) Bulletin of the Geological Society of China 20333ndash42 [In Chinese]

Maddison W P amp Maddison D R 1997 MacClade analysis ofphylogeny and character evolution Version 307 Sunderland MASinauer Associates

Mittmann B amp Scholtz G 2003 Development of the nervous systemin the lsquoheadrsquo of Limulus polyphemus (Chelicerata Xiphosura)morphological evidence for the correspondence between the seg-ments of the chelicerae and the (first) antennae of MandibulataDevelopment Genes and Evolution 213 9ndash17

Moore R C (ed) 1959 Treatise on invertebrate paleontology Part OArthropoda 1 Boulder Colorado Geological Society of America

Muller K J amp Walossek D 1986 Arthropod larvae from the UpperCambrian of Sweden Transactions of the Royal Society ofEdinburgh Earth Sciences 77 157ndash79

Muller K J amp Walossek D 1987 Morphology ontogeny and lifehabit of Agnostus pisiformis from the Upper Cambrian of SwedenFossils and Strata 19 1ndash124

Munilla T 1999 Evolucion y filogenia de los Picnogonidos InMelicA DeHaro J J Medez M amp Ribera I (eds) VolumenMonografico Evolucion y Filogenia de Arthropoda Boletin de laSociedad Entolomologica Aragonesa 26 273ndash91 [In Spanish andEnglish]

Orr P J Siveter D J Briggs D E G Siveter D J amp Sutton M D2000 A new arthropod from the Silurian Konservat-Lagerstatteof Herefordshire Proceedings of the Royal Society of LondonSeries B 267 1497ndash504

Page R D M 1996 On consensus confidence and lsquototal evidencersquoCladistics 12 83ndash92

Palmer A R amp Repina L N 1993 Through a glass darklytaxonomy phylogeny and biostratigraphy of the OlenellinaUniversity of Kansas Paleontological Contributions New Series 31ndash35

Platnick N I Griswold C E amp Coddington J A 1991 On missingentries in cladistic analysis Cladistics 7 337ndash43

Pleijel F 1995 On character coding for phylogeny reconstructionCladistics 11 309ndash15

Raasch G O 1939 Cambrian Merostomata Special Papers of theGeological Society of America 19 1ndash146

Ramskold L Chen J Edgecombe G D amp Zhou G 1996Preservational folds simulating tergite junctions in tegopeltid andnaraoiid arthropods Lethaia 29 15ndash20

Ramskold L Chen J Edgecombe G D amp Zhou G 1997Cindarella and the arachnate clade Xandarellida (ArthropodaEarly Cambrian) from China Transactions of the Royal Society ofEdinburgh Earth Sciences 88 19ndash38

Ramskold L amp Edgecombe G D 1991 Trilobite monophylyrevisited Historical Biology 4 267ndash83

Ramskold L amp Edgecombe G D 1996 Trilobite appendagestructure ndash Eoredlichia revisited Alcheringa 20 269ndash76

Raw F 1957 Origin of chelicerates Journal of Paleontology 31139ndash193

Repina L N amp Okuneva O G 1969 Cambrian arthropods of themaritime territory Paleontological Journal 1969 95ndash103

Richter R amp Richter E 1929 Weinbergina opitz ng n sp einSchwerttrager (Merost Xiphos) aus dem Devon (Rheinland)Senkenbergiana 11 193ndash209

Robison R A 1984 New occurences of the unusual trilobite Naraoiafrom the Cambrian of Idaho and Utah University of KansasPaleontological Contributions Paper 112 1ndash8

Rominger C 1887 Description of primordial fossils from MtStephens NW Territory of Canada Proceedings of the Academyof Natural Sciences of Philadelphia 1887 12ndash19

Scholtz G 1997 Cleavage germ band formation and head segmen-tation the ground pattern of the Euarthropoda In Fortey R Aamp Thomas R H (eds) Arthropod Relationships 317ndash32 LondonChapman and Hall Systematics Association Special Volume 55

Schram F R 1978 Arthropods a convergent phenomenonFieldiana Geology 39 61ndash108

Schram F R 1993 Review of Boxshall Stromberg and Dahl TheCrustacea Origin and Evolution Journal of Crustacean Biology13 820ndash2

Schram F R Vonk R amp Hof C H J 1997 Mazon CreekCycloidea Journal of Paleontology 71 261ndash84

Schram F R amp Emerson M J 1991 Arthropod Pattern Theorya new approach to arthropod phylogeny Memoirs of theQueensland Museum 31 1ndash18

Schram F R amp Hof C H J 1998 Fossils and the interrelationshipsof major crustacean groups In Edgecombe G D (ed) Arthropodfossils and phylogeny pp 233ndash302 New York NY ColumbiaUniversity Press

Shergold J H 1991 Protaspis and early meraspis growth stages of theeodiscoid trilobite Pagetia ocellata Jell and their implications forclassification Alcheringa 15 65ndash86

Shu D Geyer G Ling C amp Zhang Z 1995 Redlichiacean trilobiteswith preserved soft parts from the Lower Cambrian Chengjiangfauna (South China) Beringeria (Special Issue) 2 203ndash41

Shultz J W 1990 Evolutionary morphology and phylogeny ofArachnida Cladistics 6 1ndash38

Shultz J W 2000 Skeletomuscular anatomy of the harvestmanLeiobunum aldrichi (Weed 1893) (Arachnida OpilionesPalpatores) and its evolutionary significance Zoological Journalof the Linnean Society 128 401ndash38

Shultz J W 2001 Gross muscular anatomy of Limulus polyphemus(Xiphosura Chelicerata) and its bearing on evolution in the theArachnida Journal of Arachnology 29 283ndash303

Siewing R (ed) 1985 Lehrbuch der Zoologie II Systematik 3rd ednStuttgart Fischer

Simonetta A M 1970 Studies on the non trilobite arthropods ofthe Burgess Shale (Middle Cambrian) the genera LeanchoiliaAlalcomenaeus Opabinia Burgessia Yohoia and ActaeusPalaeontographica Italica 66 35ndash45

Simonetta A M amp Delle Cave L 1975 The Cambrian non trilobitearthropods from the Burgess Shale of British Columbia A studyof their comparative morphology taxonomy and evolutionarysignificance Palaeontographica Italica 69 1ndash37

Simonetta A M amp Delle Cave L 1981 An essay in the compara-tive and evolutionary morphology of Palaeozoic ArthropodsAccademia Nazionale dei Lincei Atti dei Convegni Lincei 49389ndash439

Starobogatov Y I 1990 Sistema i filogeniya nizshikh khelitserovykh(analiz morfologii paleozoysikh grupp) [The systematics andphylogeny of the lower chelicerates (a morphological analysis ofthe Palaeozoic groups)] Paleontologicheskii Zhurnal 1990 4ndash17

Stoslashrmer L 1944 On the relationships and phylogeny of fossil andrecent Arachnomorpha Skrifter utgitt ac Det Norske Videnskaps-Akademi i Olso I Matematisk-naturvitenskaplig Klasse 5 1ndash158

Stoslashrmer L 1951 Studies on trilobite morphology Part III Theventral cephalic structures with remarks on the zoologicalposition of the trilobites Norsk geologisk tidsskrift 29 108ndash58

Stoslashrmer L 1956 A Lower Cambrian merostome from Sweden Arkivfor Zoologi Ser 2 9 507ndash14

Stoslashrmer L 1959 Trilobitoidea In Moore R C (ed) Treatise onInvertebrate Paleontology part O Arthropoda 1 23ndash37 BoulderColorado Geological Society of America

Strong E E amp Lipscomb D 1999 Character coding and inapplicabledata Cladistics 15 363ndash71

Sturmer W amp Bergstrom J 1976 The arthropods Mimetaster andVachonisia from the Devonian Hunsruck Shale PalaontologischeZeitschrift 50 78ndash111

Sturmer W amp Bergstrom J 1978 The arthropod Cheloniellon fromthe Devonian Hunsruck Shale Palaontologische Zeitschrift 5257ndash81

Sturmer W amp Bergstrom J 1981 Weinbergina a xiphosuran fromthe Devonian Hunsruck Slate Palaontologische Zeitschrift 55237ndash55

Sutton M D Briggs D E G Siveter D J Siveter D J amp Orr PJ 2002 The arthropod Offacolus kingi (Chelicerata) from theSilurian of Herefordshire England computer based morphologi-cal reconstructions and phylogenetic affinity Proceedings of theRoyal Society of London Series B 269 1195ndash1203

Swofford D L 1999 PAUP Phylogenetic Analysis Using Parsimony(and other methods) Version 40b2a Sunderland MA SinauerAssociates

Telford M J amp Thomas R H 1998 Expression of homeobox genesshows chelicerate arthropods retain their deutocerebral segmentProceedings of the National Academy of Sciences USA 9510671ndash5

192 TREVOR J COTTON AND SIMON J BRADDY

Thorley J L amp Page R D M 2000 RadCon phylogenetic treecomparison and consensus Bioinformatics 16 486ndash7

Valentine J W 1995 Why no new phyla after the Cambrian ndashgenome and ecospace hypotheses revisited Palaios 10 190ndash194

Wagele J W 1993 Rejection of the lsquoUniramiarsquo hypothesis andimplications of the Mandibulata concept Zoologische JahrbucherAbteilung fur Systematik Geographie und Biologie der Tiere 120253ndash88

Walcott C D 1911 Middle Cambrian Merostomata Cambriangeology and palaeontology II Smithsonian MiscellaneousCollections 57 17ndash40

Walcott C D 1912 Middle Cambrian Branchiopoda MalacostracaTrilobita and Merostomata Cambrian geology and palaeontol-ogy II Smithsonian Miscellaneous Collections 57 145ndash228

Walcott C D 1918 Geological explorations in the CanadianRockies Explorations and fieldwork of the SmithsonianInstitution in 1917 Smithsonian Miscellaneous Collections 684ndash20

Walossek D 1993 The Upper Cambrian Rehbachiella kinnekullensisand the phylogeny of Branchiopoda and Crustacea Fossils andStrata 32 1ndash202

Walossek D amp Dunlop J A 2002 A larval sea spider (ArthropodaPycnogonida) from the Upper Cambrian lsquoOrstenrsquo of Swedenand the phylogenetic position of pycnogonids Palaeontology 45421ndash46

Walossek D amp Muller K J 1990 Upper Cambrian stem-lineagecrustaceans and their bearing upon the monophyletic origin ofCrustacea and the position of Agnostus Lethaia 23 409ndash27

Walossek D amp Muller K J 1997 Cambrian lsquoOrstenrsquo-type arthro-pods and the phylogeny of Crustacea In Fortey R A amp ThomasR H (eds) Arthropod Relationships 139ndash53 SystematicsAssociation Special Volume 55 London Chapman and Hall

Walossek D amp Muller K J 1998 Early arthropod phylogeny in lightof the Cambrian lsquoOrstenrsquo fossils In Edgecombe G D (ed)Arthropod fossils and phylogeny 185ndash231 New York NYColumbia University Press

Weygoldt P 1979 Significance of later embryonic stages and headdevelopment in arthropod phylogeny In Gupta A P (ed)Arthropod Phylogeny 107ndash35 New York NY Van NostrandReinhold Co

Weygoldt P 1998 Evolution and systematics of the ChelicerataExperimental and Applied Acarology 22 63ndash79

Weygoldt P amp Paulus H F 1979 Unterzuchungen aur Morpholo-gie Taxonomie und Phylogenie der Chelicerata Zeitschriftfur Zoologische Systematik und Evolutionsforschung 17 85ndash116177ndash200

Wheeler W C Cartwright P amp Hayashi C Y 1993 Arthropodphylogeny a combined approach Cladistics 9 1ndash39

Wheeler W C amp Hayashi C Y 1998 The phylogeny of extantchelicerate orders Cladistics 14 173ndash92

Whittington H B 1971 Redescription of Marrella splendens(Trilobitoidea) from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 209 1ndash24

Whittington H B 1974 Yohoia Walcott and Plenocaris n genarthropods from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 231 1ndash21

Whittington H B 1975a Trilobites with appendages from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 497ndash136

Whittington H B 1975b The enigmatic animal Opabinia regalisMiddle Cambrian Burgess Shale British Columbia PhilosophicalTransactions of the Royal Society of London Series B 271 1ndash43

Whittington H B 1977 The Middle Cambrian trilobite NaraoiaBurgess Shale British Columbia Philosophical Transactions of theRoyal Society Series B 280 409ndash43

Whittington H B 1979 Early arthropods their appendages andrelationships In House M R (ed) The origin of majorinvertebrate groups 253ndash68 Systematics Association SpecialVolume 12 New York NY Academic Press

Whittington H B 1980 Exoskeleton moult stage appendage mor-phology and habits of the Middle Cambrian trilobite Olenoidesserratus Palaeontology 23 171ndash204

Whittington H B 1981 Rare arthropods from the Burgess ShaleMiddle Cambrian British Columbia Philosophical Transactionsof the Royal Society of London Series B 292 329ndash57

Whittington H B 1985 Tegopelte gigas a second soft-bodied trilo-bite from the Burgess Shale Middle Cambrian British ColumbiaJournal of Palaeontology 23 171ndash204

Whittington H B 1989 Olenelloid trilobites type species functionalmorphology and higher classification Philosophical Transactionsof the Royal Society of London Series B 324 111ndash47

Whittington H B amp Almond J E 1987 Appendages and habits ofthe Upper Ordovician trilobite Triarthus eatoni PhilosophicalTransactions of the Royal Society of London Series B 317 1ndash46

Wilkinson M 1992 Ordered versus unordered characters Cladistics8 375ndash85

Wilkinson M 1994 Common cladistic information and its consensusrepresentation reduced Adams and reduced cladistic consensustrees and profiles Systematic Biology 43 343ndash68

Wilkinson M 1995a Coping with abundant missing entries in phylo-genetic inference using parsimony Systematic Biology 44 501ndash14

Wilkinson M 1995b TAXEQ2 software and documentation BristolSchool of Biological Sciences University of Bristol

Wilkinson M 1996 On the distribution of homoplasy and choosingamong trees Taxon 45 263ndash66

Wills M A 1997 A phylogeny of Recent and fossil Crustacea derivedfrom morphological characters In Fortey R A amp ThomasR H (eds) Arthropod Relationships 189ndash209 SystematicsAssociation Special Volume 55 London Chapman and Hall

Wills M A Briggs D E G amp Fortey R A 1994 Disparity asan evolutionary index a comparison of Cambrian and Recentarthropods Paleobiology 20 93ndash130

Wills M A Briggs D E G Fortey R A amp Wilkinson M 1995The significance of fossils in understanding arthropod evolutionVerhandlungen der deutschen zoologischen Gesselschaft 88 203ndash15

Wills M A Briggs D E G Fortey R A Wilkinson M amp SneathP H A 1998a An arthropod phylogeny based on fossil andrecent taxa In Edgecombe G D (ed) Arthropod fossils andphylogeny 33ndash105 New York NY Columbia University Press

Wills M A Briggs D E G amp Fortey R A 1998b Evolutionarycorrelates of arthropod tagmosis scrambled legs In ForteyR A amp Thomas R H (eds) Arthropod Relationships 57ndash65Systematics Association Special Volume 55 London Chapmanand Hall

Winter G 1980 Beitrage zur Morphologie und Embryologie desvorderen Korperabschnitts (Cephalosoma) der PantopodaGerstaecker 1863 I Entstehung und Struktur des Zentralnerven-systems Zeitschrift fur Zoologische Systematik und Evolutionsfor-schung 18 27ndash61

Zhang W amp Hou X 1985 Preliminary notes on the occurrence of theunusual trilobite Naraoia in Asia Acta Palaeontologica Sinica 24591ndash5 [In Chinese with English summary]

TREVOR J COTTON Department of Palaeontology The Natural History Museum CromwellRoad London SW7 5BD UKe-mail trevor_cottonhotmailcom

SIMON J BRADDY Department of Earth Sciences University of Bristol Wills MemorialBuilding Queenrsquos Road Bristol BS8 1RJ UKe-mail S J Braddybrisacuk

MS received 28 July 2003 Accepted for publication 15 March 2004

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 193

Page 8: The phylogeny of arachnomorph arthropods and the origin of the

possess a single pair of long uniramous multi- and annulatedantennae at the anterior of the head which presumably had asensory function as found in Crustacea Myriapoda and mostmembers of Hexapoda (ie proturans do not have antennae)These appendages are likely to be a synapomorphy of theEuarthropoda (eg Scholtz 1997 Walossek amp Muller 1997)and therefore the outgroup is coded as State 0 The anterior-most head appendages of most Cambrian megacheiran arthro-pods which have been called lsquogreat appendagesrsquo followingWalcott (1912) consist of a small number of robust spinosepodomeres Uniquely Fortiforceps has a pair of short anten-nae anterior to the lsquogreat appendagesrsquo (Hou amp Bergstrom 1997pp 34ndash8) Therefore it appears likely that the lsquogreat append-agesrsquo are the appendages of the second cephalic segment andthe antennae are lost in megacheirans other than FortiforcepsThis of course depends on recognising the lsquogreat appendagesrsquoas homologous in all of these taxa as discussed below(Character 2) Homology of the megacheiran anterior append-ages with the second cephalic appendages of trilobites waspreviously suggested by Stoslashrmer (1944 p 124) on the basis oftheir post-oral position

The classical view of chelicerate head segmentation main-tains that they too have lost the antennae and the cheliceraeare the appendages of the second cephalic segment Recentrevisions of arthropod phylogeny have uncritically acceptedthis homology (Edgecombe et al 2000 Giribet et al 2001)which is based largely on neuroanatomy The lsquochelicero-neuromerrsquo which innervates the chelicerae seems to be ho-mologous with the tritocerebrum associated with the secondcephalic appendage pair of mandibulates (see Weygoldt 1979Winter 1980) Although some uncertainty exists over theposition of pycnogonids within Arthropoda and the homol-ogy of their cephalic segmentation is poorly understood thisview is also supported by the presence of pre-cheliceralappendages in a pycnogonid larva (Larva D of Muller ampWalossek 1986) from the Upper Cambrian of Sweden(Walossek amp Dunlop 2002) Cheloniellon also has a pair ofpre-oral pediform appendages considered homologous to thechelicerae in addition to its antennae Thus despite recentneontological studies that indicate that the chelicerae arehomologous to the antennae of mandibulates based on pat-terns of Hox gene expression (Damen et al 1998 Telford amp

Table 2 Data matrix for cladistic analyses of arachnomorphs () missing data and (ndash) inapplicable characters Letters indicate multistateuncertainty codings as follows (A) 34 (B) 02 (C) 01 (D) 134 (E) 12

1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 3 3 3 4 4 4 4 4 4 4 4 4 4 5 5 5 5 51 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4

OUTGROUP 0 0 0 0 0 0 0 0 0 0 0 0 ndash ndash 0 0 0 0 0 0 0 0 0 0 0 4 0 ndash 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ndash 0 0 0 0 ndash ndash ndash 0 ndash 0Aglaspidida 1 0 0 1 A 0 0 0 0 0 1 0 ndash ndash ndash ndash ndash 0 0 0 1 0 0 0 0 0 0 0 0 ndash 1 1 0 0 1 0 0 0 0 0 0 0 0 1 2 1 1 0 ndash ndash ndash 1 0 1Aglaspidida 2 0 0 A 0 0 0 0 0 0 1 0 ndash ndash 0 1 0 1 0 0 0 0 0 0 0 0 ndash 1 1 0 0 1 0 0 0 0 0 0 0 0 1 2 1 1 0 ndash ndash ndash 1 0 1Alalcomenaeus 1 1 1 3 ndash 2 0 1 0 0 0 1 0 ndash ndash 1 0 0 0 0 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 0 1 0 1 ndash ndash ndash 1 1 0Buenaspis 0 1 B 0 0 1 0 0 ndash 0 1 0 1 0 0 0 0 1 0 0 0 0 ndash 0 1 1 1 0 1 0 ndash 0Burgessia 0 0 0 3 0 0 0 0 0 0 0 1 0 ndash ndash 0 0 0 1 2 ndash 0 0 0 0 0 4 0 ndash 0 0 0 0 0 0 2 0 0 0 0 0 0 ndash 0 0 0 1 ndash ndash ndash 1 0 0Cheloniellon 0 0 1 4 0 0 0 0 0 1 0 1 0 ndash ndash 0 0 1 0 1 0 0 0 0 0 0 1 0 ndash 0 1 0 0 1 0 0 0 0 0 1 0 0 1 0 1 1 0 ndash ndash ndash 0 ndash 1Cindarella 0 0 0 6 0 0 0 0 0 0 0 1 1 0 0 0 0 1 1 0 0 0 0 0 0 0 1 0 ndash 0 1 0 1 1 0 0 2 1 0 0 0 1 0 ndash 1 0 1 1 0 0 0 0 ndash 0Crustacea 0 0 0 3 0 0 0 0 0 0 0 0 0 ndash ndash 0 1 0 0 0 0 0 0 0 0 0 4 0 ndash 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ndash 0 0 0 ndash ndash ndash 1 0 0Emeraldella 0 0 1 5 0 0 0 0 0 0 0 1 1 0 0 0 0 1 0 2 ndash 0 0 0 0 0 0 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 1 1 1 1 0 1 ndash ndash ndash 1 0 1Eoredlichia 0 0 0 3 0 0 0 0 1 0 0 1 1 0 0 0 0 1 0 0 1 0 0 0 0 0 3 1 1 1 1 0 0 2 0 0 0 0 0 0 0 0 0 ndash 1 0 1 1 0 0 1 0 ndash 0Eurypterida 1 1 1 6 ndash 1 1 0 0 1 1 1 0 ndash ndash 1 0 1 0 1 0 0 0 1 0 0 4 1 0 0 1 0 0 1 0 0 0 0 0 0 0 0 1 3 0 1 0 1 ndash ndash ndash 1 0 0Fortiforceps 0 1 1 4 ndash 0 0 0 0 0 0 1 0 ndash ndash 1 0 0 1 0 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 0 1 0 1 ndash ndash ndash 1 1 0Helmetia 3 1 0 1 0 1 0 0 0 0 0 1 2 0 ndash 0 1 0 0 2 0 0 0 0 0 1 1 0 0 ndash 0 0 1 1 0 1 0 0 ndash 0Jianfengia 1 1 1 4 ndash 1 0 0 0 0 0 1 0 ndash ndash 1 0 1 1 0 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 0 1 0 1 ndash ndash ndash 1 0 0Kuamaia 0 0 0 0 0 0 0 0 0 1 1 0 1 0 0 1 0 0 0 1 0 0 0 1 2 0 ndash 0 1 0 0 2 0 0 0 0 0 1 1 0 0 ndash 0 0 1 1 0 1 0 0 ndash 0Leanchoilia 1 1 1 3 ndash 2 0 1 0 0 0 1 0 ndash ndash 1 0 1 1 B 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 1 1 0 1 ndash ndash ndash 1 0 0Lemoneites 0 1 0 0 0 0 1 1 0 1 0 0 0 0 0 0 0 0 1 2 0 0 ndash ndash ndash 1 0 Liwia 0 1 0 0 0 0 1 0 1 0 0 1 0 1 0 0 0 0 ndash 0 1 1 1 0 0 0 ndash 0Marrella 0 0 1 1 0 0 0 0 0 0 0 0 0 ndash ndash 0 1 0 0 2 ndash 0 0 0 0 0 0 ndash 0 0 0 0 0 0 2 0 0 0 0 0 0 ndash 0 0 0 0 ndash ndash ndash 0 ndash 0Mimetaster 0 0 1 2 0 0 0 0 0 0 0 0 0 ndash ndash 0 1 0 1 C 0 0 0 0 0 0 ndash 0 0 0 0 1 0 0 2 0 0 0 0 0 0 ndash 0 0 0 0 ndash ndash ndash 0 ndash 0Misszhouia 0 0 0 3 0 0 0 0 1 0 0 1 1 1 0 0 0 1 0 0 0 0 0 0 0 0 1 0 ndash 0 1 1 ndash ndash 2 0 0 0 1 0 0 0 0 ndash ndash 0 1 1 0 0 1 0 ndash 0Naraoia 0 0 0 3 1 0 0 0 1 0 0 1 1 1 0 0 0 1 0 0 0 0 0 0 0 0 1 0 ndash 0 1 1 ndash ndash 2 0 0 0 1 0 0 0 0 ndash ndash 0 1 1 0 0 0 ndash 0Olenoides 0 0 0 3 0 0 0 0 1 0 0 1 1 0 0 0 0 1 0 1 1 0 0 0 0 0 3 1 1 1 1 0 0 2 0 0 0 0 0 0 0 0 0 ndash 0 1 1 1 0 0 0 0 ndash 0Paleomerus 0 1 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0 0 0 ndash 0 0 ndash ndash ndash 1 0 Pycnogonida 1 1 1 4 ndash 1 1 0 0 1 1 1 ndash ndash ndash ndash 0 0 1 1 0 0 0 1 0 0 4 0 ndash 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 ndash ndash ndash 1 0 0Retifacies 0 0 0 3 0 0 0 0 0 0 0 1 0 ndash ndash 0 0 1 0 0 0 0 0 0 0 0 0 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 0 ndash 1 0 1 1 0 0 0 1 0 0Saperion 0 2 0 1 0 1 0 0 1 1 0 0 1 0 0 0 1 2 0 ndash 0 1 1 ndash ndash 1 1 ndash ndash 0 0 1 0 0 ndash ndash 1 1 0 0 1 0 ndash 0Sidneyia 0 0 1 0 1 0 0 0 0 1 0 1 0 ndash ndash 0 0 1 0 0 0 0 0 0 0 0 0 0 1 0 0 1 0 0 0 0 0 0 0 0 1 2 1 1 0 1 ndash ndash ndash 1 0 1Sinoburius 0 0 0 4 0 0 0 0 0 0 0 1 0 1 0 0 1 1 0 0 0 1 0 ndash 0 1 0 0 1 0 0 2 1 0 0 0 1 0 ndash 0 0 1 1 0 1 0 0 ndash 0Skioldia 0 2 0 0 0 1 0 0 0 1 2 0 ndash 0 1 1 ndash ndash 1 1 ndash ndash 0 0 1 0 0 ndash ndash 1 1 0 0 1 0 ndash 0Soomaspis 0 1 B 0 0 0 0 1 0 0 ndash 0 1 0 1 0 0 1 0 1 0 0 0 0 ndash 0 1 1 1 0 1 0 ndash 0Tariccoia 0 1 B 0 0 0 0 1 0 0 ndash 0 1 0 1 0 0 1 0 1 0 0 0 0 ndash 0 1 1 1 0 1 0 ndash 0Tegopelte 0 0 0 0 0 0 0 0 1 0 1 1 0 0 0 0 1 E 0 ndash 0 1 0 ndash ndash E 1 ndash ndash 0 0 0 0 ndash ndash 0 1 1 0 0 1 0 ndash 0Weinbergina 1 1 1 6 ndash 1 1 0 0 1 1 1 0 ndash ndash 1 0 1 0 1 0 0 0 1 0 0 4 1 0 0 1 0 0 1 0 0 0 0 0 0 0 0 1 2 0 1 0 1 ndash ndash ndash 1 0 0Xandarella 0 0 0 4 0 0 0 0 0 0 0 1 1 1 0 0 0 1 0 0 0 0 1 0 0 0 1 0 ndash 0 1 0 1 1 0 0 2 1 0 0 0 1 0 ndash 1 0 1 1 0 0 0 ndash 0Yohoia 1 1 1 4 ndash 1 0 0 0 1 1 1 0 ndash ndash 1 0 1 0 0 0 0 0 0 0 D 0 ndash 0 1 0 0 1 0 0 0 0 0 0 0 0 1 2 0 1 0 1 ndash ndash ndash 1 1 0

176 TREVOR J COTTON AND SIMON J BRADDY

Thomas 1998) and developmental data (Mittmann amp Scholtz2003) morphological evidence from fossils supports the viewthat chelicerates have lost the antennae The Hox gene evi-dence suffers from a lack of comparative data from diversemandibulates especially primitive crustaceans (see Akam2000) and the use of Hox gene expression boundaries asmarkers for segmental homology has been criticised (egAbzhanov et al 1999 although relating to posterior Hoxpatterns) Wills et al (1998a pp 43 amp 48) considered thechelicerae to be appendages of the second (deutocerebral)segment but curiously in support of this cited Schram (1978)who unambiguously followed the homology scheme used inthe present study

The anterior-most appendages of Aglaspis were originallydescribed as chelicerae (Raasch 1939) which lead to theirclassification in the Chelicerata (eg Stoslashrmer 1944) Howeverthe morphology of these appendages (see Briggs et al 1979) ismore similar to that of antennae They are narrow relative tothe thoracic endopods of relatively even diameter proximallyand the podomeres are apparently weakly defined Hesselbo(1988 1992) also argued that the anterior appendages ofAglaspis are likely to be antennae The distal parts are un-known (and hence so is their length cf Wills et al 1998ap 58) and there is no evidence that they were chelate contraryto the coding of Wills et al (1998a character 31) Nothingis known about the anterior appendages of BuenaspisLemoneites or Paleomerus and this character is accordinglycoded as missing data in these taxa

2 Form of endopod of appendages of the second segment(0) pediform and (1) anteriorly directed raptorial appendagewith reduced number of podomeres and terminal podomeresbearing spines on distal margins As discussed above cheli-cerae and lsquogreat appendagesrsquo are both likely to represent theappendages of the second segment They also differ from theassumed primitive biramous euarthropod limb in similar waysand therefore are likely to be homologous modifications ofthese appendages The homology of lsquogreat appendagesrsquo andchelicerae was originally proposed by Henriksen (1928) andsupported by Stoslashrmer (1944) but to the present authorsrsquoknowledge no modern author has discussed this possibility

Both lsquogreat appendagesrsquo (Figs 5A D E) and chelicerae(Figs 5B C) are equipped with strong spinose projections onthe outer (dorsal) side of the distal margins of the terminalpodomeres that are lacking on the proximal podomeresSecondly the number of podomeres in both chelicerae andlsquogreat appendagesrsquo is more or less reduced compared to thenumber in the endopods of biramous limbs In the case oflsquogreat appendagesrsquo they are significantly more robust thanthose of posterior endopods a situation that is matched insome chelicerates The homology of the lsquogreat appendagesrsquoof Alalcomenaeus Leanchoilia Yohoia Jiangfengia andFortiforceps is supported by all of these features and is widelyaccepted (eg Wills et al 1998a character 31 Hou ampBergstrom 1997 Bergstrom amp Hou 1998 Hou 1987a) OnlyHou amp Bergstrom (1991 p 183) have suggested albeit inpassing that the lsquogreat appendagesrsquo of all these taxa may beconvergent a view they appear to have changed The endo-pods of the appendages of other taxa are locomotory legswhich either lack strong spines or have spinose extensionsthat are invariably on the medial (ventral) surface and arestronger on the proximal podomeres than the distal ones (egMisszhouia Fig 6A) In the latter case these spines form partof the feeding system along with the gnathobases and are oftenlocated around the middle of the podomere rather than limitedto the distal margin These endopods are directed ventro-laterally as opposed to anteriorly in the case of both cheliceraeand lsquogreat appendagesrsquo The modified second appendages of

Marrella resemble this plesiomorphic condition here referredto as lsquopediformrsquo except in their anterolateral orientation Insome Recent crustaceans this appendage is modified into asecond antenna but in stem-group crustaceans it is pediform(eg Walossek amp Muller 1997 1998) This character is codedas missing data in a number of taxa for which the morphologyof the second segment appendage is unknown

Some authors (Dzik 1993 Chen amp Zhou 1997 Dewel ampDewel 1997 Budd 2002) have suggested that the lsquogreat ap-pendagesrsquo are homologous with the anterior appendages ofanomalocaridids and the anomalocaridid-like Opabinia andKerygmachela (see Budd 1996b 1999b respectively) Theanterior appendages of most anomalocaridids and otherCambrian lobopodians differ from megacheiran lsquogreat append-agesrsquo in that they consist of a large number of segmentsMoreover there is considerable morphological evidence thatanomalocaridids are part of the euarthropod stem group (Willset al 1995 1998a) and not closely related to the lsquogreatappendagersquo taxa (cf Budd 2002) with the possible exception ofParapeytoia lsquoGreat appendagersquo taxa share a suite of derivedcharacters with other crown group euarthropods includingsclerotised dorsal tergites the incorporation of more thanone pair of appendages into the head sclerotisation of theendopods and strong differentiation of the head that wereexcluded from Buddrsquos (2002) analysis The relationships ofanomalocaridids will be considered in detail later by Braddyand co-workers

3 Exopod of appendages of the second segment (0)present and (1) absent or much reduced The raptorial ap-pendages of the second segment (chelicerae and lsquogreat append-agesrsquo) are uniramous as are the corresponding pediformappendages of Marrella Mimetaster Emeraldella Cheloniellonand Sidneyia The plesiomorphic euarthropod state is found inmost other arachnomorphs including trilobites where thebiramous second segment appendages are undifferentiatedfrom those of posterior segments (Edgecombe et al 2000character 78) The exopods of these appendages are alsopresent in basal members of the crustacean crown group(Edgecombe et al 2000 character 79) and in stem groupcrustaceans (Walossek 1993 Walossek amp Muller 1997 1998)

4 Number of head segments (0) 1 (1) 2 (2) 3 (3) 4 (4) 5(5) 6 and (6) 7 The coding of this character by Edgecombe ampRamskold (1999 character 2) explicitly referred to the numberof limb pairs present in addition to the antenna Here thenumber of post-acronal segments present in the head is codedfollowing Wills et al (1998a character 29) In taxa that lack anantenna the number of somites is inferred to be one greaterthan the number of cephalic appendage pairs (cf Wills et al1998a) as described above

Sturmer amp Bergstrom (1978) suggested that the head ofCheloniellon is defined primarily on the basis of appendagetagmosis rather than the fusion of segments under a singlecephalic-shield (cf Hou amp Bergstrom 1997 pp 98ndash9) Accord-ing to this view the head consists of both the cephalic tergiteand the anteriormost free tergite that share uniramousgnathobasic appendages Wills et al (1998a) coded Cheloniel-lon as having six somites incorporated into the head andtherefore presumably accepted this suggestion There is noevidence that the first free tergite of Cheloniellon was incorpo-rated into the head (except perhaps functionally) and thepresent authors prefer to code the number of somites under thecephalic shield In Sidneyia there is a series of uniramousstrongly gnathobasic appendages similar to those of Cheloniel-lon posterior to the head shield All of these appendages wouldpresumably have to be considered part of the head based onappendage differentiation according to the view of Sturmer ampBergstrom (1978) Some autapomorphic states are included

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 177

(none for Sidneyia one for Marrella and six for Emeraldella)since these will become informative if the character is treatedas ordered

Edgecombe amp Ramskold (1999 p 265) described a novelform of cephalic tagmosis The present authors tentativelyaccept their suggestion that in trilobite-like taxa the fourthpair of biramous cephalic appendages was directly under thecephalo-thoracic junction This may explain previous confu-sion about the number of such appendages incorporated intothe trilobite cephalon (eg Cisne 1975 Whittington 1975aBergstrom amp Brassel 1984) However Edgecome amp Ramskoldaccept the view of Chen et al (1977 p 7) that only the firstthree biramous limbs of Misszhouia were structurally and

functionally part of the head Therefore it seems more accept-able to code these taxa as having only four post-acronalsomites than to use the coding scheme of Edgecombe ampRamskold (1999) The partial integration of the first thoracicsomite under the head shield could be considered to be a formof overlap of the trunk by the head shield (ie a distinct stateof character 9 of Edgecombe amp Ramskold 1999 or Character37 herein) Derived states of this character would then poten-tially be synapomorphic for a clade including xandarellidsnaraoiids helmetiids tegopeltids and trilobites Howeversince the degree of overlap in taxa with the fourth biramousappendages under the cephalo-thoracic articulation is identicalto that found between thoracic tergites (State 0 of Character

Figure 5 Diagrammatic reconstructions of raptorial second-segment appendages in lateral view (except whereotherwise stated) and approximate life orientation showing suggested homology of appendage elements (inbrackets) Roman numerals indicate position in the podomere series and do not necessarily imply homology (A)lsquoGreat appendagersquo of Fortiforceps (after Hou amp Bergstrom 1997) (B) Left chelicera of Leiobunum aldrichi(Arachnida Opiliones) (1) in medial perspective and (2) distal parts in anterior perspective (after Shultz 2000)(C) Chelifore of Nymphon (Pycnogonida) (after Child 1997) (D) lsquoGreat appendagersquo of Yohoia (after Whittington1974 fig 2) (E) lsquoGreat appendagersquo of Leanchoilia (modified after Bruton amp Whittington 1983) Not to scaleAbbreviations (apt) apotele (probably homologous with the mandibulate pretarsus) (cx) coxa (dtmrt)deutomerite and (pt) pretarsus

178 TREVOR J COTTON AND SIMON J BRADDY

37) it is here preferred to regard this as a distinct character(Character 9 herein)

The coding of Alalcomenaeus by Wills et al (1998a) ashaving four post-acronal somites is accepted here but fordifferent reasons Briggs amp Collins (1999) have recenty demon-strated that Alalcomenaeus possessed two pairs of biramousappendages on the head in addition to the great appendagesThis would equate to three head somites according to thehomology scheme of Wills et al (1998a) Here the antennaeare inferred to be lost and therefore four segments incorpo-rated into the head

Recently a number of authors have agreed that the plesio-morphic condition for euarthropods is a four-segmented headas found in stem group crustaceans (Scholtz 1997 Walossek1993 Walossek amp Muller 1997 1998 Edgecombe et al 2000)However reconstruction of the euarthropod stem groupclearly indicates that even crownward taxa had a head consist-ing of only the antennal segment and acron as found intardigrades and onychophorans Therefore it seems that thefour-segmented head is pleisomorphic only for crown groupeuarthropods Some fully arthropodised fossil taxa also have ashorter head that may be pleisomorphic such as the two-segmented head of Marella Retifacies is coded following therecent redescription of Hou amp Bergstrom (1997) rather thanon the basis of the coding by Edgecombe amp Ramskold (1999)for which they provide no explanation A partial uncertaintycoding is used for Aglaspis in which the number of post-antennal cephalic appendages is either three or four (Briggset al 1979)

5 Orientation of the antennae (0) directed anterolaterally(1) strongly deflected laterally and (2) placed well inside shieldmargin curing posteriorly from a transverse proximal element(cf Edgecombe amp Ramskold 1999 character 3) The anteriorappendages of Aglaspis (see character 1) are clearly directedanterolaterally and it is presumed that they continued past thehead shield margin The anterior appendages of arthropodstem group taxa such as Aysheaia Kerygmachela and Anoma-locaris are either directed anterolaterally or ventrally but arenot strongly-deflected laterally Therefore the outgroup iscoded as State 0 This character is coded as missing data fortaxa where the presence of antennae is equivocal (those codedas missing data for Character 1) and as inapplicable in taxawhere the antennae are considered absent (coded as State 1 forCharacter 1)

6 Length of distal spines on terminal podomeres of endo-pods of second segment appendages (0) absent or shorer thanpodomeres (1) subsequal to length of podomeres and (2)longer than the entire podomere series The spinose projectionsof the raptorial appendages described above vary in length Inchelicerae (Figs 5BC) and some lsquogreat appendagesrsquo (eg thoseof Yohoia Fig 5D) the most distal spine is equal in length tothe spinose terminal podomere forming a chelate structureIn Fortiforceps (Fig 5A) the spines are short relative tothe lengths of the podomeres but in Alalcomenaeus andLeanchoilia (Fig 5E) they are extremely long so that thespines are much longer than the entire podomere series Asdescribed above in taxa with pediform endopods of the secondsegment appendages spines on the distal podomeres are shortor entirely absent

7 Chelicerae (0) absent and (1) present Despite theirsimilarities to lsquogreat appendagesrsquo the chelicerae of the eucheli-cerates and chelifores of the pycnogonids are clearly distinctand have been widely recognised as a chelicerate synapomor-phy Chelicerae differ from any lsquogreat appendagesrsquo by thecombination of a smaller number of podomeres the presenceof only a single terminal element and only a single spinoseprojection on the dorsal side of the podomere series (Fig 5)

Amongst extant chelicerates only the pycnogonid Pallenopsishas chelicerae of four podomeres comparable to the numberin lsquogreat appendagesrsquo Bergstrom et al (1980) considered thatthe chelifores of the Devonian pycnogonid Palaeoisopus alsoconsist of four segments but this is not well supported by theirfigures

8 Distal spines of second segment endopods terminating inannulated flagellae (0) absent and (1) present It is widelyrecognised (eg Stoslashrmer 1944 Simonetta 1970 Bruton ampWhittington 1993 Briggs amp Collins 1999) that the terminalparts of the extended spines of Alalcomenaeus and Leanchoiliaformed annulated flagellae Nothing similar is known fromhomologous appendages of any other arthropod althoughsimilar processes may have operated in the transformation ofthe endopod as a whole into the second antennae of derivedcrustaceans and in the origin of multiramous antennulae (firstantennae) in malacostracans

242 Posterior appendages 9 Appendages of first thoracicsomite underneath the cephalo-thoracic articulation (0) ab-sent and (1) present (cf Edgecombe amp Ramskold 1999character 2) For a discussion of this character see Character4 herein

10 Exopods of appendages of third to fifth segments (0)present and (1) reduced or absent A number of taxa haveuniramous appendages on the third to fifth segments In mostthese segments are incorporated into the head (Yohoia cheli-cerates) but in others all (eg in Sidneyia) or some (eg inCheloniellon) of these segments are post-cephalic In all casesthe appendage is morphologically similar to the endopods ofbiramous limbs of other taxa andor other segments and it isinterpreted that the exopod is lost

11 Endopods of thoracic appendages (0) present and (1)reduced or absent The opisthosomal appendages of chelicer-ates lack endopods or have the endopods much reduced (egLimulus Siewing 1985 fig 838 Shultz 2001) a situation that isalso found in the uniramous thoracic appendages of Yohoia(Whittington 1974) It has been suggested that Helmetia(Briggs et al 1994) lacks endopods but pending a redescrip-tion of this genus and considering that they have been docu-mented in the otherwise very similar Kuamaia this is coded asuncertain

12 Exopod shaft of numerous podomeres each bearing asingle seta (0) present and (1) absent The exopods ofcrustaceans (at least plesiomorphically see eg Walossek ampMuller 1998 figs 5middot5 amp 5middot6) and of Marrella and Mimetasterhave multi-annulated shafts with each podomere bearing asingle seta In other taxa the exopod shafts consist of one oftwo lobes bearing numerous setae

The polarity of exopod segmentation is uncertain Thelateral flaps of stem group arthropods such as Opabinia(Whittington 1975b Budd 1996b) and anomalocaridids (egHou et al 1995) are certainly unsegmented but as suggestedby Buddrsquos (1996b fig 8) reconstruction this does notnecessarily suggest the form of the primitive euarthropodexopod Rather segmentation may have been a result of thesclerotization of the cuticle of the exopod shaft The outgroupwas coded as uncertain for this character According to thefirst interpretation of aglaspidid appendage morphology(coded as Aglaspidida 1) the exopods are lacking on allappendages and consequently this and all other charactersdescribing exopod morphology are coded as inapplicable

13 Exopod shaft differentiated into proximal and distallobes (0) absent and (1) present Ramskold amp Edgecombe(1996 Edgecombe amp Ramskold 1999 character 26) havediscussed the distribution of the trilobite-type bilobate exo-pods recognised by this character (Fig 6A) Among taxaincluded here that were not considered by Edgecombe amp

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 179

Ramskold (1999) exopods consist either of a single flattenedlobe or a homonomous series of podomeres (Fig 6B C) Nostem group euarthropod has a bilobate exopod and theoutgroup is consequently also coded as State 0

14 Proximal lobe of exopod (0) flattened lobe and (1)slender shaft

15 Distal lobe of exopod (0) small to moderate sized flapwith short to moderately long attachment to proximal lobeand (1) large teardrop-shaped with long attachment to proxi-mal lobe Neither of these characters can be coded for taxathat do not have an exopod differentiated into proximal anddistal lobes (Character 13 State 1) without asserting thehomology of the single-lobed exopod with one of these partsThe coding of Edgecombe amp Ramskold (1999 p 280) implic-itly homologizes the exopods of Sidneyia and Retifacies withthe proximal lobe In these taxa this view is perhaps supportedby the presence of lamellate exopod setae which match thoseof the trilobite-type proximal lobe However in other taxawith a single-lobed exopod such as Alalcomenaeus (see Briggsamp Collins 1999) the exopod is fringed with sharp spines likethe distal lobe of differentiated exopods Alternatively thebilobate exopod may have originated from the division of aprimitively single-lobed structure The single-lobed exopod isnot clearly homologous with either lobe of divided exopodsand both these characters are consequently coded as inappli-cable to taxa without differentiated proximal and distal exopodlobes Consequently (see Character 13 herein) these characterscan only be coded for taxa that were previously considered byEdgecombe amp Ramskold (1999) and their coding is followedhere except in the cases of Sidneyia and Retifacies

16 Exopod shaft is a deep rounded flap (0) absent and (1)present All bilobate (Character 13) and segmented (Character12) exopod shafts are long and relatively narrow structures (seeFig 6) whereas some single-lobed exopod shafts have theform of rounded flaps These flap-like exopod shafts are largecompared to the length of the setae and at least half as deep asthey are long This appendage structure is found in cheliceratesand lsquogreat appendagersquo arthropods

17 Medially-directed exopod setae (0) absent and (1)present Walossek amp Muller (1998 p 194) suggested that thetilting of exopod setae towards the endopod in post-antennularlimbs with a multi-annulated exopod was an autapomorphyuniting crustaceans and all members of the crustacean stemgroup (the crustacean total group sensu Ax 1986 see Fig 6C)This character is shared with the marellomorphs Marrellaand Mimetaster (see Fig 6B Sturmer amp Bergstrom 1976Bergstrom 1979 fig 1middot3A B) In other taxa the setae eithersurround the entire margin of the exopod shaft or are directeddorsally (Fig 6A) The identification of setae on the dorsalsurface of the lateral flaps in Opabinia (Budd 1996b) suggeststhat the condition in marellomorphs and crustaceans isderived

18 Lamellate exopod setae (0) absent and (1) presentLamellate exopod setae originally described from trilobitesare a classic synapomorphy of the Arachnomorpha (see egBergstrom 1992 Hou amp Bergstrom 1997 pp 42ndash3) They areperhaps best known from Misszhouia (see Chen et al 1996Hou amp Bergstrom 1997 Fig 6A) These setae differ from themore spinose setae of other taxa (Fig 6C) in that they are wideand flat and imbricate over the length of the exopod shaft Thesetae of Marrella (Fig 6B) and similar taxa have variouslybeen considered lamellate (Bergstrom 1979) or non-lamellate(Bergstrom 1992) Since they do not imbricate and do notseem to be strongly flattened (Whittington 1971 Sturmer ampBergstrom 1976 fig 9a) Marrella and Mimetaster are codedas State 0 The setae of Helmetia as shown in Briggs et al(1994 fig 141) seem to be of the lamellate type The setae ofSinoburius have been described by Hou amp Bergstrom (1997p 85) as similar to those of Misszhouia Only the setae areknown of the appendages of Buenaspis (Budd 1999a) andthese also appear to be of the trilobite-type

Figure 6 Diagrammatic reconstructions of (A) arachnomorph and(B C) non-arachnomorph biramous appendages (A) The lsquotrilobite-typersquo biramous appendage of Misszhouia longicauda (after Chen et al1997) (B) Appendage of Marrella splendens (after Whittington 1971)(C) Appendage of the stem-lineage crustacean Martinssonia elongata(after Muller amp Walossek 1997 1998) Not to scale Abbreviations(bas) basis (dle) distal lobe of exopod shaft (ls) lamellar exopod setae(pe) proximal endite or pre-coxa (ple) proximal lobe of exopod shaft(ses) segmented exopod shaft and (ss) spinose exopod setae

180 TREVOR J COTTON AND SIMON J BRADDY

The homology of the book-gill lamellae of xiphosurans withlamellar setae was supported by Walossek amp Muller (1997p 149) and Edgecombe et al (2000 p 174) but rejected bySturmer amp Berstrom (1981) on the grounds that Weinberginapossesses Limulus-like gill lamellae and fringing setaeBook-gills have also been described from a eurypterid (Braddyet al 1999) There are certainly major morphologicaldifferences between book-gills and trilobite-type exopodsFollowing most recent opinion and pending further studyWeinbergina and Eurypterida are coded as possessing lamellatesetae

The form of the setae of Yohoia is uncertain (Whittington1974) the spinose setae seen in the most common reconstruc-tion (Gould 1989 Briggs et al 1994) are not justified by thespecimens Amongst other great appendage arthropods theform of the setae appears to vary those of Jianfengia (seeChen amp Zhou 1997 p 74) and Leanchoilia (see Bruton ampWhittington 1983) are lamellate while those of Fortiforceps(Hou amp Bergstrom 1997) and Alalcomenaeus (Briggs amp Collins1999) are spinose and less densely packed

19 Gnathobase on basis andor prominent endites onendopod (0) present and (1) absent (cf Edgecombe ampRamskold 1999 character 29 Wills et al 1998a characters 36amp 59) The presence of gnathobases on the appendages of someanomalocaridids (Hou et al 1995) and their general distribu-tion amongst non-arachnomorph euarthropods suggests thatState 0 is plesiomorphic The homology of the various enditesand gnathobasic structures found in euarthropods is in need ofcareful assessment and therefore a very general coding (fol-lowing Edgecombe amp Ramskold) is used for this character

243 Eyes 20 Position of lateral facetted eyes (0)ventral and stalked (1) dorsal and sessile and (2) absent (cfEdgecombe amp Ramskold 1999 character 4) Partial uncer-tainty coding is used for a number of taxa where the dorsalsurface is known but the ventral morphology is not andtherefore the presence of ventral eyes cannot be discountedFor example there is good evidence that Leanchoilia lackeddorsal sessile eyes but ventral stalked eyes may have beenpresent (as in L hanceyi see Briggs amp Robison 1984) and a(02) partial uncertainty coding is used This is also the case inmany naraoiids such as Buenaspis However only the outlineof the head shield of Liwia is known and the absence of dorsaleyes in the reconstruction of Dzik amp Lendzion (1988) isconjectural It is unclear whether the autapomorphic conditionof stalked dorsal eyes in Mimetaster (see Sturmer amp Bergstrom1976) is homologous with State 0 or State 1 and a partialuncertainty coding is also used

Whittington (1974) was somewhat equivocal about thenature of the lateral lobes anterior to the head shield ofYohoia These more closely resemble the ventral stalked eyes ofAlacomenaeus as recently described by Briggs amp Collins(1999) than either Whittingtonrsquos (1974) or subsequent (egGould 1989 fig 3middot18 Briggs et al 1994 fig 153) reconstruc-tions suggest In a well preserved specimen showing the dorsalaspect (USNM 57696 Whittington 1974 pl 2 figs 1ndash3) and ina laterally compressed specimen (USNM 57694 Whittington1974 pl 1 fig 1) they are clearly seen to be stalked andrelatively small lobate structures That these structures arelikely to represent stalked ventral eyes was reflected in thecoding of Wills et al (1998a characters 26 amp 27)

21 Visual surface with calcified lenses bounded withcircumocular suture (0) absent and (1) present

22 Dorsal bulge in exoskeleton accommodating drop-shaped ventral eyes (0) absent and (1) present

23 Eye slits (0) absent and (1) presentCharacters 21 to 23 are used exactly as described by

Edgecombe amp Ramskold (1999 character 5) Character 21 is

inapplicable to taxa that do not possess eyes (Character 20State 2)

24 Dorsal median eyes (0) absent and (1) present Dorsalmedian eyes on a tubercle are considered a synapomorphy ofthe Chelicerata (Dunlop amp Selden 1997 Dunlop 1999) Thenature and position of the structures interpreted as dorsalmedian eyes in Mimetaster (Sturmer amp Bergstrom 1976p 83ndash4) are regarded as equivocal

244 Ventral cephalic structures Many characters of theventral surface of the euarthropod head (see Fig 7) of poten-tial phylogenetic utility are poorly known in many importanttaxa In particular the presence of pre-hypostomal frontalorgans (Fig 7A CndashE) is not coded herein (see Edgecombe ampRamskold 1999 p 272) Definitive evidence of the absence ofthese structures is available for very few taxa and the homol-ogy of these structures with the maculae of the trilobitehypostome (see Fig 7B) and the frontal organs of the crusta-cean labrum (eg see Muller amp Walossek 1987 p 39) isuncertain Secondly the detailed homology of the trilobitehypostome with that of other putative arachnomorphs isunclear and consequently a very broad definition is usedherein For example various pre-hypostomal sclerites (Fig7A D) in other arachnomorphs may have been incorporatedalong with a primitive hypostome into a single sclerite that isrecognised as the hypostome in trilobites The structure of thehypostome (eg the presence of paired anterior wings dorsal tothe antennae Fig 7B C) may also be a source of additionalcharacters

25 Expanded cephalic doublure (0) absent and (1)present maximum width more than 30 length of head shieldor more than 25 width of pygidium Chen et al (1996)suggested that the wide doublure of Soomaspis and Tariccoia isa synapomorphy uniting these taxa The doublure of Buenaspis(see Budd 1999a) is poorly known but based on the width ofthe heavily crushed region of the cephalic margin and theposition of a possible impression of the edge of the doublure inone specimen (Budd 1999a pl 1 fig 1) it also appears to berather wide (ie greater than 30 of the length of the headshield) Following Edgecombe amp Ramskoldrsquos coding ofSidneyia the present authors do not consider the postero-median expansion of the doublure of for example Emeraldellaor Retifacies (see Bruton amp Whittington 1983 Hou ampBergstrom 1999 respectively) to be homologous with State 1but to represent the hypostome (see Character 26)

26 Anteromedian margin of cephalon notched accomo-dating strongly sclerotised plate (0) notch and plate absentand (1) notch and plate present (cf Edgecombe amp Rasmkold1999 character 10) The apomorphic state (illustrated inFig 7D) is only known in the taxa discussed by Edgecombe ampRasmkold (1999) Reconstructions of Yohoia show a roundedlobe between the eyes (see Character 20) anterior to anddistinct from the head shield which resembles the anteriorsclerite recognised here This seems to be a misinterpretationSpecimens preserved in dorsal aspect (USNM 57696Whittington 1974 pl 2 figs 1ndash3) and lateral aspect (USNM57694 Whittington 1974 pl 1 fig 1 USNM 155616Whittington 1974 pl 5 fig 2) seem to clearly show that thislsquomedian lobersquo is a downward curving pointed extension of thehead shield

27 Hypostomal sclerite (0) median extension of thedoublure with no suture (1) natant sclerite not in contactwith doublure (2) with narrow overlap with pre-hypostomalsclerite (3) narrow attachment to doublure at hypostomalsuture and (4) absent The homology of hypostomes andlabrums has been discussed by Haas et al (2001) and Budd(2002) The euarthropod labrum is likely to represent a pos-teroventral extension of the pre-segmental acron (Scholtz

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 181

Figure 7 Ventral reconstructions of arachnomorph arthropod heads (A) Misszhouia longicaudata (after Chenet al 1997 figs 2ndash4 7) (B) Ceraurinella typa (after Chen et al 1997 fig 7) (C) Agnostus pisiformis (after Mulleramp Walossek 1987) (D) Kuamaia lata (after Edgecombe amp Ramskold 1999 figs 32 5 amp 6) (E) Cindarella eucala(after Ramskold et al 1997) and (F) Emeraldella brocki (after Bruton amp Whittington 1983) Not to scaleAbbreviations (aw) anterior wing beneath antennae (bs) boomerang-shaped sclerite (bl) blade-like process ofposterior wing (d) doublure of head-shield (cs) connective suture (f) fenestra (fs) facial suture (h) hypostome(hl) hypostomal lobe (emerging through fenestrae of Agnostus) (hs) hypostomal suture (if) intervening field ofhypostomal complex (lb) lateral bridge (lfo) lateral frontal organ (mb) median body (mbr) median bridge (mf)median furrow (mfb) mouth field bridge (mfo) median frontal organ (ol) ovate lobe and (phs) pre-hypostomalsclerite Post-antennal appendages and distal parts of antennae omitted

182 TREVOR J COTTON AND SIMON J BRADDY

1997) This structure is often sclerotised and covers the mouthventrally It is this sclerite that is here identified as thehypostome Therefore this character is considered absent intaxa lacking a sclerite covering the mouth irrespective of themorphology and position of the labrum itself which at leastpartly covers the mouth in all euarthropods other thanpycnogonids (see Edgecombe et al 2000 p 167) The lsquofleshylabrumrsquo of crustaceans does not appear to be sclerotised and ahypostome is consequently considered to be absent Althoughmany derived features are associated with the crustaceanlabrum this is not a good reason to consider it non-homologous with the hypostome-bearing structure of otherarthropods as Walossek amp Muller (1990) argued

In many taxa the mouth is covered by a posteromedianextension of the doublure here considered to represent thehypostome (eg Emeraldella Fig 7F Aglaspis see Hesselbo1992 fig 52) In others the hypostome is separated from thedoublure either by a suture a pre-hypostomal sclerite (egKuamaia lata Fig 7D Saperion glumaceum see Edgecombe ampRamskold 1999 figs 3 amp 4) or an intervening region ofunsclerotised cuticle (the natant condition of Fortey 1990beg Cindarella eucala Fig 7E) The homology of pre-hypostomal sclerites and hypostomes in helmetiids trilobitesand naraoiids (see Fig 7) has been discussed by Chen et al(1996) and Edgecombe amp Ramskold (1999) but it is as yetunclear whether any of the character states described here arederived from any other They are treated here as distinct andthe character as unordered pending further study

No hypostome has been identified in Yohoia (see above)Alalcomenaeus Jianfengia Fortiforceps or Leanchoilia andthere is certainly no broad posterior extension of the doublurein any of these taxa Since there is also no pre-hypostomalsclerite they have received a (134) partial uncertainty codingThe attachment of the hypostome of Tegopelte is unclear butit does not seem to be attached to any doublure (Whittington1985) and therefore it is given an uncertainty (12) coding

The lsquorostrum-like structurersquo on the ventral surface ofLemoneites appears to be similar in morphology and positionrelative to the anterior margin of the head shield (Flower 1968pl 8 figs 4 amp 13) to that described from Aglaspis and is codedas State 0 In many taxa the hypostome is unknown but inonly a small number can it be coded reliably as absentHughesrsquo (1975) suggestion that the hypostome of Burgessia isabsent is accepted preliminarily but an extension of thedoublure may be visible in some of Hughesrsquo figures Fortey(pers comm 2000) also doubts Hughesrsquo assertion

28 Visible ecdysial sutures (0) absent and (1) present Themarginal ecdysial sutures of chelicerates and the dorsal suturesof trilobites are almost certainly not homologous but thepresence of sutures and their position (see Character 29) arecoded separately to allow this to be tested Wills et al (1998aCharacter 2) coded marginal sutures as present in Sidneyiafollowing Brutonrsquos (1981) suggestion but this is consideredequivocal here

29 Position of ecdysial sutures (0) marginal and (1) dor-sal This character is inapplicable to taxa lacking ecdysialsutures (Character 28 State 0) Dorsal sutures whilst presentin the representatives of the Trilobita coded here are probablynot a synapomorphy of trilobites as a whole but for a clade oftrilobites excluding the Olenellida (Whittington 1989 Fortey1990a)

245 Exoskeletal tergites and thoracic tagmosis 30 Min-eralised cuticle (0) absent and (1) present Calcification of theexoskeleton is one of the most convincing synapomorphiesof the Trilobita (Fortey amp Whittington 1989 Ramskold ampEdgecombe 1991 Edgecombe amp Ramskold 1999 Character 1)Cuticle mineralisation is also coded as present in aglaspidids

following Briggs amp Fortey (1982) who suggested that theaglaspidid exoskeleton was originally phosphatic Paleomerusprobably also had a mineralised cuticle but Hou amp Bergstrom(1997 p 97) argued that it was likely to be calcareous Anundescribed Silurian aglaspidid may also have had an origi-nally calcitic cuticle (Fortey amp Theron 1994 p 856) Becauseof the uncertainty about the chemical composition of theexoskeleton in these forms cuticle mineralisation is coded aspotentially homologous in all these taxa and no attempt ismade to code separate states for phosphatic and calcareousmineralisation

31 Trunk tergites with expanded lateral pleurae coveringappendages dorsally (0) absent and (1) present Whilst thepresence of paratergal folds may be a synapomorphy at thelevel of the Euarthropoda (Boudreaux 1979 Wagele 1993Edgecombe et al 2000 Character 142) these are at mostsmall reflections of the margin of the tergites in most euarthro-pods In arachnomorph taxa these are expanded to form largelateral pleurae that cover the appendages dorsally (see egLimulus and Triarthus in Boudreaux 1979) This is inferred tobe the case in some taxa where dorsal features and appendagesare poorly known on the basis of their possession of tergiteswith wide pleural regions This feature was suggested as typicalof lamellipedians (=arachnomorphs) by Hou amp Bergstrom(1997 pp 42ndash3)

The arrangement of the thorax of Marrella Mimetaster andBurgessia differs from that of all the other taxa considered herein that the appendages seem to be attached to the lateralmargins of the body The trunk tergites do not cover theappendages and seem to lack pleurae entirely although theyare covered by a posterior extension of the head shield inBurgessia This character has been clearly described inMimetaster (Sturmer amp Bergstrom 1976 pp 87ndash90 fig 8) andBurgessia (Hughes 1975 p 421)

32 Free thoracic tergites (0) present and (1) absentFollowing Edgecombe amp Ramskold (1999 Character 16) anumber of taxa lack functional post-cephalic articulations andconsequently lack free thoracic tergites Despite this theycode these taxa for a number of characters relating to thestructure of thoracic tergites (eg their characters 18 and 19)These characters are treated here as inapplicable to taxawithout free tergites and the definition of this character hasbeen modified from that of Edgecombe amp Ramskold (1999) tomake this more explicit

33 Decoupling of thoracic tergites and segments (0)absent and (1) present Ramskold et al (1997) described thischaracter of Cindarella and Xandarella where the thoracictergites correspond to a variable increasing posteriorly num-ber of appendage pairs This character cannot be coded fortaxa in which free thoracic tergites are absent (Character 32State 1)

34 Tergite articulations (0) tergites non-overlapping (1)extensive overlap of tergites and (2) edge-to-edge pleuralarticulations In most of the taxa considered here the thoracictergites overlap considerably and relatively evenly over theirwidth This contrasts with the primitive euarthropod condi-tion where the tergites do not overlap medially as seen in stemgroup crustaceans In trilobites such as Helmetia and Kuamaia(see Edgecombe amp Ramskold 1999 character 18) overlap ofthoracic tergites is limited to a well-defined axial region andthe lateral pleurae meet edge-to-edge

The thoracic articulations of naraoiids with free thoracictergites are in some ways similar to those of trilobites with astrong overlap medially that is lacking abaxially andanterolateral articulating facets (Budd 1999a Ramskold ampEdgecombe 1999) However the distribution of these features

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 183

in other trilobite-like arachnomorphs is unclear and a restric-tive coding is used here The present authors do not considerthat this character can be applied reliably to the fused thoracictergites of Saperion Tegopelte and Skioldia and it is coded asinapplicable to all taxa lacking free thoracic tergites (Character32 State 1)

35 Trunk effacement (0) trunk with defined (separate orfused) tergite boundaries (1) trunk tergite boundaries effacedlaterally and (2) trunk tergite boundaries completely effaced(cf Edgecombe amp Ramskold 1999 Character 15)] Contrary toEdgecombe amp Ramskold (1999) the present authors considerSkioldia and Saperion to show a distinct form of tergiteeffacement where the fused tergite boundaries are defined byfurrows axially but effaced laterally The boundaries areeffaced at least laterally in Tegopelte but the axial region ofthe exoskeleton is unknown which accordingly is given amultistate uncertainty coding

36 Cephalic articulation fused (0) absent and (1) presentIn Tegopelte Saperion and Skioldia the articulation ofthe head with the thorax is non-functional and the entireexoskeleton forms a single tergite

37 Head shield overlap of thoracic tergites (0) overlapabsent or identical to overlap between thoracic segments (1)head shield covers first thoracic tergite only and (2) headshield covers mutliple anterior trunk tergites

38 Head shield articulates with reduced anterior thoracictergite (0) absent and (1) present Amongst taxa showing aposteriorly expanded head shield ie one that overlaps thethorax Edgecombe amp Ramskold (1999 Character 9) identifiedtwo distinct character states One of these ndash overlap of only theanteriormost thoracic tergite ndash was limited to taxa assigned tothe Liwiinae (sensu Fortey amp Theron 1994) and another ndashoverlap of multiple tergites with attachment to a reducedanterior thoracic tergite ndash to the Xandarellida None of theadditional taxa considered herein (including Buenaspis whichwas assigned to the Liwiinae by Budd 1999a) shows thesedistinctive morphological features However the expandedhead shields of Marrella Mimetaster and Burgessia which donot articulate with a narrow anterior thoracic tergite may behomologous with the expanded head shields of xandarellidsTo recognise this the degree of overlapping of the thorax andthe possession of the reduced anterior tergite are coded sepa-rately These characters are both coded as inapplicable to taxain which the head shield and thoracic tergites are fused(Character 36 State 1) The crustacean carapace which ismost similar to the head shield of Burgessia is seeminglyabsent in stem group crustaceans (Walossek amp Muller1998)

39 Trunk narrowed anteriorly relative to head shieldwidest posteriorly (0) absent and (1) present (cf Edgecombeamp Ramskold 1999 Character 14) The anterior narrowing ofthe trunk caused by the reduction of opisthosomal segment 1in xiphosurids (Weinbergina of the taxa analysed hereAndersen amp Selden 1997 Dunlop amp Selden 1997 Character14) is not considered to be homologous with the unusual shapeof the thorax in naraoiids recognised by their coding

40 Boundaries of anterior trunk segments reflexed antero-laterally (0) absent boundaries transverse or reflexed postero-laterally and (1) present

41 Joints between posterior tergites functional anteriorones variably fused (0) absent and (1) present

42 Posterior tergite bearing axial spine (0) absent and (1)present

Characters 40 to 42 are coded following Edgecombe ampRamskold (1999 characters 17 19 amp 23 respectively) Inaddition to the taxa considered here a thoracic axial spine ispresent in some olenelloid trilobites (see Lieberman 1998

Characters 73 amp 74) and in the aglaspidid Beckwithia (seeHesselbo 1989)

246 Posterior body termination 43 Postabdomen of seg-ments lacking appendages (0) absent and (1) present

44 Length of postabdomen (0) one segment (1) twosegments (2) three segments and (3) five segments In sometaxa a variable number of posterior thoracic segments bearcomplete tubular tergites and lack appendages In Aglaspisautapomorphically it seems that the posterior three thoracicsegments lack appendages (Briggs et al 1979 Hesselbo 1992)but have unmodified tergites These situations are recognisedas potentially homologous by the coding used here Character44 is coded as inapplicable to taxa lacking a postabdomen

45 Posterior tergites strongly curved in dorsal aspect com-pared to anterior tergites (0) absent and (1) present Asrecognised by Wills et al (1998a Character 17) in some taxawhere the tergites are distinct the curvature of thoracic tergitesincreases posteriorly so that the posterior tergites are highlycurved to semicircular in dorsal aspect This situation isnot known from crustaceans (except isopods) or stem grouparthropods

46 Posterior segments reduced and with highly reducedappendages (0) present and (1) absent In some taxa there area large number of posterior segments that are short sagitallyand have appendages that are much reduced in size comparedto anterior trunk appendages These somites are incorporatedinto the pygidium in some taxa but primitively they arecovered by tiny free trunk tergites In the derived state thetrunk somites and limbs are of a relatively constant size Thedistinction between these states can be seen when attempting tocount the number of segments that make up the body In taxashowing State 0 the number of segments is very high anddifficult to count whereas in other taxa the number ofpost-cephalic segments is easily assessed

47 Pygidium (0) absent and (1) present A pygidium isrecognised here as a posterior tagma consisting of a number offused segments under a single tergite which may or may notincorporate the post-segmental telson This is different to theuse of this term by Edgecombe amp Ramskold (1999) whichfollows Ramskold et al (1997) and very different to its use byWills et al (1998a p 53) as a synapomorphy of the TrilobitaThe present authorsrsquo definition recognises the situation inRetifacies where the spinose postsegmental telson is autapo-morphically not fused to the pygidium as homologous toother multisegmented posterior tagma which include thetelson The distinction between the pygidium and an expandedpost-segmental telson can also be seen in the position of theanus which is consistently in the posteriormost pre-telsonicsegment amongst putative arachnomorphs (see Character 48)In taxa with a pygidium the anal segment is fused into thepygidium (see eg Kuamaia Edgecombe amp Ramskold 1999fig 6) whereas in those taxa lacking a pygidium the anus isbetween the final trunk tergite and the telson

Ramskold et al (1997) argued that the posteriormost tergiteof xandarellids (which incorporates the anal segment) is nothomologous to the posterior tagma recognised as pygidiaherein because posterior thoracic tergites also cover multiplesegments in xandarellids (see Character 33) Evidence ofsegmentation of the pygidium in a variety of taxa suggests thatthe number of tergites fused to form the pygidium is greaterthan the number of appendages For example the pygidium ofKuamaia has two pairs of lateral spines but at least fourpairs of appendages (Hou amp Bergstrom 1997 Edgecombe ampRamskold 1999) and that of Triarthrus has only five axialrings posterior to the articulating ring but over 10 pairs ofappendages (Whittington amp Almond 1987) The fact thatdecoupling of tergites and segments is evident in the thorax of

184 TREVOR J COTTON AND SIMON J BRADDY

Xandarella and Cindarella does not necessarily suggest that asimilar more widespread decoupling in pygidia is the result ofa different developmental process and hence that they arenon-homologous Instead the situation in the xandarellidthorax is potentially homologous to that found (more widely)in pygidia and consequently the terminal xandarellid tergite isa pygidium It is unclear if decoupling of tergites and somites isprimitively limited to the terminal tergite or primitively aproperty of the arachnomorph post-cephalon as a whole

It has been suggested that the tiny pygidium of olenelloidtrilobites which probably best reflects the primitive trilobitestate consists only of the post-segmental telson (Harrington inMoore 1959) and therefore is not a true pygidium HoweverWhittington (1989) showed that the olenelloid pygidium con-sists of at least two and possibly as many as five or sixsegments and therefore is likely to be homologous with thepygidium of other trilobites

48 Position of the anus (0) terminal within telson and (1)at base of telson In crustaceans and stem group arthropodsthe anus is terminal or otherwise situated in the telson (egRehbachiella Walossek 1993 fig 15D) In putative arachno-morphs the anus is either at the junction of the posteriormostthoracic segment and the telson or ventral within a fusedpygidium In the case of taxa with a pygidium the anus isanterior to the posterior margin and where known positionedbetween the posteriormost pair of appendages suggesting thatit is anterior to the post-segmental telson (see eg OlenoidesWhittington 1980)

49 Pygidium with median keel (0) absent and (1) presentEdgecombe amp Ramskold (1999 Character 21) considered thepresence of a median keel on the pygidium as a synapomorphyuniting Soomaspis and Tarricoia They did not explain howthis structure could be distinguished from the raised pygidialaxis of trilobites Homology of these structures is not sup-ported here because the axis of more primitive trilobites(including Eoredlichia) is quite distinct morphologically fromthe naraoiid keel Budd (1999a) noted the presence of a keel onthe pygidium of Buenaspis and suggested (Budd 1999a p 102)that it may be an artefact of dorsoventral compaction How-ever contrary to the coding of Edgecombe amp Ramskold(1999) a keel is visible on the pygidium of Liwia convexa (Dzikamp Lendzion 1988 fig 4CD) preserved in full relief Thereforeit seems unlikely that the keel is a taphonomic artefact

50 Pygidium with broad-based median spine (0) absentand (1) present

51 Pygidium with lateral spines (0) present and (1) absentEdgecombe amp Ramskold (1999 Character 22) coded thepresence of a median spine and two pairs of lateral spines aspotentially homologous in Sinoburius Kuamaia and HelmetiaThe distribution of lateral spines is much wider than that ofterminal spines and therefore they are coded separately hereIn trilobites it has widely been recognised that lateral spinesrepresent the original segmentation of the pygidium Thiscannot be the case for median spines Characters 49 to 51are not applicable to taxa lacking a pygidium (Character 47State 0)

52 Expanded post-segmental telson (0) absent and (1)present In a range of taxa the posterior-most tergite is a large(relative to the thoracic tergites) structure that lacks anyevidence of segmentation and is interpreted as representing anexpanded tergite of the post-segmental telson from whichsegments are released anteriorly during ontogeny On the otherhand the posteriormost tergite of Marrella and probablyMimetaster is small compared to the thoracic tergites androunded This element probably represents the plesiomorphiceuarthropod telson The homology of this character in mosttaxa is clear and only doubtful in Retifacies in which it may

be segmented The figures of Hou amp Bergstrom (1997) providelittle unequivocal evidence for a telson in Retifacies but thepresence of this structure is very clear in colour photographs(eg Chen et al 1996 fig 198A) However these figures donot convincingly demonstrate the segmentation of the telsonRetifacies is interpreted here as being unique in possessingboth a pygidium and an expanded post-segmental telson

53 Telson shape (0) spinose and (1) paddle-shaped Theshape of the expanded telsons identified above varies frombroadly spinose to flattened and paddle-like This differencecan be recognised both on the basis of the ratio of length tobasal width (spinose telsons are relatively long) and by thechange in width posteriorly (spinose telsons reduce in widthposteriorly whereas paddle-shaped telsons increase in width)In eurypterids a wide range of telson shapes is found includ-ing both character states described here It is likely that theplesiomorphic condition is a spinose telson This character isinapplicable to taxa lacking an expanded telson

54 Post-ventral furcae (0) absent and (1) present Thischaracter recognises the potential homology of unsegmentedpaired structures that articulate with the segment immediatelyanterior to the telson The potential homology of these struc-tures (the post-ventral plates) in Emeraldella and Aglaspis wasrecognised by Wills et al (1998a Character 70) and inEmeraldella and Sidneyia by Edgecombe amp Ramskold (1999Character 25) The homology of the segmented pygidial caudalfurcae of Olenoides (see Whittington 1975a 1980) with thesestructures is considered doubtful and coded as equivocal

Despite their distinctive morphology the long caudualfurcae of Cheloniellon may be homologous with the furcae ofAglaspis Emeraldella and Sidneyia This is supported by thecaudal furcae of the Ordovician cheloniellid Duslia which aremore similar in morphology to those of Emeraldella than ofCheloniellon Chlupac (1988 pp 614ndash16) considered thesestructures to be on the terminal tergite in Duslia However afaint tergite boundary is apparent posterior to the attachmentof the furcae (see Chlupac 1988 pl 57 fig 4) Therefore theyare considered here to have been attached to the pre-telsonicsegment as in Cheloniellon Chlupac (1988) and Dunlop ampSelden (1997) supported a close relationship between Dusliaand Cheloniellon

25 MethodsAll analyses were carried out using PAUP Version 40b4a(Swofford 1999) and unless otherwise stated used heuristicsearches with one thousand random addition sequencereplicates The software packages MacClade Version 307(Maddison amp Maddison 1997) and RadCon (Thorley amp Page2000) were used for comparing trees and investigating patternsof character evolution Tree length and other statistics (theConsistency Index CI and Retention Index RI) were calcu-lated by PAUP and MacClade with uninformative charactersexcluded Analyses were run separately using each of thedifferent codings for aglaspidids

Characters were treated as unordered and of equal weight inmost analyses To assess the influence of this assumption fourof the eight multistate characters which have states that areintermediate between others (Wilkinson 1992) were treated asordered in some analyses These characters are the number ofcephalic segments (Character 4) the length of raptorialappendage spines (Character 6) the degree of overlap of thethorax by the head-shield (Character 37) and the number ofsegments making up the postabdomen (Character 44) The lastof these is uninformative when not ordered because only onetaxon shows each of states 0 1 and 3

Support for individual nodes was assessed by bootstrapanalysis (Felsenstein 1985) and by calculating Bremer support

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 185

indices (Bremer 1988 1994) These methods measure twodistinct aspects of support for phylogenetic hypotheses Treesor nodes may be considered well supported (1) to the extent towhich alternative topologies are much less parsimonious asmeasured by the support index (Wilkinson 1996) or (2) wherethey are consistent with a large proportion of characters sothat character sampling is unlikely to have had much influenceon topology as assessed by bootstrapping (Page 1996) Boot-strapping was performed with 100 bootstrap replicates eachof 10 addition sequence replicates Bremer support indicesare based on heuristic searches with 100 addition sequencereplicates

Three pairs of terminals are included in Table 2 that apartfrom missing data are coded identically These are Helmetiaand Kuamaia Soomaspis and Tariccoia and Saperion andSkioldia These pairs are lsquotaxonomic equivalentsrsquo removingthe taxon with most missing data will not influence theposition of the other They can be recoded as single terminalsaccording to the principal of safe taxonomic reduction(Wilkinson 1995a b) and were not considered separately in theanalyses presented here Therefore cladistic analyses werecarried out on the basis of 33 terminals Edgecombe ampRamskold (1999) ignored this and coded the same three pairsof taxa and Eoredlichia and Olenoides identically for theirmore limited set of characters

26 ResultsAnalysis with all characters unordered and using the lsquoAglaspi-dida 1rsquo coding found nine most parsimonious trees (MPTs)each 126 steps long (CI=0middot556 RI=0middot763) Using the lsquoAglas-pidida 2rsquo coding resulted in 18 MPTs of length 125 (CI=0middot560RI=0middot765) Nine of these 18 trees were those found with thefirst coding of aglaspidids The strict component consensus(sensu Wilkinson 1994) of the 27 unique trees found acrossboth analyses is shown in Figure 8 All trees supported themonophyly of the Marrellomorpha (Marrella and Mimetaster)and of a clade including all other taxa except Crustacea (Clade1 of Fig 8) The relationships between these two clades and theCrustacea were unresolved equal numbers of trees in bothanalyses supported each of the topologies [Clade 1 (CrustaceaMarrellomorpha)] [Crustacea (Marrellomorpha Clade 1)]and [Marrellomorpha (Crustacea Clade 1)] All trees placedBurgessia as the sister taxon to the rest of Clade 1 andsupported the division of the rest of Clade 1 into two majorsister groups The topology of one of these clades (Clade 2)which included trilobites helmetiids tegopeltids naraoiidsxandarellids and Retifacies was stable with respect to thecoding of aglaspidids The other clade (Clade 3) consists of aclade of megacheirans and chelicerates (Clade 4) and a cladeincluding aglaspidids Lemoneites Paleomerus CheloniellonEmeraldella and Sidneyia (Clade 5) This latter clade is un-resolved in the strict consensus of all 27 trees (Fig 8) and thestrict consensus of the 18 MPTs found using the lsquoAglaspidida2rsquo coding but was fully resolved in the consensus of the ninetrees found with the lsquoAglaspidida 1rsquo coding Within themegacheiran-chelicerate clade all trees found Fortiforceps tobe the sister group to all other taxa Yohoia the sister group tochelicerates and Alalcomenaeus and Leanchoilia to form aclade Jianfengia is equally parsimoniously placed as the sistergroup to the Yohoia-chelicerate clade or as the sister group tothis clade and the Alalcomenaeus-Leanchoilia clade

Treating characters 4 6 37 and 44 as ordered had a minimaleffect on most parsimonious topology Nine MPTs of 136 steps(CI=0middot515 RI=0middot743) were found with the lsquoAglaspidida 1rsquocoding and 12 MPTs 135 steps in length (CI=0middot519 RI=0middot745)with the lsquoAglaspidida 2rsquo coding These trees all supported a(Crustacea Clade 1) clade excluding the marrellomorphs

Apart from this the strict consensus of these 21 trees isidentical to the consensus of the 27 trees found with allcharacters treated as unordered

Across all analyses four different topologies for Clade 5 (ofFig 8) were found (Fig 9) The distribution of these topologiesacross the four analyses described above and across the sameanalyses repeated after reweighting of each character in pro-portion to their Rescaled Consistency Index (see Farris 1989)from the first analysis is shown in Table 3 The instability of

Figure 8 Strict consensus of 27 MPTs from four separate analyses ofarachnomorph phylogeny with both interpretations of aglaspididmorphology and with some characters treated as ordered or allcharacters unordered Clades referred to in the text are numbered

Figure 9 Alternative equally parsimonious resolutions of clade 5 ofFig 8 (see Table 3)

186 TREVOR J COTTON AND SIMON J BRADDY

this clade was to some extent caused by the inclusion ofLemoneites and Paleomerus which could each be coded foronly 27 of the 54 characters used Analysis with these two taxaexcluded and characters treated as unordered found treescompatible only with Topology D of Figure 9 irrespective ofthe coding used for aglaspidids Analysis with some characterstreated as ordered and using either aglaspidid coding foundthree trees compatible only with Topology B of Figure 9 andthree compatible only with Topology D of Figure 9 ThereforeTopology D is the preferred topology for Clade 5 as shown inTable 3

One of the MPTs resulting from the lsquoAglaspidida 2rsquo coding(with all characters treated as unordered) was chosen as thebasis for further discussion because this tree combined thepreferred topology of Clade 5 with the (Crustacea Clade 1)group that was favoured when some characters were treated asordered Bootstrap percentages and Bremer support values forthis tree (using the Aglaspidida 2 coding) are shown in Figure10 Apomorphies for all ingroup nodes of this tree are shownin Figure 10A

3 Discussion

31 Phylogenetic resultsThe results of the cladistic analyses presented above arewell-resolved and robust with respect to different analyticalparameters Most of the taxa considered form a clade (Clade 1of Fig 8) that is more closely related to chelicerates than tocrustaceans in all analyses and therefore is recognised asthe Arachnomorpha as defined by Chen et al (1996) andRamskold et al (1996) The monophyly of the Marrellomor-pha was supported in all analyses The marrellomorphs werefound to be the sister group to a Crustacea+Clade 1 group inmost MPTs but in a minority of trees formed the sister groupto Clade 1 alone According to the latter result they should beincluded within the Arachnomorpha but according to the firstexcluded from it Both of these alternatives have been sup-ported in previous studies (eg Hou amp Bergstrom 1997 Willset al 1998a respectively) In addition to the marrellomorphsthe Fuxianhuiida Bousfield 1995 and Canadaspidida havebeen placed in the Euarthropoda outside the main mandibu-late and arachnomorph clades (Hou amp Bergstrom 1997)Further study of these taxa of the crustacean or mandibulatestem group and of the marrellomorphs is necessary before the

position of the Marrellomorpha can be resolved Provisionallymarrellomorphs are excluded from the Arachnomorphafollowing the majority of analyses above and Wills et al(1994)

The Burgess Shale arthropod Burgessia was placed as thesister group to remaining arachnomorphs in all analyses OtherArachnomorpha form two major clades one consisting of thelsquotrilobite-alliedrsquo arachnomorphs analysed by Edgecombe ampRamskold (1999) here termed lsquoTrilobitomorpharsquo (defined asTrilobita and all taxa more closely related to Trilobita thanChelicerata) and the other (lsquochelicerate-alliedrsquo) clade includingchelicerates lsquogreat appendagersquo arthropods aglaspididsLemoneites Paleomerus Sidneyia and Emeraldella The latteris here termed lsquoCheliceramorpharsquo (defined as Chelicerata andall taxa more closely related to Chelicerata than Trilobita) Thetopology of the lsquotrilobite-alliedrsquo clade was similar to that foundby Edgecombe amp Ramskold (1999) but the present authorsrsquoresults are more completely resolved unambiguously support-ing the sister-group relationship between trilobites andHelmetiida suggested in the previous study Within thelsquochelicerate-alliedrsquo clade Aglaspidids Paleomerus Lemoneitesand the Burgess Shale Emeraldella and Sidneyia form a cladein opposition to megacheirans and chelicerates

The present authorsrsquo analysis supports a new hypothesis ofthe origin of the chelicerates from within a paraphyleticassemblage of megacheiran arthropods and confirms pyc-nogonids as a sister group to the euchelicerates Previoushypotheses of the chelicerate sister group (see Section 1) aremuch less parsimonious The shortest trees supporting aCheloniellon-chelicerate clade are six steps longer than theMPTs and those supporting an aglaspidid-chelicerate cladeseven steps longer All megacheirans and chelicerates areunited by the flap-shaped rounded exopods and modificationof the second cephalic segment endopods into anteriorlydirected spinose raptorial organs which share a number ofdetailed similarities A clade including all of these taxa apartfrom Fortiforceps is supported by the synapomorphic loss ofthe antennae (or antennulae of crustaceans) and the longerlength of spinose projections of the second cephalic appendageendopods Within this clade Yohoia shares the loss of cephalicexopods the loss of thoracic endopods and a postabdomen oftubular tergites with chelicerates

The poor resolution of Clade 5 (of Fig 8) was partly a resultof the poor preservation of and consequent high proportion ofmissing data for Lemoneites and Paleomerus When these two

Table 3 Numbers of most parsimonious trees supporting each of the alternative topologies for clade 4 of Fig 7 under different analyticalconditions (RCI=Revealed Consistency Index)

Aglaspididcoding

Characters 4 6 36and 43 ordered

Charactersreweighted by RCI

Paleomerus andLemoneites excluded

Topology (see Fig 8)

A B C D

1 91 X 3 3 31 X 61 X X 31 X 61 X X 3 32 9 92 X 3 3 3 32 X 62 X X 32 X 62 X X 3 3Total 6 18 12 54

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 187

taxa were excluded from the analyses only two (B and D) ofthe four topologies found in the complete analysis remainedmost parsimonious (see Fig 9 and Table 3) Secondly Bremersupport values throughout much of the lsquochelicerate-alliedrsquoclade (Clade 3 of Fig 8) were improved when these taxa wereexcluded (see Fig 11)

Dunlop (1999 p 258) suggested that in reconstructing thechelicerate stem lineage lsquowe might predict that the two mostsignificant changes towards the chelicerate condition are thereduction of the antennae and the formation of the nextappendages into a clawrsquo According to the hypothesis pre-sented here both of these adaptations were achieved in EarlyCambrian chelicerate ancestors The recognition of the loss ofthe antennae in the stem group of the chelicerates and thephylogenetic hypothesis presented here suggest that chelicer-ates are most closely related to taxa with only a singleadditional segment fused to the crown group eucheliceratehead of four fused somites Previous hypotheses have derivedchelicerates from Palaeozoic arthropods with a longer head(eg Cheloniellon Sturmer amp Bergstrom 1978 SanctacarisBriggs amp Collins 1988 Emeraldella Bruton amp Whittington1983) The plesiomorphic pattern of head segmentation ismatched in pycnogonids Bergstrom et al (1980) suggestedthat only the anterior four pairs of appendages are incorpo-rated into the head This supports the view that pycnogonidsare primitive with respect to other chelicerates which showmore or less complete fusion of this primitive head and threethoracic segments including Solpugida and Pseudoscorpiones

The present study suggests that a shift from gnathobasicbenthic feeding to a more vagrant lifestyle and raptorialappendage feeding occurred very early in the chelicerate lin-eage Appendage feeding was clearly an important adaptation

Figure 10 One of 18 equally parsimonious cladograms with lsquoAglaspidida 2rsquo coding and all characters unorderedthat is most congruent with results from other analyses Non-terminal branch lengths scaled to reflect number ofapomorphies (A) ACCTRAN apomorphy scheme Character numbers shown above boxes and states shownbelow boxes as in Table 2 and the text Character consistency indices are indicated by shading (B) Levels ofsupport for individual nodes Bremer support indices shown in bold type above branches Bootstrap percentagesin italic type below branches for all nodes with relative frequencies greater than 50

Figure 11 Bremer support values for clades within the lsquochelicerate-alliedrsquo arachnomorphs (clade 3 of Fig 8) when all taxa wereincluded (normal weight font above nodes) and with Lemoneites andPaleomerus excluded (bold type below nodes)

188 TREVOR J COTTON AND SIMON J BRADDY

for the terrestrialization of chelicerates (Dunlop 1997 p 69Dunlop amp Webster 1999) but the model proposed heresuggests that this considerably pre-dated terrestrializationGnathobasic feeding in xiphosurans and eurypterids mayrepresent a reversal to a more primitve benthic arachnomorphlifestyle from a more pycnogonid-like ancestor

32 Evolution of the arachnomorph headThe extant arthropod classes show highly conserved patternsof head segmentation (eg Wills et al 1994) In contrast therehas been a near consensus amongst Cambrian arthropodworkers (although rarely based on an explicit phylogenetichypothesis) that patterns of head segmentation are highlyhomoplastic and hence of little systematic significanceSturmer amp Bergstrom (1978 pp 78ndash9) suggested that lsquoevenclosely related forms may have different numbers of headsegments and appendagesrsquo Bruton amp Whittington (1983pp 576ndash7) that lsquodiscussion on fossil arthropod relationshipsbased on head segmentation appears to be largely irrel-evant and at best speculativersquo and Delle Cave amp Simonetta(1991 p 191) that lsquothere are no obvious phyletic affinitiesbetween genera having the same number of cephalized seg-mentsrsquo Most recently Bergstrom amp Hou (1997 p 104) con-cluded that lsquothe segmental length of the head shield seemsto be of no relevance to the discussion and discrimination ofevolutionary lineagesrsquo It has been claimed that this view issupported by cladistic analyses in which patterns of tagmosishave been found to be rather poor at defining major arthropodclades (Briggs et al 1992 Wills et al 1994)

This suggestion is not only of relevance to systematics buthas featured prominently in recent debates surrounding thenature of the lsquoCambrian explosionrsquo (see Budd amp Jensen 2000for a recent review) Gould (1989 1991) argued that theapparent plasticity of head segmentation in Cambrian com-pared to post-Cambrian arthropod evolution suggests thatbody-plan evolution was only constrained after the Cambrianexplosion According to this view the origin of new highertaxa with distinct body-plans was only possible before theonset of these constraints (eg Valentine 1995) Taxa hererecognised as arachnomorphs clearly played a major role in thedevelopment of Gouldrsquos hypothesis This suggestion has pre-viously been assessed in terms of overall morphological diver-sity which is clearly rather distinct from Gouldrsquos concept ofdisparity Wills et al (1998b) showed that there was littlecorrelation between overall morphological disparity and de-gree of limb specialisation and that the latter was phylogeneti-cally highly plastic They remarked that (Wills et al 1998bp 64) lsquothis finding has important implications for models ofarthropod phylogeny and evolution that attribute overidingimportance to head segmentationrsquo but did not explicitlyexamine head segmentation although it is likely to have beena major component of the index of tagmosis employed

The results of the present study suggest that arguments thatarthropod head segmentation was unusually labile during theCambrian are poorly founded Rather only four major pat-terns of euarthropod head segmentation are identified Theplesiomorphic euarthropod state according to both Walossekamp Muller (1997 1998) and Scholtz (1997) which may moreproperly be the plesiomorphic state for the euarthropod crowngroup only (depending on the phylogenetic position of theMarrellomorpha) consists of four post-acronal segmentsbearing the antennae and three pairs of biramous limbs Thepresent authors suggest that the term lsquocephalonrsquo be restrictedto this kind of head which is found in stem group mandibu-lates in addition to many arachnomorphs Crown groupmandibulates share a lsquobimaxillary headrsquo in which an addi-tional pair of appendages the labium or second maxillae are

incorporated into the head but in crustaceans these are notfused to the carapace (Scholtz 1997) Finally two distinctforms of head tagmosis are found in chelicerates A head withfour pairs of appendages (but without antennae giving a totalof five segments incorporated into the head) which has beencalled the lsquocephalosomarsquo is present in pycnogonids (Walossekamp Dunlop 2002 pp 434ndash5) and probably also in some euch-elicerates (J A Dunlop pers comm 2003) The lsquoprosomarsquo ofmost euchelicerates consists of this cephalosoma and twoadditional segments

In general these patterns of head segmentation are highlyphylogenetically conserved (see Fig 12) Only three homo-plastic changes namely reversal to a more primitive conditionin the Leanchoilia-Alalcomenaeus clade and the convergentorigin of a five-segmented head in Clade 3 (of Fig 8) and theXandarellida are necessary to optimise character 4 onto themost parsimonious cladogram There are also transitions toautapomorphic states in Emeraldella and Sidneyia In the caseof Sidneyia however an argument could be made (followingBergstrom amp Sturmer 1978 see discussion of Character 4) thatthe head consists of five segments as in closely related taxabut autapomorphically each segment has a separate tergiteThis would further increase the phylogenetic stability of headsegmentation Also following Bergstrom amp Sturmer (1978) thehead of Cheloniellon could be considered to consist of twotergites making it more similar to the head of Emeraldella(with six pairs of appendages)

Figure 12 Cladogram from Fig 10 showing changes in head segmen-tation Bold type shows abbreviated head segmentation formulaAbbreviations (Ant) antennae (BA) biramous appendage pairs (Ch)chelicerae (GA) great appendages (UA) uniramous appendage pairsand (X) segment without appendages Synapomorphy A is the auta-pomorphic reduction in head length to a single segment in SidneyiaAlternatively this may represent secondary division of the headshield Synapomorphy B is the increase in the length of the head inEmeraldella to six segments

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 189

4 Acknowledgements

We thank D E G Briggs J A Cotton J A Dunlop R AFortey S Harris R A Moore P Selden O E Tetlie andP Van Roy for comments on earlier drafts of this work Themanuscript was reviewed by M Wills F Schram and oneanonymous referee This research was supported by the Geo-logical Society of London Curry Research Studentshipawarded to TJC by the Department of Earth Sciences Uni-versity of Bristol and by the Department of PalaeontologyThe Natural History Museum London TJC would like tothank C Labandeira J Thompson and R Chapman fortheir hospitality at the National Museum of Natural HistoryWashington DC USA and A Olcott S Schellenberg andJ Young for making his time there so enjoyable

5 References

Abzhanov A Popadic A amp Kaufman T C 1999 Chelicerate Hoxgenes and the homology of arthropod segments Evolution andDevelopment 1 77ndash89

Akam M 2000 Arthropods developmental diversity within a (super)phylum Proceedings of the National Academy of Sciences USA97 4438ndash41

Aldridge R J Gabbott S E amp Theron J N 2001 The Soom ShaleIn Briggs D E G amp Crowther P R (eds) Palaeobiology II340ndash2 Oxford Blackwell Science

Anderson L I amp Selden P A 1997 Opisthosomal fusion andphylogeny of Palaeozoic Xiphosura Lethaia 30 19ndash31

Arango C P 2002 Morphological phylogenetics of the sea spiders(Arthropoda Pycnogonida) Organisms Diversity and Evolution2 107ndash25

Ax P 1986 Das phylogenetische System Stuttgart Gustav FischerVerlag

Babcock L E amp Zhang Wentang In press New non-mineralizedarthropods from the Chengjiang Lagerstatte Lower CambrianYunnan China Palaeontologia Cathyana

Bartels C Briggs D E G amp Brassel G 1998 The fossils ofthe Hunsruck Slate Marine life in the Devonian CambridgeCambridge University Press

Bergstrom J 1971 Paleomerus ndash merostome or merostomoid Lethaia4 393ndash401

Bergstrom J 1979 Morphology of fossil arthropods as a guideto phylogenetic relationships In Gupta A P (ed) ArthropodPhylogeny 3ndash56 New York NY Van Nostrand Reinhold Co

Bergstrom J 1992 The oldest arthropods and the origin of theCrustacea Acta Zoologica 73 287ndash91

Bergstrom J Sturmer W amp Winter G 1980 PalaeoisopusPalaeopantopodus and Palaeothea pycnogonid arthropodsfrom the Lower Devonian Hunsruck Slate West GermanyPalaontologische Zeitschrift 54 7ndash54

Bergstrom J amp Brassel G 1984 Legs in the trilobite Rhenops fromthe Lower Devonian Hunsruck Slate Lethaia 17 67ndash72

Bergstrom J amp Hou X 1998 Chengjiang arthropods and theirbearing on early arthropod evolution In Edgecombe G D(ed) Arthropod fossils and phylogeny 151ndash84 New York NYColumbia University Press

Boudreaux H B 1979 Significance of intersegmental tendon systemin arthropod phylogeny and a monophyletic classification ofArthropoda In Gupta A P (ed) Arthropod Phylogeny 551ndash86New York NY Van Nostrand Reinhold Co

Bousfield E L 1995 A contribution to the natural classification ofLower and Middle Cambrian arthropods food-gathering andfeeding mechanisms Amphipacifica 2 3ndash33

Braddy S J Aldridge R J Gabbott S E amp Theron J N 1999Lamellate book-gills in a late Ordovician eurypterid from theSoom Shale South Africa support for a eurypterid-scorpionclade Lethaia 32 72ndash4

Braddy S J amp Almond J E 1999 Eurypterid trackways from theTable Mountain Group (Ordovician) of South Africa Journal ofAfrican Earth Sciences 29 165ndash77

Bremer K 1988 The limits of amino acid sequence data in angio-sperm phylogenetic reconstruction Evolution 42 795ndash803

Bremer K 1994 Branch support and tree stability Cladistics 10295ndash304

Briggs D E G 1998 Review of Arthropods from the LowerCambrian Chengjiang fauna southwest China by HouXianguang and Jan Bergstrom Fossils and Strata 45

Scandinavian University Press Oslo Norway 1997 Palaeo-geography Palaeoclimatology Palaeoecology 143 193ndash4

Briggs D E G Bruton D L amp Whittington H B 1979 Append-ages of the arthropod Aglaspis spinifer (Upper CambrianWisconsin) and their significance Palaeontology 22 167ndash80

Briggs D E G Fortey R A amp Wills M A 1992 Morphologicaldisparity in the Cambrian Science 256 1670ndash3

Briggs D E G Fortey R A amp Wills MA 1993 How big was theCambrian explosion A taxonomic and morphologic comparisonof Cambrian and Recent arthropods In Lees D R amp EdwardsD (eds) Evolutionary patterns and processes 33ndash44 LondonLinnean Society of London Linnean Society Symposium Series

Briggs D E G Erwin D H amp Collier F J 1994 The fossils of theBurgess Shale Washington DC Smithsonian Institution Press

Briggs D E G amp Bartels C 2001 New arthropods from the LowerDevonian Hunsruck Slate (Lower Emsian Rhenish Massifwestern Germany) Palaeontology 44 275ndash304

Briggs D E G amp Bartels C 2002 Magnoculocaris a new name forthe arthropod Magnoculus Briggs and Bartels 2001 from theLower Devonian Hunsruck Slate Germany Palaeontology 45419

Briggs D E G amp Collins D 1988 A Middle Cambrian cheliceratefrom Mount Stephen British Columbia Palaeontology 31 779ndash98

Briggs D E G amp Collins D 1999 The arthropod Alalcomenaeuscambricus Simonetta from the Middle Cambrian Burgess Shale ofBritish Columbia Paleontology 42 953ndash78

Briggs D E G amp Fortey R A 1982 The cuticle of aglaspididarthropods a red-herring in the early history of vertebratesLethaia 15 25ndash9

Briggs D E G amp Fortey R A 1989 The early radiation andrelationships of the major arthropod groups Science 246 241ndash3

Briggs D E G amp Fortey R A 1992 The early Cambrian radiationof arthropods In Lipps J H amp Signor P W (eds) Origin andEarly Evolution of the Metazoa 335ndash373 New York NY PlenumPress

Briggs D E G amp Robison R A 1984 Exceptionally preservednontrilobite arthropods and Anomalocaris from the MiddleCambrian of Utah University of Kansas PaleontologicalContributions Paper 111 1ndash23

Broili F 1932 Ein neuer Crustacee aus dem rheinischen UnterdevonSitzungsberichte der Mathematisch-Naturwissenschaftlichen(Abteilung) Klasse der Bayerischen Akademie der Wissenschaftenzu Munchen 1932 27ndash38

Broili F 1933 Ein zweites Exemplar von Cheloniellon Sitzungsber-ichte der Bayerischen Akademie der Wissenschaften 1933 11ndash32

Bruton D L 1981 The arthropod Sidneyia inexpectans MiddleCambrian Burgess Shale British Columbia Philosophical Trans-actions of the Royal Society of London Series B 295 619ndash56

Bruton D L amp Whittington H B 1983 Emeraldella and Leanchoiliatwo arthropods from the Burgess Shale British Columbia Philo-sophical Transactions of the Royal Society of London Series B 300553ndash85

Budd G E 1995 Kleptothule rasmusseni gen et sp nov anolenellid-like trilobite from the Sirius Passet fauna (BuenFormation Lower Cambrian North Greenland) Transactions ofthe Royal Society of Edinburgh Earth Sciences 86 1ndash12

Budd G E 1996a Progress and problems in arthropod phylogenyTrends in Ecology and Evolution 11 356ndash8

Budd G E 1996b The morphology of Opabinia regalis and thereconstruction of the arthropod stem-group Lethaia 29 1ndash14

Budd G E 1997 Stem-group arthropods from the Lower CambrianSirius Passet fauna of North Greenland In Fortey R A ampThomas R H (eds) Arthropod Relationships 125ndash38 LondonChapman and Hall Systematics Association Special Volume 55

Budd G E 1999a A nektaspid arthropod from the Early CambrianSirius Passet fauna with a description of retrodeformation basedon functional morphology Palaeontology 42 99ndash122

Budd G E 1999b The morphology and phylogenetic significance ofKerygmachela kiergkegaardi Budd (Buen Formation LowerCambrian N Greenland) Transactions of the Royal Society ofEdinburgh Earth Sciences 89 (for 1998) 249ndash290

Budd G E 2002 A palaeontological solution to the arthropod headproblem Nature 417 271ndash5

Budd G E amp Jensen S 2000 A critical reappraisal of the fossilrecord of the bilaterian phyla Biological Reviews 75 253ndash95

Chen J Zhou G Zhu M amp Yeh K 1996 The Chengjiang biota ndasha unique window of the Cambrian explosion Taichung NationalMuseum of Natural Science [In Chinese]

Chen J Edgecombe G D amp Ramskold L 1997 Morphological andecological disparity in naraoiids (Arthropoda) from the Early

190 TREVOR J COTTON AND SIMON J BRADDY

Cambrian Chengjiang fauna China Records of the AustralianMuseum 49 1ndash24

Chen J amp Zhou G 1997 Biology of the Chengjiang fauna In ChenJ Cheng Y amp Iten H V (eds) The Cambrian explosion and thefossil record Bulletin of the National Museum of Natural ScienceTaiwan 10 11ndash106

Child C A 1997 Some deep-sea Pycnogonida from Argentine slopeand basin Proceedings of the Biological Society of Washington110 128ndash42

Chlupac I 1988 The enigmatic arthropod Duslia from the Ordovicianof Czechslovakia Palaeontology 31 611ndash20

Chlupac I 1995 Lower Cambrian arthropods from the PasekyShale (Barrandian area Czech Republic) Journal of the CzechGeological Society 40 9ndash36

Chlupac I amp Havlıcek V 1965 Kodymirus n g a new aglaspidmerostome of the Cambrian of Bohemia Sbornık GeologickechVid Paleontologie 6 7ndash20

Cisne J L 1975 Anatomy of Triarthrus and the relationships of theTrilobita Fossils and Strata 4 45ndash63

Cisne J L 1981 Triarthrus eatoni (Trilobita) anatomy of its exo-skeletal skeletomuscular and digestive systems Palaeonto-graphica Americana 9 95ndash142

Conway Morris S 1982 Atlas of the Burgess Shale London ThePalaeontological Association

Conway Morris S 1989 The persistence of Burgess Shale-type faunasimplications for the evolution of deeper-water faunas Trans-actions of the Royal Society of Edinburgh Earth Sciences 80271ndash83

Conway Morris S 1998 The crucible of creation Oxford OxfordUniversity Press

Cotton T J 1999 Review of Arthropod fossils and phylogenyPalaeogeography Palaeoclimatology Palaeoecology 151 341ndash3

Cotton T J 2002 The phylogeny and morphological evolution ofCambrian trilobites and their relatives Unpublished PhD ThesisUniversity of Bristol

Damen W G M Hausdorf M Seyfarth E A amp Tautz D 1998 Aconserved mode of head segmentation in arthropods revealed bythe expression pattern of Hox genes in a spider Proceedings of theNational Academy of Sciences USA 95 10665ndash70

Delle Cave L amp Simonetta A M 1991 Early Palaeozoic arthropodsand problems of arthropod phylogeny with some notes on taxa ofdoubtful affinities In Simonetta A M amp Conway Morris S(eds) The early evolution of Metazoa and the significance ofproblematic taxa 189ndash244 Cambridge Cambridge UniversityPress

Dewel R A amp Dewel W C 1997 The place of tardigrades inarthropod evolution In Fortey R A amp Thomas R H (eds)Arthropod Relationships 109ndash23 London Chapman and HallSystematics Association Special Volume 55

Dunlop J A 1997 Palaeozoic arachnids and their significance forarachnid phylogeny Proceedings of the 16th European Colloquiumof Arachnology 65ndash82 Siedlce Poland

Dunlop J A 1998 The origins of tetrapulmonate booklungs and theirsignificance for chelicerate phylogeny In Selden P A (ed)Proceedings of the 17th European Colloquium of ArachnologyEdinburgh 1997 pp 9ndash16

Dunlop J A 1999 Pasando revista a la evolucion de los quelicerados[A review of chelicerate evolution] In Melic A DeHaro J JMedez M amp Ribera I (eds) Volumen Monografico Evolucion yFilogenia de Arthropoda Boletin de la Sociedad EntolomologicaAragonesa 26 255ndash272 [In Spanish and English]

Dunlop J A amp Selden P A 1997 The early history and phylogenyof the chelicerates In Fortey R A amp Thomas R H (eds)Arthropod relationships Systematics Association Special Volume55 221ndash35 London Chapman and Hall

Dunlop J A amp Webster M 1999 Fossil evidence terrestrializationand arachnid phylogeny Journal of Arachnology 27 86ndash93

Dzik J 1993 Early metazoan evolution and the meaning of its fossilrecord Evolutionary Biology 27 339ndash86

Dzik J amp Lendzion K 1988 The oldest arthropods of the EastEuropean Platform Lethaia 21 29ndash38

Edgecombe G D Wilson G D F Colgan D J Gray M R ampCassis G 2000 Arthropod cladistics combined analysisof Histone H3 and U2 snRNA sequences and morphologyCladistics 16 155ndash203

Edgecombe G D amp Ramskold L 1999 Relationships of CambrianArachnata and the systematic position of Trilobita Journal ofPaleontology 73 263ndash87

Emerson M J amp Schram F R 1997 Theories patterns and realitygame plan for arthropod phylogeny In Fortey R A amp Thomas

R H (eds) Arthropod Relationships Systematics AssociationSpecial Volume 55 67ndash87 London Chapman and Hall

Farris J S 1989 The Retention Index and the Rescaled ConsistencyIndex Cladistics 5 417ndash19

Felsenstein J 1985 Confidence limits on phylogenies an approachusing the bootstrap Evolution 39 783ndash91

Flower R H 1968 Merostomes from a Cotter Horizon of the El PasoGroup New Mexico Bureau of Mines and Mineral ResourcesMemoir 22 35ndash44

Fortey R A 1990a Trilobite evolution and systematics In MikulicD G (ed) Arthropod paleobiology 44ndash65 Short Courses inPaleontology 3 Knoxville Tenessee The Paleontological Society

Fortey R A 1990b Ontogeny hypostome attachment and trilobiteclassification Palaeontology 33 529ndash76

Fortey R A 1997 Classification In Kaesler R L (ed) Treatise oninvertebrate paleontology Part O Arthropoda 1 (revised) 289ndash302Boulder Colorado amp Lawrence Kansas Geological Society ofAmerica and University of Kansas Press

Fortey R A amp Theron J N 1994 A new Ordovician arthropodSoomaspis and the agnostid problem Palaeontology 37 841ndash61

Fortey R A amp Whittington H B 1989 The Trilobita as a naturalgroup Historical Biology 2 125ndash38

Fry W G 1978 A classification within the pycnogonids ZoologicalJournal of the Linnean Society of London 63 35ndash58

Giribet G Edgecombe G D amp Wheeler W C 2001 Arthropodphylogeny based on eight molecular loci and morphology Nature413 157ndash61

Gould S J 1989 Wonderful life The Burgess Shale and the nature ofhistory New York NY Norton

Gould S J 1991 The disparity of the Burgess Shale arthropod faunaand the limits of cladistic analysis why we must strive to quantifymorphospace Paleobiology 17 411ndash23

Gurich G 1931 Mimetaster hexagonalis ein neuer Kruster aus demunterdevonischen Bundenbacher Dachschiefer PalaontologischeZeitschrift 12 204ndash38

Hammann W Laske R amp Pillola G L 1990 Tariccoia arrusensisngnsp an unusual trilobite-like arthropod Rediscovery of thelsquophylocaridrsquo beds of Taricco (1922) in the Ordovician lsquoPuddingarsquosequence of Sardinia Bolletino della Societa PaleontologicaItaliana 29 163ndash78

Haas M S Brown S J amp Beeman R W 2001 Pondering theprocephalon the segmental origin of the labrum DevelopmentGenes and Evolution 211 89ndash95

Heider K 1913 Entwicklungsgeschichte und Morphologie derWirbellosen Die Kultur der Gegenwart Theil 3 Abt 4 2 176ndash332

Henriksen K 1928 Critical notes upon Cambrian arthropodsdescribed by Charles D Walcott Videnskabelige Meddelelser fraDansk Naturhistorisk Forening 86 1ndash20

Hesselbo S P 1988 Trace fossils of Cambrian aglaspidid arthropodsLethaia 21 139ndash46

Hesselbo S P 1989 The aglaspidid arthropod Beckwithia from theCambrian of Utah and Wisconsin Journal of Paleontology 63636ndash42

Hesselbo S P 1992 Aglaspidida (Arthropoda) from the UpperCambrian of Wisconsin Journal of Paleontology 66 885ndash923

Hou X 1987a Three new large arthropods from Lower CambrianChengjiang fauna eastern Yunnan Acta Palaeontologica Sinica26 272ndash85 [In Chinese with English summary]

Hou X 1987b Two new arthropods from Lower CambrianChengjiang eastern Yunnan Acta Palaeontologica Sinica 26236ndash56 [In Chinese with English summary]

Hou X Chen J amp Lu H 1989 Early Cambrian new arthropodsfrom Chengjiang Yunnan Acta Palaeontologica Sinica 28 42ndash57[In Chinese with English summary]

Hou X Ramskold L amp Bergstrom J 1991 Composition andpreservation of the Chengjiang fauna a Lower Cambrian soft-bodied biota Zoologica Scripta 20 395ndash411

Hou X Bergstrom J amp Ahlberg P 1995 Anomalocaris and otherlarge animals in the Lower Cambrian Chengjiang fauna ofsouthwest China GFF 117 163ndash83

Hou X amp Bergstrom J 1997 Arthropods of the Lower CambrianChengjiang fauna southwest China Fossils and Strata 45 1ndash116

Hughes C P 1975 Redescription of Burgessia bella from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 4415ndash36

Ivantsov A Y 1999 Trilobite-like arthropod from the LowerCambrian of the Siberian Platform Acta PalaeontologicaPolonica 44 455ndash66

King P E 1973 Pycnogonids London St Martin Press

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 191

Lauterbach K E 1973 Schlussereignisse in der Evolution derStammgruppe der Euarthropoda Zoologische Beitrage 19259ndash99

Lauterbach K E 1980 Schlussereignisse in der Evolution derGrundplans der Arachnata (Arthropoda) Abhandlungen desNaturwissenschaftlichen Vereins in Hamburg 23 163ndash327

Lauterbach K E 1983 Synapomorphien zwischen Trilobiten- undCheliceraten-zweig der Arachnata Zoologischer Anzeiger 210213ndash38

Lendzion K 1975 Fauna of the Mobergella Zone in the Polish LowerCambrian Kwartalnik Geologiczny 19 237ndash42

Lieberman B S 1998 Cladistic analysis of the Early Cambrianolenelloid trilobites Journal of Paleontology 72 59ndash78

Lu Y 1940 On the ontogeny and phylogeny of Redlichia intermediaLu (sp nov) Bulletin of the Geological Society of China 20333ndash42 [In Chinese]

Maddison W P amp Maddison D R 1997 MacClade analysis ofphylogeny and character evolution Version 307 Sunderland MASinauer Associates

Mittmann B amp Scholtz G 2003 Development of the nervous systemin the lsquoheadrsquo of Limulus polyphemus (Chelicerata Xiphosura)morphological evidence for the correspondence between the seg-ments of the chelicerae and the (first) antennae of MandibulataDevelopment Genes and Evolution 213 9ndash17

Moore R C (ed) 1959 Treatise on invertebrate paleontology Part OArthropoda 1 Boulder Colorado Geological Society of America

Muller K J amp Walossek D 1986 Arthropod larvae from the UpperCambrian of Sweden Transactions of the Royal Society ofEdinburgh Earth Sciences 77 157ndash79

Muller K J amp Walossek D 1987 Morphology ontogeny and lifehabit of Agnostus pisiformis from the Upper Cambrian of SwedenFossils and Strata 19 1ndash124

Munilla T 1999 Evolucion y filogenia de los Picnogonidos InMelicA DeHaro J J Medez M amp Ribera I (eds) VolumenMonografico Evolucion y Filogenia de Arthropoda Boletin de laSociedad Entolomologica Aragonesa 26 273ndash91 [In Spanish andEnglish]

Orr P J Siveter D J Briggs D E G Siveter D J amp Sutton M D2000 A new arthropod from the Silurian Konservat-Lagerstatteof Herefordshire Proceedings of the Royal Society of LondonSeries B 267 1497ndash504

Page R D M 1996 On consensus confidence and lsquototal evidencersquoCladistics 12 83ndash92

Palmer A R amp Repina L N 1993 Through a glass darklytaxonomy phylogeny and biostratigraphy of the OlenellinaUniversity of Kansas Paleontological Contributions New Series 31ndash35

Platnick N I Griswold C E amp Coddington J A 1991 On missingentries in cladistic analysis Cladistics 7 337ndash43

Pleijel F 1995 On character coding for phylogeny reconstructionCladistics 11 309ndash15

Raasch G O 1939 Cambrian Merostomata Special Papers of theGeological Society of America 19 1ndash146

Ramskold L Chen J Edgecombe G D amp Zhou G 1996Preservational folds simulating tergite junctions in tegopeltid andnaraoiid arthropods Lethaia 29 15ndash20

Ramskold L Chen J Edgecombe G D amp Zhou G 1997Cindarella and the arachnate clade Xandarellida (ArthropodaEarly Cambrian) from China Transactions of the Royal Society ofEdinburgh Earth Sciences 88 19ndash38

Ramskold L amp Edgecombe G D 1991 Trilobite monophylyrevisited Historical Biology 4 267ndash83

Ramskold L amp Edgecombe G D 1996 Trilobite appendagestructure ndash Eoredlichia revisited Alcheringa 20 269ndash76

Raw F 1957 Origin of chelicerates Journal of Paleontology 31139ndash193

Repina L N amp Okuneva O G 1969 Cambrian arthropods of themaritime territory Paleontological Journal 1969 95ndash103

Richter R amp Richter E 1929 Weinbergina opitz ng n sp einSchwerttrager (Merost Xiphos) aus dem Devon (Rheinland)Senkenbergiana 11 193ndash209

Robison R A 1984 New occurences of the unusual trilobite Naraoiafrom the Cambrian of Idaho and Utah University of KansasPaleontological Contributions Paper 112 1ndash8

Rominger C 1887 Description of primordial fossils from MtStephens NW Territory of Canada Proceedings of the Academyof Natural Sciences of Philadelphia 1887 12ndash19

Scholtz G 1997 Cleavage germ band formation and head segmen-tation the ground pattern of the Euarthropoda In Fortey R Aamp Thomas R H (eds) Arthropod Relationships 317ndash32 LondonChapman and Hall Systematics Association Special Volume 55

Schram F R 1978 Arthropods a convergent phenomenonFieldiana Geology 39 61ndash108

Schram F R 1993 Review of Boxshall Stromberg and Dahl TheCrustacea Origin and Evolution Journal of Crustacean Biology13 820ndash2

Schram F R Vonk R amp Hof C H J 1997 Mazon CreekCycloidea Journal of Paleontology 71 261ndash84

Schram F R amp Emerson M J 1991 Arthropod Pattern Theorya new approach to arthropod phylogeny Memoirs of theQueensland Museum 31 1ndash18

Schram F R amp Hof C H J 1998 Fossils and the interrelationshipsof major crustacean groups In Edgecombe G D (ed) Arthropodfossils and phylogeny pp 233ndash302 New York NY ColumbiaUniversity Press

Shergold J H 1991 Protaspis and early meraspis growth stages of theeodiscoid trilobite Pagetia ocellata Jell and their implications forclassification Alcheringa 15 65ndash86

Shu D Geyer G Ling C amp Zhang Z 1995 Redlichiacean trilobiteswith preserved soft parts from the Lower Cambrian Chengjiangfauna (South China) Beringeria (Special Issue) 2 203ndash41

Shultz J W 1990 Evolutionary morphology and phylogeny ofArachnida Cladistics 6 1ndash38

Shultz J W 2000 Skeletomuscular anatomy of the harvestmanLeiobunum aldrichi (Weed 1893) (Arachnida OpilionesPalpatores) and its evolutionary significance Zoological Journalof the Linnean Society 128 401ndash38

Shultz J W 2001 Gross muscular anatomy of Limulus polyphemus(Xiphosura Chelicerata) and its bearing on evolution in the theArachnida Journal of Arachnology 29 283ndash303

Siewing R (ed) 1985 Lehrbuch der Zoologie II Systematik 3rd ednStuttgart Fischer

Simonetta A M 1970 Studies on the non trilobite arthropods ofthe Burgess Shale (Middle Cambrian) the genera LeanchoiliaAlalcomenaeus Opabinia Burgessia Yohoia and ActaeusPalaeontographica Italica 66 35ndash45

Simonetta A M amp Delle Cave L 1975 The Cambrian non trilobitearthropods from the Burgess Shale of British Columbia A studyof their comparative morphology taxonomy and evolutionarysignificance Palaeontographica Italica 69 1ndash37

Simonetta A M amp Delle Cave L 1981 An essay in the compara-tive and evolutionary morphology of Palaeozoic ArthropodsAccademia Nazionale dei Lincei Atti dei Convegni Lincei 49389ndash439

Starobogatov Y I 1990 Sistema i filogeniya nizshikh khelitserovykh(analiz morfologii paleozoysikh grupp) [The systematics andphylogeny of the lower chelicerates (a morphological analysis ofthe Palaeozoic groups)] Paleontologicheskii Zhurnal 1990 4ndash17

Stoslashrmer L 1944 On the relationships and phylogeny of fossil andrecent Arachnomorpha Skrifter utgitt ac Det Norske Videnskaps-Akademi i Olso I Matematisk-naturvitenskaplig Klasse 5 1ndash158

Stoslashrmer L 1951 Studies on trilobite morphology Part III Theventral cephalic structures with remarks on the zoologicalposition of the trilobites Norsk geologisk tidsskrift 29 108ndash58

Stoslashrmer L 1956 A Lower Cambrian merostome from Sweden Arkivfor Zoologi Ser 2 9 507ndash14

Stoslashrmer L 1959 Trilobitoidea In Moore R C (ed) Treatise onInvertebrate Paleontology part O Arthropoda 1 23ndash37 BoulderColorado Geological Society of America

Strong E E amp Lipscomb D 1999 Character coding and inapplicabledata Cladistics 15 363ndash71

Sturmer W amp Bergstrom J 1976 The arthropods Mimetaster andVachonisia from the Devonian Hunsruck Shale PalaontologischeZeitschrift 50 78ndash111

Sturmer W amp Bergstrom J 1978 The arthropod Cheloniellon fromthe Devonian Hunsruck Shale Palaontologische Zeitschrift 5257ndash81

Sturmer W amp Bergstrom J 1981 Weinbergina a xiphosuran fromthe Devonian Hunsruck Slate Palaontologische Zeitschrift 55237ndash55

Sutton M D Briggs D E G Siveter D J Siveter D J amp Orr PJ 2002 The arthropod Offacolus kingi (Chelicerata) from theSilurian of Herefordshire England computer based morphologi-cal reconstructions and phylogenetic affinity Proceedings of theRoyal Society of London Series B 269 1195ndash1203

Swofford D L 1999 PAUP Phylogenetic Analysis Using Parsimony(and other methods) Version 40b2a Sunderland MA SinauerAssociates

Telford M J amp Thomas R H 1998 Expression of homeobox genesshows chelicerate arthropods retain their deutocerebral segmentProceedings of the National Academy of Sciences USA 9510671ndash5

192 TREVOR J COTTON AND SIMON J BRADDY

Thorley J L amp Page R D M 2000 RadCon phylogenetic treecomparison and consensus Bioinformatics 16 486ndash7

Valentine J W 1995 Why no new phyla after the Cambrian ndashgenome and ecospace hypotheses revisited Palaios 10 190ndash194

Wagele J W 1993 Rejection of the lsquoUniramiarsquo hypothesis andimplications of the Mandibulata concept Zoologische JahrbucherAbteilung fur Systematik Geographie und Biologie der Tiere 120253ndash88

Walcott C D 1911 Middle Cambrian Merostomata Cambriangeology and palaeontology II Smithsonian MiscellaneousCollections 57 17ndash40

Walcott C D 1912 Middle Cambrian Branchiopoda MalacostracaTrilobita and Merostomata Cambrian geology and palaeontol-ogy II Smithsonian Miscellaneous Collections 57 145ndash228

Walcott C D 1918 Geological explorations in the CanadianRockies Explorations and fieldwork of the SmithsonianInstitution in 1917 Smithsonian Miscellaneous Collections 684ndash20

Walossek D 1993 The Upper Cambrian Rehbachiella kinnekullensisand the phylogeny of Branchiopoda and Crustacea Fossils andStrata 32 1ndash202

Walossek D amp Dunlop J A 2002 A larval sea spider (ArthropodaPycnogonida) from the Upper Cambrian lsquoOrstenrsquo of Swedenand the phylogenetic position of pycnogonids Palaeontology 45421ndash46

Walossek D amp Muller K J 1990 Upper Cambrian stem-lineagecrustaceans and their bearing upon the monophyletic origin ofCrustacea and the position of Agnostus Lethaia 23 409ndash27

Walossek D amp Muller K J 1997 Cambrian lsquoOrstenrsquo-type arthro-pods and the phylogeny of Crustacea In Fortey R A amp ThomasR H (eds) Arthropod Relationships 139ndash53 SystematicsAssociation Special Volume 55 London Chapman and Hall

Walossek D amp Muller K J 1998 Early arthropod phylogeny in lightof the Cambrian lsquoOrstenrsquo fossils In Edgecombe G D (ed)Arthropod fossils and phylogeny 185ndash231 New York NYColumbia University Press

Weygoldt P 1979 Significance of later embryonic stages and headdevelopment in arthropod phylogeny In Gupta A P (ed)Arthropod Phylogeny 107ndash35 New York NY Van NostrandReinhold Co

Weygoldt P 1998 Evolution and systematics of the ChelicerataExperimental and Applied Acarology 22 63ndash79

Weygoldt P amp Paulus H F 1979 Unterzuchungen aur Morpholo-gie Taxonomie und Phylogenie der Chelicerata Zeitschriftfur Zoologische Systematik und Evolutionsforschung 17 85ndash116177ndash200

Wheeler W C Cartwright P amp Hayashi C Y 1993 Arthropodphylogeny a combined approach Cladistics 9 1ndash39

Wheeler W C amp Hayashi C Y 1998 The phylogeny of extantchelicerate orders Cladistics 14 173ndash92

Whittington H B 1971 Redescription of Marrella splendens(Trilobitoidea) from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 209 1ndash24

Whittington H B 1974 Yohoia Walcott and Plenocaris n genarthropods from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 231 1ndash21

Whittington H B 1975a Trilobites with appendages from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 497ndash136

Whittington H B 1975b The enigmatic animal Opabinia regalisMiddle Cambrian Burgess Shale British Columbia PhilosophicalTransactions of the Royal Society of London Series B 271 1ndash43

Whittington H B 1977 The Middle Cambrian trilobite NaraoiaBurgess Shale British Columbia Philosophical Transactions of theRoyal Society Series B 280 409ndash43

Whittington H B 1979 Early arthropods their appendages andrelationships In House M R (ed) The origin of majorinvertebrate groups 253ndash68 Systematics Association SpecialVolume 12 New York NY Academic Press

Whittington H B 1980 Exoskeleton moult stage appendage mor-phology and habits of the Middle Cambrian trilobite Olenoidesserratus Palaeontology 23 171ndash204

Whittington H B 1981 Rare arthropods from the Burgess ShaleMiddle Cambrian British Columbia Philosophical Transactionsof the Royal Society of London Series B 292 329ndash57

Whittington H B 1985 Tegopelte gigas a second soft-bodied trilo-bite from the Burgess Shale Middle Cambrian British ColumbiaJournal of Palaeontology 23 171ndash204

Whittington H B 1989 Olenelloid trilobites type species functionalmorphology and higher classification Philosophical Transactionsof the Royal Society of London Series B 324 111ndash47

Whittington H B amp Almond J E 1987 Appendages and habits ofthe Upper Ordovician trilobite Triarthus eatoni PhilosophicalTransactions of the Royal Society of London Series B 317 1ndash46

Wilkinson M 1992 Ordered versus unordered characters Cladistics8 375ndash85

Wilkinson M 1994 Common cladistic information and its consensusrepresentation reduced Adams and reduced cladistic consensustrees and profiles Systematic Biology 43 343ndash68

Wilkinson M 1995a Coping with abundant missing entries in phylo-genetic inference using parsimony Systematic Biology 44 501ndash14

Wilkinson M 1995b TAXEQ2 software and documentation BristolSchool of Biological Sciences University of Bristol

Wilkinson M 1996 On the distribution of homoplasy and choosingamong trees Taxon 45 263ndash66

Wills M A 1997 A phylogeny of Recent and fossil Crustacea derivedfrom morphological characters In Fortey R A amp ThomasR H (eds) Arthropod Relationships 189ndash209 SystematicsAssociation Special Volume 55 London Chapman and Hall

Wills M A Briggs D E G amp Fortey R A 1994 Disparity asan evolutionary index a comparison of Cambrian and Recentarthropods Paleobiology 20 93ndash130

Wills M A Briggs D E G Fortey R A amp Wilkinson M 1995The significance of fossils in understanding arthropod evolutionVerhandlungen der deutschen zoologischen Gesselschaft 88 203ndash15

Wills M A Briggs D E G Fortey R A Wilkinson M amp SneathP H A 1998a An arthropod phylogeny based on fossil andrecent taxa In Edgecombe G D (ed) Arthropod fossils andphylogeny 33ndash105 New York NY Columbia University Press

Wills M A Briggs D E G amp Fortey R A 1998b Evolutionarycorrelates of arthropod tagmosis scrambled legs In ForteyR A amp Thomas R H (eds) Arthropod Relationships 57ndash65Systematics Association Special Volume 55 London Chapmanand Hall

Winter G 1980 Beitrage zur Morphologie und Embryologie desvorderen Korperabschnitts (Cephalosoma) der PantopodaGerstaecker 1863 I Entstehung und Struktur des Zentralnerven-systems Zeitschrift fur Zoologische Systematik und Evolutionsfor-schung 18 27ndash61

Zhang W amp Hou X 1985 Preliminary notes on the occurrence of theunusual trilobite Naraoia in Asia Acta Palaeontologica Sinica 24591ndash5 [In Chinese with English summary]

TREVOR J COTTON Department of Palaeontology The Natural History Museum CromwellRoad London SW7 5BD UKe-mail trevor_cottonhotmailcom

SIMON J BRADDY Department of Earth Sciences University of Bristol Wills MemorialBuilding Queenrsquos Road Bristol BS8 1RJ UKe-mail S J Braddybrisacuk

MS received 28 July 2003 Accepted for publication 15 March 2004

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 193

Page 9: The phylogeny of arachnomorph arthropods and the origin of the

Thomas 1998) and developmental data (Mittmann amp Scholtz2003) morphological evidence from fossils supports the viewthat chelicerates have lost the antennae The Hox gene evi-dence suffers from a lack of comparative data from diversemandibulates especially primitive crustaceans (see Akam2000) and the use of Hox gene expression boundaries asmarkers for segmental homology has been criticised (egAbzhanov et al 1999 although relating to posterior Hoxpatterns) Wills et al (1998a pp 43 amp 48) considered thechelicerae to be appendages of the second (deutocerebral)segment but curiously in support of this cited Schram (1978)who unambiguously followed the homology scheme used inthe present study

The anterior-most appendages of Aglaspis were originallydescribed as chelicerae (Raasch 1939) which lead to theirclassification in the Chelicerata (eg Stoslashrmer 1944) Howeverthe morphology of these appendages (see Briggs et al 1979) ismore similar to that of antennae They are narrow relative tothe thoracic endopods of relatively even diameter proximallyand the podomeres are apparently weakly defined Hesselbo(1988 1992) also argued that the anterior appendages ofAglaspis are likely to be antennae The distal parts are un-known (and hence so is their length cf Wills et al 1998ap 58) and there is no evidence that they were chelate contraryto the coding of Wills et al (1998a character 31) Nothingis known about the anterior appendages of BuenaspisLemoneites or Paleomerus and this character is accordinglycoded as missing data in these taxa

2 Form of endopod of appendages of the second segment(0) pediform and (1) anteriorly directed raptorial appendagewith reduced number of podomeres and terminal podomeresbearing spines on distal margins As discussed above cheli-cerae and lsquogreat appendagesrsquo are both likely to represent theappendages of the second segment They also differ from theassumed primitive biramous euarthropod limb in similar waysand therefore are likely to be homologous modifications ofthese appendages The homology of lsquogreat appendagesrsquo andchelicerae was originally proposed by Henriksen (1928) andsupported by Stoslashrmer (1944) but to the present authorsrsquoknowledge no modern author has discussed this possibility

Both lsquogreat appendagesrsquo (Figs 5A D E) and chelicerae(Figs 5B C) are equipped with strong spinose projections onthe outer (dorsal) side of the distal margins of the terminalpodomeres that are lacking on the proximal podomeresSecondly the number of podomeres in both chelicerae andlsquogreat appendagesrsquo is more or less reduced compared to thenumber in the endopods of biramous limbs In the case oflsquogreat appendagesrsquo they are significantly more robust thanthose of posterior endopods a situation that is matched insome chelicerates The homology of the lsquogreat appendagesrsquoof Alalcomenaeus Leanchoilia Yohoia Jiangfengia andFortiforceps is supported by all of these features and is widelyaccepted (eg Wills et al 1998a character 31 Hou ampBergstrom 1997 Bergstrom amp Hou 1998 Hou 1987a) OnlyHou amp Bergstrom (1991 p 183) have suggested albeit inpassing that the lsquogreat appendagesrsquo of all these taxa may beconvergent a view they appear to have changed The endo-pods of the appendages of other taxa are locomotory legswhich either lack strong spines or have spinose extensionsthat are invariably on the medial (ventral) surface and arestronger on the proximal podomeres than the distal ones (egMisszhouia Fig 6A) In the latter case these spines form partof the feeding system along with the gnathobases and are oftenlocated around the middle of the podomere rather than limitedto the distal margin These endopods are directed ventro-laterally as opposed to anteriorly in the case of both cheliceraeand lsquogreat appendagesrsquo The modified second appendages of

Marrella resemble this plesiomorphic condition here referredto as lsquopediformrsquo except in their anterolateral orientation Insome Recent crustaceans this appendage is modified into asecond antenna but in stem-group crustaceans it is pediform(eg Walossek amp Muller 1997 1998) This character is codedas missing data in a number of taxa for which the morphologyof the second segment appendage is unknown

Some authors (Dzik 1993 Chen amp Zhou 1997 Dewel ampDewel 1997 Budd 2002) have suggested that the lsquogreat ap-pendagesrsquo are homologous with the anterior appendages ofanomalocaridids and the anomalocaridid-like Opabinia andKerygmachela (see Budd 1996b 1999b respectively) Theanterior appendages of most anomalocaridids and otherCambrian lobopodians differ from megacheiran lsquogreat append-agesrsquo in that they consist of a large number of segmentsMoreover there is considerable morphological evidence thatanomalocaridids are part of the euarthropod stem group (Willset al 1995 1998a) and not closely related to the lsquogreatappendagersquo taxa (cf Budd 2002) with the possible exception ofParapeytoia lsquoGreat appendagersquo taxa share a suite of derivedcharacters with other crown group euarthropods includingsclerotised dorsal tergites the incorporation of more thanone pair of appendages into the head sclerotisation of theendopods and strong differentiation of the head that wereexcluded from Buddrsquos (2002) analysis The relationships ofanomalocaridids will be considered in detail later by Braddyand co-workers

3 Exopod of appendages of the second segment (0)present and (1) absent or much reduced The raptorial ap-pendages of the second segment (chelicerae and lsquogreat append-agesrsquo) are uniramous as are the corresponding pediformappendages of Marrella Mimetaster Emeraldella Cheloniellonand Sidneyia The plesiomorphic euarthropod state is found inmost other arachnomorphs including trilobites where thebiramous second segment appendages are undifferentiatedfrom those of posterior segments (Edgecombe et al 2000character 78) The exopods of these appendages are alsopresent in basal members of the crustacean crown group(Edgecombe et al 2000 character 79) and in stem groupcrustaceans (Walossek 1993 Walossek amp Muller 1997 1998)

4 Number of head segments (0) 1 (1) 2 (2) 3 (3) 4 (4) 5(5) 6 and (6) 7 The coding of this character by Edgecombe ampRamskold (1999 character 2) explicitly referred to the numberof limb pairs present in addition to the antenna Here thenumber of post-acronal segments present in the head is codedfollowing Wills et al (1998a character 29) In taxa that lack anantenna the number of somites is inferred to be one greaterthan the number of cephalic appendage pairs (cf Wills et al1998a) as described above

Sturmer amp Bergstrom (1978) suggested that the head ofCheloniellon is defined primarily on the basis of appendagetagmosis rather than the fusion of segments under a singlecephalic-shield (cf Hou amp Bergstrom 1997 pp 98ndash9) Accord-ing to this view the head consists of both the cephalic tergiteand the anteriormost free tergite that share uniramousgnathobasic appendages Wills et al (1998a) coded Cheloniel-lon as having six somites incorporated into the head andtherefore presumably accepted this suggestion There is noevidence that the first free tergite of Cheloniellon was incorpo-rated into the head (except perhaps functionally) and thepresent authors prefer to code the number of somites under thecephalic shield In Sidneyia there is a series of uniramousstrongly gnathobasic appendages similar to those of Cheloniel-lon posterior to the head shield All of these appendages wouldpresumably have to be considered part of the head based onappendage differentiation according to the view of Sturmer ampBergstrom (1978) Some autapomorphic states are included

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 177

(none for Sidneyia one for Marrella and six for Emeraldella)since these will become informative if the character is treatedas ordered

Edgecombe amp Ramskold (1999 p 265) described a novelform of cephalic tagmosis The present authors tentativelyaccept their suggestion that in trilobite-like taxa the fourthpair of biramous cephalic appendages was directly under thecephalo-thoracic junction This may explain previous confu-sion about the number of such appendages incorporated intothe trilobite cephalon (eg Cisne 1975 Whittington 1975aBergstrom amp Brassel 1984) However Edgecome amp Ramskoldaccept the view of Chen et al (1977 p 7) that only the firstthree biramous limbs of Misszhouia were structurally and

functionally part of the head Therefore it seems more accept-able to code these taxa as having only four post-acronalsomites than to use the coding scheme of Edgecombe ampRamskold (1999) The partial integration of the first thoracicsomite under the head shield could be considered to be a formof overlap of the trunk by the head shield (ie a distinct stateof character 9 of Edgecombe amp Ramskold 1999 or Character37 herein) Derived states of this character would then poten-tially be synapomorphic for a clade including xandarellidsnaraoiids helmetiids tegopeltids and trilobites Howeversince the degree of overlap in taxa with the fourth biramousappendages under the cephalo-thoracic articulation is identicalto that found between thoracic tergites (State 0 of Character

Figure 5 Diagrammatic reconstructions of raptorial second-segment appendages in lateral view (except whereotherwise stated) and approximate life orientation showing suggested homology of appendage elements (inbrackets) Roman numerals indicate position in the podomere series and do not necessarily imply homology (A)lsquoGreat appendagersquo of Fortiforceps (after Hou amp Bergstrom 1997) (B) Left chelicera of Leiobunum aldrichi(Arachnida Opiliones) (1) in medial perspective and (2) distal parts in anterior perspective (after Shultz 2000)(C) Chelifore of Nymphon (Pycnogonida) (after Child 1997) (D) lsquoGreat appendagersquo of Yohoia (after Whittington1974 fig 2) (E) lsquoGreat appendagersquo of Leanchoilia (modified after Bruton amp Whittington 1983) Not to scaleAbbreviations (apt) apotele (probably homologous with the mandibulate pretarsus) (cx) coxa (dtmrt)deutomerite and (pt) pretarsus

178 TREVOR J COTTON AND SIMON J BRADDY

37) it is here preferred to regard this as a distinct character(Character 9 herein)

The coding of Alalcomenaeus by Wills et al (1998a) ashaving four post-acronal somites is accepted here but fordifferent reasons Briggs amp Collins (1999) have recenty demon-strated that Alalcomenaeus possessed two pairs of biramousappendages on the head in addition to the great appendagesThis would equate to three head somites according to thehomology scheme of Wills et al (1998a) Here the antennaeare inferred to be lost and therefore four segments incorpo-rated into the head

Recently a number of authors have agreed that the plesio-morphic condition for euarthropods is a four-segmented headas found in stem group crustaceans (Scholtz 1997 Walossek1993 Walossek amp Muller 1997 1998 Edgecombe et al 2000)However reconstruction of the euarthropod stem groupclearly indicates that even crownward taxa had a head consist-ing of only the antennal segment and acron as found intardigrades and onychophorans Therefore it seems that thefour-segmented head is pleisomorphic only for crown groupeuarthropods Some fully arthropodised fossil taxa also have ashorter head that may be pleisomorphic such as the two-segmented head of Marella Retifacies is coded following therecent redescription of Hou amp Bergstrom (1997) rather thanon the basis of the coding by Edgecombe amp Ramskold (1999)for which they provide no explanation A partial uncertaintycoding is used for Aglaspis in which the number of post-antennal cephalic appendages is either three or four (Briggset al 1979)

5 Orientation of the antennae (0) directed anterolaterally(1) strongly deflected laterally and (2) placed well inside shieldmargin curing posteriorly from a transverse proximal element(cf Edgecombe amp Ramskold 1999 character 3) The anteriorappendages of Aglaspis (see character 1) are clearly directedanterolaterally and it is presumed that they continued past thehead shield margin The anterior appendages of arthropodstem group taxa such as Aysheaia Kerygmachela and Anoma-locaris are either directed anterolaterally or ventrally but arenot strongly-deflected laterally Therefore the outgroup iscoded as State 0 This character is coded as missing data fortaxa where the presence of antennae is equivocal (those codedas missing data for Character 1) and as inapplicable in taxawhere the antennae are considered absent (coded as State 1 forCharacter 1)

6 Length of distal spines on terminal podomeres of endo-pods of second segment appendages (0) absent or shorer thanpodomeres (1) subsequal to length of podomeres and (2)longer than the entire podomere series The spinose projectionsof the raptorial appendages described above vary in length Inchelicerae (Figs 5BC) and some lsquogreat appendagesrsquo (eg thoseof Yohoia Fig 5D) the most distal spine is equal in length tothe spinose terminal podomere forming a chelate structureIn Fortiforceps (Fig 5A) the spines are short relative tothe lengths of the podomeres but in Alalcomenaeus andLeanchoilia (Fig 5E) they are extremely long so that thespines are much longer than the entire podomere series Asdescribed above in taxa with pediform endopods of the secondsegment appendages spines on the distal podomeres are shortor entirely absent

7 Chelicerae (0) absent and (1) present Despite theirsimilarities to lsquogreat appendagesrsquo the chelicerae of the eucheli-cerates and chelifores of the pycnogonids are clearly distinctand have been widely recognised as a chelicerate synapomor-phy Chelicerae differ from any lsquogreat appendagesrsquo by thecombination of a smaller number of podomeres the presenceof only a single terminal element and only a single spinoseprojection on the dorsal side of the podomere series (Fig 5)

Amongst extant chelicerates only the pycnogonid Pallenopsishas chelicerae of four podomeres comparable to the numberin lsquogreat appendagesrsquo Bergstrom et al (1980) considered thatthe chelifores of the Devonian pycnogonid Palaeoisopus alsoconsist of four segments but this is not well supported by theirfigures

8 Distal spines of second segment endopods terminating inannulated flagellae (0) absent and (1) present It is widelyrecognised (eg Stoslashrmer 1944 Simonetta 1970 Bruton ampWhittington 1993 Briggs amp Collins 1999) that the terminalparts of the extended spines of Alalcomenaeus and Leanchoiliaformed annulated flagellae Nothing similar is known fromhomologous appendages of any other arthropod althoughsimilar processes may have operated in the transformation ofthe endopod as a whole into the second antennae of derivedcrustaceans and in the origin of multiramous antennulae (firstantennae) in malacostracans

242 Posterior appendages 9 Appendages of first thoracicsomite underneath the cephalo-thoracic articulation (0) ab-sent and (1) present (cf Edgecombe amp Ramskold 1999character 2) For a discussion of this character see Character4 herein

10 Exopods of appendages of third to fifth segments (0)present and (1) reduced or absent A number of taxa haveuniramous appendages on the third to fifth segments In mostthese segments are incorporated into the head (Yohoia cheli-cerates) but in others all (eg in Sidneyia) or some (eg inCheloniellon) of these segments are post-cephalic In all casesthe appendage is morphologically similar to the endopods ofbiramous limbs of other taxa andor other segments and it isinterpreted that the exopod is lost

11 Endopods of thoracic appendages (0) present and (1)reduced or absent The opisthosomal appendages of chelicer-ates lack endopods or have the endopods much reduced (egLimulus Siewing 1985 fig 838 Shultz 2001) a situation that isalso found in the uniramous thoracic appendages of Yohoia(Whittington 1974) It has been suggested that Helmetia(Briggs et al 1994) lacks endopods but pending a redescrip-tion of this genus and considering that they have been docu-mented in the otherwise very similar Kuamaia this is coded asuncertain

12 Exopod shaft of numerous podomeres each bearing asingle seta (0) present and (1) absent The exopods ofcrustaceans (at least plesiomorphically see eg Walossek ampMuller 1998 figs 5middot5 amp 5middot6) and of Marrella and Mimetasterhave multi-annulated shafts with each podomere bearing asingle seta In other taxa the exopod shafts consist of one oftwo lobes bearing numerous setae

The polarity of exopod segmentation is uncertain Thelateral flaps of stem group arthropods such as Opabinia(Whittington 1975b Budd 1996b) and anomalocaridids (egHou et al 1995) are certainly unsegmented but as suggestedby Buddrsquos (1996b fig 8) reconstruction this does notnecessarily suggest the form of the primitive euarthropodexopod Rather segmentation may have been a result of thesclerotization of the cuticle of the exopod shaft The outgroupwas coded as uncertain for this character According to thefirst interpretation of aglaspidid appendage morphology(coded as Aglaspidida 1) the exopods are lacking on allappendages and consequently this and all other charactersdescribing exopod morphology are coded as inapplicable

13 Exopod shaft differentiated into proximal and distallobes (0) absent and (1) present Ramskold amp Edgecombe(1996 Edgecombe amp Ramskold 1999 character 26) havediscussed the distribution of the trilobite-type bilobate exo-pods recognised by this character (Fig 6A) Among taxaincluded here that were not considered by Edgecombe amp

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 179

Ramskold (1999) exopods consist either of a single flattenedlobe or a homonomous series of podomeres (Fig 6B C) Nostem group euarthropod has a bilobate exopod and theoutgroup is consequently also coded as State 0

14 Proximal lobe of exopod (0) flattened lobe and (1)slender shaft

15 Distal lobe of exopod (0) small to moderate sized flapwith short to moderately long attachment to proximal lobeand (1) large teardrop-shaped with long attachment to proxi-mal lobe Neither of these characters can be coded for taxathat do not have an exopod differentiated into proximal anddistal lobes (Character 13 State 1) without asserting thehomology of the single-lobed exopod with one of these partsThe coding of Edgecombe amp Ramskold (1999 p 280) implic-itly homologizes the exopods of Sidneyia and Retifacies withthe proximal lobe In these taxa this view is perhaps supportedby the presence of lamellate exopod setae which match thoseof the trilobite-type proximal lobe However in other taxawith a single-lobed exopod such as Alalcomenaeus (see Briggsamp Collins 1999) the exopod is fringed with sharp spines likethe distal lobe of differentiated exopods Alternatively thebilobate exopod may have originated from the division of aprimitively single-lobed structure The single-lobed exopod isnot clearly homologous with either lobe of divided exopodsand both these characters are consequently coded as inappli-cable to taxa without differentiated proximal and distal exopodlobes Consequently (see Character 13 herein) these characterscan only be coded for taxa that were previously considered byEdgecombe amp Ramskold (1999) and their coding is followedhere except in the cases of Sidneyia and Retifacies

16 Exopod shaft is a deep rounded flap (0) absent and (1)present All bilobate (Character 13) and segmented (Character12) exopod shafts are long and relatively narrow structures (seeFig 6) whereas some single-lobed exopod shafts have theform of rounded flaps These flap-like exopod shafts are largecompared to the length of the setae and at least half as deep asthey are long This appendage structure is found in cheliceratesand lsquogreat appendagersquo arthropods

17 Medially-directed exopod setae (0) absent and (1)present Walossek amp Muller (1998 p 194) suggested that thetilting of exopod setae towards the endopod in post-antennularlimbs with a multi-annulated exopod was an autapomorphyuniting crustaceans and all members of the crustacean stemgroup (the crustacean total group sensu Ax 1986 see Fig 6C)This character is shared with the marellomorphs Marrellaand Mimetaster (see Fig 6B Sturmer amp Bergstrom 1976Bergstrom 1979 fig 1middot3A B) In other taxa the setae eithersurround the entire margin of the exopod shaft or are directeddorsally (Fig 6A) The identification of setae on the dorsalsurface of the lateral flaps in Opabinia (Budd 1996b) suggeststhat the condition in marellomorphs and crustaceans isderived

18 Lamellate exopod setae (0) absent and (1) presentLamellate exopod setae originally described from trilobitesare a classic synapomorphy of the Arachnomorpha (see egBergstrom 1992 Hou amp Bergstrom 1997 pp 42ndash3) They areperhaps best known from Misszhouia (see Chen et al 1996Hou amp Bergstrom 1997 Fig 6A) These setae differ from themore spinose setae of other taxa (Fig 6C) in that they are wideand flat and imbricate over the length of the exopod shaft Thesetae of Marrella (Fig 6B) and similar taxa have variouslybeen considered lamellate (Bergstrom 1979) or non-lamellate(Bergstrom 1992) Since they do not imbricate and do notseem to be strongly flattened (Whittington 1971 Sturmer ampBergstrom 1976 fig 9a) Marrella and Mimetaster are codedas State 0 The setae of Helmetia as shown in Briggs et al(1994 fig 141) seem to be of the lamellate type The setae ofSinoburius have been described by Hou amp Bergstrom (1997p 85) as similar to those of Misszhouia Only the setae areknown of the appendages of Buenaspis (Budd 1999a) andthese also appear to be of the trilobite-type

Figure 6 Diagrammatic reconstructions of (A) arachnomorph and(B C) non-arachnomorph biramous appendages (A) The lsquotrilobite-typersquo biramous appendage of Misszhouia longicauda (after Chen et al1997) (B) Appendage of Marrella splendens (after Whittington 1971)(C) Appendage of the stem-lineage crustacean Martinssonia elongata(after Muller amp Walossek 1997 1998) Not to scale Abbreviations(bas) basis (dle) distal lobe of exopod shaft (ls) lamellar exopod setae(pe) proximal endite or pre-coxa (ple) proximal lobe of exopod shaft(ses) segmented exopod shaft and (ss) spinose exopod setae

180 TREVOR J COTTON AND SIMON J BRADDY

The homology of the book-gill lamellae of xiphosurans withlamellar setae was supported by Walossek amp Muller (1997p 149) and Edgecombe et al (2000 p 174) but rejected bySturmer amp Berstrom (1981) on the grounds that Weinberginapossesses Limulus-like gill lamellae and fringing setaeBook-gills have also been described from a eurypterid (Braddyet al 1999) There are certainly major morphologicaldifferences between book-gills and trilobite-type exopodsFollowing most recent opinion and pending further studyWeinbergina and Eurypterida are coded as possessing lamellatesetae

The form of the setae of Yohoia is uncertain (Whittington1974) the spinose setae seen in the most common reconstruc-tion (Gould 1989 Briggs et al 1994) are not justified by thespecimens Amongst other great appendage arthropods theform of the setae appears to vary those of Jianfengia (seeChen amp Zhou 1997 p 74) and Leanchoilia (see Bruton ampWhittington 1983) are lamellate while those of Fortiforceps(Hou amp Bergstrom 1997) and Alalcomenaeus (Briggs amp Collins1999) are spinose and less densely packed

19 Gnathobase on basis andor prominent endites onendopod (0) present and (1) absent (cf Edgecombe ampRamskold 1999 character 29 Wills et al 1998a characters 36amp 59) The presence of gnathobases on the appendages of someanomalocaridids (Hou et al 1995) and their general distribu-tion amongst non-arachnomorph euarthropods suggests thatState 0 is plesiomorphic The homology of the various enditesand gnathobasic structures found in euarthropods is in need ofcareful assessment and therefore a very general coding (fol-lowing Edgecombe amp Ramskold) is used for this character

243 Eyes 20 Position of lateral facetted eyes (0)ventral and stalked (1) dorsal and sessile and (2) absent (cfEdgecombe amp Ramskold 1999 character 4) Partial uncer-tainty coding is used for a number of taxa where the dorsalsurface is known but the ventral morphology is not andtherefore the presence of ventral eyes cannot be discountedFor example there is good evidence that Leanchoilia lackeddorsal sessile eyes but ventral stalked eyes may have beenpresent (as in L hanceyi see Briggs amp Robison 1984) and a(02) partial uncertainty coding is used This is also the case inmany naraoiids such as Buenaspis However only the outlineof the head shield of Liwia is known and the absence of dorsaleyes in the reconstruction of Dzik amp Lendzion (1988) isconjectural It is unclear whether the autapomorphic conditionof stalked dorsal eyes in Mimetaster (see Sturmer amp Bergstrom1976) is homologous with State 0 or State 1 and a partialuncertainty coding is also used

Whittington (1974) was somewhat equivocal about thenature of the lateral lobes anterior to the head shield ofYohoia These more closely resemble the ventral stalked eyes ofAlacomenaeus as recently described by Briggs amp Collins(1999) than either Whittingtonrsquos (1974) or subsequent (egGould 1989 fig 3middot18 Briggs et al 1994 fig 153) reconstruc-tions suggest In a well preserved specimen showing the dorsalaspect (USNM 57696 Whittington 1974 pl 2 figs 1ndash3) and ina laterally compressed specimen (USNM 57694 Whittington1974 pl 1 fig 1) they are clearly seen to be stalked andrelatively small lobate structures That these structures arelikely to represent stalked ventral eyes was reflected in thecoding of Wills et al (1998a characters 26 amp 27)

21 Visual surface with calcified lenses bounded withcircumocular suture (0) absent and (1) present

22 Dorsal bulge in exoskeleton accommodating drop-shaped ventral eyes (0) absent and (1) present

23 Eye slits (0) absent and (1) presentCharacters 21 to 23 are used exactly as described by

Edgecombe amp Ramskold (1999 character 5) Character 21 is

inapplicable to taxa that do not possess eyes (Character 20State 2)

24 Dorsal median eyes (0) absent and (1) present Dorsalmedian eyes on a tubercle are considered a synapomorphy ofthe Chelicerata (Dunlop amp Selden 1997 Dunlop 1999) Thenature and position of the structures interpreted as dorsalmedian eyes in Mimetaster (Sturmer amp Bergstrom 1976p 83ndash4) are regarded as equivocal

244 Ventral cephalic structures Many characters of theventral surface of the euarthropod head (see Fig 7) of poten-tial phylogenetic utility are poorly known in many importanttaxa In particular the presence of pre-hypostomal frontalorgans (Fig 7A CndashE) is not coded herein (see Edgecombe ampRamskold 1999 p 272) Definitive evidence of the absence ofthese structures is available for very few taxa and the homol-ogy of these structures with the maculae of the trilobitehypostome (see Fig 7B) and the frontal organs of the crusta-cean labrum (eg see Muller amp Walossek 1987 p 39) isuncertain Secondly the detailed homology of the trilobitehypostome with that of other putative arachnomorphs isunclear and consequently a very broad definition is usedherein For example various pre-hypostomal sclerites (Fig7A D) in other arachnomorphs may have been incorporatedalong with a primitive hypostome into a single sclerite that isrecognised as the hypostome in trilobites The structure of thehypostome (eg the presence of paired anterior wings dorsal tothe antennae Fig 7B C) may also be a source of additionalcharacters

25 Expanded cephalic doublure (0) absent and (1)present maximum width more than 30 length of head shieldor more than 25 width of pygidium Chen et al (1996)suggested that the wide doublure of Soomaspis and Tariccoia isa synapomorphy uniting these taxa The doublure of Buenaspis(see Budd 1999a) is poorly known but based on the width ofthe heavily crushed region of the cephalic margin and theposition of a possible impression of the edge of the doublure inone specimen (Budd 1999a pl 1 fig 1) it also appears to berather wide (ie greater than 30 of the length of the headshield) Following Edgecombe amp Ramskoldrsquos coding ofSidneyia the present authors do not consider the postero-median expansion of the doublure of for example Emeraldellaor Retifacies (see Bruton amp Whittington 1983 Hou ampBergstrom 1999 respectively) to be homologous with State 1but to represent the hypostome (see Character 26)

26 Anteromedian margin of cephalon notched accomo-dating strongly sclerotised plate (0) notch and plate absentand (1) notch and plate present (cf Edgecombe amp Rasmkold1999 character 10) The apomorphic state (illustrated inFig 7D) is only known in the taxa discussed by Edgecombe ampRasmkold (1999) Reconstructions of Yohoia show a roundedlobe between the eyes (see Character 20) anterior to anddistinct from the head shield which resembles the anteriorsclerite recognised here This seems to be a misinterpretationSpecimens preserved in dorsal aspect (USNM 57696Whittington 1974 pl 2 figs 1ndash3) and lateral aspect (USNM57694 Whittington 1974 pl 1 fig 1 USNM 155616Whittington 1974 pl 5 fig 2) seem to clearly show that thislsquomedian lobersquo is a downward curving pointed extension of thehead shield

27 Hypostomal sclerite (0) median extension of thedoublure with no suture (1) natant sclerite not in contactwith doublure (2) with narrow overlap with pre-hypostomalsclerite (3) narrow attachment to doublure at hypostomalsuture and (4) absent The homology of hypostomes andlabrums has been discussed by Haas et al (2001) and Budd(2002) The euarthropod labrum is likely to represent a pos-teroventral extension of the pre-segmental acron (Scholtz

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 181

Figure 7 Ventral reconstructions of arachnomorph arthropod heads (A) Misszhouia longicaudata (after Chenet al 1997 figs 2ndash4 7) (B) Ceraurinella typa (after Chen et al 1997 fig 7) (C) Agnostus pisiformis (after Mulleramp Walossek 1987) (D) Kuamaia lata (after Edgecombe amp Ramskold 1999 figs 32 5 amp 6) (E) Cindarella eucala(after Ramskold et al 1997) and (F) Emeraldella brocki (after Bruton amp Whittington 1983) Not to scaleAbbreviations (aw) anterior wing beneath antennae (bs) boomerang-shaped sclerite (bl) blade-like process ofposterior wing (d) doublure of head-shield (cs) connective suture (f) fenestra (fs) facial suture (h) hypostome(hl) hypostomal lobe (emerging through fenestrae of Agnostus) (hs) hypostomal suture (if) intervening field ofhypostomal complex (lb) lateral bridge (lfo) lateral frontal organ (mb) median body (mbr) median bridge (mf)median furrow (mfb) mouth field bridge (mfo) median frontal organ (ol) ovate lobe and (phs) pre-hypostomalsclerite Post-antennal appendages and distal parts of antennae omitted

182 TREVOR J COTTON AND SIMON J BRADDY

1997) This structure is often sclerotised and covers the mouthventrally It is this sclerite that is here identified as thehypostome Therefore this character is considered absent intaxa lacking a sclerite covering the mouth irrespective of themorphology and position of the labrum itself which at leastpartly covers the mouth in all euarthropods other thanpycnogonids (see Edgecombe et al 2000 p 167) The lsquofleshylabrumrsquo of crustaceans does not appear to be sclerotised and ahypostome is consequently considered to be absent Althoughmany derived features are associated with the crustaceanlabrum this is not a good reason to consider it non-homologous with the hypostome-bearing structure of otherarthropods as Walossek amp Muller (1990) argued

In many taxa the mouth is covered by a posteromedianextension of the doublure here considered to represent thehypostome (eg Emeraldella Fig 7F Aglaspis see Hesselbo1992 fig 52) In others the hypostome is separated from thedoublure either by a suture a pre-hypostomal sclerite (egKuamaia lata Fig 7D Saperion glumaceum see Edgecombe ampRamskold 1999 figs 3 amp 4) or an intervening region ofunsclerotised cuticle (the natant condition of Fortey 1990beg Cindarella eucala Fig 7E) The homology of pre-hypostomal sclerites and hypostomes in helmetiids trilobitesand naraoiids (see Fig 7) has been discussed by Chen et al(1996) and Edgecombe amp Ramskold (1999) but it is as yetunclear whether any of the character states described here arederived from any other They are treated here as distinct andthe character as unordered pending further study

No hypostome has been identified in Yohoia (see above)Alalcomenaeus Jianfengia Fortiforceps or Leanchoilia andthere is certainly no broad posterior extension of the doublurein any of these taxa Since there is also no pre-hypostomalsclerite they have received a (134) partial uncertainty codingThe attachment of the hypostome of Tegopelte is unclear butit does not seem to be attached to any doublure (Whittington1985) and therefore it is given an uncertainty (12) coding

The lsquorostrum-like structurersquo on the ventral surface ofLemoneites appears to be similar in morphology and positionrelative to the anterior margin of the head shield (Flower 1968pl 8 figs 4 amp 13) to that described from Aglaspis and is codedas State 0 In many taxa the hypostome is unknown but inonly a small number can it be coded reliably as absentHughesrsquo (1975) suggestion that the hypostome of Burgessia isabsent is accepted preliminarily but an extension of thedoublure may be visible in some of Hughesrsquo figures Fortey(pers comm 2000) also doubts Hughesrsquo assertion

28 Visible ecdysial sutures (0) absent and (1) present Themarginal ecdysial sutures of chelicerates and the dorsal suturesof trilobites are almost certainly not homologous but thepresence of sutures and their position (see Character 29) arecoded separately to allow this to be tested Wills et al (1998aCharacter 2) coded marginal sutures as present in Sidneyiafollowing Brutonrsquos (1981) suggestion but this is consideredequivocal here

29 Position of ecdysial sutures (0) marginal and (1) dor-sal This character is inapplicable to taxa lacking ecdysialsutures (Character 28 State 0) Dorsal sutures whilst presentin the representatives of the Trilobita coded here are probablynot a synapomorphy of trilobites as a whole but for a clade oftrilobites excluding the Olenellida (Whittington 1989 Fortey1990a)

245 Exoskeletal tergites and thoracic tagmosis 30 Min-eralised cuticle (0) absent and (1) present Calcification of theexoskeleton is one of the most convincing synapomorphiesof the Trilobita (Fortey amp Whittington 1989 Ramskold ampEdgecombe 1991 Edgecombe amp Ramskold 1999 Character 1)Cuticle mineralisation is also coded as present in aglaspidids

following Briggs amp Fortey (1982) who suggested that theaglaspidid exoskeleton was originally phosphatic Paleomerusprobably also had a mineralised cuticle but Hou amp Bergstrom(1997 p 97) argued that it was likely to be calcareous Anundescribed Silurian aglaspidid may also have had an origi-nally calcitic cuticle (Fortey amp Theron 1994 p 856) Becauseof the uncertainty about the chemical composition of theexoskeleton in these forms cuticle mineralisation is coded aspotentially homologous in all these taxa and no attempt ismade to code separate states for phosphatic and calcareousmineralisation

31 Trunk tergites with expanded lateral pleurae coveringappendages dorsally (0) absent and (1) present Whilst thepresence of paratergal folds may be a synapomorphy at thelevel of the Euarthropoda (Boudreaux 1979 Wagele 1993Edgecombe et al 2000 Character 142) these are at mostsmall reflections of the margin of the tergites in most euarthro-pods In arachnomorph taxa these are expanded to form largelateral pleurae that cover the appendages dorsally (see egLimulus and Triarthus in Boudreaux 1979) This is inferred tobe the case in some taxa where dorsal features and appendagesare poorly known on the basis of their possession of tergiteswith wide pleural regions This feature was suggested as typicalof lamellipedians (=arachnomorphs) by Hou amp Bergstrom(1997 pp 42ndash3)

The arrangement of the thorax of Marrella Mimetaster andBurgessia differs from that of all the other taxa considered herein that the appendages seem to be attached to the lateralmargins of the body The trunk tergites do not cover theappendages and seem to lack pleurae entirely although theyare covered by a posterior extension of the head shield inBurgessia This character has been clearly described inMimetaster (Sturmer amp Bergstrom 1976 pp 87ndash90 fig 8) andBurgessia (Hughes 1975 p 421)

32 Free thoracic tergites (0) present and (1) absentFollowing Edgecombe amp Ramskold (1999 Character 16) anumber of taxa lack functional post-cephalic articulations andconsequently lack free thoracic tergites Despite this theycode these taxa for a number of characters relating to thestructure of thoracic tergites (eg their characters 18 and 19)These characters are treated here as inapplicable to taxawithout free tergites and the definition of this character hasbeen modified from that of Edgecombe amp Ramskold (1999) tomake this more explicit

33 Decoupling of thoracic tergites and segments (0)absent and (1) present Ramskold et al (1997) described thischaracter of Cindarella and Xandarella where the thoracictergites correspond to a variable increasing posteriorly num-ber of appendage pairs This character cannot be coded fortaxa in which free thoracic tergites are absent (Character 32State 1)

34 Tergite articulations (0) tergites non-overlapping (1)extensive overlap of tergites and (2) edge-to-edge pleuralarticulations In most of the taxa considered here the thoracictergites overlap considerably and relatively evenly over theirwidth This contrasts with the primitive euarthropod condi-tion where the tergites do not overlap medially as seen in stemgroup crustaceans In trilobites such as Helmetia and Kuamaia(see Edgecombe amp Ramskold 1999 character 18) overlap ofthoracic tergites is limited to a well-defined axial region andthe lateral pleurae meet edge-to-edge

The thoracic articulations of naraoiids with free thoracictergites are in some ways similar to those of trilobites with astrong overlap medially that is lacking abaxially andanterolateral articulating facets (Budd 1999a Ramskold ampEdgecombe 1999) However the distribution of these features

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 183

in other trilobite-like arachnomorphs is unclear and a restric-tive coding is used here The present authors do not considerthat this character can be applied reliably to the fused thoracictergites of Saperion Tegopelte and Skioldia and it is coded asinapplicable to all taxa lacking free thoracic tergites (Character32 State 1)

35 Trunk effacement (0) trunk with defined (separate orfused) tergite boundaries (1) trunk tergite boundaries effacedlaterally and (2) trunk tergite boundaries completely effaced(cf Edgecombe amp Ramskold 1999 Character 15)] Contrary toEdgecombe amp Ramskold (1999) the present authors considerSkioldia and Saperion to show a distinct form of tergiteeffacement where the fused tergite boundaries are defined byfurrows axially but effaced laterally The boundaries areeffaced at least laterally in Tegopelte but the axial region ofthe exoskeleton is unknown which accordingly is given amultistate uncertainty coding

36 Cephalic articulation fused (0) absent and (1) presentIn Tegopelte Saperion and Skioldia the articulation ofthe head with the thorax is non-functional and the entireexoskeleton forms a single tergite

37 Head shield overlap of thoracic tergites (0) overlapabsent or identical to overlap between thoracic segments (1)head shield covers first thoracic tergite only and (2) headshield covers mutliple anterior trunk tergites

38 Head shield articulates with reduced anterior thoracictergite (0) absent and (1) present Amongst taxa showing aposteriorly expanded head shield ie one that overlaps thethorax Edgecombe amp Ramskold (1999 Character 9) identifiedtwo distinct character states One of these ndash overlap of only theanteriormost thoracic tergite ndash was limited to taxa assigned tothe Liwiinae (sensu Fortey amp Theron 1994) and another ndashoverlap of multiple tergites with attachment to a reducedanterior thoracic tergite ndash to the Xandarellida None of theadditional taxa considered herein (including Buenaspis whichwas assigned to the Liwiinae by Budd 1999a) shows thesedistinctive morphological features However the expandedhead shields of Marrella Mimetaster and Burgessia which donot articulate with a narrow anterior thoracic tergite may behomologous with the expanded head shields of xandarellidsTo recognise this the degree of overlapping of the thorax andthe possession of the reduced anterior tergite are coded sepa-rately These characters are both coded as inapplicable to taxain which the head shield and thoracic tergites are fused(Character 36 State 1) The crustacean carapace which ismost similar to the head shield of Burgessia is seeminglyabsent in stem group crustaceans (Walossek amp Muller1998)

39 Trunk narrowed anteriorly relative to head shieldwidest posteriorly (0) absent and (1) present (cf Edgecombeamp Ramskold 1999 Character 14) The anterior narrowing ofthe trunk caused by the reduction of opisthosomal segment 1in xiphosurids (Weinbergina of the taxa analysed hereAndersen amp Selden 1997 Dunlop amp Selden 1997 Character14) is not considered to be homologous with the unusual shapeof the thorax in naraoiids recognised by their coding

40 Boundaries of anterior trunk segments reflexed antero-laterally (0) absent boundaries transverse or reflexed postero-laterally and (1) present

41 Joints between posterior tergites functional anteriorones variably fused (0) absent and (1) present

42 Posterior tergite bearing axial spine (0) absent and (1)present

Characters 40 to 42 are coded following Edgecombe ampRamskold (1999 characters 17 19 amp 23 respectively) Inaddition to the taxa considered here a thoracic axial spine ispresent in some olenelloid trilobites (see Lieberman 1998

Characters 73 amp 74) and in the aglaspidid Beckwithia (seeHesselbo 1989)

246 Posterior body termination 43 Postabdomen of seg-ments lacking appendages (0) absent and (1) present

44 Length of postabdomen (0) one segment (1) twosegments (2) three segments and (3) five segments In sometaxa a variable number of posterior thoracic segments bearcomplete tubular tergites and lack appendages In Aglaspisautapomorphically it seems that the posterior three thoracicsegments lack appendages (Briggs et al 1979 Hesselbo 1992)but have unmodified tergites These situations are recognisedas potentially homologous by the coding used here Character44 is coded as inapplicable to taxa lacking a postabdomen

45 Posterior tergites strongly curved in dorsal aspect com-pared to anterior tergites (0) absent and (1) present Asrecognised by Wills et al (1998a Character 17) in some taxawhere the tergites are distinct the curvature of thoracic tergitesincreases posteriorly so that the posterior tergites are highlycurved to semicircular in dorsal aspect This situation isnot known from crustaceans (except isopods) or stem grouparthropods

46 Posterior segments reduced and with highly reducedappendages (0) present and (1) absent In some taxa there area large number of posterior segments that are short sagitallyand have appendages that are much reduced in size comparedto anterior trunk appendages These somites are incorporatedinto the pygidium in some taxa but primitively they arecovered by tiny free trunk tergites In the derived state thetrunk somites and limbs are of a relatively constant size Thedistinction between these states can be seen when attempting tocount the number of segments that make up the body In taxashowing State 0 the number of segments is very high anddifficult to count whereas in other taxa the number ofpost-cephalic segments is easily assessed

47 Pygidium (0) absent and (1) present A pygidium isrecognised here as a posterior tagma consisting of a number offused segments under a single tergite which may or may notincorporate the post-segmental telson This is different to theuse of this term by Edgecombe amp Ramskold (1999) whichfollows Ramskold et al (1997) and very different to its use byWills et al (1998a p 53) as a synapomorphy of the TrilobitaThe present authorsrsquo definition recognises the situation inRetifacies where the spinose postsegmental telson is autapo-morphically not fused to the pygidium as homologous toother multisegmented posterior tagma which include thetelson The distinction between the pygidium and an expandedpost-segmental telson can also be seen in the position of theanus which is consistently in the posteriormost pre-telsonicsegment amongst putative arachnomorphs (see Character 48)In taxa with a pygidium the anal segment is fused into thepygidium (see eg Kuamaia Edgecombe amp Ramskold 1999fig 6) whereas in those taxa lacking a pygidium the anus isbetween the final trunk tergite and the telson

Ramskold et al (1997) argued that the posteriormost tergiteof xandarellids (which incorporates the anal segment) is nothomologous to the posterior tagma recognised as pygidiaherein because posterior thoracic tergites also cover multiplesegments in xandarellids (see Character 33) Evidence ofsegmentation of the pygidium in a variety of taxa suggests thatthe number of tergites fused to form the pygidium is greaterthan the number of appendages For example the pygidium ofKuamaia has two pairs of lateral spines but at least fourpairs of appendages (Hou amp Bergstrom 1997 Edgecombe ampRamskold 1999) and that of Triarthrus has only five axialrings posterior to the articulating ring but over 10 pairs ofappendages (Whittington amp Almond 1987) The fact thatdecoupling of tergites and segments is evident in the thorax of

184 TREVOR J COTTON AND SIMON J BRADDY

Xandarella and Cindarella does not necessarily suggest that asimilar more widespread decoupling in pygidia is the result ofa different developmental process and hence that they arenon-homologous Instead the situation in the xandarellidthorax is potentially homologous to that found (more widely)in pygidia and consequently the terminal xandarellid tergite isa pygidium It is unclear if decoupling of tergites and somites isprimitively limited to the terminal tergite or primitively aproperty of the arachnomorph post-cephalon as a whole

It has been suggested that the tiny pygidium of olenelloidtrilobites which probably best reflects the primitive trilobitestate consists only of the post-segmental telson (Harrington inMoore 1959) and therefore is not a true pygidium HoweverWhittington (1989) showed that the olenelloid pygidium con-sists of at least two and possibly as many as five or sixsegments and therefore is likely to be homologous with thepygidium of other trilobites

48 Position of the anus (0) terminal within telson and (1)at base of telson In crustaceans and stem group arthropodsthe anus is terminal or otherwise situated in the telson (egRehbachiella Walossek 1993 fig 15D) In putative arachno-morphs the anus is either at the junction of the posteriormostthoracic segment and the telson or ventral within a fusedpygidium In the case of taxa with a pygidium the anus isanterior to the posterior margin and where known positionedbetween the posteriormost pair of appendages suggesting thatit is anterior to the post-segmental telson (see eg OlenoidesWhittington 1980)

49 Pygidium with median keel (0) absent and (1) presentEdgecombe amp Ramskold (1999 Character 21) considered thepresence of a median keel on the pygidium as a synapomorphyuniting Soomaspis and Tarricoia They did not explain howthis structure could be distinguished from the raised pygidialaxis of trilobites Homology of these structures is not sup-ported here because the axis of more primitive trilobites(including Eoredlichia) is quite distinct morphologically fromthe naraoiid keel Budd (1999a) noted the presence of a keel onthe pygidium of Buenaspis and suggested (Budd 1999a p 102)that it may be an artefact of dorsoventral compaction How-ever contrary to the coding of Edgecombe amp Ramskold(1999) a keel is visible on the pygidium of Liwia convexa (Dzikamp Lendzion 1988 fig 4CD) preserved in full relief Thereforeit seems unlikely that the keel is a taphonomic artefact

50 Pygidium with broad-based median spine (0) absentand (1) present

51 Pygidium with lateral spines (0) present and (1) absentEdgecombe amp Ramskold (1999 Character 22) coded thepresence of a median spine and two pairs of lateral spines aspotentially homologous in Sinoburius Kuamaia and HelmetiaThe distribution of lateral spines is much wider than that ofterminal spines and therefore they are coded separately hereIn trilobites it has widely been recognised that lateral spinesrepresent the original segmentation of the pygidium Thiscannot be the case for median spines Characters 49 to 51are not applicable to taxa lacking a pygidium (Character 47State 0)

52 Expanded post-segmental telson (0) absent and (1)present In a range of taxa the posterior-most tergite is a large(relative to the thoracic tergites) structure that lacks anyevidence of segmentation and is interpreted as representing anexpanded tergite of the post-segmental telson from whichsegments are released anteriorly during ontogeny On the otherhand the posteriormost tergite of Marrella and probablyMimetaster is small compared to the thoracic tergites androunded This element probably represents the plesiomorphiceuarthropod telson The homology of this character in mosttaxa is clear and only doubtful in Retifacies in which it may

be segmented The figures of Hou amp Bergstrom (1997) providelittle unequivocal evidence for a telson in Retifacies but thepresence of this structure is very clear in colour photographs(eg Chen et al 1996 fig 198A) However these figures donot convincingly demonstrate the segmentation of the telsonRetifacies is interpreted here as being unique in possessingboth a pygidium and an expanded post-segmental telson

53 Telson shape (0) spinose and (1) paddle-shaped Theshape of the expanded telsons identified above varies frombroadly spinose to flattened and paddle-like This differencecan be recognised both on the basis of the ratio of length tobasal width (spinose telsons are relatively long) and by thechange in width posteriorly (spinose telsons reduce in widthposteriorly whereas paddle-shaped telsons increase in width)In eurypterids a wide range of telson shapes is found includ-ing both character states described here It is likely that theplesiomorphic condition is a spinose telson This character isinapplicable to taxa lacking an expanded telson

54 Post-ventral furcae (0) absent and (1) present Thischaracter recognises the potential homology of unsegmentedpaired structures that articulate with the segment immediatelyanterior to the telson The potential homology of these struc-tures (the post-ventral plates) in Emeraldella and Aglaspis wasrecognised by Wills et al (1998a Character 70) and inEmeraldella and Sidneyia by Edgecombe amp Ramskold (1999Character 25) The homology of the segmented pygidial caudalfurcae of Olenoides (see Whittington 1975a 1980) with thesestructures is considered doubtful and coded as equivocal

Despite their distinctive morphology the long caudualfurcae of Cheloniellon may be homologous with the furcae ofAglaspis Emeraldella and Sidneyia This is supported by thecaudal furcae of the Ordovician cheloniellid Duslia which aremore similar in morphology to those of Emeraldella than ofCheloniellon Chlupac (1988 pp 614ndash16) considered thesestructures to be on the terminal tergite in Duslia However afaint tergite boundary is apparent posterior to the attachmentof the furcae (see Chlupac 1988 pl 57 fig 4) Therefore theyare considered here to have been attached to the pre-telsonicsegment as in Cheloniellon Chlupac (1988) and Dunlop ampSelden (1997) supported a close relationship between Dusliaand Cheloniellon

25 MethodsAll analyses were carried out using PAUP Version 40b4a(Swofford 1999) and unless otherwise stated used heuristicsearches with one thousand random addition sequencereplicates The software packages MacClade Version 307(Maddison amp Maddison 1997) and RadCon (Thorley amp Page2000) were used for comparing trees and investigating patternsof character evolution Tree length and other statistics (theConsistency Index CI and Retention Index RI) were calcu-lated by PAUP and MacClade with uninformative charactersexcluded Analyses were run separately using each of thedifferent codings for aglaspidids

Characters were treated as unordered and of equal weight inmost analyses To assess the influence of this assumption fourof the eight multistate characters which have states that areintermediate between others (Wilkinson 1992) were treated asordered in some analyses These characters are the number ofcephalic segments (Character 4) the length of raptorialappendage spines (Character 6) the degree of overlap of thethorax by the head-shield (Character 37) and the number ofsegments making up the postabdomen (Character 44) The lastof these is uninformative when not ordered because only onetaxon shows each of states 0 1 and 3

Support for individual nodes was assessed by bootstrapanalysis (Felsenstein 1985) and by calculating Bremer support

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 185

indices (Bremer 1988 1994) These methods measure twodistinct aspects of support for phylogenetic hypotheses Treesor nodes may be considered well supported (1) to the extent towhich alternative topologies are much less parsimonious asmeasured by the support index (Wilkinson 1996) or (2) wherethey are consistent with a large proportion of characters sothat character sampling is unlikely to have had much influenceon topology as assessed by bootstrapping (Page 1996) Boot-strapping was performed with 100 bootstrap replicates eachof 10 addition sequence replicates Bremer support indicesare based on heuristic searches with 100 addition sequencereplicates

Three pairs of terminals are included in Table 2 that apartfrom missing data are coded identically These are Helmetiaand Kuamaia Soomaspis and Tariccoia and Saperion andSkioldia These pairs are lsquotaxonomic equivalentsrsquo removingthe taxon with most missing data will not influence theposition of the other They can be recoded as single terminalsaccording to the principal of safe taxonomic reduction(Wilkinson 1995a b) and were not considered separately in theanalyses presented here Therefore cladistic analyses werecarried out on the basis of 33 terminals Edgecombe ampRamskold (1999) ignored this and coded the same three pairsof taxa and Eoredlichia and Olenoides identically for theirmore limited set of characters

26 ResultsAnalysis with all characters unordered and using the lsquoAglaspi-dida 1rsquo coding found nine most parsimonious trees (MPTs)each 126 steps long (CI=0middot556 RI=0middot763) Using the lsquoAglas-pidida 2rsquo coding resulted in 18 MPTs of length 125 (CI=0middot560RI=0middot765) Nine of these 18 trees were those found with thefirst coding of aglaspidids The strict component consensus(sensu Wilkinson 1994) of the 27 unique trees found acrossboth analyses is shown in Figure 8 All trees supported themonophyly of the Marrellomorpha (Marrella and Mimetaster)and of a clade including all other taxa except Crustacea (Clade1 of Fig 8) The relationships between these two clades and theCrustacea were unresolved equal numbers of trees in bothanalyses supported each of the topologies [Clade 1 (CrustaceaMarrellomorpha)] [Crustacea (Marrellomorpha Clade 1)]and [Marrellomorpha (Crustacea Clade 1)] All trees placedBurgessia as the sister taxon to the rest of Clade 1 andsupported the division of the rest of Clade 1 into two majorsister groups The topology of one of these clades (Clade 2)which included trilobites helmetiids tegopeltids naraoiidsxandarellids and Retifacies was stable with respect to thecoding of aglaspidids The other clade (Clade 3) consists of aclade of megacheirans and chelicerates (Clade 4) and a cladeincluding aglaspidids Lemoneites Paleomerus CheloniellonEmeraldella and Sidneyia (Clade 5) This latter clade is un-resolved in the strict consensus of all 27 trees (Fig 8) and thestrict consensus of the 18 MPTs found using the lsquoAglaspidida2rsquo coding but was fully resolved in the consensus of the ninetrees found with the lsquoAglaspidida 1rsquo coding Within themegacheiran-chelicerate clade all trees found Fortiforceps tobe the sister group to all other taxa Yohoia the sister group tochelicerates and Alalcomenaeus and Leanchoilia to form aclade Jianfengia is equally parsimoniously placed as the sistergroup to the Yohoia-chelicerate clade or as the sister group tothis clade and the Alalcomenaeus-Leanchoilia clade

Treating characters 4 6 37 and 44 as ordered had a minimaleffect on most parsimonious topology Nine MPTs of 136 steps(CI=0middot515 RI=0middot743) were found with the lsquoAglaspidida 1rsquocoding and 12 MPTs 135 steps in length (CI=0middot519 RI=0middot745)with the lsquoAglaspidida 2rsquo coding These trees all supported a(Crustacea Clade 1) clade excluding the marrellomorphs

Apart from this the strict consensus of these 21 trees isidentical to the consensus of the 27 trees found with allcharacters treated as unordered

Across all analyses four different topologies for Clade 5 (ofFig 8) were found (Fig 9) The distribution of these topologiesacross the four analyses described above and across the sameanalyses repeated after reweighting of each character in pro-portion to their Rescaled Consistency Index (see Farris 1989)from the first analysis is shown in Table 3 The instability of

Figure 8 Strict consensus of 27 MPTs from four separate analyses ofarachnomorph phylogeny with both interpretations of aglaspididmorphology and with some characters treated as ordered or allcharacters unordered Clades referred to in the text are numbered

Figure 9 Alternative equally parsimonious resolutions of clade 5 ofFig 8 (see Table 3)

186 TREVOR J COTTON AND SIMON J BRADDY

this clade was to some extent caused by the inclusion ofLemoneites and Paleomerus which could each be coded foronly 27 of the 54 characters used Analysis with these two taxaexcluded and characters treated as unordered found treescompatible only with Topology D of Figure 9 irrespective ofthe coding used for aglaspidids Analysis with some characterstreated as ordered and using either aglaspidid coding foundthree trees compatible only with Topology B of Figure 9 andthree compatible only with Topology D of Figure 9 ThereforeTopology D is the preferred topology for Clade 5 as shown inTable 3

One of the MPTs resulting from the lsquoAglaspidida 2rsquo coding(with all characters treated as unordered) was chosen as thebasis for further discussion because this tree combined thepreferred topology of Clade 5 with the (Crustacea Clade 1)group that was favoured when some characters were treated asordered Bootstrap percentages and Bremer support values forthis tree (using the Aglaspidida 2 coding) are shown in Figure10 Apomorphies for all ingroup nodes of this tree are shownin Figure 10A

3 Discussion

31 Phylogenetic resultsThe results of the cladistic analyses presented above arewell-resolved and robust with respect to different analyticalparameters Most of the taxa considered form a clade (Clade 1of Fig 8) that is more closely related to chelicerates than tocrustaceans in all analyses and therefore is recognised asthe Arachnomorpha as defined by Chen et al (1996) andRamskold et al (1996) The monophyly of the Marrellomor-pha was supported in all analyses The marrellomorphs werefound to be the sister group to a Crustacea+Clade 1 group inmost MPTs but in a minority of trees formed the sister groupto Clade 1 alone According to the latter result they should beincluded within the Arachnomorpha but according to the firstexcluded from it Both of these alternatives have been sup-ported in previous studies (eg Hou amp Bergstrom 1997 Willset al 1998a respectively) In addition to the marrellomorphsthe Fuxianhuiida Bousfield 1995 and Canadaspidida havebeen placed in the Euarthropoda outside the main mandibu-late and arachnomorph clades (Hou amp Bergstrom 1997)Further study of these taxa of the crustacean or mandibulatestem group and of the marrellomorphs is necessary before the

position of the Marrellomorpha can be resolved Provisionallymarrellomorphs are excluded from the Arachnomorphafollowing the majority of analyses above and Wills et al(1994)

The Burgess Shale arthropod Burgessia was placed as thesister group to remaining arachnomorphs in all analyses OtherArachnomorpha form two major clades one consisting of thelsquotrilobite-alliedrsquo arachnomorphs analysed by Edgecombe ampRamskold (1999) here termed lsquoTrilobitomorpharsquo (defined asTrilobita and all taxa more closely related to Trilobita thanChelicerata) and the other (lsquochelicerate-alliedrsquo) clade includingchelicerates lsquogreat appendagersquo arthropods aglaspididsLemoneites Paleomerus Sidneyia and Emeraldella The latteris here termed lsquoCheliceramorpharsquo (defined as Chelicerata andall taxa more closely related to Chelicerata than Trilobita) Thetopology of the lsquotrilobite-alliedrsquo clade was similar to that foundby Edgecombe amp Ramskold (1999) but the present authorsrsquoresults are more completely resolved unambiguously support-ing the sister-group relationship between trilobites andHelmetiida suggested in the previous study Within thelsquochelicerate-alliedrsquo clade Aglaspidids Paleomerus Lemoneitesand the Burgess Shale Emeraldella and Sidneyia form a cladein opposition to megacheirans and chelicerates

The present authorsrsquo analysis supports a new hypothesis ofthe origin of the chelicerates from within a paraphyleticassemblage of megacheiran arthropods and confirms pyc-nogonids as a sister group to the euchelicerates Previoushypotheses of the chelicerate sister group (see Section 1) aremuch less parsimonious The shortest trees supporting aCheloniellon-chelicerate clade are six steps longer than theMPTs and those supporting an aglaspidid-chelicerate cladeseven steps longer All megacheirans and chelicerates areunited by the flap-shaped rounded exopods and modificationof the second cephalic segment endopods into anteriorlydirected spinose raptorial organs which share a number ofdetailed similarities A clade including all of these taxa apartfrom Fortiforceps is supported by the synapomorphic loss ofthe antennae (or antennulae of crustaceans) and the longerlength of spinose projections of the second cephalic appendageendopods Within this clade Yohoia shares the loss of cephalicexopods the loss of thoracic endopods and a postabdomen oftubular tergites with chelicerates

The poor resolution of Clade 5 (of Fig 8) was partly a resultof the poor preservation of and consequent high proportion ofmissing data for Lemoneites and Paleomerus When these two

Table 3 Numbers of most parsimonious trees supporting each of the alternative topologies for clade 4 of Fig 7 under different analyticalconditions (RCI=Revealed Consistency Index)

Aglaspididcoding

Characters 4 6 36and 43 ordered

Charactersreweighted by RCI

Paleomerus andLemoneites excluded

Topology (see Fig 8)

A B C D

1 91 X 3 3 31 X 61 X X 31 X 61 X X 3 32 9 92 X 3 3 3 32 X 62 X X 32 X 62 X X 3 3Total 6 18 12 54

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 187

taxa were excluded from the analyses only two (B and D) ofthe four topologies found in the complete analysis remainedmost parsimonious (see Fig 9 and Table 3) Secondly Bremersupport values throughout much of the lsquochelicerate-alliedrsquoclade (Clade 3 of Fig 8) were improved when these taxa wereexcluded (see Fig 11)

Dunlop (1999 p 258) suggested that in reconstructing thechelicerate stem lineage lsquowe might predict that the two mostsignificant changes towards the chelicerate condition are thereduction of the antennae and the formation of the nextappendages into a clawrsquo According to the hypothesis pre-sented here both of these adaptations were achieved in EarlyCambrian chelicerate ancestors The recognition of the loss ofthe antennae in the stem group of the chelicerates and thephylogenetic hypothesis presented here suggest that chelicer-ates are most closely related to taxa with only a singleadditional segment fused to the crown group eucheliceratehead of four fused somites Previous hypotheses have derivedchelicerates from Palaeozoic arthropods with a longer head(eg Cheloniellon Sturmer amp Bergstrom 1978 SanctacarisBriggs amp Collins 1988 Emeraldella Bruton amp Whittington1983) The plesiomorphic pattern of head segmentation ismatched in pycnogonids Bergstrom et al (1980) suggestedthat only the anterior four pairs of appendages are incorpo-rated into the head This supports the view that pycnogonidsare primitive with respect to other chelicerates which showmore or less complete fusion of this primitive head and threethoracic segments including Solpugida and Pseudoscorpiones

The present study suggests that a shift from gnathobasicbenthic feeding to a more vagrant lifestyle and raptorialappendage feeding occurred very early in the chelicerate lin-eage Appendage feeding was clearly an important adaptation

Figure 10 One of 18 equally parsimonious cladograms with lsquoAglaspidida 2rsquo coding and all characters unorderedthat is most congruent with results from other analyses Non-terminal branch lengths scaled to reflect number ofapomorphies (A) ACCTRAN apomorphy scheme Character numbers shown above boxes and states shownbelow boxes as in Table 2 and the text Character consistency indices are indicated by shading (B) Levels ofsupport for individual nodes Bremer support indices shown in bold type above branches Bootstrap percentagesin italic type below branches for all nodes with relative frequencies greater than 50

Figure 11 Bremer support values for clades within the lsquochelicerate-alliedrsquo arachnomorphs (clade 3 of Fig 8) when all taxa wereincluded (normal weight font above nodes) and with Lemoneites andPaleomerus excluded (bold type below nodes)

188 TREVOR J COTTON AND SIMON J BRADDY

for the terrestrialization of chelicerates (Dunlop 1997 p 69Dunlop amp Webster 1999) but the model proposed heresuggests that this considerably pre-dated terrestrializationGnathobasic feeding in xiphosurans and eurypterids mayrepresent a reversal to a more primitve benthic arachnomorphlifestyle from a more pycnogonid-like ancestor

32 Evolution of the arachnomorph headThe extant arthropod classes show highly conserved patternsof head segmentation (eg Wills et al 1994) In contrast therehas been a near consensus amongst Cambrian arthropodworkers (although rarely based on an explicit phylogenetichypothesis) that patterns of head segmentation are highlyhomoplastic and hence of little systematic significanceSturmer amp Bergstrom (1978 pp 78ndash9) suggested that lsquoevenclosely related forms may have different numbers of headsegments and appendagesrsquo Bruton amp Whittington (1983pp 576ndash7) that lsquodiscussion on fossil arthropod relationshipsbased on head segmentation appears to be largely irrel-evant and at best speculativersquo and Delle Cave amp Simonetta(1991 p 191) that lsquothere are no obvious phyletic affinitiesbetween genera having the same number of cephalized seg-mentsrsquo Most recently Bergstrom amp Hou (1997 p 104) con-cluded that lsquothe segmental length of the head shield seemsto be of no relevance to the discussion and discrimination ofevolutionary lineagesrsquo It has been claimed that this view issupported by cladistic analyses in which patterns of tagmosishave been found to be rather poor at defining major arthropodclades (Briggs et al 1992 Wills et al 1994)

This suggestion is not only of relevance to systematics buthas featured prominently in recent debates surrounding thenature of the lsquoCambrian explosionrsquo (see Budd amp Jensen 2000for a recent review) Gould (1989 1991) argued that theapparent plasticity of head segmentation in Cambrian com-pared to post-Cambrian arthropod evolution suggests thatbody-plan evolution was only constrained after the Cambrianexplosion According to this view the origin of new highertaxa with distinct body-plans was only possible before theonset of these constraints (eg Valentine 1995) Taxa hererecognised as arachnomorphs clearly played a major role in thedevelopment of Gouldrsquos hypothesis This suggestion has pre-viously been assessed in terms of overall morphological diver-sity which is clearly rather distinct from Gouldrsquos concept ofdisparity Wills et al (1998b) showed that there was littlecorrelation between overall morphological disparity and de-gree of limb specialisation and that the latter was phylogeneti-cally highly plastic They remarked that (Wills et al 1998bp 64) lsquothis finding has important implications for models ofarthropod phylogeny and evolution that attribute overidingimportance to head segmentationrsquo but did not explicitlyexamine head segmentation although it is likely to have beena major component of the index of tagmosis employed

The results of the present study suggest that arguments thatarthropod head segmentation was unusually labile during theCambrian are poorly founded Rather only four major pat-terns of euarthropod head segmentation are identified Theplesiomorphic euarthropod state according to both Walossekamp Muller (1997 1998) and Scholtz (1997) which may moreproperly be the plesiomorphic state for the euarthropod crowngroup only (depending on the phylogenetic position of theMarrellomorpha) consists of four post-acronal segmentsbearing the antennae and three pairs of biramous limbs Thepresent authors suggest that the term lsquocephalonrsquo be restrictedto this kind of head which is found in stem group mandibu-lates in addition to many arachnomorphs Crown groupmandibulates share a lsquobimaxillary headrsquo in which an addi-tional pair of appendages the labium or second maxillae are

incorporated into the head but in crustaceans these are notfused to the carapace (Scholtz 1997) Finally two distinctforms of head tagmosis are found in chelicerates A head withfour pairs of appendages (but without antennae giving a totalof five segments incorporated into the head) which has beencalled the lsquocephalosomarsquo is present in pycnogonids (Walossekamp Dunlop 2002 pp 434ndash5) and probably also in some euch-elicerates (J A Dunlop pers comm 2003) The lsquoprosomarsquo ofmost euchelicerates consists of this cephalosoma and twoadditional segments

In general these patterns of head segmentation are highlyphylogenetically conserved (see Fig 12) Only three homo-plastic changes namely reversal to a more primitive conditionin the Leanchoilia-Alalcomenaeus clade and the convergentorigin of a five-segmented head in Clade 3 (of Fig 8) and theXandarellida are necessary to optimise character 4 onto themost parsimonious cladogram There are also transitions toautapomorphic states in Emeraldella and Sidneyia In the caseof Sidneyia however an argument could be made (followingBergstrom amp Sturmer 1978 see discussion of Character 4) thatthe head consists of five segments as in closely related taxabut autapomorphically each segment has a separate tergiteThis would further increase the phylogenetic stability of headsegmentation Also following Bergstrom amp Sturmer (1978) thehead of Cheloniellon could be considered to consist of twotergites making it more similar to the head of Emeraldella(with six pairs of appendages)

Figure 12 Cladogram from Fig 10 showing changes in head segmen-tation Bold type shows abbreviated head segmentation formulaAbbreviations (Ant) antennae (BA) biramous appendage pairs (Ch)chelicerae (GA) great appendages (UA) uniramous appendage pairsand (X) segment without appendages Synapomorphy A is the auta-pomorphic reduction in head length to a single segment in SidneyiaAlternatively this may represent secondary division of the headshield Synapomorphy B is the increase in the length of the head inEmeraldella to six segments

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 189

4 Acknowledgements

We thank D E G Briggs J A Cotton J A Dunlop R AFortey S Harris R A Moore P Selden O E Tetlie andP Van Roy for comments on earlier drafts of this work Themanuscript was reviewed by M Wills F Schram and oneanonymous referee This research was supported by the Geo-logical Society of London Curry Research Studentshipawarded to TJC by the Department of Earth Sciences Uni-versity of Bristol and by the Department of PalaeontologyThe Natural History Museum London TJC would like tothank C Labandeira J Thompson and R Chapman fortheir hospitality at the National Museum of Natural HistoryWashington DC USA and A Olcott S Schellenberg andJ Young for making his time there so enjoyable

5 References

Abzhanov A Popadic A amp Kaufman T C 1999 Chelicerate Hoxgenes and the homology of arthropod segments Evolution andDevelopment 1 77ndash89

Akam M 2000 Arthropods developmental diversity within a (super)phylum Proceedings of the National Academy of Sciences USA97 4438ndash41

Aldridge R J Gabbott S E amp Theron J N 2001 The Soom ShaleIn Briggs D E G amp Crowther P R (eds) Palaeobiology II340ndash2 Oxford Blackwell Science

Anderson L I amp Selden P A 1997 Opisthosomal fusion andphylogeny of Palaeozoic Xiphosura Lethaia 30 19ndash31

Arango C P 2002 Morphological phylogenetics of the sea spiders(Arthropoda Pycnogonida) Organisms Diversity and Evolution2 107ndash25

Ax P 1986 Das phylogenetische System Stuttgart Gustav FischerVerlag

Babcock L E amp Zhang Wentang In press New non-mineralizedarthropods from the Chengjiang Lagerstatte Lower CambrianYunnan China Palaeontologia Cathyana

Bartels C Briggs D E G amp Brassel G 1998 The fossils ofthe Hunsruck Slate Marine life in the Devonian CambridgeCambridge University Press

Bergstrom J 1971 Paleomerus ndash merostome or merostomoid Lethaia4 393ndash401

Bergstrom J 1979 Morphology of fossil arthropods as a guideto phylogenetic relationships In Gupta A P (ed) ArthropodPhylogeny 3ndash56 New York NY Van Nostrand Reinhold Co

Bergstrom J 1992 The oldest arthropods and the origin of theCrustacea Acta Zoologica 73 287ndash91

Bergstrom J Sturmer W amp Winter G 1980 PalaeoisopusPalaeopantopodus and Palaeothea pycnogonid arthropodsfrom the Lower Devonian Hunsruck Slate West GermanyPalaontologische Zeitschrift 54 7ndash54

Bergstrom J amp Brassel G 1984 Legs in the trilobite Rhenops fromthe Lower Devonian Hunsruck Slate Lethaia 17 67ndash72

Bergstrom J amp Hou X 1998 Chengjiang arthropods and theirbearing on early arthropod evolution In Edgecombe G D(ed) Arthropod fossils and phylogeny 151ndash84 New York NYColumbia University Press

Boudreaux H B 1979 Significance of intersegmental tendon systemin arthropod phylogeny and a monophyletic classification ofArthropoda In Gupta A P (ed) Arthropod Phylogeny 551ndash86New York NY Van Nostrand Reinhold Co

Bousfield E L 1995 A contribution to the natural classification ofLower and Middle Cambrian arthropods food-gathering andfeeding mechanisms Amphipacifica 2 3ndash33

Braddy S J Aldridge R J Gabbott S E amp Theron J N 1999Lamellate book-gills in a late Ordovician eurypterid from theSoom Shale South Africa support for a eurypterid-scorpionclade Lethaia 32 72ndash4

Braddy S J amp Almond J E 1999 Eurypterid trackways from theTable Mountain Group (Ordovician) of South Africa Journal ofAfrican Earth Sciences 29 165ndash77

Bremer K 1988 The limits of amino acid sequence data in angio-sperm phylogenetic reconstruction Evolution 42 795ndash803

Bremer K 1994 Branch support and tree stability Cladistics 10295ndash304

Briggs D E G 1998 Review of Arthropods from the LowerCambrian Chengjiang fauna southwest China by HouXianguang and Jan Bergstrom Fossils and Strata 45

Scandinavian University Press Oslo Norway 1997 Palaeo-geography Palaeoclimatology Palaeoecology 143 193ndash4

Briggs D E G Bruton D L amp Whittington H B 1979 Append-ages of the arthropod Aglaspis spinifer (Upper CambrianWisconsin) and their significance Palaeontology 22 167ndash80

Briggs D E G Fortey R A amp Wills M A 1992 Morphologicaldisparity in the Cambrian Science 256 1670ndash3

Briggs D E G Fortey R A amp Wills MA 1993 How big was theCambrian explosion A taxonomic and morphologic comparisonof Cambrian and Recent arthropods In Lees D R amp EdwardsD (eds) Evolutionary patterns and processes 33ndash44 LondonLinnean Society of London Linnean Society Symposium Series

Briggs D E G Erwin D H amp Collier F J 1994 The fossils of theBurgess Shale Washington DC Smithsonian Institution Press

Briggs D E G amp Bartels C 2001 New arthropods from the LowerDevonian Hunsruck Slate (Lower Emsian Rhenish Massifwestern Germany) Palaeontology 44 275ndash304

Briggs D E G amp Bartels C 2002 Magnoculocaris a new name forthe arthropod Magnoculus Briggs and Bartels 2001 from theLower Devonian Hunsruck Slate Germany Palaeontology 45419

Briggs D E G amp Collins D 1988 A Middle Cambrian cheliceratefrom Mount Stephen British Columbia Palaeontology 31 779ndash98

Briggs D E G amp Collins D 1999 The arthropod Alalcomenaeuscambricus Simonetta from the Middle Cambrian Burgess Shale ofBritish Columbia Paleontology 42 953ndash78

Briggs D E G amp Fortey R A 1982 The cuticle of aglaspididarthropods a red-herring in the early history of vertebratesLethaia 15 25ndash9

Briggs D E G amp Fortey R A 1989 The early radiation andrelationships of the major arthropod groups Science 246 241ndash3

Briggs D E G amp Fortey R A 1992 The early Cambrian radiationof arthropods In Lipps J H amp Signor P W (eds) Origin andEarly Evolution of the Metazoa 335ndash373 New York NY PlenumPress

Briggs D E G amp Robison R A 1984 Exceptionally preservednontrilobite arthropods and Anomalocaris from the MiddleCambrian of Utah University of Kansas PaleontologicalContributions Paper 111 1ndash23

Broili F 1932 Ein neuer Crustacee aus dem rheinischen UnterdevonSitzungsberichte der Mathematisch-Naturwissenschaftlichen(Abteilung) Klasse der Bayerischen Akademie der Wissenschaftenzu Munchen 1932 27ndash38

Broili F 1933 Ein zweites Exemplar von Cheloniellon Sitzungsber-ichte der Bayerischen Akademie der Wissenschaften 1933 11ndash32

Bruton D L 1981 The arthropod Sidneyia inexpectans MiddleCambrian Burgess Shale British Columbia Philosophical Trans-actions of the Royal Society of London Series B 295 619ndash56

Bruton D L amp Whittington H B 1983 Emeraldella and Leanchoiliatwo arthropods from the Burgess Shale British Columbia Philo-sophical Transactions of the Royal Society of London Series B 300553ndash85

Budd G E 1995 Kleptothule rasmusseni gen et sp nov anolenellid-like trilobite from the Sirius Passet fauna (BuenFormation Lower Cambrian North Greenland) Transactions ofthe Royal Society of Edinburgh Earth Sciences 86 1ndash12

Budd G E 1996a Progress and problems in arthropod phylogenyTrends in Ecology and Evolution 11 356ndash8

Budd G E 1996b The morphology of Opabinia regalis and thereconstruction of the arthropod stem-group Lethaia 29 1ndash14

Budd G E 1997 Stem-group arthropods from the Lower CambrianSirius Passet fauna of North Greenland In Fortey R A ampThomas R H (eds) Arthropod Relationships 125ndash38 LondonChapman and Hall Systematics Association Special Volume 55

Budd G E 1999a A nektaspid arthropod from the Early CambrianSirius Passet fauna with a description of retrodeformation basedon functional morphology Palaeontology 42 99ndash122

Budd G E 1999b The morphology and phylogenetic significance ofKerygmachela kiergkegaardi Budd (Buen Formation LowerCambrian N Greenland) Transactions of the Royal Society ofEdinburgh Earth Sciences 89 (for 1998) 249ndash290

Budd G E 2002 A palaeontological solution to the arthropod headproblem Nature 417 271ndash5

Budd G E amp Jensen S 2000 A critical reappraisal of the fossilrecord of the bilaterian phyla Biological Reviews 75 253ndash95

Chen J Zhou G Zhu M amp Yeh K 1996 The Chengjiang biota ndasha unique window of the Cambrian explosion Taichung NationalMuseum of Natural Science [In Chinese]

Chen J Edgecombe G D amp Ramskold L 1997 Morphological andecological disparity in naraoiids (Arthropoda) from the Early

190 TREVOR J COTTON AND SIMON J BRADDY

Cambrian Chengjiang fauna China Records of the AustralianMuseum 49 1ndash24

Chen J amp Zhou G 1997 Biology of the Chengjiang fauna In ChenJ Cheng Y amp Iten H V (eds) The Cambrian explosion and thefossil record Bulletin of the National Museum of Natural ScienceTaiwan 10 11ndash106

Child C A 1997 Some deep-sea Pycnogonida from Argentine slopeand basin Proceedings of the Biological Society of Washington110 128ndash42

Chlupac I 1988 The enigmatic arthropod Duslia from the Ordovicianof Czechslovakia Palaeontology 31 611ndash20

Chlupac I 1995 Lower Cambrian arthropods from the PasekyShale (Barrandian area Czech Republic) Journal of the CzechGeological Society 40 9ndash36

Chlupac I amp Havlıcek V 1965 Kodymirus n g a new aglaspidmerostome of the Cambrian of Bohemia Sbornık GeologickechVid Paleontologie 6 7ndash20

Cisne J L 1975 Anatomy of Triarthrus and the relationships of theTrilobita Fossils and Strata 4 45ndash63

Cisne J L 1981 Triarthrus eatoni (Trilobita) anatomy of its exo-skeletal skeletomuscular and digestive systems Palaeonto-graphica Americana 9 95ndash142

Conway Morris S 1982 Atlas of the Burgess Shale London ThePalaeontological Association

Conway Morris S 1989 The persistence of Burgess Shale-type faunasimplications for the evolution of deeper-water faunas Trans-actions of the Royal Society of Edinburgh Earth Sciences 80271ndash83

Conway Morris S 1998 The crucible of creation Oxford OxfordUniversity Press

Cotton T J 1999 Review of Arthropod fossils and phylogenyPalaeogeography Palaeoclimatology Palaeoecology 151 341ndash3

Cotton T J 2002 The phylogeny and morphological evolution ofCambrian trilobites and their relatives Unpublished PhD ThesisUniversity of Bristol

Damen W G M Hausdorf M Seyfarth E A amp Tautz D 1998 Aconserved mode of head segmentation in arthropods revealed bythe expression pattern of Hox genes in a spider Proceedings of theNational Academy of Sciences USA 95 10665ndash70

Delle Cave L amp Simonetta A M 1991 Early Palaeozoic arthropodsand problems of arthropod phylogeny with some notes on taxa ofdoubtful affinities In Simonetta A M amp Conway Morris S(eds) The early evolution of Metazoa and the significance ofproblematic taxa 189ndash244 Cambridge Cambridge UniversityPress

Dewel R A amp Dewel W C 1997 The place of tardigrades inarthropod evolution In Fortey R A amp Thomas R H (eds)Arthropod Relationships 109ndash23 London Chapman and HallSystematics Association Special Volume 55

Dunlop J A 1997 Palaeozoic arachnids and their significance forarachnid phylogeny Proceedings of the 16th European Colloquiumof Arachnology 65ndash82 Siedlce Poland

Dunlop J A 1998 The origins of tetrapulmonate booklungs and theirsignificance for chelicerate phylogeny In Selden P A (ed)Proceedings of the 17th European Colloquium of ArachnologyEdinburgh 1997 pp 9ndash16

Dunlop J A 1999 Pasando revista a la evolucion de los quelicerados[A review of chelicerate evolution] In Melic A DeHaro J JMedez M amp Ribera I (eds) Volumen Monografico Evolucion yFilogenia de Arthropoda Boletin de la Sociedad EntolomologicaAragonesa 26 255ndash272 [In Spanish and English]

Dunlop J A amp Selden P A 1997 The early history and phylogenyof the chelicerates In Fortey R A amp Thomas R H (eds)Arthropod relationships Systematics Association Special Volume55 221ndash35 London Chapman and Hall

Dunlop J A amp Webster M 1999 Fossil evidence terrestrializationand arachnid phylogeny Journal of Arachnology 27 86ndash93

Dzik J 1993 Early metazoan evolution and the meaning of its fossilrecord Evolutionary Biology 27 339ndash86

Dzik J amp Lendzion K 1988 The oldest arthropods of the EastEuropean Platform Lethaia 21 29ndash38

Edgecombe G D Wilson G D F Colgan D J Gray M R ampCassis G 2000 Arthropod cladistics combined analysisof Histone H3 and U2 snRNA sequences and morphologyCladistics 16 155ndash203

Edgecombe G D amp Ramskold L 1999 Relationships of CambrianArachnata and the systematic position of Trilobita Journal ofPaleontology 73 263ndash87

Emerson M J amp Schram F R 1997 Theories patterns and realitygame plan for arthropod phylogeny In Fortey R A amp Thomas

R H (eds) Arthropod Relationships Systematics AssociationSpecial Volume 55 67ndash87 London Chapman and Hall

Farris J S 1989 The Retention Index and the Rescaled ConsistencyIndex Cladistics 5 417ndash19

Felsenstein J 1985 Confidence limits on phylogenies an approachusing the bootstrap Evolution 39 783ndash91

Flower R H 1968 Merostomes from a Cotter Horizon of the El PasoGroup New Mexico Bureau of Mines and Mineral ResourcesMemoir 22 35ndash44

Fortey R A 1990a Trilobite evolution and systematics In MikulicD G (ed) Arthropod paleobiology 44ndash65 Short Courses inPaleontology 3 Knoxville Tenessee The Paleontological Society

Fortey R A 1990b Ontogeny hypostome attachment and trilobiteclassification Palaeontology 33 529ndash76

Fortey R A 1997 Classification In Kaesler R L (ed) Treatise oninvertebrate paleontology Part O Arthropoda 1 (revised) 289ndash302Boulder Colorado amp Lawrence Kansas Geological Society ofAmerica and University of Kansas Press

Fortey R A amp Theron J N 1994 A new Ordovician arthropodSoomaspis and the agnostid problem Palaeontology 37 841ndash61

Fortey R A amp Whittington H B 1989 The Trilobita as a naturalgroup Historical Biology 2 125ndash38

Fry W G 1978 A classification within the pycnogonids ZoologicalJournal of the Linnean Society of London 63 35ndash58

Giribet G Edgecombe G D amp Wheeler W C 2001 Arthropodphylogeny based on eight molecular loci and morphology Nature413 157ndash61

Gould S J 1989 Wonderful life The Burgess Shale and the nature ofhistory New York NY Norton

Gould S J 1991 The disparity of the Burgess Shale arthropod faunaand the limits of cladistic analysis why we must strive to quantifymorphospace Paleobiology 17 411ndash23

Gurich G 1931 Mimetaster hexagonalis ein neuer Kruster aus demunterdevonischen Bundenbacher Dachschiefer PalaontologischeZeitschrift 12 204ndash38

Hammann W Laske R amp Pillola G L 1990 Tariccoia arrusensisngnsp an unusual trilobite-like arthropod Rediscovery of thelsquophylocaridrsquo beds of Taricco (1922) in the Ordovician lsquoPuddingarsquosequence of Sardinia Bolletino della Societa PaleontologicaItaliana 29 163ndash78

Haas M S Brown S J amp Beeman R W 2001 Pondering theprocephalon the segmental origin of the labrum DevelopmentGenes and Evolution 211 89ndash95

Heider K 1913 Entwicklungsgeschichte und Morphologie derWirbellosen Die Kultur der Gegenwart Theil 3 Abt 4 2 176ndash332

Henriksen K 1928 Critical notes upon Cambrian arthropodsdescribed by Charles D Walcott Videnskabelige Meddelelser fraDansk Naturhistorisk Forening 86 1ndash20

Hesselbo S P 1988 Trace fossils of Cambrian aglaspidid arthropodsLethaia 21 139ndash46

Hesselbo S P 1989 The aglaspidid arthropod Beckwithia from theCambrian of Utah and Wisconsin Journal of Paleontology 63636ndash42

Hesselbo S P 1992 Aglaspidida (Arthropoda) from the UpperCambrian of Wisconsin Journal of Paleontology 66 885ndash923

Hou X 1987a Three new large arthropods from Lower CambrianChengjiang fauna eastern Yunnan Acta Palaeontologica Sinica26 272ndash85 [In Chinese with English summary]

Hou X 1987b Two new arthropods from Lower CambrianChengjiang eastern Yunnan Acta Palaeontologica Sinica 26236ndash56 [In Chinese with English summary]

Hou X Chen J amp Lu H 1989 Early Cambrian new arthropodsfrom Chengjiang Yunnan Acta Palaeontologica Sinica 28 42ndash57[In Chinese with English summary]

Hou X Ramskold L amp Bergstrom J 1991 Composition andpreservation of the Chengjiang fauna a Lower Cambrian soft-bodied biota Zoologica Scripta 20 395ndash411

Hou X Bergstrom J amp Ahlberg P 1995 Anomalocaris and otherlarge animals in the Lower Cambrian Chengjiang fauna ofsouthwest China GFF 117 163ndash83

Hou X amp Bergstrom J 1997 Arthropods of the Lower CambrianChengjiang fauna southwest China Fossils and Strata 45 1ndash116

Hughes C P 1975 Redescription of Burgessia bella from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 4415ndash36

Ivantsov A Y 1999 Trilobite-like arthropod from the LowerCambrian of the Siberian Platform Acta PalaeontologicaPolonica 44 455ndash66

King P E 1973 Pycnogonids London St Martin Press

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 191

Lauterbach K E 1973 Schlussereignisse in der Evolution derStammgruppe der Euarthropoda Zoologische Beitrage 19259ndash99

Lauterbach K E 1980 Schlussereignisse in der Evolution derGrundplans der Arachnata (Arthropoda) Abhandlungen desNaturwissenschaftlichen Vereins in Hamburg 23 163ndash327

Lauterbach K E 1983 Synapomorphien zwischen Trilobiten- undCheliceraten-zweig der Arachnata Zoologischer Anzeiger 210213ndash38

Lendzion K 1975 Fauna of the Mobergella Zone in the Polish LowerCambrian Kwartalnik Geologiczny 19 237ndash42

Lieberman B S 1998 Cladistic analysis of the Early Cambrianolenelloid trilobites Journal of Paleontology 72 59ndash78

Lu Y 1940 On the ontogeny and phylogeny of Redlichia intermediaLu (sp nov) Bulletin of the Geological Society of China 20333ndash42 [In Chinese]

Maddison W P amp Maddison D R 1997 MacClade analysis ofphylogeny and character evolution Version 307 Sunderland MASinauer Associates

Mittmann B amp Scholtz G 2003 Development of the nervous systemin the lsquoheadrsquo of Limulus polyphemus (Chelicerata Xiphosura)morphological evidence for the correspondence between the seg-ments of the chelicerae and the (first) antennae of MandibulataDevelopment Genes and Evolution 213 9ndash17

Moore R C (ed) 1959 Treatise on invertebrate paleontology Part OArthropoda 1 Boulder Colorado Geological Society of America

Muller K J amp Walossek D 1986 Arthropod larvae from the UpperCambrian of Sweden Transactions of the Royal Society ofEdinburgh Earth Sciences 77 157ndash79

Muller K J amp Walossek D 1987 Morphology ontogeny and lifehabit of Agnostus pisiformis from the Upper Cambrian of SwedenFossils and Strata 19 1ndash124

Munilla T 1999 Evolucion y filogenia de los Picnogonidos InMelicA DeHaro J J Medez M amp Ribera I (eds) VolumenMonografico Evolucion y Filogenia de Arthropoda Boletin de laSociedad Entolomologica Aragonesa 26 273ndash91 [In Spanish andEnglish]

Orr P J Siveter D J Briggs D E G Siveter D J amp Sutton M D2000 A new arthropod from the Silurian Konservat-Lagerstatteof Herefordshire Proceedings of the Royal Society of LondonSeries B 267 1497ndash504

Page R D M 1996 On consensus confidence and lsquototal evidencersquoCladistics 12 83ndash92

Palmer A R amp Repina L N 1993 Through a glass darklytaxonomy phylogeny and biostratigraphy of the OlenellinaUniversity of Kansas Paleontological Contributions New Series 31ndash35

Platnick N I Griswold C E amp Coddington J A 1991 On missingentries in cladistic analysis Cladistics 7 337ndash43

Pleijel F 1995 On character coding for phylogeny reconstructionCladistics 11 309ndash15

Raasch G O 1939 Cambrian Merostomata Special Papers of theGeological Society of America 19 1ndash146

Ramskold L Chen J Edgecombe G D amp Zhou G 1996Preservational folds simulating tergite junctions in tegopeltid andnaraoiid arthropods Lethaia 29 15ndash20

Ramskold L Chen J Edgecombe G D amp Zhou G 1997Cindarella and the arachnate clade Xandarellida (ArthropodaEarly Cambrian) from China Transactions of the Royal Society ofEdinburgh Earth Sciences 88 19ndash38

Ramskold L amp Edgecombe G D 1991 Trilobite monophylyrevisited Historical Biology 4 267ndash83

Ramskold L amp Edgecombe G D 1996 Trilobite appendagestructure ndash Eoredlichia revisited Alcheringa 20 269ndash76

Raw F 1957 Origin of chelicerates Journal of Paleontology 31139ndash193

Repina L N amp Okuneva O G 1969 Cambrian arthropods of themaritime territory Paleontological Journal 1969 95ndash103

Richter R amp Richter E 1929 Weinbergina opitz ng n sp einSchwerttrager (Merost Xiphos) aus dem Devon (Rheinland)Senkenbergiana 11 193ndash209

Robison R A 1984 New occurences of the unusual trilobite Naraoiafrom the Cambrian of Idaho and Utah University of KansasPaleontological Contributions Paper 112 1ndash8

Rominger C 1887 Description of primordial fossils from MtStephens NW Territory of Canada Proceedings of the Academyof Natural Sciences of Philadelphia 1887 12ndash19

Scholtz G 1997 Cleavage germ band formation and head segmen-tation the ground pattern of the Euarthropoda In Fortey R Aamp Thomas R H (eds) Arthropod Relationships 317ndash32 LondonChapman and Hall Systematics Association Special Volume 55

Schram F R 1978 Arthropods a convergent phenomenonFieldiana Geology 39 61ndash108

Schram F R 1993 Review of Boxshall Stromberg and Dahl TheCrustacea Origin and Evolution Journal of Crustacean Biology13 820ndash2

Schram F R Vonk R amp Hof C H J 1997 Mazon CreekCycloidea Journal of Paleontology 71 261ndash84

Schram F R amp Emerson M J 1991 Arthropod Pattern Theorya new approach to arthropod phylogeny Memoirs of theQueensland Museum 31 1ndash18

Schram F R amp Hof C H J 1998 Fossils and the interrelationshipsof major crustacean groups In Edgecombe G D (ed) Arthropodfossils and phylogeny pp 233ndash302 New York NY ColumbiaUniversity Press

Shergold J H 1991 Protaspis and early meraspis growth stages of theeodiscoid trilobite Pagetia ocellata Jell and their implications forclassification Alcheringa 15 65ndash86

Shu D Geyer G Ling C amp Zhang Z 1995 Redlichiacean trilobiteswith preserved soft parts from the Lower Cambrian Chengjiangfauna (South China) Beringeria (Special Issue) 2 203ndash41

Shultz J W 1990 Evolutionary morphology and phylogeny ofArachnida Cladistics 6 1ndash38

Shultz J W 2000 Skeletomuscular anatomy of the harvestmanLeiobunum aldrichi (Weed 1893) (Arachnida OpilionesPalpatores) and its evolutionary significance Zoological Journalof the Linnean Society 128 401ndash38

Shultz J W 2001 Gross muscular anatomy of Limulus polyphemus(Xiphosura Chelicerata) and its bearing on evolution in the theArachnida Journal of Arachnology 29 283ndash303

Siewing R (ed) 1985 Lehrbuch der Zoologie II Systematik 3rd ednStuttgart Fischer

Simonetta A M 1970 Studies on the non trilobite arthropods ofthe Burgess Shale (Middle Cambrian) the genera LeanchoiliaAlalcomenaeus Opabinia Burgessia Yohoia and ActaeusPalaeontographica Italica 66 35ndash45

Simonetta A M amp Delle Cave L 1975 The Cambrian non trilobitearthropods from the Burgess Shale of British Columbia A studyof their comparative morphology taxonomy and evolutionarysignificance Palaeontographica Italica 69 1ndash37

Simonetta A M amp Delle Cave L 1981 An essay in the compara-tive and evolutionary morphology of Palaeozoic ArthropodsAccademia Nazionale dei Lincei Atti dei Convegni Lincei 49389ndash439

Starobogatov Y I 1990 Sistema i filogeniya nizshikh khelitserovykh(analiz morfologii paleozoysikh grupp) [The systematics andphylogeny of the lower chelicerates (a morphological analysis ofthe Palaeozoic groups)] Paleontologicheskii Zhurnal 1990 4ndash17

Stoslashrmer L 1944 On the relationships and phylogeny of fossil andrecent Arachnomorpha Skrifter utgitt ac Det Norske Videnskaps-Akademi i Olso I Matematisk-naturvitenskaplig Klasse 5 1ndash158

Stoslashrmer L 1951 Studies on trilobite morphology Part III Theventral cephalic structures with remarks on the zoologicalposition of the trilobites Norsk geologisk tidsskrift 29 108ndash58

Stoslashrmer L 1956 A Lower Cambrian merostome from Sweden Arkivfor Zoologi Ser 2 9 507ndash14

Stoslashrmer L 1959 Trilobitoidea In Moore R C (ed) Treatise onInvertebrate Paleontology part O Arthropoda 1 23ndash37 BoulderColorado Geological Society of America

Strong E E amp Lipscomb D 1999 Character coding and inapplicabledata Cladistics 15 363ndash71

Sturmer W amp Bergstrom J 1976 The arthropods Mimetaster andVachonisia from the Devonian Hunsruck Shale PalaontologischeZeitschrift 50 78ndash111

Sturmer W amp Bergstrom J 1978 The arthropod Cheloniellon fromthe Devonian Hunsruck Shale Palaontologische Zeitschrift 5257ndash81

Sturmer W amp Bergstrom J 1981 Weinbergina a xiphosuran fromthe Devonian Hunsruck Slate Palaontologische Zeitschrift 55237ndash55

Sutton M D Briggs D E G Siveter D J Siveter D J amp Orr PJ 2002 The arthropod Offacolus kingi (Chelicerata) from theSilurian of Herefordshire England computer based morphologi-cal reconstructions and phylogenetic affinity Proceedings of theRoyal Society of London Series B 269 1195ndash1203

Swofford D L 1999 PAUP Phylogenetic Analysis Using Parsimony(and other methods) Version 40b2a Sunderland MA SinauerAssociates

Telford M J amp Thomas R H 1998 Expression of homeobox genesshows chelicerate arthropods retain their deutocerebral segmentProceedings of the National Academy of Sciences USA 9510671ndash5

192 TREVOR J COTTON AND SIMON J BRADDY

Thorley J L amp Page R D M 2000 RadCon phylogenetic treecomparison and consensus Bioinformatics 16 486ndash7

Valentine J W 1995 Why no new phyla after the Cambrian ndashgenome and ecospace hypotheses revisited Palaios 10 190ndash194

Wagele J W 1993 Rejection of the lsquoUniramiarsquo hypothesis andimplications of the Mandibulata concept Zoologische JahrbucherAbteilung fur Systematik Geographie und Biologie der Tiere 120253ndash88

Walcott C D 1911 Middle Cambrian Merostomata Cambriangeology and palaeontology II Smithsonian MiscellaneousCollections 57 17ndash40

Walcott C D 1912 Middle Cambrian Branchiopoda MalacostracaTrilobita and Merostomata Cambrian geology and palaeontol-ogy II Smithsonian Miscellaneous Collections 57 145ndash228

Walcott C D 1918 Geological explorations in the CanadianRockies Explorations and fieldwork of the SmithsonianInstitution in 1917 Smithsonian Miscellaneous Collections 684ndash20

Walossek D 1993 The Upper Cambrian Rehbachiella kinnekullensisand the phylogeny of Branchiopoda and Crustacea Fossils andStrata 32 1ndash202

Walossek D amp Dunlop J A 2002 A larval sea spider (ArthropodaPycnogonida) from the Upper Cambrian lsquoOrstenrsquo of Swedenand the phylogenetic position of pycnogonids Palaeontology 45421ndash46

Walossek D amp Muller K J 1990 Upper Cambrian stem-lineagecrustaceans and their bearing upon the monophyletic origin ofCrustacea and the position of Agnostus Lethaia 23 409ndash27

Walossek D amp Muller K J 1997 Cambrian lsquoOrstenrsquo-type arthro-pods and the phylogeny of Crustacea In Fortey R A amp ThomasR H (eds) Arthropod Relationships 139ndash53 SystematicsAssociation Special Volume 55 London Chapman and Hall

Walossek D amp Muller K J 1998 Early arthropod phylogeny in lightof the Cambrian lsquoOrstenrsquo fossils In Edgecombe G D (ed)Arthropod fossils and phylogeny 185ndash231 New York NYColumbia University Press

Weygoldt P 1979 Significance of later embryonic stages and headdevelopment in arthropod phylogeny In Gupta A P (ed)Arthropod Phylogeny 107ndash35 New York NY Van NostrandReinhold Co

Weygoldt P 1998 Evolution and systematics of the ChelicerataExperimental and Applied Acarology 22 63ndash79

Weygoldt P amp Paulus H F 1979 Unterzuchungen aur Morpholo-gie Taxonomie und Phylogenie der Chelicerata Zeitschriftfur Zoologische Systematik und Evolutionsforschung 17 85ndash116177ndash200

Wheeler W C Cartwright P amp Hayashi C Y 1993 Arthropodphylogeny a combined approach Cladistics 9 1ndash39

Wheeler W C amp Hayashi C Y 1998 The phylogeny of extantchelicerate orders Cladistics 14 173ndash92

Whittington H B 1971 Redescription of Marrella splendens(Trilobitoidea) from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 209 1ndash24

Whittington H B 1974 Yohoia Walcott and Plenocaris n genarthropods from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 231 1ndash21

Whittington H B 1975a Trilobites with appendages from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 497ndash136

Whittington H B 1975b The enigmatic animal Opabinia regalisMiddle Cambrian Burgess Shale British Columbia PhilosophicalTransactions of the Royal Society of London Series B 271 1ndash43

Whittington H B 1977 The Middle Cambrian trilobite NaraoiaBurgess Shale British Columbia Philosophical Transactions of theRoyal Society Series B 280 409ndash43

Whittington H B 1979 Early arthropods their appendages andrelationships In House M R (ed) The origin of majorinvertebrate groups 253ndash68 Systematics Association SpecialVolume 12 New York NY Academic Press

Whittington H B 1980 Exoskeleton moult stage appendage mor-phology and habits of the Middle Cambrian trilobite Olenoidesserratus Palaeontology 23 171ndash204

Whittington H B 1981 Rare arthropods from the Burgess ShaleMiddle Cambrian British Columbia Philosophical Transactionsof the Royal Society of London Series B 292 329ndash57

Whittington H B 1985 Tegopelte gigas a second soft-bodied trilo-bite from the Burgess Shale Middle Cambrian British ColumbiaJournal of Palaeontology 23 171ndash204

Whittington H B 1989 Olenelloid trilobites type species functionalmorphology and higher classification Philosophical Transactionsof the Royal Society of London Series B 324 111ndash47

Whittington H B amp Almond J E 1987 Appendages and habits ofthe Upper Ordovician trilobite Triarthus eatoni PhilosophicalTransactions of the Royal Society of London Series B 317 1ndash46

Wilkinson M 1992 Ordered versus unordered characters Cladistics8 375ndash85

Wilkinson M 1994 Common cladistic information and its consensusrepresentation reduced Adams and reduced cladistic consensustrees and profiles Systematic Biology 43 343ndash68

Wilkinson M 1995a Coping with abundant missing entries in phylo-genetic inference using parsimony Systematic Biology 44 501ndash14

Wilkinson M 1995b TAXEQ2 software and documentation BristolSchool of Biological Sciences University of Bristol

Wilkinson M 1996 On the distribution of homoplasy and choosingamong trees Taxon 45 263ndash66

Wills M A 1997 A phylogeny of Recent and fossil Crustacea derivedfrom morphological characters In Fortey R A amp ThomasR H (eds) Arthropod Relationships 189ndash209 SystematicsAssociation Special Volume 55 London Chapman and Hall

Wills M A Briggs D E G amp Fortey R A 1994 Disparity asan evolutionary index a comparison of Cambrian and Recentarthropods Paleobiology 20 93ndash130

Wills M A Briggs D E G Fortey R A amp Wilkinson M 1995The significance of fossils in understanding arthropod evolutionVerhandlungen der deutschen zoologischen Gesselschaft 88 203ndash15

Wills M A Briggs D E G Fortey R A Wilkinson M amp SneathP H A 1998a An arthropod phylogeny based on fossil andrecent taxa In Edgecombe G D (ed) Arthropod fossils andphylogeny 33ndash105 New York NY Columbia University Press

Wills M A Briggs D E G amp Fortey R A 1998b Evolutionarycorrelates of arthropod tagmosis scrambled legs In ForteyR A amp Thomas R H (eds) Arthropod Relationships 57ndash65Systematics Association Special Volume 55 London Chapmanand Hall

Winter G 1980 Beitrage zur Morphologie und Embryologie desvorderen Korperabschnitts (Cephalosoma) der PantopodaGerstaecker 1863 I Entstehung und Struktur des Zentralnerven-systems Zeitschrift fur Zoologische Systematik und Evolutionsfor-schung 18 27ndash61

Zhang W amp Hou X 1985 Preliminary notes on the occurrence of theunusual trilobite Naraoia in Asia Acta Palaeontologica Sinica 24591ndash5 [In Chinese with English summary]

TREVOR J COTTON Department of Palaeontology The Natural History Museum CromwellRoad London SW7 5BD UKe-mail trevor_cottonhotmailcom

SIMON J BRADDY Department of Earth Sciences University of Bristol Wills MemorialBuilding Queenrsquos Road Bristol BS8 1RJ UKe-mail S J Braddybrisacuk

MS received 28 July 2003 Accepted for publication 15 March 2004

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 193

Page 10: The phylogeny of arachnomorph arthropods and the origin of the

(none for Sidneyia one for Marrella and six for Emeraldella)since these will become informative if the character is treatedas ordered

Edgecombe amp Ramskold (1999 p 265) described a novelform of cephalic tagmosis The present authors tentativelyaccept their suggestion that in trilobite-like taxa the fourthpair of biramous cephalic appendages was directly under thecephalo-thoracic junction This may explain previous confu-sion about the number of such appendages incorporated intothe trilobite cephalon (eg Cisne 1975 Whittington 1975aBergstrom amp Brassel 1984) However Edgecome amp Ramskoldaccept the view of Chen et al (1977 p 7) that only the firstthree biramous limbs of Misszhouia were structurally and

functionally part of the head Therefore it seems more accept-able to code these taxa as having only four post-acronalsomites than to use the coding scheme of Edgecombe ampRamskold (1999) The partial integration of the first thoracicsomite under the head shield could be considered to be a formof overlap of the trunk by the head shield (ie a distinct stateof character 9 of Edgecombe amp Ramskold 1999 or Character37 herein) Derived states of this character would then poten-tially be synapomorphic for a clade including xandarellidsnaraoiids helmetiids tegopeltids and trilobites Howeversince the degree of overlap in taxa with the fourth biramousappendages under the cephalo-thoracic articulation is identicalto that found between thoracic tergites (State 0 of Character

Figure 5 Diagrammatic reconstructions of raptorial second-segment appendages in lateral view (except whereotherwise stated) and approximate life orientation showing suggested homology of appendage elements (inbrackets) Roman numerals indicate position in the podomere series and do not necessarily imply homology (A)lsquoGreat appendagersquo of Fortiforceps (after Hou amp Bergstrom 1997) (B) Left chelicera of Leiobunum aldrichi(Arachnida Opiliones) (1) in medial perspective and (2) distal parts in anterior perspective (after Shultz 2000)(C) Chelifore of Nymphon (Pycnogonida) (after Child 1997) (D) lsquoGreat appendagersquo of Yohoia (after Whittington1974 fig 2) (E) lsquoGreat appendagersquo of Leanchoilia (modified after Bruton amp Whittington 1983) Not to scaleAbbreviations (apt) apotele (probably homologous with the mandibulate pretarsus) (cx) coxa (dtmrt)deutomerite and (pt) pretarsus

178 TREVOR J COTTON AND SIMON J BRADDY

37) it is here preferred to regard this as a distinct character(Character 9 herein)

The coding of Alalcomenaeus by Wills et al (1998a) ashaving four post-acronal somites is accepted here but fordifferent reasons Briggs amp Collins (1999) have recenty demon-strated that Alalcomenaeus possessed two pairs of biramousappendages on the head in addition to the great appendagesThis would equate to three head somites according to thehomology scheme of Wills et al (1998a) Here the antennaeare inferred to be lost and therefore four segments incorpo-rated into the head

Recently a number of authors have agreed that the plesio-morphic condition for euarthropods is a four-segmented headas found in stem group crustaceans (Scholtz 1997 Walossek1993 Walossek amp Muller 1997 1998 Edgecombe et al 2000)However reconstruction of the euarthropod stem groupclearly indicates that even crownward taxa had a head consist-ing of only the antennal segment and acron as found intardigrades and onychophorans Therefore it seems that thefour-segmented head is pleisomorphic only for crown groupeuarthropods Some fully arthropodised fossil taxa also have ashorter head that may be pleisomorphic such as the two-segmented head of Marella Retifacies is coded following therecent redescription of Hou amp Bergstrom (1997) rather thanon the basis of the coding by Edgecombe amp Ramskold (1999)for which they provide no explanation A partial uncertaintycoding is used for Aglaspis in which the number of post-antennal cephalic appendages is either three or four (Briggset al 1979)

5 Orientation of the antennae (0) directed anterolaterally(1) strongly deflected laterally and (2) placed well inside shieldmargin curing posteriorly from a transverse proximal element(cf Edgecombe amp Ramskold 1999 character 3) The anteriorappendages of Aglaspis (see character 1) are clearly directedanterolaterally and it is presumed that they continued past thehead shield margin The anterior appendages of arthropodstem group taxa such as Aysheaia Kerygmachela and Anoma-locaris are either directed anterolaterally or ventrally but arenot strongly-deflected laterally Therefore the outgroup iscoded as State 0 This character is coded as missing data fortaxa where the presence of antennae is equivocal (those codedas missing data for Character 1) and as inapplicable in taxawhere the antennae are considered absent (coded as State 1 forCharacter 1)

6 Length of distal spines on terminal podomeres of endo-pods of second segment appendages (0) absent or shorer thanpodomeres (1) subsequal to length of podomeres and (2)longer than the entire podomere series The spinose projectionsof the raptorial appendages described above vary in length Inchelicerae (Figs 5BC) and some lsquogreat appendagesrsquo (eg thoseof Yohoia Fig 5D) the most distal spine is equal in length tothe spinose terminal podomere forming a chelate structureIn Fortiforceps (Fig 5A) the spines are short relative tothe lengths of the podomeres but in Alalcomenaeus andLeanchoilia (Fig 5E) they are extremely long so that thespines are much longer than the entire podomere series Asdescribed above in taxa with pediform endopods of the secondsegment appendages spines on the distal podomeres are shortor entirely absent

7 Chelicerae (0) absent and (1) present Despite theirsimilarities to lsquogreat appendagesrsquo the chelicerae of the eucheli-cerates and chelifores of the pycnogonids are clearly distinctand have been widely recognised as a chelicerate synapomor-phy Chelicerae differ from any lsquogreat appendagesrsquo by thecombination of a smaller number of podomeres the presenceof only a single terminal element and only a single spinoseprojection on the dorsal side of the podomere series (Fig 5)

Amongst extant chelicerates only the pycnogonid Pallenopsishas chelicerae of four podomeres comparable to the numberin lsquogreat appendagesrsquo Bergstrom et al (1980) considered thatthe chelifores of the Devonian pycnogonid Palaeoisopus alsoconsist of four segments but this is not well supported by theirfigures

8 Distal spines of second segment endopods terminating inannulated flagellae (0) absent and (1) present It is widelyrecognised (eg Stoslashrmer 1944 Simonetta 1970 Bruton ampWhittington 1993 Briggs amp Collins 1999) that the terminalparts of the extended spines of Alalcomenaeus and Leanchoiliaformed annulated flagellae Nothing similar is known fromhomologous appendages of any other arthropod althoughsimilar processes may have operated in the transformation ofthe endopod as a whole into the second antennae of derivedcrustaceans and in the origin of multiramous antennulae (firstantennae) in malacostracans

242 Posterior appendages 9 Appendages of first thoracicsomite underneath the cephalo-thoracic articulation (0) ab-sent and (1) present (cf Edgecombe amp Ramskold 1999character 2) For a discussion of this character see Character4 herein

10 Exopods of appendages of third to fifth segments (0)present and (1) reduced or absent A number of taxa haveuniramous appendages on the third to fifth segments In mostthese segments are incorporated into the head (Yohoia cheli-cerates) but in others all (eg in Sidneyia) or some (eg inCheloniellon) of these segments are post-cephalic In all casesthe appendage is morphologically similar to the endopods ofbiramous limbs of other taxa andor other segments and it isinterpreted that the exopod is lost

11 Endopods of thoracic appendages (0) present and (1)reduced or absent The opisthosomal appendages of chelicer-ates lack endopods or have the endopods much reduced (egLimulus Siewing 1985 fig 838 Shultz 2001) a situation that isalso found in the uniramous thoracic appendages of Yohoia(Whittington 1974) It has been suggested that Helmetia(Briggs et al 1994) lacks endopods but pending a redescrip-tion of this genus and considering that they have been docu-mented in the otherwise very similar Kuamaia this is coded asuncertain

12 Exopod shaft of numerous podomeres each bearing asingle seta (0) present and (1) absent The exopods ofcrustaceans (at least plesiomorphically see eg Walossek ampMuller 1998 figs 5middot5 amp 5middot6) and of Marrella and Mimetasterhave multi-annulated shafts with each podomere bearing asingle seta In other taxa the exopod shafts consist of one oftwo lobes bearing numerous setae

The polarity of exopod segmentation is uncertain Thelateral flaps of stem group arthropods such as Opabinia(Whittington 1975b Budd 1996b) and anomalocaridids (egHou et al 1995) are certainly unsegmented but as suggestedby Buddrsquos (1996b fig 8) reconstruction this does notnecessarily suggest the form of the primitive euarthropodexopod Rather segmentation may have been a result of thesclerotization of the cuticle of the exopod shaft The outgroupwas coded as uncertain for this character According to thefirst interpretation of aglaspidid appendage morphology(coded as Aglaspidida 1) the exopods are lacking on allappendages and consequently this and all other charactersdescribing exopod morphology are coded as inapplicable

13 Exopod shaft differentiated into proximal and distallobes (0) absent and (1) present Ramskold amp Edgecombe(1996 Edgecombe amp Ramskold 1999 character 26) havediscussed the distribution of the trilobite-type bilobate exo-pods recognised by this character (Fig 6A) Among taxaincluded here that were not considered by Edgecombe amp

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 179

Ramskold (1999) exopods consist either of a single flattenedlobe or a homonomous series of podomeres (Fig 6B C) Nostem group euarthropod has a bilobate exopod and theoutgroup is consequently also coded as State 0

14 Proximal lobe of exopod (0) flattened lobe and (1)slender shaft

15 Distal lobe of exopod (0) small to moderate sized flapwith short to moderately long attachment to proximal lobeand (1) large teardrop-shaped with long attachment to proxi-mal lobe Neither of these characters can be coded for taxathat do not have an exopod differentiated into proximal anddistal lobes (Character 13 State 1) without asserting thehomology of the single-lobed exopod with one of these partsThe coding of Edgecombe amp Ramskold (1999 p 280) implic-itly homologizes the exopods of Sidneyia and Retifacies withthe proximal lobe In these taxa this view is perhaps supportedby the presence of lamellate exopod setae which match thoseof the trilobite-type proximal lobe However in other taxawith a single-lobed exopod such as Alalcomenaeus (see Briggsamp Collins 1999) the exopod is fringed with sharp spines likethe distal lobe of differentiated exopods Alternatively thebilobate exopod may have originated from the division of aprimitively single-lobed structure The single-lobed exopod isnot clearly homologous with either lobe of divided exopodsand both these characters are consequently coded as inappli-cable to taxa without differentiated proximal and distal exopodlobes Consequently (see Character 13 herein) these characterscan only be coded for taxa that were previously considered byEdgecombe amp Ramskold (1999) and their coding is followedhere except in the cases of Sidneyia and Retifacies

16 Exopod shaft is a deep rounded flap (0) absent and (1)present All bilobate (Character 13) and segmented (Character12) exopod shafts are long and relatively narrow structures (seeFig 6) whereas some single-lobed exopod shafts have theform of rounded flaps These flap-like exopod shafts are largecompared to the length of the setae and at least half as deep asthey are long This appendage structure is found in cheliceratesand lsquogreat appendagersquo arthropods

17 Medially-directed exopod setae (0) absent and (1)present Walossek amp Muller (1998 p 194) suggested that thetilting of exopod setae towards the endopod in post-antennularlimbs with a multi-annulated exopod was an autapomorphyuniting crustaceans and all members of the crustacean stemgroup (the crustacean total group sensu Ax 1986 see Fig 6C)This character is shared with the marellomorphs Marrellaand Mimetaster (see Fig 6B Sturmer amp Bergstrom 1976Bergstrom 1979 fig 1middot3A B) In other taxa the setae eithersurround the entire margin of the exopod shaft or are directeddorsally (Fig 6A) The identification of setae on the dorsalsurface of the lateral flaps in Opabinia (Budd 1996b) suggeststhat the condition in marellomorphs and crustaceans isderived

18 Lamellate exopod setae (0) absent and (1) presentLamellate exopod setae originally described from trilobitesare a classic synapomorphy of the Arachnomorpha (see egBergstrom 1992 Hou amp Bergstrom 1997 pp 42ndash3) They areperhaps best known from Misszhouia (see Chen et al 1996Hou amp Bergstrom 1997 Fig 6A) These setae differ from themore spinose setae of other taxa (Fig 6C) in that they are wideand flat and imbricate over the length of the exopod shaft Thesetae of Marrella (Fig 6B) and similar taxa have variouslybeen considered lamellate (Bergstrom 1979) or non-lamellate(Bergstrom 1992) Since they do not imbricate and do notseem to be strongly flattened (Whittington 1971 Sturmer ampBergstrom 1976 fig 9a) Marrella and Mimetaster are codedas State 0 The setae of Helmetia as shown in Briggs et al(1994 fig 141) seem to be of the lamellate type The setae ofSinoburius have been described by Hou amp Bergstrom (1997p 85) as similar to those of Misszhouia Only the setae areknown of the appendages of Buenaspis (Budd 1999a) andthese also appear to be of the trilobite-type

Figure 6 Diagrammatic reconstructions of (A) arachnomorph and(B C) non-arachnomorph biramous appendages (A) The lsquotrilobite-typersquo biramous appendage of Misszhouia longicauda (after Chen et al1997) (B) Appendage of Marrella splendens (after Whittington 1971)(C) Appendage of the stem-lineage crustacean Martinssonia elongata(after Muller amp Walossek 1997 1998) Not to scale Abbreviations(bas) basis (dle) distal lobe of exopod shaft (ls) lamellar exopod setae(pe) proximal endite or pre-coxa (ple) proximal lobe of exopod shaft(ses) segmented exopod shaft and (ss) spinose exopod setae

180 TREVOR J COTTON AND SIMON J BRADDY

The homology of the book-gill lamellae of xiphosurans withlamellar setae was supported by Walossek amp Muller (1997p 149) and Edgecombe et al (2000 p 174) but rejected bySturmer amp Berstrom (1981) on the grounds that Weinberginapossesses Limulus-like gill lamellae and fringing setaeBook-gills have also been described from a eurypterid (Braddyet al 1999) There are certainly major morphologicaldifferences between book-gills and trilobite-type exopodsFollowing most recent opinion and pending further studyWeinbergina and Eurypterida are coded as possessing lamellatesetae

The form of the setae of Yohoia is uncertain (Whittington1974) the spinose setae seen in the most common reconstruc-tion (Gould 1989 Briggs et al 1994) are not justified by thespecimens Amongst other great appendage arthropods theform of the setae appears to vary those of Jianfengia (seeChen amp Zhou 1997 p 74) and Leanchoilia (see Bruton ampWhittington 1983) are lamellate while those of Fortiforceps(Hou amp Bergstrom 1997) and Alalcomenaeus (Briggs amp Collins1999) are spinose and less densely packed

19 Gnathobase on basis andor prominent endites onendopod (0) present and (1) absent (cf Edgecombe ampRamskold 1999 character 29 Wills et al 1998a characters 36amp 59) The presence of gnathobases on the appendages of someanomalocaridids (Hou et al 1995) and their general distribu-tion amongst non-arachnomorph euarthropods suggests thatState 0 is plesiomorphic The homology of the various enditesand gnathobasic structures found in euarthropods is in need ofcareful assessment and therefore a very general coding (fol-lowing Edgecombe amp Ramskold) is used for this character

243 Eyes 20 Position of lateral facetted eyes (0)ventral and stalked (1) dorsal and sessile and (2) absent (cfEdgecombe amp Ramskold 1999 character 4) Partial uncer-tainty coding is used for a number of taxa where the dorsalsurface is known but the ventral morphology is not andtherefore the presence of ventral eyes cannot be discountedFor example there is good evidence that Leanchoilia lackeddorsal sessile eyes but ventral stalked eyes may have beenpresent (as in L hanceyi see Briggs amp Robison 1984) and a(02) partial uncertainty coding is used This is also the case inmany naraoiids such as Buenaspis However only the outlineof the head shield of Liwia is known and the absence of dorsaleyes in the reconstruction of Dzik amp Lendzion (1988) isconjectural It is unclear whether the autapomorphic conditionof stalked dorsal eyes in Mimetaster (see Sturmer amp Bergstrom1976) is homologous with State 0 or State 1 and a partialuncertainty coding is also used

Whittington (1974) was somewhat equivocal about thenature of the lateral lobes anterior to the head shield ofYohoia These more closely resemble the ventral stalked eyes ofAlacomenaeus as recently described by Briggs amp Collins(1999) than either Whittingtonrsquos (1974) or subsequent (egGould 1989 fig 3middot18 Briggs et al 1994 fig 153) reconstruc-tions suggest In a well preserved specimen showing the dorsalaspect (USNM 57696 Whittington 1974 pl 2 figs 1ndash3) and ina laterally compressed specimen (USNM 57694 Whittington1974 pl 1 fig 1) they are clearly seen to be stalked andrelatively small lobate structures That these structures arelikely to represent stalked ventral eyes was reflected in thecoding of Wills et al (1998a characters 26 amp 27)

21 Visual surface with calcified lenses bounded withcircumocular suture (0) absent and (1) present

22 Dorsal bulge in exoskeleton accommodating drop-shaped ventral eyes (0) absent and (1) present

23 Eye slits (0) absent and (1) presentCharacters 21 to 23 are used exactly as described by

Edgecombe amp Ramskold (1999 character 5) Character 21 is

inapplicable to taxa that do not possess eyes (Character 20State 2)

24 Dorsal median eyes (0) absent and (1) present Dorsalmedian eyes on a tubercle are considered a synapomorphy ofthe Chelicerata (Dunlop amp Selden 1997 Dunlop 1999) Thenature and position of the structures interpreted as dorsalmedian eyes in Mimetaster (Sturmer amp Bergstrom 1976p 83ndash4) are regarded as equivocal

244 Ventral cephalic structures Many characters of theventral surface of the euarthropod head (see Fig 7) of poten-tial phylogenetic utility are poorly known in many importanttaxa In particular the presence of pre-hypostomal frontalorgans (Fig 7A CndashE) is not coded herein (see Edgecombe ampRamskold 1999 p 272) Definitive evidence of the absence ofthese structures is available for very few taxa and the homol-ogy of these structures with the maculae of the trilobitehypostome (see Fig 7B) and the frontal organs of the crusta-cean labrum (eg see Muller amp Walossek 1987 p 39) isuncertain Secondly the detailed homology of the trilobitehypostome with that of other putative arachnomorphs isunclear and consequently a very broad definition is usedherein For example various pre-hypostomal sclerites (Fig7A D) in other arachnomorphs may have been incorporatedalong with a primitive hypostome into a single sclerite that isrecognised as the hypostome in trilobites The structure of thehypostome (eg the presence of paired anterior wings dorsal tothe antennae Fig 7B C) may also be a source of additionalcharacters

25 Expanded cephalic doublure (0) absent and (1)present maximum width more than 30 length of head shieldor more than 25 width of pygidium Chen et al (1996)suggested that the wide doublure of Soomaspis and Tariccoia isa synapomorphy uniting these taxa The doublure of Buenaspis(see Budd 1999a) is poorly known but based on the width ofthe heavily crushed region of the cephalic margin and theposition of a possible impression of the edge of the doublure inone specimen (Budd 1999a pl 1 fig 1) it also appears to berather wide (ie greater than 30 of the length of the headshield) Following Edgecombe amp Ramskoldrsquos coding ofSidneyia the present authors do not consider the postero-median expansion of the doublure of for example Emeraldellaor Retifacies (see Bruton amp Whittington 1983 Hou ampBergstrom 1999 respectively) to be homologous with State 1but to represent the hypostome (see Character 26)

26 Anteromedian margin of cephalon notched accomo-dating strongly sclerotised plate (0) notch and plate absentand (1) notch and plate present (cf Edgecombe amp Rasmkold1999 character 10) The apomorphic state (illustrated inFig 7D) is only known in the taxa discussed by Edgecombe ampRasmkold (1999) Reconstructions of Yohoia show a roundedlobe between the eyes (see Character 20) anterior to anddistinct from the head shield which resembles the anteriorsclerite recognised here This seems to be a misinterpretationSpecimens preserved in dorsal aspect (USNM 57696Whittington 1974 pl 2 figs 1ndash3) and lateral aspect (USNM57694 Whittington 1974 pl 1 fig 1 USNM 155616Whittington 1974 pl 5 fig 2) seem to clearly show that thislsquomedian lobersquo is a downward curving pointed extension of thehead shield

27 Hypostomal sclerite (0) median extension of thedoublure with no suture (1) natant sclerite not in contactwith doublure (2) with narrow overlap with pre-hypostomalsclerite (3) narrow attachment to doublure at hypostomalsuture and (4) absent The homology of hypostomes andlabrums has been discussed by Haas et al (2001) and Budd(2002) The euarthropod labrum is likely to represent a pos-teroventral extension of the pre-segmental acron (Scholtz

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 181

Figure 7 Ventral reconstructions of arachnomorph arthropod heads (A) Misszhouia longicaudata (after Chenet al 1997 figs 2ndash4 7) (B) Ceraurinella typa (after Chen et al 1997 fig 7) (C) Agnostus pisiformis (after Mulleramp Walossek 1987) (D) Kuamaia lata (after Edgecombe amp Ramskold 1999 figs 32 5 amp 6) (E) Cindarella eucala(after Ramskold et al 1997) and (F) Emeraldella brocki (after Bruton amp Whittington 1983) Not to scaleAbbreviations (aw) anterior wing beneath antennae (bs) boomerang-shaped sclerite (bl) blade-like process ofposterior wing (d) doublure of head-shield (cs) connective suture (f) fenestra (fs) facial suture (h) hypostome(hl) hypostomal lobe (emerging through fenestrae of Agnostus) (hs) hypostomal suture (if) intervening field ofhypostomal complex (lb) lateral bridge (lfo) lateral frontal organ (mb) median body (mbr) median bridge (mf)median furrow (mfb) mouth field bridge (mfo) median frontal organ (ol) ovate lobe and (phs) pre-hypostomalsclerite Post-antennal appendages and distal parts of antennae omitted

182 TREVOR J COTTON AND SIMON J BRADDY

1997) This structure is often sclerotised and covers the mouthventrally It is this sclerite that is here identified as thehypostome Therefore this character is considered absent intaxa lacking a sclerite covering the mouth irrespective of themorphology and position of the labrum itself which at leastpartly covers the mouth in all euarthropods other thanpycnogonids (see Edgecombe et al 2000 p 167) The lsquofleshylabrumrsquo of crustaceans does not appear to be sclerotised and ahypostome is consequently considered to be absent Althoughmany derived features are associated with the crustaceanlabrum this is not a good reason to consider it non-homologous with the hypostome-bearing structure of otherarthropods as Walossek amp Muller (1990) argued

In many taxa the mouth is covered by a posteromedianextension of the doublure here considered to represent thehypostome (eg Emeraldella Fig 7F Aglaspis see Hesselbo1992 fig 52) In others the hypostome is separated from thedoublure either by a suture a pre-hypostomal sclerite (egKuamaia lata Fig 7D Saperion glumaceum see Edgecombe ampRamskold 1999 figs 3 amp 4) or an intervening region ofunsclerotised cuticle (the natant condition of Fortey 1990beg Cindarella eucala Fig 7E) The homology of pre-hypostomal sclerites and hypostomes in helmetiids trilobitesand naraoiids (see Fig 7) has been discussed by Chen et al(1996) and Edgecombe amp Ramskold (1999) but it is as yetunclear whether any of the character states described here arederived from any other They are treated here as distinct andthe character as unordered pending further study

No hypostome has been identified in Yohoia (see above)Alalcomenaeus Jianfengia Fortiforceps or Leanchoilia andthere is certainly no broad posterior extension of the doublurein any of these taxa Since there is also no pre-hypostomalsclerite they have received a (134) partial uncertainty codingThe attachment of the hypostome of Tegopelte is unclear butit does not seem to be attached to any doublure (Whittington1985) and therefore it is given an uncertainty (12) coding

The lsquorostrum-like structurersquo on the ventral surface ofLemoneites appears to be similar in morphology and positionrelative to the anterior margin of the head shield (Flower 1968pl 8 figs 4 amp 13) to that described from Aglaspis and is codedas State 0 In many taxa the hypostome is unknown but inonly a small number can it be coded reliably as absentHughesrsquo (1975) suggestion that the hypostome of Burgessia isabsent is accepted preliminarily but an extension of thedoublure may be visible in some of Hughesrsquo figures Fortey(pers comm 2000) also doubts Hughesrsquo assertion

28 Visible ecdysial sutures (0) absent and (1) present Themarginal ecdysial sutures of chelicerates and the dorsal suturesof trilobites are almost certainly not homologous but thepresence of sutures and their position (see Character 29) arecoded separately to allow this to be tested Wills et al (1998aCharacter 2) coded marginal sutures as present in Sidneyiafollowing Brutonrsquos (1981) suggestion but this is consideredequivocal here

29 Position of ecdysial sutures (0) marginal and (1) dor-sal This character is inapplicable to taxa lacking ecdysialsutures (Character 28 State 0) Dorsal sutures whilst presentin the representatives of the Trilobita coded here are probablynot a synapomorphy of trilobites as a whole but for a clade oftrilobites excluding the Olenellida (Whittington 1989 Fortey1990a)

245 Exoskeletal tergites and thoracic tagmosis 30 Min-eralised cuticle (0) absent and (1) present Calcification of theexoskeleton is one of the most convincing synapomorphiesof the Trilobita (Fortey amp Whittington 1989 Ramskold ampEdgecombe 1991 Edgecombe amp Ramskold 1999 Character 1)Cuticle mineralisation is also coded as present in aglaspidids

following Briggs amp Fortey (1982) who suggested that theaglaspidid exoskeleton was originally phosphatic Paleomerusprobably also had a mineralised cuticle but Hou amp Bergstrom(1997 p 97) argued that it was likely to be calcareous Anundescribed Silurian aglaspidid may also have had an origi-nally calcitic cuticle (Fortey amp Theron 1994 p 856) Becauseof the uncertainty about the chemical composition of theexoskeleton in these forms cuticle mineralisation is coded aspotentially homologous in all these taxa and no attempt ismade to code separate states for phosphatic and calcareousmineralisation

31 Trunk tergites with expanded lateral pleurae coveringappendages dorsally (0) absent and (1) present Whilst thepresence of paratergal folds may be a synapomorphy at thelevel of the Euarthropoda (Boudreaux 1979 Wagele 1993Edgecombe et al 2000 Character 142) these are at mostsmall reflections of the margin of the tergites in most euarthro-pods In arachnomorph taxa these are expanded to form largelateral pleurae that cover the appendages dorsally (see egLimulus and Triarthus in Boudreaux 1979) This is inferred tobe the case in some taxa where dorsal features and appendagesare poorly known on the basis of their possession of tergiteswith wide pleural regions This feature was suggested as typicalof lamellipedians (=arachnomorphs) by Hou amp Bergstrom(1997 pp 42ndash3)

The arrangement of the thorax of Marrella Mimetaster andBurgessia differs from that of all the other taxa considered herein that the appendages seem to be attached to the lateralmargins of the body The trunk tergites do not cover theappendages and seem to lack pleurae entirely although theyare covered by a posterior extension of the head shield inBurgessia This character has been clearly described inMimetaster (Sturmer amp Bergstrom 1976 pp 87ndash90 fig 8) andBurgessia (Hughes 1975 p 421)

32 Free thoracic tergites (0) present and (1) absentFollowing Edgecombe amp Ramskold (1999 Character 16) anumber of taxa lack functional post-cephalic articulations andconsequently lack free thoracic tergites Despite this theycode these taxa for a number of characters relating to thestructure of thoracic tergites (eg their characters 18 and 19)These characters are treated here as inapplicable to taxawithout free tergites and the definition of this character hasbeen modified from that of Edgecombe amp Ramskold (1999) tomake this more explicit

33 Decoupling of thoracic tergites and segments (0)absent and (1) present Ramskold et al (1997) described thischaracter of Cindarella and Xandarella where the thoracictergites correspond to a variable increasing posteriorly num-ber of appendage pairs This character cannot be coded fortaxa in which free thoracic tergites are absent (Character 32State 1)

34 Tergite articulations (0) tergites non-overlapping (1)extensive overlap of tergites and (2) edge-to-edge pleuralarticulations In most of the taxa considered here the thoracictergites overlap considerably and relatively evenly over theirwidth This contrasts with the primitive euarthropod condi-tion where the tergites do not overlap medially as seen in stemgroup crustaceans In trilobites such as Helmetia and Kuamaia(see Edgecombe amp Ramskold 1999 character 18) overlap ofthoracic tergites is limited to a well-defined axial region andthe lateral pleurae meet edge-to-edge

The thoracic articulations of naraoiids with free thoracictergites are in some ways similar to those of trilobites with astrong overlap medially that is lacking abaxially andanterolateral articulating facets (Budd 1999a Ramskold ampEdgecombe 1999) However the distribution of these features

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 183

in other trilobite-like arachnomorphs is unclear and a restric-tive coding is used here The present authors do not considerthat this character can be applied reliably to the fused thoracictergites of Saperion Tegopelte and Skioldia and it is coded asinapplicable to all taxa lacking free thoracic tergites (Character32 State 1)

35 Trunk effacement (0) trunk with defined (separate orfused) tergite boundaries (1) trunk tergite boundaries effacedlaterally and (2) trunk tergite boundaries completely effaced(cf Edgecombe amp Ramskold 1999 Character 15)] Contrary toEdgecombe amp Ramskold (1999) the present authors considerSkioldia and Saperion to show a distinct form of tergiteeffacement where the fused tergite boundaries are defined byfurrows axially but effaced laterally The boundaries areeffaced at least laterally in Tegopelte but the axial region ofthe exoskeleton is unknown which accordingly is given amultistate uncertainty coding

36 Cephalic articulation fused (0) absent and (1) presentIn Tegopelte Saperion and Skioldia the articulation ofthe head with the thorax is non-functional and the entireexoskeleton forms a single tergite

37 Head shield overlap of thoracic tergites (0) overlapabsent or identical to overlap between thoracic segments (1)head shield covers first thoracic tergite only and (2) headshield covers mutliple anterior trunk tergites

38 Head shield articulates with reduced anterior thoracictergite (0) absent and (1) present Amongst taxa showing aposteriorly expanded head shield ie one that overlaps thethorax Edgecombe amp Ramskold (1999 Character 9) identifiedtwo distinct character states One of these ndash overlap of only theanteriormost thoracic tergite ndash was limited to taxa assigned tothe Liwiinae (sensu Fortey amp Theron 1994) and another ndashoverlap of multiple tergites with attachment to a reducedanterior thoracic tergite ndash to the Xandarellida None of theadditional taxa considered herein (including Buenaspis whichwas assigned to the Liwiinae by Budd 1999a) shows thesedistinctive morphological features However the expandedhead shields of Marrella Mimetaster and Burgessia which donot articulate with a narrow anterior thoracic tergite may behomologous with the expanded head shields of xandarellidsTo recognise this the degree of overlapping of the thorax andthe possession of the reduced anterior tergite are coded sepa-rately These characters are both coded as inapplicable to taxain which the head shield and thoracic tergites are fused(Character 36 State 1) The crustacean carapace which ismost similar to the head shield of Burgessia is seeminglyabsent in stem group crustaceans (Walossek amp Muller1998)

39 Trunk narrowed anteriorly relative to head shieldwidest posteriorly (0) absent and (1) present (cf Edgecombeamp Ramskold 1999 Character 14) The anterior narrowing ofthe trunk caused by the reduction of opisthosomal segment 1in xiphosurids (Weinbergina of the taxa analysed hereAndersen amp Selden 1997 Dunlop amp Selden 1997 Character14) is not considered to be homologous with the unusual shapeof the thorax in naraoiids recognised by their coding

40 Boundaries of anterior trunk segments reflexed antero-laterally (0) absent boundaries transverse or reflexed postero-laterally and (1) present

41 Joints between posterior tergites functional anteriorones variably fused (0) absent and (1) present

42 Posterior tergite bearing axial spine (0) absent and (1)present

Characters 40 to 42 are coded following Edgecombe ampRamskold (1999 characters 17 19 amp 23 respectively) Inaddition to the taxa considered here a thoracic axial spine ispresent in some olenelloid trilobites (see Lieberman 1998

Characters 73 amp 74) and in the aglaspidid Beckwithia (seeHesselbo 1989)

246 Posterior body termination 43 Postabdomen of seg-ments lacking appendages (0) absent and (1) present

44 Length of postabdomen (0) one segment (1) twosegments (2) three segments and (3) five segments In sometaxa a variable number of posterior thoracic segments bearcomplete tubular tergites and lack appendages In Aglaspisautapomorphically it seems that the posterior three thoracicsegments lack appendages (Briggs et al 1979 Hesselbo 1992)but have unmodified tergites These situations are recognisedas potentially homologous by the coding used here Character44 is coded as inapplicable to taxa lacking a postabdomen

45 Posterior tergites strongly curved in dorsal aspect com-pared to anterior tergites (0) absent and (1) present Asrecognised by Wills et al (1998a Character 17) in some taxawhere the tergites are distinct the curvature of thoracic tergitesincreases posteriorly so that the posterior tergites are highlycurved to semicircular in dorsal aspect This situation isnot known from crustaceans (except isopods) or stem grouparthropods

46 Posterior segments reduced and with highly reducedappendages (0) present and (1) absent In some taxa there area large number of posterior segments that are short sagitallyand have appendages that are much reduced in size comparedto anterior trunk appendages These somites are incorporatedinto the pygidium in some taxa but primitively they arecovered by tiny free trunk tergites In the derived state thetrunk somites and limbs are of a relatively constant size Thedistinction between these states can be seen when attempting tocount the number of segments that make up the body In taxashowing State 0 the number of segments is very high anddifficult to count whereas in other taxa the number ofpost-cephalic segments is easily assessed

47 Pygidium (0) absent and (1) present A pygidium isrecognised here as a posterior tagma consisting of a number offused segments under a single tergite which may or may notincorporate the post-segmental telson This is different to theuse of this term by Edgecombe amp Ramskold (1999) whichfollows Ramskold et al (1997) and very different to its use byWills et al (1998a p 53) as a synapomorphy of the TrilobitaThe present authorsrsquo definition recognises the situation inRetifacies where the spinose postsegmental telson is autapo-morphically not fused to the pygidium as homologous toother multisegmented posterior tagma which include thetelson The distinction between the pygidium and an expandedpost-segmental telson can also be seen in the position of theanus which is consistently in the posteriormost pre-telsonicsegment amongst putative arachnomorphs (see Character 48)In taxa with a pygidium the anal segment is fused into thepygidium (see eg Kuamaia Edgecombe amp Ramskold 1999fig 6) whereas in those taxa lacking a pygidium the anus isbetween the final trunk tergite and the telson

Ramskold et al (1997) argued that the posteriormost tergiteof xandarellids (which incorporates the anal segment) is nothomologous to the posterior tagma recognised as pygidiaherein because posterior thoracic tergites also cover multiplesegments in xandarellids (see Character 33) Evidence ofsegmentation of the pygidium in a variety of taxa suggests thatthe number of tergites fused to form the pygidium is greaterthan the number of appendages For example the pygidium ofKuamaia has two pairs of lateral spines but at least fourpairs of appendages (Hou amp Bergstrom 1997 Edgecombe ampRamskold 1999) and that of Triarthrus has only five axialrings posterior to the articulating ring but over 10 pairs ofappendages (Whittington amp Almond 1987) The fact thatdecoupling of tergites and segments is evident in the thorax of

184 TREVOR J COTTON AND SIMON J BRADDY

Xandarella and Cindarella does not necessarily suggest that asimilar more widespread decoupling in pygidia is the result ofa different developmental process and hence that they arenon-homologous Instead the situation in the xandarellidthorax is potentially homologous to that found (more widely)in pygidia and consequently the terminal xandarellid tergite isa pygidium It is unclear if decoupling of tergites and somites isprimitively limited to the terminal tergite or primitively aproperty of the arachnomorph post-cephalon as a whole

It has been suggested that the tiny pygidium of olenelloidtrilobites which probably best reflects the primitive trilobitestate consists only of the post-segmental telson (Harrington inMoore 1959) and therefore is not a true pygidium HoweverWhittington (1989) showed that the olenelloid pygidium con-sists of at least two and possibly as many as five or sixsegments and therefore is likely to be homologous with thepygidium of other trilobites

48 Position of the anus (0) terminal within telson and (1)at base of telson In crustaceans and stem group arthropodsthe anus is terminal or otherwise situated in the telson (egRehbachiella Walossek 1993 fig 15D) In putative arachno-morphs the anus is either at the junction of the posteriormostthoracic segment and the telson or ventral within a fusedpygidium In the case of taxa with a pygidium the anus isanterior to the posterior margin and where known positionedbetween the posteriormost pair of appendages suggesting thatit is anterior to the post-segmental telson (see eg OlenoidesWhittington 1980)

49 Pygidium with median keel (0) absent and (1) presentEdgecombe amp Ramskold (1999 Character 21) considered thepresence of a median keel on the pygidium as a synapomorphyuniting Soomaspis and Tarricoia They did not explain howthis structure could be distinguished from the raised pygidialaxis of trilobites Homology of these structures is not sup-ported here because the axis of more primitive trilobites(including Eoredlichia) is quite distinct morphologically fromthe naraoiid keel Budd (1999a) noted the presence of a keel onthe pygidium of Buenaspis and suggested (Budd 1999a p 102)that it may be an artefact of dorsoventral compaction How-ever contrary to the coding of Edgecombe amp Ramskold(1999) a keel is visible on the pygidium of Liwia convexa (Dzikamp Lendzion 1988 fig 4CD) preserved in full relief Thereforeit seems unlikely that the keel is a taphonomic artefact

50 Pygidium with broad-based median spine (0) absentand (1) present

51 Pygidium with lateral spines (0) present and (1) absentEdgecombe amp Ramskold (1999 Character 22) coded thepresence of a median spine and two pairs of lateral spines aspotentially homologous in Sinoburius Kuamaia and HelmetiaThe distribution of lateral spines is much wider than that ofterminal spines and therefore they are coded separately hereIn trilobites it has widely been recognised that lateral spinesrepresent the original segmentation of the pygidium Thiscannot be the case for median spines Characters 49 to 51are not applicable to taxa lacking a pygidium (Character 47State 0)

52 Expanded post-segmental telson (0) absent and (1)present In a range of taxa the posterior-most tergite is a large(relative to the thoracic tergites) structure that lacks anyevidence of segmentation and is interpreted as representing anexpanded tergite of the post-segmental telson from whichsegments are released anteriorly during ontogeny On the otherhand the posteriormost tergite of Marrella and probablyMimetaster is small compared to the thoracic tergites androunded This element probably represents the plesiomorphiceuarthropod telson The homology of this character in mosttaxa is clear and only doubtful in Retifacies in which it may

be segmented The figures of Hou amp Bergstrom (1997) providelittle unequivocal evidence for a telson in Retifacies but thepresence of this structure is very clear in colour photographs(eg Chen et al 1996 fig 198A) However these figures donot convincingly demonstrate the segmentation of the telsonRetifacies is interpreted here as being unique in possessingboth a pygidium and an expanded post-segmental telson

53 Telson shape (0) spinose and (1) paddle-shaped Theshape of the expanded telsons identified above varies frombroadly spinose to flattened and paddle-like This differencecan be recognised both on the basis of the ratio of length tobasal width (spinose telsons are relatively long) and by thechange in width posteriorly (spinose telsons reduce in widthposteriorly whereas paddle-shaped telsons increase in width)In eurypterids a wide range of telson shapes is found includ-ing both character states described here It is likely that theplesiomorphic condition is a spinose telson This character isinapplicable to taxa lacking an expanded telson

54 Post-ventral furcae (0) absent and (1) present Thischaracter recognises the potential homology of unsegmentedpaired structures that articulate with the segment immediatelyanterior to the telson The potential homology of these struc-tures (the post-ventral plates) in Emeraldella and Aglaspis wasrecognised by Wills et al (1998a Character 70) and inEmeraldella and Sidneyia by Edgecombe amp Ramskold (1999Character 25) The homology of the segmented pygidial caudalfurcae of Olenoides (see Whittington 1975a 1980) with thesestructures is considered doubtful and coded as equivocal

Despite their distinctive morphology the long caudualfurcae of Cheloniellon may be homologous with the furcae ofAglaspis Emeraldella and Sidneyia This is supported by thecaudal furcae of the Ordovician cheloniellid Duslia which aremore similar in morphology to those of Emeraldella than ofCheloniellon Chlupac (1988 pp 614ndash16) considered thesestructures to be on the terminal tergite in Duslia However afaint tergite boundary is apparent posterior to the attachmentof the furcae (see Chlupac 1988 pl 57 fig 4) Therefore theyare considered here to have been attached to the pre-telsonicsegment as in Cheloniellon Chlupac (1988) and Dunlop ampSelden (1997) supported a close relationship between Dusliaand Cheloniellon

25 MethodsAll analyses were carried out using PAUP Version 40b4a(Swofford 1999) and unless otherwise stated used heuristicsearches with one thousand random addition sequencereplicates The software packages MacClade Version 307(Maddison amp Maddison 1997) and RadCon (Thorley amp Page2000) were used for comparing trees and investigating patternsof character evolution Tree length and other statistics (theConsistency Index CI and Retention Index RI) were calcu-lated by PAUP and MacClade with uninformative charactersexcluded Analyses were run separately using each of thedifferent codings for aglaspidids

Characters were treated as unordered and of equal weight inmost analyses To assess the influence of this assumption fourof the eight multistate characters which have states that areintermediate between others (Wilkinson 1992) were treated asordered in some analyses These characters are the number ofcephalic segments (Character 4) the length of raptorialappendage spines (Character 6) the degree of overlap of thethorax by the head-shield (Character 37) and the number ofsegments making up the postabdomen (Character 44) The lastof these is uninformative when not ordered because only onetaxon shows each of states 0 1 and 3

Support for individual nodes was assessed by bootstrapanalysis (Felsenstein 1985) and by calculating Bremer support

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 185

indices (Bremer 1988 1994) These methods measure twodistinct aspects of support for phylogenetic hypotheses Treesor nodes may be considered well supported (1) to the extent towhich alternative topologies are much less parsimonious asmeasured by the support index (Wilkinson 1996) or (2) wherethey are consistent with a large proportion of characters sothat character sampling is unlikely to have had much influenceon topology as assessed by bootstrapping (Page 1996) Boot-strapping was performed with 100 bootstrap replicates eachof 10 addition sequence replicates Bremer support indicesare based on heuristic searches with 100 addition sequencereplicates

Three pairs of terminals are included in Table 2 that apartfrom missing data are coded identically These are Helmetiaand Kuamaia Soomaspis and Tariccoia and Saperion andSkioldia These pairs are lsquotaxonomic equivalentsrsquo removingthe taxon with most missing data will not influence theposition of the other They can be recoded as single terminalsaccording to the principal of safe taxonomic reduction(Wilkinson 1995a b) and were not considered separately in theanalyses presented here Therefore cladistic analyses werecarried out on the basis of 33 terminals Edgecombe ampRamskold (1999) ignored this and coded the same three pairsof taxa and Eoredlichia and Olenoides identically for theirmore limited set of characters

26 ResultsAnalysis with all characters unordered and using the lsquoAglaspi-dida 1rsquo coding found nine most parsimonious trees (MPTs)each 126 steps long (CI=0middot556 RI=0middot763) Using the lsquoAglas-pidida 2rsquo coding resulted in 18 MPTs of length 125 (CI=0middot560RI=0middot765) Nine of these 18 trees were those found with thefirst coding of aglaspidids The strict component consensus(sensu Wilkinson 1994) of the 27 unique trees found acrossboth analyses is shown in Figure 8 All trees supported themonophyly of the Marrellomorpha (Marrella and Mimetaster)and of a clade including all other taxa except Crustacea (Clade1 of Fig 8) The relationships between these two clades and theCrustacea were unresolved equal numbers of trees in bothanalyses supported each of the topologies [Clade 1 (CrustaceaMarrellomorpha)] [Crustacea (Marrellomorpha Clade 1)]and [Marrellomorpha (Crustacea Clade 1)] All trees placedBurgessia as the sister taxon to the rest of Clade 1 andsupported the division of the rest of Clade 1 into two majorsister groups The topology of one of these clades (Clade 2)which included trilobites helmetiids tegopeltids naraoiidsxandarellids and Retifacies was stable with respect to thecoding of aglaspidids The other clade (Clade 3) consists of aclade of megacheirans and chelicerates (Clade 4) and a cladeincluding aglaspidids Lemoneites Paleomerus CheloniellonEmeraldella and Sidneyia (Clade 5) This latter clade is un-resolved in the strict consensus of all 27 trees (Fig 8) and thestrict consensus of the 18 MPTs found using the lsquoAglaspidida2rsquo coding but was fully resolved in the consensus of the ninetrees found with the lsquoAglaspidida 1rsquo coding Within themegacheiran-chelicerate clade all trees found Fortiforceps tobe the sister group to all other taxa Yohoia the sister group tochelicerates and Alalcomenaeus and Leanchoilia to form aclade Jianfengia is equally parsimoniously placed as the sistergroup to the Yohoia-chelicerate clade or as the sister group tothis clade and the Alalcomenaeus-Leanchoilia clade

Treating characters 4 6 37 and 44 as ordered had a minimaleffect on most parsimonious topology Nine MPTs of 136 steps(CI=0middot515 RI=0middot743) were found with the lsquoAglaspidida 1rsquocoding and 12 MPTs 135 steps in length (CI=0middot519 RI=0middot745)with the lsquoAglaspidida 2rsquo coding These trees all supported a(Crustacea Clade 1) clade excluding the marrellomorphs

Apart from this the strict consensus of these 21 trees isidentical to the consensus of the 27 trees found with allcharacters treated as unordered

Across all analyses four different topologies for Clade 5 (ofFig 8) were found (Fig 9) The distribution of these topologiesacross the four analyses described above and across the sameanalyses repeated after reweighting of each character in pro-portion to their Rescaled Consistency Index (see Farris 1989)from the first analysis is shown in Table 3 The instability of

Figure 8 Strict consensus of 27 MPTs from four separate analyses ofarachnomorph phylogeny with both interpretations of aglaspididmorphology and with some characters treated as ordered or allcharacters unordered Clades referred to in the text are numbered

Figure 9 Alternative equally parsimonious resolutions of clade 5 ofFig 8 (see Table 3)

186 TREVOR J COTTON AND SIMON J BRADDY

this clade was to some extent caused by the inclusion ofLemoneites and Paleomerus which could each be coded foronly 27 of the 54 characters used Analysis with these two taxaexcluded and characters treated as unordered found treescompatible only with Topology D of Figure 9 irrespective ofthe coding used for aglaspidids Analysis with some characterstreated as ordered and using either aglaspidid coding foundthree trees compatible only with Topology B of Figure 9 andthree compatible only with Topology D of Figure 9 ThereforeTopology D is the preferred topology for Clade 5 as shown inTable 3

One of the MPTs resulting from the lsquoAglaspidida 2rsquo coding(with all characters treated as unordered) was chosen as thebasis for further discussion because this tree combined thepreferred topology of Clade 5 with the (Crustacea Clade 1)group that was favoured when some characters were treated asordered Bootstrap percentages and Bremer support values forthis tree (using the Aglaspidida 2 coding) are shown in Figure10 Apomorphies for all ingroup nodes of this tree are shownin Figure 10A

3 Discussion

31 Phylogenetic resultsThe results of the cladistic analyses presented above arewell-resolved and robust with respect to different analyticalparameters Most of the taxa considered form a clade (Clade 1of Fig 8) that is more closely related to chelicerates than tocrustaceans in all analyses and therefore is recognised asthe Arachnomorpha as defined by Chen et al (1996) andRamskold et al (1996) The monophyly of the Marrellomor-pha was supported in all analyses The marrellomorphs werefound to be the sister group to a Crustacea+Clade 1 group inmost MPTs but in a minority of trees formed the sister groupto Clade 1 alone According to the latter result they should beincluded within the Arachnomorpha but according to the firstexcluded from it Both of these alternatives have been sup-ported in previous studies (eg Hou amp Bergstrom 1997 Willset al 1998a respectively) In addition to the marrellomorphsthe Fuxianhuiida Bousfield 1995 and Canadaspidida havebeen placed in the Euarthropoda outside the main mandibu-late and arachnomorph clades (Hou amp Bergstrom 1997)Further study of these taxa of the crustacean or mandibulatestem group and of the marrellomorphs is necessary before the

position of the Marrellomorpha can be resolved Provisionallymarrellomorphs are excluded from the Arachnomorphafollowing the majority of analyses above and Wills et al(1994)

The Burgess Shale arthropod Burgessia was placed as thesister group to remaining arachnomorphs in all analyses OtherArachnomorpha form two major clades one consisting of thelsquotrilobite-alliedrsquo arachnomorphs analysed by Edgecombe ampRamskold (1999) here termed lsquoTrilobitomorpharsquo (defined asTrilobita and all taxa more closely related to Trilobita thanChelicerata) and the other (lsquochelicerate-alliedrsquo) clade includingchelicerates lsquogreat appendagersquo arthropods aglaspididsLemoneites Paleomerus Sidneyia and Emeraldella The latteris here termed lsquoCheliceramorpharsquo (defined as Chelicerata andall taxa more closely related to Chelicerata than Trilobita) Thetopology of the lsquotrilobite-alliedrsquo clade was similar to that foundby Edgecombe amp Ramskold (1999) but the present authorsrsquoresults are more completely resolved unambiguously support-ing the sister-group relationship between trilobites andHelmetiida suggested in the previous study Within thelsquochelicerate-alliedrsquo clade Aglaspidids Paleomerus Lemoneitesand the Burgess Shale Emeraldella and Sidneyia form a cladein opposition to megacheirans and chelicerates

The present authorsrsquo analysis supports a new hypothesis ofthe origin of the chelicerates from within a paraphyleticassemblage of megacheiran arthropods and confirms pyc-nogonids as a sister group to the euchelicerates Previoushypotheses of the chelicerate sister group (see Section 1) aremuch less parsimonious The shortest trees supporting aCheloniellon-chelicerate clade are six steps longer than theMPTs and those supporting an aglaspidid-chelicerate cladeseven steps longer All megacheirans and chelicerates areunited by the flap-shaped rounded exopods and modificationof the second cephalic segment endopods into anteriorlydirected spinose raptorial organs which share a number ofdetailed similarities A clade including all of these taxa apartfrom Fortiforceps is supported by the synapomorphic loss ofthe antennae (or antennulae of crustaceans) and the longerlength of spinose projections of the second cephalic appendageendopods Within this clade Yohoia shares the loss of cephalicexopods the loss of thoracic endopods and a postabdomen oftubular tergites with chelicerates

The poor resolution of Clade 5 (of Fig 8) was partly a resultof the poor preservation of and consequent high proportion ofmissing data for Lemoneites and Paleomerus When these two

Table 3 Numbers of most parsimonious trees supporting each of the alternative topologies for clade 4 of Fig 7 under different analyticalconditions (RCI=Revealed Consistency Index)

Aglaspididcoding

Characters 4 6 36and 43 ordered

Charactersreweighted by RCI

Paleomerus andLemoneites excluded

Topology (see Fig 8)

A B C D

1 91 X 3 3 31 X 61 X X 31 X 61 X X 3 32 9 92 X 3 3 3 32 X 62 X X 32 X 62 X X 3 3Total 6 18 12 54

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 187

taxa were excluded from the analyses only two (B and D) ofthe four topologies found in the complete analysis remainedmost parsimonious (see Fig 9 and Table 3) Secondly Bremersupport values throughout much of the lsquochelicerate-alliedrsquoclade (Clade 3 of Fig 8) were improved when these taxa wereexcluded (see Fig 11)

Dunlop (1999 p 258) suggested that in reconstructing thechelicerate stem lineage lsquowe might predict that the two mostsignificant changes towards the chelicerate condition are thereduction of the antennae and the formation of the nextappendages into a clawrsquo According to the hypothesis pre-sented here both of these adaptations were achieved in EarlyCambrian chelicerate ancestors The recognition of the loss ofthe antennae in the stem group of the chelicerates and thephylogenetic hypothesis presented here suggest that chelicer-ates are most closely related to taxa with only a singleadditional segment fused to the crown group eucheliceratehead of four fused somites Previous hypotheses have derivedchelicerates from Palaeozoic arthropods with a longer head(eg Cheloniellon Sturmer amp Bergstrom 1978 SanctacarisBriggs amp Collins 1988 Emeraldella Bruton amp Whittington1983) The plesiomorphic pattern of head segmentation ismatched in pycnogonids Bergstrom et al (1980) suggestedthat only the anterior four pairs of appendages are incorpo-rated into the head This supports the view that pycnogonidsare primitive with respect to other chelicerates which showmore or less complete fusion of this primitive head and threethoracic segments including Solpugida and Pseudoscorpiones

The present study suggests that a shift from gnathobasicbenthic feeding to a more vagrant lifestyle and raptorialappendage feeding occurred very early in the chelicerate lin-eage Appendage feeding was clearly an important adaptation

Figure 10 One of 18 equally parsimonious cladograms with lsquoAglaspidida 2rsquo coding and all characters unorderedthat is most congruent with results from other analyses Non-terminal branch lengths scaled to reflect number ofapomorphies (A) ACCTRAN apomorphy scheme Character numbers shown above boxes and states shownbelow boxes as in Table 2 and the text Character consistency indices are indicated by shading (B) Levels ofsupport for individual nodes Bremer support indices shown in bold type above branches Bootstrap percentagesin italic type below branches for all nodes with relative frequencies greater than 50

Figure 11 Bremer support values for clades within the lsquochelicerate-alliedrsquo arachnomorphs (clade 3 of Fig 8) when all taxa wereincluded (normal weight font above nodes) and with Lemoneites andPaleomerus excluded (bold type below nodes)

188 TREVOR J COTTON AND SIMON J BRADDY

for the terrestrialization of chelicerates (Dunlop 1997 p 69Dunlop amp Webster 1999) but the model proposed heresuggests that this considerably pre-dated terrestrializationGnathobasic feeding in xiphosurans and eurypterids mayrepresent a reversal to a more primitve benthic arachnomorphlifestyle from a more pycnogonid-like ancestor

32 Evolution of the arachnomorph headThe extant arthropod classes show highly conserved patternsof head segmentation (eg Wills et al 1994) In contrast therehas been a near consensus amongst Cambrian arthropodworkers (although rarely based on an explicit phylogenetichypothesis) that patterns of head segmentation are highlyhomoplastic and hence of little systematic significanceSturmer amp Bergstrom (1978 pp 78ndash9) suggested that lsquoevenclosely related forms may have different numbers of headsegments and appendagesrsquo Bruton amp Whittington (1983pp 576ndash7) that lsquodiscussion on fossil arthropod relationshipsbased on head segmentation appears to be largely irrel-evant and at best speculativersquo and Delle Cave amp Simonetta(1991 p 191) that lsquothere are no obvious phyletic affinitiesbetween genera having the same number of cephalized seg-mentsrsquo Most recently Bergstrom amp Hou (1997 p 104) con-cluded that lsquothe segmental length of the head shield seemsto be of no relevance to the discussion and discrimination ofevolutionary lineagesrsquo It has been claimed that this view issupported by cladistic analyses in which patterns of tagmosishave been found to be rather poor at defining major arthropodclades (Briggs et al 1992 Wills et al 1994)

This suggestion is not only of relevance to systematics buthas featured prominently in recent debates surrounding thenature of the lsquoCambrian explosionrsquo (see Budd amp Jensen 2000for a recent review) Gould (1989 1991) argued that theapparent plasticity of head segmentation in Cambrian com-pared to post-Cambrian arthropod evolution suggests thatbody-plan evolution was only constrained after the Cambrianexplosion According to this view the origin of new highertaxa with distinct body-plans was only possible before theonset of these constraints (eg Valentine 1995) Taxa hererecognised as arachnomorphs clearly played a major role in thedevelopment of Gouldrsquos hypothesis This suggestion has pre-viously been assessed in terms of overall morphological diver-sity which is clearly rather distinct from Gouldrsquos concept ofdisparity Wills et al (1998b) showed that there was littlecorrelation between overall morphological disparity and de-gree of limb specialisation and that the latter was phylogeneti-cally highly plastic They remarked that (Wills et al 1998bp 64) lsquothis finding has important implications for models ofarthropod phylogeny and evolution that attribute overidingimportance to head segmentationrsquo but did not explicitlyexamine head segmentation although it is likely to have beena major component of the index of tagmosis employed

The results of the present study suggest that arguments thatarthropod head segmentation was unusually labile during theCambrian are poorly founded Rather only four major pat-terns of euarthropod head segmentation are identified Theplesiomorphic euarthropod state according to both Walossekamp Muller (1997 1998) and Scholtz (1997) which may moreproperly be the plesiomorphic state for the euarthropod crowngroup only (depending on the phylogenetic position of theMarrellomorpha) consists of four post-acronal segmentsbearing the antennae and three pairs of biramous limbs Thepresent authors suggest that the term lsquocephalonrsquo be restrictedto this kind of head which is found in stem group mandibu-lates in addition to many arachnomorphs Crown groupmandibulates share a lsquobimaxillary headrsquo in which an addi-tional pair of appendages the labium or second maxillae are

incorporated into the head but in crustaceans these are notfused to the carapace (Scholtz 1997) Finally two distinctforms of head tagmosis are found in chelicerates A head withfour pairs of appendages (but without antennae giving a totalof five segments incorporated into the head) which has beencalled the lsquocephalosomarsquo is present in pycnogonids (Walossekamp Dunlop 2002 pp 434ndash5) and probably also in some euch-elicerates (J A Dunlop pers comm 2003) The lsquoprosomarsquo ofmost euchelicerates consists of this cephalosoma and twoadditional segments

In general these patterns of head segmentation are highlyphylogenetically conserved (see Fig 12) Only three homo-plastic changes namely reversal to a more primitive conditionin the Leanchoilia-Alalcomenaeus clade and the convergentorigin of a five-segmented head in Clade 3 (of Fig 8) and theXandarellida are necessary to optimise character 4 onto themost parsimonious cladogram There are also transitions toautapomorphic states in Emeraldella and Sidneyia In the caseof Sidneyia however an argument could be made (followingBergstrom amp Sturmer 1978 see discussion of Character 4) thatthe head consists of five segments as in closely related taxabut autapomorphically each segment has a separate tergiteThis would further increase the phylogenetic stability of headsegmentation Also following Bergstrom amp Sturmer (1978) thehead of Cheloniellon could be considered to consist of twotergites making it more similar to the head of Emeraldella(with six pairs of appendages)

Figure 12 Cladogram from Fig 10 showing changes in head segmen-tation Bold type shows abbreviated head segmentation formulaAbbreviations (Ant) antennae (BA) biramous appendage pairs (Ch)chelicerae (GA) great appendages (UA) uniramous appendage pairsand (X) segment without appendages Synapomorphy A is the auta-pomorphic reduction in head length to a single segment in SidneyiaAlternatively this may represent secondary division of the headshield Synapomorphy B is the increase in the length of the head inEmeraldella to six segments

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 189

4 Acknowledgements

We thank D E G Briggs J A Cotton J A Dunlop R AFortey S Harris R A Moore P Selden O E Tetlie andP Van Roy for comments on earlier drafts of this work Themanuscript was reviewed by M Wills F Schram and oneanonymous referee This research was supported by the Geo-logical Society of London Curry Research Studentshipawarded to TJC by the Department of Earth Sciences Uni-versity of Bristol and by the Department of PalaeontologyThe Natural History Museum London TJC would like tothank C Labandeira J Thompson and R Chapman fortheir hospitality at the National Museum of Natural HistoryWashington DC USA and A Olcott S Schellenberg andJ Young for making his time there so enjoyable

5 References

Abzhanov A Popadic A amp Kaufman T C 1999 Chelicerate Hoxgenes and the homology of arthropod segments Evolution andDevelopment 1 77ndash89

Akam M 2000 Arthropods developmental diversity within a (super)phylum Proceedings of the National Academy of Sciences USA97 4438ndash41

Aldridge R J Gabbott S E amp Theron J N 2001 The Soom ShaleIn Briggs D E G amp Crowther P R (eds) Palaeobiology II340ndash2 Oxford Blackwell Science

Anderson L I amp Selden P A 1997 Opisthosomal fusion andphylogeny of Palaeozoic Xiphosura Lethaia 30 19ndash31

Arango C P 2002 Morphological phylogenetics of the sea spiders(Arthropoda Pycnogonida) Organisms Diversity and Evolution2 107ndash25

Ax P 1986 Das phylogenetische System Stuttgart Gustav FischerVerlag

Babcock L E amp Zhang Wentang In press New non-mineralizedarthropods from the Chengjiang Lagerstatte Lower CambrianYunnan China Palaeontologia Cathyana

Bartels C Briggs D E G amp Brassel G 1998 The fossils ofthe Hunsruck Slate Marine life in the Devonian CambridgeCambridge University Press

Bergstrom J 1971 Paleomerus ndash merostome or merostomoid Lethaia4 393ndash401

Bergstrom J 1979 Morphology of fossil arthropods as a guideto phylogenetic relationships In Gupta A P (ed) ArthropodPhylogeny 3ndash56 New York NY Van Nostrand Reinhold Co

Bergstrom J 1992 The oldest arthropods and the origin of theCrustacea Acta Zoologica 73 287ndash91

Bergstrom J Sturmer W amp Winter G 1980 PalaeoisopusPalaeopantopodus and Palaeothea pycnogonid arthropodsfrom the Lower Devonian Hunsruck Slate West GermanyPalaontologische Zeitschrift 54 7ndash54

Bergstrom J amp Brassel G 1984 Legs in the trilobite Rhenops fromthe Lower Devonian Hunsruck Slate Lethaia 17 67ndash72

Bergstrom J amp Hou X 1998 Chengjiang arthropods and theirbearing on early arthropod evolution In Edgecombe G D(ed) Arthropod fossils and phylogeny 151ndash84 New York NYColumbia University Press

Boudreaux H B 1979 Significance of intersegmental tendon systemin arthropod phylogeny and a monophyletic classification ofArthropoda In Gupta A P (ed) Arthropod Phylogeny 551ndash86New York NY Van Nostrand Reinhold Co

Bousfield E L 1995 A contribution to the natural classification ofLower and Middle Cambrian arthropods food-gathering andfeeding mechanisms Amphipacifica 2 3ndash33

Braddy S J Aldridge R J Gabbott S E amp Theron J N 1999Lamellate book-gills in a late Ordovician eurypterid from theSoom Shale South Africa support for a eurypterid-scorpionclade Lethaia 32 72ndash4

Braddy S J amp Almond J E 1999 Eurypterid trackways from theTable Mountain Group (Ordovician) of South Africa Journal ofAfrican Earth Sciences 29 165ndash77

Bremer K 1988 The limits of amino acid sequence data in angio-sperm phylogenetic reconstruction Evolution 42 795ndash803

Bremer K 1994 Branch support and tree stability Cladistics 10295ndash304

Briggs D E G 1998 Review of Arthropods from the LowerCambrian Chengjiang fauna southwest China by HouXianguang and Jan Bergstrom Fossils and Strata 45

Scandinavian University Press Oslo Norway 1997 Palaeo-geography Palaeoclimatology Palaeoecology 143 193ndash4

Briggs D E G Bruton D L amp Whittington H B 1979 Append-ages of the arthropod Aglaspis spinifer (Upper CambrianWisconsin) and their significance Palaeontology 22 167ndash80

Briggs D E G Fortey R A amp Wills M A 1992 Morphologicaldisparity in the Cambrian Science 256 1670ndash3

Briggs D E G Fortey R A amp Wills MA 1993 How big was theCambrian explosion A taxonomic and morphologic comparisonof Cambrian and Recent arthropods In Lees D R amp EdwardsD (eds) Evolutionary patterns and processes 33ndash44 LondonLinnean Society of London Linnean Society Symposium Series

Briggs D E G Erwin D H amp Collier F J 1994 The fossils of theBurgess Shale Washington DC Smithsonian Institution Press

Briggs D E G amp Bartels C 2001 New arthropods from the LowerDevonian Hunsruck Slate (Lower Emsian Rhenish Massifwestern Germany) Palaeontology 44 275ndash304

Briggs D E G amp Bartels C 2002 Magnoculocaris a new name forthe arthropod Magnoculus Briggs and Bartels 2001 from theLower Devonian Hunsruck Slate Germany Palaeontology 45419

Briggs D E G amp Collins D 1988 A Middle Cambrian cheliceratefrom Mount Stephen British Columbia Palaeontology 31 779ndash98

Briggs D E G amp Collins D 1999 The arthropod Alalcomenaeuscambricus Simonetta from the Middle Cambrian Burgess Shale ofBritish Columbia Paleontology 42 953ndash78

Briggs D E G amp Fortey R A 1982 The cuticle of aglaspididarthropods a red-herring in the early history of vertebratesLethaia 15 25ndash9

Briggs D E G amp Fortey R A 1989 The early radiation andrelationships of the major arthropod groups Science 246 241ndash3

Briggs D E G amp Fortey R A 1992 The early Cambrian radiationof arthropods In Lipps J H amp Signor P W (eds) Origin andEarly Evolution of the Metazoa 335ndash373 New York NY PlenumPress

Briggs D E G amp Robison R A 1984 Exceptionally preservednontrilobite arthropods and Anomalocaris from the MiddleCambrian of Utah University of Kansas PaleontologicalContributions Paper 111 1ndash23

Broili F 1932 Ein neuer Crustacee aus dem rheinischen UnterdevonSitzungsberichte der Mathematisch-Naturwissenschaftlichen(Abteilung) Klasse der Bayerischen Akademie der Wissenschaftenzu Munchen 1932 27ndash38

Broili F 1933 Ein zweites Exemplar von Cheloniellon Sitzungsber-ichte der Bayerischen Akademie der Wissenschaften 1933 11ndash32

Bruton D L 1981 The arthropod Sidneyia inexpectans MiddleCambrian Burgess Shale British Columbia Philosophical Trans-actions of the Royal Society of London Series B 295 619ndash56

Bruton D L amp Whittington H B 1983 Emeraldella and Leanchoiliatwo arthropods from the Burgess Shale British Columbia Philo-sophical Transactions of the Royal Society of London Series B 300553ndash85

Budd G E 1995 Kleptothule rasmusseni gen et sp nov anolenellid-like trilobite from the Sirius Passet fauna (BuenFormation Lower Cambrian North Greenland) Transactions ofthe Royal Society of Edinburgh Earth Sciences 86 1ndash12

Budd G E 1996a Progress and problems in arthropod phylogenyTrends in Ecology and Evolution 11 356ndash8

Budd G E 1996b The morphology of Opabinia regalis and thereconstruction of the arthropod stem-group Lethaia 29 1ndash14

Budd G E 1997 Stem-group arthropods from the Lower CambrianSirius Passet fauna of North Greenland In Fortey R A ampThomas R H (eds) Arthropod Relationships 125ndash38 LondonChapman and Hall Systematics Association Special Volume 55

Budd G E 1999a A nektaspid arthropod from the Early CambrianSirius Passet fauna with a description of retrodeformation basedon functional morphology Palaeontology 42 99ndash122

Budd G E 1999b The morphology and phylogenetic significance ofKerygmachela kiergkegaardi Budd (Buen Formation LowerCambrian N Greenland) Transactions of the Royal Society ofEdinburgh Earth Sciences 89 (for 1998) 249ndash290

Budd G E 2002 A palaeontological solution to the arthropod headproblem Nature 417 271ndash5

Budd G E amp Jensen S 2000 A critical reappraisal of the fossilrecord of the bilaterian phyla Biological Reviews 75 253ndash95

Chen J Zhou G Zhu M amp Yeh K 1996 The Chengjiang biota ndasha unique window of the Cambrian explosion Taichung NationalMuseum of Natural Science [In Chinese]

Chen J Edgecombe G D amp Ramskold L 1997 Morphological andecological disparity in naraoiids (Arthropoda) from the Early

190 TREVOR J COTTON AND SIMON J BRADDY

Cambrian Chengjiang fauna China Records of the AustralianMuseum 49 1ndash24

Chen J amp Zhou G 1997 Biology of the Chengjiang fauna In ChenJ Cheng Y amp Iten H V (eds) The Cambrian explosion and thefossil record Bulletin of the National Museum of Natural ScienceTaiwan 10 11ndash106

Child C A 1997 Some deep-sea Pycnogonida from Argentine slopeand basin Proceedings of the Biological Society of Washington110 128ndash42

Chlupac I 1988 The enigmatic arthropod Duslia from the Ordovicianof Czechslovakia Palaeontology 31 611ndash20

Chlupac I 1995 Lower Cambrian arthropods from the PasekyShale (Barrandian area Czech Republic) Journal of the CzechGeological Society 40 9ndash36

Chlupac I amp Havlıcek V 1965 Kodymirus n g a new aglaspidmerostome of the Cambrian of Bohemia Sbornık GeologickechVid Paleontologie 6 7ndash20

Cisne J L 1975 Anatomy of Triarthrus and the relationships of theTrilobita Fossils and Strata 4 45ndash63

Cisne J L 1981 Triarthrus eatoni (Trilobita) anatomy of its exo-skeletal skeletomuscular and digestive systems Palaeonto-graphica Americana 9 95ndash142

Conway Morris S 1982 Atlas of the Burgess Shale London ThePalaeontological Association

Conway Morris S 1989 The persistence of Burgess Shale-type faunasimplications for the evolution of deeper-water faunas Trans-actions of the Royal Society of Edinburgh Earth Sciences 80271ndash83

Conway Morris S 1998 The crucible of creation Oxford OxfordUniversity Press

Cotton T J 1999 Review of Arthropod fossils and phylogenyPalaeogeography Palaeoclimatology Palaeoecology 151 341ndash3

Cotton T J 2002 The phylogeny and morphological evolution ofCambrian trilobites and their relatives Unpublished PhD ThesisUniversity of Bristol

Damen W G M Hausdorf M Seyfarth E A amp Tautz D 1998 Aconserved mode of head segmentation in arthropods revealed bythe expression pattern of Hox genes in a spider Proceedings of theNational Academy of Sciences USA 95 10665ndash70

Delle Cave L amp Simonetta A M 1991 Early Palaeozoic arthropodsand problems of arthropod phylogeny with some notes on taxa ofdoubtful affinities In Simonetta A M amp Conway Morris S(eds) The early evolution of Metazoa and the significance ofproblematic taxa 189ndash244 Cambridge Cambridge UniversityPress

Dewel R A amp Dewel W C 1997 The place of tardigrades inarthropod evolution In Fortey R A amp Thomas R H (eds)Arthropod Relationships 109ndash23 London Chapman and HallSystematics Association Special Volume 55

Dunlop J A 1997 Palaeozoic arachnids and their significance forarachnid phylogeny Proceedings of the 16th European Colloquiumof Arachnology 65ndash82 Siedlce Poland

Dunlop J A 1998 The origins of tetrapulmonate booklungs and theirsignificance for chelicerate phylogeny In Selden P A (ed)Proceedings of the 17th European Colloquium of ArachnologyEdinburgh 1997 pp 9ndash16

Dunlop J A 1999 Pasando revista a la evolucion de los quelicerados[A review of chelicerate evolution] In Melic A DeHaro J JMedez M amp Ribera I (eds) Volumen Monografico Evolucion yFilogenia de Arthropoda Boletin de la Sociedad EntolomologicaAragonesa 26 255ndash272 [In Spanish and English]

Dunlop J A amp Selden P A 1997 The early history and phylogenyof the chelicerates In Fortey R A amp Thomas R H (eds)Arthropod relationships Systematics Association Special Volume55 221ndash35 London Chapman and Hall

Dunlop J A amp Webster M 1999 Fossil evidence terrestrializationand arachnid phylogeny Journal of Arachnology 27 86ndash93

Dzik J 1993 Early metazoan evolution and the meaning of its fossilrecord Evolutionary Biology 27 339ndash86

Dzik J amp Lendzion K 1988 The oldest arthropods of the EastEuropean Platform Lethaia 21 29ndash38

Edgecombe G D Wilson G D F Colgan D J Gray M R ampCassis G 2000 Arthropod cladistics combined analysisof Histone H3 and U2 snRNA sequences and morphologyCladistics 16 155ndash203

Edgecombe G D amp Ramskold L 1999 Relationships of CambrianArachnata and the systematic position of Trilobita Journal ofPaleontology 73 263ndash87

Emerson M J amp Schram F R 1997 Theories patterns and realitygame plan for arthropod phylogeny In Fortey R A amp Thomas

R H (eds) Arthropod Relationships Systematics AssociationSpecial Volume 55 67ndash87 London Chapman and Hall

Farris J S 1989 The Retention Index and the Rescaled ConsistencyIndex Cladistics 5 417ndash19

Felsenstein J 1985 Confidence limits on phylogenies an approachusing the bootstrap Evolution 39 783ndash91

Flower R H 1968 Merostomes from a Cotter Horizon of the El PasoGroup New Mexico Bureau of Mines and Mineral ResourcesMemoir 22 35ndash44

Fortey R A 1990a Trilobite evolution and systematics In MikulicD G (ed) Arthropod paleobiology 44ndash65 Short Courses inPaleontology 3 Knoxville Tenessee The Paleontological Society

Fortey R A 1990b Ontogeny hypostome attachment and trilobiteclassification Palaeontology 33 529ndash76

Fortey R A 1997 Classification In Kaesler R L (ed) Treatise oninvertebrate paleontology Part O Arthropoda 1 (revised) 289ndash302Boulder Colorado amp Lawrence Kansas Geological Society ofAmerica and University of Kansas Press

Fortey R A amp Theron J N 1994 A new Ordovician arthropodSoomaspis and the agnostid problem Palaeontology 37 841ndash61

Fortey R A amp Whittington H B 1989 The Trilobita as a naturalgroup Historical Biology 2 125ndash38

Fry W G 1978 A classification within the pycnogonids ZoologicalJournal of the Linnean Society of London 63 35ndash58

Giribet G Edgecombe G D amp Wheeler W C 2001 Arthropodphylogeny based on eight molecular loci and morphology Nature413 157ndash61

Gould S J 1989 Wonderful life The Burgess Shale and the nature ofhistory New York NY Norton

Gould S J 1991 The disparity of the Burgess Shale arthropod faunaand the limits of cladistic analysis why we must strive to quantifymorphospace Paleobiology 17 411ndash23

Gurich G 1931 Mimetaster hexagonalis ein neuer Kruster aus demunterdevonischen Bundenbacher Dachschiefer PalaontologischeZeitschrift 12 204ndash38

Hammann W Laske R amp Pillola G L 1990 Tariccoia arrusensisngnsp an unusual trilobite-like arthropod Rediscovery of thelsquophylocaridrsquo beds of Taricco (1922) in the Ordovician lsquoPuddingarsquosequence of Sardinia Bolletino della Societa PaleontologicaItaliana 29 163ndash78

Haas M S Brown S J amp Beeman R W 2001 Pondering theprocephalon the segmental origin of the labrum DevelopmentGenes and Evolution 211 89ndash95

Heider K 1913 Entwicklungsgeschichte und Morphologie derWirbellosen Die Kultur der Gegenwart Theil 3 Abt 4 2 176ndash332

Henriksen K 1928 Critical notes upon Cambrian arthropodsdescribed by Charles D Walcott Videnskabelige Meddelelser fraDansk Naturhistorisk Forening 86 1ndash20

Hesselbo S P 1988 Trace fossils of Cambrian aglaspidid arthropodsLethaia 21 139ndash46

Hesselbo S P 1989 The aglaspidid arthropod Beckwithia from theCambrian of Utah and Wisconsin Journal of Paleontology 63636ndash42

Hesselbo S P 1992 Aglaspidida (Arthropoda) from the UpperCambrian of Wisconsin Journal of Paleontology 66 885ndash923

Hou X 1987a Three new large arthropods from Lower CambrianChengjiang fauna eastern Yunnan Acta Palaeontologica Sinica26 272ndash85 [In Chinese with English summary]

Hou X 1987b Two new arthropods from Lower CambrianChengjiang eastern Yunnan Acta Palaeontologica Sinica 26236ndash56 [In Chinese with English summary]

Hou X Chen J amp Lu H 1989 Early Cambrian new arthropodsfrom Chengjiang Yunnan Acta Palaeontologica Sinica 28 42ndash57[In Chinese with English summary]

Hou X Ramskold L amp Bergstrom J 1991 Composition andpreservation of the Chengjiang fauna a Lower Cambrian soft-bodied biota Zoologica Scripta 20 395ndash411

Hou X Bergstrom J amp Ahlberg P 1995 Anomalocaris and otherlarge animals in the Lower Cambrian Chengjiang fauna ofsouthwest China GFF 117 163ndash83

Hou X amp Bergstrom J 1997 Arthropods of the Lower CambrianChengjiang fauna southwest China Fossils and Strata 45 1ndash116

Hughes C P 1975 Redescription of Burgessia bella from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 4415ndash36

Ivantsov A Y 1999 Trilobite-like arthropod from the LowerCambrian of the Siberian Platform Acta PalaeontologicaPolonica 44 455ndash66

King P E 1973 Pycnogonids London St Martin Press

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 191

Lauterbach K E 1973 Schlussereignisse in der Evolution derStammgruppe der Euarthropoda Zoologische Beitrage 19259ndash99

Lauterbach K E 1980 Schlussereignisse in der Evolution derGrundplans der Arachnata (Arthropoda) Abhandlungen desNaturwissenschaftlichen Vereins in Hamburg 23 163ndash327

Lauterbach K E 1983 Synapomorphien zwischen Trilobiten- undCheliceraten-zweig der Arachnata Zoologischer Anzeiger 210213ndash38

Lendzion K 1975 Fauna of the Mobergella Zone in the Polish LowerCambrian Kwartalnik Geologiczny 19 237ndash42

Lieberman B S 1998 Cladistic analysis of the Early Cambrianolenelloid trilobites Journal of Paleontology 72 59ndash78

Lu Y 1940 On the ontogeny and phylogeny of Redlichia intermediaLu (sp nov) Bulletin of the Geological Society of China 20333ndash42 [In Chinese]

Maddison W P amp Maddison D R 1997 MacClade analysis ofphylogeny and character evolution Version 307 Sunderland MASinauer Associates

Mittmann B amp Scholtz G 2003 Development of the nervous systemin the lsquoheadrsquo of Limulus polyphemus (Chelicerata Xiphosura)morphological evidence for the correspondence between the seg-ments of the chelicerae and the (first) antennae of MandibulataDevelopment Genes and Evolution 213 9ndash17

Moore R C (ed) 1959 Treatise on invertebrate paleontology Part OArthropoda 1 Boulder Colorado Geological Society of America

Muller K J amp Walossek D 1986 Arthropod larvae from the UpperCambrian of Sweden Transactions of the Royal Society ofEdinburgh Earth Sciences 77 157ndash79

Muller K J amp Walossek D 1987 Morphology ontogeny and lifehabit of Agnostus pisiformis from the Upper Cambrian of SwedenFossils and Strata 19 1ndash124

Munilla T 1999 Evolucion y filogenia de los Picnogonidos InMelicA DeHaro J J Medez M amp Ribera I (eds) VolumenMonografico Evolucion y Filogenia de Arthropoda Boletin de laSociedad Entolomologica Aragonesa 26 273ndash91 [In Spanish andEnglish]

Orr P J Siveter D J Briggs D E G Siveter D J amp Sutton M D2000 A new arthropod from the Silurian Konservat-Lagerstatteof Herefordshire Proceedings of the Royal Society of LondonSeries B 267 1497ndash504

Page R D M 1996 On consensus confidence and lsquototal evidencersquoCladistics 12 83ndash92

Palmer A R amp Repina L N 1993 Through a glass darklytaxonomy phylogeny and biostratigraphy of the OlenellinaUniversity of Kansas Paleontological Contributions New Series 31ndash35

Platnick N I Griswold C E amp Coddington J A 1991 On missingentries in cladistic analysis Cladistics 7 337ndash43

Pleijel F 1995 On character coding for phylogeny reconstructionCladistics 11 309ndash15

Raasch G O 1939 Cambrian Merostomata Special Papers of theGeological Society of America 19 1ndash146

Ramskold L Chen J Edgecombe G D amp Zhou G 1996Preservational folds simulating tergite junctions in tegopeltid andnaraoiid arthropods Lethaia 29 15ndash20

Ramskold L Chen J Edgecombe G D amp Zhou G 1997Cindarella and the arachnate clade Xandarellida (ArthropodaEarly Cambrian) from China Transactions of the Royal Society ofEdinburgh Earth Sciences 88 19ndash38

Ramskold L amp Edgecombe G D 1991 Trilobite monophylyrevisited Historical Biology 4 267ndash83

Ramskold L amp Edgecombe G D 1996 Trilobite appendagestructure ndash Eoredlichia revisited Alcheringa 20 269ndash76

Raw F 1957 Origin of chelicerates Journal of Paleontology 31139ndash193

Repina L N amp Okuneva O G 1969 Cambrian arthropods of themaritime territory Paleontological Journal 1969 95ndash103

Richter R amp Richter E 1929 Weinbergina opitz ng n sp einSchwerttrager (Merost Xiphos) aus dem Devon (Rheinland)Senkenbergiana 11 193ndash209

Robison R A 1984 New occurences of the unusual trilobite Naraoiafrom the Cambrian of Idaho and Utah University of KansasPaleontological Contributions Paper 112 1ndash8

Rominger C 1887 Description of primordial fossils from MtStephens NW Territory of Canada Proceedings of the Academyof Natural Sciences of Philadelphia 1887 12ndash19

Scholtz G 1997 Cleavage germ band formation and head segmen-tation the ground pattern of the Euarthropoda In Fortey R Aamp Thomas R H (eds) Arthropod Relationships 317ndash32 LondonChapman and Hall Systematics Association Special Volume 55

Schram F R 1978 Arthropods a convergent phenomenonFieldiana Geology 39 61ndash108

Schram F R 1993 Review of Boxshall Stromberg and Dahl TheCrustacea Origin and Evolution Journal of Crustacean Biology13 820ndash2

Schram F R Vonk R amp Hof C H J 1997 Mazon CreekCycloidea Journal of Paleontology 71 261ndash84

Schram F R amp Emerson M J 1991 Arthropod Pattern Theorya new approach to arthropod phylogeny Memoirs of theQueensland Museum 31 1ndash18

Schram F R amp Hof C H J 1998 Fossils and the interrelationshipsof major crustacean groups In Edgecombe G D (ed) Arthropodfossils and phylogeny pp 233ndash302 New York NY ColumbiaUniversity Press

Shergold J H 1991 Protaspis and early meraspis growth stages of theeodiscoid trilobite Pagetia ocellata Jell and their implications forclassification Alcheringa 15 65ndash86

Shu D Geyer G Ling C amp Zhang Z 1995 Redlichiacean trilobiteswith preserved soft parts from the Lower Cambrian Chengjiangfauna (South China) Beringeria (Special Issue) 2 203ndash41

Shultz J W 1990 Evolutionary morphology and phylogeny ofArachnida Cladistics 6 1ndash38

Shultz J W 2000 Skeletomuscular anatomy of the harvestmanLeiobunum aldrichi (Weed 1893) (Arachnida OpilionesPalpatores) and its evolutionary significance Zoological Journalof the Linnean Society 128 401ndash38

Shultz J W 2001 Gross muscular anatomy of Limulus polyphemus(Xiphosura Chelicerata) and its bearing on evolution in the theArachnida Journal of Arachnology 29 283ndash303

Siewing R (ed) 1985 Lehrbuch der Zoologie II Systematik 3rd ednStuttgart Fischer

Simonetta A M 1970 Studies on the non trilobite arthropods ofthe Burgess Shale (Middle Cambrian) the genera LeanchoiliaAlalcomenaeus Opabinia Burgessia Yohoia and ActaeusPalaeontographica Italica 66 35ndash45

Simonetta A M amp Delle Cave L 1975 The Cambrian non trilobitearthropods from the Burgess Shale of British Columbia A studyof their comparative morphology taxonomy and evolutionarysignificance Palaeontographica Italica 69 1ndash37

Simonetta A M amp Delle Cave L 1981 An essay in the compara-tive and evolutionary morphology of Palaeozoic ArthropodsAccademia Nazionale dei Lincei Atti dei Convegni Lincei 49389ndash439

Starobogatov Y I 1990 Sistema i filogeniya nizshikh khelitserovykh(analiz morfologii paleozoysikh grupp) [The systematics andphylogeny of the lower chelicerates (a morphological analysis ofthe Palaeozoic groups)] Paleontologicheskii Zhurnal 1990 4ndash17

Stoslashrmer L 1944 On the relationships and phylogeny of fossil andrecent Arachnomorpha Skrifter utgitt ac Det Norske Videnskaps-Akademi i Olso I Matematisk-naturvitenskaplig Klasse 5 1ndash158

Stoslashrmer L 1951 Studies on trilobite morphology Part III Theventral cephalic structures with remarks on the zoologicalposition of the trilobites Norsk geologisk tidsskrift 29 108ndash58

Stoslashrmer L 1956 A Lower Cambrian merostome from Sweden Arkivfor Zoologi Ser 2 9 507ndash14

Stoslashrmer L 1959 Trilobitoidea In Moore R C (ed) Treatise onInvertebrate Paleontology part O Arthropoda 1 23ndash37 BoulderColorado Geological Society of America

Strong E E amp Lipscomb D 1999 Character coding and inapplicabledata Cladistics 15 363ndash71

Sturmer W amp Bergstrom J 1976 The arthropods Mimetaster andVachonisia from the Devonian Hunsruck Shale PalaontologischeZeitschrift 50 78ndash111

Sturmer W amp Bergstrom J 1978 The arthropod Cheloniellon fromthe Devonian Hunsruck Shale Palaontologische Zeitschrift 5257ndash81

Sturmer W amp Bergstrom J 1981 Weinbergina a xiphosuran fromthe Devonian Hunsruck Slate Palaontologische Zeitschrift 55237ndash55

Sutton M D Briggs D E G Siveter D J Siveter D J amp Orr PJ 2002 The arthropod Offacolus kingi (Chelicerata) from theSilurian of Herefordshire England computer based morphologi-cal reconstructions and phylogenetic affinity Proceedings of theRoyal Society of London Series B 269 1195ndash1203

Swofford D L 1999 PAUP Phylogenetic Analysis Using Parsimony(and other methods) Version 40b2a Sunderland MA SinauerAssociates

Telford M J amp Thomas R H 1998 Expression of homeobox genesshows chelicerate arthropods retain their deutocerebral segmentProceedings of the National Academy of Sciences USA 9510671ndash5

192 TREVOR J COTTON AND SIMON J BRADDY

Thorley J L amp Page R D M 2000 RadCon phylogenetic treecomparison and consensus Bioinformatics 16 486ndash7

Valentine J W 1995 Why no new phyla after the Cambrian ndashgenome and ecospace hypotheses revisited Palaios 10 190ndash194

Wagele J W 1993 Rejection of the lsquoUniramiarsquo hypothesis andimplications of the Mandibulata concept Zoologische JahrbucherAbteilung fur Systematik Geographie und Biologie der Tiere 120253ndash88

Walcott C D 1911 Middle Cambrian Merostomata Cambriangeology and palaeontology II Smithsonian MiscellaneousCollections 57 17ndash40

Walcott C D 1912 Middle Cambrian Branchiopoda MalacostracaTrilobita and Merostomata Cambrian geology and palaeontol-ogy II Smithsonian Miscellaneous Collections 57 145ndash228

Walcott C D 1918 Geological explorations in the CanadianRockies Explorations and fieldwork of the SmithsonianInstitution in 1917 Smithsonian Miscellaneous Collections 684ndash20

Walossek D 1993 The Upper Cambrian Rehbachiella kinnekullensisand the phylogeny of Branchiopoda and Crustacea Fossils andStrata 32 1ndash202

Walossek D amp Dunlop J A 2002 A larval sea spider (ArthropodaPycnogonida) from the Upper Cambrian lsquoOrstenrsquo of Swedenand the phylogenetic position of pycnogonids Palaeontology 45421ndash46

Walossek D amp Muller K J 1990 Upper Cambrian stem-lineagecrustaceans and their bearing upon the monophyletic origin ofCrustacea and the position of Agnostus Lethaia 23 409ndash27

Walossek D amp Muller K J 1997 Cambrian lsquoOrstenrsquo-type arthro-pods and the phylogeny of Crustacea In Fortey R A amp ThomasR H (eds) Arthropod Relationships 139ndash53 SystematicsAssociation Special Volume 55 London Chapman and Hall

Walossek D amp Muller K J 1998 Early arthropod phylogeny in lightof the Cambrian lsquoOrstenrsquo fossils In Edgecombe G D (ed)Arthropod fossils and phylogeny 185ndash231 New York NYColumbia University Press

Weygoldt P 1979 Significance of later embryonic stages and headdevelopment in arthropod phylogeny In Gupta A P (ed)Arthropod Phylogeny 107ndash35 New York NY Van NostrandReinhold Co

Weygoldt P 1998 Evolution and systematics of the ChelicerataExperimental and Applied Acarology 22 63ndash79

Weygoldt P amp Paulus H F 1979 Unterzuchungen aur Morpholo-gie Taxonomie und Phylogenie der Chelicerata Zeitschriftfur Zoologische Systematik und Evolutionsforschung 17 85ndash116177ndash200

Wheeler W C Cartwright P amp Hayashi C Y 1993 Arthropodphylogeny a combined approach Cladistics 9 1ndash39

Wheeler W C amp Hayashi C Y 1998 The phylogeny of extantchelicerate orders Cladistics 14 173ndash92

Whittington H B 1971 Redescription of Marrella splendens(Trilobitoidea) from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 209 1ndash24

Whittington H B 1974 Yohoia Walcott and Plenocaris n genarthropods from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 231 1ndash21

Whittington H B 1975a Trilobites with appendages from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 497ndash136

Whittington H B 1975b The enigmatic animal Opabinia regalisMiddle Cambrian Burgess Shale British Columbia PhilosophicalTransactions of the Royal Society of London Series B 271 1ndash43

Whittington H B 1977 The Middle Cambrian trilobite NaraoiaBurgess Shale British Columbia Philosophical Transactions of theRoyal Society Series B 280 409ndash43

Whittington H B 1979 Early arthropods their appendages andrelationships In House M R (ed) The origin of majorinvertebrate groups 253ndash68 Systematics Association SpecialVolume 12 New York NY Academic Press

Whittington H B 1980 Exoskeleton moult stage appendage mor-phology and habits of the Middle Cambrian trilobite Olenoidesserratus Palaeontology 23 171ndash204

Whittington H B 1981 Rare arthropods from the Burgess ShaleMiddle Cambrian British Columbia Philosophical Transactionsof the Royal Society of London Series B 292 329ndash57

Whittington H B 1985 Tegopelte gigas a second soft-bodied trilo-bite from the Burgess Shale Middle Cambrian British ColumbiaJournal of Palaeontology 23 171ndash204

Whittington H B 1989 Olenelloid trilobites type species functionalmorphology and higher classification Philosophical Transactionsof the Royal Society of London Series B 324 111ndash47

Whittington H B amp Almond J E 1987 Appendages and habits ofthe Upper Ordovician trilobite Triarthus eatoni PhilosophicalTransactions of the Royal Society of London Series B 317 1ndash46

Wilkinson M 1992 Ordered versus unordered characters Cladistics8 375ndash85

Wilkinson M 1994 Common cladistic information and its consensusrepresentation reduced Adams and reduced cladistic consensustrees and profiles Systematic Biology 43 343ndash68

Wilkinson M 1995a Coping with abundant missing entries in phylo-genetic inference using parsimony Systematic Biology 44 501ndash14

Wilkinson M 1995b TAXEQ2 software and documentation BristolSchool of Biological Sciences University of Bristol

Wilkinson M 1996 On the distribution of homoplasy and choosingamong trees Taxon 45 263ndash66

Wills M A 1997 A phylogeny of Recent and fossil Crustacea derivedfrom morphological characters In Fortey R A amp ThomasR H (eds) Arthropod Relationships 189ndash209 SystematicsAssociation Special Volume 55 London Chapman and Hall

Wills M A Briggs D E G amp Fortey R A 1994 Disparity asan evolutionary index a comparison of Cambrian and Recentarthropods Paleobiology 20 93ndash130

Wills M A Briggs D E G Fortey R A amp Wilkinson M 1995The significance of fossils in understanding arthropod evolutionVerhandlungen der deutschen zoologischen Gesselschaft 88 203ndash15

Wills M A Briggs D E G Fortey R A Wilkinson M amp SneathP H A 1998a An arthropod phylogeny based on fossil andrecent taxa In Edgecombe G D (ed) Arthropod fossils andphylogeny 33ndash105 New York NY Columbia University Press

Wills M A Briggs D E G amp Fortey R A 1998b Evolutionarycorrelates of arthropod tagmosis scrambled legs In ForteyR A amp Thomas R H (eds) Arthropod Relationships 57ndash65Systematics Association Special Volume 55 London Chapmanand Hall

Winter G 1980 Beitrage zur Morphologie und Embryologie desvorderen Korperabschnitts (Cephalosoma) der PantopodaGerstaecker 1863 I Entstehung und Struktur des Zentralnerven-systems Zeitschrift fur Zoologische Systematik und Evolutionsfor-schung 18 27ndash61

Zhang W amp Hou X 1985 Preliminary notes on the occurrence of theunusual trilobite Naraoia in Asia Acta Palaeontologica Sinica 24591ndash5 [In Chinese with English summary]

TREVOR J COTTON Department of Palaeontology The Natural History Museum CromwellRoad London SW7 5BD UKe-mail trevor_cottonhotmailcom

SIMON J BRADDY Department of Earth Sciences University of Bristol Wills MemorialBuilding Queenrsquos Road Bristol BS8 1RJ UKe-mail S J Braddybrisacuk

MS received 28 July 2003 Accepted for publication 15 March 2004

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 193

Page 11: The phylogeny of arachnomorph arthropods and the origin of the

37) it is here preferred to regard this as a distinct character(Character 9 herein)

The coding of Alalcomenaeus by Wills et al (1998a) ashaving four post-acronal somites is accepted here but fordifferent reasons Briggs amp Collins (1999) have recenty demon-strated that Alalcomenaeus possessed two pairs of biramousappendages on the head in addition to the great appendagesThis would equate to three head somites according to thehomology scheme of Wills et al (1998a) Here the antennaeare inferred to be lost and therefore four segments incorpo-rated into the head

Recently a number of authors have agreed that the plesio-morphic condition for euarthropods is a four-segmented headas found in stem group crustaceans (Scholtz 1997 Walossek1993 Walossek amp Muller 1997 1998 Edgecombe et al 2000)However reconstruction of the euarthropod stem groupclearly indicates that even crownward taxa had a head consist-ing of only the antennal segment and acron as found intardigrades and onychophorans Therefore it seems that thefour-segmented head is pleisomorphic only for crown groupeuarthropods Some fully arthropodised fossil taxa also have ashorter head that may be pleisomorphic such as the two-segmented head of Marella Retifacies is coded following therecent redescription of Hou amp Bergstrom (1997) rather thanon the basis of the coding by Edgecombe amp Ramskold (1999)for which they provide no explanation A partial uncertaintycoding is used for Aglaspis in which the number of post-antennal cephalic appendages is either three or four (Briggset al 1979)

5 Orientation of the antennae (0) directed anterolaterally(1) strongly deflected laterally and (2) placed well inside shieldmargin curing posteriorly from a transverse proximal element(cf Edgecombe amp Ramskold 1999 character 3) The anteriorappendages of Aglaspis (see character 1) are clearly directedanterolaterally and it is presumed that they continued past thehead shield margin The anterior appendages of arthropodstem group taxa such as Aysheaia Kerygmachela and Anoma-locaris are either directed anterolaterally or ventrally but arenot strongly-deflected laterally Therefore the outgroup iscoded as State 0 This character is coded as missing data fortaxa where the presence of antennae is equivocal (those codedas missing data for Character 1) and as inapplicable in taxawhere the antennae are considered absent (coded as State 1 forCharacter 1)

6 Length of distal spines on terminal podomeres of endo-pods of second segment appendages (0) absent or shorer thanpodomeres (1) subsequal to length of podomeres and (2)longer than the entire podomere series The spinose projectionsof the raptorial appendages described above vary in length Inchelicerae (Figs 5BC) and some lsquogreat appendagesrsquo (eg thoseof Yohoia Fig 5D) the most distal spine is equal in length tothe spinose terminal podomere forming a chelate structureIn Fortiforceps (Fig 5A) the spines are short relative tothe lengths of the podomeres but in Alalcomenaeus andLeanchoilia (Fig 5E) they are extremely long so that thespines are much longer than the entire podomere series Asdescribed above in taxa with pediform endopods of the secondsegment appendages spines on the distal podomeres are shortor entirely absent

7 Chelicerae (0) absent and (1) present Despite theirsimilarities to lsquogreat appendagesrsquo the chelicerae of the eucheli-cerates and chelifores of the pycnogonids are clearly distinctand have been widely recognised as a chelicerate synapomor-phy Chelicerae differ from any lsquogreat appendagesrsquo by thecombination of a smaller number of podomeres the presenceof only a single terminal element and only a single spinoseprojection on the dorsal side of the podomere series (Fig 5)

Amongst extant chelicerates only the pycnogonid Pallenopsishas chelicerae of four podomeres comparable to the numberin lsquogreat appendagesrsquo Bergstrom et al (1980) considered thatthe chelifores of the Devonian pycnogonid Palaeoisopus alsoconsist of four segments but this is not well supported by theirfigures

8 Distal spines of second segment endopods terminating inannulated flagellae (0) absent and (1) present It is widelyrecognised (eg Stoslashrmer 1944 Simonetta 1970 Bruton ampWhittington 1993 Briggs amp Collins 1999) that the terminalparts of the extended spines of Alalcomenaeus and Leanchoiliaformed annulated flagellae Nothing similar is known fromhomologous appendages of any other arthropod althoughsimilar processes may have operated in the transformation ofthe endopod as a whole into the second antennae of derivedcrustaceans and in the origin of multiramous antennulae (firstantennae) in malacostracans

242 Posterior appendages 9 Appendages of first thoracicsomite underneath the cephalo-thoracic articulation (0) ab-sent and (1) present (cf Edgecombe amp Ramskold 1999character 2) For a discussion of this character see Character4 herein

10 Exopods of appendages of third to fifth segments (0)present and (1) reduced or absent A number of taxa haveuniramous appendages on the third to fifth segments In mostthese segments are incorporated into the head (Yohoia cheli-cerates) but in others all (eg in Sidneyia) or some (eg inCheloniellon) of these segments are post-cephalic In all casesthe appendage is morphologically similar to the endopods ofbiramous limbs of other taxa andor other segments and it isinterpreted that the exopod is lost

11 Endopods of thoracic appendages (0) present and (1)reduced or absent The opisthosomal appendages of chelicer-ates lack endopods or have the endopods much reduced (egLimulus Siewing 1985 fig 838 Shultz 2001) a situation that isalso found in the uniramous thoracic appendages of Yohoia(Whittington 1974) It has been suggested that Helmetia(Briggs et al 1994) lacks endopods but pending a redescrip-tion of this genus and considering that they have been docu-mented in the otherwise very similar Kuamaia this is coded asuncertain

12 Exopod shaft of numerous podomeres each bearing asingle seta (0) present and (1) absent The exopods ofcrustaceans (at least plesiomorphically see eg Walossek ampMuller 1998 figs 5middot5 amp 5middot6) and of Marrella and Mimetasterhave multi-annulated shafts with each podomere bearing asingle seta In other taxa the exopod shafts consist of one oftwo lobes bearing numerous setae

The polarity of exopod segmentation is uncertain Thelateral flaps of stem group arthropods such as Opabinia(Whittington 1975b Budd 1996b) and anomalocaridids (egHou et al 1995) are certainly unsegmented but as suggestedby Buddrsquos (1996b fig 8) reconstruction this does notnecessarily suggest the form of the primitive euarthropodexopod Rather segmentation may have been a result of thesclerotization of the cuticle of the exopod shaft The outgroupwas coded as uncertain for this character According to thefirst interpretation of aglaspidid appendage morphology(coded as Aglaspidida 1) the exopods are lacking on allappendages and consequently this and all other charactersdescribing exopod morphology are coded as inapplicable

13 Exopod shaft differentiated into proximal and distallobes (0) absent and (1) present Ramskold amp Edgecombe(1996 Edgecombe amp Ramskold 1999 character 26) havediscussed the distribution of the trilobite-type bilobate exo-pods recognised by this character (Fig 6A) Among taxaincluded here that were not considered by Edgecombe amp

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 179

Ramskold (1999) exopods consist either of a single flattenedlobe or a homonomous series of podomeres (Fig 6B C) Nostem group euarthropod has a bilobate exopod and theoutgroup is consequently also coded as State 0

14 Proximal lobe of exopod (0) flattened lobe and (1)slender shaft

15 Distal lobe of exopod (0) small to moderate sized flapwith short to moderately long attachment to proximal lobeand (1) large teardrop-shaped with long attachment to proxi-mal lobe Neither of these characters can be coded for taxathat do not have an exopod differentiated into proximal anddistal lobes (Character 13 State 1) without asserting thehomology of the single-lobed exopod with one of these partsThe coding of Edgecombe amp Ramskold (1999 p 280) implic-itly homologizes the exopods of Sidneyia and Retifacies withthe proximal lobe In these taxa this view is perhaps supportedby the presence of lamellate exopod setae which match thoseof the trilobite-type proximal lobe However in other taxawith a single-lobed exopod such as Alalcomenaeus (see Briggsamp Collins 1999) the exopod is fringed with sharp spines likethe distal lobe of differentiated exopods Alternatively thebilobate exopod may have originated from the division of aprimitively single-lobed structure The single-lobed exopod isnot clearly homologous with either lobe of divided exopodsand both these characters are consequently coded as inappli-cable to taxa without differentiated proximal and distal exopodlobes Consequently (see Character 13 herein) these characterscan only be coded for taxa that were previously considered byEdgecombe amp Ramskold (1999) and their coding is followedhere except in the cases of Sidneyia and Retifacies

16 Exopod shaft is a deep rounded flap (0) absent and (1)present All bilobate (Character 13) and segmented (Character12) exopod shafts are long and relatively narrow structures (seeFig 6) whereas some single-lobed exopod shafts have theform of rounded flaps These flap-like exopod shafts are largecompared to the length of the setae and at least half as deep asthey are long This appendage structure is found in cheliceratesand lsquogreat appendagersquo arthropods

17 Medially-directed exopod setae (0) absent and (1)present Walossek amp Muller (1998 p 194) suggested that thetilting of exopod setae towards the endopod in post-antennularlimbs with a multi-annulated exopod was an autapomorphyuniting crustaceans and all members of the crustacean stemgroup (the crustacean total group sensu Ax 1986 see Fig 6C)This character is shared with the marellomorphs Marrellaand Mimetaster (see Fig 6B Sturmer amp Bergstrom 1976Bergstrom 1979 fig 1middot3A B) In other taxa the setae eithersurround the entire margin of the exopod shaft or are directeddorsally (Fig 6A) The identification of setae on the dorsalsurface of the lateral flaps in Opabinia (Budd 1996b) suggeststhat the condition in marellomorphs and crustaceans isderived

18 Lamellate exopod setae (0) absent and (1) presentLamellate exopod setae originally described from trilobitesare a classic synapomorphy of the Arachnomorpha (see egBergstrom 1992 Hou amp Bergstrom 1997 pp 42ndash3) They areperhaps best known from Misszhouia (see Chen et al 1996Hou amp Bergstrom 1997 Fig 6A) These setae differ from themore spinose setae of other taxa (Fig 6C) in that they are wideand flat and imbricate over the length of the exopod shaft Thesetae of Marrella (Fig 6B) and similar taxa have variouslybeen considered lamellate (Bergstrom 1979) or non-lamellate(Bergstrom 1992) Since they do not imbricate and do notseem to be strongly flattened (Whittington 1971 Sturmer ampBergstrom 1976 fig 9a) Marrella and Mimetaster are codedas State 0 The setae of Helmetia as shown in Briggs et al(1994 fig 141) seem to be of the lamellate type The setae ofSinoburius have been described by Hou amp Bergstrom (1997p 85) as similar to those of Misszhouia Only the setae areknown of the appendages of Buenaspis (Budd 1999a) andthese also appear to be of the trilobite-type

Figure 6 Diagrammatic reconstructions of (A) arachnomorph and(B C) non-arachnomorph biramous appendages (A) The lsquotrilobite-typersquo biramous appendage of Misszhouia longicauda (after Chen et al1997) (B) Appendage of Marrella splendens (after Whittington 1971)(C) Appendage of the stem-lineage crustacean Martinssonia elongata(after Muller amp Walossek 1997 1998) Not to scale Abbreviations(bas) basis (dle) distal lobe of exopod shaft (ls) lamellar exopod setae(pe) proximal endite or pre-coxa (ple) proximal lobe of exopod shaft(ses) segmented exopod shaft and (ss) spinose exopod setae

180 TREVOR J COTTON AND SIMON J BRADDY

The homology of the book-gill lamellae of xiphosurans withlamellar setae was supported by Walossek amp Muller (1997p 149) and Edgecombe et al (2000 p 174) but rejected bySturmer amp Berstrom (1981) on the grounds that Weinberginapossesses Limulus-like gill lamellae and fringing setaeBook-gills have also been described from a eurypterid (Braddyet al 1999) There are certainly major morphologicaldifferences between book-gills and trilobite-type exopodsFollowing most recent opinion and pending further studyWeinbergina and Eurypterida are coded as possessing lamellatesetae

The form of the setae of Yohoia is uncertain (Whittington1974) the spinose setae seen in the most common reconstruc-tion (Gould 1989 Briggs et al 1994) are not justified by thespecimens Amongst other great appendage arthropods theform of the setae appears to vary those of Jianfengia (seeChen amp Zhou 1997 p 74) and Leanchoilia (see Bruton ampWhittington 1983) are lamellate while those of Fortiforceps(Hou amp Bergstrom 1997) and Alalcomenaeus (Briggs amp Collins1999) are spinose and less densely packed

19 Gnathobase on basis andor prominent endites onendopod (0) present and (1) absent (cf Edgecombe ampRamskold 1999 character 29 Wills et al 1998a characters 36amp 59) The presence of gnathobases on the appendages of someanomalocaridids (Hou et al 1995) and their general distribu-tion amongst non-arachnomorph euarthropods suggests thatState 0 is plesiomorphic The homology of the various enditesand gnathobasic structures found in euarthropods is in need ofcareful assessment and therefore a very general coding (fol-lowing Edgecombe amp Ramskold) is used for this character

243 Eyes 20 Position of lateral facetted eyes (0)ventral and stalked (1) dorsal and sessile and (2) absent (cfEdgecombe amp Ramskold 1999 character 4) Partial uncer-tainty coding is used for a number of taxa where the dorsalsurface is known but the ventral morphology is not andtherefore the presence of ventral eyes cannot be discountedFor example there is good evidence that Leanchoilia lackeddorsal sessile eyes but ventral stalked eyes may have beenpresent (as in L hanceyi see Briggs amp Robison 1984) and a(02) partial uncertainty coding is used This is also the case inmany naraoiids such as Buenaspis However only the outlineof the head shield of Liwia is known and the absence of dorsaleyes in the reconstruction of Dzik amp Lendzion (1988) isconjectural It is unclear whether the autapomorphic conditionof stalked dorsal eyes in Mimetaster (see Sturmer amp Bergstrom1976) is homologous with State 0 or State 1 and a partialuncertainty coding is also used

Whittington (1974) was somewhat equivocal about thenature of the lateral lobes anterior to the head shield ofYohoia These more closely resemble the ventral stalked eyes ofAlacomenaeus as recently described by Briggs amp Collins(1999) than either Whittingtonrsquos (1974) or subsequent (egGould 1989 fig 3middot18 Briggs et al 1994 fig 153) reconstruc-tions suggest In a well preserved specimen showing the dorsalaspect (USNM 57696 Whittington 1974 pl 2 figs 1ndash3) and ina laterally compressed specimen (USNM 57694 Whittington1974 pl 1 fig 1) they are clearly seen to be stalked andrelatively small lobate structures That these structures arelikely to represent stalked ventral eyes was reflected in thecoding of Wills et al (1998a characters 26 amp 27)

21 Visual surface with calcified lenses bounded withcircumocular suture (0) absent and (1) present

22 Dorsal bulge in exoskeleton accommodating drop-shaped ventral eyes (0) absent and (1) present

23 Eye slits (0) absent and (1) presentCharacters 21 to 23 are used exactly as described by

Edgecombe amp Ramskold (1999 character 5) Character 21 is

inapplicable to taxa that do not possess eyes (Character 20State 2)

24 Dorsal median eyes (0) absent and (1) present Dorsalmedian eyes on a tubercle are considered a synapomorphy ofthe Chelicerata (Dunlop amp Selden 1997 Dunlop 1999) Thenature and position of the structures interpreted as dorsalmedian eyes in Mimetaster (Sturmer amp Bergstrom 1976p 83ndash4) are regarded as equivocal

244 Ventral cephalic structures Many characters of theventral surface of the euarthropod head (see Fig 7) of poten-tial phylogenetic utility are poorly known in many importanttaxa In particular the presence of pre-hypostomal frontalorgans (Fig 7A CndashE) is not coded herein (see Edgecombe ampRamskold 1999 p 272) Definitive evidence of the absence ofthese structures is available for very few taxa and the homol-ogy of these structures with the maculae of the trilobitehypostome (see Fig 7B) and the frontal organs of the crusta-cean labrum (eg see Muller amp Walossek 1987 p 39) isuncertain Secondly the detailed homology of the trilobitehypostome with that of other putative arachnomorphs isunclear and consequently a very broad definition is usedherein For example various pre-hypostomal sclerites (Fig7A D) in other arachnomorphs may have been incorporatedalong with a primitive hypostome into a single sclerite that isrecognised as the hypostome in trilobites The structure of thehypostome (eg the presence of paired anterior wings dorsal tothe antennae Fig 7B C) may also be a source of additionalcharacters

25 Expanded cephalic doublure (0) absent and (1)present maximum width more than 30 length of head shieldor more than 25 width of pygidium Chen et al (1996)suggested that the wide doublure of Soomaspis and Tariccoia isa synapomorphy uniting these taxa The doublure of Buenaspis(see Budd 1999a) is poorly known but based on the width ofthe heavily crushed region of the cephalic margin and theposition of a possible impression of the edge of the doublure inone specimen (Budd 1999a pl 1 fig 1) it also appears to berather wide (ie greater than 30 of the length of the headshield) Following Edgecombe amp Ramskoldrsquos coding ofSidneyia the present authors do not consider the postero-median expansion of the doublure of for example Emeraldellaor Retifacies (see Bruton amp Whittington 1983 Hou ampBergstrom 1999 respectively) to be homologous with State 1but to represent the hypostome (see Character 26)

26 Anteromedian margin of cephalon notched accomo-dating strongly sclerotised plate (0) notch and plate absentand (1) notch and plate present (cf Edgecombe amp Rasmkold1999 character 10) The apomorphic state (illustrated inFig 7D) is only known in the taxa discussed by Edgecombe ampRasmkold (1999) Reconstructions of Yohoia show a roundedlobe between the eyes (see Character 20) anterior to anddistinct from the head shield which resembles the anteriorsclerite recognised here This seems to be a misinterpretationSpecimens preserved in dorsal aspect (USNM 57696Whittington 1974 pl 2 figs 1ndash3) and lateral aspect (USNM57694 Whittington 1974 pl 1 fig 1 USNM 155616Whittington 1974 pl 5 fig 2) seem to clearly show that thislsquomedian lobersquo is a downward curving pointed extension of thehead shield

27 Hypostomal sclerite (0) median extension of thedoublure with no suture (1) natant sclerite not in contactwith doublure (2) with narrow overlap with pre-hypostomalsclerite (3) narrow attachment to doublure at hypostomalsuture and (4) absent The homology of hypostomes andlabrums has been discussed by Haas et al (2001) and Budd(2002) The euarthropod labrum is likely to represent a pos-teroventral extension of the pre-segmental acron (Scholtz

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 181

Figure 7 Ventral reconstructions of arachnomorph arthropod heads (A) Misszhouia longicaudata (after Chenet al 1997 figs 2ndash4 7) (B) Ceraurinella typa (after Chen et al 1997 fig 7) (C) Agnostus pisiformis (after Mulleramp Walossek 1987) (D) Kuamaia lata (after Edgecombe amp Ramskold 1999 figs 32 5 amp 6) (E) Cindarella eucala(after Ramskold et al 1997) and (F) Emeraldella brocki (after Bruton amp Whittington 1983) Not to scaleAbbreviations (aw) anterior wing beneath antennae (bs) boomerang-shaped sclerite (bl) blade-like process ofposterior wing (d) doublure of head-shield (cs) connective suture (f) fenestra (fs) facial suture (h) hypostome(hl) hypostomal lobe (emerging through fenestrae of Agnostus) (hs) hypostomal suture (if) intervening field ofhypostomal complex (lb) lateral bridge (lfo) lateral frontal organ (mb) median body (mbr) median bridge (mf)median furrow (mfb) mouth field bridge (mfo) median frontal organ (ol) ovate lobe and (phs) pre-hypostomalsclerite Post-antennal appendages and distal parts of antennae omitted

182 TREVOR J COTTON AND SIMON J BRADDY

1997) This structure is often sclerotised and covers the mouthventrally It is this sclerite that is here identified as thehypostome Therefore this character is considered absent intaxa lacking a sclerite covering the mouth irrespective of themorphology and position of the labrum itself which at leastpartly covers the mouth in all euarthropods other thanpycnogonids (see Edgecombe et al 2000 p 167) The lsquofleshylabrumrsquo of crustaceans does not appear to be sclerotised and ahypostome is consequently considered to be absent Althoughmany derived features are associated with the crustaceanlabrum this is not a good reason to consider it non-homologous with the hypostome-bearing structure of otherarthropods as Walossek amp Muller (1990) argued

In many taxa the mouth is covered by a posteromedianextension of the doublure here considered to represent thehypostome (eg Emeraldella Fig 7F Aglaspis see Hesselbo1992 fig 52) In others the hypostome is separated from thedoublure either by a suture a pre-hypostomal sclerite (egKuamaia lata Fig 7D Saperion glumaceum see Edgecombe ampRamskold 1999 figs 3 amp 4) or an intervening region ofunsclerotised cuticle (the natant condition of Fortey 1990beg Cindarella eucala Fig 7E) The homology of pre-hypostomal sclerites and hypostomes in helmetiids trilobitesand naraoiids (see Fig 7) has been discussed by Chen et al(1996) and Edgecombe amp Ramskold (1999) but it is as yetunclear whether any of the character states described here arederived from any other They are treated here as distinct andthe character as unordered pending further study

No hypostome has been identified in Yohoia (see above)Alalcomenaeus Jianfengia Fortiforceps or Leanchoilia andthere is certainly no broad posterior extension of the doublurein any of these taxa Since there is also no pre-hypostomalsclerite they have received a (134) partial uncertainty codingThe attachment of the hypostome of Tegopelte is unclear butit does not seem to be attached to any doublure (Whittington1985) and therefore it is given an uncertainty (12) coding

The lsquorostrum-like structurersquo on the ventral surface ofLemoneites appears to be similar in morphology and positionrelative to the anterior margin of the head shield (Flower 1968pl 8 figs 4 amp 13) to that described from Aglaspis and is codedas State 0 In many taxa the hypostome is unknown but inonly a small number can it be coded reliably as absentHughesrsquo (1975) suggestion that the hypostome of Burgessia isabsent is accepted preliminarily but an extension of thedoublure may be visible in some of Hughesrsquo figures Fortey(pers comm 2000) also doubts Hughesrsquo assertion

28 Visible ecdysial sutures (0) absent and (1) present Themarginal ecdysial sutures of chelicerates and the dorsal suturesof trilobites are almost certainly not homologous but thepresence of sutures and their position (see Character 29) arecoded separately to allow this to be tested Wills et al (1998aCharacter 2) coded marginal sutures as present in Sidneyiafollowing Brutonrsquos (1981) suggestion but this is consideredequivocal here

29 Position of ecdysial sutures (0) marginal and (1) dor-sal This character is inapplicable to taxa lacking ecdysialsutures (Character 28 State 0) Dorsal sutures whilst presentin the representatives of the Trilobita coded here are probablynot a synapomorphy of trilobites as a whole but for a clade oftrilobites excluding the Olenellida (Whittington 1989 Fortey1990a)

245 Exoskeletal tergites and thoracic tagmosis 30 Min-eralised cuticle (0) absent and (1) present Calcification of theexoskeleton is one of the most convincing synapomorphiesof the Trilobita (Fortey amp Whittington 1989 Ramskold ampEdgecombe 1991 Edgecombe amp Ramskold 1999 Character 1)Cuticle mineralisation is also coded as present in aglaspidids

following Briggs amp Fortey (1982) who suggested that theaglaspidid exoskeleton was originally phosphatic Paleomerusprobably also had a mineralised cuticle but Hou amp Bergstrom(1997 p 97) argued that it was likely to be calcareous Anundescribed Silurian aglaspidid may also have had an origi-nally calcitic cuticle (Fortey amp Theron 1994 p 856) Becauseof the uncertainty about the chemical composition of theexoskeleton in these forms cuticle mineralisation is coded aspotentially homologous in all these taxa and no attempt ismade to code separate states for phosphatic and calcareousmineralisation

31 Trunk tergites with expanded lateral pleurae coveringappendages dorsally (0) absent and (1) present Whilst thepresence of paratergal folds may be a synapomorphy at thelevel of the Euarthropoda (Boudreaux 1979 Wagele 1993Edgecombe et al 2000 Character 142) these are at mostsmall reflections of the margin of the tergites in most euarthro-pods In arachnomorph taxa these are expanded to form largelateral pleurae that cover the appendages dorsally (see egLimulus and Triarthus in Boudreaux 1979) This is inferred tobe the case in some taxa where dorsal features and appendagesare poorly known on the basis of their possession of tergiteswith wide pleural regions This feature was suggested as typicalof lamellipedians (=arachnomorphs) by Hou amp Bergstrom(1997 pp 42ndash3)

The arrangement of the thorax of Marrella Mimetaster andBurgessia differs from that of all the other taxa considered herein that the appendages seem to be attached to the lateralmargins of the body The trunk tergites do not cover theappendages and seem to lack pleurae entirely although theyare covered by a posterior extension of the head shield inBurgessia This character has been clearly described inMimetaster (Sturmer amp Bergstrom 1976 pp 87ndash90 fig 8) andBurgessia (Hughes 1975 p 421)

32 Free thoracic tergites (0) present and (1) absentFollowing Edgecombe amp Ramskold (1999 Character 16) anumber of taxa lack functional post-cephalic articulations andconsequently lack free thoracic tergites Despite this theycode these taxa for a number of characters relating to thestructure of thoracic tergites (eg their characters 18 and 19)These characters are treated here as inapplicable to taxawithout free tergites and the definition of this character hasbeen modified from that of Edgecombe amp Ramskold (1999) tomake this more explicit

33 Decoupling of thoracic tergites and segments (0)absent and (1) present Ramskold et al (1997) described thischaracter of Cindarella and Xandarella where the thoracictergites correspond to a variable increasing posteriorly num-ber of appendage pairs This character cannot be coded fortaxa in which free thoracic tergites are absent (Character 32State 1)

34 Tergite articulations (0) tergites non-overlapping (1)extensive overlap of tergites and (2) edge-to-edge pleuralarticulations In most of the taxa considered here the thoracictergites overlap considerably and relatively evenly over theirwidth This contrasts with the primitive euarthropod condi-tion where the tergites do not overlap medially as seen in stemgroup crustaceans In trilobites such as Helmetia and Kuamaia(see Edgecombe amp Ramskold 1999 character 18) overlap ofthoracic tergites is limited to a well-defined axial region andthe lateral pleurae meet edge-to-edge

The thoracic articulations of naraoiids with free thoracictergites are in some ways similar to those of trilobites with astrong overlap medially that is lacking abaxially andanterolateral articulating facets (Budd 1999a Ramskold ampEdgecombe 1999) However the distribution of these features

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 183

in other trilobite-like arachnomorphs is unclear and a restric-tive coding is used here The present authors do not considerthat this character can be applied reliably to the fused thoracictergites of Saperion Tegopelte and Skioldia and it is coded asinapplicable to all taxa lacking free thoracic tergites (Character32 State 1)

35 Trunk effacement (0) trunk with defined (separate orfused) tergite boundaries (1) trunk tergite boundaries effacedlaterally and (2) trunk tergite boundaries completely effaced(cf Edgecombe amp Ramskold 1999 Character 15)] Contrary toEdgecombe amp Ramskold (1999) the present authors considerSkioldia and Saperion to show a distinct form of tergiteeffacement where the fused tergite boundaries are defined byfurrows axially but effaced laterally The boundaries areeffaced at least laterally in Tegopelte but the axial region ofthe exoskeleton is unknown which accordingly is given amultistate uncertainty coding

36 Cephalic articulation fused (0) absent and (1) presentIn Tegopelte Saperion and Skioldia the articulation ofthe head with the thorax is non-functional and the entireexoskeleton forms a single tergite

37 Head shield overlap of thoracic tergites (0) overlapabsent or identical to overlap between thoracic segments (1)head shield covers first thoracic tergite only and (2) headshield covers mutliple anterior trunk tergites

38 Head shield articulates with reduced anterior thoracictergite (0) absent and (1) present Amongst taxa showing aposteriorly expanded head shield ie one that overlaps thethorax Edgecombe amp Ramskold (1999 Character 9) identifiedtwo distinct character states One of these ndash overlap of only theanteriormost thoracic tergite ndash was limited to taxa assigned tothe Liwiinae (sensu Fortey amp Theron 1994) and another ndashoverlap of multiple tergites with attachment to a reducedanterior thoracic tergite ndash to the Xandarellida None of theadditional taxa considered herein (including Buenaspis whichwas assigned to the Liwiinae by Budd 1999a) shows thesedistinctive morphological features However the expandedhead shields of Marrella Mimetaster and Burgessia which donot articulate with a narrow anterior thoracic tergite may behomologous with the expanded head shields of xandarellidsTo recognise this the degree of overlapping of the thorax andthe possession of the reduced anterior tergite are coded sepa-rately These characters are both coded as inapplicable to taxain which the head shield and thoracic tergites are fused(Character 36 State 1) The crustacean carapace which ismost similar to the head shield of Burgessia is seeminglyabsent in stem group crustaceans (Walossek amp Muller1998)

39 Trunk narrowed anteriorly relative to head shieldwidest posteriorly (0) absent and (1) present (cf Edgecombeamp Ramskold 1999 Character 14) The anterior narrowing ofthe trunk caused by the reduction of opisthosomal segment 1in xiphosurids (Weinbergina of the taxa analysed hereAndersen amp Selden 1997 Dunlop amp Selden 1997 Character14) is not considered to be homologous with the unusual shapeof the thorax in naraoiids recognised by their coding

40 Boundaries of anterior trunk segments reflexed antero-laterally (0) absent boundaries transverse or reflexed postero-laterally and (1) present

41 Joints between posterior tergites functional anteriorones variably fused (0) absent and (1) present

42 Posterior tergite bearing axial spine (0) absent and (1)present

Characters 40 to 42 are coded following Edgecombe ampRamskold (1999 characters 17 19 amp 23 respectively) Inaddition to the taxa considered here a thoracic axial spine ispresent in some olenelloid trilobites (see Lieberman 1998

Characters 73 amp 74) and in the aglaspidid Beckwithia (seeHesselbo 1989)

246 Posterior body termination 43 Postabdomen of seg-ments lacking appendages (0) absent and (1) present

44 Length of postabdomen (0) one segment (1) twosegments (2) three segments and (3) five segments In sometaxa a variable number of posterior thoracic segments bearcomplete tubular tergites and lack appendages In Aglaspisautapomorphically it seems that the posterior three thoracicsegments lack appendages (Briggs et al 1979 Hesselbo 1992)but have unmodified tergites These situations are recognisedas potentially homologous by the coding used here Character44 is coded as inapplicable to taxa lacking a postabdomen

45 Posterior tergites strongly curved in dorsal aspect com-pared to anterior tergites (0) absent and (1) present Asrecognised by Wills et al (1998a Character 17) in some taxawhere the tergites are distinct the curvature of thoracic tergitesincreases posteriorly so that the posterior tergites are highlycurved to semicircular in dorsal aspect This situation isnot known from crustaceans (except isopods) or stem grouparthropods

46 Posterior segments reduced and with highly reducedappendages (0) present and (1) absent In some taxa there area large number of posterior segments that are short sagitallyand have appendages that are much reduced in size comparedto anterior trunk appendages These somites are incorporatedinto the pygidium in some taxa but primitively they arecovered by tiny free trunk tergites In the derived state thetrunk somites and limbs are of a relatively constant size Thedistinction between these states can be seen when attempting tocount the number of segments that make up the body In taxashowing State 0 the number of segments is very high anddifficult to count whereas in other taxa the number ofpost-cephalic segments is easily assessed

47 Pygidium (0) absent and (1) present A pygidium isrecognised here as a posterior tagma consisting of a number offused segments under a single tergite which may or may notincorporate the post-segmental telson This is different to theuse of this term by Edgecombe amp Ramskold (1999) whichfollows Ramskold et al (1997) and very different to its use byWills et al (1998a p 53) as a synapomorphy of the TrilobitaThe present authorsrsquo definition recognises the situation inRetifacies where the spinose postsegmental telson is autapo-morphically not fused to the pygidium as homologous toother multisegmented posterior tagma which include thetelson The distinction between the pygidium and an expandedpost-segmental telson can also be seen in the position of theanus which is consistently in the posteriormost pre-telsonicsegment amongst putative arachnomorphs (see Character 48)In taxa with a pygidium the anal segment is fused into thepygidium (see eg Kuamaia Edgecombe amp Ramskold 1999fig 6) whereas in those taxa lacking a pygidium the anus isbetween the final trunk tergite and the telson

Ramskold et al (1997) argued that the posteriormost tergiteof xandarellids (which incorporates the anal segment) is nothomologous to the posterior tagma recognised as pygidiaherein because posterior thoracic tergites also cover multiplesegments in xandarellids (see Character 33) Evidence ofsegmentation of the pygidium in a variety of taxa suggests thatthe number of tergites fused to form the pygidium is greaterthan the number of appendages For example the pygidium ofKuamaia has two pairs of lateral spines but at least fourpairs of appendages (Hou amp Bergstrom 1997 Edgecombe ampRamskold 1999) and that of Triarthrus has only five axialrings posterior to the articulating ring but over 10 pairs ofappendages (Whittington amp Almond 1987) The fact thatdecoupling of tergites and segments is evident in the thorax of

184 TREVOR J COTTON AND SIMON J BRADDY

Xandarella and Cindarella does not necessarily suggest that asimilar more widespread decoupling in pygidia is the result ofa different developmental process and hence that they arenon-homologous Instead the situation in the xandarellidthorax is potentially homologous to that found (more widely)in pygidia and consequently the terminal xandarellid tergite isa pygidium It is unclear if decoupling of tergites and somites isprimitively limited to the terminal tergite or primitively aproperty of the arachnomorph post-cephalon as a whole

It has been suggested that the tiny pygidium of olenelloidtrilobites which probably best reflects the primitive trilobitestate consists only of the post-segmental telson (Harrington inMoore 1959) and therefore is not a true pygidium HoweverWhittington (1989) showed that the olenelloid pygidium con-sists of at least two and possibly as many as five or sixsegments and therefore is likely to be homologous with thepygidium of other trilobites

48 Position of the anus (0) terminal within telson and (1)at base of telson In crustaceans and stem group arthropodsthe anus is terminal or otherwise situated in the telson (egRehbachiella Walossek 1993 fig 15D) In putative arachno-morphs the anus is either at the junction of the posteriormostthoracic segment and the telson or ventral within a fusedpygidium In the case of taxa with a pygidium the anus isanterior to the posterior margin and where known positionedbetween the posteriormost pair of appendages suggesting thatit is anterior to the post-segmental telson (see eg OlenoidesWhittington 1980)

49 Pygidium with median keel (0) absent and (1) presentEdgecombe amp Ramskold (1999 Character 21) considered thepresence of a median keel on the pygidium as a synapomorphyuniting Soomaspis and Tarricoia They did not explain howthis structure could be distinguished from the raised pygidialaxis of trilobites Homology of these structures is not sup-ported here because the axis of more primitive trilobites(including Eoredlichia) is quite distinct morphologically fromthe naraoiid keel Budd (1999a) noted the presence of a keel onthe pygidium of Buenaspis and suggested (Budd 1999a p 102)that it may be an artefact of dorsoventral compaction How-ever contrary to the coding of Edgecombe amp Ramskold(1999) a keel is visible on the pygidium of Liwia convexa (Dzikamp Lendzion 1988 fig 4CD) preserved in full relief Thereforeit seems unlikely that the keel is a taphonomic artefact

50 Pygidium with broad-based median spine (0) absentand (1) present

51 Pygidium with lateral spines (0) present and (1) absentEdgecombe amp Ramskold (1999 Character 22) coded thepresence of a median spine and two pairs of lateral spines aspotentially homologous in Sinoburius Kuamaia and HelmetiaThe distribution of lateral spines is much wider than that ofterminal spines and therefore they are coded separately hereIn trilobites it has widely been recognised that lateral spinesrepresent the original segmentation of the pygidium Thiscannot be the case for median spines Characters 49 to 51are not applicable to taxa lacking a pygidium (Character 47State 0)

52 Expanded post-segmental telson (0) absent and (1)present In a range of taxa the posterior-most tergite is a large(relative to the thoracic tergites) structure that lacks anyevidence of segmentation and is interpreted as representing anexpanded tergite of the post-segmental telson from whichsegments are released anteriorly during ontogeny On the otherhand the posteriormost tergite of Marrella and probablyMimetaster is small compared to the thoracic tergites androunded This element probably represents the plesiomorphiceuarthropod telson The homology of this character in mosttaxa is clear and only doubtful in Retifacies in which it may

be segmented The figures of Hou amp Bergstrom (1997) providelittle unequivocal evidence for a telson in Retifacies but thepresence of this structure is very clear in colour photographs(eg Chen et al 1996 fig 198A) However these figures donot convincingly demonstrate the segmentation of the telsonRetifacies is interpreted here as being unique in possessingboth a pygidium and an expanded post-segmental telson

53 Telson shape (0) spinose and (1) paddle-shaped Theshape of the expanded telsons identified above varies frombroadly spinose to flattened and paddle-like This differencecan be recognised both on the basis of the ratio of length tobasal width (spinose telsons are relatively long) and by thechange in width posteriorly (spinose telsons reduce in widthposteriorly whereas paddle-shaped telsons increase in width)In eurypterids a wide range of telson shapes is found includ-ing both character states described here It is likely that theplesiomorphic condition is a spinose telson This character isinapplicable to taxa lacking an expanded telson

54 Post-ventral furcae (0) absent and (1) present Thischaracter recognises the potential homology of unsegmentedpaired structures that articulate with the segment immediatelyanterior to the telson The potential homology of these struc-tures (the post-ventral plates) in Emeraldella and Aglaspis wasrecognised by Wills et al (1998a Character 70) and inEmeraldella and Sidneyia by Edgecombe amp Ramskold (1999Character 25) The homology of the segmented pygidial caudalfurcae of Olenoides (see Whittington 1975a 1980) with thesestructures is considered doubtful and coded as equivocal

Despite their distinctive morphology the long caudualfurcae of Cheloniellon may be homologous with the furcae ofAglaspis Emeraldella and Sidneyia This is supported by thecaudal furcae of the Ordovician cheloniellid Duslia which aremore similar in morphology to those of Emeraldella than ofCheloniellon Chlupac (1988 pp 614ndash16) considered thesestructures to be on the terminal tergite in Duslia However afaint tergite boundary is apparent posterior to the attachmentof the furcae (see Chlupac 1988 pl 57 fig 4) Therefore theyare considered here to have been attached to the pre-telsonicsegment as in Cheloniellon Chlupac (1988) and Dunlop ampSelden (1997) supported a close relationship between Dusliaand Cheloniellon

25 MethodsAll analyses were carried out using PAUP Version 40b4a(Swofford 1999) and unless otherwise stated used heuristicsearches with one thousand random addition sequencereplicates The software packages MacClade Version 307(Maddison amp Maddison 1997) and RadCon (Thorley amp Page2000) were used for comparing trees and investigating patternsof character evolution Tree length and other statistics (theConsistency Index CI and Retention Index RI) were calcu-lated by PAUP and MacClade with uninformative charactersexcluded Analyses were run separately using each of thedifferent codings for aglaspidids

Characters were treated as unordered and of equal weight inmost analyses To assess the influence of this assumption fourof the eight multistate characters which have states that areintermediate between others (Wilkinson 1992) were treated asordered in some analyses These characters are the number ofcephalic segments (Character 4) the length of raptorialappendage spines (Character 6) the degree of overlap of thethorax by the head-shield (Character 37) and the number ofsegments making up the postabdomen (Character 44) The lastof these is uninformative when not ordered because only onetaxon shows each of states 0 1 and 3

Support for individual nodes was assessed by bootstrapanalysis (Felsenstein 1985) and by calculating Bremer support

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 185

indices (Bremer 1988 1994) These methods measure twodistinct aspects of support for phylogenetic hypotheses Treesor nodes may be considered well supported (1) to the extent towhich alternative topologies are much less parsimonious asmeasured by the support index (Wilkinson 1996) or (2) wherethey are consistent with a large proportion of characters sothat character sampling is unlikely to have had much influenceon topology as assessed by bootstrapping (Page 1996) Boot-strapping was performed with 100 bootstrap replicates eachof 10 addition sequence replicates Bremer support indicesare based on heuristic searches with 100 addition sequencereplicates

Three pairs of terminals are included in Table 2 that apartfrom missing data are coded identically These are Helmetiaand Kuamaia Soomaspis and Tariccoia and Saperion andSkioldia These pairs are lsquotaxonomic equivalentsrsquo removingthe taxon with most missing data will not influence theposition of the other They can be recoded as single terminalsaccording to the principal of safe taxonomic reduction(Wilkinson 1995a b) and were not considered separately in theanalyses presented here Therefore cladistic analyses werecarried out on the basis of 33 terminals Edgecombe ampRamskold (1999) ignored this and coded the same three pairsof taxa and Eoredlichia and Olenoides identically for theirmore limited set of characters

26 ResultsAnalysis with all characters unordered and using the lsquoAglaspi-dida 1rsquo coding found nine most parsimonious trees (MPTs)each 126 steps long (CI=0middot556 RI=0middot763) Using the lsquoAglas-pidida 2rsquo coding resulted in 18 MPTs of length 125 (CI=0middot560RI=0middot765) Nine of these 18 trees were those found with thefirst coding of aglaspidids The strict component consensus(sensu Wilkinson 1994) of the 27 unique trees found acrossboth analyses is shown in Figure 8 All trees supported themonophyly of the Marrellomorpha (Marrella and Mimetaster)and of a clade including all other taxa except Crustacea (Clade1 of Fig 8) The relationships between these two clades and theCrustacea were unresolved equal numbers of trees in bothanalyses supported each of the topologies [Clade 1 (CrustaceaMarrellomorpha)] [Crustacea (Marrellomorpha Clade 1)]and [Marrellomorpha (Crustacea Clade 1)] All trees placedBurgessia as the sister taxon to the rest of Clade 1 andsupported the division of the rest of Clade 1 into two majorsister groups The topology of one of these clades (Clade 2)which included trilobites helmetiids tegopeltids naraoiidsxandarellids and Retifacies was stable with respect to thecoding of aglaspidids The other clade (Clade 3) consists of aclade of megacheirans and chelicerates (Clade 4) and a cladeincluding aglaspidids Lemoneites Paleomerus CheloniellonEmeraldella and Sidneyia (Clade 5) This latter clade is un-resolved in the strict consensus of all 27 trees (Fig 8) and thestrict consensus of the 18 MPTs found using the lsquoAglaspidida2rsquo coding but was fully resolved in the consensus of the ninetrees found with the lsquoAglaspidida 1rsquo coding Within themegacheiran-chelicerate clade all trees found Fortiforceps tobe the sister group to all other taxa Yohoia the sister group tochelicerates and Alalcomenaeus and Leanchoilia to form aclade Jianfengia is equally parsimoniously placed as the sistergroup to the Yohoia-chelicerate clade or as the sister group tothis clade and the Alalcomenaeus-Leanchoilia clade

Treating characters 4 6 37 and 44 as ordered had a minimaleffect on most parsimonious topology Nine MPTs of 136 steps(CI=0middot515 RI=0middot743) were found with the lsquoAglaspidida 1rsquocoding and 12 MPTs 135 steps in length (CI=0middot519 RI=0middot745)with the lsquoAglaspidida 2rsquo coding These trees all supported a(Crustacea Clade 1) clade excluding the marrellomorphs

Apart from this the strict consensus of these 21 trees isidentical to the consensus of the 27 trees found with allcharacters treated as unordered

Across all analyses four different topologies for Clade 5 (ofFig 8) were found (Fig 9) The distribution of these topologiesacross the four analyses described above and across the sameanalyses repeated after reweighting of each character in pro-portion to their Rescaled Consistency Index (see Farris 1989)from the first analysis is shown in Table 3 The instability of

Figure 8 Strict consensus of 27 MPTs from four separate analyses ofarachnomorph phylogeny with both interpretations of aglaspididmorphology and with some characters treated as ordered or allcharacters unordered Clades referred to in the text are numbered

Figure 9 Alternative equally parsimonious resolutions of clade 5 ofFig 8 (see Table 3)

186 TREVOR J COTTON AND SIMON J BRADDY

this clade was to some extent caused by the inclusion ofLemoneites and Paleomerus which could each be coded foronly 27 of the 54 characters used Analysis with these two taxaexcluded and characters treated as unordered found treescompatible only with Topology D of Figure 9 irrespective ofthe coding used for aglaspidids Analysis with some characterstreated as ordered and using either aglaspidid coding foundthree trees compatible only with Topology B of Figure 9 andthree compatible only with Topology D of Figure 9 ThereforeTopology D is the preferred topology for Clade 5 as shown inTable 3

One of the MPTs resulting from the lsquoAglaspidida 2rsquo coding(with all characters treated as unordered) was chosen as thebasis for further discussion because this tree combined thepreferred topology of Clade 5 with the (Crustacea Clade 1)group that was favoured when some characters were treated asordered Bootstrap percentages and Bremer support values forthis tree (using the Aglaspidida 2 coding) are shown in Figure10 Apomorphies for all ingroup nodes of this tree are shownin Figure 10A

3 Discussion

31 Phylogenetic resultsThe results of the cladistic analyses presented above arewell-resolved and robust with respect to different analyticalparameters Most of the taxa considered form a clade (Clade 1of Fig 8) that is more closely related to chelicerates than tocrustaceans in all analyses and therefore is recognised asthe Arachnomorpha as defined by Chen et al (1996) andRamskold et al (1996) The monophyly of the Marrellomor-pha was supported in all analyses The marrellomorphs werefound to be the sister group to a Crustacea+Clade 1 group inmost MPTs but in a minority of trees formed the sister groupto Clade 1 alone According to the latter result they should beincluded within the Arachnomorpha but according to the firstexcluded from it Both of these alternatives have been sup-ported in previous studies (eg Hou amp Bergstrom 1997 Willset al 1998a respectively) In addition to the marrellomorphsthe Fuxianhuiida Bousfield 1995 and Canadaspidida havebeen placed in the Euarthropoda outside the main mandibu-late and arachnomorph clades (Hou amp Bergstrom 1997)Further study of these taxa of the crustacean or mandibulatestem group and of the marrellomorphs is necessary before the

position of the Marrellomorpha can be resolved Provisionallymarrellomorphs are excluded from the Arachnomorphafollowing the majority of analyses above and Wills et al(1994)

The Burgess Shale arthropod Burgessia was placed as thesister group to remaining arachnomorphs in all analyses OtherArachnomorpha form two major clades one consisting of thelsquotrilobite-alliedrsquo arachnomorphs analysed by Edgecombe ampRamskold (1999) here termed lsquoTrilobitomorpharsquo (defined asTrilobita and all taxa more closely related to Trilobita thanChelicerata) and the other (lsquochelicerate-alliedrsquo) clade includingchelicerates lsquogreat appendagersquo arthropods aglaspididsLemoneites Paleomerus Sidneyia and Emeraldella The latteris here termed lsquoCheliceramorpharsquo (defined as Chelicerata andall taxa more closely related to Chelicerata than Trilobita) Thetopology of the lsquotrilobite-alliedrsquo clade was similar to that foundby Edgecombe amp Ramskold (1999) but the present authorsrsquoresults are more completely resolved unambiguously support-ing the sister-group relationship between trilobites andHelmetiida suggested in the previous study Within thelsquochelicerate-alliedrsquo clade Aglaspidids Paleomerus Lemoneitesand the Burgess Shale Emeraldella and Sidneyia form a cladein opposition to megacheirans and chelicerates

The present authorsrsquo analysis supports a new hypothesis ofthe origin of the chelicerates from within a paraphyleticassemblage of megacheiran arthropods and confirms pyc-nogonids as a sister group to the euchelicerates Previoushypotheses of the chelicerate sister group (see Section 1) aremuch less parsimonious The shortest trees supporting aCheloniellon-chelicerate clade are six steps longer than theMPTs and those supporting an aglaspidid-chelicerate cladeseven steps longer All megacheirans and chelicerates areunited by the flap-shaped rounded exopods and modificationof the second cephalic segment endopods into anteriorlydirected spinose raptorial organs which share a number ofdetailed similarities A clade including all of these taxa apartfrom Fortiforceps is supported by the synapomorphic loss ofthe antennae (or antennulae of crustaceans) and the longerlength of spinose projections of the second cephalic appendageendopods Within this clade Yohoia shares the loss of cephalicexopods the loss of thoracic endopods and a postabdomen oftubular tergites with chelicerates

The poor resolution of Clade 5 (of Fig 8) was partly a resultof the poor preservation of and consequent high proportion ofmissing data for Lemoneites and Paleomerus When these two

Table 3 Numbers of most parsimonious trees supporting each of the alternative topologies for clade 4 of Fig 7 under different analyticalconditions (RCI=Revealed Consistency Index)

Aglaspididcoding

Characters 4 6 36and 43 ordered

Charactersreweighted by RCI

Paleomerus andLemoneites excluded

Topology (see Fig 8)

A B C D

1 91 X 3 3 31 X 61 X X 31 X 61 X X 3 32 9 92 X 3 3 3 32 X 62 X X 32 X 62 X X 3 3Total 6 18 12 54

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 187

taxa were excluded from the analyses only two (B and D) ofthe four topologies found in the complete analysis remainedmost parsimonious (see Fig 9 and Table 3) Secondly Bremersupport values throughout much of the lsquochelicerate-alliedrsquoclade (Clade 3 of Fig 8) were improved when these taxa wereexcluded (see Fig 11)

Dunlop (1999 p 258) suggested that in reconstructing thechelicerate stem lineage lsquowe might predict that the two mostsignificant changes towards the chelicerate condition are thereduction of the antennae and the formation of the nextappendages into a clawrsquo According to the hypothesis pre-sented here both of these adaptations were achieved in EarlyCambrian chelicerate ancestors The recognition of the loss ofthe antennae in the stem group of the chelicerates and thephylogenetic hypothesis presented here suggest that chelicer-ates are most closely related to taxa with only a singleadditional segment fused to the crown group eucheliceratehead of four fused somites Previous hypotheses have derivedchelicerates from Palaeozoic arthropods with a longer head(eg Cheloniellon Sturmer amp Bergstrom 1978 SanctacarisBriggs amp Collins 1988 Emeraldella Bruton amp Whittington1983) The plesiomorphic pattern of head segmentation ismatched in pycnogonids Bergstrom et al (1980) suggestedthat only the anterior four pairs of appendages are incorpo-rated into the head This supports the view that pycnogonidsare primitive with respect to other chelicerates which showmore or less complete fusion of this primitive head and threethoracic segments including Solpugida and Pseudoscorpiones

The present study suggests that a shift from gnathobasicbenthic feeding to a more vagrant lifestyle and raptorialappendage feeding occurred very early in the chelicerate lin-eage Appendage feeding was clearly an important adaptation

Figure 10 One of 18 equally parsimonious cladograms with lsquoAglaspidida 2rsquo coding and all characters unorderedthat is most congruent with results from other analyses Non-terminal branch lengths scaled to reflect number ofapomorphies (A) ACCTRAN apomorphy scheme Character numbers shown above boxes and states shownbelow boxes as in Table 2 and the text Character consistency indices are indicated by shading (B) Levels ofsupport for individual nodes Bremer support indices shown in bold type above branches Bootstrap percentagesin italic type below branches for all nodes with relative frequencies greater than 50

Figure 11 Bremer support values for clades within the lsquochelicerate-alliedrsquo arachnomorphs (clade 3 of Fig 8) when all taxa wereincluded (normal weight font above nodes) and with Lemoneites andPaleomerus excluded (bold type below nodes)

188 TREVOR J COTTON AND SIMON J BRADDY

for the terrestrialization of chelicerates (Dunlop 1997 p 69Dunlop amp Webster 1999) but the model proposed heresuggests that this considerably pre-dated terrestrializationGnathobasic feeding in xiphosurans and eurypterids mayrepresent a reversal to a more primitve benthic arachnomorphlifestyle from a more pycnogonid-like ancestor

32 Evolution of the arachnomorph headThe extant arthropod classes show highly conserved patternsof head segmentation (eg Wills et al 1994) In contrast therehas been a near consensus amongst Cambrian arthropodworkers (although rarely based on an explicit phylogenetichypothesis) that patterns of head segmentation are highlyhomoplastic and hence of little systematic significanceSturmer amp Bergstrom (1978 pp 78ndash9) suggested that lsquoevenclosely related forms may have different numbers of headsegments and appendagesrsquo Bruton amp Whittington (1983pp 576ndash7) that lsquodiscussion on fossil arthropod relationshipsbased on head segmentation appears to be largely irrel-evant and at best speculativersquo and Delle Cave amp Simonetta(1991 p 191) that lsquothere are no obvious phyletic affinitiesbetween genera having the same number of cephalized seg-mentsrsquo Most recently Bergstrom amp Hou (1997 p 104) con-cluded that lsquothe segmental length of the head shield seemsto be of no relevance to the discussion and discrimination ofevolutionary lineagesrsquo It has been claimed that this view issupported by cladistic analyses in which patterns of tagmosishave been found to be rather poor at defining major arthropodclades (Briggs et al 1992 Wills et al 1994)

This suggestion is not only of relevance to systematics buthas featured prominently in recent debates surrounding thenature of the lsquoCambrian explosionrsquo (see Budd amp Jensen 2000for a recent review) Gould (1989 1991) argued that theapparent plasticity of head segmentation in Cambrian com-pared to post-Cambrian arthropod evolution suggests thatbody-plan evolution was only constrained after the Cambrianexplosion According to this view the origin of new highertaxa with distinct body-plans was only possible before theonset of these constraints (eg Valentine 1995) Taxa hererecognised as arachnomorphs clearly played a major role in thedevelopment of Gouldrsquos hypothesis This suggestion has pre-viously been assessed in terms of overall morphological diver-sity which is clearly rather distinct from Gouldrsquos concept ofdisparity Wills et al (1998b) showed that there was littlecorrelation between overall morphological disparity and de-gree of limb specialisation and that the latter was phylogeneti-cally highly plastic They remarked that (Wills et al 1998bp 64) lsquothis finding has important implications for models ofarthropod phylogeny and evolution that attribute overidingimportance to head segmentationrsquo but did not explicitlyexamine head segmentation although it is likely to have beena major component of the index of tagmosis employed

The results of the present study suggest that arguments thatarthropod head segmentation was unusually labile during theCambrian are poorly founded Rather only four major pat-terns of euarthropod head segmentation are identified Theplesiomorphic euarthropod state according to both Walossekamp Muller (1997 1998) and Scholtz (1997) which may moreproperly be the plesiomorphic state for the euarthropod crowngroup only (depending on the phylogenetic position of theMarrellomorpha) consists of four post-acronal segmentsbearing the antennae and three pairs of biramous limbs Thepresent authors suggest that the term lsquocephalonrsquo be restrictedto this kind of head which is found in stem group mandibu-lates in addition to many arachnomorphs Crown groupmandibulates share a lsquobimaxillary headrsquo in which an addi-tional pair of appendages the labium or second maxillae are

incorporated into the head but in crustaceans these are notfused to the carapace (Scholtz 1997) Finally two distinctforms of head tagmosis are found in chelicerates A head withfour pairs of appendages (but without antennae giving a totalof five segments incorporated into the head) which has beencalled the lsquocephalosomarsquo is present in pycnogonids (Walossekamp Dunlop 2002 pp 434ndash5) and probably also in some euch-elicerates (J A Dunlop pers comm 2003) The lsquoprosomarsquo ofmost euchelicerates consists of this cephalosoma and twoadditional segments

In general these patterns of head segmentation are highlyphylogenetically conserved (see Fig 12) Only three homo-plastic changes namely reversal to a more primitive conditionin the Leanchoilia-Alalcomenaeus clade and the convergentorigin of a five-segmented head in Clade 3 (of Fig 8) and theXandarellida are necessary to optimise character 4 onto themost parsimonious cladogram There are also transitions toautapomorphic states in Emeraldella and Sidneyia In the caseof Sidneyia however an argument could be made (followingBergstrom amp Sturmer 1978 see discussion of Character 4) thatthe head consists of five segments as in closely related taxabut autapomorphically each segment has a separate tergiteThis would further increase the phylogenetic stability of headsegmentation Also following Bergstrom amp Sturmer (1978) thehead of Cheloniellon could be considered to consist of twotergites making it more similar to the head of Emeraldella(with six pairs of appendages)

Figure 12 Cladogram from Fig 10 showing changes in head segmen-tation Bold type shows abbreviated head segmentation formulaAbbreviations (Ant) antennae (BA) biramous appendage pairs (Ch)chelicerae (GA) great appendages (UA) uniramous appendage pairsand (X) segment without appendages Synapomorphy A is the auta-pomorphic reduction in head length to a single segment in SidneyiaAlternatively this may represent secondary division of the headshield Synapomorphy B is the increase in the length of the head inEmeraldella to six segments

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 189

4 Acknowledgements

We thank D E G Briggs J A Cotton J A Dunlop R AFortey S Harris R A Moore P Selden O E Tetlie andP Van Roy for comments on earlier drafts of this work Themanuscript was reviewed by M Wills F Schram and oneanonymous referee This research was supported by the Geo-logical Society of London Curry Research Studentshipawarded to TJC by the Department of Earth Sciences Uni-versity of Bristol and by the Department of PalaeontologyThe Natural History Museum London TJC would like tothank C Labandeira J Thompson and R Chapman fortheir hospitality at the National Museum of Natural HistoryWashington DC USA and A Olcott S Schellenberg andJ Young for making his time there so enjoyable

5 References

Abzhanov A Popadic A amp Kaufman T C 1999 Chelicerate Hoxgenes and the homology of arthropod segments Evolution andDevelopment 1 77ndash89

Akam M 2000 Arthropods developmental diversity within a (super)phylum Proceedings of the National Academy of Sciences USA97 4438ndash41

Aldridge R J Gabbott S E amp Theron J N 2001 The Soom ShaleIn Briggs D E G amp Crowther P R (eds) Palaeobiology II340ndash2 Oxford Blackwell Science

Anderson L I amp Selden P A 1997 Opisthosomal fusion andphylogeny of Palaeozoic Xiphosura Lethaia 30 19ndash31

Arango C P 2002 Morphological phylogenetics of the sea spiders(Arthropoda Pycnogonida) Organisms Diversity and Evolution2 107ndash25

Ax P 1986 Das phylogenetische System Stuttgart Gustav FischerVerlag

Babcock L E amp Zhang Wentang In press New non-mineralizedarthropods from the Chengjiang Lagerstatte Lower CambrianYunnan China Palaeontologia Cathyana

Bartels C Briggs D E G amp Brassel G 1998 The fossils ofthe Hunsruck Slate Marine life in the Devonian CambridgeCambridge University Press

Bergstrom J 1971 Paleomerus ndash merostome or merostomoid Lethaia4 393ndash401

Bergstrom J 1979 Morphology of fossil arthropods as a guideto phylogenetic relationships In Gupta A P (ed) ArthropodPhylogeny 3ndash56 New York NY Van Nostrand Reinhold Co

Bergstrom J 1992 The oldest arthropods and the origin of theCrustacea Acta Zoologica 73 287ndash91

Bergstrom J Sturmer W amp Winter G 1980 PalaeoisopusPalaeopantopodus and Palaeothea pycnogonid arthropodsfrom the Lower Devonian Hunsruck Slate West GermanyPalaontologische Zeitschrift 54 7ndash54

Bergstrom J amp Brassel G 1984 Legs in the trilobite Rhenops fromthe Lower Devonian Hunsruck Slate Lethaia 17 67ndash72

Bergstrom J amp Hou X 1998 Chengjiang arthropods and theirbearing on early arthropod evolution In Edgecombe G D(ed) Arthropod fossils and phylogeny 151ndash84 New York NYColumbia University Press

Boudreaux H B 1979 Significance of intersegmental tendon systemin arthropod phylogeny and a monophyletic classification ofArthropoda In Gupta A P (ed) Arthropod Phylogeny 551ndash86New York NY Van Nostrand Reinhold Co

Bousfield E L 1995 A contribution to the natural classification ofLower and Middle Cambrian arthropods food-gathering andfeeding mechanisms Amphipacifica 2 3ndash33

Braddy S J Aldridge R J Gabbott S E amp Theron J N 1999Lamellate book-gills in a late Ordovician eurypterid from theSoom Shale South Africa support for a eurypterid-scorpionclade Lethaia 32 72ndash4

Braddy S J amp Almond J E 1999 Eurypterid trackways from theTable Mountain Group (Ordovician) of South Africa Journal ofAfrican Earth Sciences 29 165ndash77

Bremer K 1988 The limits of amino acid sequence data in angio-sperm phylogenetic reconstruction Evolution 42 795ndash803

Bremer K 1994 Branch support and tree stability Cladistics 10295ndash304

Briggs D E G 1998 Review of Arthropods from the LowerCambrian Chengjiang fauna southwest China by HouXianguang and Jan Bergstrom Fossils and Strata 45

Scandinavian University Press Oslo Norway 1997 Palaeo-geography Palaeoclimatology Palaeoecology 143 193ndash4

Briggs D E G Bruton D L amp Whittington H B 1979 Append-ages of the arthropod Aglaspis spinifer (Upper CambrianWisconsin) and their significance Palaeontology 22 167ndash80

Briggs D E G Fortey R A amp Wills M A 1992 Morphologicaldisparity in the Cambrian Science 256 1670ndash3

Briggs D E G Fortey R A amp Wills MA 1993 How big was theCambrian explosion A taxonomic and morphologic comparisonof Cambrian and Recent arthropods In Lees D R amp EdwardsD (eds) Evolutionary patterns and processes 33ndash44 LondonLinnean Society of London Linnean Society Symposium Series

Briggs D E G Erwin D H amp Collier F J 1994 The fossils of theBurgess Shale Washington DC Smithsonian Institution Press

Briggs D E G amp Bartels C 2001 New arthropods from the LowerDevonian Hunsruck Slate (Lower Emsian Rhenish Massifwestern Germany) Palaeontology 44 275ndash304

Briggs D E G amp Bartels C 2002 Magnoculocaris a new name forthe arthropod Magnoculus Briggs and Bartels 2001 from theLower Devonian Hunsruck Slate Germany Palaeontology 45419

Briggs D E G amp Collins D 1988 A Middle Cambrian cheliceratefrom Mount Stephen British Columbia Palaeontology 31 779ndash98

Briggs D E G amp Collins D 1999 The arthropod Alalcomenaeuscambricus Simonetta from the Middle Cambrian Burgess Shale ofBritish Columbia Paleontology 42 953ndash78

Briggs D E G amp Fortey R A 1982 The cuticle of aglaspididarthropods a red-herring in the early history of vertebratesLethaia 15 25ndash9

Briggs D E G amp Fortey R A 1989 The early radiation andrelationships of the major arthropod groups Science 246 241ndash3

Briggs D E G amp Fortey R A 1992 The early Cambrian radiationof arthropods In Lipps J H amp Signor P W (eds) Origin andEarly Evolution of the Metazoa 335ndash373 New York NY PlenumPress

Briggs D E G amp Robison R A 1984 Exceptionally preservednontrilobite arthropods and Anomalocaris from the MiddleCambrian of Utah University of Kansas PaleontologicalContributions Paper 111 1ndash23

Broili F 1932 Ein neuer Crustacee aus dem rheinischen UnterdevonSitzungsberichte der Mathematisch-Naturwissenschaftlichen(Abteilung) Klasse der Bayerischen Akademie der Wissenschaftenzu Munchen 1932 27ndash38

Broili F 1933 Ein zweites Exemplar von Cheloniellon Sitzungsber-ichte der Bayerischen Akademie der Wissenschaften 1933 11ndash32

Bruton D L 1981 The arthropod Sidneyia inexpectans MiddleCambrian Burgess Shale British Columbia Philosophical Trans-actions of the Royal Society of London Series B 295 619ndash56

Bruton D L amp Whittington H B 1983 Emeraldella and Leanchoiliatwo arthropods from the Burgess Shale British Columbia Philo-sophical Transactions of the Royal Society of London Series B 300553ndash85

Budd G E 1995 Kleptothule rasmusseni gen et sp nov anolenellid-like trilobite from the Sirius Passet fauna (BuenFormation Lower Cambrian North Greenland) Transactions ofthe Royal Society of Edinburgh Earth Sciences 86 1ndash12

Budd G E 1996a Progress and problems in arthropod phylogenyTrends in Ecology and Evolution 11 356ndash8

Budd G E 1996b The morphology of Opabinia regalis and thereconstruction of the arthropod stem-group Lethaia 29 1ndash14

Budd G E 1997 Stem-group arthropods from the Lower CambrianSirius Passet fauna of North Greenland In Fortey R A ampThomas R H (eds) Arthropod Relationships 125ndash38 LondonChapman and Hall Systematics Association Special Volume 55

Budd G E 1999a A nektaspid arthropod from the Early CambrianSirius Passet fauna with a description of retrodeformation basedon functional morphology Palaeontology 42 99ndash122

Budd G E 1999b The morphology and phylogenetic significance ofKerygmachela kiergkegaardi Budd (Buen Formation LowerCambrian N Greenland) Transactions of the Royal Society ofEdinburgh Earth Sciences 89 (for 1998) 249ndash290

Budd G E 2002 A palaeontological solution to the arthropod headproblem Nature 417 271ndash5

Budd G E amp Jensen S 2000 A critical reappraisal of the fossilrecord of the bilaterian phyla Biological Reviews 75 253ndash95

Chen J Zhou G Zhu M amp Yeh K 1996 The Chengjiang biota ndasha unique window of the Cambrian explosion Taichung NationalMuseum of Natural Science [In Chinese]

Chen J Edgecombe G D amp Ramskold L 1997 Morphological andecological disparity in naraoiids (Arthropoda) from the Early

190 TREVOR J COTTON AND SIMON J BRADDY

Cambrian Chengjiang fauna China Records of the AustralianMuseum 49 1ndash24

Chen J amp Zhou G 1997 Biology of the Chengjiang fauna In ChenJ Cheng Y amp Iten H V (eds) The Cambrian explosion and thefossil record Bulletin of the National Museum of Natural ScienceTaiwan 10 11ndash106

Child C A 1997 Some deep-sea Pycnogonida from Argentine slopeand basin Proceedings of the Biological Society of Washington110 128ndash42

Chlupac I 1988 The enigmatic arthropod Duslia from the Ordovicianof Czechslovakia Palaeontology 31 611ndash20

Chlupac I 1995 Lower Cambrian arthropods from the PasekyShale (Barrandian area Czech Republic) Journal of the CzechGeological Society 40 9ndash36

Chlupac I amp Havlıcek V 1965 Kodymirus n g a new aglaspidmerostome of the Cambrian of Bohemia Sbornık GeologickechVid Paleontologie 6 7ndash20

Cisne J L 1975 Anatomy of Triarthrus and the relationships of theTrilobita Fossils and Strata 4 45ndash63

Cisne J L 1981 Triarthrus eatoni (Trilobita) anatomy of its exo-skeletal skeletomuscular and digestive systems Palaeonto-graphica Americana 9 95ndash142

Conway Morris S 1982 Atlas of the Burgess Shale London ThePalaeontological Association

Conway Morris S 1989 The persistence of Burgess Shale-type faunasimplications for the evolution of deeper-water faunas Trans-actions of the Royal Society of Edinburgh Earth Sciences 80271ndash83

Conway Morris S 1998 The crucible of creation Oxford OxfordUniversity Press

Cotton T J 1999 Review of Arthropod fossils and phylogenyPalaeogeography Palaeoclimatology Palaeoecology 151 341ndash3

Cotton T J 2002 The phylogeny and morphological evolution ofCambrian trilobites and their relatives Unpublished PhD ThesisUniversity of Bristol

Damen W G M Hausdorf M Seyfarth E A amp Tautz D 1998 Aconserved mode of head segmentation in arthropods revealed bythe expression pattern of Hox genes in a spider Proceedings of theNational Academy of Sciences USA 95 10665ndash70

Delle Cave L amp Simonetta A M 1991 Early Palaeozoic arthropodsand problems of arthropod phylogeny with some notes on taxa ofdoubtful affinities In Simonetta A M amp Conway Morris S(eds) The early evolution of Metazoa and the significance ofproblematic taxa 189ndash244 Cambridge Cambridge UniversityPress

Dewel R A amp Dewel W C 1997 The place of tardigrades inarthropod evolution In Fortey R A amp Thomas R H (eds)Arthropod Relationships 109ndash23 London Chapman and HallSystematics Association Special Volume 55

Dunlop J A 1997 Palaeozoic arachnids and their significance forarachnid phylogeny Proceedings of the 16th European Colloquiumof Arachnology 65ndash82 Siedlce Poland

Dunlop J A 1998 The origins of tetrapulmonate booklungs and theirsignificance for chelicerate phylogeny In Selden P A (ed)Proceedings of the 17th European Colloquium of ArachnologyEdinburgh 1997 pp 9ndash16

Dunlop J A 1999 Pasando revista a la evolucion de los quelicerados[A review of chelicerate evolution] In Melic A DeHaro J JMedez M amp Ribera I (eds) Volumen Monografico Evolucion yFilogenia de Arthropoda Boletin de la Sociedad EntolomologicaAragonesa 26 255ndash272 [In Spanish and English]

Dunlop J A amp Selden P A 1997 The early history and phylogenyof the chelicerates In Fortey R A amp Thomas R H (eds)Arthropod relationships Systematics Association Special Volume55 221ndash35 London Chapman and Hall

Dunlop J A amp Webster M 1999 Fossil evidence terrestrializationand arachnid phylogeny Journal of Arachnology 27 86ndash93

Dzik J 1993 Early metazoan evolution and the meaning of its fossilrecord Evolutionary Biology 27 339ndash86

Dzik J amp Lendzion K 1988 The oldest arthropods of the EastEuropean Platform Lethaia 21 29ndash38

Edgecombe G D Wilson G D F Colgan D J Gray M R ampCassis G 2000 Arthropod cladistics combined analysisof Histone H3 and U2 snRNA sequences and morphologyCladistics 16 155ndash203

Edgecombe G D amp Ramskold L 1999 Relationships of CambrianArachnata and the systematic position of Trilobita Journal ofPaleontology 73 263ndash87

Emerson M J amp Schram F R 1997 Theories patterns and realitygame plan for arthropod phylogeny In Fortey R A amp Thomas

R H (eds) Arthropod Relationships Systematics AssociationSpecial Volume 55 67ndash87 London Chapman and Hall

Farris J S 1989 The Retention Index and the Rescaled ConsistencyIndex Cladistics 5 417ndash19

Felsenstein J 1985 Confidence limits on phylogenies an approachusing the bootstrap Evolution 39 783ndash91

Flower R H 1968 Merostomes from a Cotter Horizon of the El PasoGroup New Mexico Bureau of Mines and Mineral ResourcesMemoir 22 35ndash44

Fortey R A 1990a Trilobite evolution and systematics In MikulicD G (ed) Arthropod paleobiology 44ndash65 Short Courses inPaleontology 3 Knoxville Tenessee The Paleontological Society

Fortey R A 1990b Ontogeny hypostome attachment and trilobiteclassification Palaeontology 33 529ndash76

Fortey R A 1997 Classification In Kaesler R L (ed) Treatise oninvertebrate paleontology Part O Arthropoda 1 (revised) 289ndash302Boulder Colorado amp Lawrence Kansas Geological Society ofAmerica and University of Kansas Press

Fortey R A amp Theron J N 1994 A new Ordovician arthropodSoomaspis and the agnostid problem Palaeontology 37 841ndash61

Fortey R A amp Whittington H B 1989 The Trilobita as a naturalgroup Historical Biology 2 125ndash38

Fry W G 1978 A classification within the pycnogonids ZoologicalJournal of the Linnean Society of London 63 35ndash58

Giribet G Edgecombe G D amp Wheeler W C 2001 Arthropodphylogeny based on eight molecular loci and morphology Nature413 157ndash61

Gould S J 1989 Wonderful life The Burgess Shale and the nature ofhistory New York NY Norton

Gould S J 1991 The disparity of the Burgess Shale arthropod faunaand the limits of cladistic analysis why we must strive to quantifymorphospace Paleobiology 17 411ndash23

Gurich G 1931 Mimetaster hexagonalis ein neuer Kruster aus demunterdevonischen Bundenbacher Dachschiefer PalaontologischeZeitschrift 12 204ndash38

Hammann W Laske R amp Pillola G L 1990 Tariccoia arrusensisngnsp an unusual trilobite-like arthropod Rediscovery of thelsquophylocaridrsquo beds of Taricco (1922) in the Ordovician lsquoPuddingarsquosequence of Sardinia Bolletino della Societa PaleontologicaItaliana 29 163ndash78

Haas M S Brown S J amp Beeman R W 2001 Pondering theprocephalon the segmental origin of the labrum DevelopmentGenes and Evolution 211 89ndash95

Heider K 1913 Entwicklungsgeschichte und Morphologie derWirbellosen Die Kultur der Gegenwart Theil 3 Abt 4 2 176ndash332

Henriksen K 1928 Critical notes upon Cambrian arthropodsdescribed by Charles D Walcott Videnskabelige Meddelelser fraDansk Naturhistorisk Forening 86 1ndash20

Hesselbo S P 1988 Trace fossils of Cambrian aglaspidid arthropodsLethaia 21 139ndash46

Hesselbo S P 1989 The aglaspidid arthropod Beckwithia from theCambrian of Utah and Wisconsin Journal of Paleontology 63636ndash42

Hesselbo S P 1992 Aglaspidida (Arthropoda) from the UpperCambrian of Wisconsin Journal of Paleontology 66 885ndash923

Hou X 1987a Three new large arthropods from Lower CambrianChengjiang fauna eastern Yunnan Acta Palaeontologica Sinica26 272ndash85 [In Chinese with English summary]

Hou X 1987b Two new arthropods from Lower CambrianChengjiang eastern Yunnan Acta Palaeontologica Sinica 26236ndash56 [In Chinese with English summary]

Hou X Chen J amp Lu H 1989 Early Cambrian new arthropodsfrom Chengjiang Yunnan Acta Palaeontologica Sinica 28 42ndash57[In Chinese with English summary]

Hou X Ramskold L amp Bergstrom J 1991 Composition andpreservation of the Chengjiang fauna a Lower Cambrian soft-bodied biota Zoologica Scripta 20 395ndash411

Hou X Bergstrom J amp Ahlberg P 1995 Anomalocaris and otherlarge animals in the Lower Cambrian Chengjiang fauna ofsouthwest China GFF 117 163ndash83

Hou X amp Bergstrom J 1997 Arthropods of the Lower CambrianChengjiang fauna southwest China Fossils and Strata 45 1ndash116

Hughes C P 1975 Redescription of Burgessia bella from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 4415ndash36

Ivantsov A Y 1999 Trilobite-like arthropod from the LowerCambrian of the Siberian Platform Acta PalaeontologicaPolonica 44 455ndash66

King P E 1973 Pycnogonids London St Martin Press

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 191

Lauterbach K E 1973 Schlussereignisse in der Evolution derStammgruppe der Euarthropoda Zoologische Beitrage 19259ndash99

Lauterbach K E 1980 Schlussereignisse in der Evolution derGrundplans der Arachnata (Arthropoda) Abhandlungen desNaturwissenschaftlichen Vereins in Hamburg 23 163ndash327

Lauterbach K E 1983 Synapomorphien zwischen Trilobiten- undCheliceraten-zweig der Arachnata Zoologischer Anzeiger 210213ndash38

Lendzion K 1975 Fauna of the Mobergella Zone in the Polish LowerCambrian Kwartalnik Geologiczny 19 237ndash42

Lieberman B S 1998 Cladistic analysis of the Early Cambrianolenelloid trilobites Journal of Paleontology 72 59ndash78

Lu Y 1940 On the ontogeny and phylogeny of Redlichia intermediaLu (sp nov) Bulletin of the Geological Society of China 20333ndash42 [In Chinese]

Maddison W P amp Maddison D R 1997 MacClade analysis ofphylogeny and character evolution Version 307 Sunderland MASinauer Associates

Mittmann B amp Scholtz G 2003 Development of the nervous systemin the lsquoheadrsquo of Limulus polyphemus (Chelicerata Xiphosura)morphological evidence for the correspondence between the seg-ments of the chelicerae and the (first) antennae of MandibulataDevelopment Genes and Evolution 213 9ndash17

Moore R C (ed) 1959 Treatise on invertebrate paleontology Part OArthropoda 1 Boulder Colorado Geological Society of America

Muller K J amp Walossek D 1986 Arthropod larvae from the UpperCambrian of Sweden Transactions of the Royal Society ofEdinburgh Earth Sciences 77 157ndash79

Muller K J amp Walossek D 1987 Morphology ontogeny and lifehabit of Agnostus pisiformis from the Upper Cambrian of SwedenFossils and Strata 19 1ndash124

Munilla T 1999 Evolucion y filogenia de los Picnogonidos InMelicA DeHaro J J Medez M amp Ribera I (eds) VolumenMonografico Evolucion y Filogenia de Arthropoda Boletin de laSociedad Entolomologica Aragonesa 26 273ndash91 [In Spanish andEnglish]

Orr P J Siveter D J Briggs D E G Siveter D J amp Sutton M D2000 A new arthropod from the Silurian Konservat-Lagerstatteof Herefordshire Proceedings of the Royal Society of LondonSeries B 267 1497ndash504

Page R D M 1996 On consensus confidence and lsquototal evidencersquoCladistics 12 83ndash92

Palmer A R amp Repina L N 1993 Through a glass darklytaxonomy phylogeny and biostratigraphy of the OlenellinaUniversity of Kansas Paleontological Contributions New Series 31ndash35

Platnick N I Griswold C E amp Coddington J A 1991 On missingentries in cladistic analysis Cladistics 7 337ndash43

Pleijel F 1995 On character coding for phylogeny reconstructionCladistics 11 309ndash15

Raasch G O 1939 Cambrian Merostomata Special Papers of theGeological Society of America 19 1ndash146

Ramskold L Chen J Edgecombe G D amp Zhou G 1996Preservational folds simulating tergite junctions in tegopeltid andnaraoiid arthropods Lethaia 29 15ndash20

Ramskold L Chen J Edgecombe G D amp Zhou G 1997Cindarella and the arachnate clade Xandarellida (ArthropodaEarly Cambrian) from China Transactions of the Royal Society ofEdinburgh Earth Sciences 88 19ndash38

Ramskold L amp Edgecombe G D 1991 Trilobite monophylyrevisited Historical Biology 4 267ndash83

Ramskold L amp Edgecombe G D 1996 Trilobite appendagestructure ndash Eoredlichia revisited Alcheringa 20 269ndash76

Raw F 1957 Origin of chelicerates Journal of Paleontology 31139ndash193

Repina L N amp Okuneva O G 1969 Cambrian arthropods of themaritime territory Paleontological Journal 1969 95ndash103

Richter R amp Richter E 1929 Weinbergina opitz ng n sp einSchwerttrager (Merost Xiphos) aus dem Devon (Rheinland)Senkenbergiana 11 193ndash209

Robison R A 1984 New occurences of the unusual trilobite Naraoiafrom the Cambrian of Idaho and Utah University of KansasPaleontological Contributions Paper 112 1ndash8

Rominger C 1887 Description of primordial fossils from MtStephens NW Territory of Canada Proceedings of the Academyof Natural Sciences of Philadelphia 1887 12ndash19

Scholtz G 1997 Cleavage germ band formation and head segmen-tation the ground pattern of the Euarthropoda In Fortey R Aamp Thomas R H (eds) Arthropod Relationships 317ndash32 LondonChapman and Hall Systematics Association Special Volume 55

Schram F R 1978 Arthropods a convergent phenomenonFieldiana Geology 39 61ndash108

Schram F R 1993 Review of Boxshall Stromberg and Dahl TheCrustacea Origin and Evolution Journal of Crustacean Biology13 820ndash2

Schram F R Vonk R amp Hof C H J 1997 Mazon CreekCycloidea Journal of Paleontology 71 261ndash84

Schram F R amp Emerson M J 1991 Arthropod Pattern Theorya new approach to arthropod phylogeny Memoirs of theQueensland Museum 31 1ndash18

Schram F R amp Hof C H J 1998 Fossils and the interrelationshipsof major crustacean groups In Edgecombe G D (ed) Arthropodfossils and phylogeny pp 233ndash302 New York NY ColumbiaUniversity Press

Shergold J H 1991 Protaspis and early meraspis growth stages of theeodiscoid trilobite Pagetia ocellata Jell and their implications forclassification Alcheringa 15 65ndash86

Shu D Geyer G Ling C amp Zhang Z 1995 Redlichiacean trilobiteswith preserved soft parts from the Lower Cambrian Chengjiangfauna (South China) Beringeria (Special Issue) 2 203ndash41

Shultz J W 1990 Evolutionary morphology and phylogeny ofArachnida Cladistics 6 1ndash38

Shultz J W 2000 Skeletomuscular anatomy of the harvestmanLeiobunum aldrichi (Weed 1893) (Arachnida OpilionesPalpatores) and its evolutionary significance Zoological Journalof the Linnean Society 128 401ndash38

Shultz J W 2001 Gross muscular anatomy of Limulus polyphemus(Xiphosura Chelicerata) and its bearing on evolution in the theArachnida Journal of Arachnology 29 283ndash303

Siewing R (ed) 1985 Lehrbuch der Zoologie II Systematik 3rd ednStuttgart Fischer

Simonetta A M 1970 Studies on the non trilobite arthropods ofthe Burgess Shale (Middle Cambrian) the genera LeanchoiliaAlalcomenaeus Opabinia Burgessia Yohoia and ActaeusPalaeontographica Italica 66 35ndash45

Simonetta A M amp Delle Cave L 1975 The Cambrian non trilobitearthropods from the Burgess Shale of British Columbia A studyof their comparative morphology taxonomy and evolutionarysignificance Palaeontographica Italica 69 1ndash37

Simonetta A M amp Delle Cave L 1981 An essay in the compara-tive and evolutionary morphology of Palaeozoic ArthropodsAccademia Nazionale dei Lincei Atti dei Convegni Lincei 49389ndash439

Starobogatov Y I 1990 Sistema i filogeniya nizshikh khelitserovykh(analiz morfologii paleozoysikh grupp) [The systematics andphylogeny of the lower chelicerates (a morphological analysis ofthe Palaeozoic groups)] Paleontologicheskii Zhurnal 1990 4ndash17

Stoslashrmer L 1944 On the relationships and phylogeny of fossil andrecent Arachnomorpha Skrifter utgitt ac Det Norske Videnskaps-Akademi i Olso I Matematisk-naturvitenskaplig Klasse 5 1ndash158

Stoslashrmer L 1951 Studies on trilobite morphology Part III Theventral cephalic structures with remarks on the zoologicalposition of the trilobites Norsk geologisk tidsskrift 29 108ndash58

Stoslashrmer L 1956 A Lower Cambrian merostome from Sweden Arkivfor Zoologi Ser 2 9 507ndash14

Stoslashrmer L 1959 Trilobitoidea In Moore R C (ed) Treatise onInvertebrate Paleontology part O Arthropoda 1 23ndash37 BoulderColorado Geological Society of America

Strong E E amp Lipscomb D 1999 Character coding and inapplicabledata Cladistics 15 363ndash71

Sturmer W amp Bergstrom J 1976 The arthropods Mimetaster andVachonisia from the Devonian Hunsruck Shale PalaontologischeZeitschrift 50 78ndash111

Sturmer W amp Bergstrom J 1978 The arthropod Cheloniellon fromthe Devonian Hunsruck Shale Palaontologische Zeitschrift 5257ndash81

Sturmer W amp Bergstrom J 1981 Weinbergina a xiphosuran fromthe Devonian Hunsruck Slate Palaontologische Zeitschrift 55237ndash55

Sutton M D Briggs D E G Siveter D J Siveter D J amp Orr PJ 2002 The arthropod Offacolus kingi (Chelicerata) from theSilurian of Herefordshire England computer based morphologi-cal reconstructions and phylogenetic affinity Proceedings of theRoyal Society of London Series B 269 1195ndash1203

Swofford D L 1999 PAUP Phylogenetic Analysis Using Parsimony(and other methods) Version 40b2a Sunderland MA SinauerAssociates

Telford M J amp Thomas R H 1998 Expression of homeobox genesshows chelicerate arthropods retain their deutocerebral segmentProceedings of the National Academy of Sciences USA 9510671ndash5

192 TREVOR J COTTON AND SIMON J BRADDY

Thorley J L amp Page R D M 2000 RadCon phylogenetic treecomparison and consensus Bioinformatics 16 486ndash7

Valentine J W 1995 Why no new phyla after the Cambrian ndashgenome and ecospace hypotheses revisited Palaios 10 190ndash194

Wagele J W 1993 Rejection of the lsquoUniramiarsquo hypothesis andimplications of the Mandibulata concept Zoologische JahrbucherAbteilung fur Systematik Geographie und Biologie der Tiere 120253ndash88

Walcott C D 1911 Middle Cambrian Merostomata Cambriangeology and palaeontology II Smithsonian MiscellaneousCollections 57 17ndash40

Walcott C D 1912 Middle Cambrian Branchiopoda MalacostracaTrilobita and Merostomata Cambrian geology and palaeontol-ogy II Smithsonian Miscellaneous Collections 57 145ndash228

Walcott C D 1918 Geological explorations in the CanadianRockies Explorations and fieldwork of the SmithsonianInstitution in 1917 Smithsonian Miscellaneous Collections 684ndash20

Walossek D 1993 The Upper Cambrian Rehbachiella kinnekullensisand the phylogeny of Branchiopoda and Crustacea Fossils andStrata 32 1ndash202

Walossek D amp Dunlop J A 2002 A larval sea spider (ArthropodaPycnogonida) from the Upper Cambrian lsquoOrstenrsquo of Swedenand the phylogenetic position of pycnogonids Palaeontology 45421ndash46

Walossek D amp Muller K J 1990 Upper Cambrian stem-lineagecrustaceans and their bearing upon the monophyletic origin ofCrustacea and the position of Agnostus Lethaia 23 409ndash27

Walossek D amp Muller K J 1997 Cambrian lsquoOrstenrsquo-type arthro-pods and the phylogeny of Crustacea In Fortey R A amp ThomasR H (eds) Arthropod Relationships 139ndash53 SystematicsAssociation Special Volume 55 London Chapman and Hall

Walossek D amp Muller K J 1998 Early arthropod phylogeny in lightof the Cambrian lsquoOrstenrsquo fossils In Edgecombe G D (ed)Arthropod fossils and phylogeny 185ndash231 New York NYColumbia University Press

Weygoldt P 1979 Significance of later embryonic stages and headdevelopment in arthropod phylogeny In Gupta A P (ed)Arthropod Phylogeny 107ndash35 New York NY Van NostrandReinhold Co

Weygoldt P 1998 Evolution and systematics of the ChelicerataExperimental and Applied Acarology 22 63ndash79

Weygoldt P amp Paulus H F 1979 Unterzuchungen aur Morpholo-gie Taxonomie und Phylogenie der Chelicerata Zeitschriftfur Zoologische Systematik und Evolutionsforschung 17 85ndash116177ndash200

Wheeler W C Cartwright P amp Hayashi C Y 1993 Arthropodphylogeny a combined approach Cladistics 9 1ndash39

Wheeler W C amp Hayashi C Y 1998 The phylogeny of extantchelicerate orders Cladistics 14 173ndash92

Whittington H B 1971 Redescription of Marrella splendens(Trilobitoidea) from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 209 1ndash24

Whittington H B 1974 Yohoia Walcott and Plenocaris n genarthropods from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 231 1ndash21

Whittington H B 1975a Trilobites with appendages from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 497ndash136

Whittington H B 1975b The enigmatic animal Opabinia regalisMiddle Cambrian Burgess Shale British Columbia PhilosophicalTransactions of the Royal Society of London Series B 271 1ndash43

Whittington H B 1977 The Middle Cambrian trilobite NaraoiaBurgess Shale British Columbia Philosophical Transactions of theRoyal Society Series B 280 409ndash43

Whittington H B 1979 Early arthropods their appendages andrelationships In House M R (ed) The origin of majorinvertebrate groups 253ndash68 Systematics Association SpecialVolume 12 New York NY Academic Press

Whittington H B 1980 Exoskeleton moult stage appendage mor-phology and habits of the Middle Cambrian trilobite Olenoidesserratus Palaeontology 23 171ndash204

Whittington H B 1981 Rare arthropods from the Burgess ShaleMiddle Cambrian British Columbia Philosophical Transactionsof the Royal Society of London Series B 292 329ndash57

Whittington H B 1985 Tegopelte gigas a second soft-bodied trilo-bite from the Burgess Shale Middle Cambrian British ColumbiaJournal of Palaeontology 23 171ndash204

Whittington H B 1989 Olenelloid trilobites type species functionalmorphology and higher classification Philosophical Transactionsof the Royal Society of London Series B 324 111ndash47

Whittington H B amp Almond J E 1987 Appendages and habits ofthe Upper Ordovician trilobite Triarthus eatoni PhilosophicalTransactions of the Royal Society of London Series B 317 1ndash46

Wilkinson M 1992 Ordered versus unordered characters Cladistics8 375ndash85

Wilkinson M 1994 Common cladistic information and its consensusrepresentation reduced Adams and reduced cladistic consensustrees and profiles Systematic Biology 43 343ndash68

Wilkinson M 1995a Coping with abundant missing entries in phylo-genetic inference using parsimony Systematic Biology 44 501ndash14

Wilkinson M 1995b TAXEQ2 software and documentation BristolSchool of Biological Sciences University of Bristol

Wilkinson M 1996 On the distribution of homoplasy and choosingamong trees Taxon 45 263ndash66

Wills M A 1997 A phylogeny of Recent and fossil Crustacea derivedfrom morphological characters In Fortey R A amp ThomasR H (eds) Arthropod Relationships 189ndash209 SystematicsAssociation Special Volume 55 London Chapman and Hall

Wills M A Briggs D E G amp Fortey R A 1994 Disparity asan evolutionary index a comparison of Cambrian and Recentarthropods Paleobiology 20 93ndash130

Wills M A Briggs D E G Fortey R A amp Wilkinson M 1995The significance of fossils in understanding arthropod evolutionVerhandlungen der deutschen zoologischen Gesselschaft 88 203ndash15

Wills M A Briggs D E G Fortey R A Wilkinson M amp SneathP H A 1998a An arthropod phylogeny based on fossil andrecent taxa In Edgecombe G D (ed) Arthropod fossils andphylogeny 33ndash105 New York NY Columbia University Press

Wills M A Briggs D E G amp Fortey R A 1998b Evolutionarycorrelates of arthropod tagmosis scrambled legs In ForteyR A amp Thomas R H (eds) Arthropod Relationships 57ndash65Systematics Association Special Volume 55 London Chapmanand Hall

Winter G 1980 Beitrage zur Morphologie und Embryologie desvorderen Korperabschnitts (Cephalosoma) der PantopodaGerstaecker 1863 I Entstehung und Struktur des Zentralnerven-systems Zeitschrift fur Zoologische Systematik und Evolutionsfor-schung 18 27ndash61

Zhang W amp Hou X 1985 Preliminary notes on the occurrence of theunusual trilobite Naraoia in Asia Acta Palaeontologica Sinica 24591ndash5 [In Chinese with English summary]

TREVOR J COTTON Department of Palaeontology The Natural History Museum CromwellRoad London SW7 5BD UKe-mail trevor_cottonhotmailcom

SIMON J BRADDY Department of Earth Sciences University of Bristol Wills MemorialBuilding Queenrsquos Road Bristol BS8 1RJ UKe-mail S J Braddybrisacuk

MS received 28 July 2003 Accepted for publication 15 March 2004

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 193

Page 12: The phylogeny of arachnomorph arthropods and the origin of the

Ramskold (1999) exopods consist either of a single flattenedlobe or a homonomous series of podomeres (Fig 6B C) Nostem group euarthropod has a bilobate exopod and theoutgroup is consequently also coded as State 0

14 Proximal lobe of exopod (0) flattened lobe and (1)slender shaft

15 Distal lobe of exopod (0) small to moderate sized flapwith short to moderately long attachment to proximal lobeand (1) large teardrop-shaped with long attachment to proxi-mal lobe Neither of these characters can be coded for taxathat do not have an exopod differentiated into proximal anddistal lobes (Character 13 State 1) without asserting thehomology of the single-lobed exopod with one of these partsThe coding of Edgecombe amp Ramskold (1999 p 280) implic-itly homologizes the exopods of Sidneyia and Retifacies withthe proximal lobe In these taxa this view is perhaps supportedby the presence of lamellate exopod setae which match thoseof the trilobite-type proximal lobe However in other taxawith a single-lobed exopod such as Alalcomenaeus (see Briggsamp Collins 1999) the exopod is fringed with sharp spines likethe distal lobe of differentiated exopods Alternatively thebilobate exopod may have originated from the division of aprimitively single-lobed structure The single-lobed exopod isnot clearly homologous with either lobe of divided exopodsand both these characters are consequently coded as inappli-cable to taxa without differentiated proximal and distal exopodlobes Consequently (see Character 13 herein) these characterscan only be coded for taxa that were previously considered byEdgecombe amp Ramskold (1999) and their coding is followedhere except in the cases of Sidneyia and Retifacies

16 Exopod shaft is a deep rounded flap (0) absent and (1)present All bilobate (Character 13) and segmented (Character12) exopod shafts are long and relatively narrow structures (seeFig 6) whereas some single-lobed exopod shafts have theform of rounded flaps These flap-like exopod shafts are largecompared to the length of the setae and at least half as deep asthey are long This appendage structure is found in cheliceratesand lsquogreat appendagersquo arthropods

17 Medially-directed exopod setae (0) absent and (1)present Walossek amp Muller (1998 p 194) suggested that thetilting of exopod setae towards the endopod in post-antennularlimbs with a multi-annulated exopod was an autapomorphyuniting crustaceans and all members of the crustacean stemgroup (the crustacean total group sensu Ax 1986 see Fig 6C)This character is shared with the marellomorphs Marrellaand Mimetaster (see Fig 6B Sturmer amp Bergstrom 1976Bergstrom 1979 fig 1middot3A B) In other taxa the setae eithersurround the entire margin of the exopod shaft or are directeddorsally (Fig 6A) The identification of setae on the dorsalsurface of the lateral flaps in Opabinia (Budd 1996b) suggeststhat the condition in marellomorphs and crustaceans isderived

18 Lamellate exopod setae (0) absent and (1) presentLamellate exopod setae originally described from trilobitesare a classic synapomorphy of the Arachnomorpha (see egBergstrom 1992 Hou amp Bergstrom 1997 pp 42ndash3) They areperhaps best known from Misszhouia (see Chen et al 1996Hou amp Bergstrom 1997 Fig 6A) These setae differ from themore spinose setae of other taxa (Fig 6C) in that they are wideand flat and imbricate over the length of the exopod shaft Thesetae of Marrella (Fig 6B) and similar taxa have variouslybeen considered lamellate (Bergstrom 1979) or non-lamellate(Bergstrom 1992) Since they do not imbricate and do notseem to be strongly flattened (Whittington 1971 Sturmer ampBergstrom 1976 fig 9a) Marrella and Mimetaster are codedas State 0 The setae of Helmetia as shown in Briggs et al(1994 fig 141) seem to be of the lamellate type The setae ofSinoburius have been described by Hou amp Bergstrom (1997p 85) as similar to those of Misszhouia Only the setae areknown of the appendages of Buenaspis (Budd 1999a) andthese also appear to be of the trilobite-type

Figure 6 Diagrammatic reconstructions of (A) arachnomorph and(B C) non-arachnomorph biramous appendages (A) The lsquotrilobite-typersquo biramous appendage of Misszhouia longicauda (after Chen et al1997) (B) Appendage of Marrella splendens (after Whittington 1971)(C) Appendage of the stem-lineage crustacean Martinssonia elongata(after Muller amp Walossek 1997 1998) Not to scale Abbreviations(bas) basis (dle) distal lobe of exopod shaft (ls) lamellar exopod setae(pe) proximal endite or pre-coxa (ple) proximal lobe of exopod shaft(ses) segmented exopod shaft and (ss) spinose exopod setae

180 TREVOR J COTTON AND SIMON J BRADDY

The homology of the book-gill lamellae of xiphosurans withlamellar setae was supported by Walossek amp Muller (1997p 149) and Edgecombe et al (2000 p 174) but rejected bySturmer amp Berstrom (1981) on the grounds that Weinberginapossesses Limulus-like gill lamellae and fringing setaeBook-gills have also been described from a eurypterid (Braddyet al 1999) There are certainly major morphologicaldifferences between book-gills and trilobite-type exopodsFollowing most recent opinion and pending further studyWeinbergina and Eurypterida are coded as possessing lamellatesetae

The form of the setae of Yohoia is uncertain (Whittington1974) the spinose setae seen in the most common reconstruc-tion (Gould 1989 Briggs et al 1994) are not justified by thespecimens Amongst other great appendage arthropods theform of the setae appears to vary those of Jianfengia (seeChen amp Zhou 1997 p 74) and Leanchoilia (see Bruton ampWhittington 1983) are lamellate while those of Fortiforceps(Hou amp Bergstrom 1997) and Alalcomenaeus (Briggs amp Collins1999) are spinose and less densely packed

19 Gnathobase on basis andor prominent endites onendopod (0) present and (1) absent (cf Edgecombe ampRamskold 1999 character 29 Wills et al 1998a characters 36amp 59) The presence of gnathobases on the appendages of someanomalocaridids (Hou et al 1995) and their general distribu-tion amongst non-arachnomorph euarthropods suggests thatState 0 is plesiomorphic The homology of the various enditesand gnathobasic structures found in euarthropods is in need ofcareful assessment and therefore a very general coding (fol-lowing Edgecombe amp Ramskold) is used for this character

243 Eyes 20 Position of lateral facetted eyes (0)ventral and stalked (1) dorsal and sessile and (2) absent (cfEdgecombe amp Ramskold 1999 character 4) Partial uncer-tainty coding is used for a number of taxa where the dorsalsurface is known but the ventral morphology is not andtherefore the presence of ventral eyes cannot be discountedFor example there is good evidence that Leanchoilia lackeddorsal sessile eyes but ventral stalked eyes may have beenpresent (as in L hanceyi see Briggs amp Robison 1984) and a(02) partial uncertainty coding is used This is also the case inmany naraoiids such as Buenaspis However only the outlineof the head shield of Liwia is known and the absence of dorsaleyes in the reconstruction of Dzik amp Lendzion (1988) isconjectural It is unclear whether the autapomorphic conditionof stalked dorsal eyes in Mimetaster (see Sturmer amp Bergstrom1976) is homologous with State 0 or State 1 and a partialuncertainty coding is also used

Whittington (1974) was somewhat equivocal about thenature of the lateral lobes anterior to the head shield ofYohoia These more closely resemble the ventral stalked eyes ofAlacomenaeus as recently described by Briggs amp Collins(1999) than either Whittingtonrsquos (1974) or subsequent (egGould 1989 fig 3middot18 Briggs et al 1994 fig 153) reconstruc-tions suggest In a well preserved specimen showing the dorsalaspect (USNM 57696 Whittington 1974 pl 2 figs 1ndash3) and ina laterally compressed specimen (USNM 57694 Whittington1974 pl 1 fig 1) they are clearly seen to be stalked andrelatively small lobate structures That these structures arelikely to represent stalked ventral eyes was reflected in thecoding of Wills et al (1998a characters 26 amp 27)

21 Visual surface with calcified lenses bounded withcircumocular suture (0) absent and (1) present

22 Dorsal bulge in exoskeleton accommodating drop-shaped ventral eyes (0) absent and (1) present

23 Eye slits (0) absent and (1) presentCharacters 21 to 23 are used exactly as described by

Edgecombe amp Ramskold (1999 character 5) Character 21 is

inapplicable to taxa that do not possess eyes (Character 20State 2)

24 Dorsal median eyes (0) absent and (1) present Dorsalmedian eyes on a tubercle are considered a synapomorphy ofthe Chelicerata (Dunlop amp Selden 1997 Dunlop 1999) Thenature and position of the structures interpreted as dorsalmedian eyes in Mimetaster (Sturmer amp Bergstrom 1976p 83ndash4) are regarded as equivocal

244 Ventral cephalic structures Many characters of theventral surface of the euarthropod head (see Fig 7) of poten-tial phylogenetic utility are poorly known in many importanttaxa In particular the presence of pre-hypostomal frontalorgans (Fig 7A CndashE) is not coded herein (see Edgecombe ampRamskold 1999 p 272) Definitive evidence of the absence ofthese structures is available for very few taxa and the homol-ogy of these structures with the maculae of the trilobitehypostome (see Fig 7B) and the frontal organs of the crusta-cean labrum (eg see Muller amp Walossek 1987 p 39) isuncertain Secondly the detailed homology of the trilobitehypostome with that of other putative arachnomorphs isunclear and consequently a very broad definition is usedherein For example various pre-hypostomal sclerites (Fig7A D) in other arachnomorphs may have been incorporatedalong with a primitive hypostome into a single sclerite that isrecognised as the hypostome in trilobites The structure of thehypostome (eg the presence of paired anterior wings dorsal tothe antennae Fig 7B C) may also be a source of additionalcharacters

25 Expanded cephalic doublure (0) absent and (1)present maximum width more than 30 length of head shieldor more than 25 width of pygidium Chen et al (1996)suggested that the wide doublure of Soomaspis and Tariccoia isa synapomorphy uniting these taxa The doublure of Buenaspis(see Budd 1999a) is poorly known but based on the width ofthe heavily crushed region of the cephalic margin and theposition of a possible impression of the edge of the doublure inone specimen (Budd 1999a pl 1 fig 1) it also appears to berather wide (ie greater than 30 of the length of the headshield) Following Edgecombe amp Ramskoldrsquos coding ofSidneyia the present authors do not consider the postero-median expansion of the doublure of for example Emeraldellaor Retifacies (see Bruton amp Whittington 1983 Hou ampBergstrom 1999 respectively) to be homologous with State 1but to represent the hypostome (see Character 26)

26 Anteromedian margin of cephalon notched accomo-dating strongly sclerotised plate (0) notch and plate absentand (1) notch and plate present (cf Edgecombe amp Rasmkold1999 character 10) The apomorphic state (illustrated inFig 7D) is only known in the taxa discussed by Edgecombe ampRasmkold (1999) Reconstructions of Yohoia show a roundedlobe between the eyes (see Character 20) anterior to anddistinct from the head shield which resembles the anteriorsclerite recognised here This seems to be a misinterpretationSpecimens preserved in dorsal aspect (USNM 57696Whittington 1974 pl 2 figs 1ndash3) and lateral aspect (USNM57694 Whittington 1974 pl 1 fig 1 USNM 155616Whittington 1974 pl 5 fig 2) seem to clearly show that thislsquomedian lobersquo is a downward curving pointed extension of thehead shield

27 Hypostomal sclerite (0) median extension of thedoublure with no suture (1) natant sclerite not in contactwith doublure (2) with narrow overlap with pre-hypostomalsclerite (3) narrow attachment to doublure at hypostomalsuture and (4) absent The homology of hypostomes andlabrums has been discussed by Haas et al (2001) and Budd(2002) The euarthropod labrum is likely to represent a pos-teroventral extension of the pre-segmental acron (Scholtz

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 181

Figure 7 Ventral reconstructions of arachnomorph arthropod heads (A) Misszhouia longicaudata (after Chenet al 1997 figs 2ndash4 7) (B) Ceraurinella typa (after Chen et al 1997 fig 7) (C) Agnostus pisiformis (after Mulleramp Walossek 1987) (D) Kuamaia lata (after Edgecombe amp Ramskold 1999 figs 32 5 amp 6) (E) Cindarella eucala(after Ramskold et al 1997) and (F) Emeraldella brocki (after Bruton amp Whittington 1983) Not to scaleAbbreviations (aw) anterior wing beneath antennae (bs) boomerang-shaped sclerite (bl) blade-like process ofposterior wing (d) doublure of head-shield (cs) connective suture (f) fenestra (fs) facial suture (h) hypostome(hl) hypostomal lobe (emerging through fenestrae of Agnostus) (hs) hypostomal suture (if) intervening field ofhypostomal complex (lb) lateral bridge (lfo) lateral frontal organ (mb) median body (mbr) median bridge (mf)median furrow (mfb) mouth field bridge (mfo) median frontal organ (ol) ovate lobe and (phs) pre-hypostomalsclerite Post-antennal appendages and distal parts of antennae omitted

182 TREVOR J COTTON AND SIMON J BRADDY

1997) This structure is often sclerotised and covers the mouthventrally It is this sclerite that is here identified as thehypostome Therefore this character is considered absent intaxa lacking a sclerite covering the mouth irrespective of themorphology and position of the labrum itself which at leastpartly covers the mouth in all euarthropods other thanpycnogonids (see Edgecombe et al 2000 p 167) The lsquofleshylabrumrsquo of crustaceans does not appear to be sclerotised and ahypostome is consequently considered to be absent Althoughmany derived features are associated with the crustaceanlabrum this is not a good reason to consider it non-homologous with the hypostome-bearing structure of otherarthropods as Walossek amp Muller (1990) argued

In many taxa the mouth is covered by a posteromedianextension of the doublure here considered to represent thehypostome (eg Emeraldella Fig 7F Aglaspis see Hesselbo1992 fig 52) In others the hypostome is separated from thedoublure either by a suture a pre-hypostomal sclerite (egKuamaia lata Fig 7D Saperion glumaceum see Edgecombe ampRamskold 1999 figs 3 amp 4) or an intervening region ofunsclerotised cuticle (the natant condition of Fortey 1990beg Cindarella eucala Fig 7E) The homology of pre-hypostomal sclerites and hypostomes in helmetiids trilobitesand naraoiids (see Fig 7) has been discussed by Chen et al(1996) and Edgecombe amp Ramskold (1999) but it is as yetunclear whether any of the character states described here arederived from any other They are treated here as distinct andthe character as unordered pending further study

No hypostome has been identified in Yohoia (see above)Alalcomenaeus Jianfengia Fortiforceps or Leanchoilia andthere is certainly no broad posterior extension of the doublurein any of these taxa Since there is also no pre-hypostomalsclerite they have received a (134) partial uncertainty codingThe attachment of the hypostome of Tegopelte is unclear butit does not seem to be attached to any doublure (Whittington1985) and therefore it is given an uncertainty (12) coding

The lsquorostrum-like structurersquo on the ventral surface ofLemoneites appears to be similar in morphology and positionrelative to the anterior margin of the head shield (Flower 1968pl 8 figs 4 amp 13) to that described from Aglaspis and is codedas State 0 In many taxa the hypostome is unknown but inonly a small number can it be coded reliably as absentHughesrsquo (1975) suggestion that the hypostome of Burgessia isabsent is accepted preliminarily but an extension of thedoublure may be visible in some of Hughesrsquo figures Fortey(pers comm 2000) also doubts Hughesrsquo assertion

28 Visible ecdysial sutures (0) absent and (1) present Themarginal ecdysial sutures of chelicerates and the dorsal suturesof trilobites are almost certainly not homologous but thepresence of sutures and their position (see Character 29) arecoded separately to allow this to be tested Wills et al (1998aCharacter 2) coded marginal sutures as present in Sidneyiafollowing Brutonrsquos (1981) suggestion but this is consideredequivocal here

29 Position of ecdysial sutures (0) marginal and (1) dor-sal This character is inapplicable to taxa lacking ecdysialsutures (Character 28 State 0) Dorsal sutures whilst presentin the representatives of the Trilobita coded here are probablynot a synapomorphy of trilobites as a whole but for a clade oftrilobites excluding the Olenellida (Whittington 1989 Fortey1990a)

245 Exoskeletal tergites and thoracic tagmosis 30 Min-eralised cuticle (0) absent and (1) present Calcification of theexoskeleton is one of the most convincing synapomorphiesof the Trilobita (Fortey amp Whittington 1989 Ramskold ampEdgecombe 1991 Edgecombe amp Ramskold 1999 Character 1)Cuticle mineralisation is also coded as present in aglaspidids

following Briggs amp Fortey (1982) who suggested that theaglaspidid exoskeleton was originally phosphatic Paleomerusprobably also had a mineralised cuticle but Hou amp Bergstrom(1997 p 97) argued that it was likely to be calcareous Anundescribed Silurian aglaspidid may also have had an origi-nally calcitic cuticle (Fortey amp Theron 1994 p 856) Becauseof the uncertainty about the chemical composition of theexoskeleton in these forms cuticle mineralisation is coded aspotentially homologous in all these taxa and no attempt ismade to code separate states for phosphatic and calcareousmineralisation

31 Trunk tergites with expanded lateral pleurae coveringappendages dorsally (0) absent and (1) present Whilst thepresence of paratergal folds may be a synapomorphy at thelevel of the Euarthropoda (Boudreaux 1979 Wagele 1993Edgecombe et al 2000 Character 142) these are at mostsmall reflections of the margin of the tergites in most euarthro-pods In arachnomorph taxa these are expanded to form largelateral pleurae that cover the appendages dorsally (see egLimulus and Triarthus in Boudreaux 1979) This is inferred tobe the case in some taxa where dorsal features and appendagesare poorly known on the basis of their possession of tergiteswith wide pleural regions This feature was suggested as typicalof lamellipedians (=arachnomorphs) by Hou amp Bergstrom(1997 pp 42ndash3)

The arrangement of the thorax of Marrella Mimetaster andBurgessia differs from that of all the other taxa considered herein that the appendages seem to be attached to the lateralmargins of the body The trunk tergites do not cover theappendages and seem to lack pleurae entirely although theyare covered by a posterior extension of the head shield inBurgessia This character has been clearly described inMimetaster (Sturmer amp Bergstrom 1976 pp 87ndash90 fig 8) andBurgessia (Hughes 1975 p 421)

32 Free thoracic tergites (0) present and (1) absentFollowing Edgecombe amp Ramskold (1999 Character 16) anumber of taxa lack functional post-cephalic articulations andconsequently lack free thoracic tergites Despite this theycode these taxa for a number of characters relating to thestructure of thoracic tergites (eg their characters 18 and 19)These characters are treated here as inapplicable to taxawithout free tergites and the definition of this character hasbeen modified from that of Edgecombe amp Ramskold (1999) tomake this more explicit

33 Decoupling of thoracic tergites and segments (0)absent and (1) present Ramskold et al (1997) described thischaracter of Cindarella and Xandarella where the thoracictergites correspond to a variable increasing posteriorly num-ber of appendage pairs This character cannot be coded fortaxa in which free thoracic tergites are absent (Character 32State 1)

34 Tergite articulations (0) tergites non-overlapping (1)extensive overlap of tergites and (2) edge-to-edge pleuralarticulations In most of the taxa considered here the thoracictergites overlap considerably and relatively evenly over theirwidth This contrasts with the primitive euarthropod condi-tion where the tergites do not overlap medially as seen in stemgroup crustaceans In trilobites such as Helmetia and Kuamaia(see Edgecombe amp Ramskold 1999 character 18) overlap ofthoracic tergites is limited to a well-defined axial region andthe lateral pleurae meet edge-to-edge

The thoracic articulations of naraoiids with free thoracictergites are in some ways similar to those of trilobites with astrong overlap medially that is lacking abaxially andanterolateral articulating facets (Budd 1999a Ramskold ampEdgecombe 1999) However the distribution of these features

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 183

in other trilobite-like arachnomorphs is unclear and a restric-tive coding is used here The present authors do not considerthat this character can be applied reliably to the fused thoracictergites of Saperion Tegopelte and Skioldia and it is coded asinapplicable to all taxa lacking free thoracic tergites (Character32 State 1)

35 Trunk effacement (0) trunk with defined (separate orfused) tergite boundaries (1) trunk tergite boundaries effacedlaterally and (2) trunk tergite boundaries completely effaced(cf Edgecombe amp Ramskold 1999 Character 15)] Contrary toEdgecombe amp Ramskold (1999) the present authors considerSkioldia and Saperion to show a distinct form of tergiteeffacement where the fused tergite boundaries are defined byfurrows axially but effaced laterally The boundaries areeffaced at least laterally in Tegopelte but the axial region ofthe exoskeleton is unknown which accordingly is given amultistate uncertainty coding

36 Cephalic articulation fused (0) absent and (1) presentIn Tegopelte Saperion and Skioldia the articulation ofthe head with the thorax is non-functional and the entireexoskeleton forms a single tergite

37 Head shield overlap of thoracic tergites (0) overlapabsent or identical to overlap between thoracic segments (1)head shield covers first thoracic tergite only and (2) headshield covers mutliple anterior trunk tergites

38 Head shield articulates with reduced anterior thoracictergite (0) absent and (1) present Amongst taxa showing aposteriorly expanded head shield ie one that overlaps thethorax Edgecombe amp Ramskold (1999 Character 9) identifiedtwo distinct character states One of these ndash overlap of only theanteriormost thoracic tergite ndash was limited to taxa assigned tothe Liwiinae (sensu Fortey amp Theron 1994) and another ndashoverlap of multiple tergites with attachment to a reducedanterior thoracic tergite ndash to the Xandarellida None of theadditional taxa considered herein (including Buenaspis whichwas assigned to the Liwiinae by Budd 1999a) shows thesedistinctive morphological features However the expandedhead shields of Marrella Mimetaster and Burgessia which donot articulate with a narrow anterior thoracic tergite may behomologous with the expanded head shields of xandarellidsTo recognise this the degree of overlapping of the thorax andthe possession of the reduced anterior tergite are coded sepa-rately These characters are both coded as inapplicable to taxain which the head shield and thoracic tergites are fused(Character 36 State 1) The crustacean carapace which ismost similar to the head shield of Burgessia is seeminglyabsent in stem group crustaceans (Walossek amp Muller1998)

39 Trunk narrowed anteriorly relative to head shieldwidest posteriorly (0) absent and (1) present (cf Edgecombeamp Ramskold 1999 Character 14) The anterior narrowing ofthe trunk caused by the reduction of opisthosomal segment 1in xiphosurids (Weinbergina of the taxa analysed hereAndersen amp Selden 1997 Dunlop amp Selden 1997 Character14) is not considered to be homologous with the unusual shapeof the thorax in naraoiids recognised by their coding

40 Boundaries of anterior trunk segments reflexed antero-laterally (0) absent boundaries transverse or reflexed postero-laterally and (1) present

41 Joints between posterior tergites functional anteriorones variably fused (0) absent and (1) present

42 Posterior tergite bearing axial spine (0) absent and (1)present

Characters 40 to 42 are coded following Edgecombe ampRamskold (1999 characters 17 19 amp 23 respectively) Inaddition to the taxa considered here a thoracic axial spine ispresent in some olenelloid trilobites (see Lieberman 1998

Characters 73 amp 74) and in the aglaspidid Beckwithia (seeHesselbo 1989)

246 Posterior body termination 43 Postabdomen of seg-ments lacking appendages (0) absent and (1) present

44 Length of postabdomen (0) one segment (1) twosegments (2) three segments and (3) five segments In sometaxa a variable number of posterior thoracic segments bearcomplete tubular tergites and lack appendages In Aglaspisautapomorphically it seems that the posterior three thoracicsegments lack appendages (Briggs et al 1979 Hesselbo 1992)but have unmodified tergites These situations are recognisedas potentially homologous by the coding used here Character44 is coded as inapplicable to taxa lacking a postabdomen

45 Posterior tergites strongly curved in dorsal aspect com-pared to anterior tergites (0) absent and (1) present Asrecognised by Wills et al (1998a Character 17) in some taxawhere the tergites are distinct the curvature of thoracic tergitesincreases posteriorly so that the posterior tergites are highlycurved to semicircular in dorsal aspect This situation isnot known from crustaceans (except isopods) or stem grouparthropods

46 Posterior segments reduced and with highly reducedappendages (0) present and (1) absent In some taxa there area large number of posterior segments that are short sagitallyand have appendages that are much reduced in size comparedto anterior trunk appendages These somites are incorporatedinto the pygidium in some taxa but primitively they arecovered by tiny free trunk tergites In the derived state thetrunk somites and limbs are of a relatively constant size Thedistinction between these states can be seen when attempting tocount the number of segments that make up the body In taxashowing State 0 the number of segments is very high anddifficult to count whereas in other taxa the number ofpost-cephalic segments is easily assessed

47 Pygidium (0) absent and (1) present A pygidium isrecognised here as a posterior tagma consisting of a number offused segments under a single tergite which may or may notincorporate the post-segmental telson This is different to theuse of this term by Edgecombe amp Ramskold (1999) whichfollows Ramskold et al (1997) and very different to its use byWills et al (1998a p 53) as a synapomorphy of the TrilobitaThe present authorsrsquo definition recognises the situation inRetifacies where the spinose postsegmental telson is autapo-morphically not fused to the pygidium as homologous toother multisegmented posterior tagma which include thetelson The distinction between the pygidium and an expandedpost-segmental telson can also be seen in the position of theanus which is consistently in the posteriormost pre-telsonicsegment amongst putative arachnomorphs (see Character 48)In taxa with a pygidium the anal segment is fused into thepygidium (see eg Kuamaia Edgecombe amp Ramskold 1999fig 6) whereas in those taxa lacking a pygidium the anus isbetween the final trunk tergite and the telson

Ramskold et al (1997) argued that the posteriormost tergiteof xandarellids (which incorporates the anal segment) is nothomologous to the posterior tagma recognised as pygidiaherein because posterior thoracic tergites also cover multiplesegments in xandarellids (see Character 33) Evidence ofsegmentation of the pygidium in a variety of taxa suggests thatthe number of tergites fused to form the pygidium is greaterthan the number of appendages For example the pygidium ofKuamaia has two pairs of lateral spines but at least fourpairs of appendages (Hou amp Bergstrom 1997 Edgecombe ampRamskold 1999) and that of Triarthrus has only five axialrings posterior to the articulating ring but over 10 pairs ofappendages (Whittington amp Almond 1987) The fact thatdecoupling of tergites and segments is evident in the thorax of

184 TREVOR J COTTON AND SIMON J BRADDY

Xandarella and Cindarella does not necessarily suggest that asimilar more widespread decoupling in pygidia is the result ofa different developmental process and hence that they arenon-homologous Instead the situation in the xandarellidthorax is potentially homologous to that found (more widely)in pygidia and consequently the terminal xandarellid tergite isa pygidium It is unclear if decoupling of tergites and somites isprimitively limited to the terminal tergite or primitively aproperty of the arachnomorph post-cephalon as a whole

It has been suggested that the tiny pygidium of olenelloidtrilobites which probably best reflects the primitive trilobitestate consists only of the post-segmental telson (Harrington inMoore 1959) and therefore is not a true pygidium HoweverWhittington (1989) showed that the olenelloid pygidium con-sists of at least two and possibly as many as five or sixsegments and therefore is likely to be homologous with thepygidium of other trilobites

48 Position of the anus (0) terminal within telson and (1)at base of telson In crustaceans and stem group arthropodsthe anus is terminal or otherwise situated in the telson (egRehbachiella Walossek 1993 fig 15D) In putative arachno-morphs the anus is either at the junction of the posteriormostthoracic segment and the telson or ventral within a fusedpygidium In the case of taxa with a pygidium the anus isanterior to the posterior margin and where known positionedbetween the posteriormost pair of appendages suggesting thatit is anterior to the post-segmental telson (see eg OlenoidesWhittington 1980)

49 Pygidium with median keel (0) absent and (1) presentEdgecombe amp Ramskold (1999 Character 21) considered thepresence of a median keel on the pygidium as a synapomorphyuniting Soomaspis and Tarricoia They did not explain howthis structure could be distinguished from the raised pygidialaxis of trilobites Homology of these structures is not sup-ported here because the axis of more primitive trilobites(including Eoredlichia) is quite distinct morphologically fromthe naraoiid keel Budd (1999a) noted the presence of a keel onthe pygidium of Buenaspis and suggested (Budd 1999a p 102)that it may be an artefact of dorsoventral compaction How-ever contrary to the coding of Edgecombe amp Ramskold(1999) a keel is visible on the pygidium of Liwia convexa (Dzikamp Lendzion 1988 fig 4CD) preserved in full relief Thereforeit seems unlikely that the keel is a taphonomic artefact

50 Pygidium with broad-based median spine (0) absentand (1) present

51 Pygidium with lateral spines (0) present and (1) absentEdgecombe amp Ramskold (1999 Character 22) coded thepresence of a median spine and two pairs of lateral spines aspotentially homologous in Sinoburius Kuamaia and HelmetiaThe distribution of lateral spines is much wider than that ofterminal spines and therefore they are coded separately hereIn trilobites it has widely been recognised that lateral spinesrepresent the original segmentation of the pygidium Thiscannot be the case for median spines Characters 49 to 51are not applicable to taxa lacking a pygidium (Character 47State 0)

52 Expanded post-segmental telson (0) absent and (1)present In a range of taxa the posterior-most tergite is a large(relative to the thoracic tergites) structure that lacks anyevidence of segmentation and is interpreted as representing anexpanded tergite of the post-segmental telson from whichsegments are released anteriorly during ontogeny On the otherhand the posteriormost tergite of Marrella and probablyMimetaster is small compared to the thoracic tergites androunded This element probably represents the plesiomorphiceuarthropod telson The homology of this character in mosttaxa is clear and only doubtful in Retifacies in which it may

be segmented The figures of Hou amp Bergstrom (1997) providelittle unequivocal evidence for a telson in Retifacies but thepresence of this structure is very clear in colour photographs(eg Chen et al 1996 fig 198A) However these figures donot convincingly demonstrate the segmentation of the telsonRetifacies is interpreted here as being unique in possessingboth a pygidium and an expanded post-segmental telson

53 Telson shape (0) spinose and (1) paddle-shaped Theshape of the expanded telsons identified above varies frombroadly spinose to flattened and paddle-like This differencecan be recognised both on the basis of the ratio of length tobasal width (spinose telsons are relatively long) and by thechange in width posteriorly (spinose telsons reduce in widthposteriorly whereas paddle-shaped telsons increase in width)In eurypterids a wide range of telson shapes is found includ-ing both character states described here It is likely that theplesiomorphic condition is a spinose telson This character isinapplicable to taxa lacking an expanded telson

54 Post-ventral furcae (0) absent and (1) present Thischaracter recognises the potential homology of unsegmentedpaired structures that articulate with the segment immediatelyanterior to the telson The potential homology of these struc-tures (the post-ventral plates) in Emeraldella and Aglaspis wasrecognised by Wills et al (1998a Character 70) and inEmeraldella and Sidneyia by Edgecombe amp Ramskold (1999Character 25) The homology of the segmented pygidial caudalfurcae of Olenoides (see Whittington 1975a 1980) with thesestructures is considered doubtful and coded as equivocal

Despite their distinctive morphology the long caudualfurcae of Cheloniellon may be homologous with the furcae ofAglaspis Emeraldella and Sidneyia This is supported by thecaudal furcae of the Ordovician cheloniellid Duslia which aremore similar in morphology to those of Emeraldella than ofCheloniellon Chlupac (1988 pp 614ndash16) considered thesestructures to be on the terminal tergite in Duslia However afaint tergite boundary is apparent posterior to the attachmentof the furcae (see Chlupac 1988 pl 57 fig 4) Therefore theyare considered here to have been attached to the pre-telsonicsegment as in Cheloniellon Chlupac (1988) and Dunlop ampSelden (1997) supported a close relationship between Dusliaand Cheloniellon

25 MethodsAll analyses were carried out using PAUP Version 40b4a(Swofford 1999) and unless otherwise stated used heuristicsearches with one thousand random addition sequencereplicates The software packages MacClade Version 307(Maddison amp Maddison 1997) and RadCon (Thorley amp Page2000) were used for comparing trees and investigating patternsof character evolution Tree length and other statistics (theConsistency Index CI and Retention Index RI) were calcu-lated by PAUP and MacClade with uninformative charactersexcluded Analyses were run separately using each of thedifferent codings for aglaspidids

Characters were treated as unordered and of equal weight inmost analyses To assess the influence of this assumption fourof the eight multistate characters which have states that areintermediate between others (Wilkinson 1992) were treated asordered in some analyses These characters are the number ofcephalic segments (Character 4) the length of raptorialappendage spines (Character 6) the degree of overlap of thethorax by the head-shield (Character 37) and the number ofsegments making up the postabdomen (Character 44) The lastof these is uninformative when not ordered because only onetaxon shows each of states 0 1 and 3

Support for individual nodes was assessed by bootstrapanalysis (Felsenstein 1985) and by calculating Bremer support

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 185

indices (Bremer 1988 1994) These methods measure twodistinct aspects of support for phylogenetic hypotheses Treesor nodes may be considered well supported (1) to the extent towhich alternative topologies are much less parsimonious asmeasured by the support index (Wilkinson 1996) or (2) wherethey are consistent with a large proportion of characters sothat character sampling is unlikely to have had much influenceon topology as assessed by bootstrapping (Page 1996) Boot-strapping was performed with 100 bootstrap replicates eachof 10 addition sequence replicates Bremer support indicesare based on heuristic searches with 100 addition sequencereplicates

Three pairs of terminals are included in Table 2 that apartfrom missing data are coded identically These are Helmetiaand Kuamaia Soomaspis and Tariccoia and Saperion andSkioldia These pairs are lsquotaxonomic equivalentsrsquo removingthe taxon with most missing data will not influence theposition of the other They can be recoded as single terminalsaccording to the principal of safe taxonomic reduction(Wilkinson 1995a b) and were not considered separately in theanalyses presented here Therefore cladistic analyses werecarried out on the basis of 33 terminals Edgecombe ampRamskold (1999) ignored this and coded the same three pairsof taxa and Eoredlichia and Olenoides identically for theirmore limited set of characters

26 ResultsAnalysis with all characters unordered and using the lsquoAglaspi-dida 1rsquo coding found nine most parsimonious trees (MPTs)each 126 steps long (CI=0middot556 RI=0middot763) Using the lsquoAglas-pidida 2rsquo coding resulted in 18 MPTs of length 125 (CI=0middot560RI=0middot765) Nine of these 18 trees were those found with thefirst coding of aglaspidids The strict component consensus(sensu Wilkinson 1994) of the 27 unique trees found acrossboth analyses is shown in Figure 8 All trees supported themonophyly of the Marrellomorpha (Marrella and Mimetaster)and of a clade including all other taxa except Crustacea (Clade1 of Fig 8) The relationships between these two clades and theCrustacea were unresolved equal numbers of trees in bothanalyses supported each of the topologies [Clade 1 (CrustaceaMarrellomorpha)] [Crustacea (Marrellomorpha Clade 1)]and [Marrellomorpha (Crustacea Clade 1)] All trees placedBurgessia as the sister taxon to the rest of Clade 1 andsupported the division of the rest of Clade 1 into two majorsister groups The topology of one of these clades (Clade 2)which included trilobites helmetiids tegopeltids naraoiidsxandarellids and Retifacies was stable with respect to thecoding of aglaspidids The other clade (Clade 3) consists of aclade of megacheirans and chelicerates (Clade 4) and a cladeincluding aglaspidids Lemoneites Paleomerus CheloniellonEmeraldella and Sidneyia (Clade 5) This latter clade is un-resolved in the strict consensus of all 27 trees (Fig 8) and thestrict consensus of the 18 MPTs found using the lsquoAglaspidida2rsquo coding but was fully resolved in the consensus of the ninetrees found with the lsquoAglaspidida 1rsquo coding Within themegacheiran-chelicerate clade all trees found Fortiforceps tobe the sister group to all other taxa Yohoia the sister group tochelicerates and Alalcomenaeus and Leanchoilia to form aclade Jianfengia is equally parsimoniously placed as the sistergroup to the Yohoia-chelicerate clade or as the sister group tothis clade and the Alalcomenaeus-Leanchoilia clade

Treating characters 4 6 37 and 44 as ordered had a minimaleffect on most parsimonious topology Nine MPTs of 136 steps(CI=0middot515 RI=0middot743) were found with the lsquoAglaspidida 1rsquocoding and 12 MPTs 135 steps in length (CI=0middot519 RI=0middot745)with the lsquoAglaspidida 2rsquo coding These trees all supported a(Crustacea Clade 1) clade excluding the marrellomorphs

Apart from this the strict consensus of these 21 trees isidentical to the consensus of the 27 trees found with allcharacters treated as unordered

Across all analyses four different topologies for Clade 5 (ofFig 8) were found (Fig 9) The distribution of these topologiesacross the four analyses described above and across the sameanalyses repeated after reweighting of each character in pro-portion to their Rescaled Consistency Index (see Farris 1989)from the first analysis is shown in Table 3 The instability of

Figure 8 Strict consensus of 27 MPTs from four separate analyses ofarachnomorph phylogeny with both interpretations of aglaspididmorphology and with some characters treated as ordered or allcharacters unordered Clades referred to in the text are numbered

Figure 9 Alternative equally parsimonious resolutions of clade 5 ofFig 8 (see Table 3)

186 TREVOR J COTTON AND SIMON J BRADDY

this clade was to some extent caused by the inclusion ofLemoneites and Paleomerus which could each be coded foronly 27 of the 54 characters used Analysis with these two taxaexcluded and characters treated as unordered found treescompatible only with Topology D of Figure 9 irrespective ofthe coding used for aglaspidids Analysis with some characterstreated as ordered and using either aglaspidid coding foundthree trees compatible only with Topology B of Figure 9 andthree compatible only with Topology D of Figure 9 ThereforeTopology D is the preferred topology for Clade 5 as shown inTable 3

One of the MPTs resulting from the lsquoAglaspidida 2rsquo coding(with all characters treated as unordered) was chosen as thebasis for further discussion because this tree combined thepreferred topology of Clade 5 with the (Crustacea Clade 1)group that was favoured when some characters were treated asordered Bootstrap percentages and Bremer support values forthis tree (using the Aglaspidida 2 coding) are shown in Figure10 Apomorphies for all ingroup nodes of this tree are shownin Figure 10A

3 Discussion

31 Phylogenetic resultsThe results of the cladistic analyses presented above arewell-resolved and robust with respect to different analyticalparameters Most of the taxa considered form a clade (Clade 1of Fig 8) that is more closely related to chelicerates than tocrustaceans in all analyses and therefore is recognised asthe Arachnomorpha as defined by Chen et al (1996) andRamskold et al (1996) The monophyly of the Marrellomor-pha was supported in all analyses The marrellomorphs werefound to be the sister group to a Crustacea+Clade 1 group inmost MPTs but in a minority of trees formed the sister groupto Clade 1 alone According to the latter result they should beincluded within the Arachnomorpha but according to the firstexcluded from it Both of these alternatives have been sup-ported in previous studies (eg Hou amp Bergstrom 1997 Willset al 1998a respectively) In addition to the marrellomorphsthe Fuxianhuiida Bousfield 1995 and Canadaspidida havebeen placed in the Euarthropoda outside the main mandibu-late and arachnomorph clades (Hou amp Bergstrom 1997)Further study of these taxa of the crustacean or mandibulatestem group and of the marrellomorphs is necessary before the

position of the Marrellomorpha can be resolved Provisionallymarrellomorphs are excluded from the Arachnomorphafollowing the majority of analyses above and Wills et al(1994)

The Burgess Shale arthropod Burgessia was placed as thesister group to remaining arachnomorphs in all analyses OtherArachnomorpha form two major clades one consisting of thelsquotrilobite-alliedrsquo arachnomorphs analysed by Edgecombe ampRamskold (1999) here termed lsquoTrilobitomorpharsquo (defined asTrilobita and all taxa more closely related to Trilobita thanChelicerata) and the other (lsquochelicerate-alliedrsquo) clade includingchelicerates lsquogreat appendagersquo arthropods aglaspididsLemoneites Paleomerus Sidneyia and Emeraldella The latteris here termed lsquoCheliceramorpharsquo (defined as Chelicerata andall taxa more closely related to Chelicerata than Trilobita) Thetopology of the lsquotrilobite-alliedrsquo clade was similar to that foundby Edgecombe amp Ramskold (1999) but the present authorsrsquoresults are more completely resolved unambiguously support-ing the sister-group relationship between trilobites andHelmetiida suggested in the previous study Within thelsquochelicerate-alliedrsquo clade Aglaspidids Paleomerus Lemoneitesand the Burgess Shale Emeraldella and Sidneyia form a cladein opposition to megacheirans and chelicerates

The present authorsrsquo analysis supports a new hypothesis ofthe origin of the chelicerates from within a paraphyleticassemblage of megacheiran arthropods and confirms pyc-nogonids as a sister group to the euchelicerates Previoushypotheses of the chelicerate sister group (see Section 1) aremuch less parsimonious The shortest trees supporting aCheloniellon-chelicerate clade are six steps longer than theMPTs and those supporting an aglaspidid-chelicerate cladeseven steps longer All megacheirans and chelicerates areunited by the flap-shaped rounded exopods and modificationof the second cephalic segment endopods into anteriorlydirected spinose raptorial organs which share a number ofdetailed similarities A clade including all of these taxa apartfrom Fortiforceps is supported by the synapomorphic loss ofthe antennae (or antennulae of crustaceans) and the longerlength of spinose projections of the second cephalic appendageendopods Within this clade Yohoia shares the loss of cephalicexopods the loss of thoracic endopods and a postabdomen oftubular tergites with chelicerates

The poor resolution of Clade 5 (of Fig 8) was partly a resultof the poor preservation of and consequent high proportion ofmissing data for Lemoneites and Paleomerus When these two

Table 3 Numbers of most parsimonious trees supporting each of the alternative topologies for clade 4 of Fig 7 under different analyticalconditions (RCI=Revealed Consistency Index)

Aglaspididcoding

Characters 4 6 36and 43 ordered

Charactersreweighted by RCI

Paleomerus andLemoneites excluded

Topology (see Fig 8)

A B C D

1 91 X 3 3 31 X 61 X X 31 X 61 X X 3 32 9 92 X 3 3 3 32 X 62 X X 32 X 62 X X 3 3Total 6 18 12 54

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 187

taxa were excluded from the analyses only two (B and D) ofthe four topologies found in the complete analysis remainedmost parsimonious (see Fig 9 and Table 3) Secondly Bremersupport values throughout much of the lsquochelicerate-alliedrsquoclade (Clade 3 of Fig 8) were improved when these taxa wereexcluded (see Fig 11)

Dunlop (1999 p 258) suggested that in reconstructing thechelicerate stem lineage lsquowe might predict that the two mostsignificant changes towards the chelicerate condition are thereduction of the antennae and the formation of the nextappendages into a clawrsquo According to the hypothesis pre-sented here both of these adaptations were achieved in EarlyCambrian chelicerate ancestors The recognition of the loss ofthe antennae in the stem group of the chelicerates and thephylogenetic hypothesis presented here suggest that chelicer-ates are most closely related to taxa with only a singleadditional segment fused to the crown group eucheliceratehead of four fused somites Previous hypotheses have derivedchelicerates from Palaeozoic arthropods with a longer head(eg Cheloniellon Sturmer amp Bergstrom 1978 SanctacarisBriggs amp Collins 1988 Emeraldella Bruton amp Whittington1983) The plesiomorphic pattern of head segmentation ismatched in pycnogonids Bergstrom et al (1980) suggestedthat only the anterior four pairs of appendages are incorpo-rated into the head This supports the view that pycnogonidsare primitive with respect to other chelicerates which showmore or less complete fusion of this primitive head and threethoracic segments including Solpugida and Pseudoscorpiones

The present study suggests that a shift from gnathobasicbenthic feeding to a more vagrant lifestyle and raptorialappendage feeding occurred very early in the chelicerate lin-eage Appendage feeding was clearly an important adaptation

Figure 10 One of 18 equally parsimonious cladograms with lsquoAglaspidida 2rsquo coding and all characters unorderedthat is most congruent with results from other analyses Non-terminal branch lengths scaled to reflect number ofapomorphies (A) ACCTRAN apomorphy scheme Character numbers shown above boxes and states shownbelow boxes as in Table 2 and the text Character consistency indices are indicated by shading (B) Levels ofsupport for individual nodes Bremer support indices shown in bold type above branches Bootstrap percentagesin italic type below branches for all nodes with relative frequencies greater than 50

Figure 11 Bremer support values for clades within the lsquochelicerate-alliedrsquo arachnomorphs (clade 3 of Fig 8) when all taxa wereincluded (normal weight font above nodes) and with Lemoneites andPaleomerus excluded (bold type below nodes)

188 TREVOR J COTTON AND SIMON J BRADDY

for the terrestrialization of chelicerates (Dunlop 1997 p 69Dunlop amp Webster 1999) but the model proposed heresuggests that this considerably pre-dated terrestrializationGnathobasic feeding in xiphosurans and eurypterids mayrepresent a reversal to a more primitve benthic arachnomorphlifestyle from a more pycnogonid-like ancestor

32 Evolution of the arachnomorph headThe extant arthropod classes show highly conserved patternsof head segmentation (eg Wills et al 1994) In contrast therehas been a near consensus amongst Cambrian arthropodworkers (although rarely based on an explicit phylogenetichypothesis) that patterns of head segmentation are highlyhomoplastic and hence of little systematic significanceSturmer amp Bergstrom (1978 pp 78ndash9) suggested that lsquoevenclosely related forms may have different numbers of headsegments and appendagesrsquo Bruton amp Whittington (1983pp 576ndash7) that lsquodiscussion on fossil arthropod relationshipsbased on head segmentation appears to be largely irrel-evant and at best speculativersquo and Delle Cave amp Simonetta(1991 p 191) that lsquothere are no obvious phyletic affinitiesbetween genera having the same number of cephalized seg-mentsrsquo Most recently Bergstrom amp Hou (1997 p 104) con-cluded that lsquothe segmental length of the head shield seemsto be of no relevance to the discussion and discrimination ofevolutionary lineagesrsquo It has been claimed that this view issupported by cladistic analyses in which patterns of tagmosishave been found to be rather poor at defining major arthropodclades (Briggs et al 1992 Wills et al 1994)

This suggestion is not only of relevance to systematics buthas featured prominently in recent debates surrounding thenature of the lsquoCambrian explosionrsquo (see Budd amp Jensen 2000for a recent review) Gould (1989 1991) argued that theapparent plasticity of head segmentation in Cambrian com-pared to post-Cambrian arthropod evolution suggests thatbody-plan evolution was only constrained after the Cambrianexplosion According to this view the origin of new highertaxa with distinct body-plans was only possible before theonset of these constraints (eg Valentine 1995) Taxa hererecognised as arachnomorphs clearly played a major role in thedevelopment of Gouldrsquos hypothesis This suggestion has pre-viously been assessed in terms of overall morphological diver-sity which is clearly rather distinct from Gouldrsquos concept ofdisparity Wills et al (1998b) showed that there was littlecorrelation between overall morphological disparity and de-gree of limb specialisation and that the latter was phylogeneti-cally highly plastic They remarked that (Wills et al 1998bp 64) lsquothis finding has important implications for models ofarthropod phylogeny and evolution that attribute overidingimportance to head segmentationrsquo but did not explicitlyexamine head segmentation although it is likely to have beena major component of the index of tagmosis employed

The results of the present study suggest that arguments thatarthropod head segmentation was unusually labile during theCambrian are poorly founded Rather only four major pat-terns of euarthropod head segmentation are identified Theplesiomorphic euarthropod state according to both Walossekamp Muller (1997 1998) and Scholtz (1997) which may moreproperly be the plesiomorphic state for the euarthropod crowngroup only (depending on the phylogenetic position of theMarrellomorpha) consists of four post-acronal segmentsbearing the antennae and three pairs of biramous limbs Thepresent authors suggest that the term lsquocephalonrsquo be restrictedto this kind of head which is found in stem group mandibu-lates in addition to many arachnomorphs Crown groupmandibulates share a lsquobimaxillary headrsquo in which an addi-tional pair of appendages the labium or second maxillae are

incorporated into the head but in crustaceans these are notfused to the carapace (Scholtz 1997) Finally two distinctforms of head tagmosis are found in chelicerates A head withfour pairs of appendages (but without antennae giving a totalof five segments incorporated into the head) which has beencalled the lsquocephalosomarsquo is present in pycnogonids (Walossekamp Dunlop 2002 pp 434ndash5) and probably also in some euch-elicerates (J A Dunlop pers comm 2003) The lsquoprosomarsquo ofmost euchelicerates consists of this cephalosoma and twoadditional segments

In general these patterns of head segmentation are highlyphylogenetically conserved (see Fig 12) Only three homo-plastic changes namely reversal to a more primitive conditionin the Leanchoilia-Alalcomenaeus clade and the convergentorigin of a five-segmented head in Clade 3 (of Fig 8) and theXandarellida are necessary to optimise character 4 onto themost parsimonious cladogram There are also transitions toautapomorphic states in Emeraldella and Sidneyia In the caseof Sidneyia however an argument could be made (followingBergstrom amp Sturmer 1978 see discussion of Character 4) thatthe head consists of five segments as in closely related taxabut autapomorphically each segment has a separate tergiteThis would further increase the phylogenetic stability of headsegmentation Also following Bergstrom amp Sturmer (1978) thehead of Cheloniellon could be considered to consist of twotergites making it more similar to the head of Emeraldella(with six pairs of appendages)

Figure 12 Cladogram from Fig 10 showing changes in head segmen-tation Bold type shows abbreviated head segmentation formulaAbbreviations (Ant) antennae (BA) biramous appendage pairs (Ch)chelicerae (GA) great appendages (UA) uniramous appendage pairsand (X) segment without appendages Synapomorphy A is the auta-pomorphic reduction in head length to a single segment in SidneyiaAlternatively this may represent secondary division of the headshield Synapomorphy B is the increase in the length of the head inEmeraldella to six segments

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 189

4 Acknowledgements

We thank D E G Briggs J A Cotton J A Dunlop R AFortey S Harris R A Moore P Selden O E Tetlie andP Van Roy for comments on earlier drafts of this work Themanuscript was reviewed by M Wills F Schram and oneanonymous referee This research was supported by the Geo-logical Society of London Curry Research Studentshipawarded to TJC by the Department of Earth Sciences Uni-versity of Bristol and by the Department of PalaeontologyThe Natural History Museum London TJC would like tothank C Labandeira J Thompson and R Chapman fortheir hospitality at the National Museum of Natural HistoryWashington DC USA and A Olcott S Schellenberg andJ Young for making his time there so enjoyable

5 References

Abzhanov A Popadic A amp Kaufman T C 1999 Chelicerate Hoxgenes and the homology of arthropod segments Evolution andDevelopment 1 77ndash89

Akam M 2000 Arthropods developmental diversity within a (super)phylum Proceedings of the National Academy of Sciences USA97 4438ndash41

Aldridge R J Gabbott S E amp Theron J N 2001 The Soom ShaleIn Briggs D E G amp Crowther P R (eds) Palaeobiology II340ndash2 Oxford Blackwell Science

Anderson L I amp Selden P A 1997 Opisthosomal fusion andphylogeny of Palaeozoic Xiphosura Lethaia 30 19ndash31

Arango C P 2002 Morphological phylogenetics of the sea spiders(Arthropoda Pycnogonida) Organisms Diversity and Evolution2 107ndash25

Ax P 1986 Das phylogenetische System Stuttgart Gustav FischerVerlag

Babcock L E amp Zhang Wentang In press New non-mineralizedarthropods from the Chengjiang Lagerstatte Lower CambrianYunnan China Palaeontologia Cathyana

Bartels C Briggs D E G amp Brassel G 1998 The fossils ofthe Hunsruck Slate Marine life in the Devonian CambridgeCambridge University Press

Bergstrom J 1971 Paleomerus ndash merostome or merostomoid Lethaia4 393ndash401

Bergstrom J 1979 Morphology of fossil arthropods as a guideto phylogenetic relationships In Gupta A P (ed) ArthropodPhylogeny 3ndash56 New York NY Van Nostrand Reinhold Co

Bergstrom J 1992 The oldest arthropods and the origin of theCrustacea Acta Zoologica 73 287ndash91

Bergstrom J Sturmer W amp Winter G 1980 PalaeoisopusPalaeopantopodus and Palaeothea pycnogonid arthropodsfrom the Lower Devonian Hunsruck Slate West GermanyPalaontologische Zeitschrift 54 7ndash54

Bergstrom J amp Brassel G 1984 Legs in the trilobite Rhenops fromthe Lower Devonian Hunsruck Slate Lethaia 17 67ndash72

Bergstrom J amp Hou X 1998 Chengjiang arthropods and theirbearing on early arthropod evolution In Edgecombe G D(ed) Arthropod fossils and phylogeny 151ndash84 New York NYColumbia University Press

Boudreaux H B 1979 Significance of intersegmental tendon systemin arthropod phylogeny and a monophyletic classification ofArthropoda In Gupta A P (ed) Arthropod Phylogeny 551ndash86New York NY Van Nostrand Reinhold Co

Bousfield E L 1995 A contribution to the natural classification ofLower and Middle Cambrian arthropods food-gathering andfeeding mechanisms Amphipacifica 2 3ndash33

Braddy S J Aldridge R J Gabbott S E amp Theron J N 1999Lamellate book-gills in a late Ordovician eurypterid from theSoom Shale South Africa support for a eurypterid-scorpionclade Lethaia 32 72ndash4

Braddy S J amp Almond J E 1999 Eurypterid trackways from theTable Mountain Group (Ordovician) of South Africa Journal ofAfrican Earth Sciences 29 165ndash77

Bremer K 1988 The limits of amino acid sequence data in angio-sperm phylogenetic reconstruction Evolution 42 795ndash803

Bremer K 1994 Branch support and tree stability Cladistics 10295ndash304

Briggs D E G 1998 Review of Arthropods from the LowerCambrian Chengjiang fauna southwest China by HouXianguang and Jan Bergstrom Fossils and Strata 45

Scandinavian University Press Oslo Norway 1997 Palaeo-geography Palaeoclimatology Palaeoecology 143 193ndash4

Briggs D E G Bruton D L amp Whittington H B 1979 Append-ages of the arthropod Aglaspis spinifer (Upper CambrianWisconsin) and their significance Palaeontology 22 167ndash80

Briggs D E G Fortey R A amp Wills M A 1992 Morphologicaldisparity in the Cambrian Science 256 1670ndash3

Briggs D E G Fortey R A amp Wills MA 1993 How big was theCambrian explosion A taxonomic and morphologic comparisonof Cambrian and Recent arthropods In Lees D R amp EdwardsD (eds) Evolutionary patterns and processes 33ndash44 LondonLinnean Society of London Linnean Society Symposium Series

Briggs D E G Erwin D H amp Collier F J 1994 The fossils of theBurgess Shale Washington DC Smithsonian Institution Press

Briggs D E G amp Bartels C 2001 New arthropods from the LowerDevonian Hunsruck Slate (Lower Emsian Rhenish Massifwestern Germany) Palaeontology 44 275ndash304

Briggs D E G amp Bartels C 2002 Magnoculocaris a new name forthe arthropod Magnoculus Briggs and Bartels 2001 from theLower Devonian Hunsruck Slate Germany Palaeontology 45419

Briggs D E G amp Collins D 1988 A Middle Cambrian cheliceratefrom Mount Stephen British Columbia Palaeontology 31 779ndash98

Briggs D E G amp Collins D 1999 The arthropod Alalcomenaeuscambricus Simonetta from the Middle Cambrian Burgess Shale ofBritish Columbia Paleontology 42 953ndash78

Briggs D E G amp Fortey R A 1982 The cuticle of aglaspididarthropods a red-herring in the early history of vertebratesLethaia 15 25ndash9

Briggs D E G amp Fortey R A 1989 The early radiation andrelationships of the major arthropod groups Science 246 241ndash3

Briggs D E G amp Fortey R A 1992 The early Cambrian radiationof arthropods In Lipps J H amp Signor P W (eds) Origin andEarly Evolution of the Metazoa 335ndash373 New York NY PlenumPress

Briggs D E G amp Robison R A 1984 Exceptionally preservednontrilobite arthropods and Anomalocaris from the MiddleCambrian of Utah University of Kansas PaleontologicalContributions Paper 111 1ndash23

Broili F 1932 Ein neuer Crustacee aus dem rheinischen UnterdevonSitzungsberichte der Mathematisch-Naturwissenschaftlichen(Abteilung) Klasse der Bayerischen Akademie der Wissenschaftenzu Munchen 1932 27ndash38

Broili F 1933 Ein zweites Exemplar von Cheloniellon Sitzungsber-ichte der Bayerischen Akademie der Wissenschaften 1933 11ndash32

Bruton D L 1981 The arthropod Sidneyia inexpectans MiddleCambrian Burgess Shale British Columbia Philosophical Trans-actions of the Royal Society of London Series B 295 619ndash56

Bruton D L amp Whittington H B 1983 Emeraldella and Leanchoiliatwo arthropods from the Burgess Shale British Columbia Philo-sophical Transactions of the Royal Society of London Series B 300553ndash85

Budd G E 1995 Kleptothule rasmusseni gen et sp nov anolenellid-like trilobite from the Sirius Passet fauna (BuenFormation Lower Cambrian North Greenland) Transactions ofthe Royal Society of Edinburgh Earth Sciences 86 1ndash12

Budd G E 1996a Progress and problems in arthropod phylogenyTrends in Ecology and Evolution 11 356ndash8

Budd G E 1996b The morphology of Opabinia regalis and thereconstruction of the arthropod stem-group Lethaia 29 1ndash14

Budd G E 1997 Stem-group arthropods from the Lower CambrianSirius Passet fauna of North Greenland In Fortey R A ampThomas R H (eds) Arthropod Relationships 125ndash38 LondonChapman and Hall Systematics Association Special Volume 55

Budd G E 1999a A nektaspid arthropod from the Early CambrianSirius Passet fauna with a description of retrodeformation basedon functional morphology Palaeontology 42 99ndash122

Budd G E 1999b The morphology and phylogenetic significance ofKerygmachela kiergkegaardi Budd (Buen Formation LowerCambrian N Greenland) Transactions of the Royal Society ofEdinburgh Earth Sciences 89 (for 1998) 249ndash290

Budd G E 2002 A palaeontological solution to the arthropod headproblem Nature 417 271ndash5

Budd G E amp Jensen S 2000 A critical reappraisal of the fossilrecord of the bilaterian phyla Biological Reviews 75 253ndash95

Chen J Zhou G Zhu M amp Yeh K 1996 The Chengjiang biota ndasha unique window of the Cambrian explosion Taichung NationalMuseum of Natural Science [In Chinese]

Chen J Edgecombe G D amp Ramskold L 1997 Morphological andecological disparity in naraoiids (Arthropoda) from the Early

190 TREVOR J COTTON AND SIMON J BRADDY

Cambrian Chengjiang fauna China Records of the AustralianMuseum 49 1ndash24

Chen J amp Zhou G 1997 Biology of the Chengjiang fauna In ChenJ Cheng Y amp Iten H V (eds) The Cambrian explosion and thefossil record Bulletin of the National Museum of Natural ScienceTaiwan 10 11ndash106

Child C A 1997 Some deep-sea Pycnogonida from Argentine slopeand basin Proceedings of the Biological Society of Washington110 128ndash42

Chlupac I 1988 The enigmatic arthropod Duslia from the Ordovicianof Czechslovakia Palaeontology 31 611ndash20

Chlupac I 1995 Lower Cambrian arthropods from the PasekyShale (Barrandian area Czech Republic) Journal of the CzechGeological Society 40 9ndash36

Chlupac I amp Havlıcek V 1965 Kodymirus n g a new aglaspidmerostome of the Cambrian of Bohemia Sbornık GeologickechVid Paleontologie 6 7ndash20

Cisne J L 1975 Anatomy of Triarthrus and the relationships of theTrilobita Fossils and Strata 4 45ndash63

Cisne J L 1981 Triarthrus eatoni (Trilobita) anatomy of its exo-skeletal skeletomuscular and digestive systems Palaeonto-graphica Americana 9 95ndash142

Conway Morris S 1982 Atlas of the Burgess Shale London ThePalaeontological Association

Conway Morris S 1989 The persistence of Burgess Shale-type faunasimplications for the evolution of deeper-water faunas Trans-actions of the Royal Society of Edinburgh Earth Sciences 80271ndash83

Conway Morris S 1998 The crucible of creation Oxford OxfordUniversity Press

Cotton T J 1999 Review of Arthropod fossils and phylogenyPalaeogeography Palaeoclimatology Palaeoecology 151 341ndash3

Cotton T J 2002 The phylogeny and morphological evolution ofCambrian trilobites and their relatives Unpublished PhD ThesisUniversity of Bristol

Damen W G M Hausdorf M Seyfarth E A amp Tautz D 1998 Aconserved mode of head segmentation in arthropods revealed bythe expression pattern of Hox genes in a spider Proceedings of theNational Academy of Sciences USA 95 10665ndash70

Delle Cave L amp Simonetta A M 1991 Early Palaeozoic arthropodsand problems of arthropod phylogeny with some notes on taxa ofdoubtful affinities In Simonetta A M amp Conway Morris S(eds) The early evolution of Metazoa and the significance ofproblematic taxa 189ndash244 Cambridge Cambridge UniversityPress

Dewel R A amp Dewel W C 1997 The place of tardigrades inarthropod evolution In Fortey R A amp Thomas R H (eds)Arthropod Relationships 109ndash23 London Chapman and HallSystematics Association Special Volume 55

Dunlop J A 1997 Palaeozoic arachnids and their significance forarachnid phylogeny Proceedings of the 16th European Colloquiumof Arachnology 65ndash82 Siedlce Poland

Dunlop J A 1998 The origins of tetrapulmonate booklungs and theirsignificance for chelicerate phylogeny In Selden P A (ed)Proceedings of the 17th European Colloquium of ArachnologyEdinburgh 1997 pp 9ndash16

Dunlop J A 1999 Pasando revista a la evolucion de los quelicerados[A review of chelicerate evolution] In Melic A DeHaro J JMedez M amp Ribera I (eds) Volumen Monografico Evolucion yFilogenia de Arthropoda Boletin de la Sociedad EntolomologicaAragonesa 26 255ndash272 [In Spanish and English]

Dunlop J A amp Selden P A 1997 The early history and phylogenyof the chelicerates In Fortey R A amp Thomas R H (eds)Arthropod relationships Systematics Association Special Volume55 221ndash35 London Chapman and Hall

Dunlop J A amp Webster M 1999 Fossil evidence terrestrializationand arachnid phylogeny Journal of Arachnology 27 86ndash93

Dzik J 1993 Early metazoan evolution and the meaning of its fossilrecord Evolutionary Biology 27 339ndash86

Dzik J amp Lendzion K 1988 The oldest arthropods of the EastEuropean Platform Lethaia 21 29ndash38

Edgecombe G D Wilson G D F Colgan D J Gray M R ampCassis G 2000 Arthropod cladistics combined analysisof Histone H3 and U2 snRNA sequences and morphologyCladistics 16 155ndash203

Edgecombe G D amp Ramskold L 1999 Relationships of CambrianArachnata and the systematic position of Trilobita Journal ofPaleontology 73 263ndash87

Emerson M J amp Schram F R 1997 Theories patterns and realitygame plan for arthropod phylogeny In Fortey R A amp Thomas

R H (eds) Arthropod Relationships Systematics AssociationSpecial Volume 55 67ndash87 London Chapman and Hall

Farris J S 1989 The Retention Index and the Rescaled ConsistencyIndex Cladistics 5 417ndash19

Felsenstein J 1985 Confidence limits on phylogenies an approachusing the bootstrap Evolution 39 783ndash91

Flower R H 1968 Merostomes from a Cotter Horizon of the El PasoGroup New Mexico Bureau of Mines and Mineral ResourcesMemoir 22 35ndash44

Fortey R A 1990a Trilobite evolution and systematics In MikulicD G (ed) Arthropod paleobiology 44ndash65 Short Courses inPaleontology 3 Knoxville Tenessee The Paleontological Society

Fortey R A 1990b Ontogeny hypostome attachment and trilobiteclassification Palaeontology 33 529ndash76

Fortey R A 1997 Classification In Kaesler R L (ed) Treatise oninvertebrate paleontology Part O Arthropoda 1 (revised) 289ndash302Boulder Colorado amp Lawrence Kansas Geological Society ofAmerica and University of Kansas Press

Fortey R A amp Theron J N 1994 A new Ordovician arthropodSoomaspis and the agnostid problem Palaeontology 37 841ndash61

Fortey R A amp Whittington H B 1989 The Trilobita as a naturalgroup Historical Biology 2 125ndash38

Fry W G 1978 A classification within the pycnogonids ZoologicalJournal of the Linnean Society of London 63 35ndash58

Giribet G Edgecombe G D amp Wheeler W C 2001 Arthropodphylogeny based on eight molecular loci and morphology Nature413 157ndash61

Gould S J 1989 Wonderful life The Burgess Shale and the nature ofhistory New York NY Norton

Gould S J 1991 The disparity of the Burgess Shale arthropod faunaand the limits of cladistic analysis why we must strive to quantifymorphospace Paleobiology 17 411ndash23

Gurich G 1931 Mimetaster hexagonalis ein neuer Kruster aus demunterdevonischen Bundenbacher Dachschiefer PalaontologischeZeitschrift 12 204ndash38

Hammann W Laske R amp Pillola G L 1990 Tariccoia arrusensisngnsp an unusual trilobite-like arthropod Rediscovery of thelsquophylocaridrsquo beds of Taricco (1922) in the Ordovician lsquoPuddingarsquosequence of Sardinia Bolletino della Societa PaleontologicaItaliana 29 163ndash78

Haas M S Brown S J amp Beeman R W 2001 Pondering theprocephalon the segmental origin of the labrum DevelopmentGenes and Evolution 211 89ndash95

Heider K 1913 Entwicklungsgeschichte und Morphologie derWirbellosen Die Kultur der Gegenwart Theil 3 Abt 4 2 176ndash332

Henriksen K 1928 Critical notes upon Cambrian arthropodsdescribed by Charles D Walcott Videnskabelige Meddelelser fraDansk Naturhistorisk Forening 86 1ndash20

Hesselbo S P 1988 Trace fossils of Cambrian aglaspidid arthropodsLethaia 21 139ndash46

Hesselbo S P 1989 The aglaspidid arthropod Beckwithia from theCambrian of Utah and Wisconsin Journal of Paleontology 63636ndash42

Hesselbo S P 1992 Aglaspidida (Arthropoda) from the UpperCambrian of Wisconsin Journal of Paleontology 66 885ndash923

Hou X 1987a Three new large arthropods from Lower CambrianChengjiang fauna eastern Yunnan Acta Palaeontologica Sinica26 272ndash85 [In Chinese with English summary]

Hou X 1987b Two new arthropods from Lower CambrianChengjiang eastern Yunnan Acta Palaeontologica Sinica 26236ndash56 [In Chinese with English summary]

Hou X Chen J amp Lu H 1989 Early Cambrian new arthropodsfrom Chengjiang Yunnan Acta Palaeontologica Sinica 28 42ndash57[In Chinese with English summary]

Hou X Ramskold L amp Bergstrom J 1991 Composition andpreservation of the Chengjiang fauna a Lower Cambrian soft-bodied biota Zoologica Scripta 20 395ndash411

Hou X Bergstrom J amp Ahlberg P 1995 Anomalocaris and otherlarge animals in the Lower Cambrian Chengjiang fauna ofsouthwest China GFF 117 163ndash83

Hou X amp Bergstrom J 1997 Arthropods of the Lower CambrianChengjiang fauna southwest China Fossils and Strata 45 1ndash116

Hughes C P 1975 Redescription of Burgessia bella from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 4415ndash36

Ivantsov A Y 1999 Trilobite-like arthropod from the LowerCambrian of the Siberian Platform Acta PalaeontologicaPolonica 44 455ndash66

King P E 1973 Pycnogonids London St Martin Press

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 191

Lauterbach K E 1973 Schlussereignisse in der Evolution derStammgruppe der Euarthropoda Zoologische Beitrage 19259ndash99

Lauterbach K E 1980 Schlussereignisse in der Evolution derGrundplans der Arachnata (Arthropoda) Abhandlungen desNaturwissenschaftlichen Vereins in Hamburg 23 163ndash327

Lauterbach K E 1983 Synapomorphien zwischen Trilobiten- undCheliceraten-zweig der Arachnata Zoologischer Anzeiger 210213ndash38

Lendzion K 1975 Fauna of the Mobergella Zone in the Polish LowerCambrian Kwartalnik Geologiczny 19 237ndash42

Lieberman B S 1998 Cladistic analysis of the Early Cambrianolenelloid trilobites Journal of Paleontology 72 59ndash78

Lu Y 1940 On the ontogeny and phylogeny of Redlichia intermediaLu (sp nov) Bulletin of the Geological Society of China 20333ndash42 [In Chinese]

Maddison W P amp Maddison D R 1997 MacClade analysis ofphylogeny and character evolution Version 307 Sunderland MASinauer Associates

Mittmann B amp Scholtz G 2003 Development of the nervous systemin the lsquoheadrsquo of Limulus polyphemus (Chelicerata Xiphosura)morphological evidence for the correspondence between the seg-ments of the chelicerae and the (first) antennae of MandibulataDevelopment Genes and Evolution 213 9ndash17

Moore R C (ed) 1959 Treatise on invertebrate paleontology Part OArthropoda 1 Boulder Colorado Geological Society of America

Muller K J amp Walossek D 1986 Arthropod larvae from the UpperCambrian of Sweden Transactions of the Royal Society ofEdinburgh Earth Sciences 77 157ndash79

Muller K J amp Walossek D 1987 Morphology ontogeny and lifehabit of Agnostus pisiformis from the Upper Cambrian of SwedenFossils and Strata 19 1ndash124

Munilla T 1999 Evolucion y filogenia de los Picnogonidos InMelicA DeHaro J J Medez M amp Ribera I (eds) VolumenMonografico Evolucion y Filogenia de Arthropoda Boletin de laSociedad Entolomologica Aragonesa 26 273ndash91 [In Spanish andEnglish]

Orr P J Siveter D J Briggs D E G Siveter D J amp Sutton M D2000 A new arthropod from the Silurian Konservat-Lagerstatteof Herefordshire Proceedings of the Royal Society of LondonSeries B 267 1497ndash504

Page R D M 1996 On consensus confidence and lsquototal evidencersquoCladistics 12 83ndash92

Palmer A R amp Repina L N 1993 Through a glass darklytaxonomy phylogeny and biostratigraphy of the OlenellinaUniversity of Kansas Paleontological Contributions New Series 31ndash35

Platnick N I Griswold C E amp Coddington J A 1991 On missingentries in cladistic analysis Cladistics 7 337ndash43

Pleijel F 1995 On character coding for phylogeny reconstructionCladistics 11 309ndash15

Raasch G O 1939 Cambrian Merostomata Special Papers of theGeological Society of America 19 1ndash146

Ramskold L Chen J Edgecombe G D amp Zhou G 1996Preservational folds simulating tergite junctions in tegopeltid andnaraoiid arthropods Lethaia 29 15ndash20

Ramskold L Chen J Edgecombe G D amp Zhou G 1997Cindarella and the arachnate clade Xandarellida (ArthropodaEarly Cambrian) from China Transactions of the Royal Society ofEdinburgh Earth Sciences 88 19ndash38

Ramskold L amp Edgecombe G D 1991 Trilobite monophylyrevisited Historical Biology 4 267ndash83

Ramskold L amp Edgecombe G D 1996 Trilobite appendagestructure ndash Eoredlichia revisited Alcheringa 20 269ndash76

Raw F 1957 Origin of chelicerates Journal of Paleontology 31139ndash193

Repina L N amp Okuneva O G 1969 Cambrian arthropods of themaritime territory Paleontological Journal 1969 95ndash103

Richter R amp Richter E 1929 Weinbergina opitz ng n sp einSchwerttrager (Merost Xiphos) aus dem Devon (Rheinland)Senkenbergiana 11 193ndash209

Robison R A 1984 New occurences of the unusual trilobite Naraoiafrom the Cambrian of Idaho and Utah University of KansasPaleontological Contributions Paper 112 1ndash8

Rominger C 1887 Description of primordial fossils from MtStephens NW Territory of Canada Proceedings of the Academyof Natural Sciences of Philadelphia 1887 12ndash19

Scholtz G 1997 Cleavage germ band formation and head segmen-tation the ground pattern of the Euarthropoda In Fortey R Aamp Thomas R H (eds) Arthropod Relationships 317ndash32 LondonChapman and Hall Systematics Association Special Volume 55

Schram F R 1978 Arthropods a convergent phenomenonFieldiana Geology 39 61ndash108

Schram F R 1993 Review of Boxshall Stromberg and Dahl TheCrustacea Origin and Evolution Journal of Crustacean Biology13 820ndash2

Schram F R Vonk R amp Hof C H J 1997 Mazon CreekCycloidea Journal of Paleontology 71 261ndash84

Schram F R amp Emerson M J 1991 Arthropod Pattern Theorya new approach to arthropod phylogeny Memoirs of theQueensland Museum 31 1ndash18

Schram F R amp Hof C H J 1998 Fossils and the interrelationshipsof major crustacean groups In Edgecombe G D (ed) Arthropodfossils and phylogeny pp 233ndash302 New York NY ColumbiaUniversity Press

Shergold J H 1991 Protaspis and early meraspis growth stages of theeodiscoid trilobite Pagetia ocellata Jell and their implications forclassification Alcheringa 15 65ndash86

Shu D Geyer G Ling C amp Zhang Z 1995 Redlichiacean trilobiteswith preserved soft parts from the Lower Cambrian Chengjiangfauna (South China) Beringeria (Special Issue) 2 203ndash41

Shultz J W 1990 Evolutionary morphology and phylogeny ofArachnida Cladistics 6 1ndash38

Shultz J W 2000 Skeletomuscular anatomy of the harvestmanLeiobunum aldrichi (Weed 1893) (Arachnida OpilionesPalpatores) and its evolutionary significance Zoological Journalof the Linnean Society 128 401ndash38

Shultz J W 2001 Gross muscular anatomy of Limulus polyphemus(Xiphosura Chelicerata) and its bearing on evolution in the theArachnida Journal of Arachnology 29 283ndash303

Siewing R (ed) 1985 Lehrbuch der Zoologie II Systematik 3rd ednStuttgart Fischer

Simonetta A M 1970 Studies on the non trilobite arthropods ofthe Burgess Shale (Middle Cambrian) the genera LeanchoiliaAlalcomenaeus Opabinia Burgessia Yohoia and ActaeusPalaeontographica Italica 66 35ndash45

Simonetta A M amp Delle Cave L 1975 The Cambrian non trilobitearthropods from the Burgess Shale of British Columbia A studyof their comparative morphology taxonomy and evolutionarysignificance Palaeontographica Italica 69 1ndash37

Simonetta A M amp Delle Cave L 1981 An essay in the compara-tive and evolutionary morphology of Palaeozoic ArthropodsAccademia Nazionale dei Lincei Atti dei Convegni Lincei 49389ndash439

Starobogatov Y I 1990 Sistema i filogeniya nizshikh khelitserovykh(analiz morfologii paleozoysikh grupp) [The systematics andphylogeny of the lower chelicerates (a morphological analysis ofthe Palaeozoic groups)] Paleontologicheskii Zhurnal 1990 4ndash17

Stoslashrmer L 1944 On the relationships and phylogeny of fossil andrecent Arachnomorpha Skrifter utgitt ac Det Norske Videnskaps-Akademi i Olso I Matematisk-naturvitenskaplig Klasse 5 1ndash158

Stoslashrmer L 1951 Studies on trilobite morphology Part III Theventral cephalic structures with remarks on the zoologicalposition of the trilobites Norsk geologisk tidsskrift 29 108ndash58

Stoslashrmer L 1956 A Lower Cambrian merostome from Sweden Arkivfor Zoologi Ser 2 9 507ndash14

Stoslashrmer L 1959 Trilobitoidea In Moore R C (ed) Treatise onInvertebrate Paleontology part O Arthropoda 1 23ndash37 BoulderColorado Geological Society of America

Strong E E amp Lipscomb D 1999 Character coding and inapplicabledata Cladistics 15 363ndash71

Sturmer W amp Bergstrom J 1976 The arthropods Mimetaster andVachonisia from the Devonian Hunsruck Shale PalaontologischeZeitschrift 50 78ndash111

Sturmer W amp Bergstrom J 1978 The arthropod Cheloniellon fromthe Devonian Hunsruck Shale Palaontologische Zeitschrift 5257ndash81

Sturmer W amp Bergstrom J 1981 Weinbergina a xiphosuran fromthe Devonian Hunsruck Slate Palaontologische Zeitschrift 55237ndash55

Sutton M D Briggs D E G Siveter D J Siveter D J amp Orr PJ 2002 The arthropod Offacolus kingi (Chelicerata) from theSilurian of Herefordshire England computer based morphologi-cal reconstructions and phylogenetic affinity Proceedings of theRoyal Society of London Series B 269 1195ndash1203

Swofford D L 1999 PAUP Phylogenetic Analysis Using Parsimony(and other methods) Version 40b2a Sunderland MA SinauerAssociates

Telford M J amp Thomas R H 1998 Expression of homeobox genesshows chelicerate arthropods retain their deutocerebral segmentProceedings of the National Academy of Sciences USA 9510671ndash5

192 TREVOR J COTTON AND SIMON J BRADDY

Thorley J L amp Page R D M 2000 RadCon phylogenetic treecomparison and consensus Bioinformatics 16 486ndash7

Valentine J W 1995 Why no new phyla after the Cambrian ndashgenome and ecospace hypotheses revisited Palaios 10 190ndash194

Wagele J W 1993 Rejection of the lsquoUniramiarsquo hypothesis andimplications of the Mandibulata concept Zoologische JahrbucherAbteilung fur Systematik Geographie und Biologie der Tiere 120253ndash88

Walcott C D 1911 Middle Cambrian Merostomata Cambriangeology and palaeontology II Smithsonian MiscellaneousCollections 57 17ndash40

Walcott C D 1912 Middle Cambrian Branchiopoda MalacostracaTrilobita and Merostomata Cambrian geology and palaeontol-ogy II Smithsonian Miscellaneous Collections 57 145ndash228

Walcott C D 1918 Geological explorations in the CanadianRockies Explorations and fieldwork of the SmithsonianInstitution in 1917 Smithsonian Miscellaneous Collections 684ndash20

Walossek D 1993 The Upper Cambrian Rehbachiella kinnekullensisand the phylogeny of Branchiopoda and Crustacea Fossils andStrata 32 1ndash202

Walossek D amp Dunlop J A 2002 A larval sea spider (ArthropodaPycnogonida) from the Upper Cambrian lsquoOrstenrsquo of Swedenand the phylogenetic position of pycnogonids Palaeontology 45421ndash46

Walossek D amp Muller K J 1990 Upper Cambrian stem-lineagecrustaceans and their bearing upon the monophyletic origin ofCrustacea and the position of Agnostus Lethaia 23 409ndash27

Walossek D amp Muller K J 1997 Cambrian lsquoOrstenrsquo-type arthro-pods and the phylogeny of Crustacea In Fortey R A amp ThomasR H (eds) Arthropod Relationships 139ndash53 SystematicsAssociation Special Volume 55 London Chapman and Hall

Walossek D amp Muller K J 1998 Early arthropod phylogeny in lightof the Cambrian lsquoOrstenrsquo fossils In Edgecombe G D (ed)Arthropod fossils and phylogeny 185ndash231 New York NYColumbia University Press

Weygoldt P 1979 Significance of later embryonic stages and headdevelopment in arthropod phylogeny In Gupta A P (ed)Arthropod Phylogeny 107ndash35 New York NY Van NostrandReinhold Co

Weygoldt P 1998 Evolution and systematics of the ChelicerataExperimental and Applied Acarology 22 63ndash79

Weygoldt P amp Paulus H F 1979 Unterzuchungen aur Morpholo-gie Taxonomie und Phylogenie der Chelicerata Zeitschriftfur Zoologische Systematik und Evolutionsforschung 17 85ndash116177ndash200

Wheeler W C Cartwright P amp Hayashi C Y 1993 Arthropodphylogeny a combined approach Cladistics 9 1ndash39

Wheeler W C amp Hayashi C Y 1998 The phylogeny of extantchelicerate orders Cladistics 14 173ndash92

Whittington H B 1971 Redescription of Marrella splendens(Trilobitoidea) from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 209 1ndash24

Whittington H B 1974 Yohoia Walcott and Plenocaris n genarthropods from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 231 1ndash21

Whittington H B 1975a Trilobites with appendages from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 497ndash136

Whittington H B 1975b The enigmatic animal Opabinia regalisMiddle Cambrian Burgess Shale British Columbia PhilosophicalTransactions of the Royal Society of London Series B 271 1ndash43

Whittington H B 1977 The Middle Cambrian trilobite NaraoiaBurgess Shale British Columbia Philosophical Transactions of theRoyal Society Series B 280 409ndash43

Whittington H B 1979 Early arthropods their appendages andrelationships In House M R (ed) The origin of majorinvertebrate groups 253ndash68 Systematics Association SpecialVolume 12 New York NY Academic Press

Whittington H B 1980 Exoskeleton moult stage appendage mor-phology and habits of the Middle Cambrian trilobite Olenoidesserratus Palaeontology 23 171ndash204

Whittington H B 1981 Rare arthropods from the Burgess ShaleMiddle Cambrian British Columbia Philosophical Transactionsof the Royal Society of London Series B 292 329ndash57

Whittington H B 1985 Tegopelte gigas a second soft-bodied trilo-bite from the Burgess Shale Middle Cambrian British ColumbiaJournal of Palaeontology 23 171ndash204

Whittington H B 1989 Olenelloid trilobites type species functionalmorphology and higher classification Philosophical Transactionsof the Royal Society of London Series B 324 111ndash47

Whittington H B amp Almond J E 1987 Appendages and habits ofthe Upper Ordovician trilobite Triarthus eatoni PhilosophicalTransactions of the Royal Society of London Series B 317 1ndash46

Wilkinson M 1992 Ordered versus unordered characters Cladistics8 375ndash85

Wilkinson M 1994 Common cladistic information and its consensusrepresentation reduced Adams and reduced cladistic consensustrees and profiles Systematic Biology 43 343ndash68

Wilkinson M 1995a Coping with abundant missing entries in phylo-genetic inference using parsimony Systematic Biology 44 501ndash14

Wilkinson M 1995b TAXEQ2 software and documentation BristolSchool of Biological Sciences University of Bristol

Wilkinson M 1996 On the distribution of homoplasy and choosingamong trees Taxon 45 263ndash66

Wills M A 1997 A phylogeny of Recent and fossil Crustacea derivedfrom morphological characters In Fortey R A amp ThomasR H (eds) Arthropod Relationships 189ndash209 SystematicsAssociation Special Volume 55 London Chapman and Hall

Wills M A Briggs D E G amp Fortey R A 1994 Disparity asan evolutionary index a comparison of Cambrian and Recentarthropods Paleobiology 20 93ndash130

Wills M A Briggs D E G Fortey R A amp Wilkinson M 1995The significance of fossils in understanding arthropod evolutionVerhandlungen der deutschen zoologischen Gesselschaft 88 203ndash15

Wills M A Briggs D E G Fortey R A Wilkinson M amp SneathP H A 1998a An arthropod phylogeny based on fossil andrecent taxa In Edgecombe G D (ed) Arthropod fossils andphylogeny 33ndash105 New York NY Columbia University Press

Wills M A Briggs D E G amp Fortey R A 1998b Evolutionarycorrelates of arthropod tagmosis scrambled legs In ForteyR A amp Thomas R H (eds) Arthropod Relationships 57ndash65Systematics Association Special Volume 55 London Chapmanand Hall

Winter G 1980 Beitrage zur Morphologie und Embryologie desvorderen Korperabschnitts (Cephalosoma) der PantopodaGerstaecker 1863 I Entstehung und Struktur des Zentralnerven-systems Zeitschrift fur Zoologische Systematik und Evolutionsfor-schung 18 27ndash61

Zhang W amp Hou X 1985 Preliminary notes on the occurrence of theunusual trilobite Naraoia in Asia Acta Palaeontologica Sinica 24591ndash5 [In Chinese with English summary]

TREVOR J COTTON Department of Palaeontology The Natural History Museum CromwellRoad London SW7 5BD UKe-mail trevor_cottonhotmailcom

SIMON J BRADDY Department of Earth Sciences University of Bristol Wills MemorialBuilding Queenrsquos Road Bristol BS8 1RJ UKe-mail S J Braddybrisacuk

MS received 28 July 2003 Accepted for publication 15 March 2004

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 193

Page 13: The phylogeny of arachnomorph arthropods and the origin of the

The homology of the book-gill lamellae of xiphosurans withlamellar setae was supported by Walossek amp Muller (1997p 149) and Edgecombe et al (2000 p 174) but rejected bySturmer amp Berstrom (1981) on the grounds that Weinberginapossesses Limulus-like gill lamellae and fringing setaeBook-gills have also been described from a eurypterid (Braddyet al 1999) There are certainly major morphologicaldifferences between book-gills and trilobite-type exopodsFollowing most recent opinion and pending further studyWeinbergina and Eurypterida are coded as possessing lamellatesetae

The form of the setae of Yohoia is uncertain (Whittington1974) the spinose setae seen in the most common reconstruc-tion (Gould 1989 Briggs et al 1994) are not justified by thespecimens Amongst other great appendage arthropods theform of the setae appears to vary those of Jianfengia (seeChen amp Zhou 1997 p 74) and Leanchoilia (see Bruton ampWhittington 1983) are lamellate while those of Fortiforceps(Hou amp Bergstrom 1997) and Alalcomenaeus (Briggs amp Collins1999) are spinose and less densely packed

19 Gnathobase on basis andor prominent endites onendopod (0) present and (1) absent (cf Edgecombe ampRamskold 1999 character 29 Wills et al 1998a characters 36amp 59) The presence of gnathobases on the appendages of someanomalocaridids (Hou et al 1995) and their general distribu-tion amongst non-arachnomorph euarthropods suggests thatState 0 is plesiomorphic The homology of the various enditesand gnathobasic structures found in euarthropods is in need ofcareful assessment and therefore a very general coding (fol-lowing Edgecombe amp Ramskold) is used for this character

243 Eyes 20 Position of lateral facetted eyes (0)ventral and stalked (1) dorsal and sessile and (2) absent (cfEdgecombe amp Ramskold 1999 character 4) Partial uncer-tainty coding is used for a number of taxa where the dorsalsurface is known but the ventral morphology is not andtherefore the presence of ventral eyes cannot be discountedFor example there is good evidence that Leanchoilia lackeddorsal sessile eyes but ventral stalked eyes may have beenpresent (as in L hanceyi see Briggs amp Robison 1984) and a(02) partial uncertainty coding is used This is also the case inmany naraoiids such as Buenaspis However only the outlineof the head shield of Liwia is known and the absence of dorsaleyes in the reconstruction of Dzik amp Lendzion (1988) isconjectural It is unclear whether the autapomorphic conditionof stalked dorsal eyes in Mimetaster (see Sturmer amp Bergstrom1976) is homologous with State 0 or State 1 and a partialuncertainty coding is also used

Whittington (1974) was somewhat equivocal about thenature of the lateral lobes anterior to the head shield ofYohoia These more closely resemble the ventral stalked eyes ofAlacomenaeus as recently described by Briggs amp Collins(1999) than either Whittingtonrsquos (1974) or subsequent (egGould 1989 fig 3middot18 Briggs et al 1994 fig 153) reconstruc-tions suggest In a well preserved specimen showing the dorsalaspect (USNM 57696 Whittington 1974 pl 2 figs 1ndash3) and ina laterally compressed specimen (USNM 57694 Whittington1974 pl 1 fig 1) they are clearly seen to be stalked andrelatively small lobate structures That these structures arelikely to represent stalked ventral eyes was reflected in thecoding of Wills et al (1998a characters 26 amp 27)

21 Visual surface with calcified lenses bounded withcircumocular suture (0) absent and (1) present

22 Dorsal bulge in exoskeleton accommodating drop-shaped ventral eyes (0) absent and (1) present

23 Eye slits (0) absent and (1) presentCharacters 21 to 23 are used exactly as described by

Edgecombe amp Ramskold (1999 character 5) Character 21 is

inapplicable to taxa that do not possess eyes (Character 20State 2)

24 Dorsal median eyes (0) absent and (1) present Dorsalmedian eyes on a tubercle are considered a synapomorphy ofthe Chelicerata (Dunlop amp Selden 1997 Dunlop 1999) Thenature and position of the structures interpreted as dorsalmedian eyes in Mimetaster (Sturmer amp Bergstrom 1976p 83ndash4) are regarded as equivocal

244 Ventral cephalic structures Many characters of theventral surface of the euarthropod head (see Fig 7) of poten-tial phylogenetic utility are poorly known in many importanttaxa In particular the presence of pre-hypostomal frontalorgans (Fig 7A CndashE) is not coded herein (see Edgecombe ampRamskold 1999 p 272) Definitive evidence of the absence ofthese structures is available for very few taxa and the homol-ogy of these structures with the maculae of the trilobitehypostome (see Fig 7B) and the frontal organs of the crusta-cean labrum (eg see Muller amp Walossek 1987 p 39) isuncertain Secondly the detailed homology of the trilobitehypostome with that of other putative arachnomorphs isunclear and consequently a very broad definition is usedherein For example various pre-hypostomal sclerites (Fig7A D) in other arachnomorphs may have been incorporatedalong with a primitive hypostome into a single sclerite that isrecognised as the hypostome in trilobites The structure of thehypostome (eg the presence of paired anterior wings dorsal tothe antennae Fig 7B C) may also be a source of additionalcharacters

25 Expanded cephalic doublure (0) absent and (1)present maximum width more than 30 length of head shieldor more than 25 width of pygidium Chen et al (1996)suggested that the wide doublure of Soomaspis and Tariccoia isa synapomorphy uniting these taxa The doublure of Buenaspis(see Budd 1999a) is poorly known but based on the width ofthe heavily crushed region of the cephalic margin and theposition of a possible impression of the edge of the doublure inone specimen (Budd 1999a pl 1 fig 1) it also appears to berather wide (ie greater than 30 of the length of the headshield) Following Edgecombe amp Ramskoldrsquos coding ofSidneyia the present authors do not consider the postero-median expansion of the doublure of for example Emeraldellaor Retifacies (see Bruton amp Whittington 1983 Hou ampBergstrom 1999 respectively) to be homologous with State 1but to represent the hypostome (see Character 26)

26 Anteromedian margin of cephalon notched accomo-dating strongly sclerotised plate (0) notch and plate absentand (1) notch and plate present (cf Edgecombe amp Rasmkold1999 character 10) The apomorphic state (illustrated inFig 7D) is only known in the taxa discussed by Edgecombe ampRasmkold (1999) Reconstructions of Yohoia show a roundedlobe between the eyes (see Character 20) anterior to anddistinct from the head shield which resembles the anteriorsclerite recognised here This seems to be a misinterpretationSpecimens preserved in dorsal aspect (USNM 57696Whittington 1974 pl 2 figs 1ndash3) and lateral aspect (USNM57694 Whittington 1974 pl 1 fig 1 USNM 155616Whittington 1974 pl 5 fig 2) seem to clearly show that thislsquomedian lobersquo is a downward curving pointed extension of thehead shield

27 Hypostomal sclerite (0) median extension of thedoublure with no suture (1) natant sclerite not in contactwith doublure (2) with narrow overlap with pre-hypostomalsclerite (3) narrow attachment to doublure at hypostomalsuture and (4) absent The homology of hypostomes andlabrums has been discussed by Haas et al (2001) and Budd(2002) The euarthropod labrum is likely to represent a pos-teroventral extension of the pre-segmental acron (Scholtz

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 181

Figure 7 Ventral reconstructions of arachnomorph arthropod heads (A) Misszhouia longicaudata (after Chenet al 1997 figs 2ndash4 7) (B) Ceraurinella typa (after Chen et al 1997 fig 7) (C) Agnostus pisiformis (after Mulleramp Walossek 1987) (D) Kuamaia lata (after Edgecombe amp Ramskold 1999 figs 32 5 amp 6) (E) Cindarella eucala(after Ramskold et al 1997) and (F) Emeraldella brocki (after Bruton amp Whittington 1983) Not to scaleAbbreviations (aw) anterior wing beneath antennae (bs) boomerang-shaped sclerite (bl) blade-like process ofposterior wing (d) doublure of head-shield (cs) connective suture (f) fenestra (fs) facial suture (h) hypostome(hl) hypostomal lobe (emerging through fenestrae of Agnostus) (hs) hypostomal suture (if) intervening field ofhypostomal complex (lb) lateral bridge (lfo) lateral frontal organ (mb) median body (mbr) median bridge (mf)median furrow (mfb) mouth field bridge (mfo) median frontal organ (ol) ovate lobe and (phs) pre-hypostomalsclerite Post-antennal appendages and distal parts of antennae omitted

182 TREVOR J COTTON AND SIMON J BRADDY

1997) This structure is often sclerotised and covers the mouthventrally It is this sclerite that is here identified as thehypostome Therefore this character is considered absent intaxa lacking a sclerite covering the mouth irrespective of themorphology and position of the labrum itself which at leastpartly covers the mouth in all euarthropods other thanpycnogonids (see Edgecombe et al 2000 p 167) The lsquofleshylabrumrsquo of crustaceans does not appear to be sclerotised and ahypostome is consequently considered to be absent Althoughmany derived features are associated with the crustaceanlabrum this is not a good reason to consider it non-homologous with the hypostome-bearing structure of otherarthropods as Walossek amp Muller (1990) argued

In many taxa the mouth is covered by a posteromedianextension of the doublure here considered to represent thehypostome (eg Emeraldella Fig 7F Aglaspis see Hesselbo1992 fig 52) In others the hypostome is separated from thedoublure either by a suture a pre-hypostomal sclerite (egKuamaia lata Fig 7D Saperion glumaceum see Edgecombe ampRamskold 1999 figs 3 amp 4) or an intervening region ofunsclerotised cuticle (the natant condition of Fortey 1990beg Cindarella eucala Fig 7E) The homology of pre-hypostomal sclerites and hypostomes in helmetiids trilobitesand naraoiids (see Fig 7) has been discussed by Chen et al(1996) and Edgecombe amp Ramskold (1999) but it is as yetunclear whether any of the character states described here arederived from any other They are treated here as distinct andthe character as unordered pending further study

No hypostome has been identified in Yohoia (see above)Alalcomenaeus Jianfengia Fortiforceps or Leanchoilia andthere is certainly no broad posterior extension of the doublurein any of these taxa Since there is also no pre-hypostomalsclerite they have received a (134) partial uncertainty codingThe attachment of the hypostome of Tegopelte is unclear butit does not seem to be attached to any doublure (Whittington1985) and therefore it is given an uncertainty (12) coding

The lsquorostrum-like structurersquo on the ventral surface ofLemoneites appears to be similar in morphology and positionrelative to the anterior margin of the head shield (Flower 1968pl 8 figs 4 amp 13) to that described from Aglaspis and is codedas State 0 In many taxa the hypostome is unknown but inonly a small number can it be coded reliably as absentHughesrsquo (1975) suggestion that the hypostome of Burgessia isabsent is accepted preliminarily but an extension of thedoublure may be visible in some of Hughesrsquo figures Fortey(pers comm 2000) also doubts Hughesrsquo assertion

28 Visible ecdysial sutures (0) absent and (1) present Themarginal ecdysial sutures of chelicerates and the dorsal suturesof trilobites are almost certainly not homologous but thepresence of sutures and their position (see Character 29) arecoded separately to allow this to be tested Wills et al (1998aCharacter 2) coded marginal sutures as present in Sidneyiafollowing Brutonrsquos (1981) suggestion but this is consideredequivocal here

29 Position of ecdysial sutures (0) marginal and (1) dor-sal This character is inapplicable to taxa lacking ecdysialsutures (Character 28 State 0) Dorsal sutures whilst presentin the representatives of the Trilobita coded here are probablynot a synapomorphy of trilobites as a whole but for a clade oftrilobites excluding the Olenellida (Whittington 1989 Fortey1990a)

245 Exoskeletal tergites and thoracic tagmosis 30 Min-eralised cuticle (0) absent and (1) present Calcification of theexoskeleton is one of the most convincing synapomorphiesof the Trilobita (Fortey amp Whittington 1989 Ramskold ampEdgecombe 1991 Edgecombe amp Ramskold 1999 Character 1)Cuticle mineralisation is also coded as present in aglaspidids

following Briggs amp Fortey (1982) who suggested that theaglaspidid exoskeleton was originally phosphatic Paleomerusprobably also had a mineralised cuticle but Hou amp Bergstrom(1997 p 97) argued that it was likely to be calcareous Anundescribed Silurian aglaspidid may also have had an origi-nally calcitic cuticle (Fortey amp Theron 1994 p 856) Becauseof the uncertainty about the chemical composition of theexoskeleton in these forms cuticle mineralisation is coded aspotentially homologous in all these taxa and no attempt ismade to code separate states for phosphatic and calcareousmineralisation

31 Trunk tergites with expanded lateral pleurae coveringappendages dorsally (0) absent and (1) present Whilst thepresence of paratergal folds may be a synapomorphy at thelevel of the Euarthropoda (Boudreaux 1979 Wagele 1993Edgecombe et al 2000 Character 142) these are at mostsmall reflections of the margin of the tergites in most euarthro-pods In arachnomorph taxa these are expanded to form largelateral pleurae that cover the appendages dorsally (see egLimulus and Triarthus in Boudreaux 1979) This is inferred tobe the case in some taxa where dorsal features and appendagesare poorly known on the basis of their possession of tergiteswith wide pleural regions This feature was suggested as typicalof lamellipedians (=arachnomorphs) by Hou amp Bergstrom(1997 pp 42ndash3)

The arrangement of the thorax of Marrella Mimetaster andBurgessia differs from that of all the other taxa considered herein that the appendages seem to be attached to the lateralmargins of the body The trunk tergites do not cover theappendages and seem to lack pleurae entirely although theyare covered by a posterior extension of the head shield inBurgessia This character has been clearly described inMimetaster (Sturmer amp Bergstrom 1976 pp 87ndash90 fig 8) andBurgessia (Hughes 1975 p 421)

32 Free thoracic tergites (0) present and (1) absentFollowing Edgecombe amp Ramskold (1999 Character 16) anumber of taxa lack functional post-cephalic articulations andconsequently lack free thoracic tergites Despite this theycode these taxa for a number of characters relating to thestructure of thoracic tergites (eg their characters 18 and 19)These characters are treated here as inapplicable to taxawithout free tergites and the definition of this character hasbeen modified from that of Edgecombe amp Ramskold (1999) tomake this more explicit

33 Decoupling of thoracic tergites and segments (0)absent and (1) present Ramskold et al (1997) described thischaracter of Cindarella and Xandarella where the thoracictergites correspond to a variable increasing posteriorly num-ber of appendage pairs This character cannot be coded fortaxa in which free thoracic tergites are absent (Character 32State 1)

34 Tergite articulations (0) tergites non-overlapping (1)extensive overlap of tergites and (2) edge-to-edge pleuralarticulations In most of the taxa considered here the thoracictergites overlap considerably and relatively evenly over theirwidth This contrasts with the primitive euarthropod condi-tion where the tergites do not overlap medially as seen in stemgroup crustaceans In trilobites such as Helmetia and Kuamaia(see Edgecombe amp Ramskold 1999 character 18) overlap ofthoracic tergites is limited to a well-defined axial region andthe lateral pleurae meet edge-to-edge

The thoracic articulations of naraoiids with free thoracictergites are in some ways similar to those of trilobites with astrong overlap medially that is lacking abaxially andanterolateral articulating facets (Budd 1999a Ramskold ampEdgecombe 1999) However the distribution of these features

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 183

in other trilobite-like arachnomorphs is unclear and a restric-tive coding is used here The present authors do not considerthat this character can be applied reliably to the fused thoracictergites of Saperion Tegopelte and Skioldia and it is coded asinapplicable to all taxa lacking free thoracic tergites (Character32 State 1)

35 Trunk effacement (0) trunk with defined (separate orfused) tergite boundaries (1) trunk tergite boundaries effacedlaterally and (2) trunk tergite boundaries completely effaced(cf Edgecombe amp Ramskold 1999 Character 15)] Contrary toEdgecombe amp Ramskold (1999) the present authors considerSkioldia and Saperion to show a distinct form of tergiteeffacement where the fused tergite boundaries are defined byfurrows axially but effaced laterally The boundaries areeffaced at least laterally in Tegopelte but the axial region ofthe exoskeleton is unknown which accordingly is given amultistate uncertainty coding

36 Cephalic articulation fused (0) absent and (1) presentIn Tegopelte Saperion and Skioldia the articulation ofthe head with the thorax is non-functional and the entireexoskeleton forms a single tergite

37 Head shield overlap of thoracic tergites (0) overlapabsent or identical to overlap between thoracic segments (1)head shield covers first thoracic tergite only and (2) headshield covers mutliple anterior trunk tergites

38 Head shield articulates with reduced anterior thoracictergite (0) absent and (1) present Amongst taxa showing aposteriorly expanded head shield ie one that overlaps thethorax Edgecombe amp Ramskold (1999 Character 9) identifiedtwo distinct character states One of these ndash overlap of only theanteriormost thoracic tergite ndash was limited to taxa assigned tothe Liwiinae (sensu Fortey amp Theron 1994) and another ndashoverlap of multiple tergites with attachment to a reducedanterior thoracic tergite ndash to the Xandarellida None of theadditional taxa considered herein (including Buenaspis whichwas assigned to the Liwiinae by Budd 1999a) shows thesedistinctive morphological features However the expandedhead shields of Marrella Mimetaster and Burgessia which donot articulate with a narrow anterior thoracic tergite may behomologous with the expanded head shields of xandarellidsTo recognise this the degree of overlapping of the thorax andthe possession of the reduced anterior tergite are coded sepa-rately These characters are both coded as inapplicable to taxain which the head shield and thoracic tergites are fused(Character 36 State 1) The crustacean carapace which ismost similar to the head shield of Burgessia is seeminglyabsent in stem group crustaceans (Walossek amp Muller1998)

39 Trunk narrowed anteriorly relative to head shieldwidest posteriorly (0) absent and (1) present (cf Edgecombeamp Ramskold 1999 Character 14) The anterior narrowing ofthe trunk caused by the reduction of opisthosomal segment 1in xiphosurids (Weinbergina of the taxa analysed hereAndersen amp Selden 1997 Dunlop amp Selden 1997 Character14) is not considered to be homologous with the unusual shapeof the thorax in naraoiids recognised by their coding

40 Boundaries of anterior trunk segments reflexed antero-laterally (0) absent boundaries transverse or reflexed postero-laterally and (1) present

41 Joints between posterior tergites functional anteriorones variably fused (0) absent and (1) present

42 Posterior tergite bearing axial spine (0) absent and (1)present

Characters 40 to 42 are coded following Edgecombe ampRamskold (1999 characters 17 19 amp 23 respectively) Inaddition to the taxa considered here a thoracic axial spine ispresent in some olenelloid trilobites (see Lieberman 1998

Characters 73 amp 74) and in the aglaspidid Beckwithia (seeHesselbo 1989)

246 Posterior body termination 43 Postabdomen of seg-ments lacking appendages (0) absent and (1) present

44 Length of postabdomen (0) one segment (1) twosegments (2) three segments and (3) five segments In sometaxa a variable number of posterior thoracic segments bearcomplete tubular tergites and lack appendages In Aglaspisautapomorphically it seems that the posterior three thoracicsegments lack appendages (Briggs et al 1979 Hesselbo 1992)but have unmodified tergites These situations are recognisedas potentially homologous by the coding used here Character44 is coded as inapplicable to taxa lacking a postabdomen

45 Posterior tergites strongly curved in dorsal aspect com-pared to anterior tergites (0) absent and (1) present Asrecognised by Wills et al (1998a Character 17) in some taxawhere the tergites are distinct the curvature of thoracic tergitesincreases posteriorly so that the posterior tergites are highlycurved to semicircular in dorsal aspect This situation isnot known from crustaceans (except isopods) or stem grouparthropods

46 Posterior segments reduced and with highly reducedappendages (0) present and (1) absent In some taxa there area large number of posterior segments that are short sagitallyand have appendages that are much reduced in size comparedto anterior trunk appendages These somites are incorporatedinto the pygidium in some taxa but primitively they arecovered by tiny free trunk tergites In the derived state thetrunk somites and limbs are of a relatively constant size Thedistinction between these states can be seen when attempting tocount the number of segments that make up the body In taxashowing State 0 the number of segments is very high anddifficult to count whereas in other taxa the number ofpost-cephalic segments is easily assessed

47 Pygidium (0) absent and (1) present A pygidium isrecognised here as a posterior tagma consisting of a number offused segments under a single tergite which may or may notincorporate the post-segmental telson This is different to theuse of this term by Edgecombe amp Ramskold (1999) whichfollows Ramskold et al (1997) and very different to its use byWills et al (1998a p 53) as a synapomorphy of the TrilobitaThe present authorsrsquo definition recognises the situation inRetifacies where the spinose postsegmental telson is autapo-morphically not fused to the pygidium as homologous toother multisegmented posterior tagma which include thetelson The distinction between the pygidium and an expandedpost-segmental telson can also be seen in the position of theanus which is consistently in the posteriormost pre-telsonicsegment amongst putative arachnomorphs (see Character 48)In taxa with a pygidium the anal segment is fused into thepygidium (see eg Kuamaia Edgecombe amp Ramskold 1999fig 6) whereas in those taxa lacking a pygidium the anus isbetween the final trunk tergite and the telson

Ramskold et al (1997) argued that the posteriormost tergiteof xandarellids (which incorporates the anal segment) is nothomologous to the posterior tagma recognised as pygidiaherein because posterior thoracic tergites also cover multiplesegments in xandarellids (see Character 33) Evidence ofsegmentation of the pygidium in a variety of taxa suggests thatthe number of tergites fused to form the pygidium is greaterthan the number of appendages For example the pygidium ofKuamaia has two pairs of lateral spines but at least fourpairs of appendages (Hou amp Bergstrom 1997 Edgecombe ampRamskold 1999) and that of Triarthrus has only five axialrings posterior to the articulating ring but over 10 pairs ofappendages (Whittington amp Almond 1987) The fact thatdecoupling of tergites and segments is evident in the thorax of

184 TREVOR J COTTON AND SIMON J BRADDY

Xandarella and Cindarella does not necessarily suggest that asimilar more widespread decoupling in pygidia is the result ofa different developmental process and hence that they arenon-homologous Instead the situation in the xandarellidthorax is potentially homologous to that found (more widely)in pygidia and consequently the terminal xandarellid tergite isa pygidium It is unclear if decoupling of tergites and somites isprimitively limited to the terminal tergite or primitively aproperty of the arachnomorph post-cephalon as a whole

It has been suggested that the tiny pygidium of olenelloidtrilobites which probably best reflects the primitive trilobitestate consists only of the post-segmental telson (Harrington inMoore 1959) and therefore is not a true pygidium HoweverWhittington (1989) showed that the olenelloid pygidium con-sists of at least two and possibly as many as five or sixsegments and therefore is likely to be homologous with thepygidium of other trilobites

48 Position of the anus (0) terminal within telson and (1)at base of telson In crustaceans and stem group arthropodsthe anus is terminal or otherwise situated in the telson (egRehbachiella Walossek 1993 fig 15D) In putative arachno-morphs the anus is either at the junction of the posteriormostthoracic segment and the telson or ventral within a fusedpygidium In the case of taxa with a pygidium the anus isanterior to the posterior margin and where known positionedbetween the posteriormost pair of appendages suggesting thatit is anterior to the post-segmental telson (see eg OlenoidesWhittington 1980)

49 Pygidium with median keel (0) absent and (1) presentEdgecombe amp Ramskold (1999 Character 21) considered thepresence of a median keel on the pygidium as a synapomorphyuniting Soomaspis and Tarricoia They did not explain howthis structure could be distinguished from the raised pygidialaxis of trilobites Homology of these structures is not sup-ported here because the axis of more primitive trilobites(including Eoredlichia) is quite distinct morphologically fromthe naraoiid keel Budd (1999a) noted the presence of a keel onthe pygidium of Buenaspis and suggested (Budd 1999a p 102)that it may be an artefact of dorsoventral compaction How-ever contrary to the coding of Edgecombe amp Ramskold(1999) a keel is visible on the pygidium of Liwia convexa (Dzikamp Lendzion 1988 fig 4CD) preserved in full relief Thereforeit seems unlikely that the keel is a taphonomic artefact

50 Pygidium with broad-based median spine (0) absentand (1) present

51 Pygidium with lateral spines (0) present and (1) absentEdgecombe amp Ramskold (1999 Character 22) coded thepresence of a median spine and two pairs of lateral spines aspotentially homologous in Sinoburius Kuamaia and HelmetiaThe distribution of lateral spines is much wider than that ofterminal spines and therefore they are coded separately hereIn trilobites it has widely been recognised that lateral spinesrepresent the original segmentation of the pygidium Thiscannot be the case for median spines Characters 49 to 51are not applicable to taxa lacking a pygidium (Character 47State 0)

52 Expanded post-segmental telson (0) absent and (1)present In a range of taxa the posterior-most tergite is a large(relative to the thoracic tergites) structure that lacks anyevidence of segmentation and is interpreted as representing anexpanded tergite of the post-segmental telson from whichsegments are released anteriorly during ontogeny On the otherhand the posteriormost tergite of Marrella and probablyMimetaster is small compared to the thoracic tergites androunded This element probably represents the plesiomorphiceuarthropod telson The homology of this character in mosttaxa is clear and only doubtful in Retifacies in which it may

be segmented The figures of Hou amp Bergstrom (1997) providelittle unequivocal evidence for a telson in Retifacies but thepresence of this structure is very clear in colour photographs(eg Chen et al 1996 fig 198A) However these figures donot convincingly demonstrate the segmentation of the telsonRetifacies is interpreted here as being unique in possessingboth a pygidium and an expanded post-segmental telson

53 Telson shape (0) spinose and (1) paddle-shaped Theshape of the expanded telsons identified above varies frombroadly spinose to flattened and paddle-like This differencecan be recognised both on the basis of the ratio of length tobasal width (spinose telsons are relatively long) and by thechange in width posteriorly (spinose telsons reduce in widthposteriorly whereas paddle-shaped telsons increase in width)In eurypterids a wide range of telson shapes is found includ-ing both character states described here It is likely that theplesiomorphic condition is a spinose telson This character isinapplicable to taxa lacking an expanded telson

54 Post-ventral furcae (0) absent and (1) present Thischaracter recognises the potential homology of unsegmentedpaired structures that articulate with the segment immediatelyanterior to the telson The potential homology of these struc-tures (the post-ventral plates) in Emeraldella and Aglaspis wasrecognised by Wills et al (1998a Character 70) and inEmeraldella and Sidneyia by Edgecombe amp Ramskold (1999Character 25) The homology of the segmented pygidial caudalfurcae of Olenoides (see Whittington 1975a 1980) with thesestructures is considered doubtful and coded as equivocal

Despite their distinctive morphology the long caudualfurcae of Cheloniellon may be homologous with the furcae ofAglaspis Emeraldella and Sidneyia This is supported by thecaudal furcae of the Ordovician cheloniellid Duslia which aremore similar in morphology to those of Emeraldella than ofCheloniellon Chlupac (1988 pp 614ndash16) considered thesestructures to be on the terminal tergite in Duslia However afaint tergite boundary is apparent posterior to the attachmentof the furcae (see Chlupac 1988 pl 57 fig 4) Therefore theyare considered here to have been attached to the pre-telsonicsegment as in Cheloniellon Chlupac (1988) and Dunlop ampSelden (1997) supported a close relationship between Dusliaand Cheloniellon

25 MethodsAll analyses were carried out using PAUP Version 40b4a(Swofford 1999) and unless otherwise stated used heuristicsearches with one thousand random addition sequencereplicates The software packages MacClade Version 307(Maddison amp Maddison 1997) and RadCon (Thorley amp Page2000) were used for comparing trees and investigating patternsof character evolution Tree length and other statistics (theConsistency Index CI and Retention Index RI) were calcu-lated by PAUP and MacClade with uninformative charactersexcluded Analyses were run separately using each of thedifferent codings for aglaspidids

Characters were treated as unordered and of equal weight inmost analyses To assess the influence of this assumption fourof the eight multistate characters which have states that areintermediate between others (Wilkinson 1992) were treated asordered in some analyses These characters are the number ofcephalic segments (Character 4) the length of raptorialappendage spines (Character 6) the degree of overlap of thethorax by the head-shield (Character 37) and the number ofsegments making up the postabdomen (Character 44) The lastof these is uninformative when not ordered because only onetaxon shows each of states 0 1 and 3

Support for individual nodes was assessed by bootstrapanalysis (Felsenstein 1985) and by calculating Bremer support

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 185

indices (Bremer 1988 1994) These methods measure twodistinct aspects of support for phylogenetic hypotheses Treesor nodes may be considered well supported (1) to the extent towhich alternative topologies are much less parsimonious asmeasured by the support index (Wilkinson 1996) or (2) wherethey are consistent with a large proportion of characters sothat character sampling is unlikely to have had much influenceon topology as assessed by bootstrapping (Page 1996) Boot-strapping was performed with 100 bootstrap replicates eachof 10 addition sequence replicates Bremer support indicesare based on heuristic searches with 100 addition sequencereplicates

Three pairs of terminals are included in Table 2 that apartfrom missing data are coded identically These are Helmetiaand Kuamaia Soomaspis and Tariccoia and Saperion andSkioldia These pairs are lsquotaxonomic equivalentsrsquo removingthe taxon with most missing data will not influence theposition of the other They can be recoded as single terminalsaccording to the principal of safe taxonomic reduction(Wilkinson 1995a b) and were not considered separately in theanalyses presented here Therefore cladistic analyses werecarried out on the basis of 33 terminals Edgecombe ampRamskold (1999) ignored this and coded the same three pairsof taxa and Eoredlichia and Olenoides identically for theirmore limited set of characters

26 ResultsAnalysis with all characters unordered and using the lsquoAglaspi-dida 1rsquo coding found nine most parsimonious trees (MPTs)each 126 steps long (CI=0middot556 RI=0middot763) Using the lsquoAglas-pidida 2rsquo coding resulted in 18 MPTs of length 125 (CI=0middot560RI=0middot765) Nine of these 18 trees were those found with thefirst coding of aglaspidids The strict component consensus(sensu Wilkinson 1994) of the 27 unique trees found acrossboth analyses is shown in Figure 8 All trees supported themonophyly of the Marrellomorpha (Marrella and Mimetaster)and of a clade including all other taxa except Crustacea (Clade1 of Fig 8) The relationships between these two clades and theCrustacea were unresolved equal numbers of trees in bothanalyses supported each of the topologies [Clade 1 (CrustaceaMarrellomorpha)] [Crustacea (Marrellomorpha Clade 1)]and [Marrellomorpha (Crustacea Clade 1)] All trees placedBurgessia as the sister taxon to the rest of Clade 1 andsupported the division of the rest of Clade 1 into two majorsister groups The topology of one of these clades (Clade 2)which included trilobites helmetiids tegopeltids naraoiidsxandarellids and Retifacies was stable with respect to thecoding of aglaspidids The other clade (Clade 3) consists of aclade of megacheirans and chelicerates (Clade 4) and a cladeincluding aglaspidids Lemoneites Paleomerus CheloniellonEmeraldella and Sidneyia (Clade 5) This latter clade is un-resolved in the strict consensus of all 27 trees (Fig 8) and thestrict consensus of the 18 MPTs found using the lsquoAglaspidida2rsquo coding but was fully resolved in the consensus of the ninetrees found with the lsquoAglaspidida 1rsquo coding Within themegacheiran-chelicerate clade all trees found Fortiforceps tobe the sister group to all other taxa Yohoia the sister group tochelicerates and Alalcomenaeus and Leanchoilia to form aclade Jianfengia is equally parsimoniously placed as the sistergroup to the Yohoia-chelicerate clade or as the sister group tothis clade and the Alalcomenaeus-Leanchoilia clade

Treating characters 4 6 37 and 44 as ordered had a minimaleffect on most parsimonious topology Nine MPTs of 136 steps(CI=0middot515 RI=0middot743) were found with the lsquoAglaspidida 1rsquocoding and 12 MPTs 135 steps in length (CI=0middot519 RI=0middot745)with the lsquoAglaspidida 2rsquo coding These trees all supported a(Crustacea Clade 1) clade excluding the marrellomorphs

Apart from this the strict consensus of these 21 trees isidentical to the consensus of the 27 trees found with allcharacters treated as unordered

Across all analyses four different topologies for Clade 5 (ofFig 8) were found (Fig 9) The distribution of these topologiesacross the four analyses described above and across the sameanalyses repeated after reweighting of each character in pro-portion to their Rescaled Consistency Index (see Farris 1989)from the first analysis is shown in Table 3 The instability of

Figure 8 Strict consensus of 27 MPTs from four separate analyses ofarachnomorph phylogeny with both interpretations of aglaspididmorphology and with some characters treated as ordered or allcharacters unordered Clades referred to in the text are numbered

Figure 9 Alternative equally parsimonious resolutions of clade 5 ofFig 8 (see Table 3)

186 TREVOR J COTTON AND SIMON J BRADDY

this clade was to some extent caused by the inclusion ofLemoneites and Paleomerus which could each be coded foronly 27 of the 54 characters used Analysis with these two taxaexcluded and characters treated as unordered found treescompatible only with Topology D of Figure 9 irrespective ofthe coding used for aglaspidids Analysis with some characterstreated as ordered and using either aglaspidid coding foundthree trees compatible only with Topology B of Figure 9 andthree compatible only with Topology D of Figure 9 ThereforeTopology D is the preferred topology for Clade 5 as shown inTable 3

One of the MPTs resulting from the lsquoAglaspidida 2rsquo coding(with all characters treated as unordered) was chosen as thebasis for further discussion because this tree combined thepreferred topology of Clade 5 with the (Crustacea Clade 1)group that was favoured when some characters were treated asordered Bootstrap percentages and Bremer support values forthis tree (using the Aglaspidida 2 coding) are shown in Figure10 Apomorphies for all ingroup nodes of this tree are shownin Figure 10A

3 Discussion

31 Phylogenetic resultsThe results of the cladistic analyses presented above arewell-resolved and robust with respect to different analyticalparameters Most of the taxa considered form a clade (Clade 1of Fig 8) that is more closely related to chelicerates than tocrustaceans in all analyses and therefore is recognised asthe Arachnomorpha as defined by Chen et al (1996) andRamskold et al (1996) The monophyly of the Marrellomor-pha was supported in all analyses The marrellomorphs werefound to be the sister group to a Crustacea+Clade 1 group inmost MPTs but in a minority of trees formed the sister groupto Clade 1 alone According to the latter result they should beincluded within the Arachnomorpha but according to the firstexcluded from it Both of these alternatives have been sup-ported in previous studies (eg Hou amp Bergstrom 1997 Willset al 1998a respectively) In addition to the marrellomorphsthe Fuxianhuiida Bousfield 1995 and Canadaspidida havebeen placed in the Euarthropoda outside the main mandibu-late and arachnomorph clades (Hou amp Bergstrom 1997)Further study of these taxa of the crustacean or mandibulatestem group and of the marrellomorphs is necessary before the

position of the Marrellomorpha can be resolved Provisionallymarrellomorphs are excluded from the Arachnomorphafollowing the majority of analyses above and Wills et al(1994)

The Burgess Shale arthropod Burgessia was placed as thesister group to remaining arachnomorphs in all analyses OtherArachnomorpha form two major clades one consisting of thelsquotrilobite-alliedrsquo arachnomorphs analysed by Edgecombe ampRamskold (1999) here termed lsquoTrilobitomorpharsquo (defined asTrilobita and all taxa more closely related to Trilobita thanChelicerata) and the other (lsquochelicerate-alliedrsquo) clade includingchelicerates lsquogreat appendagersquo arthropods aglaspididsLemoneites Paleomerus Sidneyia and Emeraldella The latteris here termed lsquoCheliceramorpharsquo (defined as Chelicerata andall taxa more closely related to Chelicerata than Trilobita) Thetopology of the lsquotrilobite-alliedrsquo clade was similar to that foundby Edgecombe amp Ramskold (1999) but the present authorsrsquoresults are more completely resolved unambiguously support-ing the sister-group relationship between trilobites andHelmetiida suggested in the previous study Within thelsquochelicerate-alliedrsquo clade Aglaspidids Paleomerus Lemoneitesand the Burgess Shale Emeraldella and Sidneyia form a cladein opposition to megacheirans and chelicerates

The present authorsrsquo analysis supports a new hypothesis ofthe origin of the chelicerates from within a paraphyleticassemblage of megacheiran arthropods and confirms pyc-nogonids as a sister group to the euchelicerates Previoushypotheses of the chelicerate sister group (see Section 1) aremuch less parsimonious The shortest trees supporting aCheloniellon-chelicerate clade are six steps longer than theMPTs and those supporting an aglaspidid-chelicerate cladeseven steps longer All megacheirans and chelicerates areunited by the flap-shaped rounded exopods and modificationof the second cephalic segment endopods into anteriorlydirected spinose raptorial organs which share a number ofdetailed similarities A clade including all of these taxa apartfrom Fortiforceps is supported by the synapomorphic loss ofthe antennae (or antennulae of crustaceans) and the longerlength of spinose projections of the second cephalic appendageendopods Within this clade Yohoia shares the loss of cephalicexopods the loss of thoracic endopods and a postabdomen oftubular tergites with chelicerates

The poor resolution of Clade 5 (of Fig 8) was partly a resultof the poor preservation of and consequent high proportion ofmissing data for Lemoneites and Paleomerus When these two

Table 3 Numbers of most parsimonious trees supporting each of the alternative topologies for clade 4 of Fig 7 under different analyticalconditions (RCI=Revealed Consistency Index)

Aglaspididcoding

Characters 4 6 36and 43 ordered

Charactersreweighted by RCI

Paleomerus andLemoneites excluded

Topology (see Fig 8)

A B C D

1 91 X 3 3 31 X 61 X X 31 X 61 X X 3 32 9 92 X 3 3 3 32 X 62 X X 32 X 62 X X 3 3Total 6 18 12 54

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 187

taxa were excluded from the analyses only two (B and D) ofthe four topologies found in the complete analysis remainedmost parsimonious (see Fig 9 and Table 3) Secondly Bremersupport values throughout much of the lsquochelicerate-alliedrsquoclade (Clade 3 of Fig 8) were improved when these taxa wereexcluded (see Fig 11)

Dunlop (1999 p 258) suggested that in reconstructing thechelicerate stem lineage lsquowe might predict that the two mostsignificant changes towards the chelicerate condition are thereduction of the antennae and the formation of the nextappendages into a clawrsquo According to the hypothesis pre-sented here both of these adaptations were achieved in EarlyCambrian chelicerate ancestors The recognition of the loss ofthe antennae in the stem group of the chelicerates and thephylogenetic hypothesis presented here suggest that chelicer-ates are most closely related to taxa with only a singleadditional segment fused to the crown group eucheliceratehead of four fused somites Previous hypotheses have derivedchelicerates from Palaeozoic arthropods with a longer head(eg Cheloniellon Sturmer amp Bergstrom 1978 SanctacarisBriggs amp Collins 1988 Emeraldella Bruton amp Whittington1983) The plesiomorphic pattern of head segmentation ismatched in pycnogonids Bergstrom et al (1980) suggestedthat only the anterior four pairs of appendages are incorpo-rated into the head This supports the view that pycnogonidsare primitive with respect to other chelicerates which showmore or less complete fusion of this primitive head and threethoracic segments including Solpugida and Pseudoscorpiones

The present study suggests that a shift from gnathobasicbenthic feeding to a more vagrant lifestyle and raptorialappendage feeding occurred very early in the chelicerate lin-eage Appendage feeding was clearly an important adaptation

Figure 10 One of 18 equally parsimonious cladograms with lsquoAglaspidida 2rsquo coding and all characters unorderedthat is most congruent with results from other analyses Non-terminal branch lengths scaled to reflect number ofapomorphies (A) ACCTRAN apomorphy scheme Character numbers shown above boxes and states shownbelow boxes as in Table 2 and the text Character consistency indices are indicated by shading (B) Levels ofsupport for individual nodes Bremer support indices shown in bold type above branches Bootstrap percentagesin italic type below branches for all nodes with relative frequencies greater than 50

Figure 11 Bremer support values for clades within the lsquochelicerate-alliedrsquo arachnomorphs (clade 3 of Fig 8) when all taxa wereincluded (normal weight font above nodes) and with Lemoneites andPaleomerus excluded (bold type below nodes)

188 TREVOR J COTTON AND SIMON J BRADDY

for the terrestrialization of chelicerates (Dunlop 1997 p 69Dunlop amp Webster 1999) but the model proposed heresuggests that this considerably pre-dated terrestrializationGnathobasic feeding in xiphosurans and eurypterids mayrepresent a reversal to a more primitve benthic arachnomorphlifestyle from a more pycnogonid-like ancestor

32 Evolution of the arachnomorph headThe extant arthropod classes show highly conserved patternsof head segmentation (eg Wills et al 1994) In contrast therehas been a near consensus amongst Cambrian arthropodworkers (although rarely based on an explicit phylogenetichypothesis) that patterns of head segmentation are highlyhomoplastic and hence of little systematic significanceSturmer amp Bergstrom (1978 pp 78ndash9) suggested that lsquoevenclosely related forms may have different numbers of headsegments and appendagesrsquo Bruton amp Whittington (1983pp 576ndash7) that lsquodiscussion on fossil arthropod relationshipsbased on head segmentation appears to be largely irrel-evant and at best speculativersquo and Delle Cave amp Simonetta(1991 p 191) that lsquothere are no obvious phyletic affinitiesbetween genera having the same number of cephalized seg-mentsrsquo Most recently Bergstrom amp Hou (1997 p 104) con-cluded that lsquothe segmental length of the head shield seemsto be of no relevance to the discussion and discrimination ofevolutionary lineagesrsquo It has been claimed that this view issupported by cladistic analyses in which patterns of tagmosishave been found to be rather poor at defining major arthropodclades (Briggs et al 1992 Wills et al 1994)

This suggestion is not only of relevance to systematics buthas featured prominently in recent debates surrounding thenature of the lsquoCambrian explosionrsquo (see Budd amp Jensen 2000for a recent review) Gould (1989 1991) argued that theapparent plasticity of head segmentation in Cambrian com-pared to post-Cambrian arthropod evolution suggests thatbody-plan evolution was only constrained after the Cambrianexplosion According to this view the origin of new highertaxa with distinct body-plans was only possible before theonset of these constraints (eg Valentine 1995) Taxa hererecognised as arachnomorphs clearly played a major role in thedevelopment of Gouldrsquos hypothesis This suggestion has pre-viously been assessed in terms of overall morphological diver-sity which is clearly rather distinct from Gouldrsquos concept ofdisparity Wills et al (1998b) showed that there was littlecorrelation between overall morphological disparity and de-gree of limb specialisation and that the latter was phylogeneti-cally highly plastic They remarked that (Wills et al 1998bp 64) lsquothis finding has important implications for models ofarthropod phylogeny and evolution that attribute overidingimportance to head segmentationrsquo but did not explicitlyexamine head segmentation although it is likely to have beena major component of the index of tagmosis employed

The results of the present study suggest that arguments thatarthropod head segmentation was unusually labile during theCambrian are poorly founded Rather only four major pat-terns of euarthropod head segmentation are identified Theplesiomorphic euarthropod state according to both Walossekamp Muller (1997 1998) and Scholtz (1997) which may moreproperly be the plesiomorphic state for the euarthropod crowngroup only (depending on the phylogenetic position of theMarrellomorpha) consists of four post-acronal segmentsbearing the antennae and three pairs of biramous limbs Thepresent authors suggest that the term lsquocephalonrsquo be restrictedto this kind of head which is found in stem group mandibu-lates in addition to many arachnomorphs Crown groupmandibulates share a lsquobimaxillary headrsquo in which an addi-tional pair of appendages the labium or second maxillae are

incorporated into the head but in crustaceans these are notfused to the carapace (Scholtz 1997) Finally two distinctforms of head tagmosis are found in chelicerates A head withfour pairs of appendages (but without antennae giving a totalof five segments incorporated into the head) which has beencalled the lsquocephalosomarsquo is present in pycnogonids (Walossekamp Dunlop 2002 pp 434ndash5) and probably also in some euch-elicerates (J A Dunlop pers comm 2003) The lsquoprosomarsquo ofmost euchelicerates consists of this cephalosoma and twoadditional segments

In general these patterns of head segmentation are highlyphylogenetically conserved (see Fig 12) Only three homo-plastic changes namely reversal to a more primitive conditionin the Leanchoilia-Alalcomenaeus clade and the convergentorigin of a five-segmented head in Clade 3 (of Fig 8) and theXandarellida are necessary to optimise character 4 onto themost parsimonious cladogram There are also transitions toautapomorphic states in Emeraldella and Sidneyia In the caseof Sidneyia however an argument could be made (followingBergstrom amp Sturmer 1978 see discussion of Character 4) thatthe head consists of five segments as in closely related taxabut autapomorphically each segment has a separate tergiteThis would further increase the phylogenetic stability of headsegmentation Also following Bergstrom amp Sturmer (1978) thehead of Cheloniellon could be considered to consist of twotergites making it more similar to the head of Emeraldella(with six pairs of appendages)

Figure 12 Cladogram from Fig 10 showing changes in head segmen-tation Bold type shows abbreviated head segmentation formulaAbbreviations (Ant) antennae (BA) biramous appendage pairs (Ch)chelicerae (GA) great appendages (UA) uniramous appendage pairsand (X) segment without appendages Synapomorphy A is the auta-pomorphic reduction in head length to a single segment in SidneyiaAlternatively this may represent secondary division of the headshield Synapomorphy B is the increase in the length of the head inEmeraldella to six segments

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 189

4 Acknowledgements

We thank D E G Briggs J A Cotton J A Dunlop R AFortey S Harris R A Moore P Selden O E Tetlie andP Van Roy for comments on earlier drafts of this work Themanuscript was reviewed by M Wills F Schram and oneanonymous referee This research was supported by the Geo-logical Society of London Curry Research Studentshipawarded to TJC by the Department of Earth Sciences Uni-versity of Bristol and by the Department of PalaeontologyThe Natural History Museum London TJC would like tothank C Labandeira J Thompson and R Chapman fortheir hospitality at the National Museum of Natural HistoryWashington DC USA and A Olcott S Schellenberg andJ Young for making his time there so enjoyable

5 References

Abzhanov A Popadic A amp Kaufman T C 1999 Chelicerate Hoxgenes and the homology of arthropod segments Evolution andDevelopment 1 77ndash89

Akam M 2000 Arthropods developmental diversity within a (super)phylum Proceedings of the National Academy of Sciences USA97 4438ndash41

Aldridge R J Gabbott S E amp Theron J N 2001 The Soom ShaleIn Briggs D E G amp Crowther P R (eds) Palaeobiology II340ndash2 Oxford Blackwell Science

Anderson L I amp Selden P A 1997 Opisthosomal fusion andphylogeny of Palaeozoic Xiphosura Lethaia 30 19ndash31

Arango C P 2002 Morphological phylogenetics of the sea spiders(Arthropoda Pycnogonida) Organisms Diversity and Evolution2 107ndash25

Ax P 1986 Das phylogenetische System Stuttgart Gustav FischerVerlag

Babcock L E amp Zhang Wentang In press New non-mineralizedarthropods from the Chengjiang Lagerstatte Lower CambrianYunnan China Palaeontologia Cathyana

Bartels C Briggs D E G amp Brassel G 1998 The fossils ofthe Hunsruck Slate Marine life in the Devonian CambridgeCambridge University Press

Bergstrom J 1971 Paleomerus ndash merostome or merostomoid Lethaia4 393ndash401

Bergstrom J 1979 Morphology of fossil arthropods as a guideto phylogenetic relationships In Gupta A P (ed) ArthropodPhylogeny 3ndash56 New York NY Van Nostrand Reinhold Co

Bergstrom J 1992 The oldest arthropods and the origin of theCrustacea Acta Zoologica 73 287ndash91

Bergstrom J Sturmer W amp Winter G 1980 PalaeoisopusPalaeopantopodus and Palaeothea pycnogonid arthropodsfrom the Lower Devonian Hunsruck Slate West GermanyPalaontologische Zeitschrift 54 7ndash54

Bergstrom J amp Brassel G 1984 Legs in the trilobite Rhenops fromthe Lower Devonian Hunsruck Slate Lethaia 17 67ndash72

Bergstrom J amp Hou X 1998 Chengjiang arthropods and theirbearing on early arthropod evolution In Edgecombe G D(ed) Arthropod fossils and phylogeny 151ndash84 New York NYColumbia University Press

Boudreaux H B 1979 Significance of intersegmental tendon systemin arthropod phylogeny and a monophyletic classification ofArthropoda In Gupta A P (ed) Arthropod Phylogeny 551ndash86New York NY Van Nostrand Reinhold Co

Bousfield E L 1995 A contribution to the natural classification ofLower and Middle Cambrian arthropods food-gathering andfeeding mechanisms Amphipacifica 2 3ndash33

Braddy S J Aldridge R J Gabbott S E amp Theron J N 1999Lamellate book-gills in a late Ordovician eurypterid from theSoom Shale South Africa support for a eurypterid-scorpionclade Lethaia 32 72ndash4

Braddy S J amp Almond J E 1999 Eurypterid trackways from theTable Mountain Group (Ordovician) of South Africa Journal ofAfrican Earth Sciences 29 165ndash77

Bremer K 1988 The limits of amino acid sequence data in angio-sperm phylogenetic reconstruction Evolution 42 795ndash803

Bremer K 1994 Branch support and tree stability Cladistics 10295ndash304

Briggs D E G 1998 Review of Arthropods from the LowerCambrian Chengjiang fauna southwest China by HouXianguang and Jan Bergstrom Fossils and Strata 45

Scandinavian University Press Oslo Norway 1997 Palaeo-geography Palaeoclimatology Palaeoecology 143 193ndash4

Briggs D E G Bruton D L amp Whittington H B 1979 Append-ages of the arthropod Aglaspis spinifer (Upper CambrianWisconsin) and their significance Palaeontology 22 167ndash80

Briggs D E G Fortey R A amp Wills M A 1992 Morphologicaldisparity in the Cambrian Science 256 1670ndash3

Briggs D E G Fortey R A amp Wills MA 1993 How big was theCambrian explosion A taxonomic and morphologic comparisonof Cambrian and Recent arthropods In Lees D R amp EdwardsD (eds) Evolutionary patterns and processes 33ndash44 LondonLinnean Society of London Linnean Society Symposium Series

Briggs D E G Erwin D H amp Collier F J 1994 The fossils of theBurgess Shale Washington DC Smithsonian Institution Press

Briggs D E G amp Bartels C 2001 New arthropods from the LowerDevonian Hunsruck Slate (Lower Emsian Rhenish Massifwestern Germany) Palaeontology 44 275ndash304

Briggs D E G amp Bartels C 2002 Magnoculocaris a new name forthe arthropod Magnoculus Briggs and Bartels 2001 from theLower Devonian Hunsruck Slate Germany Palaeontology 45419

Briggs D E G amp Collins D 1988 A Middle Cambrian cheliceratefrom Mount Stephen British Columbia Palaeontology 31 779ndash98

Briggs D E G amp Collins D 1999 The arthropod Alalcomenaeuscambricus Simonetta from the Middle Cambrian Burgess Shale ofBritish Columbia Paleontology 42 953ndash78

Briggs D E G amp Fortey R A 1982 The cuticle of aglaspididarthropods a red-herring in the early history of vertebratesLethaia 15 25ndash9

Briggs D E G amp Fortey R A 1989 The early radiation andrelationships of the major arthropod groups Science 246 241ndash3

Briggs D E G amp Fortey R A 1992 The early Cambrian radiationof arthropods In Lipps J H amp Signor P W (eds) Origin andEarly Evolution of the Metazoa 335ndash373 New York NY PlenumPress

Briggs D E G amp Robison R A 1984 Exceptionally preservednontrilobite arthropods and Anomalocaris from the MiddleCambrian of Utah University of Kansas PaleontologicalContributions Paper 111 1ndash23

Broili F 1932 Ein neuer Crustacee aus dem rheinischen UnterdevonSitzungsberichte der Mathematisch-Naturwissenschaftlichen(Abteilung) Klasse der Bayerischen Akademie der Wissenschaftenzu Munchen 1932 27ndash38

Broili F 1933 Ein zweites Exemplar von Cheloniellon Sitzungsber-ichte der Bayerischen Akademie der Wissenschaften 1933 11ndash32

Bruton D L 1981 The arthropod Sidneyia inexpectans MiddleCambrian Burgess Shale British Columbia Philosophical Trans-actions of the Royal Society of London Series B 295 619ndash56

Bruton D L amp Whittington H B 1983 Emeraldella and Leanchoiliatwo arthropods from the Burgess Shale British Columbia Philo-sophical Transactions of the Royal Society of London Series B 300553ndash85

Budd G E 1995 Kleptothule rasmusseni gen et sp nov anolenellid-like trilobite from the Sirius Passet fauna (BuenFormation Lower Cambrian North Greenland) Transactions ofthe Royal Society of Edinburgh Earth Sciences 86 1ndash12

Budd G E 1996a Progress and problems in arthropod phylogenyTrends in Ecology and Evolution 11 356ndash8

Budd G E 1996b The morphology of Opabinia regalis and thereconstruction of the arthropod stem-group Lethaia 29 1ndash14

Budd G E 1997 Stem-group arthropods from the Lower CambrianSirius Passet fauna of North Greenland In Fortey R A ampThomas R H (eds) Arthropod Relationships 125ndash38 LondonChapman and Hall Systematics Association Special Volume 55

Budd G E 1999a A nektaspid arthropod from the Early CambrianSirius Passet fauna with a description of retrodeformation basedon functional morphology Palaeontology 42 99ndash122

Budd G E 1999b The morphology and phylogenetic significance ofKerygmachela kiergkegaardi Budd (Buen Formation LowerCambrian N Greenland) Transactions of the Royal Society ofEdinburgh Earth Sciences 89 (for 1998) 249ndash290

Budd G E 2002 A palaeontological solution to the arthropod headproblem Nature 417 271ndash5

Budd G E amp Jensen S 2000 A critical reappraisal of the fossilrecord of the bilaterian phyla Biological Reviews 75 253ndash95

Chen J Zhou G Zhu M amp Yeh K 1996 The Chengjiang biota ndasha unique window of the Cambrian explosion Taichung NationalMuseum of Natural Science [In Chinese]

Chen J Edgecombe G D amp Ramskold L 1997 Morphological andecological disparity in naraoiids (Arthropoda) from the Early

190 TREVOR J COTTON AND SIMON J BRADDY

Cambrian Chengjiang fauna China Records of the AustralianMuseum 49 1ndash24

Chen J amp Zhou G 1997 Biology of the Chengjiang fauna In ChenJ Cheng Y amp Iten H V (eds) The Cambrian explosion and thefossil record Bulletin of the National Museum of Natural ScienceTaiwan 10 11ndash106

Child C A 1997 Some deep-sea Pycnogonida from Argentine slopeand basin Proceedings of the Biological Society of Washington110 128ndash42

Chlupac I 1988 The enigmatic arthropod Duslia from the Ordovicianof Czechslovakia Palaeontology 31 611ndash20

Chlupac I 1995 Lower Cambrian arthropods from the PasekyShale (Barrandian area Czech Republic) Journal of the CzechGeological Society 40 9ndash36

Chlupac I amp Havlıcek V 1965 Kodymirus n g a new aglaspidmerostome of the Cambrian of Bohemia Sbornık GeologickechVid Paleontologie 6 7ndash20

Cisne J L 1975 Anatomy of Triarthrus and the relationships of theTrilobita Fossils and Strata 4 45ndash63

Cisne J L 1981 Triarthrus eatoni (Trilobita) anatomy of its exo-skeletal skeletomuscular and digestive systems Palaeonto-graphica Americana 9 95ndash142

Conway Morris S 1982 Atlas of the Burgess Shale London ThePalaeontological Association

Conway Morris S 1989 The persistence of Burgess Shale-type faunasimplications for the evolution of deeper-water faunas Trans-actions of the Royal Society of Edinburgh Earth Sciences 80271ndash83

Conway Morris S 1998 The crucible of creation Oxford OxfordUniversity Press

Cotton T J 1999 Review of Arthropod fossils and phylogenyPalaeogeography Palaeoclimatology Palaeoecology 151 341ndash3

Cotton T J 2002 The phylogeny and morphological evolution ofCambrian trilobites and their relatives Unpublished PhD ThesisUniversity of Bristol

Damen W G M Hausdorf M Seyfarth E A amp Tautz D 1998 Aconserved mode of head segmentation in arthropods revealed bythe expression pattern of Hox genes in a spider Proceedings of theNational Academy of Sciences USA 95 10665ndash70

Delle Cave L amp Simonetta A M 1991 Early Palaeozoic arthropodsand problems of arthropod phylogeny with some notes on taxa ofdoubtful affinities In Simonetta A M amp Conway Morris S(eds) The early evolution of Metazoa and the significance ofproblematic taxa 189ndash244 Cambridge Cambridge UniversityPress

Dewel R A amp Dewel W C 1997 The place of tardigrades inarthropod evolution In Fortey R A amp Thomas R H (eds)Arthropod Relationships 109ndash23 London Chapman and HallSystematics Association Special Volume 55

Dunlop J A 1997 Palaeozoic arachnids and their significance forarachnid phylogeny Proceedings of the 16th European Colloquiumof Arachnology 65ndash82 Siedlce Poland

Dunlop J A 1998 The origins of tetrapulmonate booklungs and theirsignificance for chelicerate phylogeny In Selden P A (ed)Proceedings of the 17th European Colloquium of ArachnologyEdinburgh 1997 pp 9ndash16

Dunlop J A 1999 Pasando revista a la evolucion de los quelicerados[A review of chelicerate evolution] In Melic A DeHaro J JMedez M amp Ribera I (eds) Volumen Monografico Evolucion yFilogenia de Arthropoda Boletin de la Sociedad EntolomologicaAragonesa 26 255ndash272 [In Spanish and English]

Dunlop J A amp Selden P A 1997 The early history and phylogenyof the chelicerates In Fortey R A amp Thomas R H (eds)Arthropod relationships Systematics Association Special Volume55 221ndash35 London Chapman and Hall

Dunlop J A amp Webster M 1999 Fossil evidence terrestrializationand arachnid phylogeny Journal of Arachnology 27 86ndash93

Dzik J 1993 Early metazoan evolution and the meaning of its fossilrecord Evolutionary Biology 27 339ndash86

Dzik J amp Lendzion K 1988 The oldest arthropods of the EastEuropean Platform Lethaia 21 29ndash38

Edgecombe G D Wilson G D F Colgan D J Gray M R ampCassis G 2000 Arthropod cladistics combined analysisof Histone H3 and U2 snRNA sequences and morphologyCladistics 16 155ndash203

Edgecombe G D amp Ramskold L 1999 Relationships of CambrianArachnata and the systematic position of Trilobita Journal ofPaleontology 73 263ndash87

Emerson M J amp Schram F R 1997 Theories patterns and realitygame plan for arthropod phylogeny In Fortey R A amp Thomas

R H (eds) Arthropod Relationships Systematics AssociationSpecial Volume 55 67ndash87 London Chapman and Hall

Farris J S 1989 The Retention Index and the Rescaled ConsistencyIndex Cladistics 5 417ndash19

Felsenstein J 1985 Confidence limits on phylogenies an approachusing the bootstrap Evolution 39 783ndash91

Flower R H 1968 Merostomes from a Cotter Horizon of the El PasoGroup New Mexico Bureau of Mines and Mineral ResourcesMemoir 22 35ndash44

Fortey R A 1990a Trilobite evolution and systematics In MikulicD G (ed) Arthropod paleobiology 44ndash65 Short Courses inPaleontology 3 Knoxville Tenessee The Paleontological Society

Fortey R A 1990b Ontogeny hypostome attachment and trilobiteclassification Palaeontology 33 529ndash76

Fortey R A 1997 Classification In Kaesler R L (ed) Treatise oninvertebrate paleontology Part O Arthropoda 1 (revised) 289ndash302Boulder Colorado amp Lawrence Kansas Geological Society ofAmerica and University of Kansas Press

Fortey R A amp Theron J N 1994 A new Ordovician arthropodSoomaspis and the agnostid problem Palaeontology 37 841ndash61

Fortey R A amp Whittington H B 1989 The Trilobita as a naturalgroup Historical Biology 2 125ndash38

Fry W G 1978 A classification within the pycnogonids ZoologicalJournal of the Linnean Society of London 63 35ndash58

Giribet G Edgecombe G D amp Wheeler W C 2001 Arthropodphylogeny based on eight molecular loci and morphology Nature413 157ndash61

Gould S J 1989 Wonderful life The Burgess Shale and the nature ofhistory New York NY Norton

Gould S J 1991 The disparity of the Burgess Shale arthropod faunaand the limits of cladistic analysis why we must strive to quantifymorphospace Paleobiology 17 411ndash23

Gurich G 1931 Mimetaster hexagonalis ein neuer Kruster aus demunterdevonischen Bundenbacher Dachschiefer PalaontologischeZeitschrift 12 204ndash38

Hammann W Laske R amp Pillola G L 1990 Tariccoia arrusensisngnsp an unusual trilobite-like arthropod Rediscovery of thelsquophylocaridrsquo beds of Taricco (1922) in the Ordovician lsquoPuddingarsquosequence of Sardinia Bolletino della Societa PaleontologicaItaliana 29 163ndash78

Haas M S Brown S J amp Beeman R W 2001 Pondering theprocephalon the segmental origin of the labrum DevelopmentGenes and Evolution 211 89ndash95

Heider K 1913 Entwicklungsgeschichte und Morphologie derWirbellosen Die Kultur der Gegenwart Theil 3 Abt 4 2 176ndash332

Henriksen K 1928 Critical notes upon Cambrian arthropodsdescribed by Charles D Walcott Videnskabelige Meddelelser fraDansk Naturhistorisk Forening 86 1ndash20

Hesselbo S P 1988 Trace fossils of Cambrian aglaspidid arthropodsLethaia 21 139ndash46

Hesselbo S P 1989 The aglaspidid arthropod Beckwithia from theCambrian of Utah and Wisconsin Journal of Paleontology 63636ndash42

Hesselbo S P 1992 Aglaspidida (Arthropoda) from the UpperCambrian of Wisconsin Journal of Paleontology 66 885ndash923

Hou X 1987a Three new large arthropods from Lower CambrianChengjiang fauna eastern Yunnan Acta Palaeontologica Sinica26 272ndash85 [In Chinese with English summary]

Hou X 1987b Two new arthropods from Lower CambrianChengjiang eastern Yunnan Acta Palaeontologica Sinica 26236ndash56 [In Chinese with English summary]

Hou X Chen J amp Lu H 1989 Early Cambrian new arthropodsfrom Chengjiang Yunnan Acta Palaeontologica Sinica 28 42ndash57[In Chinese with English summary]

Hou X Ramskold L amp Bergstrom J 1991 Composition andpreservation of the Chengjiang fauna a Lower Cambrian soft-bodied biota Zoologica Scripta 20 395ndash411

Hou X Bergstrom J amp Ahlberg P 1995 Anomalocaris and otherlarge animals in the Lower Cambrian Chengjiang fauna ofsouthwest China GFF 117 163ndash83

Hou X amp Bergstrom J 1997 Arthropods of the Lower CambrianChengjiang fauna southwest China Fossils and Strata 45 1ndash116

Hughes C P 1975 Redescription of Burgessia bella from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 4415ndash36

Ivantsov A Y 1999 Trilobite-like arthropod from the LowerCambrian of the Siberian Platform Acta PalaeontologicaPolonica 44 455ndash66

King P E 1973 Pycnogonids London St Martin Press

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 191

Lauterbach K E 1973 Schlussereignisse in der Evolution derStammgruppe der Euarthropoda Zoologische Beitrage 19259ndash99

Lauterbach K E 1980 Schlussereignisse in der Evolution derGrundplans der Arachnata (Arthropoda) Abhandlungen desNaturwissenschaftlichen Vereins in Hamburg 23 163ndash327

Lauterbach K E 1983 Synapomorphien zwischen Trilobiten- undCheliceraten-zweig der Arachnata Zoologischer Anzeiger 210213ndash38

Lendzion K 1975 Fauna of the Mobergella Zone in the Polish LowerCambrian Kwartalnik Geologiczny 19 237ndash42

Lieberman B S 1998 Cladistic analysis of the Early Cambrianolenelloid trilobites Journal of Paleontology 72 59ndash78

Lu Y 1940 On the ontogeny and phylogeny of Redlichia intermediaLu (sp nov) Bulletin of the Geological Society of China 20333ndash42 [In Chinese]

Maddison W P amp Maddison D R 1997 MacClade analysis ofphylogeny and character evolution Version 307 Sunderland MASinauer Associates

Mittmann B amp Scholtz G 2003 Development of the nervous systemin the lsquoheadrsquo of Limulus polyphemus (Chelicerata Xiphosura)morphological evidence for the correspondence between the seg-ments of the chelicerae and the (first) antennae of MandibulataDevelopment Genes and Evolution 213 9ndash17

Moore R C (ed) 1959 Treatise on invertebrate paleontology Part OArthropoda 1 Boulder Colorado Geological Society of America

Muller K J amp Walossek D 1986 Arthropod larvae from the UpperCambrian of Sweden Transactions of the Royal Society ofEdinburgh Earth Sciences 77 157ndash79

Muller K J amp Walossek D 1987 Morphology ontogeny and lifehabit of Agnostus pisiformis from the Upper Cambrian of SwedenFossils and Strata 19 1ndash124

Munilla T 1999 Evolucion y filogenia de los Picnogonidos InMelicA DeHaro J J Medez M amp Ribera I (eds) VolumenMonografico Evolucion y Filogenia de Arthropoda Boletin de laSociedad Entolomologica Aragonesa 26 273ndash91 [In Spanish andEnglish]

Orr P J Siveter D J Briggs D E G Siveter D J amp Sutton M D2000 A new arthropod from the Silurian Konservat-Lagerstatteof Herefordshire Proceedings of the Royal Society of LondonSeries B 267 1497ndash504

Page R D M 1996 On consensus confidence and lsquototal evidencersquoCladistics 12 83ndash92

Palmer A R amp Repina L N 1993 Through a glass darklytaxonomy phylogeny and biostratigraphy of the OlenellinaUniversity of Kansas Paleontological Contributions New Series 31ndash35

Platnick N I Griswold C E amp Coddington J A 1991 On missingentries in cladistic analysis Cladistics 7 337ndash43

Pleijel F 1995 On character coding for phylogeny reconstructionCladistics 11 309ndash15

Raasch G O 1939 Cambrian Merostomata Special Papers of theGeological Society of America 19 1ndash146

Ramskold L Chen J Edgecombe G D amp Zhou G 1996Preservational folds simulating tergite junctions in tegopeltid andnaraoiid arthropods Lethaia 29 15ndash20

Ramskold L Chen J Edgecombe G D amp Zhou G 1997Cindarella and the arachnate clade Xandarellida (ArthropodaEarly Cambrian) from China Transactions of the Royal Society ofEdinburgh Earth Sciences 88 19ndash38

Ramskold L amp Edgecombe G D 1991 Trilobite monophylyrevisited Historical Biology 4 267ndash83

Ramskold L amp Edgecombe G D 1996 Trilobite appendagestructure ndash Eoredlichia revisited Alcheringa 20 269ndash76

Raw F 1957 Origin of chelicerates Journal of Paleontology 31139ndash193

Repina L N amp Okuneva O G 1969 Cambrian arthropods of themaritime territory Paleontological Journal 1969 95ndash103

Richter R amp Richter E 1929 Weinbergina opitz ng n sp einSchwerttrager (Merost Xiphos) aus dem Devon (Rheinland)Senkenbergiana 11 193ndash209

Robison R A 1984 New occurences of the unusual trilobite Naraoiafrom the Cambrian of Idaho and Utah University of KansasPaleontological Contributions Paper 112 1ndash8

Rominger C 1887 Description of primordial fossils from MtStephens NW Territory of Canada Proceedings of the Academyof Natural Sciences of Philadelphia 1887 12ndash19

Scholtz G 1997 Cleavage germ band formation and head segmen-tation the ground pattern of the Euarthropoda In Fortey R Aamp Thomas R H (eds) Arthropod Relationships 317ndash32 LondonChapman and Hall Systematics Association Special Volume 55

Schram F R 1978 Arthropods a convergent phenomenonFieldiana Geology 39 61ndash108

Schram F R 1993 Review of Boxshall Stromberg and Dahl TheCrustacea Origin and Evolution Journal of Crustacean Biology13 820ndash2

Schram F R Vonk R amp Hof C H J 1997 Mazon CreekCycloidea Journal of Paleontology 71 261ndash84

Schram F R amp Emerson M J 1991 Arthropod Pattern Theorya new approach to arthropod phylogeny Memoirs of theQueensland Museum 31 1ndash18

Schram F R amp Hof C H J 1998 Fossils and the interrelationshipsof major crustacean groups In Edgecombe G D (ed) Arthropodfossils and phylogeny pp 233ndash302 New York NY ColumbiaUniversity Press

Shergold J H 1991 Protaspis and early meraspis growth stages of theeodiscoid trilobite Pagetia ocellata Jell and their implications forclassification Alcheringa 15 65ndash86

Shu D Geyer G Ling C amp Zhang Z 1995 Redlichiacean trilobiteswith preserved soft parts from the Lower Cambrian Chengjiangfauna (South China) Beringeria (Special Issue) 2 203ndash41

Shultz J W 1990 Evolutionary morphology and phylogeny ofArachnida Cladistics 6 1ndash38

Shultz J W 2000 Skeletomuscular anatomy of the harvestmanLeiobunum aldrichi (Weed 1893) (Arachnida OpilionesPalpatores) and its evolutionary significance Zoological Journalof the Linnean Society 128 401ndash38

Shultz J W 2001 Gross muscular anatomy of Limulus polyphemus(Xiphosura Chelicerata) and its bearing on evolution in the theArachnida Journal of Arachnology 29 283ndash303

Siewing R (ed) 1985 Lehrbuch der Zoologie II Systematik 3rd ednStuttgart Fischer

Simonetta A M 1970 Studies on the non trilobite arthropods ofthe Burgess Shale (Middle Cambrian) the genera LeanchoiliaAlalcomenaeus Opabinia Burgessia Yohoia and ActaeusPalaeontographica Italica 66 35ndash45

Simonetta A M amp Delle Cave L 1975 The Cambrian non trilobitearthropods from the Burgess Shale of British Columbia A studyof their comparative morphology taxonomy and evolutionarysignificance Palaeontographica Italica 69 1ndash37

Simonetta A M amp Delle Cave L 1981 An essay in the compara-tive and evolutionary morphology of Palaeozoic ArthropodsAccademia Nazionale dei Lincei Atti dei Convegni Lincei 49389ndash439

Starobogatov Y I 1990 Sistema i filogeniya nizshikh khelitserovykh(analiz morfologii paleozoysikh grupp) [The systematics andphylogeny of the lower chelicerates (a morphological analysis ofthe Palaeozoic groups)] Paleontologicheskii Zhurnal 1990 4ndash17

Stoslashrmer L 1944 On the relationships and phylogeny of fossil andrecent Arachnomorpha Skrifter utgitt ac Det Norske Videnskaps-Akademi i Olso I Matematisk-naturvitenskaplig Klasse 5 1ndash158

Stoslashrmer L 1951 Studies on trilobite morphology Part III Theventral cephalic structures with remarks on the zoologicalposition of the trilobites Norsk geologisk tidsskrift 29 108ndash58

Stoslashrmer L 1956 A Lower Cambrian merostome from Sweden Arkivfor Zoologi Ser 2 9 507ndash14

Stoslashrmer L 1959 Trilobitoidea In Moore R C (ed) Treatise onInvertebrate Paleontology part O Arthropoda 1 23ndash37 BoulderColorado Geological Society of America

Strong E E amp Lipscomb D 1999 Character coding and inapplicabledata Cladistics 15 363ndash71

Sturmer W amp Bergstrom J 1976 The arthropods Mimetaster andVachonisia from the Devonian Hunsruck Shale PalaontologischeZeitschrift 50 78ndash111

Sturmer W amp Bergstrom J 1978 The arthropod Cheloniellon fromthe Devonian Hunsruck Shale Palaontologische Zeitschrift 5257ndash81

Sturmer W amp Bergstrom J 1981 Weinbergina a xiphosuran fromthe Devonian Hunsruck Slate Palaontologische Zeitschrift 55237ndash55

Sutton M D Briggs D E G Siveter D J Siveter D J amp Orr PJ 2002 The arthropod Offacolus kingi (Chelicerata) from theSilurian of Herefordshire England computer based morphologi-cal reconstructions and phylogenetic affinity Proceedings of theRoyal Society of London Series B 269 1195ndash1203

Swofford D L 1999 PAUP Phylogenetic Analysis Using Parsimony(and other methods) Version 40b2a Sunderland MA SinauerAssociates

Telford M J amp Thomas R H 1998 Expression of homeobox genesshows chelicerate arthropods retain their deutocerebral segmentProceedings of the National Academy of Sciences USA 9510671ndash5

192 TREVOR J COTTON AND SIMON J BRADDY

Thorley J L amp Page R D M 2000 RadCon phylogenetic treecomparison and consensus Bioinformatics 16 486ndash7

Valentine J W 1995 Why no new phyla after the Cambrian ndashgenome and ecospace hypotheses revisited Palaios 10 190ndash194

Wagele J W 1993 Rejection of the lsquoUniramiarsquo hypothesis andimplications of the Mandibulata concept Zoologische JahrbucherAbteilung fur Systematik Geographie und Biologie der Tiere 120253ndash88

Walcott C D 1911 Middle Cambrian Merostomata Cambriangeology and palaeontology II Smithsonian MiscellaneousCollections 57 17ndash40

Walcott C D 1912 Middle Cambrian Branchiopoda MalacostracaTrilobita and Merostomata Cambrian geology and palaeontol-ogy II Smithsonian Miscellaneous Collections 57 145ndash228

Walcott C D 1918 Geological explorations in the CanadianRockies Explorations and fieldwork of the SmithsonianInstitution in 1917 Smithsonian Miscellaneous Collections 684ndash20

Walossek D 1993 The Upper Cambrian Rehbachiella kinnekullensisand the phylogeny of Branchiopoda and Crustacea Fossils andStrata 32 1ndash202

Walossek D amp Dunlop J A 2002 A larval sea spider (ArthropodaPycnogonida) from the Upper Cambrian lsquoOrstenrsquo of Swedenand the phylogenetic position of pycnogonids Palaeontology 45421ndash46

Walossek D amp Muller K J 1990 Upper Cambrian stem-lineagecrustaceans and their bearing upon the monophyletic origin ofCrustacea and the position of Agnostus Lethaia 23 409ndash27

Walossek D amp Muller K J 1997 Cambrian lsquoOrstenrsquo-type arthro-pods and the phylogeny of Crustacea In Fortey R A amp ThomasR H (eds) Arthropod Relationships 139ndash53 SystematicsAssociation Special Volume 55 London Chapman and Hall

Walossek D amp Muller K J 1998 Early arthropod phylogeny in lightof the Cambrian lsquoOrstenrsquo fossils In Edgecombe G D (ed)Arthropod fossils and phylogeny 185ndash231 New York NYColumbia University Press

Weygoldt P 1979 Significance of later embryonic stages and headdevelopment in arthropod phylogeny In Gupta A P (ed)Arthropod Phylogeny 107ndash35 New York NY Van NostrandReinhold Co

Weygoldt P 1998 Evolution and systematics of the ChelicerataExperimental and Applied Acarology 22 63ndash79

Weygoldt P amp Paulus H F 1979 Unterzuchungen aur Morpholo-gie Taxonomie und Phylogenie der Chelicerata Zeitschriftfur Zoologische Systematik und Evolutionsforschung 17 85ndash116177ndash200

Wheeler W C Cartwright P amp Hayashi C Y 1993 Arthropodphylogeny a combined approach Cladistics 9 1ndash39

Wheeler W C amp Hayashi C Y 1998 The phylogeny of extantchelicerate orders Cladistics 14 173ndash92

Whittington H B 1971 Redescription of Marrella splendens(Trilobitoidea) from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 209 1ndash24

Whittington H B 1974 Yohoia Walcott and Plenocaris n genarthropods from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 231 1ndash21

Whittington H B 1975a Trilobites with appendages from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 497ndash136

Whittington H B 1975b The enigmatic animal Opabinia regalisMiddle Cambrian Burgess Shale British Columbia PhilosophicalTransactions of the Royal Society of London Series B 271 1ndash43

Whittington H B 1977 The Middle Cambrian trilobite NaraoiaBurgess Shale British Columbia Philosophical Transactions of theRoyal Society Series B 280 409ndash43

Whittington H B 1979 Early arthropods their appendages andrelationships In House M R (ed) The origin of majorinvertebrate groups 253ndash68 Systematics Association SpecialVolume 12 New York NY Academic Press

Whittington H B 1980 Exoskeleton moult stage appendage mor-phology and habits of the Middle Cambrian trilobite Olenoidesserratus Palaeontology 23 171ndash204

Whittington H B 1981 Rare arthropods from the Burgess ShaleMiddle Cambrian British Columbia Philosophical Transactionsof the Royal Society of London Series B 292 329ndash57

Whittington H B 1985 Tegopelte gigas a second soft-bodied trilo-bite from the Burgess Shale Middle Cambrian British ColumbiaJournal of Palaeontology 23 171ndash204

Whittington H B 1989 Olenelloid trilobites type species functionalmorphology and higher classification Philosophical Transactionsof the Royal Society of London Series B 324 111ndash47

Whittington H B amp Almond J E 1987 Appendages and habits ofthe Upper Ordovician trilobite Triarthus eatoni PhilosophicalTransactions of the Royal Society of London Series B 317 1ndash46

Wilkinson M 1992 Ordered versus unordered characters Cladistics8 375ndash85

Wilkinson M 1994 Common cladistic information and its consensusrepresentation reduced Adams and reduced cladistic consensustrees and profiles Systematic Biology 43 343ndash68

Wilkinson M 1995a Coping with abundant missing entries in phylo-genetic inference using parsimony Systematic Biology 44 501ndash14

Wilkinson M 1995b TAXEQ2 software and documentation BristolSchool of Biological Sciences University of Bristol

Wilkinson M 1996 On the distribution of homoplasy and choosingamong trees Taxon 45 263ndash66

Wills M A 1997 A phylogeny of Recent and fossil Crustacea derivedfrom morphological characters In Fortey R A amp ThomasR H (eds) Arthropod Relationships 189ndash209 SystematicsAssociation Special Volume 55 London Chapman and Hall

Wills M A Briggs D E G amp Fortey R A 1994 Disparity asan evolutionary index a comparison of Cambrian and Recentarthropods Paleobiology 20 93ndash130

Wills M A Briggs D E G Fortey R A amp Wilkinson M 1995The significance of fossils in understanding arthropod evolutionVerhandlungen der deutschen zoologischen Gesselschaft 88 203ndash15

Wills M A Briggs D E G Fortey R A Wilkinson M amp SneathP H A 1998a An arthropod phylogeny based on fossil andrecent taxa In Edgecombe G D (ed) Arthropod fossils andphylogeny 33ndash105 New York NY Columbia University Press

Wills M A Briggs D E G amp Fortey R A 1998b Evolutionarycorrelates of arthropod tagmosis scrambled legs In ForteyR A amp Thomas R H (eds) Arthropod Relationships 57ndash65Systematics Association Special Volume 55 London Chapmanand Hall

Winter G 1980 Beitrage zur Morphologie und Embryologie desvorderen Korperabschnitts (Cephalosoma) der PantopodaGerstaecker 1863 I Entstehung und Struktur des Zentralnerven-systems Zeitschrift fur Zoologische Systematik und Evolutionsfor-schung 18 27ndash61

Zhang W amp Hou X 1985 Preliminary notes on the occurrence of theunusual trilobite Naraoia in Asia Acta Palaeontologica Sinica 24591ndash5 [In Chinese with English summary]

TREVOR J COTTON Department of Palaeontology The Natural History Museum CromwellRoad London SW7 5BD UKe-mail trevor_cottonhotmailcom

SIMON J BRADDY Department of Earth Sciences University of Bristol Wills MemorialBuilding Queenrsquos Road Bristol BS8 1RJ UKe-mail S J Braddybrisacuk

MS received 28 July 2003 Accepted for publication 15 March 2004

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 193

Page 14: The phylogeny of arachnomorph arthropods and the origin of the

Figure 7 Ventral reconstructions of arachnomorph arthropod heads (A) Misszhouia longicaudata (after Chenet al 1997 figs 2ndash4 7) (B) Ceraurinella typa (after Chen et al 1997 fig 7) (C) Agnostus pisiformis (after Mulleramp Walossek 1987) (D) Kuamaia lata (after Edgecombe amp Ramskold 1999 figs 32 5 amp 6) (E) Cindarella eucala(after Ramskold et al 1997) and (F) Emeraldella brocki (after Bruton amp Whittington 1983) Not to scaleAbbreviations (aw) anterior wing beneath antennae (bs) boomerang-shaped sclerite (bl) blade-like process ofposterior wing (d) doublure of head-shield (cs) connective suture (f) fenestra (fs) facial suture (h) hypostome(hl) hypostomal lobe (emerging through fenestrae of Agnostus) (hs) hypostomal suture (if) intervening field ofhypostomal complex (lb) lateral bridge (lfo) lateral frontal organ (mb) median body (mbr) median bridge (mf)median furrow (mfb) mouth field bridge (mfo) median frontal organ (ol) ovate lobe and (phs) pre-hypostomalsclerite Post-antennal appendages and distal parts of antennae omitted

182 TREVOR J COTTON AND SIMON J BRADDY

1997) This structure is often sclerotised and covers the mouthventrally It is this sclerite that is here identified as thehypostome Therefore this character is considered absent intaxa lacking a sclerite covering the mouth irrespective of themorphology and position of the labrum itself which at leastpartly covers the mouth in all euarthropods other thanpycnogonids (see Edgecombe et al 2000 p 167) The lsquofleshylabrumrsquo of crustaceans does not appear to be sclerotised and ahypostome is consequently considered to be absent Althoughmany derived features are associated with the crustaceanlabrum this is not a good reason to consider it non-homologous with the hypostome-bearing structure of otherarthropods as Walossek amp Muller (1990) argued

In many taxa the mouth is covered by a posteromedianextension of the doublure here considered to represent thehypostome (eg Emeraldella Fig 7F Aglaspis see Hesselbo1992 fig 52) In others the hypostome is separated from thedoublure either by a suture a pre-hypostomal sclerite (egKuamaia lata Fig 7D Saperion glumaceum see Edgecombe ampRamskold 1999 figs 3 amp 4) or an intervening region ofunsclerotised cuticle (the natant condition of Fortey 1990beg Cindarella eucala Fig 7E) The homology of pre-hypostomal sclerites and hypostomes in helmetiids trilobitesand naraoiids (see Fig 7) has been discussed by Chen et al(1996) and Edgecombe amp Ramskold (1999) but it is as yetunclear whether any of the character states described here arederived from any other They are treated here as distinct andthe character as unordered pending further study

No hypostome has been identified in Yohoia (see above)Alalcomenaeus Jianfengia Fortiforceps or Leanchoilia andthere is certainly no broad posterior extension of the doublurein any of these taxa Since there is also no pre-hypostomalsclerite they have received a (134) partial uncertainty codingThe attachment of the hypostome of Tegopelte is unclear butit does not seem to be attached to any doublure (Whittington1985) and therefore it is given an uncertainty (12) coding

The lsquorostrum-like structurersquo on the ventral surface ofLemoneites appears to be similar in morphology and positionrelative to the anterior margin of the head shield (Flower 1968pl 8 figs 4 amp 13) to that described from Aglaspis and is codedas State 0 In many taxa the hypostome is unknown but inonly a small number can it be coded reliably as absentHughesrsquo (1975) suggestion that the hypostome of Burgessia isabsent is accepted preliminarily but an extension of thedoublure may be visible in some of Hughesrsquo figures Fortey(pers comm 2000) also doubts Hughesrsquo assertion

28 Visible ecdysial sutures (0) absent and (1) present Themarginal ecdysial sutures of chelicerates and the dorsal suturesof trilobites are almost certainly not homologous but thepresence of sutures and their position (see Character 29) arecoded separately to allow this to be tested Wills et al (1998aCharacter 2) coded marginal sutures as present in Sidneyiafollowing Brutonrsquos (1981) suggestion but this is consideredequivocal here

29 Position of ecdysial sutures (0) marginal and (1) dor-sal This character is inapplicable to taxa lacking ecdysialsutures (Character 28 State 0) Dorsal sutures whilst presentin the representatives of the Trilobita coded here are probablynot a synapomorphy of trilobites as a whole but for a clade oftrilobites excluding the Olenellida (Whittington 1989 Fortey1990a)

245 Exoskeletal tergites and thoracic tagmosis 30 Min-eralised cuticle (0) absent and (1) present Calcification of theexoskeleton is one of the most convincing synapomorphiesof the Trilobita (Fortey amp Whittington 1989 Ramskold ampEdgecombe 1991 Edgecombe amp Ramskold 1999 Character 1)Cuticle mineralisation is also coded as present in aglaspidids

following Briggs amp Fortey (1982) who suggested that theaglaspidid exoskeleton was originally phosphatic Paleomerusprobably also had a mineralised cuticle but Hou amp Bergstrom(1997 p 97) argued that it was likely to be calcareous Anundescribed Silurian aglaspidid may also have had an origi-nally calcitic cuticle (Fortey amp Theron 1994 p 856) Becauseof the uncertainty about the chemical composition of theexoskeleton in these forms cuticle mineralisation is coded aspotentially homologous in all these taxa and no attempt ismade to code separate states for phosphatic and calcareousmineralisation

31 Trunk tergites with expanded lateral pleurae coveringappendages dorsally (0) absent and (1) present Whilst thepresence of paratergal folds may be a synapomorphy at thelevel of the Euarthropoda (Boudreaux 1979 Wagele 1993Edgecombe et al 2000 Character 142) these are at mostsmall reflections of the margin of the tergites in most euarthro-pods In arachnomorph taxa these are expanded to form largelateral pleurae that cover the appendages dorsally (see egLimulus and Triarthus in Boudreaux 1979) This is inferred tobe the case in some taxa where dorsal features and appendagesare poorly known on the basis of their possession of tergiteswith wide pleural regions This feature was suggested as typicalof lamellipedians (=arachnomorphs) by Hou amp Bergstrom(1997 pp 42ndash3)

The arrangement of the thorax of Marrella Mimetaster andBurgessia differs from that of all the other taxa considered herein that the appendages seem to be attached to the lateralmargins of the body The trunk tergites do not cover theappendages and seem to lack pleurae entirely although theyare covered by a posterior extension of the head shield inBurgessia This character has been clearly described inMimetaster (Sturmer amp Bergstrom 1976 pp 87ndash90 fig 8) andBurgessia (Hughes 1975 p 421)

32 Free thoracic tergites (0) present and (1) absentFollowing Edgecombe amp Ramskold (1999 Character 16) anumber of taxa lack functional post-cephalic articulations andconsequently lack free thoracic tergites Despite this theycode these taxa for a number of characters relating to thestructure of thoracic tergites (eg their characters 18 and 19)These characters are treated here as inapplicable to taxawithout free tergites and the definition of this character hasbeen modified from that of Edgecombe amp Ramskold (1999) tomake this more explicit

33 Decoupling of thoracic tergites and segments (0)absent and (1) present Ramskold et al (1997) described thischaracter of Cindarella and Xandarella where the thoracictergites correspond to a variable increasing posteriorly num-ber of appendage pairs This character cannot be coded fortaxa in which free thoracic tergites are absent (Character 32State 1)

34 Tergite articulations (0) tergites non-overlapping (1)extensive overlap of tergites and (2) edge-to-edge pleuralarticulations In most of the taxa considered here the thoracictergites overlap considerably and relatively evenly over theirwidth This contrasts with the primitive euarthropod condi-tion where the tergites do not overlap medially as seen in stemgroup crustaceans In trilobites such as Helmetia and Kuamaia(see Edgecombe amp Ramskold 1999 character 18) overlap ofthoracic tergites is limited to a well-defined axial region andthe lateral pleurae meet edge-to-edge

The thoracic articulations of naraoiids with free thoracictergites are in some ways similar to those of trilobites with astrong overlap medially that is lacking abaxially andanterolateral articulating facets (Budd 1999a Ramskold ampEdgecombe 1999) However the distribution of these features

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 183

in other trilobite-like arachnomorphs is unclear and a restric-tive coding is used here The present authors do not considerthat this character can be applied reliably to the fused thoracictergites of Saperion Tegopelte and Skioldia and it is coded asinapplicable to all taxa lacking free thoracic tergites (Character32 State 1)

35 Trunk effacement (0) trunk with defined (separate orfused) tergite boundaries (1) trunk tergite boundaries effacedlaterally and (2) trunk tergite boundaries completely effaced(cf Edgecombe amp Ramskold 1999 Character 15)] Contrary toEdgecombe amp Ramskold (1999) the present authors considerSkioldia and Saperion to show a distinct form of tergiteeffacement where the fused tergite boundaries are defined byfurrows axially but effaced laterally The boundaries areeffaced at least laterally in Tegopelte but the axial region ofthe exoskeleton is unknown which accordingly is given amultistate uncertainty coding

36 Cephalic articulation fused (0) absent and (1) presentIn Tegopelte Saperion and Skioldia the articulation ofthe head with the thorax is non-functional and the entireexoskeleton forms a single tergite

37 Head shield overlap of thoracic tergites (0) overlapabsent or identical to overlap between thoracic segments (1)head shield covers first thoracic tergite only and (2) headshield covers mutliple anterior trunk tergites

38 Head shield articulates with reduced anterior thoracictergite (0) absent and (1) present Amongst taxa showing aposteriorly expanded head shield ie one that overlaps thethorax Edgecombe amp Ramskold (1999 Character 9) identifiedtwo distinct character states One of these ndash overlap of only theanteriormost thoracic tergite ndash was limited to taxa assigned tothe Liwiinae (sensu Fortey amp Theron 1994) and another ndashoverlap of multiple tergites with attachment to a reducedanterior thoracic tergite ndash to the Xandarellida None of theadditional taxa considered herein (including Buenaspis whichwas assigned to the Liwiinae by Budd 1999a) shows thesedistinctive morphological features However the expandedhead shields of Marrella Mimetaster and Burgessia which donot articulate with a narrow anterior thoracic tergite may behomologous with the expanded head shields of xandarellidsTo recognise this the degree of overlapping of the thorax andthe possession of the reduced anterior tergite are coded sepa-rately These characters are both coded as inapplicable to taxain which the head shield and thoracic tergites are fused(Character 36 State 1) The crustacean carapace which ismost similar to the head shield of Burgessia is seeminglyabsent in stem group crustaceans (Walossek amp Muller1998)

39 Trunk narrowed anteriorly relative to head shieldwidest posteriorly (0) absent and (1) present (cf Edgecombeamp Ramskold 1999 Character 14) The anterior narrowing ofthe trunk caused by the reduction of opisthosomal segment 1in xiphosurids (Weinbergina of the taxa analysed hereAndersen amp Selden 1997 Dunlop amp Selden 1997 Character14) is not considered to be homologous with the unusual shapeof the thorax in naraoiids recognised by their coding

40 Boundaries of anterior trunk segments reflexed antero-laterally (0) absent boundaries transverse or reflexed postero-laterally and (1) present

41 Joints between posterior tergites functional anteriorones variably fused (0) absent and (1) present

42 Posterior tergite bearing axial spine (0) absent and (1)present

Characters 40 to 42 are coded following Edgecombe ampRamskold (1999 characters 17 19 amp 23 respectively) Inaddition to the taxa considered here a thoracic axial spine ispresent in some olenelloid trilobites (see Lieberman 1998

Characters 73 amp 74) and in the aglaspidid Beckwithia (seeHesselbo 1989)

246 Posterior body termination 43 Postabdomen of seg-ments lacking appendages (0) absent and (1) present

44 Length of postabdomen (0) one segment (1) twosegments (2) three segments and (3) five segments In sometaxa a variable number of posterior thoracic segments bearcomplete tubular tergites and lack appendages In Aglaspisautapomorphically it seems that the posterior three thoracicsegments lack appendages (Briggs et al 1979 Hesselbo 1992)but have unmodified tergites These situations are recognisedas potentially homologous by the coding used here Character44 is coded as inapplicable to taxa lacking a postabdomen

45 Posterior tergites strongly curved in dorsal aspect com-pared to anterior tergites (0) absent and (1) present Asrecognised by Wills et al (1998a Character 17) in some taxawhere the tergites are distinct the curvature of thoracic tergitesincreases posteriorly so that the posterior tergites are highlycurved to semicircular in dorsal aspect This situation isnot known from crustaceans (except isopods) or stem grouparthropods

46 Posterior segments reduced and with highly reducedappendages (0) present and (1) absent In some taxa there area large number of posterior segments that are short sagitallyand have appendages that are much reduced in size comparedto anterior trunk appendages These somites are incorporatedinto the pygidium in some taxa but primitively they arecovered by tiny free trunk tergites In the derived state thetrunk somites and limbs are of a relatively constant size Thedistinction between these states can be seen when attempting tocount the number of segments that make up the body In taxashowing State 0 the number of segments is very high anddifficult to count whereas in other taxa the number ofpost-cephalic segments is easily assessed

47 Pygidium (0) absent and (1) present A pygidium isrecognised here as a posterior tagma consisting of a number offused segments under a single tergite which may or may notincorporate the post-segmental telson This is different to theuse of this term by Edgecombe amp Ramskold (1999) whichfollows Ramskold et al (1997) and very different to its use byWills et al (1998a p 53) as a synapomorphy of the TrilobitaThe present authorsrsquo definition recognises the situation inRetifacies where the spinose postsegmental telson is autapo-morphically not fused to the pygidium as homologous toother multisegmented posterior tagma which include thetelson The distinction between the pygidium and an expandedpost-segmental telson can also be seen in the position of theanus which is consistently in the posteriormost pre-telsonicsegment amongst putative arachnomorphs (see Character 48)In taxa with a pygidium the anal segment is fused into thepygidium (see eg Kuamaia Edgecombe amp Ramskold 1999fig 6) whereas in those taxa lacking a pygidium the anus isbetween the final trunk tergite and the telson

Ramskold et al (1997) argued that the posteriormost tergiteof xandarellids (which incorporates the anal segment) is nothomologous to the posterior tagma recognised as pygidiaherein because posterior thoracic tergites also cover multiplesegments in xandarellids (see Character 33) Evidence ofsegmentation of the pygidium in a variety of taxa suggests thatthe number of tergites fused to form the pygidium is greaterthan the number of appendages For example the pygidium ofKuamaia has two pairs of lateral spines but at least fourpairs of appendages (Hou amp Bergstrom 1997 Edgecombe ampRamskold 1999) and that of Triarthrus has only five axialrings posterior to the articulating ring but over 10 pairs ofappendages (Whittington amp Almond 1987) The fact thatdecoupling of tergites and segments is evident in the thorax of

184 TREVOR J COTTON AND SIMON J BRADDY

Xandarella and Cindarella does not necessarily suggest that asimilar more widespread decoupling in pygidia is the result ofa different developmental process and hence that they arenon-homologous Instead the situation in the xandarellidthorax is potentially homologous to that found (more widely)in pygidia and consequently the terminal xandarellid tergite isa pygidium It is unclear if decoupling of tergites and somites isprimitively limited to the terminal tergite or primitively aproperty of the arachnomorph post-cephalon as a whole

It has been suggested that the tiny pygidium of olenelloidtrilobites which probably best reflects the primitive trilobitestate consists only of the post-segmental telson (Harrington inMoore 1959) and therefore is not a true pygidium HoweverWhittington (1989) showed that the olenelloid pygidium con-sists of at least two and possibly as many as five or sixsegments and therefore is likely to be homologous with thepygidium of other trilobites

48 Position of the anus (0) terminal within telson and (1)at base of telson In crustaceans and stem group arthropodsthe anus is terminal or otherwise situated in the telson (egRehbachiella Walossek 1993 fig 15D) In putative arachno-morphs the anus is either at the junction of the posteriormostthoracic segment and the telson or ventral within a fusedpygidium In the case of taxa with a pygidium the anus isanterior to the posterior margin and where known positionedbetween the posteriormost pair of appendages suggesting thatit is anterior to the post-segmental telson (see eg OlenoidesWhittington 1980)

49 Pygidium with median keel (0) absent and (1) presentEdgecombe amp Ramskold (1999 Character 21) considered thepresence of a median keel on the pygidium as a synapomorphyuniting Soomaspis and Tarricoia They did not explain howthis structure could be distinguished from the raised pygidialaxis of trilobites Homology of these structures is not sup-ported here because the axis of more primitive trilobites(including Eoredlichia) is quite distinct morphologically fromthe naraoiid keel Budd (1999a) noted the presence of a keel onthe pygidium of Buenaspis and suggested (Budd 1999a p 102)that it may be an artefact of dorsoventral compaction How-ever contrary to the coding of Edgecombe amp Ramskold(1999) a keel is visible on the pygidium of Liwia convexa (Dzikamp Lendzion 1988 fig 4CD) preserved in full relief Thereforeit seems unlikely that the keel is a taphonomic artefact

50 Pygidium with broad-based median spine (0) absentand (1) present

51 Pygidium with lateral spines (0) present and (1) absentEdgecombe amp Ramskold (1999 Character 22) coded thepresence of a median spine and two pairs of lateral spines aspotentially homologous in Sinoburius Kuamaia and HelmetiaThe distribution of lateral spines is much wider than that ofterminal spines and therefore they are coded separately hereIn trilobites it has widely been recognised that lateral spinesrepresent the original segmentation of the pygidium Thiscannot be the case for median spines Characters 49 to 51are not applicable to taxa lacking a pygidium (Character 47State 0)

52 Expanded post-segmental telson (0) absent and (1)present In a range of taxa the posterior-most tergite is a large(relative to the thoracic tergites) structure that lacks anyevidence of segmentation and is interpreted as representing anexpanded tergite of the post-segmental telson from whichsegments are released anteriorly during ontogeny On the otherhand the posteriormost tergite of Marrella and probablyMimetaster is small compared to the thoracic tergites androunded This element probably represents the plesiomorphiceuarthropod telson The homology of this character in mosttaxa is clear and only doubtful in Retifacies in which it may

be segmented The figures of Hou amp Bergstrom (1997) providelittle unequivocal evidence for a telson in Retifacies but thepresence of this structure is very clear in colour photographs(eg Chen et al 1996 fig 198A) However these figures donot convincingly demonstrate the segmentation of the telsonRetifacies is interpreted here as being unique in possessingboth a pygidium and an expanded post-segmental telson

53 Telson shape (0) spinose and (1) paddle-shaped Theshape of the expanded telsons identified above varies frombroadly spinose to flattened and paddle-like This differencecan be recognised both on the basis of the ratio of length tobasal width (spinose telsons are relatively long) and by thechange in width posteriorly (spinose telsons reduce in widthposteriorly whereas paddle-shaped telsons increase in width)In eurypterids a wide range of telson shapes is found includ-ing both character states described here It is likely that theplesiomorphic condition is a spinose telson This character isinapplicable to taxa lacking an expanded telson

54 Post-ventral furcae (0) absent and (1) present Thischaracter recognises the potential homology of unsegmentedpaired structures that articulate with the segment immediatelyanterior to the telson The potential homology of these struc-tures (the post-ventral plates) in Emeraldella and Aglaspis wasrecognised by Wills et al (1998a Character 70) and inEmeraldella and Sidneyia by Edgecombe amp Ramskold (1999Character 25) The homology of the segmented pygidial caudalfurcae of Olenoides (see Whittington 1975a 1980) with thesestructures is considered doubtful and coded as equivocal

Despite their distinctive morphology the long caudualfurcae of Cheloniellon may be homologous with the furcae ofAglaspis Emeraldella and Sidneyia This is supported by thecaudal furcae of the Ordovician cheloniellid Duslia which aremore similar in morphology to those of Emeraldella than ofCheloniellon Chlupac (1988 pp 614ndash16) considered thesestructures to be on the terminal tergite in Duslia However afaint tergite boundary is apparent posterior to the attachmentof the furcae (see Chlupac 1988 pl 57 fig 4) Therefore theyare considered here to have been attached to the pre-telsonicsegment as in Cheloniellon Chlupac (1988) and Dunlop ampSelden (1997) supported a close relationship between Dusliaand Cheloniellon

25 MethodsAll analyses were carried out using PAUP Version 40b4a(Swofford 1999) and unless otherwise stated used heuristicsearches with one thousand random addition sequencereplicates The software packages MacClade Version 307(Maddison amp Maddison 1997) and RadCon (Thorley amp Page2000) were used for comparing trees and investigating patternsof character evolution Tree length and other statistics (theConsistency Index CI and Retention Index RI) were calcu-lated by PAUP and MacClade with uninformative charactersexcluded Analyses were run separately using each of thedifferent codings for aglaspidids

Characters were treated as unordered and of equal weight inmost analyses To assess the influence of this assumption fourof the eight multistate characters which have states that areintermediate between others (Wilkinson 1992) were treated asordered in some analyses These characters are the number ofcephalic segments (Character 4) the length of raptorialappendage spines (Character 6) the degree of overlap of thethorax by the head-shield (Character 37) and the number ofsegments making up the postabdomen (Character 44) The lastof these is uninformative when not ordered because only onetaxon shows each of states 0 1 and 3

Support for individual nodes was assessed by bootstrapanalysis (Felsenstein 1985) and by calculating Bremer support

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 185

indices (Bremer 1988 1994) These methods measure twodistinct aspects of support for phylogenetic hypotheses Treesor nodes may be considered well supported (1) to the extent towhich alternative topologies are much less parsimonious asmeasured by the support index (Wilkinson 1996) or (2) wherethey are consistent with a large proportion of characters sothat character sampling is unlikely to have had much influenceon topology as assessed by bootstrapping (Page 1996) Boot-strapping was performed with 100 bootstrap replicates eachof 10 addition sequence replicates Bremer support indicesare based on heuristic searches with 100 addition sequencereplicates

Three pairs of terminals are included in Table 2 that apartfrom missing data are coded identically These are Helmetiaand Kuamaia Soomaspis and Tariccoia and Saperion andSkioldia These pairs are lsquotaxonomic equivalentsrsquo removingthe taxon with most missing data will not influence theposition of the other They can be recoded as single terminalsaccording to the principal of safe taxonomic reduction(Wilkinson 1995a b) and were not considered separately in theanalyses presented here Therefore cladistic analyses werecarried out on the basis of 33 terminals Edgecombe ampRamskold (1999) ignored this and coded the same three pairsof taxa and Eoredlichia and Olenoides identically for theirmore limited set of characters

26 ResultsAnalysis with all characters unordered and using the lsquoAglaspi-dida 1rsquo coding found nine most parsimonious trees (MPTs)each 126 steps long (CI=0middot556 RI=0middot763) Using the lsquoAglas-pidida 2rsquo coding resulted in 18 MPTs of length 125 (CI=0middot560RI=0middot765) Nine of these 18 trees were those found with thefirst coding of aglaspidids The strict component consensus(sensu Wilkinson 1994) of the 27 unique trees found acrossboth analyses is shown in Figure 8 All trees supported themonophyly of the Marrellomorpha (Marrella and Mimetaster)and of a clade including all other taxa except Crustacea (Clade1 of Fig 8) The relationships between these two clades and theCrustacea were unresolved equal numbers of trees in bothanalyses supported each of the topologies [Clade 1 (CrustaceaMarrellomorpha)] [Crustacea (Marrellomorpha Clade 1)]and [Marrellomorpha (Crustacea Clade 1)] All trees placedBurgessia as the sister taxon to the rest of Clade 1 andsupported the division of the rest of Clade 1 into two majorsister groups The topology of one of these clades (Clade 2)which included trilobites helmetiids tegopeltids naraoiidsxandarellids and Retifacies was stable with respect to thecoding of aglaspidids The other clade (Clade 3) consists of aclade of megacheirans and chelicerates (Clade 4) and a cladeincluding aglaspidids Lemoneites Paleomerus CheloniellonEmeraldella and Sidneyia (Clade 5) This latter clade is un-resolved in the strict consensus of all 27 trees (Fig 8) and thestrict consensus of the 18 MPTs found using the lsquoAglaspidida2rsquo coding but was fully resolved in the consensus of the ninetrees found with the lsquoAglaspidida 1rsquo coding Within themegacheiran-chelicerate clade all trees found Fortiforceps tobe the sister group to all other taxa Yohoia the sister group tochelicerates and Alalcomenaeus and Leanchoilia to form aclade Jianfengia is equally parsimoniously placed as the sistergroup to the Yohoia-chelicerate clade or as the sister group tothis clade and the Alalcomenaeus-Leanchoilia clade

Treating characters 4 6 37 and 44 as ordered had a minimaleffect on most parsimonious topology Nine MPTs of 136 steps(CI=0middot515 RI=0middot743) were found with the lsquoAglaspidida 1rsquocoding and 12 MPTs 135 steps in length (CI=0middot519 RI=0middot745)with the lsquoAglaspidida 2rsquo coding These trees all supported a(Crustacea Clade 1) clade excluding the marrellomorphs

Apart from this the strict consensus of these 21 trees isidentical to the consensus of the 27 trees found with allcharacters treated as unordered

Across all analyses four different topologies for Clade 5 (ofFig 8) were found (Fig 9) The distribution of these topologiesacross the four analyses described above and across the sameanalyses repeated after reweighting of each character in pro-portion to their Rescaled Consistency Index (see Farris 1989)from the first analysis is shown in Table 3 The instability of

Figure 8 Strict consensus of 27 MPTs from four separate analyses ofarachnomorph phylogeny with both interpretations of aglaspididmorphology and with some characters treated as ordered or allcharacters unordered Clades referred to in the text are numbered

Figure 9 Alternative equally parsimonious resolutions of clade 5 ofFig 8 (see Table 3)

186 TREVOR J COTTON AND SIMON J BRADDY

this clade was to some extent caused by the inclusion ofLemoneites and Paleomerus which could each be coded foronly 27 of the 54 characters used Analysis with these two taxaexcluded and characters treated as unordered found treescompatible only with Topology D of Figure 9 irrespective ofthe coding used for aglaspidids Analysis with some characterstreated as ordered and using either aglaspidid coding foundthree trees compatible only with Topology B of Figure 9 andthree compatible only with Topology D of Figure 9 ThereforeTopology D is the preferred topology for Clade 5 as shown inTable 3

One of the MPTs resulting from the lsquoAglaspidida 2rsquo coding(with all characters treated as unordered) was chosen as thebasis for further discussion because this tree combined thepreferred topology of Clade 5 with the (Crustacea Clade 1)group that was favoured when some characters were treated asordered Bootstrap percentages and Bremer support values forthis tree (using the Aglaspidida 2 coding) are shown in Figure10 Apomorphies for all ingroup nodes of this tree are shownin Figure 10A

3 Discussion

31 Phylogenetic resultsThe results of the cladistic analyses presented above arewell-resolved and robust with respect to different analyticalparameters Most of the taxa considered form a clade (Clade 1of Fig 8) that is more closely related to chelicerates than tocrustaceans in all analyses and therefore is recognised asthe Arachnomorpha as defined by Chen et al (1996) andRamskold et al (1996) The monophyly of the Marrellomor-pha was supported in all analyses The marrellomorphs werefound to be the sister group to a Crustacea+Clade 1 group inmost MPTs but in a minority of trees formed the sister groupto Clade 1 alone According to the latter result they should beincluded within the Arachnomorpha but according to the firstexcluded from it Both of these alternatives have been sup-ported in previous studies (eg Hou amp Bergstrom 1997 Willset al 1998a respectively) In addition to the marrellomorphsthe Fuxianhuiida Bousfield 1995 and Canadaspidida havebeen placed in the Euarthropoda outside the main mandibu-late and arachnomorph clades (Hou amp Bergstrom 1997)Further study of these taxa of the crustacean or mandibulatestem group and of the marrellomorphs is necessary before the

position of the Marrellomorpha can be resolved Provisionallymarrellomorphs are excluded from the Arachnomorphafollowing the majority of analyses above and Wills et al(1994)

The Burgess Shale arthropod Burgessia was placed as thesister group to remaining arachnomorphs in all analyses OtherArachnomorpha form two major clades one consisting of thelsquotrilobite-alliedrsquo arachnomorphs analysed by Edgecombe ampRamskold (1999) here termed lsquoTrilobitomorpharsquo (defined asTrilobita and all taxa more closely related to Trilobita thanChelicerata) and the other (lsquochelicerate-alliedrsquo) clade includingchelicerates lsquogreat appendagersquo arthropods aglaspididsLemoneites Paleomerus Sidneyia and Emeraldella The latteris here termed lsquoCheliceramorpharsquo (defined as Chelicerata andall taxa more closely related to Chelicerata than Trilobita) Thetopology of the lsquotrilobite-alliedrsquo clade was similar to that foundby Edgecombe amp Ramskold (1999) but the present authorsrsquoresults are more completely resolved unambiguously support-ing the sister-group relationship between trilobites andHelmetiida suggested in the previous study Within thelsquochelicerate-alliedrsquo clade Aglaspidids Paleomerus Lemoneitesand the Burgess Shale Emeraldella and Sidneyia form a cladein opposition to megacheirans and chelicerates

The present authorsrsquo analysis supports a new hypothesis ofthe origin of the chelicerates from within a paraphyleticassemblage of megacheiran arthropods and confirms pyc-nogonids as a sister group to the euchelicerates Previoushypotheses of the chelicerate sister group (see Section 1) aremuch less parsimonious The shortest trees supporting aCheloniellon-chelicerate clade are six steps longer than theMPTs and those supporting an aglaspidid-chelicerate cladeseven steps longer All megacheirans and chelicerates areunited by the flap-shaped rounded exopods and modificationof the second cephalic segment endopods into anteriorlydirected spinose raptorial organs which share a number ofdetailed similarities A clade including all of these taxa apartfrom Fortiforceps is supported by the synapomorphic loss ofthe antennae (or antennulae of crustaceans) and the longerlength of spinose projections of the second cephalic appendageendopods Within this clade Yohoia shares the loss of cephalicexopods the loss of thoracic endopods and a postabdomen oftubular tergites with chelicerates

The poor resolution of Clade 5 (of Fig 8) was partly a resultof the poor preservation of and consequent high proportion ofmissing data for Lemoneites and Paleomerus When these two

Table 3 Numbers of most parsimonious trees supporting each of the alternative topologies for clade 4 of Fig 7 under different analyticalconditions (RCI=Revealed Consistency Index)

Aglaspididcoding

Characters 4 6 36and 43 ordered

Charactersreweighted by RCI

Paleomerus andLemoneites excluded

Topology (see Fig 8)

A B C D

1 91 X 3 3 31 X 61 X X 31 X 61 X X 3 32 9 92 X 3 3 3 32 X 62 X X 32 X 62 X X 3 3Total 6 18 12 54

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 187

taxa were excluded from the analyses only two (B and D) ofthe four topologies found in the complete analysis remainedmost parsimonious (see Fig 9 and Table 3) Secondly Bremersupport values throughout much of the lsquochelicerate-alliedrsquoclade (Clade 3 of Fig 8) were improved when these taxa wereexcluded (see Fig 11)

Dunlop (1999 p 258) suggested that in reconstructing thechelicerate stem lineage lsquowe might predict that the two mostsignificant changes towards the chelicerate condition are thereduction of the antennae and the formation of the nextappendages into a clawrsquo According to the hypothesis pre-sented here both of these adaptations were achieved in EarlyCambrian chelicerate ancestors The recognition of the loss ofthe antennae in the stem group of the chelicerates and thephylogenetic hypothesis presented here suggest that chelicer-ates are most closely related to taxa with only a singleadditional segment fused to the crown group eucheliceratehead of four fused somites Previous hypotheses have derivedchelicerates from Palaeozoic arthropods with a longer head(eg Cheloniellon Sturmer amp Bergstrom 1978 SanctacarisBriggs amp Collins 1988 Emeraldella Bruton amp Whittington1983) The plesiomorphic pattern of head segmentation ismatched in pycnogonids Bergstrom et al (1980) suggestedthat only the anterior four pairs of appendages are incorpo-rated into the head This supports the view that pycnogonidsare primitive with respect to other chelicerates which showmore or less complete fusion of this primitive head and threethoracic segments including Solpugida and Pseudoscorpiones

The present study suggests that a shift from gnathobasicbenthic feeding to a more vagrant lifestyle and raptorialappendage feeding occurred very early in the chelicerate lin-eage Appendage feeding was clearly an important adaptation

Figure 10 One of 18 equally parsimonious cladograms with lsquoAglaspidida 2rsquo coding and all characters unorderedthat is most congruent with results from other analyses Non-terminal branch lengths scaled to reflect number ofapomorphies (A) ACCTRAN apomorphy scheme Character numbers shown above boxes and states shownbelow boxes as in Table 2 and the text Character consistency indices are indicated by shading (B) Levels ofsupport for individual nodes Bremer support indices shown in bold type above branches Bootstrap percentagesin italic type below branches for all nodes with relative frequencies greater than 50

Figure 11 Bremer support values for clades within the lsquochelicerate-alliedrsquo arachnomorphs (clade 3 of Fig 8) when all taxa wereincluded (normal weight font above nodes) and with Lemoneites andPaleomerus excluded (bold type below nodes)

188 TREVOR J COTTON AND SIMON J BRADDY

for the terrestrialization of chelicerates (Dunlop 1997 p 69Dunlop amp Webster 1999) but the model proposed heresuggests that this considerably pre-dated terrestrializationGnathobasic feeding in xiphosurans and eurypterids mayrepresent a reversal to a more primitve benthic arachnomorphlifestyle from a more pycnogonid-like ancestor

32 Evolution of the arachnomorph headThe extant arthropod classes show highly conserved patternsof head segmentation (eg Wills et al 1994) In contrast therehas been a near consensus amongst Cambrian arthropodworkers (although rarely based on an explicit phylogenetichypothesis) that patterns of head segmentation are highlyhomoplastic and hence of little systematic significanceSturmer amp Bergstrom (1978 pp 78ndash9) suggested that lsquoevenclosely related forms may have different numbers of headsegments and appendagesrsquo Bruton amp Whittington (1983pp 576ndash7) that lsquodiscussion on fossil arthropod relationshipsbased on head segmentation appears to be largely irrel-evant and at best speculativersquo and Delle Cave amp Simonetta(1991 p 191) that lsquothere are no obvious phyletic affinitiesbetween genera having the same number of cephalized seg-mentsrsquo Most recently Bergstrom amp Hou (1997 p 104) con-cluded that lsquothe segmental length of the head shield seemsto be of no relevance to the discussion and discrimination ofevolutionary lineagesrsquo It has been claimed that this view issupported by cladistic analyses in which patterns of tagmosishave been found to be rather poor at defining major arthropodclades (Briggs et al 1992 Wills et al 1994)

This suggestion is not only of relevance to systematics buthas featured prominently in recent debates surrounding thenature of the lsquoCambrian explosionrsquo (see Budd amp Jensen 2000for a recent review) Gould (1989 1991) argued that theapparent plasticity of head segmentation in Cambrian com-pared to post-Cambrian arthropod evolution suggests thatbody-plan evolution was only constrained after the Cambrianexplosion According to this view the origin of new highertaxa with distinct body-plans was only possible before theonset of these constraints (eg Valentine 1995) Taxa hererecognised as arachnomorphs clearly played a major role in thedevelopment of Gouldrsquos hypothesis This suggestion has pre-viously been assessed in terms of overall morphological diver-sity which is clearly rather distinct from Gouldrsquos concept ofdisparity Wills et al (1998b) showed that there was littlecorrelation between overall morphological disparity and de-gree of limb specialisation and that the latter was phylogeneti-cally highly plastic They remarked that (Wills et al 1998bp 64) lsquothis finding has important implications for models ofarthropod phylogeny and evolution that attribute overidingimportance to head segmentationrsquo but did not explicitlyexamine head segmentation although it is likely to have beena major component of the index of tagmosis employed

The results of the present study suggest that arguments thatarthropod head segmentation was unusually labile during theCambrian are poorly founded Rather only four major pat-terns of euarthropod head segmentation are identified Theplesiomorphic euarthropod state according to both Walossekamp Muller (1997 1998) and Scholtz (1997) which may moreproperly be the plesiomorphic state for the euarthropod crowngroup only (depending on the phylogenetic position of theMarrellomorpha) consists of four post-acronal segmentsbearing the antennae and three pairs of biramous limbs Thepresent authors suggest that the term lsquocephalonrsquo be restrictedto this kind of head which is found in stem group mandibu-lates in addition to many arachnomorphs Crown groupmandibulates share a lsquobimaxillary headrsquo in which an addi-tional pair of appendages the labium or second maxillae are

incorporated into the head but in crustaceans these are notfused to the carapace (Scholtz 1997) Finally two distinctforms of head tagmosis are found in chelicerates A head withfour pairs of appendages (but without antennae giving a totalof five segments incorporated into the head) which has beencalled the lsquocephalosomarsquo is present in pycnogonids (Walossekamp Dunlop 2002 pp 434ndash5) and probably also in some euch-elicerates (J A Dunlop pers comm 2003) The lsquoprosomarsquo ofmost euchelicerates consists of this cephalosoma and twoadditional segments

In general these patterns of head segmentation are highlyphylogenetically conserved (see Fig 12) Only three homo-plastic changes namely reversal to a more primitive conditionin the Leanchoilia-Alalcomenaeus clade and the convergentorigin of a five-segmented head in Clade 3 (of Fig 8) and theXandarellida are necessary to optimise character 4 onto themost parsimonious cladogram There are also transitions toautapomorphic states in Emeraldella and Sidneyia In the caseof Sidneyia however an argument could be made (followingBergstrom amp Sturmer 1978 see discussion of Character 4) thatthe head consists of five segments as in closely related taxabut autapomorphically each segment has a separate tergiteThis would further increase the phylogenetic stability of headsegmentation Also following Bergstrom amp Sturmer (1978) thehead of Cheloniellon could be considered to consist of twotergites making it more similar to the head of Emeraldella(with six pairs of appendages)

Figure 12 Cladogram from Fig 10 showing changes in head segmen-tation Bold type shows abbreviated head segmentation formulaAbbreviations (Ant) antennae (BA) biramous appendage pairs (Ch)chelicerae (GA) great appendages (UA) uniramous appendage pairsand (X) segment without appendages Synapomorphy A is the auta-pomorphic reduction in head length to a single segment in SidneyiaAlternatively this may represent secondary division of the headshield Synapomorphy B is the increase in the length of the head inEmeraldella to six segments

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 189

4 Acknowledgements

We thank D E G Briggs J A Cotton J A Dunlop R AFortey S Harris R A Moore P Selden O E Tetlie andP Van Roy for comments on earlier drafts of this work Themanuscript was reviewed by M Wills F Schram and oneanonymous referee This research was supported by the Geo-logical Society of London Curry Research Studentshipawarded to TJC by the Department of Earth Sciences Uni-versity of Bristol and by the Department of PalaeontologyThe Natural History Museum London TJC would like tothank C Labandeira J Thompson and R Chapman fortheir hospitality at the National Museum of Natural HistoryWashington DC USA and A Olcott S Schellenberg andJ Young for making his time there so enjoyable

5 References

Abzhanov A Popadic A amp Kaufman T C 1999 Chelicerate Hoxgenes and the homology of arthropod segments Evolution andDevelopment 1 77ndash89

Akam M 2000 Arthropods developmental diversity within a (super)phylum Proceedings of the National Academy of Sciences USA97 4438ndash41

Aldridge R J Gabbott S E amp Theron J N 2001 The Soom ShaleIn Briggs D E G amp Crowther P R (eds) Palaeobiology II340ndash2 Oxford Blackwell Science

Anderson L I amp Selden P A 1997 Opisthosomal fusion andphylogeny of Palaeozoic Xiphosura Lethaia 30 19ndash31

Arango C P 2002 Morphological phylogenetics of the sea spiders(Arthropoda Pycnogonida) Organisms Diversity and Evolution2 107ndash25

Ax P 1986 Das phylogenetische System Stuttgart Gustav FischerVerlag

Babcock L E amp Zhang Wentang In press New non-mineralizedarthropods from the Chengjiang Lagerstatte Lower CambrianYunnan China Palaeontologia Cathyana

Bartels C Briggs D E G amp Brassel G 1998 The fossils ofthe Hunsruck Slate Marine life in the Devonian CambridgeCambridge University Press

Bergstrom J 1971 Paleomerus ndash merostome or merostomoid Lethaia4 393ndash401

Bergstrom J 1979 Morphology of fossil arthropods as a guideto phylogenetic relationships In Gupta A P (ed) ArthropodPhylogeny 3ndash56 New York NY Van Nostrand Reinhold Co

Bergstrom J 1992 The oldest arthropods and the origin of theCrustacea Acta Zoologica 73 287ndash91

Bergstrom J Sturmer W amp Winter G 1980 PalaeoisopusPalaeopantopodus and Palaeothea pycnogonid arthropodsfrom the Lower Devonian Hunsruck Slate West GermanyPalaontologische Zeitschrift 54 7ndash54

Bergstrom J amp Brassel G 1984 Legs in the trilobite Rhenops fromthe Lower Devonian Hunsruck Slate Lethaia 17 67ndash72

Bergstrom J amp Hou X 1998 Chengjiang arthropods and theirbearing on early arthropod evolution In Edgecombe G D(ed) Arthropod fossils and phylogeny 151ndash84 New York NYColumbia University Press

Boudreaux H B 1979 Significance of intersegmental tendon systemin arthropod phylogeny and a monophyletic classification ofArthropoda In Gupta A P (ed) Arthropod Phylogeny 551ndash86New York NY Van Nostrand Reinhold Co

Bousfield E L 1995 A contribution to the natural classification ofLower and Middle Cambrian arthropods food-gathering andfeeding mechanisms Amphipacifica 2 3ndash33

Braddy S J Aldridge R J Gabbott S E amp Theron J N 1999Lamellate book-gills in a late Ordovician eurypterid from theSoom Shale South Africa support for a eurypterid-scorpionclade Lethaia 32 72ndash4

Braddy S J amp Almond J E 1999 Eurypterid trackways from theTable Mountain Group (Ordovician) of South Africa Journal ofAfrican Earth Sciences 29 165ndash77

Bremer K 1988 The limits of amino acid sequence data in angio-sperm phylogenetic reconstruction Evolution 42 795ndash803

Bremer K 1994 Branch support and tree stability Cladistics 10295ndash304

Briggs D E G 1998 Review of Arthropods from the LowerCambrian Chengjiang fauna southwest China by HouXianguang and Jan Bergstrom Fossils and Strata 45

Scandinavian University Press Oslo Norway 1997 Palaeo-geography Palaeoclimatology Palaeoecology 143 193ndash4

Briggs D E G Bruton D L amp Whittington H B 1979 Append-ages of the arthropod Aglaspis spinifer (Upper CambrianWisconsin) and their significance Palaeontology 22 167ndash80

Briggs D E G Fortey R A amp Wills M A 1992 Morphologicaldisparity in the Cambrian Science 256 1670ndash3

Briggs D E G Fortey R A amp Wills MA 1993 How big was theCambrian explosion A taxonomic and morphologic comparisonof Cambrian and Recent arthropods In Lees D R amp EdwardsD (eds) Evolutionary patterns and processes 33ndash44 LondonLinnean Society of London Linnean Society Symposium Series

Briggs D E G Erwin D H amp Collier F J 1994 The fossils of theBurgess Shale Washington DC Smithsonian Institution Press

Briggs D E G amp Bartels C 2001 New arthropods from the LowerDevonian Hunsruck Slate (Lower Emsian Rhenish Massifwestern Germany) Palaeontology 44 275ndash304

Briggs D E G amp Bartels C 2002 Magnoculocaris a new name forthe arthropod Magnoculus Briggs and Bartels 2001 from theLower Devonian Hunsruck Slate Germany Palaeontology 45419

Briggs D E G amp Collins D 1988 A Middle Cambrian cheliceratefrom Mount Stephen British Columbia Palaeontology 31 779ndash98

Briggs D E G amp Collins D 1999 The arthropod Alalcomenaeuscambricus Simonetta from the Middle Cambrian Burgess Shale ofBritish Columbia Paleontology 42 953ndash78

Briggs D E G amp Fortey R A 1982 The cuticle of aglaspididarthropods a red-herring in the early history of vertebratesLethaia 15 25ndash9

Briggs D E G amp Fortey R A 1989 The early radiation andrelationships of the major arthropod groups Science 246 241ndash3

Briggs D E G amp Fortey R A 1992 The early Cambrian radiationof arthropods In Lipps J H amp Signor P W (eds) Origin andEarly Evolution of the Metazoa 335ndash373 New York NY PlenumPress

Briggs D E G amp Robison R A 1984 Exceptionally preservednontrilobite arthropods and Anomalocaris from the MiddleCambrian of Utah University of Kansas PaleontologicalContributions Paper 111 1ndash23

Broili F 1932 Ein neuer Crustacee aus dem rheinischen UnterdevonSitzungsberichte der Mathematisch-Naturwissenschaftlichen(Abteilung) Klasse der Bayerischen Akademie der Wissenschaftenzu Munchen 1932 27ndash38

Broili F 1933 Ein zweites Exemplar von Cheloniellon Sitzungsber-ichte der Bayerischen Akademie der Wissenschaften 1933 11ndash32

Bruton D L 1981 The arthropod Sidneyia inexpectans MiddleCambrian Burgess Shale British Columbia Philosophical Trans-actions of the Royal Society of London Series B 295 619ndash56

Bruton D L amp Whittington H B 1983 Emeraldella and Leanchoiliatwo arthropods from the Burgess Shale British Columbia Philo-sophical Transactions of the Royal Society of London Series B 300553ndash85

Budd G E 1995 Kleptothule rasmusseni gen et sp nov anolenellid-like trilobite from the Sirius Passet fauna (BuenFormation Lower Cambrian North Greenland) Transactions ofthe Royal Society of Edinburgh Earth Sciences 86 1ndash12

Budd G E 1996a Progress and problems in arthropod phylogenyTrends in Ecology and Evolution 11 356ndash8

Budd G E 1996b The morphology of Opabinia regalis and thereconstruction of the arthropod stem-group Lethaia 29 1ndash14

Budd G E 1997 Stem-group arthropods from the Lower CambrianSirius Passet fauna of North Greenland In Fortey R A ampThomas R H (eds) Arthropod Relationships 125ndash38 LondonChapman and Hall Systematics Association Special Volume 55

Budd G E 1999a A nektaspid arthropod from the Early CambrianSirius Passet fauna with a description of retrodeformation basedon functional morphology Palaeontology 42 99ndash122

Budd G E 1999b The morphology and phylogenetic significance ofKerygmachela kiergkegaardi Budd (Buen Formation LowerCambrian N Greenland) Transactions of the Royal Society ofEdinburgh Earth Sciences 89 (for 1998) 249ndash290

Budd G E 2002 A palaeontological solution to the arthropod headproblem Nature 417 271ndash5

Budd G E amp Jensen S 2000 A critical reappraisal of the fossilrecord of the bilaterian phyla Biological Reviews 75 253ndash95

Chen J Zhou G Zhu M amp Yeh K 1996 The Chengjiang biota ndasha unique window of the Cambrian explosion Taichung NationalMuseum of Natural Science [In Chinese]

Chen J Edgecombe G D amp Ramskold L 1997 Morphological andecological disparity in naraoiids (Arthropoda) from the Early

190 TREVOR J COTTON AND SIMON J BRADDY

Cambrian Chengjiang fauna China Records of the AustralianMuseum 49 1ndash24

Chen J amp Zhou G 1997 Biology of the Chengjiang fauna In ChenJ Cheng Y amp Iten H V (eds) The Cambrian explosion and thefossil record Bulletin of the National Museum of Natural ScienceTaiwan 10 11ndash106

Child C A 1997 Some deep-sea Pycnogonida from Argentine slopeand basin Proceedings of the Biological Society of Washington110 128ndash42

Chlupac I 1988 The enigmatic arthropod Duslia from the Ordovicianof Czechslovakia Palaeontology 31 611ndash20

Chlupac I 1995 Lower Cambrian arthropods from the PasekyShale (Barrandian area Czech Republic) Journal of the CzechGeological Society 40 9ndash36

Chlupac I amp Havlıcek V 1965 Kodymirus n g a new aglaspidmerostome of the Cambrian of Bohemia Sbornık GeologickechVid Paleontologie 6 7ndash20

Cisne J L 1975 Anatomy of Triarthrus and the relationships of theTrilobita Fossils and Strata 4 45ndash63

Cisne J L 1981 Triarthrus eatoni (Trilobita) anatomy of its exo-skeletal skeletomuscular and digestive systems Palaeonto-graphica Americana 9 95ndash142

Conway Morris S 1982 Atlas of the Burgess Shale London ThePalaeontological Association

Conway Morris S 1989 The persistence of Burgess Shale-type faunasimplications for the evolution of deeper-water faunas Trans-actions of the Royal Society of Edinburgh Earth Sciences 80271ndash83

Conway Morris S 1998 The crucible of creation Oxford OxfordUniversity Press

Cotton T J 1999 Review of Arthropod fossils and phylogenyPalaeogeography Palaeoclimatology Palaeoecology 151 341ndash3

Cotton T J 2002 The phylogeny and morphological evolution ofCambrian trilobites and their relatives Unpublished PhD ThesisUniversity of Bristol

Damen W G M Hausdorf M Seyfarth E A amp Tautz D 1998 Aconserved mode of head segmentation in arthropods revealed bythe expression pattern of Hox genes in a spider Proceedings of theNational Academy of Sciences USA 95 10665ndash70

Delle Cave L amp Simonetta A M 1991 Early Palaeozoic arthropodsand problems of arthropod phylogeny with some notes on taxa ofdoubtful affinities In Simonetta A M amp Conway Morris S(eds) The early evolution of Metazoa and the significance ofproblematic taxa 189ndash244 Cambridge Cambridge UniversityPress

Dewel R A amp Dewel W C 1997 The place of tardigrades inarthropod evolution In Fortey R A amp Thomas R H (eds)Arthropod Relationships 109ndash23 London Chapman and HallSystematics Association Special Volume 55

Dunlop J A 1997 Palaeozoic arachnids and their significance forarachnid phylogeny Proceedings of the 16th European Colloquiumof Arachnology 65ndash82 Siedlce Poland

Dunlop J A 1998 The origins of tetrapulmonate booklungs and theirsignificance for chelicerate phylogeny In Selden P A (ed)Proceedings of the 17th European Colloquium of ArachnologyEdinburgh 1997 pp 9ndash16

Dunlop J A 1999 Pasando revista a la evolucion de los quelicerados[A review of chelicerate evolution] In Melic A DeHaro J JMedez M amp Ribera I (eds) Volumen Monografico Evolucion yFilogenia de Arthropoda Boletin de la Sociedad EntolomologicaAragonesa 26 255ndash272 [In Spanish and English]

Dunlop J A amp Selden P A 1997 The early history and phylogenyof the chelicerates In Fortey R A amp Thomas R H (eds)Arthropod relationships Systematics Association Special Volume55 221ndash35 London Chapman and Hall

Dunlop J A amp Webster M 1999 Fossil evidence terrestrializationand arachnid phylogeny Journal of Arachnology 27 86ndash93

Dzik J 1993 Early metazoan evolution and the meaning of its fossilrecord Evolutionary Biology 27 339ndash86

Dzik J amp Lendzion K 1988 The oldest arthropods of the EastEuropean Platform Lethaia 21 29ndash38

Edgecombe G D Wilson G D F Colgan D J Gray M R ampCassis G 2000 Arthropod cladistics combined analysisof Histone H3 and U2 snRNA sequences and morphologyCladistics 16 155ndash203

Edgecombe G D amp Ramskold L 1999 Relationships of CambrianArachnata and the systematic position of Trilobita Journal ofPaleontology 73 263ndash87

Emerson M J amp Schram F R 1997 Theories patterns and realitygame plan for arthropod phylogeny In Fortey R A amp Thomas

R H (eds) Arthropod Relationships Systematics AssociationSpecial Volume 55 67ndash87 London Chapman and Hall

Farris J S 1989 The Retention Index and the Rescaled ConsistencyIndex Cladistics 5 417ndash19

Felsenstein J 1985 Confidence limits on phylogenies an approachusing the bootstrap Evolution 39 783ndash91

Flower R H 1968 Merostomes from a Cotter Horizon of the El PasoGroup New Mexico Bureau of Mines and Mineral ResourcesMemoir 22 35ndash44

Fortey R A 1990a Trilobite evolution and systematics In MikulicD G (ed) Arthropod paleobiology 44ndash65 Short Courses inPaleontology 3 Knoxville Tenessee The Paleontological Society

Fortey R A 1990b Ontogeny hypostome attachment and trilobiteclassification Palaeontology 33 529ndash76

Fortey R A 1997 Classification In Kaesler R L (ed) Treatise oninvertebrate paleontology Part O Arthropoda 1 (revised) 289ndash302Boulder Colorado amp Lawrence Kansas Geological Society ofAmerica and University of Kansas Press

Fortey R A amp Theron J N 1994 A new Ordovician arthropodSoomaspis and the agnostid problem Palaeontology 37 841ndash61

Fortey R A amp Whittington H B 1989 The Trilobita as a naturalgroup Historical Biology 2 125ndash38

Fry W G 1978 A classification within the pycnogonids ZoologicalJournal of the Linnean Society of London 63 35ndash58

Giribet G Edgecombe G D amp Wheeler W C 2001 Arthropodphylogeny based on eight molecular loci and morphology Nature413 157ndash61

Gould S J 1989 Wonderful life The Burgess Shale and the nature ofhistory New York NY Norton

Gould S J 1991 The disparity of the Burgess Shale arthropod faunaand the limits of cladistic analysis why we must strive to quantifymorphospace Paleobiology 17 411ndash23

Gurich G 1931 Mimetaster hexagonalis ein neuer Kruster aus demunterdevonischen Bundenbacher Dachschiefer PalaontologischeZeitschrift 12 204ndash38

Hammann W Laske R amp Pillola G L 1990 Tariccoia arrusensisngnsp an unusual trilobite-like arthropod Rediscovery of thelsquophylocaridrsquo beds of Taricco (1922) in the Ordovician lsquoPuddingarsquosequence of Sardinia Bolletino della Societa PaleontologicaItaliana 29 163ndash78

Haas M S Brown S J amp Beeman R W 2001 Pondering theprocephalon the segmental origin of the labrum DevelopmentGenes and Evolution 211 89ndash95

Heider K 1913 Entwicklungsgeschichte und Morphologie derWirbellosen Die Kultur der Gegenwart Theil 3 Abt 4 2 176ndash332

Henriksen K 1928 Critical notes upon Cambrian arthropodsdescribed by Charles D Walcott Videnskabelige Meddelelser fraDansk Naturhistorisk Forening 86 1ndash20

Hesselbo S P 1988 Trace fossils of Cambrian aglaspidid arthropodsLethaia 21 139ndash46

Hesselbo S P 1989 The aglaspidid arthropod Beckwithia from theCambrian of Utah and Wisconsin Journal of Paleontology 63636ndash42

Hesselbo S P 1992 Aglaspidida (Arthropoda) from the UpperCambrian of Wisconsin Journal of Paleontology 66 885ndash923

Hou X 1987a Three new large arthropods from Lower CambrianChengjiang fauna eastern Yunnan Acta Palaeontologica Sinica26 272ndash85 [In Chinese with English summary]

Hou X 1987b Two new arthropods from Lower CambrianChengjiang eastern Yunnan Acta Palaeontologica Sinica 26236ndash56 [In Chinese with English summary]

Hou X Chen J amp Lu H 1989 Early Cambrian new arthropodsfrom Chengjiang Yunnan Acta Palaeontologica Sinica 28 42ndash57[In Chinese with English summary]

Hou X Ramskold L amp Bergstrom J 1991 Composition andpreservation of the Chengjiang fauna a Lower Cambrian soft-bodied biota Zoologica Scripta 20 395ndash411

Hou X Bergstrom J amp Ahlberg P 1995 Anomalocaris and otherlarge animals in the Lower Cambrian Chengjiang fauna ofsouthwest China GFF 117 163ndash83

Hou X amp Bergstrom J 1997 Arthropods of the Lower CambrianChengjiang fauna southwest China Fossils and Strata 45 1ndash116

Hughes C P 1975 Redescription of Burgessia bella from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 4415ndash36

Ivantsov A Y 1999 Trilobite-like arthropod from the LowerCambrian of the Siberian Platform Acta PalaeontologicaPolonica 44 455ndash66

King P E 1973 Pycnogonids London St Martin Press

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 191

Lauterbach K E 1973 Schlussereignisse in der Evolution derStammgruppe der Euarthropoda Zoologische Beitrage 19259ndash99

Lauterbach K E 1980 Schlussereignisse in der Evolution derGrundplans der Arachnata (Arthropoda) Abhandlungen desNaturwissenschaftlichen Vereins in Hamburg 23 163ndash327

Lauterbach K E 1983 Synapomorphien zwischen Trilobiten- undCheliceraten-zweig der Arachnata Zoologischer Anzeiger 210213ndash38

Lendzion K 1975 Fauna of the Mobergella Zone in the Polish LowerCambrian Kwartalnik Geologiczny 19 237ndash42

Lieberman B S 1998 Cladistic analysis of the Early Cambrianolenelloid trilobites Journal of Paleontology 72 59ndash78

Lu Y 1940 On the ontogeny and phylogeny of Redlichia intermediaLu (sp nov) Bulletin of the Geological Society of China 20333ndash42 [In Chinese]

Maddison W P amp Maddison D R 1997 MacClade analysis ofphylogeny and character evolution Version 307 Sunderland MASinauer Associates

Mittmann B amp Scholtz G 2003 Development of the nervous systemin the lsquoheadrsquo of Limulus polyphemus (Chelicerata Xiphosura)morphological evidence for the correspondence between the seg-ments of the chelicerae and the (first) antennae of MandibulataDevelopment Genes and Evolution 213 9ndash17

Moore R C (ed) 1959 Treatise on invertebrate paleontology Part OArthropoda 1 Boulder Colorado Geological Society of America

Muller K J amp Walossek D 1986 Arthropod larvae from the UpperCambrian of Sweden Transactions of the Royal Society ofEdinburgh Earth Sciences 77 157ndash79

Muller K J amp Walossek D 1987 Morphology ontogeny and lifehabit of Agnostus pisiformis from the Upper Cambrian of SwedenFossils and Strata 19 1ndash124

Munilla T 1999 Evolucion y filogenia de los Picnogonidos InMelicA DeHaro J J Medez M amp Ribera I (eds) VolumenMonografico Evolucion y Filogenia de Arthropoda Boletin de laSociedad Entolomologica Aragonesa 26 273ndash91 [In Spanish andEnglish]

Orr P J Siveter D J Briggs D E G Siveter D J amp Sutton M D2000 A new arthropod from the Silurian Konservat-Lagerstatteof Herefordshire Proceedings of the Royal Society of LondonSeries B 267 1497ndash504

Page R D M 1996 On consensus confidence and lsquototal evidencersquoCladistics 12 83ndash92

Palmer A R amp Repina L N 1993 Through a glass darklytaxonomy phylogeny and biostratigraphy of the OlenellinaUniversity of Kansas Paleontological Contributions New Series 31ndash35

Platnick N I Griswold C E amp Coddington J A 1991 On missingentries in cladistic analysis Cladistics 7 337ndash43

Pleijel F 1995 On character coding for phylogeny reconstructionCladistics 11 309ndash15

Raasch G O 1939 Cambrian Merostomata Special Papers of theGeological Society of America 19 1ndash146

Ramskold L Chen J Edgecombe G D amp Zhou G 1996Preservational folds simulating tergite junctions in tegopeltid andnaraoiid arthropods Lethaia 29 15ndash20

Ramskold L Chen J Edgecombe G D amp Zhou G 1997Cindarella and the arachnate clade Xandarellida (ArthropodaEarly Cambrian) from China Transactions of the Royal Society ofEdinburgh Earth Sciences 88 19ndash38

Ramskold L amp Edgecombe G D 1991 Trilobite monophylyrevisited Historical Biology 4 267ndash83

Ramskold L amp Edgecombe G D 1996 Trilobite appendagestructure ndash Eoredlichia revisited Alcheringa 20 269ndash76

Raw F 1957 Origin of chelicerates Journal of Paleontology 31139ndash193

Repina L N amp Okuneva O G 1969 Cambrian arthropods of themaritime territory Paleontological Journal 1969 95ndash103

Richter R amp Richter E 1929 Weinbergina opitz ng n sp einSchwerttrager (Merost Xiphos) aus dem Devon (Rheinland)Senkenbergiana 11 193ndash209

Robison R A 1984 New occurences of the unusual trilobite Naraoiafrom the Cambrian of Idaho and Utah University of KansasPaleontological Contributions Paper 112 1ndash8

Rominger C 1887 Description of primordial fossils from MtStephens NW Territory of Canada Proceedings of the Academyof Natural Sciences of Philadelphia 1887 12ndash19

Scholtz G 1997 Cleavage germ band formation and head segmen-tation the ground pattern of the Euarthropoda In Fortey R Aamp Thomas R H (eds) Arthropod Relationships 317ndash32 LondonChapman and Hall Systematics Association Special Volume 55

Schram F R 1978 Arthropods a convergent phenomenonFieldiana Geology 39 61ndash108

Schram F R 1993 Review of Boxshall Stromberg and Dahl TheCrustacea Origin and Evolution Journal of Crustacean Biology13 820ndash2

Schram F R Vonk R amp Hof C H J 1997 Mazon CreekCycloidea Journal of Paleontology 71 261ndash84

Schram F R amp Emerson M J 1991 Arthropod Pattern Theorya new approach to arthropod phylogeny Memoirs of theQueensland Museum 31 1ndash18

Schram F R amp Hof C H J 1998 Fossils and the interrelationshipsof major crustacean groups In Edgecombe G D (ed) Arthropodfossils and phylogeny pp 233ndash302 New York NY ColumbiaUniversity Press

Shergold J H 1991 Protaspis and early meraspis growth stages of theeodiscoid trilobite Pagetia ocellata Jell and their implications forclassification Alcheringa 15 65ndash86

Shu D Geyer G Ling C amp Zhang Z 1995 Redlichiacean trilobiteswith preserved soft parts from the Lower Cambrian Chengjiangfauna (South China) Beringeria (Special Issue) 2 203ndash41

Shultz J W 1990 Evolutionary morphology and phylogeny ofArachnida Cladistics 6 1ndash38

Shultz J W 2000 Skeletomuscular anatomy of the harvestmanLeiobunum aldrichi (Weed 1893) (Arachnida OpilionesPalpatores) and its evolutionary significance Zoological Journalof the Linnean Society 128 401ndash38

Shultz J W 2001 Gross muscular anatomy of Limulus polyphemus(Xiphosura Chelicerata) and its bearing on evolution in the theArachnida Journal of Arachnology 29 283ndash303

Siewing R (ed) 1985 Lehrbuch der Zoologie II Systematik 3rd ednStuttgart Fischer

Simonetta A M 1970 Studies on the non trilobite arthropods ofthe Burgess Shale (Middle Cambrian) the genera LeanchoiliaAlalcomenaeus Opabinia Burgessia Yohoia and ActaeusPalaeontographica Italica 66 35ndash45

Simonetta A M amp Delle Cave L 1975 The Cambrian non trilobitearthropods from the Burgess Shale of British Columbia A studyof their comparative morphology taxonomy and evolutionarysignificance Palaeontographica Italica 69 1ndash37

Simonetta A M amp Delle Cave L 1981 An essay in the compara-tive and evolutionary morphology of Palaeozoic ArthropodsAccademia Nazionale dei Lincei Atti dei Convegni Lincei 49389ndash439

Starobogatov Y I 1990 Sistema i filogeniya nizshikh khelitserovykh(analiz morfologii paleozoysikh grupp) [The systematics andphylogeny of the lower chelicerates (a morphological analysis ofthe Palaeozoic groups)] Paleontologicheskii Zhurnal 1990 4ndash17

Stoslashrmer L 1944 On the relationships and phylogeny of fossil andrecent Arachnomorpha Skrifter utgitt ac Det Norske Videnskaps-Akademi i Olso I Matematisk-naturvitenskaplig Klasse 5 1ndash158

Stoslashrmer L 1951 Studies on trilobite morphology Part III Theventral cephalic structures with remarks on the zoologicalposition of the trilobites Norsk geologisk tidsskrift 29 108ndash58

Stoslashrmer L 1956 A Lower Cambrian merostome from Sweden Arkivfor Zoologi Ser 2 9 507ndash14

Stoslashrmer L 1959 Trilobitoidea In Moore R C (ed) Treatise onInvertebrate Paleontology part O Arthropoda 1 23ndash37 BoulderColorado Geological Society of America

Strong E E amp Lipscomb D 1999 Character coding and inapplicabledata Cladistics 15 363ndash71

Sturmer W amp Bergstrom J 1976 The arthropods Mimetaster andVachonisia from the Devonian Hunsruck Shale PalaontologischeZeitschrift 50 78ndash111

Sturmer W amp Bergstrom J 1978 The arthropod Cheloniellon fromthe Devonian Hunsruck Shale Palaontologische Zeitschrift 5257ndash81

Sturmer W amp Bergstrom J 1981 Weinbergina a xiphosuran fromthe Devonian Hunsruck Slate Palaontologische Zeitschrift 55237ndash55

Sutton M D Briggs D E G Siveter D J Siveter D J amp Orr PJ 2002 The arthropod Offacolus kingi (Chelicerata) from theSilurian of Herefordshire England computer based morphologi-cal reconstructions and phylogenetic affinity Proceedings of theRoyal Society of London Series B 269 1195ndash1203

Swofford D L 1999 PAUP Phylogenetic Analysis Using Parsimony(and other methods) Version 40b2a Sunderland MA SinauerAssociates

Telford M J amp Thomas R H 1998 Expression of homeobox genesshows chelicerate arthropods retain their deutocerebral segmentProceedings of the National Academy of Sciences USA 9510671ndash5

192 TREVOR J COTTON AND SIMON J BRADDY

Thorley J L amp Page R D M 2000 RadCon phylogenetic treecomparison and consensus Bioinformatics 16 486ndash7

Valentine J W 1995 Why no new phyla after the Cambrian ndashgenome and ecospace hypotheses revisited Palaios 10 190ndash194

Wagele J W 1993 Rejection of the lsquoUniramiarsquo hypothesis andimplications of the Mandibulata concept Zoologische JahrbucherAbteilung fur Systematik Geographie und Biologie der Tiere 120253ndash88

Walcott C D 1911 Middle Cambrian Merostomata Cambriangeology and palaeontology II Smithsonian MiscellaneousCollections 57 17ndash40

Walcott C D 1912 Middle Cambrian Branchiopoda MalacostracaTrilobita and Merostomata Cambrian geology and palaeontol-ogy II Smithsonian Miscellaneous Collections 57 145ndash228

Walcott C D 1918 Geological explorations in the CanadianRockies Explorations and fieldwork of the SmithsonianInstitution in 1917 Smithsonian Miscellaneous Collections 684ndash20

Walossek D 1993 The Upper Cambrian Rehbachiella kinnekullensisand the phylogeny of Branchiopoda and Crustacea Fossils andStrata 32 1ndash202

Walossek D amp Dunlop J A 2002 A larval sea spider (ArthropodaPycnogonida) from the Upper Cambrian lsquoOrstenrsquo of Swedenand the phylogenetic position of pycnogonids Palaeontology 45421ndash46

Walossek D amp Muller K J 1990 Upper Cambrian stem-lineagecrustaceans and their bearing upon the monophyletic origin ofCrustacea and the position of Agnostus Lethaia 23 409ndash27

Walossek D amp Muller K J 1997 Cambrian lsquoOrstenrsquo-type arthro-pods and the phylogeny of Crustacea In Fortey R A amp ThomasR H (eds) Arthropod Relationships 139ndash53 SystematicsAssociation Special Volume 55 London Chapman and Hall

Walossek D amp Muller K J 1998 Early arthropod phylogeny in lightof the Cambrian lsquoOrstenrsquo fossils In Edgecombe G D (ed)Arthropod fossils and phylogeny 185ndash231 New York NYColumbia University Press

Weygoldt P 1979 Significance of later embryonic stages and headdevelopment in arthropod phylogeny In Gupta A P (ed)Arthropod Phylogeny 107ndash35 New York NY Van NostrandReinhold Co

Weygoldt P 1998 Evolution and systematics of the ChelicerataExperimental and Applied Acarology 22 63ndash79

Weygoldt P amp Paulus H F 1979 Unterzuchungen aur Morpholo-gie Taxonomie und Phylogenie der Chelicerata Zeitschriftfur Zoologische Systematik und Evolutionsforschung 17 85ndash116177ndash200

Wheeler W C Cartwright P amp Hayashi C Y 1993 Arthropodphylogeny a combined approach Cladistics 9 1ndash39

Wheeler W C amp Hayashi C Y 1998 The phylogeny of extantchelicerate orders Cladistics 14 173ndash92

Whittington H B 1971 Redescription of Marrella splendens(Trilobitoidea) from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 209 1ndash24

Whittington H B 1974 Yohoia Walcott and Plenocaris n genarthropods from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 231 1ndash21

Whittington H B 1975a Trilobites with appendages from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 497ndash136

Whittington H B 1975b The enigmatic animal Opabinia regalisMiddle Cambrian Burgess Shale British Columbia PhilosophicalTransactions of the Royal Society of London Series B 271 1ndash43

Whittington H B 1977 The Middle Cambrian trilobite NaraoiaBurgess Shale British Columbia Philosophical Transactions of theRoyal Society Series B 280 409ndash43

Whittington H B 1979 Early arthropods their appendages andrelationships In House M R (ed) The origin of majorinvertebrate groups 253ndash68 Systematics Association SpecialVolume 12 New York NY Academic Press

Whittington H B 1980 Exoskeleton moult stage appendage mor-phology and habits of the Middle Cambrian trilobite Olenoidesserratus Palaeontology 23 171ndash204

Whittington H B 1981 Rare arthropods from the Burgess ShaleMiddle Cambrian British Columbia Philosophical Transactionsof the Royal Society of London Series B 292 329ndash57

Whittington H B 1985 Tegopelte gigas a second soft-bodied trilo-bite from the Burgess Shale Middle Cambrian British ColumbiaJournal of Palaeontology 23 171ndash204

Whittington H B 1989 Olenelloid trilobites type species functionalmorphology and higher classification Philosophical Transactionsof the Royal Society of London Series B 324 111ndash47

Whittington H B amp Almond J E 1987 Appendages and habits ofthe Upper Ordovician trilobite Triarthus eatoni PhilosophicalTransactions of the Royal Society of London Series B 317 1ndash46

Wilkinson M 1992 Ordered versus unordered characters Cladistics8 375ndash85

Wilkinson M 1994 Common cladistic information and its consensusrepresentation reduced Adams and reduced cladistic consensustrees and profiles Systematic Biology 43 343ndash68

Wilkinson M 1995a Coping with abundant missing entries in phylo-genetic inference using parsimony Systematic Biology 44 501ndash14

Wilkinson M 1995b TAXEQ2 software and documentation BristolSchool of Biological Sciences University of Bristol

Wilkinson M 1996 On the distribution of homoplasy and choosingamong trees Taxon 45 263ndash66

Wills M A 1997 A phylogeny of Recent and fossil Crustacea derivedfrom morphological characters In Fortey R A amp ThomasR H (eds) Arthropod Relationships 189ndash209 SystematicsAssociation Special Volume 55 London Chapman and Hall

Wills M A Briggs D E G amp Fortey R A 1994 Disparity asan evolutionary index a comparison of Cambrian and Recentarthropods Paleobiology 20 93ndash130

Wills M A Briggs D E G Fortey R A amp Wilkinson M 1995The significance of fossils in understanding arthropod evolutionVerhandlungen der deutschen zoologischen Gesselschaft 88 203ndash15

Wills M A Briggs D E G Fortey R A Wilkinson M amp SneathP H A 1998a An arthropod phylogeny based on fossil andrecent taxa In Edgecombe G D (ed) Arthropod fossils andphylogeny 33ndash105 New York NY Columbia University Press

Wills M A Briggs D E G amp Fortey R A 1998b Evolutionarycorrelates of arthropod tagmosis scrambled legs In ForteyR A amp Thomas R H (eds) Arthropod Relationships 57ndash65Systematics Association Special Volume 55 London Chapmanand Hall

Winter G 1980 Beitrage zur Morphologie und Embryologie desvorderen Korperabschnitts (Cephalosoma) der PantopodaGerstaecker 1863 I Entstehung und Struktur des Zentralnerven-systems Zeitschrift fur Zoologische Systematik und Evolutionsfor-schung 18 27ndash61

Zhang W amp Hou X 1985 Preliminary notes on the occurrence of theunusual trilobite Naraoia in Asia Acta Palaeontologica Sinica 24591ndash5 [In Chinese with English summary]

TREVOR J COTTON Department of Palaeontology The Natural History Museum CromwellRoad London SW7 5BD UKe-mail trevor_cottonhotmailcom

SIMON J BRADDY Department of Earth Sciences University of Bristol Wills MemorialBuilding Queenrsquos Road Bristol BS8 1RJ UKe-mail S J Braddybrisacuk

MS received 28 July 2003 Accepted for publication 15 March 2004

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 193

Page 15: The phylogeny of arachnomorph arthropods and the origin of the

1997) This structure is often sclerotised and covers the mouthventrally It is this sclerite that is here identified as thehypostome Therefore this character is considered absent intaxa lacking a sclerite covering the mouth irrespective of themorphology and position of the labrum itself which at leastpartly covers the mouth in all euarthropods other thanpycnogonids (see Edgecombe et al 2000 p 167) The lsquofleshylabrumrsquo of crustaceans does not appear to be sclerotised and ahypostome is consequently considered to be absent Althoughmany derived features are associated with the crustaceanlabrum this is not a good reason to consider it non-homologous with the hypostome-bearing structure of otherarthropods as Walossek amp Muller (1990) argued

In many taxa the mouth is covered by a posteromedianextension of the doublure here considered to represent thehypostome (eg Emeraldella Fig 7F Aglaspis see Hesselbo1992 fig 52) In others the hypostome is separated from thedoublure either by a suture a pre-hypostomal sclerite (egKuamaia lata Fig 7D Saperion glumaceum see Edgecombe ampRamskold 1999 figs 3 amp 4) or an intervening region ofunsclerotised cuticle (the natant condition of Fortey 1990beg Cindarella eucala Fig 7E) The homology of pre-hypostomal sclerites and hypostomes in helmetiids trilobitesand naraoiids (see Fig 7) has been discussed by Chen et al(1996) and Edgecombe amp Ramskold (1999) but it is as yetunclear whether any of the character states described here arederived from any other They are treated here as distinct andthe character as unordered pending further study

No hypostome has been identified in Yohoia (see above)Alalcomenaeus Jianfengia Fortiforceps or Leanchoilia andthere is certainly no broad posterior extension of the doublurein any of these taxa Since there is also no pre-hypostomalsclerite they have received a (134) partial uncertainty codingThe attachment of the hypostome of Tegopelte is unclear butit does not seem to be attached to any doublure (Whittington1985) and therefore it is given an uncertainty (12) coding

The lsquorostrum-like structurersquo on the ventral surface ofLemoneites appears to be similar in morphology and positionrelative to the anterior margin of the head shield (Flower 1968pl 8 figs 4 amp 13) to that described from Aglaspis and is codedas State 0 In many taxa the hypostome is unknown but inonly a small number can it be coded reliably as absentHughesrsquo (1975) suggestion that the hypostome of Burgessia isabsent is accepted preliminarily but an extension of thedoublure may be visible in some of Hughesrsquo figures Fortey(pers comm 2000) also doubts Hughesrsquo assertion

28 Visible ecdysial sutures (0) absent and (1) present Themarginal ecdysial sutures of chelicerates and the dorsal suturesof trilobites are almost certainly not homologous but thepresence of sutures and their position (see Character 29) arecoded separately to allow this to be tested Wills et al (1998aCharacter 2) coded marginal sutures as present in Sidneyiafollowing Brutonrsquos (1981) suggestion but this is consideredequivocal here

29 Position of ecdysial sutures (0) marginal and (1) dor-sal This character is inapplicable to taxa lacking ecdysialsutures (Character 28 State 0) Dorsal sutures whilst presentin the representatives of the Trilobita coded here are probablynot a synapomorphy of trilobites as a whole but for a clade oftrilobites excluding the Olenellida (Whittington 1989 Fortey1990a)

245 Exoskeletal tergites and thoracic tagmosis 30 Min-eralised cuticle (0) absent and (1) present Calcification of theexoskeleton is one of the most convincing synapomorphiesof the Trilobita (Fortey amp Whittington 1989 Ramskold ampEdgecombe 1991 Edgecombe amp Ramskold 1999 Character 1)Cuticle mineralisation is also coded as present in aglaspidids

following Briggs amp Fortey (1982) who suggested that theaglaspidid exoskeleton was originally phosphatic Paleomerusprobably also had a mineralised cuticle but Hou amp Bergstrom(1997 p 97) argued that it was likely to be calcareous Anundescribed Silurian aglaspidid may also have had an origi-nally calcitic cuticle (Fortey amp Theron 1994 p 856) Becauseof the uncertainty about the chemical composition of theexoskeleton in these forms cuticle mineralisation is coded aspotentially homologous in all these taxa and no attempt ismade to code separate states for phosphatic and calcareousmineralisation

31 Trunk tergites with expanded lateral pleurae coveringappendages dorsally (0) absent and (1) present Whilst thepresence of paratergal folds may be a synapomorphy at thelevel of the Euarthropoda (Boudreaux 1979 Wagele 1993Edgecombe et al 2000 Character 142) these are at mostsmall reflections of the margin of the tergites in most euarthro-pods In arachnomorph taxa these are expanded to form largelateral pleurae that cover the appendages dorsally (see egLimulus and Triarthus in Boudreaux 1979) This is inferred tobe the case in some taxa where dorsal features and appendagesare poorly known on the basis of their possession of tergiteswith wide pleural regions This feature was suggested as typicalof lamellipedians (=arachnomorphs) by Hou amp Bergstrom(1997 pp 42ndash3)

The arrangement of the thorax of Marrella Mimetaster andBurgessia differs from that of all the other taxa considered herein that the appendages seem to be attached to the lateralmargins of the body The trunk tergites do not cover theappendages and seem to lack pleurae entirely although theyare covered by a posterior extension of the head shield inBurgessia This character has been clearly described inMimetaster (Sturmer amp Bergstrom 1976 pp 87ndash90 fig 8) andBurgessia (Hughes 1975 p 421)

32 Free thoracic tergites (0) present and (1) absentFollowing Edgecombe amp Ramskold (1999 Character 16) anumber of taxa lack functional post-cephalic articulations andconsequently lack free thoracic tergites Despite this theycode these taxa for a number of characters relating to thestructure of thoracic tergites (eg their characters 18 and 19)These characters are treated here as inapplicable to taxawithout free tergites and the definition of this character hasbeen modified from that of Edgecombe amp Ramskold (1999) tomake this more explicit

33 Decoupling of thoracic tergites and segments (0)absent and (1) present Ramskold et al (1997) described thischaracter of Cindarella and Xandarella where the thoracictergites correspond to a variable increasing posteriorly num-ber of appendage pairs This character cannot be coded fortaxa in which free thoracic tergites are absent (Character 32State 1)

34 Tergite articulations (0) tergites non-overlapping (1)extensive overlap of tergites and (2) edge-to-edge pleuralarticulations In most of the taxa considered here the thoracictergites overlap considerably and relatively evenly over theirwidth This contrasts with the primitive euarthropod condi-tion where the tergites do not overlap medially as seen in stemgroup crustaceans In trilobites such as Helmetia and Kuamaia(see Edgecombe amp Ramskold 1999 character 18) overlap ofthoracic tergites is limited to a well-defined axial region andthe lateral pleurae meet edge-to-edge

The thoracic articulations of naraoiids with free thoracictergites are in some ways similar to those of trilobites with astrong overlap medially that is lacking abaxially andanterolateral articulating facets (Budd 1999a Ramskold ampEdgecombe 1999) However the distribution of these features

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 183

in other trilobite-like arachnomorphs is unclear and a restric-tive coding is used here The present authors do not considerthat this character can be applied reliably to the fused thoracictergites of Saperion Tegopelte and Skioldia and it is coded asinapplicable to all taxa lacking free thoracic tergites (Character32 State 1)

35 Trunk effacement (0) trunk with defined (separate orfused) tergite boundaries (1) trunk tergite boundaries effacedlaterally and (2) trunk tergite boundaries completely effaced(cf Edgecombe amp Ramskold 1999 Character 15)] Contrary toEdgecombe amp Ramskold (1999) the present authors considerSkioldia and Saperion to show a distinct form of tergiteeffacement where the fused tergite boundaries are defined byfurrows axially but effaced laterally The boundaries areeffaced at least laterally in Tegopelte but the axial region ofthe exoskeleton is unknown which accordingly is given amultistate uncertainty coding

36 Cephalic articulation fused (0) absent and (1) presentIn Tegopelte Saperion and Skioldia the articulation ofthe head with the thorax is non-functional and the entireexoskeleton forms a single tergite

37 Head shield overlap of thoracic tergites (0) overlapabsent or identical to overlap between thoracic segments (1)head shield covers first thoracic tergite only and (2) headshield covers mutliple anterior trunk tergites

38 Head shield articulates with reduced anterior thoracictergite (0) absent and (1) present Amongst taxa showing aposteriorly expanded head shield ie one that overlaps thethorax Edgecombe amp Ramskold (1999 Character 9) identifiedtwo distinct character states One of these ndash overlap of only theanteriormost thoracic tergite ndash was limited to taxa assigned tothe Liwiinae (sensu Fortey amp Theron 1994) and another ndashoverlap of multiple tergites with attachment to a reducedanterior thoracic tergite ndash to the Xandarellida None of theadditional taxa considered herein (including Buenaspis whichwas assigned to the Liwiinae by Budd 1999a) shows thesedistinctive morphological features However the expandedhead shields of Marrella Mimetaster and Burgessia which donot articulate with a narrow anterior thoracic tergite may behomologous with the expanded head shields of xandarellidsTo recognise this the degree of overlapping of the thorax andthe possession of the reduced anterior tergite are coded sepa-rately These characters are both coded as inapplicable to taxain which the head shield and thoracic tergites are fused(Character 36 State 1) The crustacean carapace which ismost similar to the head shield of Burgessia is seeminglyabsent in stem group crustaceans (Walossek amp Muller1998)

39 Trunk narrowed anteriorly relative to head shieldwidest posteriorly (0) absent and (1) present (cf Edgecombeamp Ramskold 1999 Character 14) The anterior narrowing ofthe trunk caused by the reduction of opisthosomal segment 1in xiphosurids (Weinbergina of the taxa analysed hereAndersen amp Selden 1997 Dunlop amp Selden 1997 Character14) is not considered to be homologous with the unusual shapeof the thorax in naraoiids recognised by their coding

40 Boundaries of anterior trunk segments reflexed antero-laterally (0) absent boundaries transverse or reflexed postero-laterally and (1) present

41 Joints between posterior tergites functional anteriorones variably fused (0) absent and (1) present

42 Posterior tergite bearing axial spine (0) absent and (1)present

Characters 40 to 42 are coded following Edgecombe ampRamskold (1999 characters 17 19 amp 23 respectively) Inaddition to the taxa considered here a thoracic axial spine ispresent in some olenelloid trilobites (see Lieberman 1998

Characters 73 amp 74) and in the aglaspidid Beckwithia (seeHesselbo 1989)

246 Posterior body termination 43 Postabdomen of seg-ments lacking appendages (0) absent and (1) present

44 Length of postabdomen (0) one segment (1) twosegments (2) three segments and (3) five segments In sometaxa a variable number of posterior thoracic segments bearcomplete tubular tergites and lack appendages In Aglaspisautapomorphically it seems that the posterior three thoracicsegments lack appendages (Briggs et al 1979 Hesselbo 1992)but have unmodified tergites These situations are recognisedas potentially homologous by the coding used here Character44 is coded as inapplicable to taxa lacking a postabdomen

45 Posterior tergites strongly curved in dorsal aspect com-pared to anterior tergites (0) absent and (1) present Asrecognised by Wills et al (1998a Character 17) in some taxawhere the tergites are distinct the curvature of thoracic tergitesincreases posteriorly so that the posterior tergites are highlycurved to semicircular in dorsal aspect This situation isnot known from crustaceans (except isopods) or stem grouparthropods

46 Posterior segments reduced and with highly reducedappendages (0) present and (1) absent In some taxa there area large number of posterior segments that are short sagitallyand have appendages that are much reduced in size comparedto anterior trunk appendages These somites are incorporatedinto the pygidium in some taxa but primitively they arecovered by tiny free trunk tergites In the derived state thetrunk somites and limbs are of a relatively constant size Thedistinction between these states can be seen when attempting tocount the number of segments that make up the body In taxashowing State 0 the number of segments is very high anddifficult to count whereas in other taxa the number ofpost-cephalic segments is easily assessed

47 Pygidium (0) absent and (1) present A pygidium isrecognised here as a posterior tagma consisting of a number offused segments under a single tergite which may or may notincorporate the post-segmental telson This is different to theuse of this term by Edgecombe amp Ramskold (1999) whichfollows Ramskold et al (1997) and very different to its use byWills et al (1998a p 53) as a synapomorphy of the TrilobitaThe present authorsrsquo definition recognises the situation inRetifacies where the spinose postsegmental telson is autapo-morphically not fused to the pygidium as homologous toother multisegmented posterior tagma which include thetelson The distinction between the pygidium and an expandedpost-segmental telson can also be seen in the position of theanus which is consistently in the posteriormost pre-telsonicsegment amongst putative arachnomorphs (see Character 48)In taxa with a pygidium the anal segment is fused into thepygidium (see eg Kuamaia Edgecombe amp Ramskold 1999fig 6) whereas in those taxa lacking a pygidium the anus isbetween the final trunk tergite and the telson

Ramskold et al (1997) argued that the posteriormost tergiteof xandarellids (which incorporates the anal segment) is nothomologous to the posterior tagma recognised as pygidiaherein because posterior thoracic tergites also cover multiplesegments in xandarellids (see Character 33) Evidence ofsegmentation of the pygidium in a variety of taxa suggests thatthe number of tergites fused to form the pygidium is greaterthan the number of appendages For example the pygidium ofKuamaia has two pairs of lateral spines but at least fourpairs of appendages (Hou amp Bergstrom 1997 Edgecombe ampRamskold 1999) and that of Triarthrus has only five axialrings posterior to the articulating ring but over 10 pairs ofappendages (Whittington amp Almond 1987) The fact thatdecoupling of tergites and segments is evident in the thorax of

184 TREVOR J COTTON AND SIMON J BRADDY

Xandarella and Cindarella does not necessarily suggest that asimilar more widespread decoupling in pygidia is the result ofa different developmental process and hence that they arenon-homologous Instead the situation in the xandarellidthorax is potentially homologous to that found (more widely)in pygidia and consequently the terminal xandarellid tergite isa pygidium It is unclear if decoupling of tergites and somites isprimitively limited to the terminal tergite or primitively aproperty of the arachnomorph post-cephalon as a whole

It has been suggested that the tiny pygidium of olenelloidtrilobites which probably best reflects the primitive trilobitestate consists only of the post-segmental telson (Harrington inMoore 1959) and therefore is not a true pygidium HoweverWhittington (1989) showed that the olenelloid pygidium con-sists of at least two and possibly as many as five or sixsegments and therefore is likely to be homologous with thepygidium of other trilobites

48 Position of the anus (0) terminal within telson and (1)at base of telson In crustaceans and stem group arthropodsthe anus is terminal or otherwise situated in the telson (egRehbachiella Walossek 1993 fig 15D) In putative arachno-morphs the anus is either at the junction of the posteriormostthoracic segment and the telson or ventral within a fusedpygidium In the case of taxa with a pygidium the anus isanterior to the posterior margin and where known positionedbetween the posteriormost pair of appendages suggesting thatit is anterior to the post-segmental telson (see eg OlenoidesWhittington 1980)

49 Pygidium with median keel (0) absent and (1) presentEdgecombe amp Ramskold (1999 Character 21) considered thepresence of a median keel on the pygidium as a synapomorphyuniting Soomaspis and Tarricoia They did not explain howthis structure could be distinguished from the raised pygidialaxis of trilobites Homology of these structures is not sup-ported here because the axis of more primitive trilobites(including Eoredlichia) is quite distinct morphologically fromthe naraoiid keel Budd (1999a) noted the presence of a keel onthe pygidium of Buenaspis and suggested (Budd 1999a p 102)that it may be an artefact of dorsoventral compaction How-ever contrary to the coding of Edgecombe amp Ramskold(1999) a keel is visible on the pygidium of Liwia convexa (Dzikamp Lendzion 1988 fig 4CD) preserved in full relief Thereforeit seems unlikely that the keel is a taphonomic artefact

50 Pygidium with broad-based median spine (0) absentand (1) present

51 Pygidium with lateral spines (0) present and (1) absentEdgecombe amp Ramskold (1999 Character 22) coded thepresence of a median spine and two pairs of lateral spines aspotentially homologous in Sinoburius Kuamaia and HelmetiaThe distribution of lateral spines is much wider than that ofterminal spines and therefore they are coded separately hereIn trilobites it has widely been recognised that lateral spinesrepresent the original segmentation of the pygidium Thiscannot be the case for median spines Characters 49 to 51are not applicable to taxa lacking a pygidium (Character 47State 0)

52 Expanded post-segmental telson (0) absent and (1)present In a range of taxa the posterior-most tergite is a large(relative to the thoracic tergites) structure that lacks anyevidence of segmentation and is interpreted as representing anexpanded tergite of the post-segmental telson from whichsegments are released anteriorly during ontogeny On the otherhand the posteriormost tergite of Marrella and probablyMimetaster is small compared to the thoracic tergites androunded This element probably represents the plesiomorphiceuarthropod telson The homology of this character in mosttaxa is clear and only doubtful in Retifacies in which it may

be segmented The figures of Hou amp Bergstrom (1997) providelittle unequivocal evidence for a telson in Retifacies but thepresence of this structure is very clear in colour photographs(eg Chen et al 1996 fig 198A) However these figures donot convincingly demonstrate the segmentation of the telsonRetifacies is interpreted here as being unique in possessingboth a pygidium and an expanded post-segmental telson

53 Telson shape (0) spinose and (1) paddle-shaped Theshape of the expanded telsons identified above varies frombroadly spinose to flattened and paddle-like This differencecan be recognised both on the basis of the ratio of length tobasal width (spinose telsons are relatively long) and by thechange in width posteriorly (spinose telsons reduce in widthposteriorly whereas paddle-shaped telsons increase in width)In eurypterids a wide range of telson shapes is found includ-ing both character states described here It is likely that theplesiomorphic condition is a spinose telson This character isinapplicable to taxa lacking an expanded telson

54 Post-ventral furcae (0) absent and (1) present Thischaracter recognises the potential homology of unsegmentedpaired structures that articulate with the segment immediatelyanterior to the telson The potential homology of these struc-tures (the post-ventral plates) in Emeraldella and Aglaspis wasrecognised by Wills et al (1998a Character 70) and inEmeraldella and Sidneyia by Edgecombe amp Ramskold (1999Character 25) The homology of the segmented pygidial caudalfurcae of Olenoides (see Whittington 1975a 1980) with thesestructures is considered doubtful and coded as equivocal

Despite their distinctive morphology the long caudualfurcae of Cheloniellon may be homologous with the furcae ofAglaspis Emeraldella and Sidneyia This is supported by thecaudal furcae of the Ordovician cheloniellid Duslia which aremore similar in morphology to those of Emeraldella than ofCheloniellon Chlupac (1988 pp 614ndash16) considered thesestructures to be on the terminal tergite in Duslia However afaint tergite boundary is apparent posterior to the attachmentof the furcae (see Chlupac 1988 pl 57 fig 4) Therefore theyare considered here to have been attached to the pre-telsonicsegment as in Cheloniellon Chlupac (1988) and Dunlop ampSelden (1997) supported a close relationship between Dusliaand Cheloniellon

25 MethodsAll analyses were carried out using PAUP Version 40b4a(Swofford 1999) and unless otherwise stated used heuristicsearches with one thousand random addition sequencereplicates The software packages MacClade Version 307(Maddison amp Maddison 1997) and RadCon (Thorley amp Page2000) were used for comparing trees and investigating patternsof character evolution Tree length and other statistics (theConsistency Index CI and Retention Index RI) were calcu-lated by PAUP and MacClade with uninformative charactersexcluded Analyses were run separately using each of thedifferent codings for aglaspidids

Characters were treated as unordered and of equal weight inmost analyses To assess the influence of this assumption fourof the eight multistate characters which have states that areintermediate between others (Wilkinson 1992) were treated asordered in some analyses These characters are the number ofcephalic segments (Character 4) the length of raptorialappendage spines (Character 6) the degree of overlap of thethorax by the head-shield (Character 37) and the number ofsegments making up the postabdomen (Character 44) The lastof these is uninformative when not ordered because only onetaxon shows each of states 0 1 and 3

Support for individual nodes was assessed by bootstrapanalysis (Felsenstein 1985) and by calculating Bremer support

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 185

indices (Bremer 1988 1994) These methods measure twodistinct aspects of support for phylogenetic hypotheses Treesor nodes may be considered well supported (1) to the extent towhich alternative topologies are much less parsimonious asmeasured by the support index (Wilkinson 1996) or (2) wherethey are consistent with a large proportion of characters sothat character sampling is unlikely to have had much influenceon topology as assessed by bootstrapping (Page 1996) Boot-strapping was performed with 100 bootstrap replicates eachof 10 addition sequence replicates Bremer support indicesare based on heuristic searches with 100 addition sequencereplicates

Three pairs of terminals are included in Table 2 that apartfrom missing data are coded identically These are Helmetiaand Kuamaia Soomaspis and Tariccoia and Saperion andSkioldia These pairs are lsquotaxonomic equivalentsrsquo removingthe taxon with most missing data will not influence theposition of the other They can be recoded as single terminalsaccording to the principal of safe taxonomic reduction(Wilkinson 1995a b) and were not considered separately in theanalyses presented here Therefore cladistic analyses werecarried out on the basis of 33 terminals Edgecombe ampRamskold (1999) ignored this and coded the same three pairsof taxa and Eoredlichia and Olenoides identically for theirmore limited set of characters

26 ResultsAnalysis with all characters unordered and using the lsquoAglaspi-dida 1rsquo coding found nine most parsimonious trees (MPTs)each 126 steps long (CI=0middot556 RI=0middot763) Using the lsquoAglas-pidida 2rsquo coding resulted in 18 MPTs of length 125 (CI=0middot560RI=0middot765) Nine of these 18 trees were those found with thefirst coding of aglaspidids The strict component consensus(sensu Wilkinson 1994) of the 27 unique trees found acrossboth analyses is shown in Figure 8 All trees supported themonophyly of the Marrellomorpha (Marrella and Mimetaster)and of a clade including all other taxa except Crustacea (Clade1 of Fig 8) The relationships between these two clades and theCrustacea were unresolved equal numbers of trees in bothanalyses supported each of the topologies [Clade 1 (CrustaceaMarrellomorpha)] [Crustacea (Marrellomorpha Clade 1)]and [Marrellomorpha (Crustacea Clade 1)] All trees placedBurgessia as the sister taxon to the rest of Clade 1 andsupported the division of the rest of Clade 1 into two majorsister groups The topology of one of these clades (Clade 2)which included trilobites helmetiids tegopeltids naraoiidsxandarellids and Retifacies was stable with respect to thecoding of aglaspidids The other clade (Clade 3) consists of aclade of megacheirans and chelicerates (Clade 4) and a cladeincluding aglaspidids Lemoneites Paleomerus CheloniellonEmeraldella and Sidneyia (Clade 5) This latter clade is un-resolved in the strict consensus of all 27 trees (Fig 8) and thestrict consensus of the 18 MPTs found using the lsquoAglaspidida2rsquo coding but was fully resolved in the consensus of the ninetrees found with the lsquoAglaspidida 1rsquo coding Within themegacheiran-chelicerate clade all trees found Fortiforceps tobe the sister group to all other taxa Yohoia the sister group tochelicerates and Alalcomenaeus and Leanchoilia to form aclade Jianfengia is equally parsimoniously placed as the sistergroup to the Yohoia-chelicerate clade or as the sister group tothis clade and the Alalcomenaeus-Leanchoilia clade

Treating characters 4 6 37 and 44 as ordered had a minimaleffect on most parsimonious topology Nine MPTs of 136 steps(CI=0middot515 RI=0middot743) were found with the lsquoAglaspidida 1rsquocoding and 12 MPTs 135 steps in length (CI=0middot519 RI=0middot745)with the lsquoAglaspidida 2rsquo coding These trees all supported a(Crustacea Clade 1) clade excluding the marrellomorphs

Apart from this the strict consensus of these 21 trees isidentical to the consensus of the 27 trees found with allcharacters treated as unordered

Across all analyses four different topologies for Clade 5 (ofFig 8) were found (Fig 9) The distribution of these topologiesacross the four analyses described above and across the sameanalyses repeated after reweighting of each character in pro-portion to their Rescaled Consistency Index (see Farris 1989)from the first analysis is shown in Table 3 The instability of

Figure 8 Strict consensus of 27 MPTs from four separate analyses ofarachnomorph phylogeny with both interpretations of aglaspididmorphology and with some characters treated as ordered or allcharacters unordered Clades referred to in the text are numbered

Figure 9 Alternative equally parsimonious resolutions of clade 5 ofFig 8 (see Table 3)

186 TREVOR J COTTON AND SIMON J BRADDY

this clade was to some extent caused by the inclusion ofLemoneites and Paleomerus which could each be coded foronly 27 of the 54 characters used Analysis with these two taxaexcluded and characters treated as unordered found treescompatible only with Topology D of Figure 9 irrespective ofthe coding used for aglaspidids Analysis with some characterstreated as ordered and using either aglaspidid coding foundthree trees compatible only with Topology B of Figure 9 andthree compatible only with Topology D of Figure 9 ThereforeTopology D is the preferred topology for Clade 5 as shown inTable 3

One of the MPTs resulting from the lsquoAglaspidida 2rsquo coding(with all characters treated as unordered) was chosen as thebasis for further discussion because this tree combined thepreferred topology of Clade 5 with the (Crustacea Clade 1)group that was favoured when some characters were treated asordered Bootstrap percentages and Bremer support values forthis tree (using the Aglaspidida 2 coding) are shown in Figure10 Apomorphies for all ingroup nodes of this tree are shownin Figure 10A

3 Discussion

31 Phylogenetic resultsThe results of the cladistic analyses presented above arewell-resolved and robust with respect to different analyticalparameters Most of the taxa considered form a clade (Clade 1of Fig 8) that is more closely related to chelicerates than tocrustaceans in all analyses and therefore is recognised asthe Arachnomorpha as defined by Chen et al (1996) andRamskold et al (1996) The monophyly of the Marrellomor-pha was supported in all analyses The marrellomorphs werefound to be the sister group to a Crustacea+Clade 1 group inmost MPTs but in a minority of trees formed the sister groupto Clade 1 alone According to the latter result they should beincluded within the Arachnomorpha but according to the firstexcluded from it Both of these alternatives have been sup-ported in previous studies (eg Hou amp Bergstrom 1997 Willset al 1998a respectively) In addition to the marrellomorphsthe Fuxianhuiida Bousfield 1995 and Canadaspidida havebeen placed in the Euarthropoda outside the main mandibu-late and arachnomorph clades (Hou amp Bergstrom 1997)Further study of these taxa of the crustacean or mandibulatestem group and of the marrellomorphs is necessary before the

position of the Marrellomorpha can be resolved Provisionallymarrellomorphs are excluded from the Arachnomorphafollowing the majority of analyses above and Wills et al(1994)

The Burgess Shale arthropod Burgessia was placed as thesister group to remaining arachnomorphs in all analyses OtherArachnomorpha form two major clades one consisting of thelsquotrilobite-alliedrsquo arachnomorphs analysed by Edgecombe ampRamskold (1999) here termed lsquoTrilobitomorpharsquo (defined asTrilobita and all taxa more closely related to Trilobita thanChelicerata) and the other (lsquochelicerate-alliedrsquo) clade includingchelicerates lsquogreat appendagersquo arthropods aglaspididsLemoneites Paleomerus Sidneyia and Emeraldella The latteris here termed lsquoCheliceramorpharsquo (defined as Chelicerata andall taxa more closely related to Chelicerata than Trilobita) Thetopology of the lsquotrilobite-alliedrsquo clade was similar to that foundby Edgecombe amp Ramskold (1999) but the present authorsrsquoresults are more completely resolved unambiguously support-ing the sister-group relationship between trilobites andHelmetiida suggested in the previous study Within thelsquochelicerate-alliedrsquo clade Aglaspidids Paleomerus Lemoneitesand the Burgess Shale Emeraldella and Sidneyia form a cladein opposition to megacheirans and chelicerates

The present authorsrsquo analysis supports a new hypothesis ofthe origin of the chelicerates from within a paraphyleticassemblage of megacheiran arthropods and confirms pyc-nogonids as a sister group to the euchelicerates Previoushypotheses of the chelicerate sister group (see Section 1) aremuch less parsimonious The shortest trees supporting aCheloniellon-chelicerate clade are six steps longer than theMPTs and those supporting an aglaspidid-chelicerate cladeseven steps longer All megacheirans and chelicerates areunited by the flap-shaped rounded exopods and modificationof the second cephalic segment endopods into anteriorlydirected spinose raptorial organs which share a number ofdetailed similarities A clade including all of these taxa apartfrom Fortiforceps is supported by the synapomorphic loss ofthe antennae (or antennulae of crustaceans) and the longerlength of spinose projections of the second cephalic appendageendopods Within this clade Yohoia shares the loss of cephalicexopods the loss of thoracic endopods and a postabdomen oftubular tergites with chelicerates

The poor resolution of Clade 5 (of Fig 8) was partly a resultof the poor preservation of and consequent high proportion ofmissing data for Lemoneites and Paleomerus When these two

Table 3 Numbers of most parsimonious trees supporting each of the alternative topologies for clade 4 of Fig 7 under different analyticalconditions (RCI=Revealed Consistency Index)

Aglaspididcoding

Characters 4 6 36and 43 ordered

Charactersreweighted by RCI

Paleomerus andLemoneites excluded

Topology (see Fig 8)

A B C D

1 91 X 3 3 31 X 61 X X 31 X 61 X X 3 32 9 92 X 3 3 3 32 X 62 X X 32 X 62 X X 3 3Total 6 18 12 54

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 187

taxa were excluded from the analyses only two (B and D) ofthe four topologies found in the complete analysis remainedmost parsimonious (see Fig 9 and Table 3) Secondly Bremersupport values throughout much of the lsquochelicerate-alliedrsquoclade (Clade 3 of Fig 8) were improved when these taxa wereexcluded (see Fig 11)

Dunlop (1999 p 258) suggested that in reconstructing thechelicerate stem lineage lsquowe might predict that the two mostsignificant changes towards the chelicerate condition are thereduction of the antennae and the formation of the nextappendages into a clawrsquo According to the hypothesis pre-sented here both of these adaptations were achieved in EarlyCambrian chelicerate ancestors The recognition of the loss ofthe antennae in the stem group of the chelicerates and thephylogenetic hypothesis presented here suggest that chelicer-ates are most closely related to taxa with only a singleadditional segment fused to the crown group eucheliceratehead of four fused somites Previous hypotheses have derivedchelicerates from Palaeozoic arthropods with a longer head(eg Cheloniellon Sturmer amp Bergstrom 1978 SanctacarisBriggs amp Collins 1988 Emeraldella Bruton amp Whittington1983) The plesiomorphic pattern of head segmentation ismatched in pycnogonids Bergstrom et al (1980) suggestedthat only the anterior four pairs of appendages are incorpo-rated into the head This supports the view that pycnogonidsare primitive with respect to other chelicerates which showmore or less complete fusion of this primitive head and threethoracic segments including Solpugida and Pseudoscorpiones

The present study suggests that a shift from gnathobasicbenthic feeding to a more vagrant lifestyle and raptorialappendage feeding occurred very early in the chelicerate lin-eage Appendage feeding was clearly an important adaptation

Figure 10 One of 18 equally parsimonious cladograms with lsquoAglaspidida 2rsquo coding and all characters unorderedthat is most congruent with results from other analyses Non-terminal branch lengths scaled to reflect number ofapomorphies (A) ACCTRAN apomorphy scheme Character numbers shown above boxes and states shownbelow boxes as in Table 2 and the text Character consistency indices are indicated by shading (B) Levels ofsupport for individual nodes Bremer support indices shown in bold type above branches Bootstrap percentagesin italic type below branches for all nodes with relative frequencies greater than 50

Figure 11 Bremer support values for clades within the lsquochelicerate-alliedrsquo arachnomorphs (clade 3 of Fig 8) when all taxa wereincluded (normal weight font above nodes) and with Lemoneites andPaleomerus excluded (bold type below nodes)

188 TREVOR J COTTON AND SIMON J BRADDY

for the terrestrialization of chelicerates (Dunlop 1997 p 69Dunlop amp Webster 1999) but the model proposed heresuggests that this considerably pre-dated terrestrializationGnathobasic feeding in xiphosurans and eurypterids mayrepresent a reversal to a more primitve benthic arachnomorphlifestyle from a more pycnogonid-like ancestor

32 Evolution of the arachnomorph headThe extant arthropod classes show highly conserved patternsof head segmentation (eg Wills et al 1994) In contrast therehas been a near consensus amongst Cambrian arthropodworkers (although rarely based on an explicit phylogenetichypothesis) that patterns of head segmentation are highlyhomoplastic and hence of little systematic significanceSturmer amp Bergstrom (1978 pp 78ndash9) suggested that lsquoevenclosely related forms may have different numbers of headsegments and appendagesrsquo Bruton amp Whittington (1983pp 576ndash7) that lsquodiscussion on fossil arthropod relationshipsbased on head segmentation appears to be largely irrel-evant and at best speculativersquo and Delle Cave amp Simonetta(1991 p 191) that lsquothere are no obvious phyletic affinitiesbetween genera having the same number of cephalized seg-mentsrsquo Most recently Bergstrom amp Hou (1997 p 104) con-cluded that lsquothe segmental length of the head shield seemsto be of no relevance to the discussion and discrimination ofevolutionary lineagesrsquo It has been claimed that this view issupported by cladistic analyses in which patterns of tagmosishave been found to be rather poor at defining major arthropodclades (Briggs et al 1992 Wills et al 1994)

This suggestion is not only of relevance to systematics buthas featured prominently in recent debates surrounding thenature of the lsquoCambrian explosionrsquo (see Budd amp Jensen 2000for a recent review) Gould (1989 1991) argued that theapparent plasticity of head segmentation in Cambrian com-pared to post-Cambrian arthropod evolution suggests thatbody-plan evolution was only constrained after the Cambrianexplosion According to this view the origin of new highertaxa with distinct body-plans was only possible before theonset of these constraints (eg Valentine 1995) Taxa hererecognised as arachnomorphs clearly played a major role in thedevelopment of Gouldrsquos hypothesis This suggestion has pre-viously been assessed in terms of overall morphological diver-sity which is clearly rather distinct from Gouldrsquos concept ofdisparity Wills et al (1998b) showed that there was littlecorrelation between overall morphological disparity and de-gree of limb specialisation and that the latter was phylogeneti-cally highly plastic They remarked that (Wills et al 1998bp 64) lsquothis finding has important implications for models ofarthropod phylogeny and evolution that attribute overidingimportance to head segmentationrsquo but did not explicitlyexamine head segmentation although it is likely to have beena major component of the index of tagmosis employed

The results of the present study suggest that arguments thatarthropod head segmentation was unusually labile during theCambrian are poorly founded Rather only four major pat-terns of euarthropod head segmentation are identified Theplesiomorphic euarthropod state according to both Walossekamp Muller (1997 1998) and Scholtz (1997) which may moreproperly be the plesiomorphic state for the euarthropod crowngroup only (depending on the phylogenetic position of theMarrellomorpha) consists of four post-acronal segmentsbearing the antennae and three pairs of biramous limbs Thepresent authors suggest that the term lsquocephalonrsquo be restrictedto this kind of head which is found in stem group mandibu-lates in addition to many arachnomorphs Crown groupmandibulates share a lsquobimaxillary headrsquo in which an addi-tional pair of appendages the labium or second maxillae are

incorporated into the head but in crustaceans these are notfused to the carapace (Scholtz 1997) Finally two distinctforms of head tagmosis are found in chelicerates A head withfour pairs of appendages (but without antennae giving a totalof five segments incorporated into the head) which has beencalled the lsquocephalosomarsquo is present in pycnogonids (Walossekamp Dunlop 2002 pp 434ndash5) and probably also in some euch-elicerates (J A Dunlop pers comm 2003) The lsquoprosomarsquo ofmost euchelicerates consists of this cephalosoma and twoadditional segments

In general these patterns of head segmentation are highlyphylogenetically conserved (see Fig 12) Only three homo-plastic changes namely reversal to a more primitive conditionin the Leanchoilia-Alalcomenaeus clade and the convergentorigin of a five-segmented head in Clade 3 (of Fig 8) and theXandarellida are necessary to optimise character 4 onto themost parsimonious cladogram There are also transitions toautapomorphic states in Emeraldella and Sidneyia In the caseof Sidneyia however an argument could be made (followingBergstrom amp Sturmer 1978 see discussion of Character 4) thatthe head consists of five segments as in closely related taxabut autapomorphically each segment has a separate tergiteThis would further increase the phylogenetic stability of headsegmentation Also following Bergstrom amp Sturmer (1978) thehead of Cheloniellon could be considered to consist of twotergites making it more similar to the head of Emeraldella(with six pairs of appendages)

Figure 12 Cladogram from Fig 10 showing changes in head segmen-tation Bold type shows abbreviated head segmentation formulaAbbreviations (Ant) antennae (BA) biramous appendage pairs (Ch)chelicerae (GA) great appendages (UA) uniramous appendage pairsand (X) segment without appendages Synapomorphy A is the auta-pomorphic reduction in head length to a single segment in SidneyiaAlternatively this may represent secondary division of the headshield Synapomorphy B is the increase in the length of the head inEmeraldella to six segments

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 189

4 Acknowledgements

We thank D E G Briggs J A Cotton J A Dunlop R AFortey S Harris R A Moore P Selden O E Tetlie andP Van Roy for comments on earlier drafts of this work Themanuscript was reviewed by M Wills F Schram and oneanonymous referee This research was supported by the Geo-logical Society of London Curry Research Studentshipawarded to TJC by the Department of Earth Sciences Uni-versity of Bristol and by the Department of PalaeontologyThe Natural History Museum London TJC would like tothank C Labandeira J Thompson and R Chapman fortheir hospitality at the National Museum of Natural HistoryWashington DC USA and A Olcott S Schellenberg andJ Young for making his time there so enjoyable

5 References

Abzhanov A Popadic A amp Kaufman T C 1999 Chelicerate Hoxgenes and the homology of arthropod segments Evolution andDevelopment 1 77ndash89

Akam M 2000 Arthropods developmental diversity within a (super)phylum Proceedings of the National Academy of Sciences USA97 4438ndash41

Aldridge R J Gabbott S E amp Theron J N 2001 The Soom ShaleIn Briggs D E G amp Crowther P R (eds) Palaeobiology II340ndash2 Oxford Blackwell Science

Anderson L I amp Selden P A 1997 Opisthosomal fusion andphylogeny of Palaeozoic Xiphosura Lethaia 30 19ndash31

Arango C P 2002 Morphological phylogenetics of the sea spiders(Arthropoda Pycnogonida) Organisms Diversity and Evolution2 107ndash25

Ax P 1986 Das phylogenetische System Stuttgart Gustav FischerVerlag

Babcock L E amp Zhang Wentang In press New non-mineralizedarthropods from the Chengjiang Lagerstatte Lower CambrianYunnan China Palaeontologia Cathyana

Bartels C Briggs D E G amp Brassel G 1998 The fossils ofthe Hunsruck Slate Marine life in the Devonian CambridgeCambridge University Press

Bergstrom J 1971 Paleomerus ndash merostome or merostomoid Lethaia4 393ndash401

Bergstrom J 1979 Morphology of fossil arthropods as a guideto phylogenetic relationships In Gupta A P (ed) ArthropodPhylogeny 3ndash56 New York NY Van Nostrand Reinhold Co

Bergstrom J 1992 The oldest arthropods and the origin of theCrustacea Acta Zoologica 73 287ndash91

Bergstrom J Sturmer W amp Winter G 1980 PalaeoisopusPalaeopantopodus and Palaeothea pycnogonid arthropodsfrom the Lower Devonian Hunsruck Slate West GermanyPalaontologische Zeitschrift 54 7ndash54

Bergstrom J amp Brassel G 1984 Legs in the trilobite Rhenops fromthe Lower Devonian Hunsruck Slate Lethaia 17 67ndash72

Bergstrom J amp Hou X 1998 Chengjiang arthropods and theirbearing on early arthropod evolution In Edgecombe G D(ed) Arthropod fossils and phylogeny 151ndash84 New York NYColumbia University Press

Boudreaux H B 1979 Significance of intersegmental tendon systemin arthropod phylogeny and a monophyletic classification ofArthropoda In Gupta A P (ed) Arthropod Phylogeny 551ndash86New York NY Van Nostrand Reinhold Co

Bousfield E L 1995 A contribution to the natural classification ofLower and Middle Cambrian arthropods food-gathering andfeeding mechanisms Amphipacifica 2 3ndash33

Braddy S J Aldridge R J Gabbott S E amp Theron J N 1999Lamellate book-gills in a late Ordovician eurypterid from theSoom Shale South Africa support for a eurypterid-scorpionclade Lethaia 32 72ndash4

Braddy S J amp Almond J E 1999 Eurypterid trackways from theTable Mountain Group (Ordovician) of South Africa Journal ofAfrican Earth Sciences 29 165ndash77

Bremer K 1988 The limits of amino acid sequence data in angio-sperm phylogenetic reconstruction Evolution 42 795ndash803

Bremer K 1994 Branch support and tree stability Cladistics 10295ndash304

Briggs D E G 1998 Review of Arthropods from the LowerCambrian Chengjiang fauna southwest China by HouXianguang and Jan Bergstrom Fossils and Strata 45

Scandinavian University Press Oslo Norway 1997 Palaeo-geography Palaeoclimatology Palaeoecology 143 193ndash4

Briggs D E G Bruton D L amp Whittington H B 1979 Append-ages of the arthropod Aglaspis spinifer (Upper CambrianWisconsin) and their significance Palaeontology 22 167ndash80

Briggs D E G Fortey R A amp Wills M A 1992 Morphologicaldisparity in the Cambrian Science 256 1670ndash3

Briggs D E G Fortey R A amp Wills MA 1993 How big was theCambrian explosion A taxonomic and morphologic comparisonof Cambrian and Recent arthropods In Lees D R amp EdwardsD (eds) Evolutionary patterns and processes 33ndash44 LondonLinnean Society of London Linnean Society Symposium Series

Briggs D E G Erwin D H amp Collier F J 1994 The fossils of theBurgess Shale Washington DC Smithsonian Institution Press

Briggs D E G amp Bartels C 2001 New arthropods from the LowerDevonian Hunsruck Slate (Lower Emsian Rhenish Massifwestern Germany) Palaeontology 44 275ndash304

Briggs D E G amp Bartels C 2002 Magnoculocaris a new name forthe arthropod Magnoculus Briggs and Bartels 2001 from theLower Devonian Hunsruck Slate Germany Palaeontology 45419

Briggs D E G amp Collins D 1988 A Middle Cambrian cheliceratefrom Mount Stephen British Columbia Palaeontology 31 779ndash98

Briggs D E G amp Collins D 1999 The arthropod Alalcomenaeuscambricus Simonetta from the Middle Cambrian Burgess Shale ofBritish Columbia Paleontology 42 953ndash78

Briggs D E G amp Fortey R A 1982 The cuticle of aglaspididarthropods a red-herring in the early history of vertebratesLethaia 15 25ndash9

Briggs D E G amp Fortey R A 1989 The early radiation andrelationships of the major arthropod groups Science 246 241ndash3

Briggs D E G amp Fortey R A 1992 The early Cambrian radiationof arthropods In Lipps J H amp Signor P W (eds) Origin andEarly Evolution of the Metazoa 335ndash373 New York NY PlenumPress

Briggs D E G amp Robison R A 1984 Exceptionally preservednontrilobite arthropods and Anomalocaris from the MiddleCambrian of Utah University of Kansas PaleontologicalContributions Paper 111 1ndash23

Broili F 1932 Ein neuer Crustacee aus dem rheinischen UnterdevonSitzungsberichte der Mathematisch-Naturwissenschaftlichen(Abteilung) Klasse der Bayerischen Akademie der Wissenschaftenzu Munchen 1932 27ndash38

Broili F 1933 Ein zweites Exemplar von Cheloniellon Sitzungsber-ichte der Bayerischen Akademie der Wissenschaften 1933 11ndash32

Bruton D L 1981 The arthropod Sidneyia inexpectans MiddleCambrian Burgess Shale British Columbia Philosophical Trans-actions of the Royal Society of London Series B 295 619ndash56

Bruton D L amp Whittington H B 1983 Emeraldella and Leanchoiliatwo arthropods from the Burgess Shale British Columbia Philo-sophical Transactions of the Royal Society of London Series B 300553ndash85

Budd G E 1995 Kleptothule rasmusseni gen et sp nov anolenellid-like trilobite from the Sirius Passet fauna (BuenFormation Lower Cambrian North Greenland) Transactions ofthe Royal Society of Edinburgh Earth Sciences 86 1ndash12

Budd G E 1996a Progress and problems in arthropod phylogenyTrends in Ecology and Evolution 11 356ndash8

Budd G E 1996b The morphology of Opabinia regalis and thereconstruction of the arthropod stem-group Lethaia 29 1ndash14

Budd G E 1997 Stem-group arthropods from the Lower CambrianSirius Passet fauna of North Greenland In Fortey R A ampThomas R H (eds) Arthropod Relationships 125ndash38 LondonChapman and Hall Systematics Association Special Volume 55

Budd G E 1999a A nektaspid arthropod from the Early CambrianSirius Passet fauna with a description of retrodeformation basedon functional morphology Palaeontology 42 99ndash122

Budd G E 1999b The morphology and phylogenetic significance ofKerygmachela kiergkegaardi Budd (Buen Formation LowerCambrian N Greenland) Transactions of the Royal Society ofEdinburgh Earth Sciences 89 (for 1998) 249ndash290

Budd G E 2002 A palaeontological solution to the arthropod headproblem Nature 417 271ndash5

Budd G E amp Jensen S 2000 A critical reappraisal of the fossilrecord of the bilaterian phyla Biological Reviews 75 253ndash95

Chen J Zhou G Zhu M amp Yeh K 1996 The Chengjiang biota ndasha unique window of the Cambrian explosion Taichung NationalMuseum of Natural Science [In Chinese]

Chen J Edgecombe G D amp Ramskold L 1997 Morphological andecological disparity in naraoiids (Arthropoda) from the Early

190 TREVOR J COTTON AND SIMON J BRADDY

Cambrian Chengjiang fauna China Records of the AustralianMuseum 49 1ndash24

Chen J amp Zhou G 1997 Biology of the Chengjiang fauna In ChenJ Cheng Y amp Iten H V (eds) The Cambrian explosion and thefossil record Bulletin of the National Museum of Natural ScienceTaiwan 10 11ndash106

Child C A 1997 Some deep-sea Pycnogonida from Argentine slopeand basin Proceedings of the Biological Society of Washington110 128ndash42

Chlupac I 1988 The enigmatic arthropod Duslia from the Ordovicianof Czechslovakia Palaeontology 31 611ndash20

Chlupac I 1995 Lower Cambrian arthropods from the PasekyShale (Barrandian area Czech Republic) Journal of the CzechGeological Society 40 9ndash36

Chlupac I amp Havlıcek V 1965 Kodymirus n g a new aglaspidmerostome of the Cambrian of Bohemia Sbornık GeologickechVid Paleontologie 6 7ndash20

Cisne J L 1975 Anatomy of Triarthrus and the relationships of theTrilobita Fossils and Strata 4 45ndash63

Cisne J L 1981 Triarthrus eatoni (Trilobita) anatomy of its exo-skeletal skeletomuscular and digestive systems Palaeonto-graphica Americana 9 95ndash142

Conway Morris S 1982 Atlas of the Burgess Shale London ThePalaeontological Association

Conway Morris S 1989 The persistence of Burgess Shale-type faunasimplications for the evolution of deeper-water faunas Trans-actions of the Royal Society of Edinburgh Earth Sciences 80271ndash83

Conway Morris S 1998 The crucible of creation Oxford OxfordUniversity Press

Cotton T J 1999 Review of Arthropod fossils and phylogenyPalaeogeography Palaeoclimatology Palaeoecology 151 341ndash3

Cotton T J 2002 The phylogeny and morphological evolution ofCambrian trilobites and their relatives Unpublished PhD ThesisUniversity of Bristol

Damen W G M Hausdorf M Seyfarth E A amp Tautz D 1998 Aconserved mode of head segmentation in arthropods revealed bythe expression pattern of Hox genes in a spider Proceedings of theNational Academy of Sciences USA 95 10665ndash70

Delle Cave L amp Simonetta A M 1991 Early Palaeozoic arthropodsand problems of arthropod phylogeny with some notes on taxa ofdoubtful affinities In Simonetta A M amp Conway Morris S(eds) The early evolution of Metazoa and the significance ofproblematic taxa 189ndash244 Cambridge Cambridge UniversityPress

Dewel R A amp Dewel W C 1997 The place of tardigrades inarthropod evolution In Fortey R A amp Thomas R H (eds)Arthropod Relationships 109ndash23 London Chapman and HallSystematics Association Special Volume 55

Dunlop J A 1997 Palaeozoic arachnids and their significance forarachnid phylogeny Proceedings of the 16th European Colloquiumof Arachnology 65ndash82 Siedlce Poland

Dunlop J A 1998 The origins of tetrapulmonate booklungs and theirsignificance for chelicerate phylogeny In Selden P A (ed)Proceedings of the 17th European Colloquium of ArachnologyEdinburgh 1997 pp 9ndash16

Dunlop J A 1999 Pasando revista a la evolucion de los quelicerados[A review of chelicerate evolution] In Melic A DeHaro J JMedez M amp Ribera I (eds) Volumen Monografico Evolucion yFilogenia de Arthropoda Boletin de la Sociedad EntolomologicaAragonesa 26 255ndash272 [In Spanish and English]

Dunlop J A amp Selden P A 1997 The early history and phylogenyof the chelicerates In Fortey R A amp Thomas R H (eds)Arthropod relationships Systematics Association Special Volume55 221ndash35 London Chapman and Hall

Dunlop J A amp Webster M 1999 Fossil evidence terrestrializationand arachnid phylogeny Journal of Arachnology 27 86ndash93

Dzik J 1993 Early metazoan evolution and the meaning of its fossilrecord Evolutionary Biology 27 339ndash86

Dzik J amp Lendzion K 1988 The oldest arthropods of the EastEuropean Platform Lethaia 21 29ndash38

Edgecombe G D Wilson G D F Colgan D J Gray M R ampCassis G 2000 Arthropod cladistics combined analysisof Histone H3 and U2 snRNA sequences and morphologyCladistics 16 155ndash203

Edgecombe G D amp Ramskold L 1999 Relationships of CambrianArachnata and the systematic position of Trilobita Journal ofPaleontology 73 263ndash87

Emerson M J amp Schram F R 1997 Theories patterns and realitygame plan for arthropod phylogeny In Fortey R A amp Thomas

R H (eds) Arthropod Relationships Systematics AssociationSpecial Volume 55 67ndash87 London Chapman and Hall

Farris J S 1989 The Retention Index and the Rescaled ConsistencyIndex Cladistics 5 417ndash19

Felsenstein J 1985 Confidence limits on phylogenies an approachusing the bootstrap Evolution 39 783ndash91

Flower R H 1968 Merostomes from a Cotter Horizon of the El PasoGroup New Mexico Bureau of Mines and Mineral ResourcesMemoir 22 35ndash44

Fortey R A 1990a Trilobite evolution and systematics In MikulicD G (ed) Arthropod paleobiology 44ndash65 Short Courses inPaleontology 3 Knoxville Tenessee The Paleontological Society

Fortey R A 1990b Ontogeny hypostome attachment and trilobiteclassification Palaeontology 33 529ndash76

Fortey R A 1997 Classification In Kaesler R L (ed) Treatise oninvertebrate paleontology Part O Arthropoda 1 (revised) 289ndash302Boulder Colorado amp Lawrence Kansas Geological Society ofAmerica and University of Kansas Press

Fortey R A amp Theron J N 1994 A new Ordovician arthropodSoomaspis and the agnostid problem Palaeontology 37 841ndash61

Fortey R A amp Whittington H B 1989 The Trilobita as a naturalgroup Historical Biology 2 125ndash38

Fry W G 1978 A classification within the pycnogonids ZoologicalJournal of the Linnean Society of London 63 35ndash58

Giribet G Edgecombe G D amp Wheeler W C 2001 Arthropodphylogeny based on eight molecular loci and morphology Nature413 157ndash61

Gould S J 1989 Wonderful life The Burgess Shale and the nature ofhistory New York NY Norton

Gould S J 1991 The disparity of the Burgess Shale arthropod faunaand the limits of cladistic analysis why we must strive to quantifymorphospace Paleobiology 17 411ndash23

Gurich G 1931 Mimetaster hexagonalis ein neuer Kruster aus demunterdevonischen Bundenbacher Dachschiefer PalaontologischeZeitschrift 12 204ndash38

Hammann W Laske R amp Pillola G L 1990 Tariccoia arrusensisngnsp an unusual trilobite-like arthropod Rediscovery of thelsquophylocaridrsquo beds of Taricco (1922) in the Ordovician lsquoPuddingarsquosequence of Sardinia Bolletino della Societa PaleontologicaItaliana 29 163ndash78

Haas M S Brown S J amp Beeman R W 2001 Pondering theprocephalon the segmental origin of the labrum DevelopmentGenes and Evolution 211 89ndash95

Heider K 1913 Entwicklungsgeschichte und Morphologie derWirbellosen Die Kultur der Gegenwart Theil 3 Abt 4 2 176ndash332

Henriksen K 1928 Critical notes upon Cambrian arthropodsdescribed by Charles D Walcott Videnskabelige Meddelelser fraDansk Naturhistorisk Forening 86 1ndash20

Hesselbo S P 1988 Trace fossils of Cambrian aglaspidid arthropodsLethaia 21 139ndash46

Hesselbo S P 1989 The aglaspidid arthropod Beckwithia from theCambrian of Utah and Wisconsin Journal of Paleontology 63636ndash42

Hesselbo S P 1992 Aglaspidida (Arthropoda) from the UpperCambrian of Wisconsin Journal of Paleontology 66 885ndash923

Hou X 1987a Three new large arthropods from Lower CambrianChengjiang fauna eastern Yunnan Acta Palaeontologica Sinica26 272ndash85 [In Chinese with English summary]

Hou X 1987b Two new arthropods from Lower CambrianChengjiang eastern Yunnan Acta Palaeontologica Sinica 26236ndash56 [In Chinese with English summary]

Hou X Chen J amp Lu H 1989 Early Cambrian new arthropodsfrom Chengjiang Yunnan Acta Palaeontologica Sinica 28 42ndash57[In Chinese with English summary]

Hou X Ramskold L amp Bergstrom J 1991 Composition andpreservation of the Chengjiang fauna a Lower Cambrian soft-bodied biota Zoologica Scripta 20 395ndash411

Hou X Bergstrom J amp Ahlberg P 1995 Anomalocaris and otherlarge animals in the Lower Cambrian Chengjiang fauna ofsouthwest China GFF 117 163ndash83

Hou X amp Bergstrom J 1997 Arthropods of the Lower CambrianChengjiang fauna southwest China Fossils and Strata 45 1ndash116

Hughes C P 1975 Redescription of Burgessia bella from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 4415ndash36

Ivantsov A Y 1999 Trilobite-like arthropod from the LowerCambrian of the Siberian Platform Acta PalaeontologicaPolonica 44 455ndash66

King P E 1973 Pycnogonids London St Martin Press

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 191

Lauterbach K E 1973 Schlussereignisse in der Evolution derStammgruppe der Euarthropoda Zoologische Beitrage 19259ndash99

Lauterbach K E 1980 Schlussereignisse in der Evolution derGrundplans der Arachnata (Arthropoda) Abhandlungen desNaturwissenschaftlichen Vereins in Hamburg 23 163ndash327

Lauterbach K E 1983 Synapomorphien zwischen Trilobiten- undCheliceraten-zweig der Arachnata Zoologischer Anzeiger 210213ndash38

Lendzion K 1975 Fauna of the Mobergella Zone in the Polish LowerCambrian Kwartalnik Geologiczny 19 237ndash42

Lieberman B S 1998 Cladistic analysis of the Early Cambrianolenelloid trilobites Journal of Paleontology 72 59ndash78

Lu Y 1940 On the ontogeny and phylogeny of Redlichia intermediaLu (sp nov) Bulletin of the Geological Society of China 20333ndash42 [In Chinese]

Maddison W P amp Maddison D R 1997 MacClade analysis ofphylogeny and character evolution Version 307 Sunderland MASinauer Associates

Mittmann B amp Scholtz G 2003 Development of the nervous systemin the lsquoheadrsquo of Limulus polyphemus (Chelicerata Xiphosura)morphological evidence for the correspondence between the seg-ments of the chelicerae and the (first) antennae of MandibulataDevelopment Genes and Evolution 213 9ndash17

Moore R C (ed) 1959 Treatise on invertebrate paleontology Part OArthropoda 1 Boulder Colorado Geological Society of America

Muller K J amp Walossek D 1986 Arthropod larvae from the UpperCambrian of Sweden Transactions of the Royal Society ofEdinburgh Earth Sciences 77 157ndash79

Muller K J amp Walossek D 1987 Morphology ontogeny and lifehabit of Agnostus pisiformis from the Upper Cambrian of SwedenFossils and Strata 19 1ndash124

Munilla T 1999 Evolucion y filogenia de los Picnogonidos InMelicA DeHaro J J Medez M amp Ribera I (eds) VolumenMonografico Evolucion y Filogenia de Arthropoda Boletin de laSociedad Entolomologica Aragonesa 26 273ndash91 [In Spanish andEnglish]

Orr P J Siveter D J Briggs D E G Siveter D J amp Sutton M D2000 A new arthropod from the Silurian Konservat-Lagerstatteof Herefordshire Proceedings of the Royal Society of LondonSeries B 267 1497ndash504

Page R D M 1996 On consensus confidence and lsquototal evidencersquoCladistics 12 83ndash92

Palmer A R amp Repina L N 1993 Through a glass darklytaxonomy phylogeny and biostratigraphy of the OlenellinaUniversity of Kansas Paleontological Contributions New Series 31ndash35

Platnick N I Griswold C E amp Coddington J A 1991 On missingentries in cladistic analysis Cladistics 7 337ndash43

Pleijel F 1995 On character coding for phylogeny reconstructionCladistics 11 309ndash15

Raasch G O 1939 Cambrian Merostomata Special Papers of theGeological Society of America 19 1ndash146

Ramskold L Chen J Edgecombe G D amp Zhou G 1996Preservational folds simulating tergite junctions in tegopeltid andnaraoiid arthropods Lethaia 29 15ndash20

Ramskold L Chen J Edgecombe G D amp Zhou G 1997Cindarella and the arachnate clade Xandarellida (ArthropodaEarly Cambrian) from China Transactions of the Royal Society ofEdinburgh Earth Sciences 88 19ndash38

Ramskold L amp Edgecombe G D 1991 Trilobite monophylyrevisited Historical Biology 4 267ndash83

Ramskold L amp Edgecombe G D 1996 Trilobite appendagestructure ndash Eoredlichia revisited Alcheringa 20 269ndash76

Raw F 1957 Origin of chelicerates Journal of Paleontology 31139ndash193

Repina L N amp Okuneva O G 1969 Cambrian arthropods of themaritime territory Paleontological Journal 1969 95ndash103

Richter R amp Richter E 1929 Weinbergina opitz ng n sp einSchwerttrager (Merost Xiphos) aus dem Devon (Rheinland)Senkenbergiana 11 193ndash209

Robison R A 1984 New occurences of the unusual trilobite Naraoiafrom the Cambrian of Idaho and Utah University of KansasPaleontological Contributions Paper 112 1ndash8

Rominger C 1887 Description of primordial fossils from MtStephens NW Territory of Canada Proceedings of the Academyof Natural Sciences of Philadelphia 1887 12ndash19

Scholtz G 1997 Cleavage germ band formation and head segmen-tation the ground pattern of the Euarthropoda In Fortey R Aamp Thomas R H (eds) Arthropod Relationships 317ndash32 LondonChapman and Hall Systematics Association Special Volume 55

Schram F R 1978 Arthropods a convergent phenomenonFieldiana Geology 39 61ndash108

Schram F R 1993 Review of Boxshall Stromberg and Dahl TheCrustacea Origin and Evolution Journal of Crustacean Biology13 820ndash2

Schram F R Vonk R amp Hof C H J 1997 Mazon CreekCycloidea Journal of Paleontology 71 261ndash84

Schram F R amp Emerson M J 1991 Arthropod Pattern Theorya new approach to arthropod phylogeny Memoirs of theQueensland Museum 31 1ndash18

Schram F R amp Hof C H J 1998 Fossils and the interrelationshipsof major crustacean groups In Edgecombe G D (ed) Arthropodfossils and phylogeny pp 233ndash302 New York NY ColumbiaUniversity Press

Shergold J H 1991 Protaspis and early meraspis growth stages of theeodiscoid trilobite Pagetia ocellata Jell and their implications forclassification Alcheringa 15 65ndash86

Shu D Geyer G Ling C amp Zhang Z 1995 Redlichiacean trilobiteswith preserved soft parts from the Lower Cambrian Chengjiangfauna (South China) Beringeria (Special Issue) 2 203ndash41

Shultz J W 1990 Evolutionary morphology and phylogeny ofArachnida Cladistics 6 1ndash38

Shultz J W 2000 Skeletomuscular anatomy of the harvestmanLeiobunum aldrichi (Weed 1893) (Arachnida OpilionesPalpatores) and its evolutionary significance Zoological Journalof the Linnean Society 128 401ndash38

Shultz J W 2001 Gross muscular anatomy of Limulus polyphemus(Xiphosura Chelicerata) and its bearing on evolution in the theArachnida Journal of Arachnology 29 283ndash303

Siewing R (ed) 1985 Lehrbuch der Zoologie II Systematik 3rd ednStuttgart Fischer

Simonetta A M 1970 Studies on the non trilobite arthropods ofthe Burgess Shale (Middle Cambrian) the genera LeanchoiliaAlalcomenaeus Opabinia Burgessia Yohoia and ActaeusPalaeontographica Italica 66 35ndash45

Simonetta A M amp Delle Cave L 1975 The Cambrian non trilobitearthropods from the Burgess Shale of British Columbia A studyof their comparative morphology taxonomy and evolutionarysignificance Palaeontographica Italica 69 1ndash37

Simonetta A M amp Delle Cave L 1981 An essay in the compara-tive and evolutionary morphology of Palaeozoic ArthropodsAccademia Nazionale dei Lincei Atti dei Convegni Lincei 49389ndash439

Starobogatov Y I 1990 Sistema i filogeniya nizshikh khelitserovykh(analiz morfologii paleozoysikh grupp) [The systematics andphylogeny of the lower chelicerates (a morphological analysis ofthe Palaeozoic groups)] Paleontologicheskii Zhurnal 1990 4ndash17

Stoslashrmer L 1944 On the relationships and phylogeny of fossil andrecent Arachnomorpha Skrifter utgitt ac Det Norske Videnskaps-Akademi i Olso I Matematisk-naturvitenskaplig Klasse 5 1ndash158

Stoslashrmer L 1951 Studies on trilobite morphology Part III Theventral cephalic structures with remarks on the zoologicalposition of the trilobites Norsk geologisk tidsskrift 29 108ndash58

Stoslashrmer L 1956 A Lower Cambrian merostome from Sweden Arkivfor Zoologi Ser 2 9 507ndash14

Stoslashrmer L 1959 Trilobitoidea In Moore R C (ed) Treatise onInvertebrate Paleontology part O Arthropoda 1 23ndash37 BoulderColorado Geological Society of America

Strong E E amp Lipscomb D 1999 Character coding and inapplicabledata Cladistics 15 363ndash71

Sturmer W amp Bergstrom J 1976 The arthropods Mimetaster andVachonisia from the Devonian Hunsruck Shale PalaontologischeZeitschrift 50 78ndash111

Sturmer W amp Bergstrom J 1978 The arthropod Cheloniellon fromthe Devonian Hunsruck Shale Palaontologische Zeitschrift 5257ndash81

Sturmer W amp Bergstrom J 1981 Weinbergina a xiphosuran fromthe Devonian Hunsruck Slate Palaontologische Zeitschrift 55237ndash55

Sutton M D Briggs D E G Siveter D J Siveter D J amp Orr PJ 2002 The arthropod Offacolus kingi (Chelicerata) from theSilurian of Herefordshire England computer based morphologi-cal reconstructions and phylogenetic affinity Proceedings of theRoyal Society of London Series B 269 1195ndash1203

Swofford D L 1999 PAUP Phylogenetic Analysis Using Parsimony(and other methods) Version 40b2a Sunderland MA SinauerAssociates

Telford M J amp Thomas R H 1998 Expression of homeobox genesshows chelicerate arthropods retain their deutocerebral segmentProceedings of the National Academy of Sciences USA 9510671ndash5

192 TREVOR J COTTON AND SIMON J BRADDY

Thorley J L amp Page R D M 2000 RadCon phylogenetic treecomparison and consensus Bioinformatics 16 486ndash7

Valentine J W 1995 Why no new phyla after the Cambrian ndashgenome and ecospace hypotheses revisited Palaios 10 190ndash194

Wagele J W 1993 Rejection of the lsquoUniramiarsquo hypothesis andimplications of the Mandibulata concept Zoologische JahrbucherAbteilung fur Systematik Geographie und Biologie der Tiere 120253ndash88

Walcott C D 1911 Middle Cambrian Merostomata Cambriangeology and palaeontology II Smithsonian MiscellaneousCollections 57 17ndash40

Walcott C D 1912 Middle Cambrian Branchiopoda MalacostracaTrilobita and Merostomata Cambrian geology and palaeontol-ogy II Smithsonian Miscellaneous Collections 57 145ndash228

Walcott C D 1918 Geological explorations in the CanadianRockies Explorations and fieldwork of the SmithsonianInstitution in 1917 Smithsonian Miscellaneous Collections 684ndash20

Walossek D 1993 The Upper Cambrian Rehbachiella kinnekullensisand the phylogeny of Branchiopoda and Crustacea Fossils andStrata 32 1ndash202

Walossek D amp Dunlop J A 2002 A larval sea spider (ArthropodaPycnogonida) from the Upper Cambrian lsquoOrstenrsquo of Swedenand the phylogenetic position of pycnogonids Palaeontology 45421ndash46

Walossek D amp Muller K J 1990 Upper Cambrian stem-lineagecrustaceans and their bearing upon the monophyletic origin ofCrustacea and the position of Agnostus Lethaia 23 409ndash27

Walossek D amp Muller K J 1997 Cambrian lsquoOrstenrsquo-type arthro-pods and the phylogeny of Crustacea In Fortey R A amp ThomasR H (eds) Arthropod Relationships 139ndash53 SystematicsAssociation Special Volume 55 London Chapman and Hall

Walossek D amp Muller K J 1998 Early arthropod phylogeny in lightof the Cambrian lsquoOrstenrsquo fossils In Edgecombe G D (ed)Arthropod fossils and phylogeny 185ndash231 New York NYColumbia University Press

Weygoldt P 1979 Significance of later embryonic stages and headdevelopment in arthropod phylogeny In Gupta A P (ed)Arthropod Phylogeny 107ndash35 New York NY Van NostrandReinhold Co

Weygoldt P 1998 Evolution and systematics of the ChelicerataExperimental and Applied Acarology 22 63ndash79

Weygoldt P amp Paulus H F 1979 Unterzuchungen aur Morpholo-gie Taxonomie und Phylogenie der Chelicerata Zeitschriftfur Zoologische Systematik und Evolutionsforschung 17 85ndash116177ndash200

Wheeler W C Cartwright P amp Hayashi C Y 1993 Arthropodphylogeny a combined approach Cladistics 9 1ndash39

Wheeler W C amp Hayashi C Y 1998 The phylogeny of extantchelicerate orders Cladistics 14 173ndash92

Whittington H B 1971 Redescription of Marrella splendens(Trilobitoidea) from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 209 1ndash24

Whittington H B 1974 Yohoia Walcott and Plenocaris n genarthropods from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 231 1ndash21

Whittington H B 1975a Trilobites with appendages from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 497ndash136

Whittington H B 1975b The enigmatic animal Opabinia regalisMiddle Cambrian Burgess Shale British Columbia PhilosophicalTransactions of the Royal Society of London Series B 271 1ndash43

Whittington H B 1977 The Middle Cambrian trilobite NaraoiaBurgess Shale British Columbia Philosophical Transactions of theRoyal Society Series B 280 409ndash43

Whittington H B 1979 Early arthropods their appendages andrelationships In House M R (ed) The origin of majorinvertebrate groups 253ndash68 Systematics Association SpecialVolume 12 New York NY Academic Press

Whittington H B 1980 Exoskeleton moult stage appendage mor-phology and habits of the Middle Cambrian trilobite Olenoidesserratus Palaeontology 23 171ndash204

Whittington H B 1981 Rare arthropods from the Burgess ShaleMiddle Cambrian British Columbia Philosophical Transactionsof the Royal Society of London Series B 292 329ndash57

Whittington H B 1985 Tegopelte gigas a second soft-bodied trilo-bite from the Burgess Shale Middle Cambrian British ColumbiaJournal of Palaeontology 23 171ndash204

Whittington H B 1989 Olenelloid trilobites type species functionalmorphology and higher classification Philosophical Transactionsof the Royal Society of London Series B 324 111ndash47

Whittington H B amp Almond J E 1987 Appendages and habits ofthe Upper Ordovician trilobite Triarthus eatoni PhilosophicalTransactions of the Royal Society of London Series B 317 1ndash46

Wilkinson M 1992 Ordered versus unordered characters Cladistics8 375ndash85

Wilkinson M 1994 Common cladistic information and its consensusrepresentation reduced Adams and reduced cladistic consensustrees and profiles Systematic Biology 43 343ndash68

Wilkinson M 1995a Coping with abundant missing entries in phylo-genetic inference using parsimony Systematic Biology 44 501ndash14

Wilkinson M 1995b TAXEQ2 software and documentation BristolSchool of Biological Sciences University of Bristol

Wilkinson M 1996 On the distribution of homoplasy and choosingamong trees Taxon 45 263ndash66

Wills M A 1997 A phylogeny of Recent and fossil Crustacea derivedfrom morphological characters In Fortey R A amp ThomasR H (eds) Arthropod Relationships 189ndash209 SystematicsAssociation Special Volume 55 London Chapman and Hall

Wills M A Briggs D E G amp Fortey R A 1994 Disparity asan evolutionary index a comparison of Cambrian and Recentarthropods Paleobiology 20 93ndash130

Wills M A Briggs D E G Fortey R A amp Wilkinson M 1995The significance of fossils in understanding arthropod evolutionVerhandlungen der deutschen zoologischen Gesselschaft 88 203ndash15

Wills M A Briggs D E G Fortey R A Wilkinson M amp SneathP H A 1998a An arthropod phylogeny based on fossil andrecent taxa In Edgecombe G D (ed) Arthropod fossils andphylogeny 33ndash105 New York NY Columbia University Press

Wills M A Briggs D E G amp Fortey R A 1998b Evolutionarycorrelates of arthropod tagmosis scrambled legs In ForteyR A amp Thomas R H (eds) Arthropod Relationships 57ndash65Systematics Association Special Volume 55 London Chapmanand Hall

Winter G 1980 Beitrage zur Morphologie und Embryologie desvorderen Korperabschnitts (Cephalosoma) der PantopodaGerstaecker 1863 I Entstehung und Struktur des Zentralnerven-systems Zeitschrift fur Zoologische Systematik und Evolutionsfor-schung 18 27ndash61

Zhang W amp Hou X 1985 Preliminary notes on the occurrence of theunusual trilobite Naraoia in Asia Acta Palaeontologica Sinica 24591ndash5 [In Chinese with English summary]

TREVOR J COTTON Department of Palaeontology The Natural History Museum CromwellRoad London SW7 5BD UKe-mail trevor_cottonhotmailcom

SIMON J BRADDY Department of Earth Sciences University of Bristol Wills MemorialBuilding Queenrsquos Road Bristol BS8 1RJ UKe-mail S J Braddybrisacuk

MS received 28 July 2003 Accepted for publication 15 March 2004

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 193

Page 16: The phylogeny of arachnomorph arthropods and the origin of the

in other trilobite-like arachnomorphs is unclear and a restric-tive coding is used here The present authors do not considerthat this character can be applied reliably to the fused thoracictergites of Saperion Tegopelte and Skioldia and it is coded asinapplicable to all taxa lacking free thoracic tergites (Character32 State 1)

35 Trunk effacement (0) trunk with defined (separate orfused) tergite boundaries (1) trunk tergite boundaries effacedlaterally and (2) trunk tergite boundaries completely effaced(cf Edgecombe amp Ramskold 1999 Character 15)] Contrary toEdgecombe amp Ramskold (1999) the present authors considerSkioldia and Saperion to show a distinct form of tergiteeffacement where the fused tergite boundaries are defined byfurrows axially but effaced laterally The boundaries areeffaced at least laterally in Tegopelte but the axial region ofthe exoskeleton is unknown which accordingly is given amultistate uncertainty coding

36 Cephalic articulation fused (0) absent and (1) presentIn Tegopelte Saperion and Skioldia the articulation ofthe head with the thorax is non-functional and the entireexoskeleton forms a single tergite

37 Head shield overlap of thoracic tergites (0) overlapabsent or identical to overlap between thoracic segments (1)head shield covers first thoracic tergite only and (2) headshield covers mutliple anterior trunk tergites

38 Head shield articulates with reduced anterior thoracictergite (0) absent and (1) present Amongst taxa showing aposteriorly expanded head shield ie one that overlaps thethorax Edgecombe amp Ramskold (1999 Character 9) identifiedtwo distinct character states One of these ndash overlap of only theanteriormost thoracic tergite ndash was limited to taxa assigned tothe Liwiinae (sensu Fortey amp Theron 1994) and another ndashoverlap of multiple tergites with attachment to a reducedanterior thoracic tergite ndash to the Xandarellida None of theadditional taxa considered herein (including Buenaspis whichwas assigned to the Liwiinae by Budd 1999a) shows thesedistinctive morphological features However the expandedhead shields of Marrella Mimetaster and Burgessia which donot articulate with a narrow anterior thoracic tergite may behomologous with the expanded head shields of xandarellidsTo recognise this the degree of overlapping of the thorax andthe possession of the reduced anterior tergite are coded sepa-rately These characters are both coded as inapplicable to taxain which the head shield and thoracic tergites are fused(Character 36 State 1) The crustacean carapace which ismost similar to the head shield of Burgessia is seeminglyabsent in stem group crustaceans (Walossek amp Muller1998)

39 Trunk narrowed anteriorly relative to head shieldwidest posteriorly (0) absent and (1) present (cf Edgecombeamp Ramskold 1999 Character 14) The anterior narrowing ofthe trunk caused by the reduction of opisthosomal segment 1in xiphosurids (Weinbergina of the taxa analysed hereAndersen amp Selden 1997 Dunlop amp Selden 1997 Character14) is not considered to be homologous with the unusual shapeof the thorax in naraoiids recognised by their coding

40 Boundaries of anterior trunk segments reflexed antero-laterally (0) absent boundaries transverse or reflexed postero-laterally and (1) present

41 Joints between posterior tergites functional anteriorones variably fused (0) absent and (1) present

42 Posterior tergite bearing axial spine (0) absent and (1)present

Characters 40 to 42 are coded following Edgecombe ampRamskold (1999 characters 17 19 amp 23 respectively) Inaddition to the taxa considered here a thoracic axial spine ispresent in some olenelloid trilobites (see Lieberman 1998

Characters 73 amp 74) and in the aglaspidid Beckwithia (seeHesselbo 1989)

246 Posterior body termination 43 Postabdomen of seg-ments lacking appendages (0) absent and (1) present

44 Length of postabdomen (0) one segment (1) twosegments (2) three segments and (3) five segments In sometaxa a variable number of posterior thoracic segments bearcomplete tubular tergites and lack appendages In Aglaspisautapomorphically it seems that the posterior three thoracicsegments lack appendages (Briggs et al 1979 Hesselbo 1992)but have unmodified tergites These situations are recognisedas potentially homologous by the coding used here Character44 is coded as inapplicable to taxa lacking a postabdomen

45 Posterior tergites strongly curved in dorsal aspect com-pared to anterior tergites (0) absent and (1) present Asrecognised by Wills et al (1998a Character 17) in some taxawhere the tergites are distinct the curvature of thoracic tergitesincreases posteriorly so that the posterior tergites are highlycurved to semicircular in dorsal aspect This situation isnot known from crustaceans (except isopods) or stem grouparthropods

46 Posterior segments reduced and with highly reducedappendages (0) present and (1) absent In some taxa there area large number of posterior segments that are short sagitallyand have appendages that are much reduced in size comparedto anterior trunk appendages These somites are incorporatedinto the pygidium in some taxa but primitively they arecovered by tiny free trunk tergites In the derived state thetrunk somites and limbs are of a relatively constant size Thedistinction between these states can be seen when attempting tocount the number of segments that make up the body In taxashowing State 0 the number of segments is very high anddifficult to count whereas in other taxa the number ofpost-cephalic segments is easily assessed

47 Pygidium (0) absent and (1) present A pygidium isrecognised here as a posterior tagma consisting of a number offused segments under a single tergite which may or may notincorporate the post-segmental telson This is different to theuse of this term by Edgecombe amp Ramskold (1999) whichfollows Ramskold et al (1997) and very different to its use byWills et al (1998a p 53) as a synapomorphy of the TrilobitaThe present authorsrsquo definition recognises the situation inRetifacies where the spinose postsegmental telson is autapo-morphically not fused to the pygidium as homologous toother multisegmented posterior tagma which include thetelson The distinction between the pygidium and an expandedpost-segmental telson can also be seen in the position of theanus which is consistently in the posteriormost pre-telsonicsegment amongst putative arachnomorphs (see Character 48)In taxa with a pygidium the anal segment is fused into thepygidium (see eg Kuamaia Edgecombe amp Ramskold 1999fig 6) whereas in those taxa lacking a pygidium the anus isbetween the final trunk tergite and the telson

Ramskold et al (1997) argued that the posteriormost tergiteof xandarellids (which incorporates the anal segment) is nothomologous to the posterior tagma recognised as pygidiaherein because posterior thoracic tergites also cover multiplesegments in xandarellids (see Character 33) Evidence ofsegmentation of the pygidium in a variety of taxa suggests thatthe number of tergites fused to form the pygidium is greaterthan the number of appendages For example the pygidium ofKuamaia has two pairs of lateral spines but at least fourpairs of appendages (Hou amp Bergstrom 1997 Edgecombe ampRamskold 1999) and that of Triarthrus has only five axialrings posterior to the articulating ring but over 10 pairs ofappendages (Whittington amp Almond 1987) The fact thatdecoupling of tergites and segments is evident in the thorax of

184 TREVOR J COTTON AND SIMON J BRADDY

Xandarella and Cindarella does not necessarily suggest that asimilar more widespread decoupling in pygidia is the result ofa different developmental process and hence that they arenon-homologous Instead the situation in the xandarellidthorax is potentially homologous to that found (more widely)in pygidia and consequently the terminal xandarellid tergite isa pygidium It is unclear if decoupling of tergites and somites isprimitively limited to the terminal tergite or primitively aproperty of the arachnomorph post-cephalon as a whole

It has been suggested that the tiny pygidium of olenelloidtrilobites which probably best reflects the primitive trilobitestate consists only of the post-segmental telson (Harrington inMoore 1959) and therefore is not a true pygidium HoweverWhittington (1989) showed that the olenelloid pygidium con-sists of at least two and possibly as many as five or sixsegments and therefore is likely to be homologous with thepygidium of other trilobites

48 Position of the anus (0) terminal within telson and (1)at base of telson In crustaceans and stem group arthropodsthe anus is terminal or otherwise situated in the telson (egRehbachiella Walossek 1993 fig 15D) In putative arachno-morphs the anus is either at the junction of the posteriormostthoracic segment and the telson or ventral within a fusedpygidium In the case of taxa with a pygidium the anus isanterior to the posterior margin and where known positionedbetween the posteriormost pair of appendages suggesting thatit is anterior to the post-segmental telson (see eg OlenoidesWhittington 1980)

49 Pygidium with median keel (0) absent and (1) presentEdgecombe amp Ramskold (1999 Character 21) considered thepresence of a median keel on the pygidium as a synapomorphyuniting Soomaspis and Tarricoia They did not explain howthis structure could be distinguished from the raised pygidialaxis of trilobites Homology of these structures is not sup-ported here because the axis of more primitive trilobites(including Eoredlichia) is quite distinct morphologically fromthe naraoiid keel Budd (1999a) noted the presence of a keel onthe pygidium of Buenaspis and suggested (Budd 1999a p 102)that it may be an artefact of dorsoventral compaction How-ever contrary to the coding of Edgecombe amp Ramskold(1999) a keel is visible on the pygidium of Liwia convexa (Dzikamp Lendzion 1988 fig 4CD) preserved in full relief Thereforeit seems unlikely that the keel is a taphonomic artefact

50 Pygidium with broad-based median spine (0) absentand (1) present

51 Pygidium with lateral spines (0) present and (1) absentEdgecombe amp Ramskold (1999 Character 22) coded thepresence of a median spine and two pairs of lateral spines aspotentially homologous in Sinoburius Kuamaia and HelmetiaThe distribution of lateral spines is much wider than that ofterminal spines and therefore they are coded separately hereIn trilobites it has widely been recognised that lateral spinesrepresent the original segmentation of the pygidium Thiscannot be the case for median spines Characters 49 to 51are not applicable to taxa lacking a pygidium (Character 47State 0)

52 Expanded post-segmental telson (0) absent and (1)present In a range of taxa the posterior-most tergite is a large(relative to the thoracic tergites) structure that lacks anyevidence of segmentation and is interpreted as representing anexpanded tergite of the post-segmental telson from whichsegments are released anteriorly during ontogeny On the otherhand the posteriormost tergite of Marrella and probablyMimetaster is small compared to the thoracic tergites androunded This element probably represents the plesiomorphiceuarthropod telson The homology of this character in mosttaxa is clear and only doubtful in Retifacies in which it may

be segmented The figures of Hou amp Bergstrom (1997) providelittle unequivocal evidence for a telson in Retifacies but thepresence of this structure is very clear in colour photographs(eg Chen et al 1996 fig 198A) However these figures donot convincingly demonstrate the segmentation of the telsonRetifacies is interpreted here as being unique in possessingboth a pygidium and an expanded post-segmental telson

53 Telson shape (0) spinose and (1) paddle-shaped Theshape of the expanded telsons identified above varies frombroadly spinose to flattened and paddle-like This differencecan be recognised both on the basis of the ratio of length tobasal width (spinose telsons are relatively long) and by thechange in width posteriorly (spinose telsons reduce in widthposteriorly whereas paddle-shaped telsons increase in width)In eurypterids a wide range of telson shapes is found includ-ing both character states described here It is likely that theplesiomorphic condition is a spinose telson This character isinapplicable to taxa lacking an expanded telson

54 Post-ventral furcae (0) absent and (1) present Thischaracter recognises the potential homology of unsegmentedpaired structures that articulate with the segment immediatelyanterior to the telson The potential homology of these struc-tures (the post-ventral plates) in Emeraldella and Aglaspis wasrecognised by Wills et al (1998a Character 70) and inEmeraldella and Sidneyia by Edgecombe amp Ramskold (1999Character 25) The homology of the segmented pygidial caudalfurcae of Olenoides (see Whittington 1975a 1980) with thesestructures is considered doubtful and coded as equivocal

Despite their distinctive morphology the long caudualfurcae of Cheloniellon may be homologous with the furcae ofAglaspis Emeraldella and Sidneyia This is supported by thecaudal furcae of the Ordovician cheloniellid Duslia which aremore similar in morphology to those of Emeraldella than ofCheloniellon Chlupac (1988 pp 614ndash16) considered thesestructures to be on the terminal tergite in Duslia However afaint tergite boundary is apparent posterior to the attachmentof the furcae (see Chlupac 1988 pl 57 fig 4) Therefore theyare considered here to have been attached to the pre-telsonicsegment as in Cheloniellon Chlupac (1988) and Dunlop ampSelden (1997) supported a close relationship between Dusliaand Cheloniellon

25 MethodsAll analyses were carried out using PAUP Version 40b4a(Swofford 1999) and unless otherwise stated used heuristicsearches with one thousand random addition sequencereplicates The software packages MacClade Version 307(Maddison amp Maddison 1997) and RadCon (Thorley amp Page2000) were used for comparing trees and investigating patternsof character evolution Tree length and other statistics (theConsistency Index CI and Retention Index RI) were calcu-lated by PAUP and MacClade with uninformative charactersexcluded Analyses were run separately using each of thedifferent codings for aglaspidids

Characters were treated as unordered and of equal weight inmost analyses To assess the influence of this assumption fourof the eight multistate characters which have states that areintermediate between others (Wilkinson 1992) were treated asordered in some analyses These characters are the number ofcephalic segments (Character 4) the length of raptorialappendage spines (Character 6) the degree of overlap of thethorax by the head-shield (Character 37) and the number ofsegments making up the postabdomen (Character 44) The lastof these is uninformative when not ordered because only onetaxon shows each of states 0 1 and 3

Support for individual nodes was assessed by bootstrapanalysis (Felsenstein 1985) and by calculating Bremer support

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 185

indices (Bremer 1988 1994) These methods measure twodistinct aspects of support for phylogenetic hypotheses Treesor nodes may be considered well supported (1) to the extent towhich alternative topologies are much less parsimonious asmeasured by the support index (Wilkinson 1996) or (2) wherethey are consistent with a large proportion of characters sothat character sampling is unlikely to have had much influenceon topology as assessed by bootstrapping (Page 1996) Boot-strapping was performed with 100 bootstrap replicates eachof 10 addition sequence replicates Bremer support indicesare based on heuristic searches with 100 addition sequencereplicates

Three pairs of terminals are included in Table 2 that apartfrom missing data are coded identically These are Helmetiaand Kuamaia Soomaspis and Tariccoia and Saperion andSkioldia These pairs are lsquotaxonomic equivalentsrsquo removingthe taxon with most missing data will not influence theposition of the other They can be recoded as single terminalsaccording to the principal of safe taxonomic reduction(Wilkinson 1995a b) and were not considered separately in theanalyses presented here Therefore cladistic analyses werecarried out on the basis of 33 terminals Edgecombe ampRamskold (1999) ignored this and coded the same three pairsof taxa and Eoredlichia and Olenoides identically for theirmore limited set of characters

26 ResultsAnalysis with all characters unordered and using the lsquoAglaspi-dida 1rsquo coding found nine most parsimonious trees (MPTs)each 126 steps long (CI=0middot556 RI=0middot763) Using the lsquoAglas-pidida 2rsquo coding resulted in 18 MPTs of length 125 (CI=0middot560RI=0middot765) Nine of these 18 trees were those found with thefirst coding of aglaspidids The strict component consensus(sensu Wilkinson 1994) of the 27 unique trees found acrossboth analyses is shown in Figure 8 All trees supported themonophyly of the Marrellomorpha (Marrella and Mimetaster)and of a clade including all other taxa except Crustacea (Clade1 of Fig 8) The relationships between these two clades and theCrustacea were unresolved equal numbers of trees in bothanalyses supported each of the topologies [Clade 1 (CrustaceaMarrellomorpha)] [Crustacea (Marrellomorpha Clade 1)]and [Marrellomorpha (Crustacea Clade 1)] All trees placedBurgessia as the sister taxon to the rest of Clade 1 andsupported the division of the rest of Clade 1 into two majorsister groups The topology of one of these clades (Clade 2)which included trilobites helmetiids tegopeltids naraoiidsxandarellids and Retifacies was stable with respect to thecoding of aglaspidids The other clade (Clade 3) consists of aclade of megacheirans and chelicerates (Clade 4) and a cladeincluding aglaspidids Lemoneites Paleomerus CheloniellonEmeraldella and Sidneyia (Clade 5) This latter clade is un-resolved in the strict consensus of all 27 trees (Fig 8) and thestrict consensus of the 18 MPTs found using the lsquoAglaspidida2rsquo coding but was fully resolved in the consensus of the ninetrees found with the lsquoAglaspidida 1rsquo coding Within themegacheiran-chelicerate clade all trees found Fortiforceps tobe the sister group to all other taxa Yohoia the sister group tochelicerates and Alalcomenaeus and Leanchoilia to form aclade Jianfengia is equally parsimoniously placed as the sistergroup to the Yohoia-chelicerate clade or as the sister group tothis clade and the Alalcomenaeus-Leanchoilia clade

Treating characters 4 6 37 and 44 as ordered had a minimaleffect on most parsimonious topology Nine MPTs of 136 steps(CI=0middot515 RI=0middot743) were found with the lsquoAglaspidida 1rsquocoding and 12 MPTs 135 steps in length (CI=0middot519 RI=0middot745)with the lsquoAglaspidida 2rsquo coding These trees all supported a(Crustacea Clade 1) clade excluding the marrellomorphs

Apart from this the strict consensus of these 21 trees isidentical to the consensus of the 27 trees found with allcharacters treated as unordered

Across all analyses four different topologies for Clade 5 (ofFig 8) were found (Fig 9) The distribution of these topologiesacross the four analyses described above and across the sameanalyses repeated after reweighting of each character in pro-portion to their Rescaled Consistency Index (see Farris 1989)from the first analysis is shown in Table 3 The instability of

Figure 8 Strict consensus of 27 MPTs from four separate analyses ofarachnomorph phylogeny with both interpretations of aglaspididmorphology and with some characters treated as ordered or allcharacters unordered Clades referred to in the text are numbered

Figure 9 Alternative equally parsimonious resolutions of clade 5 ofFig 8 (see Table 3)

186 TREVOR J COTTON AND SIMON J BRADDY

this clade was to some extent caused by the inclusion ofLemoneites and Paleomerus which could each be coded foronly 27 of the 54 characters used Analysis with these two taxaexcluded and characters treated as unordered found treescompatible only with Topology D of Figure 9 irrespective ofthe coding used for aglaspidids Analysis with some characterstreated as ordered and using either aglaspidid coding foundthree trees compatible only with Topology B of Figure 9 andthree compatible only with Topology D of Figure 9 ThereforeTopology D is the preferred topology for Clade 5 as shown inTable 3

One of the MPTs resulting from the lsquoAglaspidida 2rsquo coding(with all characters treated as unordered) was chosen as thebasis for further discussion because this tree combined thepreferred topology of Clade 5 with the (Crustacea Clade 1)group that was favoured when some characters were treated asordered Bootstrap percentages and Bremer support values forthis tree (using the Aglaspidida 2 coding) are shown in Figure10 Apomorphies for all ingroup nodes of this tree are shownin Figure 10A

3 Discussion

31 Phylogenetic resultsThe results of the cladistic analyses presented above arewell-resolved and robust with respect to different analyticalparameters Most of the taxa considered form a clade (Clade 1of Fig 8) that is more closely related to chelicerates than tocrustaceans in all analyses and therefore is recognised asthe Arachnomorpha as defined by Chen et al (1996) andRamskold et al (1996) The monophyly of the Marrellomor-pha was supported in all analyses The marrellomorphs werefound to be the sister group to a Crustacea+Clade 1 group inmost MPTs but in a minority of trees formed the sister groupto Clade 1 alone According to the latter result they should beincluded within the Arachnomorpha but according to the firstexcluded from it Both of these alternatives have been sup-ported in previous studies (eg Hou amp Bergstrom 1997 Willset al 1998a respectively) In addition to the marrellomorphsthe Fuxianhuiida Bousfield 1995 and Canadaspidida havebeen placed in the Euarthropoda outside the main mandibu-late and arachnomorph clades (Hou amp Bergstrom 1997)Further study of these taxa of the crustacean or mandibulatestem group and of the marrellomorphs is necessary before the

position of the Marrellomorpha can be resolved Provisionallymarrellomorphs are excluded from the Arachnomorphafollowing the majority of analyses above and Wills et al(1994)

The Burgess Shale arthropod Burgessia was placed as thesister group to remaining arachnomorphs in all analyses OtherArachnomorpha form two major clades one consisting of thelsquotrilobite-alliedrsquo arachnomorphs analysed by Edgecombe ampRamskold (1999) here termed lsquoTrilobitomorpharsquo (defined asTrilobita and all taxa more closely related to Trilobita thanChelicerata) and the other (lsquochelicerate-alliedrsquo) clade includingchelicerates lsquogreat appendagersquo arthropods aglaspididsLemoneites Paleomerus Sidneyia and Emeraldella The latteris here termed lsquoCheliceramorpharsquo (defined as Chelicerata andall taxa more closely related to Chelicerata than Trilobita) Thetopology of the lsquotrilobite-alliedrsquo clade was similar to that foundby Edgecombe amp Ramskold (1999) but the present authorsrsquoresults are more completely resolved unambiguously support-ing the sister-group relationship between trilobites andHelmetiida suggested in the previous study Within thelsquochelicerate-alliedrsquo clade Aglaspidids Paleomerus Lemoneitesand the Burgess Shale Emeraldella and Sidneyia form a cladein opposition to megacheirans and chelicerates

The present authorsrsquo analysis supports a new hypothesis ofthe origin of the chelicerates from within a paraphyleticassemblage of megacheiran arthropods and confirms pyc-nogonids as a sister group to the euchelicerates Previoushypotheses of the chelicerate sister group (see Section 1) aremuch less parsimonious The shortest trees supporting aCheloniellon-chelicerate clade are six steps longer than theMPTs and those supporting an aglaspidid-chelicerate cladeseven steps longer All megacheirans and chelicerates areunited by the flap-shaped rounded exopods and modificationof the second cephalic segment endopods into anteriorlydirected spinose raptorial organs which share a number ofdetailed similarities A clade including all of these taxa apartfrom Fortiforceps is supported by the synapomorphic loss ofthe antennae (or antennulae of crustaceans) and the longerlength of spinose projections of the second cephalic appendageendopods Within this clade Yohoia shares the loss of cephalicexopods the loss of thoracic endopods and a postabdomen oftubular tergites with chelicerates

The poor resolution of Clade 5 (of Fig 8) was partly a resultof the poor preservation of and consequent high proportion ofmissing data for Lemoneites and Paleomerus When these two

Table 3 Numbers of most parsimonious trees supporting each of the alternative topologies for clade 4 of Fig 7 under different analyticalconditions (RCI=Revealed Consistency Index)

Aglaspididcoding

Characters 4 6 36and 43 ordered

Charactersreweighted by RCI

Paleomerus andLemoneites excluded

Topology (see Fig 8)

A B C D

1 91 X 3 3 31 X 61 X X 31 X 61 X X 3 32 9 92 X 3 3 3 32 X 62 X X 32 X 62 X X 3 3Total 6 18 12 54

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 187

taxa were excluded from the analyses only two (B and D) ofthe four topologies found in the complete analysis remainedmost parsimonious (see Fig 9 and Table 3) Secondly Bremersupport values throughout much of the lsquochelicerate-alliedrsquoclade (Clade 3 of Fig 8) were improved when these taxa wereexcluded (see Fig 11)

Dunlop (1999 p 258) suggested that in reconstructing thechelicerate stem lineage lsquowe might predict that the two mostsignificant changes towards the chelicerate condition are thereduction of the antennae and the formation of the nextappendages into a clawrsquo According to the hypothesis pre-sented here both of these adaptations were achieved in EarlyCambrian chelicerate ancestors The recognition of the loss ofthe antennae in the stem group of the chelicerates and thephylogenetic hypothesis presented here suggest that chelicer-ates are most closely related to taxa with only a singleadditional segment fused to the crown group eucheliceratehead of four fused somites Previous hypotheses have derivedchelicerates from Palaeozoic arthropods with a longer head(eg Cheloniellon Sturmer amp Bergstrom 1978 SanctacarisBriggs amp Collins 1988 Emeraldella Bruton amp Whittington1983) The plesiomorphic pattern of head segmentation ismatched in pycnogonids Bergstrom et al (1980) suggestedthat only the anterior four pairs of appendages are incorpo-rated into the head This supports the view that pycnogonidsare primitive with respect to other chelicerates which showmore or less complete fusion of this primitive head and threethoracic segments including Solpugida and Pseudoscorpiones

The present study suggests that a shift from gnathobasicbenthic feeding to a more vagrant lifestyle and raptorialappendage feeding occurred very early in the chelicerate lin-eage Appendage feeding was clearly an important adaptation

Figure 10 One of 18 equally parsimonious cladograms with lsquoAglaspidida 2rsquo coding and all characters unorderedthat is most congruent with results from other analyses Non-terminal branch lengths scaled to reflect number ofapomorphies (A) ACCTRAN apomorphy scheme Character numbers shown above boxes and states shownbelow boxes as in Table 2 and the text Character consistency indices are indicated by shading (B) Levels ofsupport for individual nodes Bremer support indices shown in bold type above branches Bootstrap percentagesin italic type below branches for all nodes with relative frequencies greater than 50

Figure 11 Bremer support values for clades within the lsquochelicerate-alliedrsquo arachnomorphs (clade 3 of Fig 8) when all taxa wereincluded (normal weight font above nodes) and with Lemoneites andPaleomerus excluded (bold type below nodes)

188 TREVOR J COTTON AND SIMON J BRADDY

for the terrestrialization of chelicerates (Dunlop 1997 p 69Dunlop amp Webster 1999) but the model proposed heresuggests that this considerably pre-dated terrestrializationGnathobasic feeding in xiphosurans and eurypterids mayrepresent a reversal to a more primitve benthic arachnomorphlifestyle from a more pycnogonid-like ancestor

32 Evolution of the arachnomorph headThe extant arthropod classes show highly conserved patternsof head segmentation (eg Wills et al 1994) In contrast therehas been a near consensus amongst Cambrian arthropodworkers (although rarely based on an explicit phylogenetichypothesis) that patterns of head segmentation are highlyhomoplastic and hence of little systematic significanceSturmer amp Bergstrom (1978 pp 78ndash9) suggested that lsquoevenclosely related forms may have different numbers of headsegments and appendagesrsquo Bruton amp Whittington (1983pp 576ndash7) that lsquodiscussion on fossil arthropod relationshipsbased on head segmentation appears to be largely irrel-evant and at best speculativersquo and Delle Cave amp Simonetta(1991 p 191) that lsquothere are no obvious phyletic affinitiesbetween genera having the same number of cephalized seg-mentsrsquo Most recently Bergstrom amp Hou (1997 p 104) con-cluded that lsquothe segmental length of the head shield seemsto be of no relevance to the discussion and discrimination ofevolutionary lineagesrsquo It has been claimed that this view issupported by cladistic analyses in which patterns of tagmosishave been found to be rather poor at defining major arthropodclades (Briggs et al 1992 Wills et al 1994)

This suggestion is not only of relevance to systematics buthas featured prominently in recent debates surrounding thenature of the lsquoCambrian explosionrsquo (see Budd amp Jensen 2000for a recent review) Gould (1989 1991) argued that theapparent plasticity of head segmentation in Cambrian com-pared to post-Cambrian arthropod evolution suggests thatbody-plan evolution was only constrained after the Cambrianexplosion According to this view the origin of new highertaxa with distinct body-plans was only possible before theonset of these constraints (eg Valentine 1995) Taxa hererecognised as arachnomorphs clearly played a major role in thedevelopment of Gouldrsquos hypothesis This suggestion has pre-viously been assessed in terms of overall morphological diver-sity which is clearly rather distinct from Gouldrsquos concept ofdisparity Wills et al (1998b) showed that there was littlecorrelation between overall morphological disparity and de-gree of limb specialisation and that the latter was phylogeneti-cally highly plastic They remarked that (Wills et al 1998bp 64) lsquothis finding has important implications for models ofarthropod phylogeny and evolution that attribute overidingimportance to head segmentationrsquo but did not explicitlyexamine head segmentation although it is likely to have beena major component of the index of tagmosis employed

The results of the present study suggest that arguments thatarthropod head segmentation was unusually labile during theCambrian are poorly founded Rather only four major pat-terns of euarthropod head segmentation are identified Theplesiomorphic euarthropod state according to both Walossekamp Muller (1997 1998) and Scholtz (1997) which may moreproperly be the plesiomorphic state for the euarthropod crowngroup only (depending on the phylogenetic position of theMarrellomorpha) consists of four post-acronal segmentsbearing the antennae and three pairs of biramous limbs Thepresent authors suggest that the term lsquocephalonrsquo be restrictedto this kind of head which is found in stem group mandibu-lates in addition to many arachnomorphs Crown groupmandibulates share a lsquobimaxillary headrsquo in which an addi-tional pair of appendages the labium or second maxillae are

incorporated into the head but in crustaceans these are notfused to the carapace (Scholtz 1997) Finally two distinctforms of head tagmosis are found in chelicerates A head withfour pairs of appendages (but without antennae giving a totalof five segments incorporated into the head) which has beencalled the lsquocephalosomarsquo is present in pycnogonids (Walossekamp Dunlop 2002 pp 434ndash5) and probably also in some euch-elicerates (J A Dunlop pers comm 2003) The lsquoprosomarsquo ofmost euchelicerates consists of this cephalosoma and twoadditional segments

In general these patterns of head segmentation are highlyphylogenetically conserved (see Fig 12) Only three homo-plastic changes namely reversal to a more primitive conditionin the Leanchoilia-Alalcomenaeus clade and the convergentorigin of a five-segmented head in Clade 3 (of Fig 8) and theXandarellida are necessary to optimise character 4 onto themost parsimonious cladogram There are also transitions toautapomorphic states in Emeraldella and Sidneyia In the caseof Sidneyia however an argument could be made (followingBergstrom amp Sturmer 1978 see discussion of Character 4) thatthe head consists of five segments as in closely related taxabut autapomorphically each segment has a separate tergiteThis would further increase the phylogenetic stability of headsegmentation Also following Bergstrom amp Sturmer (1978) thehead of Cheloniellon could be considered to consist of twotergites making it more similar to the head of Emeraldella(with six pairs of appendages)

Figure 12 Cladogram from Fig 10 showing changes in head segmen-tation Bold type shows abbreviated head segmentation formulaAbbreviations (Ant) antennae (BA) biramous appendage pairs (Ch)chelicerae (GA) great appendages (UA) uniramous appendage pairsand (X) segment without appendages Synapomorphy A is the auta-pomorphic reduction in head length to a single segment in SidneyiaAlternatively this may represent secondary division of the headshield Synapomorphy B is the increase in the length of the head inEmeraldella to six segments

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 189

4 Acknowledgements

We thank D E G Briggs J A Cotton J A Dunlop R AFortey S Harris R A Moore P Selden O E Tetlie andP Van Roy for comments on earlier drafts of this work Themanuscript was reviewed by M Wills F Schram and oneanonymous referee This research was supported by the Geo-logical Society of London Curry Research Studentshipawarded to TJC by the Department of Earth Sciences Uni-versity of Bristol and by the Department of PalaeontologyThe Natural History Museum London TJC would like tothank C Labandeira J Thompson and R Chapman fortheir hospitality at the National Museum of Natural HistoryWashington DC USA and A Olcott S Schellenberg andJ Young for making his time there so enjoyable

5 References

Abzhanov A Popadic A amp Kaufman T C 1999 Chelicerate Hoxgenes and the homology of arthropod segments Evolution andDevelopment 1 77ndash89

Akam M 2000 Arthropods developmental diversity within a (super)phylum Proceedings of the National Academy of Sciences USA97 4438ndash41

Aldridge R J Gabbott S E amp Theron J N 2001 The Soom ShaleIn Briggs D E G amp Crowther P R (eds) Palaeobiology II340ndash2 Oxford Blackwell Science

Anderson L I amp Selden P A 1997 Opisthosomal fusion andphylogeny of Palaeozoic Xiphosura Lethaia 30 19ndash31

Arango C P 2002 Morphological phylogenetics of the sea spiders(Arthropoda Pycnogonida) Organisms Diversity and Evolution2 107ndash25

Ax P 1986 Das phylogenetische System Stuttgart Gustav FischerVerlag

Babcock L E amp Zhang Wentang In press New non-mineralizedarthropods from the Chengjiang Lagerstatte Lower CambrianYunnan China Palaeontologia Cathyana

Bartels C Briggs D E G amp Brassel G 1998 The fossils ofthe Hunsruck Slate Marine life in the Devonian CambridgeCambridge University Press

Bergstrom J 1971 Paleomerus ndash merostome or merostomoid Lethaia4 393ndash401

Bergstrom J 1979 Morphology of fossil arthropods as a guideto phylogenetic relationships In Gupta A P (ed) ArthropodPhylogeny 3ndash56 New York NY Van Nostrand Reinhold Co

Bergstrom J 1992 The oldest arthropods and the origin of theCrustacea Acta Zoologica 73 287ndash91

Bergstrom J Sturmer W amp Winter G 1980 PalaeoisopusPalaeopantopodus and Palaeothea pycnogonid arthropodsfrom the Lower Devonian Hunsruck Slate West GermanyPalaontologische Zeitschrift 54 7ndash54

Bergstrom J amp Brassel G 1984 Legs in the trilobite Rhenops fromthe Lower Devonian Hunsruck Slate Lethaia 17 67ndash72

Bergstrom J amp Hou X 1998 Chengjiang arthropods and theirbearing on early arthropod evolution In Edgecombe G D(ed) Arthropod fossils and phylogeny 151ndash84 New York NYColumbia University Press

Boudreaux H B 1979 Significance of intersegmental tendon systemin arthropod phylogeny and a monophyletic classification ofArthropoda In Gupta A P (ed) Arthropod Phylogeny 551ndash86New York NY Van Nostrand Reinhold Co

Bousfield E L 1995 A contribution to the natural classification ofLower and Middle Cambrian arthropods food-gathering andfeeding mechanisms Amphipacifica 2 3ndash33

Braddy S J Aldridge R J Gabbott S E amp Theron J N 1999Lamellate book-gills in a late Ordovician eurypterid from theSoom Shale South Africa support for a eurypterid-scorpionclade Lethaia 32 72ndash4

Braddy S J amp Almond J E 1999 Eurypterid trackways from theTable Mountain Group (Ordovician) of South Africa Journal ofAfrican Earth Sciences 29 165ndash77

Bremer K 1988 The limits of amino acid sequence data in angio-sperm phylogenetic reconstruction Evolution 42 795ndash803

Bremer K 1994 Branch support and tree stability Cladistics 10295ndash304

Briggs D E G 1998 Review of Arthropods from the LowerCambrian Chengjiang fauna southwest China by HouXianguang and Jan Bergstrom Fossils and Strata 45

Scandinavian University Press Oslo Norway 1997 Palaeo-geography Palaeoclimatology Palaeoecology 143 193ndash4

Briggs D E G Bruton D L amp Whittington H B 1979 Append-ages of the arthropod Aglaspis spinifer (Upper CambrianWisconsin) and their significance Palaeontology 22 167ndash80

Briggs D E G Fortey R A amp Wills M A 1992 Morphologicaldisparity in the Cambrian Science 256 1670ndash3

Briggs D E G Fortey R A amp Wills MA 1993 How big was theCambrian explosion A taxonomic and morphologic comparisonof Cambrian and Recent arthropods In Lees D R amp EdwardsD (eds) Evolutionary patterns and processes 33ndash44 LondonLinnean Society of London Linnean Society Symposium Series

Briggs D E G Erwin D H amp Collier F J 1994 The fossils of theBurgess Shale Washington DC Smithsonian Institution Press

Briggs D E G amp Bartels C 2001 New arthropods from the LowerDevonian Hunsruck Slate (Lower Emsian Rhenish Massifwestern Germany) Palaeontology 44 275ndash304

Briggs D E G amp Bartels C 2002 Magnoculocaris a new name forthe arthropod Magnoculus Briggs and Bartels 2001 from theLower Devonian Hunsruck Slate Germany Palaeontology 45419

Briggs D E G amp Collins D 1988 A Middle Cambrian cheliceratefrom Mount Stephen British Columbia Palaeontology 31 779ndash98

Briggs D E G amp Collins D 1999 The arthropod Alalcomenaeuscambricus Simonetta from the Middle Cambrian Burgess Shale ofBritish Columbia Paleontology 42 953ndash78

Briggs D E G amp Fortey R A 1982 The cuticle of aglaspididarthropods a red-herring in the early history of vertebratesLethaia 15 25ndash9

Briggs D E G amp Fortey R A 1989 The early radiation andrelationships of the major arthropod groups Science 246 241ndash3

Briggs D E G amp Fortey R A 1992 The early Cambrian radiationof arthropods In Lipps J H amp Signor P W (eds) Origin andEarly Evolution of the Metazoa 335ndash373 New York NY PlenumPress

Briggs D E G amp Robison R A 1984 Exceptionally preservednontrilobite arthropods and Anomalocaris from the MiddleCambrian of Utah University of Kansas PaleontologicalContributions Paper 111 1ndash23

Broili F 1932 Ein neuer Crustacee aus dem rheinischen UnterdevonSitzungsberichte der Mathematisch-Naturwissenschaftlichen(Abteilung) Klasse der Bayerischen Akademie der Wissenschaftenzu Munchen 1932 27ndash38

Broili F 1933 Ein zweites Exemplar von Cheloniellon Sitzungsber-ichte der Bayerischen Akademie der Wissenschaften 1933 11ndash32

Bruton D L 1981 The arthropod Sidneyia inexpectans MiddleCambrian Burgess Shale British Columbia Philosophical Trans-actions of the Royal Society of London Series B 295 619ndash56

Bruton D L amp Whittington H B 1983 Emeraldella and Leanchoiliatwo arthropods from the Burgess Shale British Columbia Philo-sophical Transactions of the Royal Society of London Series B 300553ndash85

Budd G E 1995 Kleptothule rasmusseni gen et sp nov anolenellid-like trilobite from the Sirius Passet fauna (BuenFormation Lower Cambrian North Greenland) Transactions ofthe Royal Society of Edinburgh Earth Sciences 86 1ndash12

Budd G E 1996a Progress and problems in arthropod phylogenyTrends in Ecology and Evolution 11 356ndash8

Budd G E 1996b The morphology of Opabinia regalis and thereconstruction of the arthropod stem-group Lethaia 29 1ndash14

Budd G E 1997 Stem-group arthropods from the Lower CambrianSirius Passet fauna of North Greenland In Fortey R A ampThomas R H (eds) Arthropod Relationships 125ndash38 LondonChapman and Hall Systematics Association Special Volume 55

Budd G E 1999a A nektaspid arthropod from the Early CambrianSirius Passet fauna with a description of retrodeformation basedon functional morphology Palaeontology 42 99ndash122

Budd G E 1999b The morphology and phylogenetic significance ofKerygmachela kiergkegaardi Budd (Buen Formation LowerCambrian N Greenland) Transactions of the Royal Society ofEdinburgh Earth Sciences 89 (for 1998) 249ndash290

Budd G E 2002 A palaeontological solution to the arthropod headproblem Nature 417 271ndash5

Budd G E amp Jensen S 2000 A critical reappraisal of the fossilrecord of the bilaterian phyla Biological Reviews 75 253ndash95

Chen J Zhou G Zhu M amp Yeh K 1996 The Chengjiang biota ndasha unique window of the Cambrian explosion Taichung NationalMuseum of Natural Science [In Chinese]

Chen J Edgecombe G D amp Ramskold L 1997 Morphological andecological disparity in naraoiids (Arthropoda) from the Early

190 TREVOR J COTTON AND SIMON J BRADDY

Cambrian Chengjiang fauna China Records of the AustralianMuseum 49 1ndash24

Chen J amp Zhou G 1997 Biology of the Chengjiang fauna In ChenJ Cheng Y amp Iten H V (eds) The Cambrian explosion and thefossil record Bulletin of the National Museum of Natural ScienceTaiwan 10 11ndash106

Child C A 1997 Some deep-sea Pycnogonida from Argentine slopeand basin Proceedings of the Biological Society of Washington110 128ndash42

Chlupac I 1988 The enigmatic arthropod Duslia from the Ordovicianof Czechslovakia Palaeontology 31 611ndash20

Chlupac I 1995 Lower Cambrian arthropods from the PasekyShale (Barrandian area Czech Republic) Journal of the CzechGeological Society 40 9ndash36

Chlupac I amp Havlıcek V 1965 Kodymirus n g a new aglaspidmerostome of the Cambrian of Bohemia Sbornık GeologickechVid Paleontologie 6 7ndash20

Cisne J L 1975 Anatomy of Triarthrus and the relationships of theTrilobita Fossils and Strata 4 45ndash63

Cisne J L 1981 Triarthrus eatoni (Trilobita) anatomy of its exo-skeletal skeletomuscular and digestive systems Palaeonto-graphica Americana 9 95ndash142

Conway Morris S 1982 Atlas of the Burgess Shale London ThePalaeontological Association

Conway Morris S 1989 The persistence of Burgess Shale-type faunasimplications for the evolution of deeper-water faunas Trans-actions of the Royal Society of Edinburgh Earth Sciences 80271ndash83

Conway Morris S 1998 The crucible of creation Oxford OxfordUniversity Press

Cotton T J 1999 Review of Arthropod fossils and phylogenyPalaeogeography Palaeoclimatology Palaeoecology 151 341ndash3

Cotton T J 2002 The phylogeny and morphological evolution ofCambrian trilobites and their relatives Unpublished PhD ThesisUniversity of Bristol

Damen W G M Hausdorf M Seyfarth E A amp Tautz D 1998 Aconserved mode of head segmentation in arthropods revealed bythe expression pattern of Hox genes in a spider Proceedings of theNational Academy of Sciences USA 95 10665ndash70

Delle Cave L amp Simonetta A M 1991 Early Palaeozoic arthropodsand problems of arthropod phylogeny with some notes on taxa ofdoubtful affinities In Simonetta A M amp Conway Morris S(eds) The early evolution of Metazoa and the significance ofproblematic taxa 189ndash244 Cambridge Cambridge UniversityPress

Dewel R A amp Dewel W C 1997 The place of tardigrades inarthropod evolution In Fortey R A amp Thomas R H (eds)Arthropod Relationships 109ndash23 London Chapman and HallSystematics Association Special Volume 55

Dunlop J A 1997 Palaeozoic arachnids and their significance forarachnid phylogeny Proceedings of the 16th European Colloquiumof Arachnology 65ndash82 Siedlce Poland

Dunlop J A 1998 The origins of tetrapulmonate booklungs and theirsignificance for chelicerate phylogeny In Selden P A (ed)Proceedings of the 17th European Colloquium of ArachnologyEdinburgh 1997 pp 9ndash16

Dunlop J A 1999 Pasando revista a la evolucion de los quelicerados[A review of chelicerate evolution] In Melic A DeHaro J JMedez M amp Ribera I (eds) Volumen Monografico Evolucion yFilogenia de Arthropoda Boletin de la Sociedad EntolomologicaAragonesa 26 255ndash272 [In Spanish and English]

Dunlop J A amp Selden P A 1997 The early history and phylogenyof the chelicerates In Fortey R A amp Thomas R H (eds)Arthropod relationships Systematics Association Special Volume55 221ndash35 London Chapman and Hall

Dunlop J A amp Webster M 1999 Fossil evidence terrestrializationand arachnid phylogeny Journal of Arachnology 27 86ndash93

Dzik J 1993 Early metazoan evolution and the meaning of its fossilrecord Evolutionary Biology 27 339ndash86

Dzik J amp Lendzion K 1988 The oldest arthropods of the EastEuropean Platform Lethaia 21 29ndash38

Edgecombe G D Wilson G D F Colgan D J Gray M R ampCassis G 2000 Arthropod cladistics combined analysisof Histone H3 and U2 snRNA sequences and morphologyCladistics 16 155ndash203

Edgecombe G D amp Ramskold L 1999 Relationships of CambrianArachnata and the systematic position of Trilobita Journal ofPaleontology 73 263ndash87

Emerson M J amp Schram F R 1997 Theories patterns and realitygame plan for arthropod phylogeny In Fortey R A amp Thomas

R H (eds) Arthropod Relationships Systematics AssociationSpecial Volume 55 67ndash87 London Chapman and Hall

Farris J S 1989 The Retention Index and the Rescaled ConsistencyIndex Cladistics 5 417ndash19

Felsenstein J 1985 Confidence limits on phylogenies an approachusing the bootstrap Evolution 39 783ndash91

Flower R H 1968 Merostomes from a Cotter Horizon of the El PasoGroup New Mexico Bureau of Mines and Mineral ResourcesMemoir 22 35ndash44

Fortey R A 1990a Trilobite evolution and systematics In MikulicD G (ed) Arthropod paleobiology 44ndash65 Short Courses inPaleontology 3 Knoxville Tenessee The Paleontological Society

Fortey R A 1990b Ontogeny hypostome attachment and trilobiteclassification Palaeontology 33 529ndash76

Fortey R A 1997 Classification In Kaesler R L (ed) Treatise oninvertebrate paleontology Part O Arthropoda 1 (revised) 289ndash302Boulder Colorado amp Lawrence Kansas Geological Society ofAmerica and University of Kansas Press

Fortey R A amp Theron J N 1994 A new Ordovician arthropodSoomaspis and the agnostid problem Palaeontology 37 841ndash61

Fortey R A amp Whittington H B 1989 The Trilobita as a naturalgroup Historical Biology 2 125ndash38

Fry W G 1978 A classification within the pycnogonids ZoologicalJournal of the Linnean Society of London 63 35ndash58

Giribet G Edgecombe G D amp Wheeler W C 2001 Arthropodphylogeny based on eight molecular loci and morphology Nature413 157ndash61

Gould S J 1989 Wonderful life The Burgess Shale and the nature ofhistory New York NY Norton

Gould S J 1991 The disparity of the Burgess Shale arthropod faunaand the limits of cladistic analysis why we must strive to quantifymorphospace Paleobiology 17 411ndash23

Gurich G 1931 Mimetaster hexagonalis ein neuer Kruster aus demunterdevonischen Bundenbacher Dachschiefer PalaontologischeZeitschrift 12 204ndash38

Hammann W Laske R amp Pillola G L 1990 Tariccoia arrusensisngnsp an unusual trilobite-like arthropod Rediscovery of thelsquophylocaridrsquo beds of Taricco (1922) in the Ordovician lsquoPuddingarsquosequence of Sardinia Bolletino della Societa PaleontologicaItaliana 29 163ndash78

Haas M S Brown S J amp Beeman R W 2001 Pondering theprocephalon the segmental origin of the labrum DevelopmentGenes and Evolution 211 89ndash95

Heider K 1913 Entwicklungsgeschichte und Morphologie derWirbellosen Die Kultur der Gegenwart Theil 3 Abt 4 2 176ndash332

Henriksen K 1928 Critical notes upon Cambrian arthropodsdescribed by Charles D Walcott Videnskabelige Meddelelser fraDansk Naturhistorisk Forening 86 1ndash20

Hesselbo S P 1988 Trace fossils of Cambrian aglaspidid arthropodsLethaia 21 139ndash46

Hesselbo S P 1989 The aglaspidid arthropod Beckwithia from theCambrian of Utah and Wisconsin Journal of Paleontology 63636ndash42

Hesselbo S P 1992 Aglaspidida (Arthropoda) from the UpperCambrian of Wisconsin Journal of Paleontology 66 885ndash923

Hou X 1987a Three new large arthropods from Lower CambrianChengjiang fauna eastern Yunnan Acta Palaeontologica Sinica26 272ndash85 [In Chinese with English summary]

Hou X 1987b Two new arthropods from Lower CambrianChengjiang eastern Yunnan Acta Palaeontologica Sinica 26236ndash56 [In Chinese with English summary]

Hou X Chen J amp Lu H 1989 Early Cambrian new arthropodsfrom Chengjiang Yunnan Acta Palaeontologica Sinica 28 42ndash57[In Chinese with English summary]

Hou X Ramskold L amp Bergstrom J 1991 Composition andpreservation of the Chengjiang fauna a Lower Cambrian soft-bodied biota Zoologica Scripta 20 395ndash411

Hou X Bergstrom J amp Ahlberg P 1995 Anomalocaris and otherlarge animals in the Lower Cambrian Chengjiang fauna ofsouthwest China GFF 117 163ndash83

Hou X amp Bergstrom J 1997 Arthropods of the Lower CambrianChengjiang fauna southwest China Fossils and Strata 45 1ndash116

Hughes C P 1975 Redescription of Burgessia bella from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 4415ndash36

Ivantsov A Y 1999 Trilobite-like arthropod from the LowerCambrian of the Siberian Platform Acta PalaeontologicaPolonica 44 455ndash66

King P E 1973 Pycnogonids London St Martin Press

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 191

Lauterbach K E 1973 Schlussereignisse in der Evolution derStammgruppe der Euarthropoda Zoologische Beitrage 19259ndash99

Lauterbach K E 1980 Schlussereignisse in der Evolution derGrundplans der Arachnata (Arthropoda) Abhandlungen desNaturwissenschaftlichen Vereins in Hamburg 23 163ndash327

Lauterbach K E 1983 Synapomorphien zwischen Trilobiten- undCheliceraten-zweig der Arachnata Zoologischer Anzeiger 210213ndash38

Lendzion K 1975 Fauna of the Mobergella Zone in the Polish LowerCambrian Kwartalnik Geologiczny 19 237ndash42

Lieberman B S 1998 Cladistic analysis of the Early Cambrianolenelloid trilobites Journal of Paleontology 72 59ndash78

Lu Y 1940 On the ontogeny and phylogeny of Redlichia intermediaLu (sp nov) Bulletin of the Geological Society of China 20333ndash42 [In Chinese]

Maddison W P amp Maddison D R 1997 MacClade analysis ofphylogeny and character evolution Version 307 Sunderland MASinauer Associates

Mittmann B amp Scholtz G 2003 Development of the nervous systemin the lsquoheadrsquo of Limulus polyphemus (Chelicerata Xiphosura)morphological evidence for the correspondence between the seg-ments of the chelicerae and the (first) antennae of MandibulataDevelopment Genes and Evolution 213 9ndash17

Moore R C (ed) 1959 Treatise on invertebrate paleontology Part OArthropoda 1 Boulder Colorado Geological Society of America

Muller K J amp Walossek D 1986 Arthropod larvae from the UpperCambrian of Sweden Transactions of the Royal Society ofEdinburgh Earth Sciences 77 157ndash79

Muller K J amp Walossek D 1987 Morphology ontogeny and lifehabit of Agnostus pisiformis from the Upper Cambrian of SwedenFossils and Strata 19 1ndash124

Munilla T 1999 Evolucion y filogenia de los Picnogonidos InMelicA DeHaro J J Medez M amp Ribera I (eds) VolumenMonografico Evolucion y Filogenia de Arthropoda Boletin de laSociedad Entolomologica Aragonesa 26 273ndash91 [In Spanish andEnglish]

Orr P J Siveter D J Briggs D E G Siveter D J amp Sutton M D2000 A new arthropod from the Silurian Konservat-Lagerstatteof Herefordshire Proceedings of the Royal Society of LondonSeries B 267 1497ndash504

Page R D M 1996 On consensus confidence and lsquototal evidencersquoCladistics 12 83ndash92

Palmer A R amp Repina L N 1993 Through a glass darklytaxonomy phylogeny and biostratigraphy of the OlenellinaUniversity of Kansas Paleontological Contributions New Series 31ndash35

Platnick N I Griswold C E amp Coddington J A 1991 On missingentries in cladistic analysis Cladistics 7 337ndash43

Pleijel F 1995 On character coding for phylogeny reconstructionCladistics 11 309ndash15

Raasch G O 1939 Cambrian Merostomata Special Papers of theGeological Society of America 19 1ndash146

Ramskold L Chen J Edgecombe G D amp Zhou G 1996Preservational folds simulating tergite junctions in tegopeltid andnaraoiid arthropods Lethaia 29 15ndash20

Ramskold L Chen J Edgecombe G D amp Zhou G 1997Cindarella and the arachnate clade Xandarellida (ArthropodaEarly Cambrian) from China Transactions of the Royal Society ofEdinburgh Earth Sciences 88 19ndash38

Ramskold L amp Edgecombe G D 1991 Trilobite monophylyrevisited Historical Biology 4 267ndash83

Ramskold L amp Edgecombe G D 1996 Trilobite appendagestructure ndash Eoredlichia revisited Alcheringa 20 269ndash76

Raw F 1957 Origin of chelicerates Journal of Paleontology 31139ndash193

Repina L N amp Okuneva O G 1969 Cambrian arthropods of themaritime territory Paleontological Journal 1969 95ndash103

Richter R amp Richter E 1929 Weinbergina opitz ng n sp einSchwerttrager (Merost Xiphos) aus dem Devon (Rheinland)Senkenbergiana 11 193ndash209

Robison R A 1984 New occurences of the unusual trilobite Naraoiafrom the Cambrian of Idaho and Utah University of KansasPaleontological Contributions Paper 112 1ndash8

Rominger C 1887 Description of primordial fossils from MtStephens NW Territory of Canada Proceedings of the Academyof Natural Sciences of Philadelphia 1887 12ndash19

Scholtz G 1997 Cleavage germ band formation and head segmen-tation the ground pattern of the Euarthropoda In Fortey R Aamp Thomas R H (eds) Arthropod Relationships 317ndash32 LondonChapman and Hall Systematics Association Special Volume 55

Schram F R 1978 Arthropods a convergent phenomenonFieldiana Geology 39 61ndash108

Schram F R 1993 Review of Boxshall Stromberg and Dahl TheCrustacea Origin and Evolution Journal of Crustacean Biology13 820ndash2

Schram F R Vonk R amp Hof C H J 1997 Mazon CreekCycloidea Journal of Paleontology 71 261ndash84

Schram F R amp Emerson M J 1991 Arthropod Pattern Theorya new approach to arthropod phylogeny Memoirs of theQueensland Museum 31 1ndash18

Schram F R amp Hof C H J 1998 Fossils and the interrelationshipsof major crustacean groups In Edgecombe G D (ed) Arthropodfossils and phylogeny pp 233ndash302 New York NY ColumbiaUniversity Press

Shergold J H 1991 Protaspis and early meraspis growth stages of theeodiscoid trilobite Pagetia ocellata Jell and their implications forclassification Alcheringa 15 65ndash86

Shu D Geyer G Ling C amp Zhang Z 1995 Redlichiacean trilobiteswith preserved soft parts from the Lower Cambrian Chengjiangfauna (South China) Beringeria (Special Issue) 2 203ndash41

Shultz J W 1990 Evolutionary morphology and phylogeny ofArachnida Cladistics 6 1ndash38

Shultz J W 2000 Skeletomuscular anatomy of the harvestmanLeiobunum aldrichi (Weed 1893) (Arachnida OpilionesPalpatores) and its evolutionary significance Zoological Journalof the Linnean Society 128 401ndash38

Shultz J W 2001 Gross muscular anatomy of Limulus polyphemus(Xiphosura Chelicerata) and its bearing on evolution in the theArachnida Journal of Arachnology 29 283ndash303

Siewing R (ed) 1985 Lehrbuch der Zoologie II Systematik 3rd ednStuttgart Fischer

Simonetta A M 1970 Studies on the non trilobite arthropods ofthe Burgess Shale (Middle Cambrian) the genera LeanchoiliaAlalcomenaeus Opabinia Burgessia Yohoia and ActaeusPalaeontographica Italica 66 35ndash45

Simonetta A M amp Delle Cave L 1975 The Cambrian non trilobitearthropods from the Burgess Shale of British Columbia A studyof their comparative morphology taxonomy and evolutionarysignificance Palaeontographica Italica 69 1ndash37

Simonetta A M amp Delle Cave L 1981 An essay in the compara-tive and evolutionary morphology of Palaeozoic ArthropodsAccademia Nazionale dei Lincei Atti dei Convegni Lincei 49389ndash439

Starobogatov Y I 1990 Sistema i filogeniya nizshikh khelitserovykh(analiz morfologii paleozoysikh grupp) [The systematics andphylogeny of the lower chelicerates (a morphological analysis ofthe Palaeozoic groups)] Paleontologicheskii Zhurnal 1990 4ndash17

Stoslashrmer L 1944 On the relationships and phylogeny of fossil andrecent Arachnomorpha Skrifter utgitt ac Det Norske Videnskaps-Akademi i Olso I Matematisk-naturvitenskaplig Klasse 5 1ndash158

Stoslashrmer L 1951 Studies on trilobite morphology Part III Theventral cephalic structures with remarks on the zoologicalposition of the trilobites Norsk geologisk tidsskrift 29 108ndash58

Stoslashrmer L 1956 A Lower Cambrian merostome from Sweden Arkivfor Zoologi Ser 2 9 507ndash14

Stoslashrmer L 1959 Trilobitoidea In Moore R C (ed) Treatise onInvertebrate Paleontology part O Arthropoda 1 23ndash37 BoulderColorado Geological Society of America

Strong E E amp Lipscomb D 1999 Character coding and inapplicabledata Cladistics 15 363ndash71

Sturmer W amp Bergstrom J 1976 The arthropods Mimetaster andVachonisia from the Devonian Hunsruck Shale PalaontologischeZeitschrift 50 78ndash111

Sturmer W amp Bergstrom J 1978 The arthropod Cheloniellon fromthe Devonian Hunsruck Shale Palaontologische Zeitschrift 5257ndash81

Sturmer W amp Bergstrom J 1981 Weinbergina a xiphosuran fromthe Devonian Hunsruck Slate Palaontologische Zeitschrift 55237ndash55

Sutton M D Briggs D E G Siveter D J Siveter D J amp Orr PJ 2002 The arthropod Offacolus kingi (Chelicerata) from theSilurian of Herefordshire England computer based morphologi-cal reconstructions and phylogenetic affinity Proceedings of theRoyal Society of London Series B 269 1195ndash1203

Swofford D L 1999 PAUP Phylogenetic Analysis Using Parsimony(and other methods) Version 40b2a Sunderland MA SinauerAssociates

Telford M J amp Thomas R H 1998 Expression of homeobox genesshows chelicerate arthropods retain their deutocerebral segmentProceedings of the National Academy of Sciences USA 9510671ndash5

192 TREVOR J COTTON AND SIMON J BRADDY

Thorley J L amp Page R D M 2000 RadCon phylogenetic treecomparison and consensus Bioinformatics 16 486ndash7

Valentine J W 1995 Why no new phyla after the Cambrian ndashgenome and ecospace hypotheses revisited Palaios 10 190ndash194

Wagele J W 1993 Rejection of the lsquoUniramiarsquo hypothesis andimplications of the Mandibulata concept Zoologische JahrbucherAbteilung fur Systematik Geographie und Biologie der Tiere 120253ndash88

Walcott C D 1911 Middle Cambrian Merostomata Cambriangeology and palaeontology II Smithsonian MiscellaneousCollections 57 17ndash40

Walcott C D 1912 Middle Cambrian Branchiopoda MalacostracaTrilobita and Merostomata Cambrian geology and palaeontol-ogy II Smithsonian Miscellaneous Collections 57 145ndash228

Walcott C D 1918 Geological explorations in the CanadianRockies Explorations and fieldwork of the SmithsonianInstitution in 1917 Smithsonian Miscellaneous Collections 684ndash20

Walossek D 1993 The Upper Cambrian Rehbachiella kinnekullensisand the phylogeny of Branchiopoda and Crustacea Fossils andStrata 32 1ndash202

Walossek D amp Dunlop J A 2002 A larval sea spider (ArthropodaPycnogonida) from the Upper Cambrian lsquoOrstenrsquo of Swedenand the phylogenetic position of pycnogonids Palaeontology 45421ndash46

Walossek D amp Muller K J 1990 Upper Cambrian stem-lineagecrustaceans and their bearing upon the monophyletic origin ofCrustacea and the position of Agnostus Lethaia 23 409ndash27

Walossek D amp Muller K J 1997 Cambrian lsquoOrstenrsquo-type arthro-pods and the phylogeny of Crustacea In Fortey R A amp ThomasR H (eds) Arthropod Relationships 139ndash53 SystematicsAssociation Special Volume 55 London Chapman and Hall

Walossek D amp Muller K J 1998 Early arthropod phylogeny in lightof the Cambrian lsquoOrstenrsquo fossils In Edgecombe G D (ed)Arthropod fossils and phylogeny 185ndash231 New York NYColumbia University Press

Weygoldt P 1979 Significance of later embryonic stages and headdevelopment in arthropod phylogeny In Gupta A P (ed)Arthropod Phylogeny 107ndash35 New York NY Van NostrandReinhold Co

Weygoldt P 1998 Evolution and systematics of the ChelicerataExperimental and Applied Acarology 22 63ndash79

Weygoldt P amp Paulus H F 1979 Unterzuchungen aur Morpholo-gie Taxonomie und Phylogenie der Chelicerata Zeitschriftfur Zoologische Systematik und Evolutionsforschung 17 85ndash116177ndash200

Wheeler W C Cartwright P amp Hayashi C Y 1993 Arthropodphylogeny a combined approach Cladistics 9 1ndash39

Wheeler W C amp Hayashi C Y 1998 The phylogeny of extantchelicerate orders Cladistics 14 173ndash92

Whittington H B 1971 Redescription of Marrella splendens(Trilobitoidea) from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 209 1ndash24

Whittington H B 1974 Yohoia Walcott and Plenocaris n genarthropods from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 231 1ndash21

Whittington H B 1975a Trilobites with appendages from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 497ndash136

Whittington H B 1975b The enigmatic animal Opabinia regalisMiddle Cambrian Burgess Shale British Columbia PhilosophicalTransactions of the Royal Society of London Series B 271 1ndash43

Whittington H B 1977 The Middle Cambrian trilobite NaraoiaBurgess Shale British Columbia Philosophical Transactions of theRoyal Society Series B 280 409ndash43

Whittington H B 1979 Early arthropods their appendages andrelationships In House M R (ed) The origin of majorinvertebrate groups 253ndash68 Systematics Association SpecialVolume 12 New York NY Academic Press

Whittington H B 1980 Exoskeleton moult stage appendage mor-phology and habits of the Middle Cambrian trilobite Olenoidesserratus Palaeontology 23 171ndash204

Whittington H B 1981 Rare arthropods from the Burgess ShaleMiddle Cambrian British Columbia Philosophical Transactionsof the Royal Society of London Series B 292 329ndash57

Whittington H B 1985 Tegopelte gigas a second soft-bodied trilo-bite from the Burgess Shale Middle Cambrian British ColumbiaJournal of Palaeontology 23 171ndash204

Whittington H B 1989 Olenelloid trilobites type species functionalmorphology and higher classification Philosophical Transactionsof the Royal Society of London Series B 324 111ndash47

Whittington H B amp Almond J E 1987 Appendages and habits ofthe Upper Ordovician trilobite Triarthus eatoni PhilosophicalTransactions of the Royal Society of London Series B 317 1ndash46

Wilkinson M 1992 Ordered versus unordered characters Cladistics8 375ndash85

Wilkinson M 1994 Common cladistic information and its consensusrepresentation reduced Adams and reduced cladistic consensustrees and profiles Systematic Biology 43 343ndash68

Wilkinson M 1995a Coping with abundant missing entries in phylo-genetic inference using parsimony Systematic Biology 44 501ndash14

Wilkinson M 1995b TAXEQ2 software and documentation BristolSchool of Biological Sciences University of Bristol

Wilkinson M 1996 On the distribution of homoplasy and choosingamong trees Taxon 45 263ndash66

Wills M A 1997 A phylogeny of Recent and fossil Crustacea derivedfrom morphological characters In Fortey R A amp ThomasR H (eds) Arthropod Relationships 189ndash209 SystematicsAssociation Special Volume 55 London Chapman and Hall

Wills M A Briggs D E G amp Fortey R A 1994 Disparity asan evolutionary index a comparison of Cambrian and Recentarthropods Paleobiology 20 93ndash130

Wills M A Briggs D E G Fortey R A amp Wilkinson M 1995The significance of fossils in understanding arthropod evolutionVerhandlungen der deutschen zoologischen Gesselschaft 88 203ndash15

Wills M A Briggs D E G Fortey R A Wilkinson M amp SneathP H A 1998a An arthropod phylogeny based on fossil andrecent taxa In Edgecombe G D (ed) Arthropod fossils andphylogeny 33ndash105 New York NY Columbia University Press

Wills M A Briggs D E G amp Fortey R A 1998b Evolutionarycorrelates of arthropod tagmosis scrambled legs In ForteyR A amp Thomas R H (eds) Arthropod Relationships 57ndash65Systematics Association Special Volume 55 London Chapmanand Hall

Winter G 1980 Beitrage zur Morphologie und Embryologie desvorderen Korperabschnitts (Cephalosoma) der PantopodaGerstaecker 1863 I Entstehung und Struktur des Zentralnerven-systems Zeitschrift fur Zoologische Systematik und Evolutionsfor-schung 18 27ndash61

Zhang W amp Hou X 1985 Preliminary notes on the occurrence of theunusual trilobite Naraoia in Asia Acta Palaeontologica Sinica 24591ndash5 [In Chinese with English summary]

TREVOR J COTTON Department of Palaeontology The Natural History Museum CromwellRoad London SW7 5BD UKe-mail trevor_cottonhotmailcom

SIMON J BRADDY Department of Earth Sciences University of Bristol Wills MemorialBuilding Queenrsquos Road Bristol BS8 1RJ UKe-mail S J Braddybrisacuk

MS received 28 July 2003 Accepted for publication 15 March 2004

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 193

Page 17: The phylogeny of arachnomorph arthropods and the origin of the

Xandarella and Cindarella does not necessarily suggest that asimilar more widespread decoupling in pygidia is the result ofa different developmental process and hence that they arenon-homologous Instead the situation in the xandarellidthorax is potentially homologous to that found (more widely)in pygidia and consequently the terminal xandarellid tergite isa pygidium It is unclear if decoupling of tergites and somites isprimitively limited to the terminal tergite or primitively aproperty of the arachnomorph post-cephalon as a whole

It has been suggested that the tiny pygidium of olenelloidtrilobites which probably best reflects the primitive trilobitestate consists only of the post-segmental telson (Harrington inMoore 1959) and therefore is not a true pygidium HoweverWhittington (1989) showed that the olenelloid pygidium con-sists of at least two and possibly as many as five or sixsegments and therefore is likely to be homologous with thepygidium of other trilobites

48 Position of the anus (0) terminal within telson and (1)at base of telson In crustaceans and stem group arthropodsthe anus is terminal or otherwise situated in the telson (egRehbachiella Walossek 1993 fig 15D) In putative arachno-morphs the anus is either at the junction of the posteriormostthoracic segment and the telson or ventral within a fusedpygidium In the case of taxa with a pygidium the anus isanterior to the posterior margin and where known positionedbetween the posteriormost pair of appendages suggesting thatit is anterior to the post-segmental telson (see eg OlenoidesWhittington 1980)

49 Pygidium with median keel (0) absent and (1) presentEdgecombe amp Ramskold (1999 Character 21) considered thepresence of a median keel on the pygidium as a synapomorphyuniting Soomaspis and Tarricoia They did not explain howthis structure could be distinguished from the raised pygidialaxis of trilobites Homology of these structures is not sup-ported here because the axis of more primitive trilobites(including Eoredlichia) is quite distinct morphologically fromthe naraoiid keel Budd (1999a) noted the presence of a keel onthe pygidium of Buenaspis and suggested (Budd 1999a p 102)that it may be an artefact of dorsoventral compaction How-ever contrary to the coding of Edgecombe amp Ramskold(1999) a keel is visible on the pygidium of Liwia convexa (Dzikamp Lendzion 1988 fig 4CD) preserved in full relief Thereforeit seems unlikely that the keel is a taphonomic artefact

50 Pygidium with broad-based median spine (0) absentand (1) present

51 Pygidium with lateral spines (0) present and (1) absentEdgecombe amp Ramskold (1999 Character 22) coded thepresence of a median spine and two pairs of lateral spines aspotentially homologous in Sinoburius Kuamaia and HelmetiaThe distribution of lateral spines is much wider than that ofterminal spines and therefore they are coded separately hereIn trilobites it has widely been recognised that lateral spinesrepresent the original segmentation of the pygidium Thiscannot be the case for median spines Characters 49 to 51are not applicable to taxa lacking a pygidium (Character 47State 0)

52 Expanded post-segmental telson (0) absent and (1)present In a range of taxa the posterior-most tergite is a large(relative to the thoracic tergites) structure that lacks anyevidence of segmentation and is interpreted as representing anexpanded tergite of the post-segmental telson from whichsegments are released anteriorly during ontogeny On the otherhand the posteriormost tergite of Marrella and probablyMimetaster is small compared to the thoracic tergites androunded This element probably represents the plesiomorphiceuarthropod telson The homology of this character in mosttaxa is clear and only doubtful in Retifacies in which it may

be segmented The figures of Hou amp Bergstrom (1997) providelittle unequivocal evidence for a telson in Retifacies but thepresence of this structure is very clear in colour photographs(eg Chen et al 1996 fig 198A) However these figures donot convincingly demonstrate the segmentation of the telsonRetifacies is interpreted here as being unique in possessingboth a pygidium and an expanded post-segmental telson

53 Telson shape (0) spinose and (1) paddle-shaped Theshape of the expanded telsons identified above varies frombroadly spinose to flattened and paddle-like This differencecan be recognised both on the basis of the ratio of length tobasal width (spinose telsons are relatively long) and by thechange in width posteriorly (spinose telsons reduce in widthposteriorly whereas paddle-shaped telsons increase in width)In eurypterids a wide range of telson shapes is found includ-ing both character states described here It is likely that theplesiomorphic condition is a spinose telson This character isinapplicable to taxa lacking an expanded telson

54 Post-ventral furcae (0) absent and (1) present Thischaracter recognises the potential homology of unsegmentedpaired structures that articulate with the segment immediatelyanterior to the telson The potential homology of these struc-tures (the post-ventral plates) in Emeraldella and Aglaspis wasrecognised by Wills et al (1998a Character 70) and inEmeraldella and Sidneyia by Edgecombe amp Ramskold (1999Character 25) The homology of the segmented pygidial caudalfurcae of Olenoides (see Whittington 1975a 1980) with thesestructures is considered doubtful and coded as equivocal

Despite their distinctive morphology the long caudualfurcae of Cheloniellon may be homologous with the furcae ofAglaspis Emeraldella and Sidneyia This is supported by thecaudal furcae of the Ordovician cheloniellid Duslia which aremore similar in morphology to those of Emeraldella than ofCheloniellon Chlupac (1988 pp 614ndash16) considered thesestructures to be on the terminal tergite in Duslia However afaint tergite boundary is apparent posterior to the attachmentof the furcae (see Chlupac 1988 pl 57 fig 4) Therefore theyare considered here to have been attached to the pre-telsonicsegment as in Cheloniellon Chlupac (1988) and Dunlop ampSelden (1997) supported a close relationship between Dusliaand Cheloniellon

25 MethodsAll analyses were carried out using PAUP Version 40b4a(Swofford 1999) and unless otherwise stated used heuristicsearches with one thousand random addition sequencereplicates The software packages MacClade Version 307(Maddison amp Maddison 1997) and RadCon (Thorley amp Page2000) were used for comparing trees and investigating patternsof character evolution Tree length and other statistics (theConsistency Index CI and Retention Index RI) were calcu-lated by PAUP and MacClade with uninformative charactersexcluded Analyses were run separately using each of thedifferent codings for aglaspidids

Characters were treated as unordered and of equal weight inmost analyses To assess the influence of this assumption fourof the eight multistate characters which have states that areintermediate between others (Wilkinson 1992) were treated asordered in some analyses These characters are the number ofcephalic segments (Character 4) the length of raptorialappendage spines (Character 6) the degree of overlap of thethorax by the head-shield (Character 37) and the number ofsegments making up the postabdomen (Character 44) The lastof these is uninformative when not ordered because only onetaxon shows each of states 0 1 and 3

Support for individual nodes was assessed by bootstrapanalysis (Felsenstein 1985) and by calculating Bremer support

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 185

indices (Bremer 1988 1994) These methods measure twodistinct aspects of support for phylogenetic hypotheses Treesor nodes may be considered well supported (1) to the extent towhich alternative topologies are much less parsimonious asmeasured by the support index (Wilkinson 1996) or (2) wherethey are consistent with a large proportion of characters sothat character sampling is unlikely to have had much influenceon topology as assessed by bootstrapping (Page 1996) Boot-strapping was performed with 100 bootstrap replicates eachof 10 addition sequence replicates Bremer support indicesare based on heuristic searches with 100 addition sequencereplicates

Three pairs of terminals are included in Table 2 that apartfrom missing data are coded identically These are Helmetiaand Kuamaia Soomaspis and Tariccoia and Saperion andSkioldia These pairs are lsquotaxonomic equivalentsrsquo removingthe taxon with most missing data will not influence theposition of the other They can be recoded as single terminalsaccording to the principal of safe taxonomic reduction(Wilkinson 1995a b) and were not considered separately in theanalyses presented here Therefore cladistic analyses werecarried out on the basis of 33 terminals Edgecombe ampRamskold (1999) ignored this and coded the same three pairsof taxa and Eoredlichia and Olenoides identically for theirmore limited set of characters

26 ResultsAnalysis with all characters unordered and using the lsquoAglaspi-dida 1rsquo coding found nine most parsimonious trees (MPTs)each 126 steps long (CI=0middot556 RI=0middot763) Using the lsquoAglas-pidida 2rsquo coding resulted in 18 MPTs of length 125 (CI=0middot560RI=0middot765) Nine of these 18 trees were those found with thefirst coding of aglaspidids The strict component consensus(sensu Wilkinson 1994) of the 27 unique trees found acrossboth analyses is shown in Figure 8 All trees supported themonophyly of the Marrellomorpha (Marrella and Mimetaster)and of a clade including all other taxa except Crustacea (Clade1 of Fig 8) The relationships between these two clades and theCrustacea were unresolved equal numbers of trees in bothanalyses supported each of the topologies [Clade 1 (CrustaceaMarrellomorpha)] [Crustacea (Marrellomorpha Clade 1)]and [Marrellomorpha (Crustacea Clade 1)] All trees placedBurgessia as the sister taxon to the rest of Clade 1 andsupported the division of the rest of Clade 1 into two majorsister groups The topology of one of these clades (Clade 2)which included trilobites helmetiids tegopeltids naraoiidsxandarellids and Retifacies was stable with respect to thecoding of aglaspidids The other clade (Clade 3) consists of aclade of megacheirans and chelicerates (Clade 4) and a cladeincluding aglaspidids Lemoneites Paleomerus CheloniellonEmeraldella and Sidneyia (Clade 5) This latter clade is un-resolved in the strict consensus of all 27 trees (Fig 8) and thestrict consensus of the 18 MPTs found using the lsquoAglaspidida2rsquo coding but was fully resolved in the consensus of the ninetrees found with the lsquoAglaspidida 1rsquo coding Within themegacheiran-chelicerate clade all trees found Fortiforceps tobe the sister group to all other taxa Yohoia the sister group tochelicerates and Alalcomenaeus and Leanchoilia to form aclade Jianfengia is equally parsimoniously placed as the sistergroup to the Yohoia-chelicerate clade or as the sister group tothis clade and the Alalcomenaeus-Leanchoilia clade

Treating characters 4 6 37 and 44 as ordered had a minimaleffect on most parsimonious topology Nine MPTs of 136 steps(CI=0middot515 RI=0middot743) were found with the lsquoAglaspidida 1rsquocoding and 12 MPTs 135 steps in length (CI=0middot519 RI=0middot745)with the lsquoAglaspidida 2rsquo coding These trees all supported a(Crustacea Clade 1) clade excluding the marrellomorphs

Apart from this the strict consensus of these 21 trees isidentical to the consensus of the 27 trees found with allcharacters treated as unordered

Across all analyses four different topologies for Clade 5 (ofFig 8) were found (Fig 9) The distribution of these topologiesacross the four analyses described above and across the sameanalyses repeated after reweighting of each character in pro-portion to their Rescaled Consistency Index (see Farris 1989)from the first analysis is shown in Table 3 The instability of

Figure 8 Strict consensus of 27 MPTs from four separate analyses ofarachnomorph phylogeny with both interpretations of aglaspididmorphology and with some characters treated as ordered or allcharacters unordered Clades referred to in the text are numbered

Figure 9 Alternative equally parsimonious resolutions of clade 5 ofFig 8 (see Table 3)

186 TREVOR J COTTON AND SIMON J BRADDY

this clade was to some extent caused by the inclusion ofLemoneites and Paleomerus which could each be coded foronly 27 of the 54 characters used Analysis with these two taxaexcluded and characters treated as unordered found treescompatible only with Topology D of Figure 9 irrespective ofthe coding used for aglaspidids Analysis with some characterstreated as ordered and using either aglaspidid coding foundthree trees compatible only with Topology B of Figure 9 andthree compatible only with Topology D of Figure 9 ThereforeTopology D is the preferred topology for Clade 5 as shown inTable 3

One of the MPTs resulting from the lsquoAglaspidida 2rsquo coding(with all characters treated as unordered) was chosen as thebasis for further discussion because this tree combined thepreferred topology of Clade 5 with the (Crustacea Clade 1)group that was favoured when some characters were treated asordered Bootstrap percentages and Bremer support values forthis tree (using the Aglaspidida 2 coding) are shown in Figure10 Apomorphies for all ingroup nodes of this tree are shownin Figure 10A

3 Discussion

31 Phylogenetic resultsThe results of the cladistic analyses presented above arewell-resolved and robust with respect to different analyticalparameters Most of the taxa considered form a clade (Clade 1of Fig 8) that is more closely related to chelicerates than tocrustaceans in all analyses and therefore is recognised asthe Arachnomorpha as defined by Chen et al (1996) andRamskold et al (1996) The monophyly of the Marrellomor-pha was supported in all analyses The marrellomorphs werefound to be the sister group to a Crustacea+Clade 1 group inmost MPTs but in a minority of trees formed the sister groupto Clade 1 alone According to the latter result they should beincluded within the Arachnomorpha but according to the firstexcluded from it Both of these alternatives have been sup-ported in previous studies (eg Hou amp Bergstrom 1997 Willset al 1998a respectively) In addition to the marrellomorphsthe Fuxianhuiida Bousfield 1995 and Canadaspidida havebeen placed in the Euarthropoda outside the main mandibu-late and arachnomorph clades (Hou amp Bergstrom 1997)Further study of these taxa of the crustacean or mandibulatestem group and of the marrellomorphs is necessary before the

position of the Marrellomorpha can be resolved Provisionallymarrellomorphs are excluded from the Arachnomorphafollowing the majority of analyses above and Wills et al(1994)

The Burgess Shale arthropod Burgessia was placed as thesister group to remaining arachnomorphs in all analyses OtherArachnomorpha form two major clades one consisting of thelsquotrilobite-alliedrsquo arachnomorphs analysed by Edgecombe ampRamskold (1999) here termed lsquoTrilobitomorpharsquo (defined asTrilobita and all taxa more closely related to Trilobita thanChelicerata) and the other (lsquochelicerate-alliedrsquo) clade includingchelicerates lsquogreat appendagersquo arthropods aglaspididsLemoneites Paleomerus Sidneyia and Emeraldella The latteris here termed lsquoCheliceramorpharsquo (defined as Chelicerata andall taxa more closely related to Chelicerata than Trilobita) Thetopology of the lsquotrilobite-alliedrsquo clade was similar to that foundby Edgecombe amp Ramskold (1999) but the present authorsrsquoresults are more completely resolved unambiguously support-ing the sister-group relationship between trilobites andHelmetiida suggested in the previous study Within thelsquochelicerate-alliedrsquo clade Aglaspidids Paleomerus Lemoneitesand the Burgess Shale Emeraldella and Sidneyia form a cladein opposition to megacheirans and chelicerates

The present authorsrsquo analysis supports a new hypothesis ofthe origin of the chelicerates from within a paraphyleticassemblage of megacheiran arthropods and confirms pyc-nogonids as a sister group to the euchelicerates Previoushypotheses of the chelicerate sister group (see Section 1) aremuch less parsimonious The shortest trees supporting aCheloniellon-chelicerate clade are six steps longer than theMPTs and those supporting an aglaspidid-chelicerate cladeseven steps longer All megacheirans and chelicerates areunited by the flap-shaped rounded exopods and modificationof the second cephalic segment endopods into anteriorlydirected spinose raptorial organs which share a number ofdetailed similarities A clade including all of these taxa apartfrom Fortiforceps is supported by the synapomorphic loss ofthe antennae (or antennulae of crustaceans) and the longerlength of spinose projections of the second cephalic appendageendopods Within this clade Yohoia shares the loss of cephalicexopods the loss of thoracic endopods and a postabdomen oftubular tergites with chelicerates

The poor resolution of Clade 5 (of Fig 8) was partly a resultof the poor preservation of and consequent high proportion ofmissing data for Lemoneites and Paleomerus When these two

Table 3 Numbers of most parsimonious trees supporting each of the alternative topologies for clade 4 of Fig 7 under different analyticalconditions (RCI=Revealed Consistency Index)

Aglaspididcoding

Characters 4 6 36and 43 ordered

Charactersreweighted by RCI

Paleomerus andLemoneites excluded

Topology (see Fig 8)

A B C D

1 91 X 3 3 31 X 61 X X 31 X 61 X X 3 32 9 92 X 3 3 3 32 X 62 X X 32 X 62 X X 3 3Total 6 18 12 54

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 187

taxa were excluded from the analyses only two (B and D) ofthe four topologies found in the complete analysis remainedmost parsimonious (see Fig 9 and Table 3) Secondly Bremersupport values throughout much of the lsquochelicerate-alliedrsquoclade (Clade 3 of Fig 8) were improved when these taxa wereexcluded (see Fig 11)

Dunlop (1999 p 258) suggested that in reconstructing thechelicerate stem lineage lsquowe might predict that the two mostsignificant changes towards the chelicerate condition are thereduction of the antennae and the formation of the nextappendages into a clawrsquo According to the hypothesis pre-sented here both of these adaptations were achieved in EarlyCambrian chelicerate ancestors The recognition of the loss ofthe antennae in the stem group of the chelicerates and thephylogenetic hypothesis presented here suggest that chelicer-ates are most closely related to taxa with only a singleadditional segment fused to the crown group eucheliceratehead of four fused somites Previous hypotheses have derivedchelicerates from Palaeozoic arthropods with a longer head(eg Cheloniellon Sturmer amp Bergstrom 1978 SanctacarisBriggs amp Collins 1988 Emeraldella Bruton amp Whittington1983) The plesiomorphic pattern of head segmentation ismatched in pycnogonids Bergstrom et al (1980) suggestedthat only the anterior four pairs of appendages are incorpo-rated into the head This supports the view that pycnogonidsare primitive with respect to other chelicerates which showmore or less complete fusion of this primitive head and threethoracic segments including Solpugida and Pseudoscorpiones

The present study suggests that a shift from gnathobasicbenthic feeding to a more vagrant lifestyle and raptorialappendage feeding occurred very early in the chelicerate lin-eage Appendage feeding was clearly an important adaptation

Figure 10 One of 18 equally parsimonious cladograms with lsquoAglaspidida 2rsquo coding and all characters unorderedthat is most congruent with results from other analyses Non-terminal branch lengths scaled to reflect number ofapomorphies (A) ACCTRAN apomorphy scheme Character numbers shown above boxes and states shownbelow boxes as in Table 2 and the text Character consistency indices are indicated by shading (B) Levels ofsupport for individual nodes Bremer support indices shown in bold type above branches Bootstrap percentagesin italic type below branches for all nodes with relative frequencies greater than 50

Figure 11 Bremer support values for clades within the lsquochelicerate-alliedrsquo arachnomorphs (clade 3 of Fig 8) when all taxa wereincluded (normal weight font above nodes) and with Lemoneites andPaleomerus excluded (bold type below nodes)

188 TREVOR J COTTON AND SIMON J BRADDY

for the terrestrialization of chelicerates (Dunlop 1997 p 69Dunlop amp Webster 1999) but the model proposed heresuggests that this considerably pre-dated terrestrializationGnathobasic feeding in xiphosurans and eurypterids mayrepresent a reversal to a more primitve benthic arachnomorphlifestyle from a more pycnogonid-like ancestor

32 Evolution of the arachnomorph headThe extant arthropod classes show highly conserved patternsof head segmentation (eg Wills et al 1994) In contrast therehas been a near consensus amongst Cambrian arthropodworkers (although rarely based on an explicit phylogenetichypothesis) that patterns of head segmentation are highlyhomoplastic and hence of little systematic significanceSturmer amp Bergstrom (1978 pp 78ndash9) suggested that lsquoevenclosely related forms may have different numbers of headsegments and appendagesrsquo Bruton amp Whittington (1983pp 576ndash7) that lsquodiscussion on fossil arthropod relationshipsbased on head segmentation appears to be largely irrel-evant and at best speculativersquo and Delle Cave amp Simonetta(1991 p 191) that lsquothere are no obvious phyletic affinitiesbetween genera having the same number of cephalized seg-mentsrsquo Most recently Bergstrom amp Hou (1997 p 104) con-cluded that lsquothe segmental length of the head shield seemsto be of no relevance to the discussion and discrimination ofevolutionary lineagesrsquo It has been claimed that this view issupported by cladistic analyses in which patterns of tagmosishave been found to be rather poor at defining major arthropodclades (Briggs et al 1992 Wills et al 1994)

This suggestion is not only of relevance to systematics buthas featured prominently in recent debates surrounding thenature of the lsquoCambrian explosionrsquo (see Budd amp Jensen 2000for a recent review) Gould (1989 1991) argued that theapparent plasticity of head segmentation in Cambrian com-pared to post-Cambrian arthropod evolution suggests thatbody-plan evolution was only constrained after the Cambrianexplosion According to this view the origin of new highertaxa with distinct body-plans was only possible before theonset of these constraints (eg Valentine 1995) Taxa hererecognised as arachnomorphs clearly played a major role in thedevelopment of Gouldrsquos hypothesis This suggestion has pre-viously been assessed in terms of overall morphological diver-sity which is clearly rather distinct from Gouldrsquos concept ofdisparity Wills et al (1998b) showed that there was littlecorrelation between overall morphological disparity and de-gree of limb specialisation and that the latter was phylogeneti-cally highly plastic They remarked that (Wills et al 1998bp 64) lsquothis finding has important implications for models ofarthropod phylogeny and evolution that attribute overidingimportance to head segmentationrsquo but did not explicitlyexamine head segmentation although it is likely to have beena major component of the index of tagmosis employed

The results of the present study suggest that arguments thatarthropod head segmentation was unusually labile during theCambrian are poorly founded Rather only four major pat-terns of euarthropod head segmentation are identified Theplesiomorphic euarthropod state according to both Walossekamp Muller (1997 1998) and Scholtz (1997) which may moreproperly be the plesiomorphic state for the euarthropod crowngroup only (depending on the phylogenetic position of theMarrellomorpha) consists of four post-acronal segmentsbearing the antennae and three pairs of biramous limbs Thepresent authors suggest that the term lsquocephalonrsquo be restrictedto this kind of head which is found in stem group mandibu-lates in addition to many arachnomorphs Crown groupmandibulates share a lsquobimaxillary headrsquo in which an addi-tional pair of appendages the labium or second maxillae are

incorporated into the head but in crustaceans these are notfused to the carapace (Scholtz 1997) Finally two distinctforms of head tagmosis are found in chelicerates A head withfour pairs of appendages (but without antennae giving a totalof five segments incorporated into the head) which has beencalled the lsquocephalosomarsquo is present in pycnogonids (Walossekamp Dunlop 2002 pp 434ndash5) and probably also in some euch-elicerates (J A Dunlop pers comm 2003) The lsquoprosomarsquo ofmost euchelicerates consists of this cephalosoma and twoadditional segments

In general these patterns of head segmentation are highlyphylogenetically conserved (see Fig 12) Only three homo-plastic changes namely reversal to a more primitive conditionin the Leanchoilia-Alalcomenaeus clade and the convergentorigin of a five-segmented head in Clade 3 (of Fig 8) and theXandarellida are necessary to optimise character 4 onto themost parsimonious cladogram There are also transitions toautapomorphic states in Emeraldella and Sidneyia In the caseof Sidneyia however an argument could be made (followingBergstrom amp Sturmer 1978 see discussion of Character 4) thatthe head consists of five segments as in closely related taxabut autapomorphically each segment has a separate tergiteThis would further increase the phylogenetic stability of headsegmentation Also following Bergstrom amp Sturmer (1978) thehead of Cheloniellon could be considered to consist of twotergites making it more similar to the head of Emeraldella(with six pairs of appendages)

Figure 12 Cladogram from Fig 10 showing changes in head segmen-tation Bold type shows abbreviated head segmentation formulaAbbreviations (Ant) antennae (BA) biramous appendage pairs (Ch)chelicerae (GA) great appendages (UA) uniramous appendage pairsand (X) segment without appendages Synapomorphy A is the auta-pomorphic reduction in head length to a single segment in SidneyiaAlternatively this may represent secondary division of the headshield Synapomorphy B is the increase in the length of the head inEmeraldella to six segments

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 189

4 Acknowledgements

We thank D E G Briggs J A Cotton J A Dunlop R AFortey S Harris R A Moore P Selden O E Tetlie andP Van Roy for comments on earlier drafts of this work Themanuscript was reviewed by M Wills F Schram and oneanonymous referee This research was supported by the Geo-logical Society of London Curry Research Studentshipawarded to TJC by the Department of Earth Sciences Uni-versity of Bristol and by the Department of PalaeontologyThe Natural History Museum London TJC would like tothank C Labandeira J Thompson and R Chapman fortheir hospitality at the National Museum of Natural HistoryWashington DC USA and A Olcott S Schellenberg andJ Young for making his time there so enjoyable

5 References

Abzhanov A Popadic A amp Kaufman T C 1999 Chelicerate Hoxgenes and the homology of arthropod segments Evolution andDevelopment 1 77ndash89

Akam M 2000 Arthropods developmental diversity within a (super)phylum Proceedings of the National Academy of Sciences USA97 4438ndash41

Aldridge R J Gabbott S E amp Theron J N 2001 The Soom ShaleIn Briggs D E G amp Crowther P R (eds) Palaeobiology II340ndash2 Oxford Blackwell Science

Anderson L I amp Selden P A 1997 Opisthosomal fusion andphylogeny of Palaeozoic Xiphosura Lethaia 30 19ndash31

Arango C P 2002 Morphological phylogenetics of the sea spiders(Arthropoda Pycnogonida) Organisms Diversity and Evolution2 107ndash25

Ax P 1986 Das phylogenetische System Stuttgart Gustav FischerVerlag

Babcock L E amp Zhang Wentang In press New non-mineralizedarthropods from the Chengjiang Lagerstatte Lower CambrianYunnan China Palaeontologia Cathyana

Bartels C Briggs D E G amp Brassel G 1998 The fossils ofthe Hunsruck Slate Marine life in the Devonian CambridgeCambridge University Press

Bergstrom J 1971 Paleomerus ndash merostome or merostomoid Lethaia4 393ndash401

Bergstrom J 1979 Morphology of fossil arthropods as a guideto phylogenetic relationships In Gupta A P (ed) ArthropodPhylogeny 3ndash56 New York NY Van Nostrand Reinhold Co

Bergstrom J 1992 The oldest arthropods and the origin of theCrustacea Acta Zoologica 73 287ndash91

Bergstrom J Sturmer W amp Winter G 1980 PalaeoisopusPalaeopantopodus and Palaeothea pycnogonid arthropodsfrom the Lower Devonian Hunsruck Slate West GermanyPalaontologische Zeitschrift 54 7ndash54

Bergstrom J amp Brassel G 1984 Legs in the trilobite Rhenops fromthe Lower Devonian Hunsruck Slate Lethaia 17 67ndash72

Bergstrom J amp Hou X 1998 Chengjiang arthropods and theirbearing on early arthropod evolution In Edgecombe G D(ed) Arthropod fossils and phylogeny 151ndash84 New York NYColumbia University Press

Boudreaux H B 1979 Significance of intersegmental tendon systemin arthropod phylogeny and a monophyletic classification ofArthropoda In Gupta A P (ed) Arthropod Phylogeny 551ndash86New York NY Van Nostrand Reinhold Co

Bousfield E L 1995 A contribution to the natural classification ofLower and Middle Cambrian arthropods food-gathering andfeeding mechanisms Amphipacifica 2 3ndash33

Braddy S J Aldridge R J Gabbott S E amp Theron J N 1999Lamellate book-gills in a late Ordovician eurypterid from theSoom Shale South Africa support for a eurypterid-scorpionclade Lethaia 32 72ndash4

Braddy S J amp Almond J E 1999 Eurypterid trackways from theTable Mountain Group (Ordovician) of South Africa Journal ofAfrican Earth Sciences 29 165ndash77

Bremer K 1988 The limits of amino acid sequence data in angio-sperm phylogenetic reconstruction Evolution 42 795ndash803

Bremer K 1994 Branch support and tree stability Cladistics 10295ndash304

Briggs D E G 1998 Review of Arthropods from the LowerCambrian Chengjiang fauna southwest China by HouXianguang and Jan Bergstrom Fossils and Strata 45

Scandinavian University Press Oslo Norway 1997 Palaeo-geography Palaeoclimatology Palaeoecology 143 193ndash4

Briggs D E G Bruton D L amp Whittington H B 1979 Append-ages of the arthropod Aglaspis spinifer (Upper CambrianWisconsin) and their significance Palaeontology 22 167ndash80

Briggs D E G Fortey R A amp Wills M A 1992 Morphologicaldisparity in the Cambrian Science 256 1670ndash3

Briggs D E G Fortey R A amp Wills MA 1993 How big was theCambrian explosion A taxonomic and morphologic comparisonof Cambrian and Recent arthropods In Lees D R amp EdwardsD (eds) Evolutionary patterns and processes 33ndash44 LondonLinnean Society of London Linnean Society Symposium Series

Briggs D E G Erwin D H amp Collier F J 1994 The fossils of theBurgess Shale Washington DC Smithsonian Institution Press

Briggs D E G amp Bartels C 2001 New arthropods from the LowerDevonian Hunsruck Slate (Lower Emsian Rhenish Massifwestern Germany) Palaeontology 44 275ndash304

Briggs D E G amp Bartels C 2002 Magnoculocaris a new name forthe arthropod Magnoculus Briggs and Bartels 2001 from theLower Devonian Hunsruck Slate Germany Palaeontology 45419

Briggs D E G amp Collins D 1988 A Middle Cambrian cheliceratefrom Mount Stephen British Columbia Palaeontology 31 779ndash98

Briggs D E G amp Collins D 1999 The arthropod Alalcomenaeuscambricus Simonetta from the Middle Cambrian Burgess Shale ofBritish Columbia Paleontology 42 953ndash78

Briggs D E G amp Fortey R A 1982 The cuticle of aglaspididarthropods a red-herring in the early history of vertebratesLethaia 15 25ndash9

Briggs D E G amp Fortey R A 1989 The early radiation andrelationships of the major arthropod groups Science 246 241ndash3

Briggs D E G amp Fortey R A 1992 The early Cambrian radiationof arthropods In Lipps J H amp Signor P W (eds) Origin andEarly Evolution of the Metazoa 335ndash373 New York NY PlenumPress

Briggs D E G amp Robison R A 1984 Exceptionally preservednontrilobite arthropods and Anomalocaris from the MiddleCambrian of Utah University of Kansas PaleontologicalContributions Paper 111 1ndash23

Broili F 1932 Ein neuer Crustacee aus dem rheinischen UnterdevonSitzungsberichte der Mathematisch-Naturwissenschaftlichen(Abteilung) Klasse der Bayerischen Akademie der Wissenschaftenzu Munchen 1932 27ndash38

Broili F 1933 Ein zweites Exemplar von Cheloniellon Sitzungsber-ichte der Bayerischen Akademie der Wissenschaften 1933 11ndash32

Bruton D L 1981 The arthropod Sidneyia inexpectans MiddleCambrian Burgess Shale British Columbia Philosophical Trans-actions of the Royal Society of London Series B 295 619ndash56

Bruton D L amp Whittington H B 1983 Emeraldella and Leanchoiliatwo arthropods from the Burgess Shale British Columbia Philo-sophical Transactions of the Royal Society of London Series B 300553ndash85

Budd G E 1995 Kleptothule rasmusseni gen et sp nov anolenellid-like trilobite from the Sirius Passet fauna (BuenFormation Lower Cambrian North Greenland) Transactions ofthe Royal Society of Edinburgh Earth Sciences 86 1ndash12

Budd G E 1996a Progress and problems in arthropod phylogenyTrends in Ecology and Evolution 11 356ndash8

Budd G E 1996b The morphology of Opabinia regalis and thereconstruction of the arthropod stem-group Lethaia 29 1ndash14

Budd G E 1997 Stem-group arthropods from the Lower CambrianSirius Passet fauna of North Greenland In Fortey R A ampThomas R H (eds) Arthropod Relationships 125ndash38 LondonChapman and Hall Systematics Association Special Volume 55

Budd G E 1999a A nektaspid arthropod from the Early CambrianSirius Passet fauna with a description of retrodeformation basedon functional morphology Palaeontology 42 99ndash122

Budd G E 1999b The morphology and phylogenetic significance ofKerygmachela kiergkegaardi Budd (Buen Formation LowerCambrian N Greenland) Transactions of the Royal Society ofEdinburgh Earth Sciences 89 (for 1998) 249ndash290

Budd G E 2002 A palaeontological solution to the arthropod headproblem Nature 417 271ndash5

Budd G E amp Jensen S 2000 A critical reappraisal of the fossilrecord of the bilaterian phyla Biological Reviews 75 253ndash95

Chen J Zhou G Zhu M amp Yeh K 1996 The Chengjiang biota ndasha unique window of the Cambrian explosion Taichung NationalMuseum of Natural Science [In Chinese]

Chen J Edgecombe G D amp Ramskold L 1997 Morphological andecological disparity in naraoiids (Arthropoda) from the Early

190 TREVOR J COTTON AND SIMON J BRADDY

Cambrian Chengjiang fauna China Records of the AustralianMuseum 49 1ndash24

Chen J amp Zhou G 1997 Biology of the Chengjiang fauna In ChenJ Cheng Y amp Iten H V (eds) The Cambrian explosion and thefossil record Bulletin of the National Museum of Natural ScienceTaiwan 10 11ndash106

Child C A 1997 Some deep-sea Pycnogonida from Argentine slopeand basin Proceedings of the Biological Society of Washington110 128ndash42

Chlupac I 1988 The enigmatic arthropod Duslia from the Ordovicianof Czechslovakia Palaeontology 31 611ndash20

Chlupac I 1995 Lower Cambrian arthropods from the PasekyShale (Barrandian area Czech Republic) Journal of the CzechGeological Society 40 9ndash36

Chlupac I amp Havlıcek V 1965 Kodymirus n g a new aglaspidmerostome of the Cambrian of Bohemia Sbornık GeologickechVid Paleontologie 6 7ndash20

Cisne J L 1975 Anatomy of Triarthrus and the relationships of theTrilobita Fossils and Strata 4 45ndash63

Cisne J L 1981 Triarthrus eatoni (Trilobita) anatomy of its exo-skeletal skeletomuscular and digestive systems Palaeonto-graphica Americana 9 95ndash142

Conway Morris S 1982 Atlas of the Burgess Shale London ThePalaeontological Association

Conway Morris S 1989 The persistence of Burgess Shale-type faunasimplications for the evolution of deeper-water faunas Trans-actions of the Royal Society of Edinburgh Earth Sciences 80271ndash83

Conway Morris S 1998 The crucible of creation Oxford OxfordUniversity Press

Cotton T J 1999 Review of Arthropod fossils and phylogenyPalaeogeography Palaeoclimatology Palaeoecology 151 341ndash3

Cotton T J 2002 The phylogeny and morphological evolution ofCambrian trilobites and their relatives Unpublished PhD ThesisUniversity of Bristol

Damen W G M Hausdorf M Seyfarth E A amp Tautz D 1998 Aconserved mode of head segmentation in arthropods revealed bythe expression pattern of Hox genes in a spider Proceedings of theNational Academy of Sciences USA 95 10665ndash70

Delle Cave L amp Simonetta A M 1991 Early Palaeozoic arthropodsand problems of arthropod phylogeny with some notes on taxa ofdoubtful affinities In Simonetta A M amp Conway Morris S(eds) The early evolution of Metazoa and the significance ofproblematic taxa 189ndash244 Cambridge Cambridge UniversityPress

Dewel R A amp Dewel W C 1997 The place of tardigrades inarthropod evolution In Fortey R A amp Thomas R H (eds)Arthropod Relationships 109ndash23 London Chapman and HallSystematics Association Special Volume 55

Dunlop J A 1997 Palaeozoic arachnids and their significance forarachnid phylogeny Proceedings of the 16th European Colloquiumof Arachnology 65ndash82 Siedlce Poland

Dunlop J A 1998 The origins of tetrapulmonate booklungs and theirsignificance for chelicerate phylogeny In Selden P A (ed)Proceedings of the 17th European Colloquium of ArachnologyEdinburgh 1997 pp 9ndash16

Dunlop J A 1999 Pasando revista a la evolucion de los quelicerados[A review of chelicerate evolution] In Melic A DeHaro J JMedez M amp Ribera I (eds) Volumen Monografico Evolucion yFilogenia de Arthropoda Boletin de la Sociedad EntolomologicaAragonesa 26 255ndash272 [In Spanish and English]

Dunlop J A amp Selden P A 1997 The early history and phylogenyof the chelicerates In Fortey R A amp Thomas R H (eds)Arthropod relationships Systematics Association Special Volume55 221ndash35 London Chapman and Hall

Dunlop J A amp Webster M 1999 Fossil evidence terrestrializationand arachnid phylogeny Journal of Arachnology 27 86ndash93

Dzik J 1993 Early metazoan evolution and the meaning of its fossilrecord Evolutionary Biology 27 339ndash86

Dzik J amp Lendzion K 1988 The oldest arthropods of the EastEuropean Platform Lethaia 21 29ndash38

Edgecombe G D Wilson G D F Colgan D J Gray M R ampCassis G 2000 Arthropod cladistics combined analysisof Histone H3 and U2 snRNA sequences and morphologyCladistics 16 155ndash203

Edgecombe G D amp Ramskold L 1999 Relationships of CambrianArachnata and the systematic position of Trilobita Journal ofPaleontology 73 263ndash87

Emerson M J amp Schram F R 1997 Theories patterns and realitygame plan for arthropod phylogeny In Fortey R A amp Thomas

R H (eds) Arthropod Relationships Systematics AssociationSpecial Volume 55 67ndash87 London Chapman and Hall

Farris J S 1989 The Retention Index and the Rescaled ConsistencyIndex Cladistics 5 417ndash19

Felsenstein J 1985 Confidence limits on phylogenies an approachusing the bootstrap Evolution 39 783ndash91

Flower R H 1968 Merostomes from a Cotter Horizon of the El PasoGroup New Mexico Bureau of Mines and Mineral ResourcesMemoir 22 35ndash44

Fortey R A 1990a Trilobite evolution and systematics In MikulicD G (ed) Arthropod paleobiology 44ndash65 Short Courses inPaleontology 3 Knoxville Tenessee The Paleontological Society

Fortey R A 1990b Ontogeny hypostome attachment and trilobiteclassification Palaeontology 33 529ndash76

Fortey R A 1997 Classification In Kaesler R L (ed) Treatise oninvertebrate paleontology Part O Arthropoda 1 (revised) 289ndash302Boulder Colorado amp Lawrence Kansas Geological Society ofAmerica and University of Kansas Press

Fortey R A amp Theron J N 1994 A new Ordovician arthropodSoomaspis and the agnostid problem Palaeontology 37 841ndash61

Fortey R A amp Whittington H B 1989 The Trilobita as a naturalgroup Historical Biology 2 125ndash38

Fry W G 1978 A classification within the pycnogonids ZoologicalJournal of the Linnean Society of London 63 35ndash58

Giribet G Edgecombe G D amp Wheeler W C 2001 Arthropodphylogeny based on eight molecular loci and morphology Nature413 157ndash61

Gould S J 1989 Wonderful life The Burgess Shale and the nature ofhistory New York NY Norton

Gould S J 1991 The disparity of the Burgess Shale arthropod faunaand the limits of cladistic analysis why we must strive to quantifymorphospace Paleobiology 17 411ndash23

Gurich G 1931 Mimetaster hexagonalis ein neuer Kruster aus demunterdevonischen Bundenbacher Dachschiefer PalaontologischeZeitschrift 12 204ndash38

Hammann W Laske R amp Pillola G L 1990 Tariccoia arrusensisngnsp an unusual trilobite-like arthropod Rediscovery of thelsquophylocaridrsquo beds of Taricco (1922) in the Ordovician lsquoPuddingarsquosequence of Sardinia Bolletino della Societa PaleontologicaItaliana 29 163ndash78

Haas M S Brown S J amp Beeman R W 2001 Pondering theprocephalon the segmental origin of the labrum DevelopmentGenes and Evolution 211 89ndash95

Heider K 1913 Entwicklungsgeschichte und Morphologie derWirbellosen Die Kultur der Gegenwart Theil 3 Abt 4 2 176ndash332

Henriksen K 1928 Critical notes upon Cambrian arthropodsdescribed by Charles D Walcott Videnskabelige Meddelelser fraDansk Naturhistorisk Forening 86 1ndash20

Hesselbo S P 1988 Trace fossils of Cambrian aglaspidid arthropodsLethaia 21 139ndash46

Hesselbo S P 1989 The aglaspidid arthropod Beckwithia from theCambrian of Utah and Wisconsin Journal of Paleontology 63636ndash42

Hesselbo S P 1992 Aglaspidida (Arthropoda) from the UpperCambrian of Wisconsin Journal of Paleontology 66 885ndash923

Hou X 1987a Three new large arthropods from Lower CambrianChengjiang fauna eastern Yunnan Acta Palaeontologica Sinica26 272ndash85 [In Chinese with English summary]

Hou X 1987b Two new arthropods from Lower CambrianChengjiang eastern Yunnan Acta Palaeontologica Sinica 26236ndash56 [In Chinese with English summary]

Hou X Chen J amp Lu H 1989 Early Cambrian new arthropodsfrom Chengjiang Yunnan Acta Palaeontologica Sinica 28 42ndash57[In Chinese with English summary]

Hou X Ramskold L amp Bergstrom J 1991 Composition andpreservation of the Chengjiang fauna a Lower Cambrian soft-bodied biota Zoologica Scripta 20 395ndash411

Hou X Bergstrom J amp Ahlberg P 1995 Anomalocaris and otherlarge animals in the Lower Cambrian Chengjiang fauna ofsouthwest China GFF 117 163ndash83

Hou X amp Bergstrom J 1997 Arthropods of the Lower CambrianChengjiang fauna southwest China Fossils and Strata 45 1ndash116

Hughes C P 1975 Redescription of Burgessia bella from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 4415ndash36

Ivantsov A Y 1999 Trilobite-like arthropod from the LowerCambrian of the Siberian Platform Acta PalaeontologicaPolonica 44 455ndash66

King P E 1973 Pycnogonids London St Martin Press

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 191

Lauterbach K E 1973 Schlussereignisse in der Evolution derStammgruppe der Euarthropoda Zoologische Beitrage 19259ndash99

Lauterbach K E 1980 Schlussereignisse in der Evolution derGrundplans der Arachnata (Arthropoda) Abhandlungen desNaturwissenschaftlichen Vereins in Hamburg 23 163ndash327

Lauterbach K E 1983 Synapomorphien zwischen Trilobiten- undCheliceraten-zweig der Arachnata Zoologischer Anzeiger 210213ndash38

Lendzion K 1975 Fauna of the Mobergella Zone in the Polish LowerCambrian Kwartalnik Geologiczny 19 237ndash42

Lieberman B S 1998 Cladistic analysis of the Early Cambrianolenelloid trilobites Journal of Paleontology 72 59ndash78

Lu Y 1940 On the ontogeny and phylogeny of Redlichia intermediaLu (sp nov) Bulletin of the Geological Society of China 20333ndash42 [In Chinese]

Maddison W P amp Maddison D R 1997 MacClade analysis ofphylogeny and character evolution Version 307 Sunderland MASinauer Associates

Mittmann B amp Scholtz G 2003 Development of the nervous systemin the lsquoheadrsquo of Limulus polyphemus (Chelicerata Xiphosura)morphological evidence for the correspondence between the seg-ments of the chelicerae and the (first) antennae of MandibulataDevelopment Genes and Evolution 213 9ndash17

Moore R C (ed) 1959 Treatise on invertebrate paleontology Part OArthropoda 1 Boulder Colorado Geological Society of America

Muller K J amp Walossek D 1986 Arthropod larvae from the UpperCambrian of Sweden Transactions of the Royal Society ofEdinburgh Earth Sciences 77 157ndash79

Muller K J amp Walossek D 1987 Morphology ontogeny and lifehabit of Agnostus pisiformis from the Upper Cambrian of SwedenFossils and Strata 19 1ndash124

Munilla T 1999 Evolucion y filogenia de los Picnogonidos InMelicA DeHaro J J Medez M amp Ribera I (eds) VolumenMonografico Evolucion y Filogenia de Arthropoda Boletin de laSociedad Entolomologica Aragonesa 26 273ndash91 [In Spanish andEnglish]

Orr P J Siveter D J Briggs D E G Siveter D J amp Sutton M D2000 A new arthropod from the Silurian Konservat-Lagerstatteof Herefordshire Proceedings of the Royal Society of LondonSeries B 267 1497ndash504

Page R D M 1996 On consensus confidence and lsquototal evidencersquoCladistics 12 83ndash92

Palmer A R amp Repina L N 1993 Through a glass darklytaxonomy phylogeny and biostratigraphy of the OlenellinaUniversity of Kansas Paleontological Contributions New Series 31ndash35

Platnick N I Griswold C E amp Coddington J A 1991 On missingentries in cladistic analysis Cladistics 7 337ndash43

Pleijel F 1995 On character coding for phylogeny reconstructionCladistics 11 309ndash15

Raasch G O 1939 Cambrian Merostomata Special Papers of theGeological Society of America 19 1ndash146

Ramskold L Chen J Edgecombe G D amp Zhou G 1996Preservational folds simulating tergite junctions in tegopeltid andnaraoiid arthropods Lethaia 29 15ndash20

Ramskold L Chen J Edgecombe G D amp Zhou G 1997Cindarella and the arachnate clade Xandarellida (ArthropodaEarly Cambrian) from China Transactions of the Royal Society ofEdinburgh Earth Sciences 88 19ndash38

Ramskold L amp Edgecombe G D 1991 Trilobite monophylyrevisited Historical Biology 4 267ndash83

Ramskold L amp Edgecombe G D 1996 Trilobite appendagestructure ndash Eoredlichia revisited Alcheringa 20 269ndash76

Raw F 1957 Origin of chelicerates Journal of Paleontology 31139ndash193

Repina L N amp Okuneva O G 1969 Cambrian arthropods of themaritime territory Paleontological Journal 1969 95ndash103

Richter R amp Richter E 1929 Weinbergina opitz ng n sp einSchwerttrager (Merost Xiphos) aus dem Devon (Rheinland)Senkenbergiana 11 193ndash209

Robison R A 1984 New occurences of the unusual trilobite Naraoiafrom the Cambrian of Idaho and Utah University of KansasPaleontological Contributions Paper 112 1ndash8

Rominger C 1887 Description of primordial fossils from MtStephens NW Territory of Canada Proceedings of the Academyof Natural Sciences of Philadelphia 1887 12ndash19

Scholtz G 1997 Cleavage germ band formation and head segmen-tation the ground pattern of the Euarthropoda In Fortey R Aamp Thomas R H (eds) Arthropod Relationships 317ndash32 LondonChapman and Hall Systematics Association Special Volume 55

Schram F R 1978 Arthropods a convergent phenomenonFieldiana Geology 39 61ndash108

Schram F R 1993 Review of Boxshall Stromberg and Dahl TheCrustacea Origin and Evolution Journal of Crustacean Biology13 820ndash2

Schram F R Vonk R amp Hof C H J 1997 Mazon CreekCycloidea Journal of Paleontology 71 261ndash84

Schram F R amp Emerson M J 1991 Arthropod Pattern Theorya new approach to arthropod phylogeny Memoirs of theQueensland Museum 31 1ndash18

Schram F R amp Hof C H J 1998 Fossils and the interrelationshipsof major crustacean groups In Edgecombe G D (ed) Arthropodfossils and phylogeny pp 233ndash302 New York NY ColumbiaUniversity Press

Shergold J H 1991 Protaspis and early meraspis growth stages of theeodiscoid trilobite Pagetia ocellata Jell and their implications forclassification Alcheringa 15 65ndash86

Shu D Geyer G Ling C amp Zhang Z 1995 Redlichiacean trilobiteswith preserved soft parts from the Lower Cambrian Chengjiangfauna (South China) Beringeria (Special Issue) 2 203ndash41

Shultz J W 1990 Evolutionary morphology and phylogeny ofArachnida Cladistics 6 1ndash38

Shultz J W 2000 Skeletomuscular anatomy of the harvestmanLeiobunum aldrichi (Weed 1893) (Arachnida OpilionesPalpatores) and its evolutionary significance Zoological Journalof the Linnean Society 128 401ndash38

Shultz J W 2001 Gross muscular anatomy of Limulus polyphemus(Xiphosura Chelicerata) and its bearing on evolution in the theArachnida Journal of Arachnology 29 283ndash303

Siewing R (ed) 1985 Lehrbuch der Zoologie II Systematik 3rd ednStuttgart Fischer

Simonetta A M 1970 Studies on the non trilobite arthropods ofthe Burgess Shale (Middle Cambrian) the genera LeanchoiliaAlalcomenaeus Opabinia Burgessia Yohoia and ActaeusPalaeontographica Italica 66 35ndash45

Simonetta A M amp Delle Cave L 1975 The Cambrian non trilobitearthropods from the Burgess Shale of British Columbia A studyof their comparative morphology taxonomy and evolutionarysignificance Palaeontographica Italica 69 1ndash37

Simonetta A M amp Delle Cave L 1981 An essay in the compara-tive and evolutionary morphology of Palaeozoic ArthropodsAccademia Nazionale dei Lincei Atti dei Convegni Lincei 49389ndash439

Starobogatov Y I 1990 Sistema i filogeniya nizshikh khelitserovykh(analiz morfologii paleozoysikh grupp) [The systematics andphylogeny of the lower chelicerates (a morphological analysis ofthe Palaeozoic groups)] Paleontologicheskii Zhurnal 1990 4ndash17

Stoslashrmer L 1944 On the relationships and phylogeny of fossil andrecent Arachnomorpha Skrifter utgitt ac Det Norske Videnskaps-Akademi i Olso I Matematisk-naturvitenskaplig Klasse 5 1ndash158

Stoslashrmer L 1951 Studies on trilobite morphology Part III Theventral cephalic structures with remarks on the zoologicalposition of the trilobites Norsk geologisk tidsskrift 29 108ndash58

Stoslashrmer L 1956 A Lower Cambrian merostome from Sweden Arkivfor Zoologi Ser 2 9 507ndash14

Stoslashrmer L 1959 Trilobitoidea In Moore R C (ed) Treatise onInvertebrate Paleontology part O Arthropoda 1 23ndash37 BoulderColorado Geological Society of America

Strong E E amp Lipscomb D 1999 Character coding and inapplicabledata Cladistics 15 363ndash71

Sturmer W amp Bergstrom J 1976 The arthropods Mimetaster andVachonisia from the Devonian Hunsruck Shale PalaontologischeZeitschrift 50 78ndash111

Sturmer W amp Bergstrom J 1978 The arthropod Cheloniellon fromthe Devonian Hunsruck Shale Palaontologische Zeitschrift 5257ndash81

Sturmer W amp Bergstrom J 1981 Weinbergina a xiphosuran fromthe Devonian Hunsruck Slate Palaontologische Zeitschrift 55237ndash55

Sutton M D Briggs D E G Siveter D J Siveter D J amp Orr PJ 2002 The arthropod Offacolus kingi (Chelicerata) from theSilurian of Herefordshire England computer based morphologi-cal reconstructions and phylogenetic affinity Proceedings of theRoyal Society of London Series B 269 1195ndash1203

Swofford D L 1999 PAUP Phylogenetic Analysis Using Parsimony(and other methods) Version 40b2a Sunderland MA SinauerAssociates

Telford M J amp Thomas R H 1998 Expression of homeobox genesshows chelicerate arthropods retain their deutocerebral segmentProceedings of the National Academy of Sciences USA 9510671ndash5

192 TREVOR J COTTON AND SIMON J BRADDY

Thorley J L amp Page R D M 2000 RadCon phylogenetic treecomparison and consensus Bioinformatics 16 486ndash7

Valentine J W 1995 Why no new phyla after the Cambrian ndashgenome and ecospace hypotheses revisited Palaios 10 190ndash194

Wagele J W 1993 Rejection of the lsquoUniramiarsquo hypothesis andimplications of the Mandibulata concept Zoologische JahrbucherAbteilung fur Systematik Geographie und Biologie der Tiere 120253ndash88

Walcott C D 1911 Middle Cambrian Merostomata Cambriangeology and palaeontology II Smithsonian MiscellaneousCollections 57 17ndash40

Walcott C D 1912 Middle Cambrian Branchiopoda MalacostracaTrilobita and Merostomata Cambrian geology and palaeontol-ogy II Smithsonian Miscellaneous Collections 57 145ndash228

Walcott C D 1918 Geological explorations in the CanadianRockies Explorations and fieldwork of the SmithsonianInstitution in 1917 Smithsonian Miscellaneous Collections 684ndash20

Walossek D 1993 The Upper Cambrian Rehbachiella kinnekullensisand the phylogeny of Branchiopoda and Crustacea Fossils andStrata 32 1ndash202

Walossek D amp Dunlop J A 2002 A larval sea spider (ArthropodaPycnogonida) from the Upper Cambrian lsquoOrstenrsquo of Swedenand the phylogenetic position of pycnogonids Palaeontology 45421ndash46

Walossek D amp Muller K J 1990 Upper Cambrian stem-lineagecrustaceans and their bearing upon the monophyletic origin ofCrustacea and the position of Agnostus Lethaia 23 409ndash27

Walossek D amp Muller K J 1997 Cambrian lsquoOrstenrsquo-type arthro-pods and the phylogeny of Crustacea In Fortey R A amp ThomasR H (eds) Arthropod Relationships 139ndash53 SystematicsAssociation Special Volume 55 London Chapman and Hall

Walossek D amp Muller K J 1998 Early arthropod phylogeny in lightof the Cambrian lsquoOrstenrsquo fossils In Edgecombe G D (ed)Arthropod fossils and phylogeny 185ndash231 New York NYColumbia University Press

Weygoldt P 1979 Significance of later embryonic stages and headdevelopment in arthropod phylogeny In Gupta A P (ed)Arthropod Phylogeny 107ndash35 New York NY Van NostrandReinhold Co

Weygoldt P 1998 Evolution and systematics of the ChelicerataExperimental and Applied Acarology 22 63ndash79

Weygoldt P amp Paulus H F 1979 Unterzuchungen aur Morpholo-gie Taxonomie und Phylogenie der Chelicerata Zeitschriftfur Zoologische Systematik und Evolutionsforschung 17 85ndash116177ndash200

Wheeler W C Cartwright P amp Hayashi C Y 1993 Arthropodphylogeny a combined approach Cladistics 9 1ndash39

Wheeler W C amp Hayashi C Y 1998 The phylogeny of extantchelicerate orders Cladistics 14 173ndash92

Whittington H B 1971 Redescription of Marrella splendens(Trilobitoidea) from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 209 1ndash24

Whittington H B 1974 Yohoia Walcott and Plenocaris n genarthropods from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 231 1ndash21

Whittington H B 1975a Trilobites with appendages from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 497ndash136

Whittington H B 1975b The enigmatic animal Opabinia regalisMiddle Cambrian Burgess Shale British Columbia PhilosophicalTransactions of the Royal Society of London Series B 271 1ndash43

Whittington H B 1977 The Middle Cambrian trilobite NaraoiaBurgess Shale British Columbia Philosophical Transactions of theRoyal Society Series B 280 409ndash43

Whittington H B 1979 Early arthropods their appendages andrelationships In House M R (ed) The origin of majorinvertebrate groups 253ndash68 Systematics Association SpecialVolume 12 New York NY Academic Press

Whittington H B 1980 Exoskeleton moult stage appendage mor-phology and habits of the Middle Cambrian trilobite Olenoidesserratus Palaeontology 23 171ndash204

Whittington H B 1981 Rare arthropods from the Burgess ShaleMiddle Cambrian British Columbia Philosophical Transactionsof the Royal Society of London Series B 292 329ndash57

Whittington H B 1985 Tegopelte gigas a second soft-bodied trilo-bite from the Burgess Shale Middle Cambrian British ColumbiaJournal of Palaeontology 23 171ndash204

Whittington H B 1989 Olenelloid trilobites type species functionalmorphology and higher classification Philosophical Transactionsof the Royal Society of London Series B 324 111ndash47

Whittington H B amp Almond J E 1987 Appendages and habits ofthe Upper Ordovician trilobite Triarthus eatoni PhilosophicalTransactions of the Royal Society of London Series B 317 1ndash46

Wilkinson M 1992 Ordered versus unordered characters Cladistics8 375ndash85

Wilkinson M 1994 Common cladistic information and its consensusrepresentation reduced Adams and reduced cladistic consensustrees and profiles Systematic Biology 43 343ndash68

Wilkinson M 1995a Coping with abundant missing entries in phylo-genetic inference using parsimony Systematic Biology 44 501ndash14

Wilkinson M 1995b TAXEQ2 software and documentation BristolSchool of Biological Sciences University of Bristol

Wilkinson M 1996 On the distribution of homoplasy and choosingamong trees Taxon 45 263ndash66

Wills M A 1997 A phylogeny of Recent and fossil Crustacea derivedfrom morphological characters In Fortey R A amp ThomasR H (eds) Arthropod Relationships 189ndash209 SystematicsAssociation Special Volume 55 London Chapman and Hall

Wills M A Briggs D E G amp Fortey R A 1994 Disparity asan evolutionary index a comparison of Cambrian and Recentarthropods Paleobiology 20 93ndash130

Wills M A Briggs D E G Fortey R A amp Wilkinson M 1995The significance of fossils in understanding arthropod evolutionVerhandlungen der deutschen zoologischen Gesselschaft 88 203ndash15

Wills M A Briggs D E G Fortey R A Wilkinson M amp SneathP H A 1998a An arthropod phylogeny based on fossil andrecent taxa In Edgecombe G D (ed) Arthropod fossils andphylogeny 33ndash105 New York NY Columbia University Press

Wills M A Briggs D E G amp Fortey R A 1998b Evolutionarycorrelates of arthropod tagmosis scrambled legs In ForteyR A amp Thomas R H (eds) Arthropod Relationships 57ndash65Systematics Association Special Volume 55 London Chapmanand Hall

Winter G 1980 Beitrage zur Morphologie und Embryologie desvorderen Korperabschnitts (Cephalosoma) der PantopodaGerstaecker 1863 I Entstehung und Struktur des Zentralnerven-systems Zeitschrift fur Zoologische Systematik und Evolutionsfor-schung 18 27ndash61

Zhang W amp Hou X 1985 Preliminary notes on the occurrence of theunusual trilobite Naraoia in Asia Acta Palaeontologica Sinica 24591ndash5 [In Chinese with English summary]

TREVOR J COTTON Department of Palaeontology The Natural History Museum CromwellRoad London SW7 5BD UKe-mail trevor_cottonhotmailcom

SIMON J BRADDY Department of Earth Sciences University of Bristol Wills MemorialBuilding Queenrsquos Road Bristol BS8 1RJ UKe-mail S J Braddybrisacuk

MS received 28 July 2003 Accepted for publication 15 March 2004

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 193

Page 18: The phylogeny of arachnomorph arthropods and the origin of the

indices (Bremer 1988 1994) These methods measure twodistinct aspects of support for phylogenetic hypotheses Treesor nodes may be considered well supported (1) to the extent towhich alternative topologies are much less parsimonious asmeasured by the support index (Wilkinson 1996) or (2) wherethey are consistent with a large proportion of characters sothat character sampling is unlikely to have had much influenceon topology as assessed by bootstrapping (Page 1996) Boot-strapping was performed with 100 bootstrap replicates eachof 10 addition sequence replicates Bremer support indicesare based on heuristic searches with 100 addition sequencereplicates

Three pairs of terminals are included in Table 2 that apartfrom missing data are coded identically These are Helmetiaand Kuamaia Soomaspis and Tariccoia and Saperion andSkioldia These pairs are lsquotaxonomic equivalentsrsquo removingthe taxon with most missing data will not influence theposition of the other They can be recoded as single terminalsaccording to the principal of safe taxonomic reduction(Wilkinson 1995a b) and were not considered separately in theanalyses presented here Therefore cladistic analyses werecarried out on the basis of 33 terminals Edgecombe ampRamskold (1999) ignored this and coded the same three pairsof taxa and Eoredlichia and Olenoides identically for theirmore limited set of characters

26 ResultsAnalysis with all characters unordered and using the lsquoAglaspi-dida 1rsquo coding found nine most parsimonious trees (MPTs)each 126 steps long (CI=0middot556 RI=0middot763) Using the lsquoAglas-pidida 2rsquo coding resulted in 18 MPTs of length 125 (CI=0middot560RI=0middot765) Nine of these 18 trees were those found with thefirst coding of aglaspidids The strict component consensus(sensu Wilkinson 1994) of the 27 unique trees found acrossboth analyses is shown in Figure 8 All trees supported themonophyly of the Marrellomorpha (Marrella and Mimetaster)and of a clade including all other taxa except Crustacea (Clade1 of Fig 8) The relationships between these two clades and theCrustacea were unresolved equal numbers of trees in bothanalyses supported each of the topologies [Clade 1 (CrustaceaMarrellomorpha)] [Crustacea (Marrellomorpha Clade 1)]and [Marrellomorpha (Crustacea Clade 1)] All trees placedBurgessia as the sister taxon to the rest of Clade 1 andsupported the division of the rest of Clade 1 into two majorsister groups The topology of one of these clades (Clade 2)which included trilobites helmetiids tegopeltids naraoiidsxandarellids and Retifacies was stable with respect to thecoding of aglaspidids The other clade (Clade 3) consists of aclade of megacheirans and chelicerates (Clade 4) and a cladeincluding aglaspidids Lemoneites Paleomerus CheloniellonEmeraldella and Sidneyia (Clade 5) This latter clade is un-resolved in the strict consensus of all 27 trees (Fig 8) and thestrict consensus of the 18 MPTs found using the lsquoAglaspidida2rsquo coding but was fully resolved in the consensus of the ninetrees found with the lsquoAglaspidida 1rsquo coding Within themegacheiran-chelicerate clade all trees found Fortiforceps tobe the sister group to all other taxa Yohoia the sister group tochelicerates and Alalcomenaeus and Leanchoilia to form aclade Jianfengia is equally parsimoniously placed as the sistergroup to the Yohoia-chelicerate clade or as the sister group tothis clade and the Alalcomenaeus-Leanchoilia clade

Treating characters 4 6 37 and 44 as ordered had a minimaleffect on most parsimonious topology Nine MPTs of 136 steps(CI=0middot515 RI=0middot743) were found with the lsquoAglaspidida 1rsquocoding and 12 MPTs 135 steps in length (CI=0middot519 RI=0middot745)with the lsquoAglaspidida 2rsquo coding These trees all supported a(Crustacea Clade 1) clade excluding the marrellomorphs

Apart from this the strict consensus of these 21 trees isidentical to the consensus of the 27 trees found with allcharacters treated as unordered

Across all analyses four different topologies for Clade 5 (ofFig 8) were found (Fig 9) The distribution of these topologiesacross the four analyses described above and across the sameanalyses repeated after reweighting of each character in pro-portion to their Rescaled Consistency Index (see Farris 1989)from the first analysis is shown in Table 3 The instability of

Figure 8 Strict consensus of 27 MPTs from four separate analyses ofarachnomorph phylogeny with both interpretations of aglaspididmorphology and with some characters treated as ordered or allcharacters unordered Clades referred to in the text are numbered

Figure 9 Alternative equally parsimonious resolutions of clade 5 ofFig 8 (see Table 3)

186 TREVOR J COTTON AND SIMON J BRADDY

this clade was to some extent caused by the inclusion ofLemoneites and Paleomerus which could each be coded foronly 27 of the 54 characters used Analysis with these two taxaexcluded and characters treated as unordered found treescompatible only with Topology D of Figure 9 irrespective ofthe coding used for aglaspidids Analysis with some characterstreated as ordered and using either aglaspidid coding foundthree trees compatible only with Topology B of Figure 9 andthree compatible only with Topology D of Figure 9 ThereforeTopology D is the preferred topology for Clade 5 as shown inTable 3

One of the MPTs resulting from the lsquoAglaspidida 2rsquo coding(with all characters treated as unordered) was chosen as thebasis for further discussion because this tree combined thepreferred topology of Clade 5 with the (Crustacea Clade 1)group that was favoured when some characters were treated asordered Bootstrap percentages and Bremer support values forthis tree (using the Aglaspidida 2 coding) are shown in Figure10 Apomorphies for all ingroup nodes of this tree are shownin Figure 10A

3 Discussion

31 Phylogenetic resultsThe results of the cladistic analyses presented above arewell-resolved and robust with respect to different analyticalparameters Most of the taxa considered form a clade (Clade 1of Fig 8) that is more closely related to chelicerates than tocrustaceans in all analyses and therefore is recognised asthe Arachnomorpha as defined by Chen et al (1996) andRamskold et al (1996) The monophyly of the Marrellomor-pha was supported in all analyses The marrellomorphs werefound to be the sister group to a Crustacea+Clade 1 group inmost MPTs but in a minority of trees formed the sister groupto Clade 1 alone According to the latter result they should beincluded within the Arachnomorpha but according to the firstexcluded from it Both of these alternatives have been sup-ported in previous studies (eg Hou amp Bergstrom 1997 Willset al 1998a respectively) In addition to the marrellomorphsthe Fuxianhuiida Bousfield 1995 and Canadaspidida havebeen placed in the Euarthropoda outside the main mandibu-late and arachnomorph clades (Hou amp Bergstrom 1997)Further study of these taxa of the crustacean or mandibulatestem group and of the marrellomorphs is necessary before the

position of the Marrellomorpha can be resolved Provisionallymarrellomorphs are excluded from the Arachnomorphafollowing the majority of analyses above and Wills et al(1994)

The Burgess Shale arthropod Burgessia was placed as thesister group to remaining arachnomorphs in all analyses OtherArachnomorpha form two major clades one consisting of thelsquotrilobite-alliedrsquo arachnomorphs analysed by Edgecombe ampRamskold (1999) here termed lsquoTrilobitomorpharsquo (defined asTrilobita and all taxa more closely related to Trilobita thanChelicerata) and the other (lsquochelicerate-alliedrsquo) clade includingchelicerates lsquogreat appendagersquo arthropods aglaspididsLemoneites Paleomerus Sidneyia and Emeraldella The latteris here termed lsquoCheliceramorpharsquo (defined as Chelicerata andall taxa more closely related to Chelicerata than Trilobita) Thetopology of the lsquotrilobite-alliedrsquo clade was similar to that foundby Edgecombe amp Ramskold (1999) but the present authorsrsquoresults are more completely resolved unambiguously support-ing the sister-group relationship between trilobites andHelmetiida suggested in the previous study Within thelsquochelicerate-alliedrsquo clade Aglaspidids Paleomerus Lemoneitesand the Burgess Shale Emeraldella and Sidneyia form a cladein opposition to megacheirans and chelicerates

The present authorsrsquo analysis supports a new hypothesis ofthe origin of the chelicerates from within a paraphyleticassemblage of megacheiran arthropods and confirms pyc-nogonids as a sister group to the euchelicerates Previoushypotheses of the chelicerate sister group (see Section 1) aremuch less parsimonious The shortest trees supporting aCheloniellon-chelicerate clade are six steps longer than theMPTs and those supporting an aglaspidid-chelicerate cladeseven steps longer All megacheirans and chelicerates areunited by the flap-shaped rounded exopods and modificationof the second cephalic segment endopods into anteriorlydirected spinose raptorial organs which share a number ofdetailed similarities A clade including all of these taxa apartfrom Fortiforceps is supported by the synapomorphic loss ofthe antennae (or antennulae of crustaceans) and the longerlength of spinose projections of the second cephalic appendageendopods Within this clade Yohoia shares the loss of cephalicexopods the loss of thoracic endopods and a postabdomen oftubular tergites with chelicerates

The poor resolution of Clade 5 (of Fig 8) was partly a resultof the poor preservation of and consequent high proportion ofmissing data for Lemoneites and Paleomerus When these two

Table 3 Numbers of most parsimonious trees supporting each of the alternative topologies for clade 4 of Fig 7 under different analyticalconditions (RCI=Revealed Consistency Index)

Aglaspididcoding

Characters 4 6 36and 43 ordered

Charactersreweighted by RCI

Paleomerus andLemoneites excluded

Topology (see Fig 8)

A B C D

1 91 X 3 3 31 X 61 X X 31 X 61 X X 3 32 9 92 X 3 3 3 32 X 62 X X 32 X 62 X X 3 3Total 6 18 12 54

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 187

taxa were excluded from the analyses only two (B and D) ofthe four topologies found in the complete analysis remainedmost parsimonious (see Fig 9 and Table 3) Secondly Bremersupport values throughout much of the lsquochelicerate-alliedrsquoclade (Clade 3 of Fig 8) were improved when these taxa wereexcluded (see Fig 11)

Dunlop (1999 p 258) suggested that in reconstructing thechelicerate stem lineage lsquowe might predict that the two mostsignificant changes towards the chelicerate condition are thereduction of the antennae and the formation of the nextappendages into a clawrsquo According to the hypothesis pre-sented here both of these adaptations were achieved in EarlyCambrian chelicerate ancestors The recognition of the loss ofthe antennae in the stem group of the chelicerates and thephylogenetic hypothesis presented here suggest that chelicer-ates are most closely related to taxa with only a singleadditional segment fused to the crown group eucheliceratehead of four fused somites Previous hypotheses have derivedchelicerates from Palaeozoic arthropods with a longer head(eg Cheloniellon Sturmer amp Bergstrom 1978 SanctacarisBriggs amp Collins 1988 Emeraldella Bruton amp Whittington1983) The plesiomorphic pattern of head segmentation ismatched in pycnogonids Bergstrom et al (1980) suggestedthat only the anterior four pairs of appendages are incorpo-rated into the head This supports the view that pycnogonidsare primitive with respect to other chelicerates which showmore or less complete fusion of this primitive head and threethoracic segments including Solpugida and Pseudoscorpiones

The present study suggests that a shift from gnathobasicbenthic feeding to a more vagrant lifestyle and raptorialappendage feeding occurred very early in the chelicerate lin-eage Appendage feeding was clearly an important adaptation

Figure 10 One of 18 equally parsimonious cladograms with lsquoAglaspidida 2rsquo coding and all characters unorderedthat is most congruent with results from other analyses Non-terminal branch lengths scaled to reflect number ofapomorphies (A) ACCTRAN apomorphy scheme Character numbers shown above boxes and states shownbelow boxes as in Table 2 and the text Character consistency indices are indicated by shading (B) Levels ofsupport for individual nodes Bremer support indices shown in bold type above branches Bootstrap percentagesin italic type below branches for all nodes with relative frequencies greater than 50

Figure 11 Bremer support values for clades within the lsquochelicerate-alliedrsquo arachnomorphs (clade 3 of Fig 8) when all taxa wereincluded (normal weight font above nodes) and with Lemoneites andPaleomerus excluded (bold type below nodes)

188 TREVOR J COTTON AND SIMON J BRADDY

for the terrestrialization of chelicerates (Dunlop 1997 p 69Dunlop amp Webster 1999) but the model proposed heresuggests that this considerably pre-dated terrestrializationGnathobasic feeding in xiphosurans and eurypterids mayrepresent a reversal to a more primitve benthic arachnomorphlifestyle from a more pycnogonid-like ancestor

32 Evolution of the arachnomorph headThe extant arthropod classes show highly conserved patternsof head segmentation (eg Wills et al 1994) In contrast therehas been a near consensus amongst Cambrian arthropodworkers (although rarely based on an explicit phylogenetichypothesis) that patterns of head segmentation are highlyhomoplastic and hence of little systematic significanceSturmer amp Bergstrom (1978 pp 78ndash9) suggested that lsquoevenclosely related forms may have different numbers of headsegments and appendagesrsquo Bruton amp Whittington (1983pp 576ndash7) that lsquodiscussion on fossil arthropod relationshipsbased on head segmentation appears to be largely irrel-evant and at best speculativersquo and Delle Cave amp Simonetta(1991 p 191) that lsquothere are no obvious phyletic affinitiesbetween genera having the same number of cephalized seg-mentsrsquo Most recently Bergstrom amp Hou (1997 p 104) con-cluded that lsquothe segmental length of the head shield seemsto be of no relevance to the discussion and discrimination ofevolutionary lineagesrsquo It has been claimed that this view issupported by cladistic analyses in which patterns of tagmosishave been found to be rather poor at defining major arthropodclades (Briggs et al 1992 Wills et al 1994)

This suggestion is not only of relevance to systematics buthas featured prominently in recent debates surrounding thenature of the lsquoCambrian explosionrsquo (see Budd amp Jensen 2000for a recent review) Gould (1989 1991) argued that theapparent plasticity of head segmentation in Cambrian com-pared to post-Cambrian arthropod evolution suggests thatbody-plan evolution was only constrained after the Cambrianexplosion According to this view the origin of new highertaxa with distinct body-plans was only possible before theonset of these constraints (eg Valentine 1995) Taxa hererecognised as arachnomorphs clearly played a major role in thedevelopment of Gouldrsquos hypothesis This suggestion has pre-viously been assessed in terms of overall morphological diver-sity which is clearly rather distinct from Gouldrsquos concept ofdisparity Wills et al (1998b) showed that there was littlecorrelation between overall morphological disparity and de-gree of limb specialisation and that the latter was phylogeneti-cally highly plastic They remarked that (Wills et al 1998bp 64) lsquothis finding has important implications for models ofarthropod phylogeny and evolution that attribute overidingimportance to head segmentationrsquo but did not explicitlyexamine head segmentation although it is likely to have beena major component of the index of tagmosis employed

The results of the present study suggest that arguments thatarthropod head segmentation was unusually labile during theCambrian are poorly founded Rather only four major pat-terns of euarthropod head segmentation are identified Theplesiomorphic euarthropod state according to both Walossekamp Muller (1997 1998) and Scholtz (1997) which may moreproperly be the plesiomorphic state for the euarthropod crowngroup only (depending on the phylogenetic position of theMarrellomorpha) consists of four post-acronal segmentsbearing the antennae and three pairs of biramous limbs Thepresent authors suggest that the term lsquocephalonrsquo be restrictedto this kind of head which is found in stem group mandibu-lates in addition to many arachnomorphs Crown groupmandibulates share a lsquobimaxillary headrsquo in which an addi-tional pair of appendages the labium or second maxillae are

incorporated into the head but in crustaceans these are notfused to the carapace (Scholtz 1997) Finally two distinctforms of head tagmosis are found in chelicerates A head withfour pairs of appendages (but without antennae giving a totalof five segments incorporated into the head) which has beencalled the lsquocephalosomarsquo is present in pycnogonids (Walossekamp Dunlop 2002 pp 434ndash5) and probably also in some euch-elicerates (J A Dunlop pers comm 2003) The lsquoprosomarsquo ofmost euchelicerates consists of this cephalosoma and twoadditional segments

In general these patterns of head segmentation are highlyphylogenetically conserved (see Fig 12) Only three homo-plastic changes namely reversal to a more primitive conditionin the Leanchoilia-Alalcomenaeus clade and the convergentorigin of a five-segmented head in Clade 3 (of Fig 8) and theXandarellida are necessary to optimise character 4 onto themost parsimonious cladogram There are also transitions toautapomorphic states in Emeraldella and Sidneyia In the caseof Sidneyia however an argument could be made (followingBergstrom amp Sturmer 1978 see discussion of Character 4) thatthe head consists of five segments as in closely related taxabut autapomorphically each segment has a separate tergiteThis would further increase the phylogenetic stability of headsegmentation Also following Bergstrom amp Sturmer (1978) thehead of Cheloniellon could be considered to consist of twotergites making it more similar to the head of Emeraldella(with six pairs of appendages)

Figure 12 Cladogram from Fig 10 showing changes in head segmen-tation Bold type shows abbreviated head segmentation formulaAbbreviations (Ant) antennae (BA) biramous appendage pairs (Ch)chelicerae (GA) great appendages (UA) uniramous appendage pairsand (X) segment without appendages Synapomorphy A is the auta-pomorphic reduction in head length to a single segment in SidneyiaAlternatively this may represent secondary division of the headshield Synapomorphy B is the increase in the length of the head inEmeraldella to six segments

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 189

4 Acknowledgements

We thank D E G Briggs J A Cotton J A Dunlop R AFortey S Harris R A Moore P Selden O E Tetlie andP Van Roy for comments on earlier drafts of this work Themanuscript was reviewed by M Wills F Schram and oneanonymous referee This research was supported by the Geo-logical Society of London Curry Research Studentshipawarded to TJC by the Department of Earth Sciences Uni-versity of Bristol and by the Department of PalaeontologyThe Natural History Museum London TJC would like tothank C Labandeira J Thompson and R Chapman fortheir hospitality at the National Museum of Natural HistoryWashington DC USA and A Olcott S Schellenberg andJ Young for making his time there so enjoyable

5 References

Abzhanov A Popadic A amp Kaufman T C 1999 Chelicerate Hoxgenes and the homology of arthropod segments Evolution andDevelopment 1 77ndash89

Akam M 2000 Arthropods developmental diversity within a (super)phylum Proceedings of the National Academy of Sciences USA97 4438ndash41

Aldridge R J Gabbott S E amp Theron J N 2001 The Soom ShaleIn Briggs D E G amp Crowther P R (eds) Palaeobiology II340ndash2 Oxford Blackwell Science

Anderson L I amp Selden P A 1997 Opisthosomal fusion andphylogeny of Palaeozoic Xiphosura Lethaia 30 19ndash31

Arango C P 2002 Morphological phylogenetics of the sea spiders(Arthropoda Pycnogonida) Organisms Diversity and Evolution2 107ndash25

Ax P 1986 Das phylogenetische System Stuttgart Gustav FischerVerlag

Babcock L E amp Zhang Wentang In press New non-mineralizedarthropods from the Chengjiang Lagerstatte Lower CambrianYunnan China Palaeontologia Cathyana

Bartels C Briggs D E G amp Brassel G 1998 The fossils ofthe Hunsruck Slate Marine life in the Devonian CambridgeCambridge University Press

Bergstrom J 1971 Paleomerus ndash merostome or merostomoid Lethaia4 393ndash401

Bergstrom J 1979 Morphology of fossil arthropods as a guideto phylogenetic relationships In Gupta A P (ed) ArthropodPhylogeny 3ndash56 New York NY Van Nostrand Reinhold Co

Bergstrom J 1992 The oldest arthropods and the origin of theCrustacea Acta Zoologica 73 287ndash91

Bergstrom J Sturmer W amp Winter G 1980 PalaeoisopusPalaeopantopodus and Palaeothea pycnogonid arthropodsfrom the Lower Devonian Hunsruck Slate West GermanyPalaontologische Zeitschrift 54 7ndash54

Bergstrom J amp Brassel G 1984 Legs in the trilobite Rhenops fromthe Lower Devonian Hunsruck Slate Lethaia 17 67ndash72

Bergstrom J amp Hou X 1998 Chengjiang arthropods and theirbearing on early arthropod evolution In Edgecombe G D(ed) Arthropod fossils and phylogeny 151ndash84 New York NYColumbia University Press

Boudreaux H B 1979 Significance of intersegmental tendon systemin arthropod phylogeny and a monophyletic classification ofArthropoda In Gupta A P (ed) Arthropod Phylogeny 551ndash86New York NY Van Nostrand Reinhold Co

Bousfield E L 1995 A contribution to the natural classification ofLower and Middle Cambrian arthropods food-gathering andfeeding mechanisms Amphipacifica 2 3ndash33

Braddy S J Aldridge R J Gabbott S E amp Theron J N 1999Lamellate book-gills in a late Ordovician eurypterid from theSoom Shale South Africa support for a eurypterid-scorpionclade Lethaia 32 72ndash4

Braddy S J amp Almond J E 1999 Eurypterid trackways from theTable Mountain Group (Ordovician) of South Africa Journal ofAfrican Earth Sciences 29 165ndash77

Bremer K 1988 The limits of amino acid sequence data in angio-sperm phylogenetic reconstruction Evolution 42 795ndash803

Bremer K 1994 Branch support and tree stability Cladistics 10295ndash304

Briggs D E G 1998 Review of Arthropods from the LowerCambrian Chengjiang fauna southwest China by HouXianguang and Jan Bergstrom Fossils and Strata 45

Scandinavian University Press Oslo Norway 1997 Palaeo-geography Palaeoclimatology Palaeoecology 143 193ndash4

Briggs D E G Bruton D L amp Whittington H B 1979 Append-ages of the arthropod Aglaspis spinifer (Upper CambrianWisconsin) and their significance Palaeontology 22 167ndash80

Briggs D E G Fortey R A amp Wills M A 1992 Morphologicaldisparity in the Cambrian Science 256 1670ndash3

Briggs D E G Fortey R A amp Wills MA 1993 How big was theCambrian explosion A taxonomic and morphologic comparisonof Cambrian and Recent arthropods In Lees D R amp EdwardsD (eds) Evolutionary patterns and processes 33ndash44 LondonLinnean Society of London Linnean Society Symposium Series

Briggs D E G Erwin D H amp Collier F J 1994 The fossils of theBurgess Shale Washington DC Smithsonian Institution Press

Briggs D E G amp Bartels C 2001 New arthropods from the LowerDevonian Hunsruck Slate (Lower Emsian Rhenish Massifwestern Germany) Palaeontology 44 275ndash304

Briggs D E G amp Bartels C 2002 Magnoculocaris a new name forthe arthropod Magnoculus Briggs and Bartels 2001 from theLower Devonian Hunsruck Slate Germany Palaeontology 45419

Briggs D E G amp Collins D 1988 A Middle Cambrian cheliceratefrom Mount Stephen British Columbia Palaeontology 31 779ndash98

Briggs D E G amp Collins D 1999 The arthropod Alalcomenaeuscambricus Simonetta from the Middle Cambrian Burgess Shale ofBritish Columbia Paleontology 42 953ndash78

Briggs D E G amp Fortey R A 1982 The cuticle of aglaspididarthropods a red-herring in the early history of vertebratesLethaia 15 25ndash9

Briggs D E G amp Fortey R A 1989 The early radiation andrelationships of the major arthropod groups Science 246 241ndash3

Briggs D E G amp Fortey R A 1992 The early Cambrian radiationof arthropods In Lipps J H amp Signor P W (eds) Origin andEarly Evolution of the Metazoa 335ndash373 New York NY PlenumPress

Briggs D E G amp Robison R A 1984 Exceptionally preservednontrilobite arthropods and Anomalocaris from the MiddleCambrian of Utah University of Kansas PaleontologicalContributions Paper 111 1ndash23

Broili F 1932 Ein neuer Crustacee aus dem rheinischen UnterdevonSitzungsberichte der Mathematisch-Naturwissenschaftlichen(Abteilung) Klasse der Bayerischen Akademie der Wissenschaftenzu Munchen 1932 27ndash38

Broili F 1933 Ein zweites Exemplar von Cheloniellon Sitzungsber-ichte der Bayerischen Akademie der Wissenschaften 1933 11ndash32

Bruton D L 1981 The arthropod Sidneyia inexpectans MiddleCambrian Burgess Shale British Columbia Philosophical Trans-actions of the Royal Society of London Series B 295 619ndash56

Bruton D L amp Whittington H B 1983 Emeraldella and Leanchoiliatwo arthropods from the Burgess Shale British Columbia Philo-sophical Transactions of the Royal Society of London Series B 300553ndash85

Budd G E 1995 Kleptothule rasmusseni gen et sp nov anolenellid-like trilobite from the Sirius Passet fauna (BuenFormation Lower Cambrian North Greenland) Transactions ofthe Royal Society of Edinburgh Earth Sciences 86 1ndash12

Budd G E 1996a Progress and problems in arthropod phylogenyTrends in Ecology and Evolution 11 356ndash8

Budd G E 1996b The morphology of Opabinia regalis and thereconstruction of the arthropod stem-group Lethaia 29 1ndash14

Budd G E 1997 Stem-group arthropods from the Lower CambrianSirius Passet fauna of North Greenland In Fortey R A ampThomas R H (eds) Arthropod Relationships 125ndash38 LondonChapman and Hall Systematics Association Special Volume 55

Budd G E 1999a A nektaspid arthropod from the Early CambrianSirius Passet fauna with a description of retrodeformation basedon functional morphology Palaeontology 42 99ndash122

Budd G E 1999b The morphology and phylogenetic significance ofKerygmachela kiergkegaardi Budd (Buen Formation LowerCambrian N Greenland) Transactions of the Royal Society ofEdinburgh Earth Sciences 89 (for 1998) 249ndash290

Budd G E 2002 A palaeontological solution to the arthropod headproblem Nature 417 271ndash5

Budd G E amp Jensen S 2000 A critical reappraisal of the fossilrecord of the bilaterian phyla Biological Reviews 75 253ndash95

Chen J Zhou G Zhu M amp Yeh K 1996 The Chengjiang biota ndasha unique window of the Cambrian explosion Taichung NationalMuseum of Natural Science [In Chinese]

Chen J Edgecombe G D amp Ramskold L 1997 Morphological andecological disparity in naraoiids (Arthropoda) from the Early

190 TREVOR J COTTON AND SIMON J BRADDY

Cambrian Chengjiang fauna China Records of the AustralianMuseum 49 1ndash24

Chen J amp Zhou G 1997 Biology of the Chengjiang fauna In ChenJ Cheng Y amp Iten H V (eds) The Cambrian explosion and thefossil record Bulletin of the National Museum of Natural ScienceTaiwan 10 11ndash106

Child C A 1997 Some deep-sea Pycnogonida from Argentine slopeand basin Proceedings of the Biological Society of Washington110 128ndash42

Chlupac I 1988 The enigmatic arthropod Duslia from the Ordovicianof Czechslovakia Palaeontology 31 611ndash20

Chlupac I 1995 Lower Cambrian arthropods from the PasekyShale (Barrandian area Czech Republic) Journal of the CzechGeological Society 40 9ndash36

Chlupac I amp Havlıcek V 1965 Kodymirus n g a new aglaspidmerostome of the Cambrian of Bohemia Sbornık GeologickechVid Paleontologie 6 7ndash20

Cisne J L 1975 Anatomy of Triarthrus and the relationships of theTrilobita Fossils and Strata 4 45ndash63

Cisne J L 1981 Triarthrus eatoni (Trilobita) anatomy of its exo-skeletal skeletomuscular and digestive systems Palaeonto-graphica Americana 9 95ndash142

Conway Morris S 1982 Atlas of the Burgess Shale London ThePalaeontological Association

Conway Morris S 1989 The persistence of Burgess Shale-type faunasimplications for the evolution of deeper-water faunas Trans-actions of the Royal Society of Edinburgh Earth Sciences 80271ndash83

Conway Morris S 1998 The crucible of creation Oxford OxfordUniversity Press

Cotton T J 1999 Review of Arthropod fossils and phylogenyPalaeogeography Palaeoclimatology Palaeoecology 151 341ndash3

Cotton T J 2002 The phylogeny and morphological evolution ofCambrian trilobites and their relatives Unpublished PhD ThesisUniversity of Bristol

Damen W G M Hausdorf M Seyfarth E A amp Tautz D 1998 Aconserved mode of head segmentation in arthropods revealed bythe expression pattern of Hox genes in a spider Proceedings of theNational Academy of Sciences USA 95 10665ndash70

Delle Cave L amp Simonetta A M 1991 Early Palaeozoic arthropodsand problems of arthropod phylogeny with some notes on taxa ofdoubtful affinities In Simonetta A M amp Conway Morris S(eds) The early evolution of Metazoa and the significance ofproblematic taxa 189ndash244 Cambridge Cambridge UniversityPress

Dewel R A amp Dewel W C 1997 The place of tardigrades inarthropod evolution In Fortey R A amp Thomas R H (eds)Arthropod Relationships 109ndash23 London Chapman and HallSystematics Association Special Volume 55

Dunlop J A 1997 Palaeozoic arachnids and their significance forarachnid phylogeny Proceedings of the 16th European Colloquiumof Arachnology 65ndash82 Siedlce Poland

Dunlop J A 1998 The origins of tetrapulmonate booklungs and theirsignificance for chelicerate phylogeny In Selden P A (ed)Proceedings of the 17th European Colloquium of ArachnologyEdinburgh 1997 pp 9ndash16

Dunlop J A 1999 Pasando revista a la evolucion de los quelicerados[A review of chelicerate evolution] In Melic A DeHaro J JMedez M amp Ribera I (eds) Volumen Monografico Evolucion yFilogenia de Arthropoda Boletin de la Sociedad EntolomologicaAragonesa 26 255ndash272 [In Spanish and English]

Dunlop J A amp Selden P A 1997 The early history and phylogenyof the chelicerates In Fortey R A amp Thomas R H (eds)Arthropod relationships Systematics Association Special Volume55 221ndash35 London Chapman and Hall

Dunlop J A amp Webster M 1999 Fossil evidence terrestrializationand arachnid phylogeny Journal of Arachnology 27 86ndash93

Dzik J 1993 Early metazoan evolution and the meaning of its fossilrecord Evolutionary Biology 27 339ndash86

Dzik J amp Lendzion K 1988 The oldest arthropods of the EastEuropean Platform Lethaia 21 29ndash38

Edgecombe G D Wilson G D F Colgan D J Gray M R ampCassis G 2000 Arthropod cladistics combined analysisof Histone H3 and U2 snRNA sequences and morphologyCladistics 16 155ndash203

Edgecombe G D amp Ramskold L 1999 Relationships of CambrianArachnata and the systematic position of Trilobita Journal ofPaleontology 73 263ndash87

Emerson M J amp Schram F R 1997 Theories patterns and realitygame plan for arthropod phylogeny In Fortey R A amp Thomas

R H (eds) Arthropod Relationships Systematics AssociationSpecial Volume 55 67ndash87 London Chapman and Hall

Farris J S 1989 The Retention Index and the Rescaled ConsistencyIndex Cladistics 5 417ndash19

Felsenstein J 1985 Confidence limits on phylogenies an approachusing the bootstrap Evolution 39 783ndash91

Flower R H 1968 Merostomes from a Cotter Horizon of the El PasoGroup New Mexico Bureau of Mines and Mineral ResourcesMemoir 22 35ndash44

Fortey R A 1990a Trilobite evolution and systematics In MikulicD G (ed) Arthropod paleobiology 44ndash65 Short Courses inPaleontology 3 Knoxville Tenessee The Paleontological Society

Fortey R A 1990b Ontogeny hypostome attachment and trilobiteclassification Palaeontology 33 529ndash76

Fortey R A 1997 Classification In Kaesler R L (ed) Treatise oninvertebrate paleontology Part O Arthropoda 1 (revised) 289ndash302Boulder Colorado amp Lawrence Kansas Geological Society ofAmerica and University of Kansas Press

Fortey R A amp Theron J N 1994 A new Ordovician arthropodSoomaspis and the agnostid problem Palaeontology 37 841ndash61

Fortey R A amp Whittington H B 1989 The Trilobita as a naturalgroup Historical Biology 2 125ndash38

Fry W G 1978 A classification within the pycnogonids ZoologicalJournal of the Linnean Society of London 63 35ndash58

Giribet G Edgecombe G D amp Wheeler W C 2001 Arthropodphylogeny based on eight molecular loci and morphology Nature413 157ndash61

Gould S J 1989 Wonderful life The Burgess Shale and the nature ofhistory New York NY Norton

Gould S J 1991 The disparity of the Burgess Shale arthropod faunaand the limits of cladistic analysis why we must strive to quantifymorphospace Paleobiology 17 411ndash23

Gurich G 1931 Mimetaster hexagonalis ein neuer Kruster aus demunterdevonischen Bundenbacher Dachschiefer PalaontologischeZeitschrift 12 204ndash38

Hammann W Laske R amp Pillola G L 1990 Tariccoia arrusensisngnsp an unusual trilobite-like arthropod Rediscovery of thelsquophylocaridrsquo beds of Taricco (1922) in the Ordovician lsquoPuddingarsquosequence of Sardinia Bolletino della Societa PaleontologicaItaliana 29 163ndash78

Haas M S Brown S J amp Beeman R W 2001 Pondering theprocephalon the segmental origin of the labrum DevelopmentGenes and Evolution 211 89ndash95

Heider K 1913 Entwicklungsgeschichte und Morphologie derWirbellosen Die Kultur der Gegenwart Theil 3 Abt 4 2 176ndash332

Henriksen K 1928 Critical notes upon Cambrian arthropodsdescribed by Charles D Walcott Videnskabelige Meddelelser fraDansk Naturhistorisk Forening 86 1ndash20

Hesselbo S P 1988 Trace fossils of Cambrian aglaspidid arthropodsLethaia 21 139ndash46

Hesselbo S P 1989 The aglaspidid arthropod Beckwithia from theCambrian of Utah and Wisconsin Journal of Paleontology 63636ndash42

Hesselbo S P 1992 Aglaspidida (Arthropoda) from the UpperCambrian of Wisconsin Journal of Paleontology 66 885ndash923

Hou X 1987a Three new large arthropods from Lower CambrianChengjiang fauna eastern Yunnan Acta Palaeontologica Sinica26 272ndash85 [In Chinese with English summary]

Hou X 1987b Two new arthropods from Lower CambrianChengjiang eastern Yunnan Acta Palaeontologica Sinica 26236ndash56 [In Chinese with English summary]

Hou X Chen J amp Lu H 1989 Early Cambrian new arthropodsfrom Chengjiang Yunnan Acta Palaeontologica Sinica 28 42ndash57[In Chinese with English summary]

Hou X Ramskold L amp Bergstrom J 1991 Composition andpreservation of the Chengjiang fauna a Lower Cambrian soft-bodied biota Zoologica Scripta 20 395ndash411

Hou X Bergstrom J amp Ahlberg P 1995 Anomalocaris and otherlarge animals in the Lower Cambrian Chengjiang fauna ofsouthwest China GFF 117 163ndash83

Hou X amp Bergstrom J 1997 Arthropods of the Lower CambrianChengjiang fauna southwest China Fossils and Strata 45 1ndash116

Hughes C P 1975 Redescription of Burgessia bella from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 4415ndash36

Ivantsov A Y 1999 Trilobite-like arthropod from the LowerCambrian of the Siberian Platform Acta PalaeontologicaPolonica 44 455ndash66

King P E 1973 Pycnogonids London St Martin Press

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 191

Lauterbach K E 1973 Schlussereignisse in der Evolution derStammgruppe der Euarthropoda Zoologische Beitrage 19259ndash99

Lauterbach K E 1980 Schlussereignisse in der Evolution derGrundplans der Arachnata (Arthropoda) Abhandlungen desNaturwissenschaftlichen Vereins in Hamburg 23 163ndash327

Lauterbach K E 1983 Synapomorphien zwischen Trilobiten- undCheliceraten-zweig der Arachnata Zoologischer Anzeiger 210213ndash38

Lendzion K 1975 Fauna of the Mobergella Zone in the Polish LowerCambrian Kwartalnik Geologiczny 19 237ndash42

Lieberman B S 1998 Cladistic analysis of the Early Cambrianolenelloid trilobites Journal of Paleontology 72 59ndash78

Lu Y 1940 On the ontogeny and phylogeny of Redlichia intermediaLu (sp nov) Bulletin of the Geological Society of China 20333ndash42 [In Chinese]

Maddison W P amp Maddison D R 1997 MacClade analysis ofphylogeny and character evolution Version 307 Sunderland MASinauer Associates

Mittmann B amp Scholtz G 2003 Development of the nervous systemin the lsquoheadrsquo of Limulus polyphemus (Chelicerata Xiphosura)morphological evidence for the correspondence between the seg-ments of the chelicerae and the (first) antennae of MandibulataDevelopment Genes and Evolution 213 9ndash17

Moore R C (ed) 1959 Treatise on invertebrate paleontology Part OArthropoda 1 Boulder Colorado Geological Society of America

Muller K J amp Walossek D 1986 Arthropod larvae from the UpperCambrian of Sweden Transactions of the Royal Society ofEdinburgh Earth Sciences 77 157ndash79

Muller K J amp Walossek D 1987 Morphology ontogeny and lifehabit of Agnostus pisiformis from the Upper Cambrian of SwedenFossils and Strata 19 1ndash124

Munilla T 1999 Evolucion y filogenia de los Picnogonidos InMelicA DeHaro J J Medez M amp Ribera I (eds) VolumenMonografico Evolucion y Filogenia de Arthropoda Boletin de laSociedad Entolomologica Aragonesa 26 273ndash91 [In Spanish andEnglish]

Orr P J Siveter D J Briggs D E G Siveter D J amp Sutton M D2000 A new arthropod from the Silurian Konservat-Lagerstatteof Herefordshire Proceedings of the Royal Society of LondonSeries B 267 1497ndash504

Page R D M 1996 On consensus confidence and lsquototal evidencersquoCladistics 12 83ndash92

Palmer A R amp Repina L N 1993 Through a glass darklytaxonomy phylogeny and biostratigraphy of the OlenellinaUniversity of Kansas Paleontological Contributions New Series 31ndash35

Platnick N I Griswold C E amp Coddington J A 1991 On missingentries in cladistic analysis Cladistics 7 337ndash43

Pleijel F 1995 On character coding for phylogeny reconstructionCladistics 11 309ndash15

Raasch G O 1939 Cambrian Merostomata Special Papers of theGeological Society of America 19 1ndash146

Ramskold L Chen J Edgecombe G D amp Zhou G 1996Preservational folds simulating tergite junctions in tegopeltid andnaraoiid arthropods Lethaia 29 15ndash20

Ramskold L Chen J Edgecombe G D amp Zhou G 1997Cindarella and the arachnate clade Xandarellida (ArthropodaEarly Cambrian) from China Transactions of the Royal Society ofEdinburgh Earth Sciences 88 19ndash38

Ramskold L amp Edgecombe G D 1991 Trilobite monophylyrevisited Historical Biology 4 267ndash83

Ramskold L amp Edgecombe G D 1996 Trilobite appendagestructure ndash Eoredlichia revisited Alcheringa 20 269ndash76

Raw F 1957 Origin of chelicerates Journal of Paleontology 31139ndash193

Repina L N amp Okuneva O G 1969 Cambrian arthropods of themaritime territory Paleontological Journal 1969 95ndash103

Richter R amp Richter E 1929 Weinbergina opitz ng n sp einSchwerttrager (Merost Xiphos) aus dem Devon (Rheinland)Senkenbergiana 11 193ndash209

Robison R A 1984 New occurences of the unusual trilobite Naraoiafrom the Cambrian of Idaho and Utah University of KansasPaleontological Contributions Paper 112 1ndash8

Rominger C 1887 Description of primordial fossils from MtStephens NW Territory of Canada Proceedings of the Academyof Natural Sciences of Philadelphia 1887 12ndash19

Scholtz G 1997 Cleavage germ band formation and head segmen-tation the ground pattern of the Euarthropoda In Fortey R Aamp Thomas R H (eds) Arthropod Relationships 317ndash32 LondonChapman and Hall Systematics Association Special Volume 55

Schram F R 1978 Arthropods a convergent phenomenonFieldiana Geology 39 61ndash108

Schram F R 1993 Review of Boxshall Stromberg and Dahl TheCrustacea Origin and Evolution Journal of Crustacean Biology13 820ndash2

Schram F R Vonk R amp Hof C H J 1997 Mazon CreekCycloidea Journal of Paleontology 71 261ndash84

Schram F R amp Emerson M J 1991 Arthropod Pattern Theorya new approach to arthropod phylogeny Memoirs of theQueensland Museum 31 1ndash18

Schram F R amp Hof C H J 1998 Fossils and the interrelationshipsof major crustacean groups In Edgecombe G D (ed) Arthropodfossils and phylogeny pp 233ndash302 New York NY ColumbiaUniversity Press

Shergold J H 1991 Protaspis and early meraspis growth stages of theeodiscoid trilobite Pagetia ocellata Jell and their implications forclassification Alcheringa 15 65ndash86

Shu D Geyer G Ling C amp Zhang Z 1995 Redlichiacean trilobiteswith preserved soft parts from the Lower Cambrian Chengjiangfauna (South China) Beringeria (Special Issue) 2 203ndash41

Shultz J W 1990 Evolutionary morphology and phylogeny ofArachnida Cladistics 6 1ndash38

Shultz J W 2000 Skeletomuscular anatomy of the harvestmanLeiobunum aldrichi (Weed 1893) (Arachnida OpilionesPalpatores) and its evolutionary significance Zoological Journalof the Linnean Society 128 401ndash38

Shultz J W 2001 Gross muscular anatomy of Limulus polyphemus(Xiphosura Chelicerata) and its bearing on evolution in the theArachnida Journal of Arachnology 29 283ndash303

Siewing R (ed) 1985 Lehrbuch der Zoologie II Systematik 3rd ednStuttgart Fischer

Simonetta A M 1970 Studies on the non trilobite arthropods ofthe Burgess Shale (Middle Cambrian) the genera LeanchoiliaAlalcomenaeus Opabinia Burgessia Yohoia and ActaeusPalaeontographica Italica 66 35ndash45

Simonetta A M amp Delle Cave L 1975 The Cambrian non trilobitearthropods from the Burgess Shale of British Columbia A studyof their comparative morphology taxonomy and evolutionarysignificance Palaeontographica Italica 69 1ndash37

Simonetta A M amp Delle Cave L 1981 An essay in the compara-tive and evolutionary morphology of Palaeozoic ArthropodsAccademia Nazionale dei Lincei Atti dei Convegni Lincei 49389ndash439

Starobogatov Y I 1990 Sistema i filogeniya nizshikh khelitserovykh(analiz morfologii paleozoysikh grupp) [The systematics andphylogeny of the lower chelicerates (a morphological analysis ofthe Palaeozoic groups)] Paleontologicheskii Zhurnal 1990 4ndash17

Stoslashrmer L 1944 On the relationships and phylogeny of fossil andrecent Arachnomorpha Skrifter utgitt ac Det Norske Videnskaps-Akademi i Olso I Matematisk-naturvitenskaplig Klasse 5 1ndash158

Stoslashrmer L 1951 Studies on trilobite morphology Part III Theventral cephalic structures with remarks on the zoologicalposition of the trilobites Norsk geologisk tidsskrift 29 108ndash58

Stoslashrmer L 1956 A Lower Cambrian merostome from Sweden Arkivfor Zoologi Ser 2 9 507ndash14

Stoslashrmer L 1959 Trilobitoidea In Moore R C (ed) Treatise onInvertebrate Paleontology part O Arthropoda 1 23ndash37 BoulderColorado Geological Society of America

Strong E E amp Lipscomb D 1999 Character coding and inapplicabledata Cladistics 15 363ndash71

Sturmer W amp Bergstrom J 1976 The arthropods Mimetaster andVachonisia from the Devonian Hunsruck Shale PalaontologischeZeitschrift 50 78ndash111

Sturmer W amp Bergstrom J 1978 The arthropod Cheloniellon fromthe Devonian Hunsruck Shale Palaontologische Zeitschrift 5257ndash81

Sturmer W amp Bergstrom J 1981 Weinbergina a xiphosuran fromthe Devonian Hunsruck Slate Palaontologische Zeitschrift 55237ndash55

Sutton M D Briggs D E G Siveter D J Siveter D J amp Orr PJ 2002 The arthropod Offacolus kingi (Chelicerata) from theSilurian of Herefordshire England computer based morphologi-cal reconstructions and phylogenetic affinity Proceedings of theRoyal Society of London Series B 269 1195ndash1203

Swofford D L 1999 PAUP Phylogenetic Analysis Using Parsimony(and other methods) Version 40b2a Sunderland MA SinauerAssociates

Telford M J amp Thomas R H 1998 Expression of homeobox genesshows chelicerate arthropods retain their deutocerebral segmentProceedings of the National Academy of Sciences USA 9510671ndash5

192 TREVOR J COTTON AND SIMON J BRADDY

Thorley J L amp Page R D M 2000 RadCon phylogenetic treecomparison and consensus Bioinformatics 16 486ndash7

Valentine J W 1995 Why no new phyla after the Cambrian ndashgenome and ecospace hypotheses revisited Palaios 10 190ndash194

Wagele J W 1993 Rejection of the lsquoUniramiarsquo hypothesis andimplications of the Mandibulata concept Zoologische JahrbucherAbteilung fur Systematik Geographie und Biologie der Tiere 120253ndash88

Walcott C D 1911 Middle Cambrian Merostomata Cambriangeology and palaeontology II Smithsonian MiscellaneousCollections 57 17ndash40

Walcott C D 1912 Middle Cambrian Branchiopoda MalacostracaTrilobita and Merostomata Cambrian geology and palaeontol-ogy II Smithsonian Miscellaneous Collections 57 145ndash228

Walcott C D 1918 Geological explorations in the CanadianRockies Explorations and fieldwork of the SmithsonianInstitution in 1917 Smithsonian Miscellaneous Collections 684ndash20

Walossek D 1993 The Upper Cambrian Rehbachiella kinnekullensisand the phylogeny of Branchiopoda and Crustacea Fossils andStrata 32 1ndash202

Walossek D amp Dunlop J A 2002 A larval sea spider (ArthropodaPycnogonida) from the Upper Cambrian lsquoOrstenrsquo of Swedenand the phylogenetic position of pycnogonids Palaeontology 45421ndash46

Walossek D amp Muller K J 1990 Upper Cambrian stem-lineagecrustaceans and their bearing upon the monophyletic origin ofCrustacea and the position of Agnostus Lethaia 23 409ndash27

Walossek D amp Muller K J 1997 Cambrian lsquoOrstenrsquo-type arthro-pods and the phylogeny of Crustacea In Fortey R A amp ThomasR H (eds) Arthropod Relationships 139ndash53 SystematicsAssociation Special Volume 55 London Chapman and Hall

Walossek D amp Muller K J 1998 Early arthropod phylogeny in lightof the Cambrian lsquoOrstenrsquo fossils In Edgecombe G D (ed)Arthropod fossils and phylogeny 185ndash231 New York NYColumbia University Press

Weygoldt P 1979 Significance of later embryonic stages and headdevelopment in arthropod phylogeny In Gupta A P (ed)Arthropod Phylogeny 107ndash35 New York NY Van NostrandReinhold Co

Weygoldt P 1998 Evolution and systematics of the ChelicerataExperimental and Applied Acarology 22 63ndash79

Weygoldt P amp Paulus H F 1979 Unterzuchungen aur Morpholo-gie Taxonomie und Phylogenie der Chelicerata Zeitschriftfur Zoologische Systematik und Evolutionsforschung 17 85ndash116177ndash200

Wheeler W C Cartwright P amp Hayashi C Y 1993 Arthropodphylogeny a combined approach Cladistics 9 1ndash39

Wheeler W C amp Hayashi C Y 1998 The phylogeny of extantchelicerate orders Cladistics 14 173ndash92

Whittington H B 1971 Redescription of Marrella splendens(Trilobitoidea) from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 209 1ndash24

Whittington H B 1974 Yohoia Walcott and Plenocaris n genarthropods from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 231 1ndash21

Whittington H B 1975a Trilobites with appendages from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 497ndash136

Whittington H B 1975b The enigmatic animal Opabinia regalisMiddle Cambrian Burgess Shale British Columbia PhilosophicalTransactions of the Royal Society of London Series B 271 1ndash43

Whittington H B 1977 The Middle Cambrian trilobite NaraoiaBurgess Shale British Columbia Philosophical Transactions of theRoyal Society Series B 280 409ndash43

Whittington H B 1979 Early arthropods their appendages andrelationships In House M R (ed) The origin of majorinvertebrate groups 253ndash68 Systematics Association SpecialVolume 12 New York NY Academic Press

Whittington H B 1980 Exoskeleton moult stage appendage mor-phology and habits of the Middle Cambrian trilobite Olenoidesserratus Palaeontology 23 171ndash204

Whittington H B 1981 Rare arthropods from the Burgess ShaleMiddle Cambrian British Columbia Philosophical Transactionsof the Royal Society of London Series B 292 329ndash57

Whittington H B 1985 Tegopelte gigas a second soft-bodied trilo-bite from the Burgess Shale Middle Cambrian British ColumbiaJournal of Palaeontology 23 171ndash204

Whittington H B 1989 Olenelloid trilobites type species functionalmorphology and higher classification Philosophical Transactionsof the Royal Society of London Series B 324 111ndash47

Whittington H B amp Almond J E 1987 Appendages and habits ofthe Upper Ordovician trilobite Triarthus eatoni PhilosophicalTransactions of the Royal Society of London Series B 317 1ndash46

Wilkinson M 1992 Ordered versus unordered characters Cladistics8 375ndash85

Wilkinson M 1994 Common cladistic information and its consensusrepresentation reduced Adams and reduced cladistic consensustrees and profiles Systematic Biology 43 343ndash68

Wilkinson M 1995a Coping with abundant missing entries in phylo-genetic inference using parsimony Systematic Biology 44 501ndash14

Wilkinson M 1995b TAXEQ2 software and documentation BristolSchool of Biological Sciences University of Bristol

Wilkinson M 1996 On the distribution of homoplasy and choosingamong trees Taxon 45 263ndash66

Wills M A 1997 A phylogeny of Recent and fossil Crustacea derivedfrom morphological characters In Fortey R A amp ThomasR H (eds) Arthropod Relationships 189ndash209 SystematicsAssociation Special Volume 55 London Chapman and Hall

Wills M A Briggs D E G amp Fortey R A 1994 Disparity asan evolutionary index a comparison of Cambrian and Recentarthropods Paleobiology 20 93ndash130

Wills M A Briggs D E G Fortey R A amp Wilkinson M 1995The significance of fossils in understanding arthropod evolutionVerhandlungen der deutschen zoologischen Gesselschaft 88 203ndash15

Wills M A Briggs D E G Fortey R A Wilkinson M amp SneathP H A 1998a An arthropod phylogeny based on fossil andrecent taxa In Edgecombe G D (ed) Arthropod fossils andphylogeny 33ndash105 New York NY Columbia University Press

Wills M A Briggs D E G amp Fortey R A 1998b Evolutionarycorrelates of arthropod tagmosis scrambled legs In ForteyR A amp Thomas R H (eds) Arthropod Relationships 57ndash65Systematics Association Special Volume 55 London Chapmanand Hall

Winter G 1980 Beitrage zur Morphologie und Embryologie desvorderen Korperabschnitts (Cephalosoma) der PantopodaGerstaecker 1863 I Entstehung und Struktur des Zentralnerven-systems Zeitschrift fur Zoologische Systematik und Evolutionsfor-schung 18 27ndash61

Zhang W amp Hou X 1985 Preliminary notes on the occurrence of theunusual trilobite Naraoia in Asia Acta Palaeontologica Sinica 24591ndash5 [In Chinese with English summary]

TREVOR J COTTON Department of Palaeontology The Natural History Museum CromwellRoad London SW7 5BD UKe-mail trevor_cottonhotmailcom

SIMON J BRADDY Department of Earth Sciences University of Bristol Wills MemorialBuilding Queenrsquos Road Bristol BS8 1RJ UKe-mail S J Braddybrisacuk

MS received 28 July 2003 Accepted for publication 15 March 2004

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 193

Page 19: The phylogeny of arachnomorph arthropods and the origin of the

this clade was to some extent caused by the inclusion ofLemoneites and Paleomerus which could each be coded foronly 27 of the 54 characters used Analysis with these two taxaexcluded and characters treated as unordered found treescompatible only with Topology D of Figure 9 irrespective ofthe coding used for aglaspidids Analysis with some characterstreated as ordered and using either aglaspidid coding foundthree trees compatible only with Topology B of Figure 9 andthree compatible only with Topology D of Figure 9 ThereforeTopology D is the preferred topology for Clade 5 as shown inTable 3

One of the MPTs resulting from the lsquoAglaspidida 2rsquo coding(with all characters treated as unordered) was chosen as thebasis for further discussion because this tree combined thepreferred topology of Clade 5 with the (Crustacea Clade 1)group that was favoured when some characters were treated asordered Bootstrap percentages and Bremer support values forthis tree (using the Aglaspidida 2 coding) are shown in Figure10 Apomorphies for all ingroup nodes of this tree are shownin Figure 10A

3 Discussion

31 Phylogenetic resultsThe results of the cladistic analyses presented above arewell-resolved and robust with respect to different analyticalparameters Most of the taxa considered form a clade (Clade 1of Fig 8) that is more closely related to chelicerates than tocrustaceans in all analyses and therefore is recognised asthe Arachnomorpha as defined by Chen et al (1996) andRamskold et al (1996) The monophyly of the Marrellomor-pha was supported in all analyses The marrellomorphs werefound to be the sister group to a Crustacea+Clade 1 group inmost MPTs but in a minority of trees formed the sister groupto Clade 1 alone According to the latter result they should beincluded within the Arachnomorpha but according to the firstexcluded from it Both of these alternatives have been sup-ported in previous studies (eg Hou amp Bergstrom 1997 Willset al 1998a respectively) In addition to the marrellomorphsthe Fuxianhuiida Bousfield 1995 and Canadaspidida havebeen placed in the Euarthropoda outside the main mandibu-late and arachnomorph clades (Hou amp Bergstrom 1997)Further study of these taxa of the crustacean or mandibulatestem group and of the marrellomorphs is necessary before the

position of the Marrellomorpha can be resolved Provisionallymarrellomorphs are excluded from the Arachnomorphafollowing the majority of analyses above and Wills et al(1994)

The Burgess Shale arthropod Burgessia was placed as thesister group to remaining arachnomorphs in all analyses OtherArachnomorpha form two major clades one consisting of thelsquotrilobite-alliedrsquo arachnomorphs analysed by Edgecombe ampRamskold (1999) here termed lsquoTrilobitomorpharsquo (defined asTrilobita and all taxa more closely related to Trilobita thanChelicerata) and the other (lsquochelicerate-alliedrsquo) clade includingchelicerates lsquogreat appendagersquo arthropods aglaspididsLemoneites Paleomerus Sidneyia and Emeraldella The latteris here termed lsquoCheliceramorpharsquo (defined as Chelicerata andall taxa more closely related to Chelicerata than Trilobita) Thetopology of the lsquotrilobite-alliedrsquo clade was similar to that foundby Edgecombe amp Ramskold (1999) but the present authorsrsquoresults are more completely resolved unambiguously support-ing the sister-group relationship between trilobites andHelmetiida suggested in the previous study Within thelsquochelicerate-alliedrsquo clade Aglaspidids Paleomerus Lemoneitesand the Burgess Shale Emeraldella and Sidneyia form a cladein opposition to megacheirans and chelicerates

The present authorsrsquo analysis supports a new hypothesis ofthe origin of the chelicerates from within a paraphyleticassemblage of megacheiran arthropods and confirms pyc-nogonids as a sister group to the euchelicerates Previoushypotheses of the chelicerate sister group (see Section 1) aremuch less parsimonious The shortest trees supporting aCheloniellon-chelicerate clade are six steps longer than theMPTs and those supporting an aglaspidid-chelicerate cladeseven steps longer All megacheirans and chelicerates areunited by the flap-shaped rounded exopods and modificationof the second cephalic segment endopods into anteriorlydirected spinose raptorial organs which share a number ofdetailed similarities A clade including all of these taxa apartfrom Fortiforceps is supported by the synapomorphic loss ofthe antennae (or antennulae of crustaceans) and the longerlength of spinose projections of the second cephalic appendageendopods Within this clade Yohoia shares the loss of cephalicexopods the loss of thoracic endopods and a postabdomen oftubular tergites with chelicerates

The poor resolution of Clade 5 (of Fig 8) was partly a resultof the poor preservation of and consequent high proportion ofmissing data for Lemoneites and Paleomerus When these two

Table 3 Numbers of most parsimonious trees supporting each of the alternative topologies for clade 4 of Fig 7 under different analyticalconditions (RCI=Revealed Consistency Index)

Aglaspididcoding

Characters 4 6 36and 43 ordered

Charactersreweighted by RCI

Paleomerus andLemoneites excluded

Topology (see Fig 8)

A B C D

1 91 X 3 3 31 X 61 X X 31 X 61 X X 3 32 9 92 X 3 3 3 32 X 62 X X 32 X 62 X X 3 3Total 6 18 12 54

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 187

taxa were excluded from the analyses only two (B and D) ofthe four topologies found in the complete analysis remainedmost parsimonious (see Fig 9 and Table 3) Secondly Bremersupport values throughout much of the lsquochelicerate-alliedrsquoclade (Clade 3 of Fig 8) were improved when these taxa wereexcluded (see Fig 11)

Dunlop (1999 p 258) suggested that in reconstructing thechelicerate stem lineage lsquowe might predict that the two mostsignificant changes towards the chelicerate condition are thereduction of the antennae and the formation of the nextappendages into a clawrsquo According to the hypothesis pre-sented here both of these adaptations were achieved in EarlyCambrian chelicerate ancestors The recognition of the loss ofthe antennae in the stem group of the chelicerates and thephylogenetic hypothesis presented here suggest that chelicer-ates are most closely related to taxa with only a singleadditional segment fused to the crown group eucheliceratehead of four fused somites Previous hypotheses have derivedchelicerates from Palaeozoic arthropods with a longer head(eg Cheloniellon Sturmer amp Bergstrom 1978 SanctacarisBriggs amp Collins 1988 Emeraldella Bruton amp Whittington1983) The plesiomorphic pattern of head segmentation ismatched in pycnogonids Bergstrom et al (1980) suggestedthat only the anterior four pairs of appendages are incorpo-rated into the head This supports the view that pycnogonidsare primitive with respect to other chelicerates which showmore or less complete fusion of this primitive head and threethoracic segments including Solpugida and Pseudoscorpiones

The present study suggests that a shift from gnathobasicbenthic feeding to a more vagrant lifestyle and raptorialappendage feeding occurred very early in the chelicerate lin-eage Appendage feeding was clearly an important adaptation

Figure 10 One of 18 equally parsimonious cladograms with lsquoAglaspidida 2rsquo coding and all characters unorderedthat is most congruent with results from other analyses Non-terminal branch lengths scaled to reflect number ofapomorphies (A) ACCTRAN apomorphy scheme Character numbers shown above boxes and states shownbelow boxes as in Table 2 and the text Character consistency indices are indicated by shading (B) Levels ofsupport for individual nodes Bremer support indices shown in bold type above branches Bootstrap percentagesin italic type below branches for all nodes with relative frequencies greater than 50

Figure 11 Bremer support values for clades within the lsquochelicerate-alliedrsquo arachnomorphs (clade 3 of Fig 8) when all taxa wereincluded (normal weight font above nodes) and with Lemoneites andPaleomerus excluded (bold type below nodes)

188 TREVOR J COTTON AND SIMON J BRADDY

for the terrestrialization of chelicerates (Dunlop 1997 p 69Dunlop amp Webster 1999) but the model proposed heresuggests that this considerably pre-dated terrestrializationGnathobasic feeding in xiphosurans and eurypterids mayrepresent a reversal to a more primitve benthic arachnomorphlifestyle from a more pycnogonid-like ancestor

32 Evolution of the arachnomorph headThe extant arthropod classes show highly conserved patternsof head segmentation (eg Wills et al 1994) In contrast therehas been a near consensus amongst Cambrian arthropodworkers (although rarely based on an explicit phylogenetichypothesis) that patterns of head segmentation are highlyhomoplastic and hence of little systematic significanceSturmer amp Bergstrom (1978 pp 78ndash9) suggested that lsquoevenclosely related forms may have different numbers of headsegments and appendagesrsquo Bruton amp Whittington (1983pp 576ndash7) that lsquodiscussion on fossil arthropod relationshipsbased on head segmentation appears to be largely irrel-evant and at best speculativersquo and Delle Cave amp Simonetta(1991 p 191) that lsquothere are no obvious phyletic affinitiesbetween genera having the same number of cephalized seg-mentsrsquo Most recently Bergstrom amp Hou (1997 p 104) con-cluded that lsquothe segmental length of the head shield seemsto be of no relevance to the discussion and discrimination ofevolutionary lineagesrsquo It has been claimed that this view issupported by cladistic analyses in which patterns of tagmosishave been found to be rather poor at defining major arthropodclades (Briggs et al 1992 Wills et al 1994)

This suggestion is not only of relevance to systematics buthas featured prominently in recent debates surrounding thenature of the lsquoCambrian explosionrsquo (see Budd amp Jensen 2000for a recent review) Gould (1989 1991) argued that theapparent plasticity of head segmentation in Cambrian com-pared to post-Cambrian arthropod evolution suggests thatbody-plan evolution was only constrained after the Cambrianexplosion According to this view the origin of new highertaxa with distinct body-plans was only possible before theonset of these constraints (eg Valentine 1995) Taxa hererecognised as arachnomorphs clearly played a major role in thedevelopment of Gouldrsquos hypothesis This suggestion has pre-viously been assessed in terms of overall morphological diver-sity which is clearly rather distinct from Gouldrsquos concept ofdisparity Wills et al (1998b) showed that there was littlecorrelation between overall morphological disparity and de-gree of limb specialisation and that the latter was phylogeneti-cally highly plastic They remarked that (Wills et al 1998bp 64) lsquothis finding has important implications for models ofarthropod phylogeny and evolution that attribute overidingimportance to head segmentationrsquo but did not explicitlyexamine head segmentation although it is likely to have beena major component of the index of tagmosis employed

The results of the present study suggest that arguments thatarthropod head segmentation was unusually labile during theCambrian are poorly founded Rather only four major pat-terns of euarthropod head segmentation are identified Theplesiomorphic euarthropod state according to both Walossekamp Muller (1997 1998) and Scholtz (1997) which may moreproperly be the plesiomorphic state for the euarthropod crowngroup only (depending on the phylogenetic position of theMarrellomorpha) consists of four post-acronal segmentsbearing the antennae and three pairs of biramous limbs Thepresent authors suggest that the term lsquocephalonrsquo be restrictedto this kind of head which is found in stem group mandibu-lates in addition to many arachnomorphs Crown groupmandibulates share a lsquobimaxillary headrsquo in which an addi-tional pair of appendages the labium or second maxillae are

incorporated into the head but in crustaceans these are notfused to the carapace (Scholtz 1997) Finally two distinctforms of head tagmosis are found in chelicerates A head withfour pairs of appendages (but without antennae giving a totalof five segments incorporated into the head) which has beencalled the lsquocephalosomarsquo is present in pycnogonids (Walossekamp Dunlop 2002 pp 434ndash5) and probably also in some euch-elicerates (J A Dunlop pers comm 2003) The lsquoprosomarsquo ofmost euchelicerates consists of this cephalosoma and twoadditional segments

In general these patterns of head segmentation are highlyphylogenetically conserved (see Fig 12) Only three homo-plastic changes namely reversal to a more primitive conditionin the Leanchoilia-Alalcomenaeus clade and the convergentorigin of a five-segmented head in Clade 3 (of Fig 8) and theXandarellida are necessary to optimise character 4 onto themost parsimonious cladogram There are also transitions toautapomorphic states in Emeraldella and Sidneyia In the caseof Sidneyia however an argument could be made (followingBergstrom amp Sturmer 1978 see discussion of Character 4) thatthe head consists of five segments as in closely related taxabut autapomorphically each segment has a separate tergiteThis would further increase the phylogenetic stability of headsegmentation Also following Bergstrom amp Sturmer (1978) thehead of Cheloniellon could be considered to consist of twotergites making it more similar to the head of Emeraldella(with six pairs of appendages)

Figure 12 Cladogram from Fig 10 showing changes in head segmen-tation Bold type shows abbreviated head segmentation formulaAbbreviations (Ant) antennae (BA) biramous appendage pairs (Ch)chelicerae (GA) great appendages (UA) uniramous appendage pairsand (X) segment without appendages Synapomorphy A is the auta-pomorphic reduction in head length to a single segment in SidneyiaAlternatively this may represent secondary division of the headshield Synapomorphy B is the increase in the length of the head inEmeraldella to six segments

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 189

4 Acknowledgements

We thank D E G Briggs J A Cotton J A Dunlop R AFortey S Harris R A Moore P Selden O E Tetlie andP Van Roy for comments on earlier drafts of this work Themanuscript was reviewed by M Wills F Schram and oneanonymous referee This research was supported by the Geo-logical Society of London Curry Research Studentshipawarded to TJC by the Department of Earth Sciences Uni-versity of Bristol and by the Department of PalaeontologyThe Natural History Museum London TJC would like tothank C Labandeira J Thompson and R Chapman fortheir hospitality at the National Museum of Natural HistoryWashington DC USA and A Olcott S Schellenberg andJ Young for making his time there so enjoyable

5 References

Abzhanov A Popadic A amp Kaufman T C 1999 Chelicerate Hoxgenes and the homology of arthropod segments Evolution andDevelopment 1 77ndash89

Akam M 2000 Arthropods developmental diversity within a (super)phylum Proceedings of the National Academy of Sciences USA97 4438ndash41

Aldridge R J Gabbott S E amp Theron J N 2001 The Soom ShaleIn Briggs D E G amp Crowther P R (eds) Palaeobiology II340ndash2 Oxford Blackwell Science

Anderson L I amp Selden P A 1997 Opisthosomal fusion andphylogeny of Palaeozoic Xiphosura Lethaia 30 19ndash31

Arango C P 2002 Morphological phylogenetics of the sea spiders(Arthropoda Pycnogonida) Organisms Diversity and Evolution2 107ndash25

Ax P 1986 Das phylogenetische System Stuttgart Gustav FischerVerlag

Babcock L E amp Zhang Wentang In press New non-mineralizedarthropods from the Chengjiang Lagerstatte Lower CambrianYunnan China Palaeontologia Cathyana

Bartels C Briggs D E G amp Brassel G 1998 The fossils ofthe Hunsruck Slate Marine life in the Devonian CambridgeCambridge University Press

Bergstrom J 1971 Paleomerus ndash merostome or merostomoid Lethaia4 393ndash401

Bergstrom J 1979 Morphology of fossil arthropods as a guideto phylogenetic relationships In Gupta A P (ed) ArthropodPhylogeny 3ndash56 New York NY Van Nostrand Reinhold Co

Bergstrom J 1992 The oldest arthropods and the origin of theCrustacea Acta Zoologica 73 287ndash91

Bergstrom J Sturmer W amp Winter G 1980 PalaeoisopusPalaeopantopodus and Palaeothea pycnogonid arthropodsfrom the Lower Devonian Hunsruck Slate West GermanyPalaontologische Zeitschrift 54 7ndash54

Bergstrom J amp Brassel G 1984 Legs in the trilobite Rhenops fromthe Lower Devonian Hunsruck Slate Lethaia 17 67ndash72

Bergstrom J amp Hou X 1998 Chengjiang arthropods and theirbearing on early arthropod evolution In Edgecombe G D(ed) Arthropod fossils and phylogeny 151ndash84 New York NYColumbia University Press

Boudreaux H B 1979 Significance of intersegmental tendon systemin arthropod phylogeny and a monophyletic classification ofArthropoda In Gupta A P (ed) Arthropod Phylogeny 551ndash86New York NY Van Nostrand Reinhold Co

Bousfield E L 1995 A contribution to the natural classification ofLower and Middle Cambrian arthropods food-gathering andfeeding mechanisms Amphipacifica 2 3ndash33

Braddy S J Aldridge R J Gabbott S E amp Theron J N 1999Lamellate book-gills in a late Ordovician eurypterid from theSoom Shale South Africa support for a eurypterid-scorpionclade Lethaia 32 72ndash4

Braddy S J amp Almond J E 1999 Eurypterid trackways from theTable Mountain Group (Ordovician) of South Africa Journal ofAfrican Earth Sciences 29 165ndash77

Bremer K 1988 The limits of amino acid sequence data in angio-sperm phylogenetic reconstruction Evolution 42 795ndash803

Bremer K 1994 Branch support and tree stability Cladistics 10295ndash304

Briggs D E G 1998 Review of Arthropods from the LowerCambrian Chengjiang fauna southwest China by HouXianguang and Jan Bergstrom Fossils and Strata 45

Scandinavian University Press Oslo Norway 1997 Palaeo-geography Palaeoclimatology Palaeoecology 143 193ndash4

Briggs D E G Bruton D L amp Whittington H B 1979 Append-ages of the arthropod Aglaspis spinifer (Upper CambrianWisconsin) and their significance Palaeontology 22 167ndash80

Briggs D E G Fortey R A amp Wills M A 1992 Morphologicaldisparity in the Cambrian Science 256 1670ndash3

Briggs D E G Fortey R A amp Wills MA 1993 How big was theCambrian explosion A taxonomic and morphologic comparisonof Cambrian and Recent arthropods In Lees D R amp EdwardsD (eds) Evolutionary patterns and processes 33ndash44 LondonLinnean Society of London Linnean Society Symposium Series

Briggs D E G Erwin D H amp Collier F J 1994 The fossils of theBurgess Shale Washington DC Smithsonian Institution Press

Briggs D E G amp Bartels C 2001 New arthropods from the LowerDevonian Hunsruck Slate (Lower Emsian Rhenish Massifwestern Germany) Palaeontology 44 275ndash304

Briggs D E G amp Bartels C 2002 Magnoculocaris a new name forthe arthropod Magnoculus Briggs and Bartels 2001 from theLower Devonian Hunsruck Slate Germany Palaeontology 45419

Briggs D E G amp Collins D 1988 A Middle Cambrian cheliceratefrom Mount Stephen British Columbia Palaeontology 31 779ndash98

Briggs D E G amp Collins D 1999 The arthropod Alalcomenaeuscambricus Simonetta from the Middle Cambrian Burgess Shale ofBritish Columbia Paleontology 42 953ndash78

Briggs D E G amp Fortey R A 1982 The cuticle of aglaspididarthropods a red-herring in the early history of vertebratesLethaia 15 25ndash9

Briggs D E G amp Fortey R A 1989 The early radiation andrelationships of the major arthropod groups Science 246 241ndash3

Briggs D E G amp Fortey R A 1992 The early Cambrian radiationof arthropods In Lipps J H amp Signor P W (eds) Origin andEarly Evolution of the Metazoa 335ndash373 New York NY PlenumPress

Briggs D E G amp Robison R A 1984 Exceptionally preservednontrilobite arthropods and Anomalocaris from the MiddleCambrian of Utah University of Kansas PaleontologicalContributions Paper 111 1ndash23

Broili F 1932 Ein neuer Crustacee aus dem rheinischen UnterdevonSitzungsberichte der Mathematisch-Naturwissenschaftlichen(Abteilung) Klasse der Bayerischen Akademie der Wissenschaftenzu Munchen 1932 27ndash38

Broili F 1933 Ein zweites Exemplar von Cheloniellon Sitzungsber-ichte der Bayerischen Akademie der Wissenschaften 1933 11ndash32

Bruton D L 1981 The arthropod Sidneyia inexpectans MiddleCambrian Burgess Shale British Columbia Philosophical Trans-actions of the Royal Society of London Series B 295 619ndash56

Bruton D L amp Whittington H B 1983 Emeraldella and Leanchoiliatwo arthropods from the Burgess Shale British Columbia Philo-sophical Transactions of the Royal Society of London Series B 300553ndash85

Budd G E 1995 Kleptothule rasmusseni gen et sp nov anolenellid-like trilobite from the Sirius Passet fauna (BuenFormation Lower Cambrian North Greenland) Transactions ofthe Royal Society of Edinburgh Earth Sciences 86 1ndash12

Budd G E 1996a Progress and problems in arthropod phylogenyTrends in Ecology and Evolution 11 356ndash8

Budd G E 1996b The morphology of Opabinia regalis and thereconstruction of the arthropod stem-group Lethaia 29 1ndash14

Budd G E 1997 Stem-group arthropods from the Lower CambrianSirius Passet fauna of North Greenland In Fortey R A ampThomas R H (eds) Arthropod Relationships 125ndash38 LondonChapman and Hall Systematics Association Special Volume 55

Budd G E 1999a A nektaspid arthropod from the Early CambrianSirius Passet fauna with a description of retrodeformation basedon functional morphology Palaeontology 42 99ndash122

Budd G E 1999b The morphology and phylogenetic significance ofKerygmachela kiergkegaardi Budd (Buen Formation LowerCambrian N Greenland) Transactions of the Royal Society ofEdinburgh Earth Sciences 89 (for 1998) 249ndash290

Budd G E 2002 A palaeontological solution to the arthropod headproblem Nature 417 271ndash5

Budd G E amp Jensen S 2000 A critical reappraisal of the fossilrecord of the bilaterian phyla Biological Reviews 75 253ndash95

Chen J Zhou G Zhu M amp Yeh K 1996 The Chengjiang biota ndasha unique window of the Cambrian explosion Taichung NationalMuseum of Natural Science [In Chinese]

Chen J Edgecombe G D amp Ramskold L 1997 Morphological andecological disparity in naraoiids (Arthropoda) from the Early

190 TREVOR J COTTON AND SIMON J BRADDY

Cambrian Chengjiang fauna China Records of the AustralianMuseum 49 1ndash24

Chen J amp Zhou G 1997 Biology of the Chengjiang fauna In ChenJ Cheng Y amp Iten H V (eds) The Cambrian explosion and thefossil record Bulletin of the National Museum of Natural ScienceTaiwan 10 11ndash106

Child C A 1997 Some deep-sea Pycnogonida from Argentine slopeand basin Proceedings of the Biological Society of Washington110 128ndash42

Chlupac I 1988 The enigmatic arthropod Duslia from the Ordovicianof Czechslovakia Palaeontology 31 611ndash20

Chlupac I 1995 Lower Cambrian arthropods from the PasekyShale (Barrandian area Czech Republic) Journal of the CzechGeological Society 40 9ndash36

Chlupac I amp Havlıcek V 1965 Kodymirus n g a new aglaspidmerostome of the Cambrian of Bohemia Sbornık GeologickechVid Paleontologie 6 7ndash20

Cisne J L 1975 Anatomy of Triarthrus and the relationships of theTrilobita Fossils and Strata 4 45ndash63

Cisne J L 1981 Triarthrus eatoni (Trilobita) anatomy of its exo-skeletal skeletomuscular and digestive systems Palaeonto-graphica Americana 9 95ndash142

Conway Morris S 1982 Atlas of the Burgess Shale London ThePalaeontological Association

Conway Morris S 1989 The persistence of Burgess Shale-type faunasimplications for the evolution of deeper-water faunas Trans-actions of the Royal Society of Edinburgh Earth Sciences 80271ndash83

Conway Morris S 1998 The crucible of creation Oxford OxfordUniversity Press

Cotton T J 1999 Review of Arthropod fossils and phylogenyPalaeogeography Palaeoclimatology Palaeoecology 151 341ndash3

Cotton T J 2002 The phylogeny and morphological evolution ofCambrian trilobites and their relatives Unpublished PhD ThesisUniversity of Bristol

Damen W G M Hausdorf M Seyfarth E A amp Tautz D 1998 Aconserved mode of head segmentation in arthropods revealed bythe expression pattern of Hox genes in a spider Proceedings of theNational Academy of Sciences USA 95 10665ndash70

Delle Cave L amp Simonetta A M 1991 Early Palaeozoic arthropodsand problems of arthropod phylogeny with some notes on taxa ofdoubtful affinities In Simonetta A M amp Conway Morris S(eds) The early evolution of Metazoa and the significance ofproblematic taxa 189ndash244 Cambridge Cambridge UniversityPress

Dewel R A amp Dewel W C 1997 The place of tardigrades inarthropod evolution In Fortey R A amp Thomas R H (eds)Arthropod Relationships 109ndash23 London Chapman and HallSystematics Association Special Volume 55

Dunlop J A 1997 Palaeozoic arachnids and their significance forarachnid phylogeny Proceedings of the 16th European Colloquiumof Arachnology 65ndash82 Siedlce Poland

Dunlop J A 1998 The origins of tetrapulmonate booklungs and theirsignificance for chelicerate phylogeny In Selden P A (ed)Proceedings of the 17th European Colloquium of ArachnologyEdinburgh 1997 pp 9ndash16

Dunlop J A 1999 Pasando revista a la evolucion de los quelicerados[A review of chelicerate evolution] In Melic A DeHaro J JMedez M amp Ribera I (eds) Volumen Monografico Evolucion yFilogenia de Arthropoda Boletin de la Sociedad EntolomologicaAragonesa 26 255ndash272 [In Spanish and English]

Dunlop J A amp Selden P A 1997 The early history and phylogenyof the chelicerates In Fortey R A amp Thomas R H (eds)Arthropod relationships Systematics Association Special Volume55 221ndash35 London Chapman and Hall

Dunlop J A amp Webster M 1999 Fossil evidence terrestrializationand arachnid phylogeny Journal of Arachnology 27 86ndash93

Dzik J 1993 Early metazoan evolution and the meaning of its fossilrecord Evolutionary Biology 27 339ndash86

Dzik J amp Lendzion K 1988 The oldest arthropods of the EastEuropean Platform Lethaia 21 29ndash38

Edgecombe G D Wilson G D F Colgan D J Gray M R ampCassis G 2000 Arthropod cladistics combined analysisof Histone H3 and U2 snRNA sequences and morphologyCladistics 16 155ndash203

Edgecombe G D amp Ramskold L 1999 Relationships of CambrianArachnata and the systematic position of Trilobita Journal ofPaleontology 73 263ndash87

Emerson M J amp Schram F R 1997 Theories patterns and realitygame plan for arthropod phylogeny In Fortey R A amp Thomas

R H (eds) Arthropod Relationships Systematics AssociationSpecial Volume 55 67ndash87 London Chapman and Hall

Farris J S 1989 The Retention Index and the Rescaled ConsistencyIndex Cladistics 5 417ndash19

Felsenstein J 1985 Confidence limits on phylogenies an approachusing the bootstrap Evolution 39 783ndash91

Flower R H 1968 Merostomes from a Cotter Horizon of the El PasoGroup New Mexico Bureau of Mines and Mineral ResourcesMemoir 22 35ndash44

Fortey R A 1990a Trilobite evolution and systematics In MikulicD G (ed) Arthropod paleobiology 44ndash65 Short Courses inPaleontology 3 Knoxville Tenessee The Paleontological Society

Fortey R A 1990b Ontogeny hypostome attachment and trilobiteclassification Palaeontology 33 529ndash76

Fortey R A 1997 Classification In Kaesler R L (ed) Treatise oninvertebrate paleontology Part O Arthropoda 1 (revised) 289ndash302Boulder Colorado amp Lawrence Kansas Geological Society ofAmerica and University of Kansas Press

Fortey R A amp Theron J N 1994 A new Ordovician arthropodSoomaspis and the agnostid problem Palaeontology 37 841ndash61

Fortey R A amp Whittington H B 1989 The Trilobita as a naturalgroup Historical Biology 2 125ndash38

Fry W G 1978 A classification within the pycnogonids ZoologicalJournal of the Linnean Society of London 63 35ndash58

Giribet G Edgecombe G D amp Wheeler W C 2001 Arthropodphylogeny based on eight molecular loci and morphology Nature413 157ndash61

Gould S J 1989 Wonderful life The Burgess Shale and the nature ofhistory New York NY Norton

Gould S J 1991 The disparity of the Burgess Shale arthropod faunaand the limits of cladistic analysis why we must strive to quantifymorphospace Paleobiology 17 411ndash23

Gurich G 1931 Mimetaster hexagonalis ein neuer Kruster aus demunterdevonischen Bundenbacher Dachschiefer PalaontologischeZeitschrift 12 204ndash38

Hammann W Laske R amp Pillola G L 1990 Tariccoia arrusensisngnsp an unusual trilobite-like arthropod Rediscovery of thelsquophylocaridrsquo beds of Taricco (1922) in the Ordovician lsquoPuddingarsquosequence of Sardinia Bolletino della Societa PaleontologicaItaliana 29 163ndash78

Haas M S Brown S J amp Beeman R W 2001 Pondering theprocephalon the segmental origin of the labrum DevelopmentGenes and Evolution 211 89ndash95

Heider K 1913 Entwicklungsgeschichte und Morphologie derWirbellosen Die Kultur der Gegenwart Theil 3 Abt 4 2 176ndash332

Henriksen K 1928 Critical notes upon Cambrian arthropodsdescribed by Charles D Walcott Videnskabelige Meddelelser fraDansk Naturhistorisk Forening 86 1ndash20

Hesselbo S P 1988 Trace fossils of Cambrian aglaspidid arthropodsLethaia 21 139ndash46

Hesselbo S P 1989 The aglaspidid arthropod Beckwithia from theCambrian of Utah and Wisconsin Journal of Paleontology 63636ndash42

Hesselbo S P 1992 Aglaspidida (Arthropoda) from the UpperCambrian of Wisconsin Journal of Paleontology 66 885ndash923

Hou X 1987a Three new large arthropods from Lower CambrianChengjiang fauna eastern Yunnan Acta Palaeontologica Sinica26 272ndash85 [In Chinese with English summary]

Hou X 1987b Two new arthropods from Lower CambrianChengjiang eastern Yunnan Acta Palaeontologica Sinica 26236ndash56 [In Chinese with English summary]

Hou X Chen J amp Lu H 1989 Early Cambrian new arthropodsfrom Chengjiang Yunnan Acta Palaeontologica Sinica 28 42ndash57[In Chinese with English summary]

Hou X Ramskold L amp Bergstrom J 1991 Composition andpreservation of the Chengjiang fauna a Lower Cambrian soft-bodied biota Zoologica Scripta 20 395ndash411

Hou X Bergstrom J amp Ahlberg P 1995 Anomalocaris and otherlarge animals in the Lower Cambrian Chengjiang fauna ofsouthwest China GFF 117 163ndash83

Hou X amp Bergstrom J 1997 Arthropods of the Lower CambrianChengjiang fauna southwest China Fossils and Strata 45 1ndash116

Hughes C P 1975 Redescription of Burgessia bella from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 4415ndash36

Ivantsov A Y 1999 Trilobite-like arthropod from the LowerCambrian of the Siberian Platform Acta PalaeontologicaPolonica 44 455ndash66

King P E 1973 Pycnogonids London St Martin Press

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 191

Lauterbach K E 1973 Schlussereignisse in der Evolution derStammgruppe der Euarthropoda Zoologische Beitrage 19259ndash99

Lauterbach K E 1980 Schlussereignisse in der Evolution derGrundplans der Arachnata (Arthropoda) Abhandlungen desNaturwissenschaftlichen Vereins in Hamburg 23 163ndash327

Lauterbach K E 1983 Synapomorphien zwischen Trilobiten- undCheliceraten-zweig der Arachnata Zoologischer Anzeiger 210213ndash38

Lendzion K 1975 Fauna of the Mobergella Zone in the Polish LowerCambrian Kwartalnik Geologiczny 19 237ndash42

Lieberman B S 1998 Cladistic analysis of the Early Cambrianolenelloid trilobites Journal of Paleontology 72 59ndash78

Lu Y 1940 On the ontogeny and phylogeny of Redlichia intermediaLu (sp nov) Bulletin of the Geological Society of China 20333ndash42 [In Chinese]

Maddison W P amp Maddison D R 1997 MacClade analysis ofphylogeny and character evolution Version 307 Sunderland MASinauer Associates

Mittmann B amp Scholtz G 2003 Development of the nervous systemin the lsquoheadrsquo of Limulus polyphemus (Chelicerata Xiphosura)morphological evidence for the correspondence between the seg-ments of the chelicerae and the (first) antennae of MandibulataDevelopment Genes and Evolution 213 9ndash17

Moore R C (ed) 1959 Treatise on invertebrate paleontology Part OArthropoda 1 Boulder Colorado Geological Society of America

Muller K J amp Walossek D 1986 Arthropod larvae from the UpperCambrian of Sweden Transactions of the Royal Society ofEdinburgh Earth Sciences 77 157ndash79

Muller K J amp Walossek D 1987 Morphology ontogeny and lifehabit of Agnostus pisiformis from the Upper Cambrian of SwedenFossils and Strata 19 1ndash124

Munilla T 1999 Evolucion y filogenia de los Picnogonidos InMelicA DeHaro J J Medez M amp Ribera I (eds) VolumenMonografico Evolucion y Filogenia de Arthropoda Boletin de laSociedad Entolomologica Aragonesa 26 273ndash91 [In Spanish andEnglish]

Orr P J Siveter D J Briggs D E G Siveter D J amp Sutton M D2000 A new arthropod from the Silurian Konservat-Lagerstatteof Herefordshire Proceedings of the Royal Society of LondonSeries B 267 1497ndash504

Page R D M 1996 On consensus confidence and lsquototal evidencersquoCladistics 12 83ndash92

Palmer A R amp Repina L N 1993 Through a glass darklytaxonomy phylogeny and biostratigraphy of the OlenellinaUniversity of Kansas Paleontological Contributions New Series 31ndash35

Platnick N I Griswold C E amp Coddington J A 1991 On missingentries in cladistic analysis Cladistics 7 337ndash43

Pleijel F 1995 On character coding for phylogeny reconstructionCladistics 11 309ndash15

Raasch G O 1939 Cambrian Merostomata Special Papers of theGeological Society of America 19 1ndash146

Ramskold L Chen J Edgecombe G D amp Zhou G 1996Preservational folds simulating tergite junctions in tegopeltid andnaraoiid arthropods Lethaia 29 15ndash20

Ramskold L Chen J Edgecombe G D amp Zhou G 1997Cindarella and the arachnate clade Xandarellida (ArthropodaEarly Cambrian) from China Transactions of the Royal Society ofEdinburgh Earth Sciences 88 19ndash38

Ramskold L amp Edgecombe G D 1991 Trilobite monophylyrevisited Historical Biology 4 267ndash83

Ramskold L amp Edgecombe G D 1996 Trilobite appendagestructure ndash Eoredlichia revisited Alcheringa 20 269ndash76

Raw F 1957 Origin of chelicerates Journal of Paleontology 31139ndash193

Repina L N amp Okuneva O G 1969 Cambrian arthropods of themaritime territory Paleontological Journal 1969 95ndash103

Richter R amp Richter E 1929 Weinbergina opitz ng n sp einSchwerttrager (Merost Xiphos) aus dem Devon (Rheinland)Senkenbergiana 11 193ndash209

Robison R A 1984 New occurences of the unusual trilobite Naraoiafrom the Cambrian of Idaho and Utah University of KansasPaleontological Contributions Paper 112 1ndash8

Rominger C 1887 Description of primordial fossils from MtStephens NW Territory of Canada Proceedings of the Academyof Natural Sciences of Philadelphia 1887 12ndash19

Scholtz G 1997 Cleavage germ band formation and head segmen-tation the ground pattern of the Euarthropoda In Fortey R Aamp Thomas R H (eds) Arthropod Relationships 317ndash32 LondonChapman and Hall Systematics Association Special Volume 55

Schram F R 1978 Arthropods a convergent phenomenonFieldiana Geology 39 61ndash108

Schram F R 1993 Review of Boxshall Stromberg and Dahl TheCrustacea Origin and Evolution Journal of Crustacean Biology13 820ndash2

Schram F R Vonk R amp Hof C H J 1997 Mazon CreekCycloidea Journal of Paleontology 71 261ndash84

Schram F R amp Emerson M J 1991 Arthropod Pattern Theorya new approach to arthropod phylogeny Memoirs of theQueensland Museum 31 1ndash18

Schram F R amp Hof C H J 1998 Fossils and the interrelationshipsof major crustacean groups In Edgecombe G D (ed) Arthropodfossils and phylogeny pp 233ndash302 New York NY ColumbiaUniversity Press

Shergold J H 1991 Protaspis and early meraspis growth stages of theeodiscoid trilobite Pagetia ocellata Jell and their implications forclassification Alcheringa 15 65ndash86

Shu D Geyer G Ling C amp Zhang Z 1995 Redlichiacean trilobiteswith preserved soft parts from the Lower Cambrian Chengjiangfauna (South China) Beringeria (Special Issue) 2 203ndash41

Shultz J W 1990 Evolutionary morphology and phylogeny ofArachnida Cladistics 6 1ndash38

Shultz J W 2000 Skeletomuscular anatomy of the harvestmanLeiobunum aldrichi (Weed 1893) (Arachnida OpilionesPalpatores) and its evolutionary significance Zoological Journalof the Linnean Society 128 401ndash38

Shultz J W 2001 Gross muscular anatomy of Limulus polyphemus(Xiphosura Chelicerata) and its bearing on evolution in the theArachnida Journal of Arachnology 29 283ndash303

Siewing R (ed) 1985 Lehrbuch der Zoologie II Systematik 3rd ednStuttgart Fischer

Simonetta A M 1970 Studies on the non trilobite arthropods ofthe Burgess Shale (Middle Cambrian) the genera LeanchoiliaAlalcomenaeus Opabinia Burgessia Yohoia and ActaeusPalaeontographica Italica 66 35ndash45

Simonetta A M amp Delle Cave L 1975 The Cambrian non trilobitearthropods from the Burgess Shale of British Columbia A studyof their comparative morphology taxonomy and evolutionarysignificance Palaeontographica Italica 69 1ndash37

Simonetta A M amp Delle Cave L 1981 An essay in the compara-tive and evolutionary morphology of Palaeozoic ArthropodsAccademia Nazionale dei Lincei Atti dei Convegni Lincei 49389ndash439

Starobogatov Y I 1990 Sistema i filogeniya nizshikh khelitserovykh(analiz morfologii paleozoysikh grupp) [The systematics andphylogeny of the lower chelicerates (a morphological analysis ofthe Palaeozoic groups)] Paleontologicheskii Zhurnal 1990 4ndash17

Stoslashrmer L 1944 On the relationships and phylogeny of fossil andrecent Arachnomorpha Skrifter utgitt ac Det Norske Videnskaps-Akademi i Olso I Matematisk-naturvitenskaplig Klasse 5 1ndash158

Stoslashrmer L 1951 Studies on trilobite morphology Part III Theventral cephalic structures with remarks on the zoologicalposition of the trilobites Norsk geologisk tidsskrift 29 108ndash58

Stoslashrmer L 1956 A Lower Cambrian merostome from Sweden Arkivfor Zoologi Ser 2 9 507ndash14

Stoslashrmer L 1959 Trilobitoidea In Moore R C (ed) Treatise onInvertebrate Paleontology part O Arthropoda 1 23ndash37 BoulderColorado Geological Society of America

Strong E E amp Lipscomb D 1999 Character coding and inapplicabledata Cladistics 15 363ndash71

Sturmer W amp Bergstrom J 1976 The arthropods Mimetaster andVachonisia from the Devonian Hunsruck Shale PalaontologischeZeitschrift 50 78ndash111

Sturmer W amp Bergstrom J 1978 The arthropod Cheloniellon fromthe Devonian Hunsruck Shale Palaontologische Zeitschrift 5257ndash81

Sturmer W amp Bergstrom J 1981 Weinbergina a xiphosuran fromthe Devonian Hunsruck Slate Palaontologische Zeitschrift 55237ndash55

Sutton M D Briggs D E G Siveter D J Siveter D J amp Orr PJ 2002 The arthropod Offacolus kingi (Chelicerata) from theSilurian of Herefordshire England computer based morphologi-cal reconstructions and phylogenetic affinity Proceedings of theRoyal Society of London Series B 269 1195ndash1203

Swofford D L 1999 PAUP Phylogenetic Analysis Using Parsimony(and other methods) Version 40b2a Sunderland MA SinauerAssociates

Telford M J amp Thomas R H 1998 Expression of homeobox genesshows chelicerate arthropods retain their deutocerebral segmentProceedings of the National Academy of Sciences USA 9510671ndash5

192 TREVOR J COTTON AND SIMON J BRADDY

Thorley J L amp Page R D M 2000 RadCon phylogenetic treecomparison and consensus Bioinformatics 16 486ndash7

Valentine J W 1995 Why no new phyla after the Cambrian ndashgenome and ecospace hypotheses revisited Palaios 10 190ndash194

Wagele J W 1993 Rejection of the lsquoUniramiarsquo hypothesis andimplications of the Mandibulata concept Zoologische JahrbucherAbteilung fur Systematik Geographie und Biologie der Tiere 120253ndash88

Walcott C D 1911 Middle Cambrian Merostomata Cambriangeology and palaeontology II Smithsonian MiscellaneousCollections 57 17ndash40

Walcott C D 1912 Middle Cambrian Branchiopoda MalacostracaTrilobita and Merostomata Cambrian geology and palaeontol-ogy II Smithsonian Miscellaneous Collections 57 145ndash228

Walcott C D 1918 Geological explorations in the CanadianRockies Explorations and fieldwork of the SmithsonianInstitution in 1917 Smithsonian Miscellaneous Collections 684ndash20

Walossek D 1993 The Upper Cambrian Rehbachiella kinnekullensisand the phylogeny of Branchiopoda and Crustacea Fossils andStrata 32 1ndash202

Walossek D amp Dunlop J A 2002 A larval sea spider (ArthropodaPycnogonida) from the Upper Cambrian lsquoOrstenrsquo of Swedenand the phylogenetic position of pycnogonids Palaeontology 45421ndash46

Walossek D amp Muller K J 1990 Upper Cambrian stem-lineagecrustaceans and their bearing upon the monophyletic origin ofCrustacea and the position of Agnostus Lethaia 23 409ndash27

Walossek D amp Muller K J 1997 Cambrian lsquoOrstenrsquo-type arthro-pods and the phylogeny of Crustacea In Fortey R A amp ThomasR H (eds) Arthropod Relationships 139ndash53 SystematicsAssociation Special Volume 55 London Chapman and Hall

Walossek D amp Muller K J 1998 Early arthropod phylogeny in lightof the Cambrian lsquoOrstenrsquo fossils In Edgecombe G D (ed)Arthropod fossils and phylogeny 185ndash231 New York NYColumbia University Press

Weygoldt P 1979 Significance of later embryonic stages and headdevelopment in arthropod phylogeny In Gupta A P (ed)Arthropod Phylogeny 107ndash35 New York NY Van NostrandReinhold Co

Weygoldt P 1998 Evolution and systematics of the ChelicerataExperimental and Applied Acarology 22 63ndash79

Weygoldt P amp Paulus H F 1979 Unterzuchungen aur Morpholo-gie Taxonomie und Phylogenie der Chelicerata Zeitschriftfur Zoologische Systematik und Evolutionsforschung 17 85ndash116177ndash200

Wheeler W C Cartwright P amp Hayashi C Y 1993 Arthropodphylogeny a combined approach Cladistics 9 1ndash39

Wheeler W C amp Hayashi C Y 1998 The phylogeny of extantchelicerate orders Cladistics 14 173ndash92

Whittington H B 1971 Redescription of Marrella splendens(Trilobitoidea) from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 209 1ndash24

Whittington H B 1974 Yohoia Walcott and Plenocaris n genarthropods from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 231 1ndash21

Whittington H B 1975a Trilobites with appendages from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 497ndash136

Whittington H B 1975b The enigmatic animal Opabinia regalisMiddle Cambrian Burgess Shale British Columbia PhilosophicalTransactions of the Royal Society of London Series B 271 1ndash43

Whittington H B 1977 The Middle Cambrian trilobite NaraoiaBurgess Shale British Columbia Philosophical Transactions of theRoyal Society Series B 280 409ndash43

Whittington H B 1979 Early arthropods their appendages andrelationships In House M R (ed) The origin of majorinvertebrate groups 253ndash68 Systematics Association SpecialVolume 12 New York NY Academic Press

Whittington H B 1980 Exoskeleton moult stage appendage mor-phology and habits of the Middle Cambrian trilobite Olenoidesserratus Palaeontology 23 171ndash204

Whittington H B 1981 Rare arthropods from the Burgess ShaleMiddle Cambrian British Columbia Philosophical Transactionsof the Royal Society of London Series B 292 329ndash57

Whittington H B 1985 Tegopelte gigas a second soft-bodied trilo-bite from the Burgess Shale Middle Cambrian British ColumbiaJournal of Palaeontology 23 171ndash204

Whittington H B 1989 Olenelloid trilobites type species functionalmorphology and higher classification Philosophical Transactionsof the Royal Society of London Series B 324 111ndash47

Whittington H B amp Almond J E 1987 Appendages and habits ofthe Upper Ordovician trilobite Triarthus eatoni PhilosophicalTransactions of the Royal Society of London Series B 317 1ndash46

Wilkinson M 1992 Ordered versus unordered characters Cladistics8 375ndash85

Wilkinson M 1994 Common cladistic information and its consensusrepresentation reduced Adams and reduced cladistic consensustrees and profiles Systematic Biology 43 343ndash68

Wilkinson M 1995a Coping with abundant missing entries in phylo-genetic inference using parsimony Systematic Biology 44 501ndash14

Wilkinson M 1995b TAXEQ2 software and documentation BristolSchool of Biological Sciences University of Bristol

Wilkinson M 1996 On the distribution of homoplasy and choosingamong trees Taxon 45 263ndash66

Wills M A 1997 A phylogeny of Recent and fossil Crustacea derivedfrom morphological characters In Fortey R A amp ThomasR H (eds) Arthropod Relationships 189ndash209 SystematicsAssociation Special Volume 55 London Chapman and Hall

Wills M A Briggs D E G amp Fortey R A 1994 Disparity asan evolutionary index a comparison of Cambrian and Recentarthropods Paleobiology 20 93ndash130

Wills M A Briggs D E G Fortey R A amp Wilkinson M 1995The significance of fossils in understanding arthropod evolutionVerhandlungen der deutschen zoologischen Gesselschaft 88 203ndash15

Wills M A Briggs D E G Fortey R A Wilkinson M amp SneathP H A 1998a An arthropod phylogeny based on fossil andrecent taxa In Edgecombe G D (ed) Arthropod fossils andphylogeny 33ndash105 New York NY Columbia University Press

Wills M A Briggs D E G amp Fortey R A 1998b Evolutionarycorrelates of arthropod tagmosis scrambled legs In ForteyR A amp Thomas R H (eds) Arthropod Relationships 57ndash65Systematics Association Special Volume 55 London Chapmanand Hall

Winter G 1980 Beitrage zur Morphologie und Embryologie desvorderen Korperabschnitts (Cephalosoma) der PantopodaGerstaecker 1863 I Entstehung und Struktur des Zentralnerven-systems Zeitschrift fur Zoologische Systematik und Evolutionsfor-schung 18 27ndash61

Zhang W amp Hou X 1985 Preliminary notes on the occurrence of theunusual trilobite Naraoia in Asia Acta Palaeontologica Sinica 24591ndash5 [In Chinese with English summary]

TREVOR J COTTON Department of Palaeontology The Natural History Museum CromwellRoad London SW7 5BD UKe-mail trevor_cottonhotmailcom

SIMON J BRADDY Department of Earth Sciences University of Bristol Wills MemorialBuilding Queenrsquos Road Bristol BS8 1RJ UKe-mail S J Braddybrisacuk

MS received 28 July 2003 Accepted for publication 15 March 2004

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 193

Page 20: The phylogeny of arachnomorph arthropods and the origin of the

taxa were excluded from the analyses only two (B and D) ofthe four topologies found in the complete analysis remainedmost parsimonious (see Fig 9 and Table 3) Secondly Bremersupport values throughout much of the lsquochelicerate-alliedrsquoclade (Clade 3 of Fig 8) were improved when these taxa wereexcluded (see Fig 11)

Dunlop (1999 p 258) suggested that in reconstructing thechelicerate stem lineage lsquowe might predict that the two mostsignificant changes towards the chelicerate condition are thereduction of the antennae and the formation of the nextappendages into a clawrsquo According to the hypothesis pre-sented here both of these adaptations were achieved in EarlyCambrian chelicerate ancestors The recognition of the loss ofthe antennae in the stem group of the chelicerates and thephylogenetic hypothesis presented here suggest that chelicer-ates are most closely related to taxa with only a singleadditional segment fused to the crown group eucheliceratehead of four fused somites Previous hypotheses have derivedchelicerates from Palaeozoic arthropods with a longer head(eg Cheloniellon Sturmer amp Bergstrom 1978 SanctacarisBriggs amp Collins 1988 Emeraldella Bruton amp Whittington1983) The plesiomorphic pattern of head segmentation ismatched in pycnogonids Bergstrom et al (1980) suggestedthat only the anterior four pairs of appendages are incorpo-rated into the head This supports the view that pycnogonidsare primitive with respect to other chelicerates which showmore or less complete fusion of this primitive head and threethoracic segments including Solpugida and Pseudoscorpiones

The present study suggests that a shift from gnathobasicbenthic feeding to a more vagrant lifestyle and raptorialappendage feeding occurred very early in the chelicerate lin-eage Appendage feeding was clearly an important adaptation

Figure 10 One of 18 equally parsimonious cladograms with lsquoAglaspidida 2rsquo coding and all characters unorderedthat is most congruent with results from other analyses Non-terminal branch lengths scaled to reflect number ofapomorphies (A) ACCTRAN apomorphy scheme Character numbers shown above boxes and states shownbelow boxes as in Table 2 and the text Character consistency indices are indicated by shading (B) Levels ofsupport for individual nodes Bremer support indices shown in bold type above branches Bootstrap percentagesin italic type below branches for all nodes with relative frequencies greater than 50

Figure 11 Bremer support values for clades within the lsquochelicerate-alliedrsquo arachnomorphs (clade 3 of Fig 8) when all taxa wereincluded (normal weight font above nodes) and with Lemoneites andPaleomerus excluded (bold type below nodes)

188 TREVOR J COTTON AND SIMON J BRADDY

for the terrestrialization of chelicerates (Dunlop 1997 p 69Dunlop amp Webster 1999) but the model proposed heresuggests that this considerably pre-dated terrestrializationGnathobasic feeding in xiphosurans and eurypterids mayrepresent a reversal to a more primitve benthic arachnomorphlifestyle from a more pycnogonid-like ancestor

32 Evolution of the arachnomorph headThe extant arthropod classes show highly conserved patternsof head segmentation (eg Wills et al 1994) In contrast therehas been a near consensus amongst Cambrian arthropodworkers (although rarely based on an explicit phylogenetichypothesis) that patterns of head segmentation are highlyhomoplastic and hence of little systematic significanceSturmer amp Bergstrom (1978 pp 78ndash9) suggested that lsquoevenclosely related forms may have different numbers of headsegments and appendagesrsquo Bruton amp Whittington (1983pp 576ndash7) that lsquodiscussion on fossil arthropod relationshipsbased on head segmentation appears to be largely irrel-evant and at best speculativersquo and Delle Cave amp Simonetta(1991 p 191) that lsquothere are no obvious phyletic affinitiesbetween genera having the same number of cephalized seg-mentsrsquo Most recently Bergstrom amp Hou (1997 p 104) con-cluded that lsquothe segmental length of the head shield seemsto be of no relevance to the discussion and discrimination ofevolutionary lineagesrsquo It has been claimed that this view issupported by cladistic analyses in which patterns of tagmosishave been found to be rather poor at defining major arthropodclades (Briggs et al 1992 Wills et al 1994)

This suggestion is not only of relevance to systematics buthas featured prominently in recent debates surrounding thenature of the lsquoCambrian explosionrsquo (see Budd amp Jensen 2000for a recent review) Gould (1989 1991) argued that theapparent plasticity of head segmentation in Cambrian com-pared to post-Cambrian arthropod evolution suggests thatbody-plan evolution was only constrained after the Cambrianexplosion According to this view the origin of new highertaxa with distinct body-plans was only possible before theonset of these constraints (eg Valentine 1995) Taxa hererecognised as arachnomorphs clearly played a major role in thedevelopment of Gouldrsquos hypothesis This suggestion has pre-viously been assessed in terms of overall morphological diver-sity which is clearly rather distinct from Gouldrsquos concept ofdisparity Wills et al (1998b) showed that there was littlecorrelation between overall morphological disparity and de-gree of limb specialisation and that the latter was phylogeneti-cally highly plastic They remarked that (Wills et al 1998bp 64) lsquothis finding has important implications for models ofarthropod phylogeny and evolution that attribute overidingimportance to head segmentationrsquo but did not explicitlyexamine head segmentation although it is likely to have beena major component of the index of tagmosis employed

The results of the present study suggest that arguments thatarthropod head segmentation was unusually labile during theCambrian are poorly founded Rather only four major pat-terns of euarthropod head segmentation are identified Theplesiomorphic euarthropod state according to both Walossekamp Muller (1997 1998) and Scholtz (1997) which may moreproperly be the plesiomorphic state for the euarthropod crowngroup only (depending on the phylogenetic position of theMarrellomorpha) consists of four post-acronal segmentsbearing the antennae and three pairs of biramous limbs Thepresent authors suggest that the term lsquocephalonrsquo be restrictedto this kind of head which is found in stem group mandibu-lates in addition to many arachnomorphs Crown groupmandibulates share a lsquobimaxillary headrsquo in which an addi-tional pair of appendages the labium or second maxillae are

incorporated into the head but in crustaceans these are notfused to the carapace (Scholtz 1997) Finally two distinctforms of head tagmosis are found in chelicerates A head withfour pairs of appendages (but without antennae giving a totalof five segments incorporated into the head) which has beencalled the lsquocephalosomarsquo is present in pycnogonids (Walossekamp Dunlop 2002 pp 434ndash5) and probably also in some euch-elicerates (J A Dunlop pers comm 2003) The lsquoprosomarsquo ofmost euchelicerates consists of this cephalosoma and twoadditional segments

In general these patterns of head segmentation are highlyphylogenetically conserved (see Fig 12) Only three homo-plastic changes namely reversal to a more primitive conditionin the Leanchoilia-Alalcomenaeus clade and the convergentorigin of a five-segmented head in Clade 3 (of Fig 8) and theXandarellida are necessary to optimise character 4 onto themost parsimonious cladogram There are also transitions toautapomorphic states in Emeraldella and Sidneyia In the caseof Sidneyia however an argument could be made (followingBergstrom amp Sturmer 1978 see discussion of Character 4) thatthe head consists of five segments as in closely related taxabut autapomorphically each segment has a separate tergiteThis would further increase the phylogenetic stability of headsegmentation Also following Bergstrom amp Sturmer (1978) thehead of Cheloniellon could be considered to consist of twotergites making it more similar to the head of Emeraldella(with six pairs of appendages)

Figure 12 Cladogram from Fig 10 showing changes in head segmen-tation Bold type shows abbreviated head segmentation formulaAbbreviations (Ant) antennae (BA) biramous appendage pairs (Ch)chelicerae (GA) great appendages (UA) uniramous appendage pairsand (X) segment without appendages Synapomorphy A is the auta-pomorphic reduction in head length to a single segment in SidneyiaAlternatively this may represent secondary division of the headshield Synapomorphy B is the increase in the length of the head inEmeraldella to six segments

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 189

4 Acknowledgements

We thank D E G Briggs J A Cotton J A Dunlop R AFortey S Harris R A Moore P Selden O E Tetlie andP Van Roy for comments on earlier drafts of this work Themanuscript was reviewed by M Wills F Schram and oneanonymous referee This research was supported by the Geo-logical Society of London Curry Research Studentshipawarded to TJC by the Department of Earth Sciences Uni-versity of Bristol and by the Department of PalaeontologyThe Natural History Museum London TJC would like tothank C Labandeira J Thompson and R Chapman fortheir hospitality at the National Museum of Natural HistoryWashington DC USA and A Olcott S Schellenberg andJ Young for making his time there so enjoyable

5 References

Abzhanov A Popadic A amp Kaufman T C 1999 Chelicerate Hoxgenes and the homology of arthropod segments Evolution andDevelopment 1 77ndash89

Akam M 2000 Arthropods developmental diversity within a (super)phylum Proceedings of the National Academy of Sciences USA97 4438ndash41

Aldridge R J Gabbott S E amp Theron J N 2001 The Soom ShaleIn Briggs D E G amp Crowther P R (eds) Palaeobiology II340ndash2 Oxford Blackwell Science

Anderson L I amp Selden P A 1997 Opisthosomal fusion andphylogeny of Palaeozoic Xiphosura Lethaia 30 19ndash31

Arango C P 2002 Morphological phylogenetics of the sea spiders(Arthropoda Pycnogonida) Organisms Diversity and Evolution2 107ndash25

Ax P 1986 Das phylogenetische System Stuttgart Gustav FischerVerlag

Babcock L E amp Zhang Wentang In press New non-mineralizedarthropods from the Chengjiang Lagerstatte Lower CambrianYunnan China Palaeontologia Cathyana

Bartels C Briggs D E G amp Brassel G 1998 The fossils ofthe Hunsruck Slate Marine life in the Devonian CambridgeCambridge University Press

Bergstrom J 1971 Paleomerus ndash merostome or merostomoid Lethaia4 393ndash401

Bergstrom J 1979 Morphology of fossil arthropods as a guideto phylogenetic relationships In Gupta A P (ed) ArthropodPhylogeny 3ndash56 New York NY Van Nostrand Reinhold Co

Bergstrom J 1992 The oldest arthropods and the origin of theCrustacea Acta Zoologica 73 287ndash91

Bergstrom J Sturmer W amp Winter G 1980 PalaeoisopusPalaeopantopodus and Palaeothea pycnogonid arthropodsfrom the Lower Devonian Hunsruck Slate West GermanyPalaontologische Zeitschrift 54 7ndash54

Bergstrom J amp Brassel G 1984 Legs in the trilobite Rhenops fromthe Lower Devonian Hunsruck Slate Lethaia 17 67ndash72

Bergstrom J amp Hou X 1998 Chengjiang arthropods and theirbearing on early arthropod evolution In Edgecombe G D(ed) Arthropod fossils and phylogeny 151ndash84 New York NYColumbia University Press

Boudreaux H B 1979 Significance of intersegmental tendon systemin arthropod phylogeny and a monophyletic classification ofArthropoda In Gupta A P (ed) Arthropod Phylogeny 551ndash86New York NY Van Nostrand Reinhold Co

Bousfield E L 1995 A contribution to the natural classification ofLower and Middle Cambrian arthropods food-gathering andfeeding mechanisms Amphipacifica 2 3ndash33

Braddy S J Aldridge R J Gabbott S E amp Theron J N 1999Lamellate book-gills in a late Ordovician eurypterid from theSoom Shale South Africa support for a eurypterid-scorpionclade Lethaia 32 72ndash4

Braddy S J amp Almond J E 1999 Eurypterid trackways from theTable Mountain Group (Ordovician) of South Africa Journal ofAfrican Earth Sciences 29 165ndash77

Bremer K 1988 The limits of amino acid sequence data in angio-sperm phylogenetic reconstruction Evolution 42 795ndash803

Bremer K 1994 Branch support and tree stability Cladistics 10295ndash304

Briggs D E G 1998 Review of Arthropods from the LowerCambrian Chengjiang fauna southwest China by HouXianguang and Jan Bergstrom Fossils and Strata 45

Scandinavian University Press Oslo Norway 1997 Palaeo-geography Palaeoclimatology Palaeoecology 143 193ndash4

Briggs D E G Bruton D L amp Whittington H B 1979 Append-ages of the arthropod Aglaspis spinifer (Upper CambrianWisconsin) and their significance Palaeontology 22 167ndash80

Briggs D E G Fortey R A amp Wills M A 1992 Morphologicaldisparity in the Cambrian Science 256 1670ndash3

Briggs D E G Fortey R A amp Wills MA 1993 How big was theCambrian explosion A taxonomic and morphologic comparisonof Cambrian and Recent arthropods In Lees D R amp EdwardsD (eds) Evolutionary patterns and processes 33ndash44 LondonLinnean Society of London Linnean Society Symposium Series

Briggs D E G Erwin D H amp Collier F J 1994 The fossils of theBurgess Shale Washington DC Smithsonian Institution Press

Briggs D E G amp Bartels C 2001 New arthropods from the LowerDevonian Hunsruck Slate (Lower Emsian Rhenish Massifwestern Germany) Palaeontology 44 275ndash304

Briggs D E G amp Bartels C 2002 Magnoculocaris a new name forthe arthropod Magnoculus Briggs and Bartels 2001 from theLower Devonian Hunsruck Slate Germany Palaeontology 45419

Briggs D E G amp Collins D 1988 A Middle Cambrian cheliceratefrom Mount Stephen British Columbia Palaeontology 31 779ndash98

Briggs D E G amp Collins D 1999 The arthropod Alalcomenaeuscambricus Simonetta from the Middle Cambrian Burgess Shale ofBritish Columbia Paleontology 42 953ndash78

Briggs D E G amp Fortey R A 1982 The cuticle of aglaspididarthropods a red-herring in the early history of vertebratesLethaia 15 25ndash9

Briggs D E G amp Fortey R A 1989 The early radiation andrelationships of the major arthropod groups Science 246 241ndash3

Briggs D E G amp Fortey R A 1992 The early Cambrian radiationof arthropods In Lipps J H amp Signor P W (eds) Origin andEarly Evolution of the Metazoa 335ndash373 New York NY PlenumPress

Briggs D E G amp Robison R A 1984 Exceptionally preservednontrilobite arthropods and Anomalocaris from the MiddleCambrian of Utah University of Kansas PaleontologicalContributions Paper 111 1ndash23

Broili F 1932 Ein neuer Crustacee aus dem rheinischen UnterdevonSitzungsberichte der Mathematisch-Naturwissenschaftlichen(Abteilung) Klasse der Bayerischen Akademie der Wissenschaftenzu Munchen 1932 27ndash38

Broili F 1933 Ein zweites Exemplar von Cheloniellon Sitzungsber-ichte der Bayerischen Akademie der Wissenschaften 1933 11ndash32

Bruton D L 1981 The arthropod Sidneyia inexpectans MiddleCambrian Burgess Shale British Columbia Philosophical Trans-actions of the Royal Society of London Series B 295 619ndash56

Bruton D L amp Whittington H B 1983 Emeraldella and Leanchoiliatwo arthropods from the Burgess Shale British Columbia Philo-sophical Transactions of the Royal Society of London Series B 300553ndash85

Budd G E 1995 Kleptothule rasmusseni gen et sp nov anolenellid-like trilobite from the Sirius Passet fauna (BuenFormation Lower Cambrian North Greenland) Transactions ofthe Royal Society of Edinburgh Earth Sciences 86 1ndash12

Budd G E 1996a Progress and problems in arthropod phylogenyTrends in Ecology and Evolution 11 356ndash8

Budd G E 1996b The morphology of Opabinia regalis and thereconstruction of the arthropod stem-group Lethaia 29 1ndash14

Budd G E 1997 Stem-group arthropods from the Lower CambrianSirius Passet fauna of North Greenland In Fortey R A ampThomas R H (eds) Arthropod Relationships 125ndash38 LondonChapman and Hall Systematics Association Special Volume 55

Budd G E 1999a A nektaspid arthropod from the Early CambrianSirius Passet fauna with a description of retrodeformation basedon functional morphology Palaeontology 42 99ndash122

Budd G E 1999b The morphology and phylogenetic significance ofKerygmachela kiergkegaardi Budd (Buen Formation LowerCambrian N Greenland) Transactions of the Royal Society ofEdinburgh Earth Sciences 89 (for 1998) 249ndash290

Budd G E 2002 A palaeontological solution to the arthropod headproblem Nature 417 271ndash5

Budd G E amp Jensen S 2000 A critical reappraisal of the fossilrecord of the bilaterian phyla Biological Reviews 75 253ndash95

Chen J Zhou G Zhu M amp Yeh K 1996 The Chengjiang biota ndasha unique window of the Cambrian explosion Taichung NationalMuseum of Natural Science [In Chinese]

Chen J Edgecombe G D amp Ramskold L 1997 Morphological andecological disparity in naraoiids (Arthropoda) from the Early

190 TREVOR J COTTON AND SIMON J BRADDY

Cambrian Chengjiang fauna China Records of the AustralianMuseum 49 1ndash24

Chen J amp Zhou G 1997 Biology of the Chengjiang fauna In ChenJ Cheng Y amp Iten H V (eds) The Cambrian explosion and thefossil record Bulletin of the National Museum of Natural ScienceTaiwan 10 11ndash106

Child C A 1997 Some deep-sea Pycnogonida from Argentine slopeand basin Proceedings of the Biological Society of Washington110 128ndash42

Chlupac I 1988 The enigmatic arthropod Duslia from the Ordovicianof Czechslovakia Palaeontology 31 611ndash20

Chlupac I 1995 Lower Cambrian arthropods from the PasekyShale (Barrandian area Czech Republic) Journal of the CzechGeological Society 40 9ndash36

Chlupac I amp Havlıcek V 1965 Kodymirus n g a new aglaspidmerostome of the Cambrian of Bohemia Sbornık GeologickechVid Paleontologie 6 7ndash20

Cisne J L 1975 Anatomy of Triarthrus and the relationships of theTrilobita Fossils and Strata 4 45ndash63

Cisne J L 1981 Triarthrus eatoni (Trilobita) anatomy of its exo-skeletal skeletomuscular and digestive systems Palaeonto-graphica Americana 9 95ndash142

Conway Morris S 1982 Atlas of the Burgess Shale London ThePalaeontological Association

Conway Morris S 1989 The persistence of Burgess Shale-type faunasimplications for the evolution of deeper-water faunas Trans-actions of the Royal Society of Edinburgh Earth Sciences 80271ndash83

Conway Morris S 1998 The crucible of creation Oxford OxfordUniversity Press

Cotton T J 1999 Review of Arthropod fossils and phylogenyPalaeogeography Palaeoclimatology Palaeoecology 151 341ndash3

Cotton T J 2002 The phylogeny and morphological evolution ofCambrian trilobites and their relatives Unpublished PhD ThesisUniversity of Bristol

Damen W G M Hausdorf M Seyfarth E A amp Tautz D 1998 Aconserved mode of head segmentation in arthropods revealed bythe expression pattern of Hox genes in a spider Proceedings of theNational Academy of Sciences USA 95 10665ndash70

Delle Cave L amp Simonetta A M 1991 Early Palaeozoic arthropodsand problems of arthropod phylogeny with some notes on taxa ofdoubtful affinities In Simonetta A M amp Conway Morris S(eds) The early evolution of Metazoa and the significance ofproblematic taxa 189ndash244 Cambridge Cambridge UniversityPress

Dewel R A amp Dewel W C 1997 The place of tardigrades inarthropod evolution In Fortey R A amp Thomas R H (eds)Arthropod Relationships 109ndash23 London Chapman and HallSystematics Association Special Volume 55

Dunlop J A 1997 Palaeozoic arachnids and their significance forarachnid phylogeny Proceedings of the 16th European Colloquiumof Arachnology 65ndash82 Siedlce Poland

Dunlop J A 1998 The origins of tetrapulmonate booklungs and theirsignificance for chelicerate phylogeny In Selden P A (ed)Proceedings of the 17th European Colloquium of ArachnologyEdinburgh 1997 pp 9ndash16

Dunlop J A 1999 Pasando revista a la evolucion de los quelicerados[A review of chelicerate evolution] In Melic A DeHaro J JMedez M amp Ribera I (eds) Volumen Monografico Evolucion yFilogenia de Arthropoda Boletin de la Sociedad EntolomologicaAragonesa 26 255ndash272 [In Spanish and English]

Dunlop J A amp Selden P A 1997 The early history and phylogenyof the chelicerates In Fortey R A amp Thomas R H (eds)Arthropod relationships Systematics Association Special Volume55 221ndash35 London Chapman and Hall

Dunlop J A amp Webster M 1999 Fossil evidence terrestrializationand arachnid phylogeny Journal of Arachnology 27 86ndash93

Dzik J 1993 Early metazoan evolution and the meaning of its fossilrecord Evolutionary Biology 27 339ndash86

Dzik J amp Lendzion K 1988 The oldest arthropods of the EastEuropean Platform Lethaia 21 29ndash38

Edgecombe G D Wilson G D F Colgan D J Gray M R ampCassis G 2000 Arthropod cladistics combined analysisof Histone H3 and U2 snRNA sequences and morphologyCladistics 16 155ndash203

Edgecombe G D amp Ramskold L 1999 Relationships of CambrianArachnata and the systematic position of Trilobita Journal ofPaleontology 73 263ndash87

Emerson M J amp Schram F R 1997 Theories patterns and realitygame plan for arthropod phylogeny In Fortey R A amp Thomas

R H (eds) Arthropod Relationships Systematics AssociationSpecial Volume 55 67ndash87 London Chapman and Hall

Farris J S 1989 The Retention Index and the Rescaled ConsistencyIndex Cladistics 5 417ndash19

Felsenstein J 1985 Confidence limits on phylogenies an approachusing the bootstrap Evolution 39 783ndash91

Flower R H 1968 Merostomes from a Cotter Horizon of the El PasoGroup New Mexico Bureau of Mines and Mineral ResourcesMemoir 22 35ndash44

Fortey R A 1990a Trilobite evolution and systematics In MikulicD G (ed) Arthropod paleobiology 44ndash65 Short Courses inPaleontology 3 Knoxville Tenessee The Paleontological Society

Fortey R A 1990b Ontogeny hypostome attachment and trilobiteclassification Palaeontology 33 529ndash76

Fortey R A 1997 Classification In Kaesler R L (ed) Treatise oninvertebrate paleontology Part O Arthropoda 1 (revised) 289ndash302Boulder Colorado amp Lawrence Kansas Geological Society ofAmerica and University of Kansas Press

Fortey R A amp Theron J N 1994 A new Ordovician arthropodSoomaspis and the agnostid problem Palaeontology 37 841ndash61

Fortey R A amp Whittington H B 1989 The Trilobita as a naturalgroup Historical Biology 2 125ndash38

Fry W G 1978 A classification within the pycnogonids ZoologicalJournal of the Linnean Society of London 63 35ndash58

Giribet G Edgecombe G D amp Wheeler W C 2001 Arthropodphylogeny based on eight molecular loci and morphology Nature413 157ndash61

Gould S J 1989 Wonderful life The Burgess Shale and the nature ofhistory New York NY Norton

Gould S J 1991 The disparity of the Burgess Shale arthropod faunaand the limits of cladistic analysis why we must strive to quantifymorphospace Paleobiology 17 411ndash23

Gurich G 1931 Mimetaster hexagonalis ein neuer Kruster aus demunterdevonischen Bundenbacher Dachschiefer PalaontologischeZeitschrift 12 204ndash38

Hammann W Laske R amp Pillola G L 1990 Tariccoia arrusensisngnsp an unusual trilobite-like arthropod Rediscovery of thelsquophylocaridrsquo beds of Taricco (1922) in the Ordovician lsquoPuddingarsquosequence of Sardinia Bolletino della Societa PaleontologicaItaliana 29 163ndash78

Haas M S Brown S J amp Beeman R W 2001 Pondering theprocephalon the segmental origin of the labrum DevelopmentGenes and Evolution 211 89ndash95

Heider K 1913 Entwicklungsgeschichte und Morphologie derWirbellosen Die Kultur der Gegenwart Theil 3 Abt 4 2 176ndash332

Henriksen K 1928 Critical notes upon Cambrian arthropodsdescribed by Charles D Walcott Videnskabelige Meddelelser fraDansk Naturhistorisk Forening 86 1ndash20

Hesselbo S P 1988 Trace fossils of Cambrian aglaspidid arthropodsLethaia 21 139ndash46

Hesselbo S P 1989 The aglaspidid arthropod Beckwithia from theCambrian of Utah and Wisconsin Journal of Paleontology 63636ndash42

Hesselbo S P 1992 Aglaspidida (Arthropoda) from the UpperCambrian of Wisconsin Journal of Paleontology 66 885ndash923

Hou X 1987a Three new large arthropods from Lower CambrianChengjiang fauna eastern Yunnan Acta Palaeontologica Sinica26 272ndash85 [In Chinese with English summary]

Hou X 1987b Two new arthropods from Lower CambrianChengjiang eastern Yunnan Acta Palaeontologica Sinica 26236ndash56 [In Chinese with English summary]

Hou X Chen J amp Lu H 1989 Early Cambrian new arthropodsfrom Chengjiang Yunnan Acta Palaeontologica Sinica 28 42ndash57[In Chinese with English summary]

Hou X Ramskold L amp Bergstrom J 1991 Composition andpreservation of the Chengjiang fauna a Lower Cambrian soft-bodied biota Zoologica Scripta 20 395ndash411

Hou X Bergstrom J amp Ahlberg P 1995 Anomalocaris and otherlarge animals in the Lower Cambrian Chengjiang fauna ofsouthwest China GFF 117 163ndash83

Hou X amp Bergstrom J 1997 Arthropods of the Lower CambrianChengjiang fauna southwest China Fossils and Strata 45 1ndash116

Hughes C P 1975 Redescription of Burgessia bella from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 4415ndash36

Ivantsov A Y 1999 Trilobite-like arthropod from the LowerCambrian of the Siberian Platform Acta PalaeontologicaPolonica 44 455ndash66

King P E 1973 Pycnogonids London St Martin Press

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 191

Lauterbach K E 1973 Schlussereignisse in der Evolution derStammgruppe der Euarthropoda Zoologische Beitrage 19259ndash99

Lauterbach K E 1980 Schlussereignisse in der Evolution derGrundplans der Arachnata (Arthropoda) Abhandlungen desNaturwissenschaftlichen Vereins in Hamburg 23 163ndash327

Lauterbach K E 1983 Synapomorphien zwischen Trilobiten- undCheliceraten-zweig der Arachnata Zoologischer Anzeiger 210213ndash38

Lendzion K 1975 Fauna of the Mobergella Zone in the Polish LowerCambrian Kwartalnik Geologiczny 19 237ndash42

Lieberman B S 1998 Cladistic analysis of the Early Cambrianolenelloid trilobites Journal of Paleontology 72 59ndash78

Lu Y 1940 On the ontogeny and phylogeny of Redlichia intermediaLu (sp nov) Bulletin of the Geological Society of China 20333ndash42 [In Chinese]

Maddison W P amp Maddison D R 1997 MacClade analysis ofphylogeny and character evolution Version 307 Sunderland MASinauer Associates

Mittmann B amp Scholtz G 2003 Development of the nervous systemin the lsquoheadrsquo of Limulus polyphemus (Chelicerata Xiphosura)morphological evidence for the correspondence between the seg-ments of the chelicerae and the (first) antennae of MandibulataDevelopment Genes and Evolution 213 9ndash17

Moore R C (ed) 1959 Treatise on invertebrate paleontology Part OArthropoda 1 Boulder Colorado Geological Society of America

Muller K J amp Walossek D 1986 Arthropod larvae from the UpperCambrian of Sweden Transactions of the Royal Society ofEdinburgh Earth Sciences 77 157ndash79

Muller K J amp Walossek D 1987 Morphology ontogeny and lifehabit of Agnostus pisiformis from the Upper Cambrian of SwedenFossils and Strata 19 1ndash124

Munilla T 1999 Evolucion y filogenia de los Picnogonidos InMelicA DeHaro J J Medez M amp Ribera I (eds) VolumenMonografico Evolucion y Filogenia de Arthropoda Boletin de laSociedad Entolomologica Aragonesa 26 273ndash91 [In Spanish andEnglish]

Orr P J Siveter D J Briggs D E G Siveter D J amp Sutton M D2000 A new arthropod from the Silurian Konservat-Lagerstatteof Herefordshire Proceedings of the Royal Society of LondonSeries B 267 1497ndash504

Page R D M 1996 On consensus confidence and lsquototal evidencersquoCladistics 12 83ndash92

Palmer A R amp Repina L N 1993 Through a glass darklytaxonomy phylogeny and biostratigraphy of the OlenellinaUniversity of Kansas Paleontological Contributions New Series 31ndash35

Platnick N I Griswold C E amp Coddington J A 1991 On missingentries in cladistic analysis Cladistics 7 337ndash43

Pleijel F 1995 On character coding for phylogeny reconstructionCladistics 11 309ndash15

Raasch G O 1939 Cambrian Merostomata Special Papers of theGeological Society of America 19 1ndash146

Ramskold L Chen J Edgecombe G D amp Zhou G 1996Preservational folds simulating tergite junctions in tegopeltid andnaraoiid arthropods Lethaia 29 15ndash20

Ramskold L Chen J Edgecombe G D amp Zhou G 1997Cindarella and the arachnate clade Xandarellida (ArthropodaEarly Cambrian) from China Transactions of the Royal Society ofEdinburgh Earth Sciences 88 19ndash38

Ramskold L amp Edgecombe G D 1991 Trilobite monophylyrevisited Historical Biology 4 267ndash83

Ramskold L amp Edgecombe G D 1996 Trilobite appendagestructure ndash Eoredlichia revisited Alcheringa 20 269ndash76

Raw F 1957 Origin of chelicerates Journal of Paleontology 31139ndash193

Repina L N amp Okuneva O G 1969 Cambrian arthropods of themaritime territory Paleontological Journal 1969 95ndash103

Richter R amp Richter E 1929 Weinbergina opitz ng n sp einSchwerttrager (Merost Xiphos) aus dem Devon (Rheinland)Senkenbergiana 11 193ndash209

Robison R A 1984 New occurences of the unusual trilobite Naraoiafrom the Cambrian of Idaho and Utah University of KansasPaleontological Contributions Paper 112 1ndash8

Rominger C 1887 Description of primordial fossils from MtStephens NW Territory of Canada Proceedings of the Academyof Natural Sciences of Philadelphia 1887 12ndash19

Scholtz G 1997 Cleavage germ band formation and head segmen-tation the ground pattern of the Euarthropoda In Fortey R Aamp Thomas R H (eds) Arthropod Relationships 317ndash32 LondonChapman and Hall Systematics Association Special Volume 55

Schram F R 1978 Arthropods a convergent phenomenonFieldiana Geology 39 61ndash108

Schram F R 1993 Review of Boxshall Stromberg and Dahl TheCrustacea Origin and Evolution Journal of Crustacean Biology13 820ndash2

Schram F R Vonk R amp Hof C H J 1997 Mazon CreekCycloidea Journal of Paleontology 71 261ndash84

Schram F R amp Emerson M J 1991 Arthropod Pattern Theorya new approach to arthropod phylogeny Memoirs of theQueensland Museum 31 1ndash18

Schram F R amp Hof C H J 1998 Fossils and the interrelationshipsof major crustacean groups In Edgecombe G D (ed) Arthropodfossils and phylogeny pp 233ndash302 New York NY ColumbiaUniversity Press

Shergold J H 1991 Protaspis and early meraspis growth stages of theeodiscoid trilobite Pagetia ocellata Jell and their implications forclassification Alcheringa 15 65ndash86

Shu D Geyer G Ling C amp Zhang Z 1995 Redlichiacean trilobiteswith preserved soft parts from the Lower Cambrian Chengjiangfauna (South China) Beringeria (Special Issue) 2 203ndash41

Shultz J W 1990 Evolutionary morphology and phylogeny ofArachnida Cladistics 6 1ndash38

Shultz J W 2000 Skeletomuscular anatomy of the harvestmanLeiobunum aldrichi (Weed 1893) (Arachnida OpilionesPalpatores) and its evolutionary significance Zoological Journalof the Linnean Society 128 401ndash38

Shultz J W 2001 Gross muscular anatomy of Limulus polyphemus(Xiphosura Chelicerata) and its bearing on evolution in the theArachnida Journal of Arachnology 29 283ndash303

Siewing R (ed) 1985 Lehrbuch der Zoologie II Systematik 3rd ednStuttgart Fischer

Simonetta A M 1970 Studies on the non trilobite arthropods ofthe Burgess Shale (Middle Cambrian) the genera LeanchoiliaAlalcomenaeus Opabinia Burgessia Yohoia and ActaeusPalaeontographica Italica 66 35ndash45

Simonetta A M amp Delle Cave L 1975 The Cambrian non trilobitearthropods from the Burgess Shale of British Columbia A studyof their comparative morphology taxonomy and evolutionarysignificance Palaeontographica Italica 69 1ndash37

Simonetta A M amp Delle Cave L 1981 An essay in the compara-tive and evolutionary morphology of Palaeozoic ArthropodsAccademia Nazionale dei Lincei Atti dei Convegni Lincei 49389ndash439

Starobogatov Y I 1990 Sistema i filogeniya nizshikh khelitserovykh(analiz morfologii paleozoysikh grupp) [The systematics andphylogeny of the lower chelicerates (a morphological analysis ofthe Palaeozoic groups)] Paleontologicheskii Zhurnal 1990 4ndash17

Stoslashrmer L 1944 On the relationships and phylogeny of fossil andrecent Arachnomorpha Skrifter utgitt ac Det Norske Videnskaps-Akademi i Olso I Matematisk-naturvitenskaplig Klasse 5 1ndash158

Stoslashrmer L 1951 Studies on trilobite morphology Part III Theventral cephalic structures with remarks on the zoologicalposition of the trilobites Norsk geologisk tidsskrift 29 108ndash58

Stoslashrmer L 1956 A Lower Cambrian merostome from Sweden Arkivfor Zoologi Ser 2 9 507ndash14

Stoslashrmer L 1959 Trilobitoidea In Moore R C (ed) Treatise onInvertebrate Paleontology part O Arthropoda 1 23ndash37 BoulderColorado Geological Society of America

Strong E E amp Lipscomb D 1999 Character coding and inapplicabledata Cladistics 15 363ndash71

Sturmer W amp Bergstrom J 1976 The arthropods Mimetaster andVachonisia from the Devonian Hunsruck Shale PalaontologischeZeitschrift 50 78ndash111

Sturmer W amp Bergstrom J 1978 The arthropod Cheloniellon fromthe Devonian Hunsruck Shale Palaontologische Zeitschrift 5257ndash81

Sturmer W amp Bergstrom J 1981 Weinbergina a xiphosuran fromthe Devonian Hunsruck Slate Palaontologische Zeitschrift 55237ndash55

Sutton M D Briggs D E G Siveter D J Siveter D J amp Orr PJ 2002 The arthropod Offacolus kingi (Chelicerata) from theSilurian of Herefordshire England computer based morphologi-cal reconstructions and phylogenetic affinity Proceedings of theRoyal Society of London Series B 269 1195ndash1203

Swofford D L 1999 PAUP Phylogenetic Analysis Using Parsimony(and other methods) Version 40b2a Sunderland MA SinauerAssociates

Telford M J amp Thomas R H 1998 Expression of homeobox genesshows chelicerate arthropods retain their deutocerebral segmentProceedings of the National Academy of Sciences USA 9510671ndash5

192 TREVOR J COTTON AND SIMON J BRADDY

Thorley J L amp Page R D M 2000 RadCon phylogenetic treecomparison and consensus Bioinformatics 16 486ndash7

Valentine J W 1995 Why no new phyla after the Cambrian ndashgenome and ecospace hypotheses revisited Palaios 10 190ndash194

Wagele J W 1993 Rejection of the lsquoUniramiarsquo hypothesis andimplications of the Mandibulata concept Zoologische JahrbucherAbteilung fur Systematik Geographie und Biologie der Tiere 120253ndash88

Walcott C D 1911 Middle Cambrian Merostomata Cambriangeology and palaeontology II Smithsonian MiscellaneousCollections 57 17ndash40

Walcott C D 1912 Middle Cambrian Branchiopoda MalacostracaTrilobita and Merostomata Cambrian geology and palaeontol-ogy II Smithsonian Miscellaneous Collections 57 145ndash228

Walcott C D 1918 Geological explorations in the CanadianRockies Explorations and fieldwork of the SmithsonianInstitution in 1917 Smithsonian Miscellaneous Collections 684ndash20

Walossek D 1993 The Upper Cambrian Rehbachiella kinnekullensisand the phylogeny of Branchiopoda and Crustacea Fossils andStrata 32 1ndash202

Walossek D amp Dunlop J A 2002 A larval sea spider (ArthropodaPycnogonida) from the Upper Cambrian lsquoOrstenrsquo of Swedenand the phylogenetic position of pycnogonids Palaeontology 45421ndash46

Walossek D amp Muller K J 1990 Upper Cambrian stem-lineagecrustaceans and their bearing upon the monophyletic origin ofCrustacea and the position of Agnostus Lethaia 23 409ndash27

Walossek D amp Muller K J 1997 Cambrian lsquoOrstenrsquo-type arthro-pods and the phylogeny of Crustacea In Fortey R A amp ThomasR H (eds) Arthropod Relationships 139ndash53 SystematicsAssociation Special Volume 55 London Chapman and Hall

Walossek D amp Muller K J 1998 Early arthropod phylogeny in lightof the Cambrian lsquoOrstenrsquo fossils In Edgecombe G D (ed)Arthropod fossils and phylogeny 185ndash231 New York NYColumbia University Press

Weygoldt P 1979 Significance of later embryonic stages and headdevelopment in arthropod phylogeny In Gupta A P (ed)Arthropod Phylogeny 107ndash35 New York NY Van NostrandReinhold Co

Weygoldt P 1998 Evolution and systematics of the ChelicerataExperimental and Applied Acarology 22 63ndash79

Weygoldt P amp Paulus H F 1979 Unterzuchungen aur Morpholo-gie Taxonomie und Phylogenie der Chelicerata Zeitschriftfur Zoologische Systematik und Evolutionsforschung 17 85ndash116177ndash200

Wheeler W C Cartwright P amp Hayashi C Y 1993 Arthropodphylogeny a combined approach Cladistics 9 1ndash39

Wheeler W C amp Hayashi C Y 1998 The phylogeny of extantchelicerate orders Cladistics 14 173ndash92

Whittington H B 1971 Redescription of Marrella splendens(Trilobitoidea) from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 209 1ndash24

Whittington H B 1974 Yohoia Walcott and Plenocaris n genarthropods from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 231 1ndash21

Whittington H B 1975a Trilobites with appendages from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 497ndash136

Whittington H B 1975b The enigmatic animal Opabinia regalisMiddle Cambrian Burgess Shale British Columbia PhilosophicalTransactions of the Royal Society of London Series B 271 1ndash43

Whittington H B 1977 The Middle Cambrian trilobite NaraoiaBurgess Shale British Columbia Philosophical Transactions of theRoyal Society Series B 280 409ndash43

Whittington H B 1979 Early arthropods their appendages andrelationships In House M R (ed) The origin of majorinvertebrate groups 253ndash68 Systematics Association SpecialVolume 12 New York NY Academic Press

Whittington H B 1980 Exoskeleton moult stage appendage mor-phology and habits of the Middle Cambrian trilobite Olenoidesserratus Palaeontology 23 171ndash204

Whittington H B 1981 Rare arthropods from the Burgess ShaleMiddle Cambrian British Columbia Philosophical Transactionsof the Royal Society of London Series B 292 329ndash57

Whittington H B 1985 Tegopelte gigas a second soft-bodied trilo-bite from the Burgess Shale Middle Cambrian British ColumbiaJournal of Palaeontology 23 171ndash204

Whittington H B 1989 Olenelloid trilobites type species functionalmorphology and higher classification Philosophical Transactionsof the Royal Society of London Series B 324 111ndash47

Whittington H B amp Almond J E 1987 Appendages and habits ofthe Upper Ordovician trilobite Triarthus eatoni PhilosophicalTransactions of the Royal Society of London Series B 317 1ndash46

Wilkinson M 1992 Ordered versus unordered characters Cladistics8 375ndash85

Wilkinson M 1994 Common cladistic information and its consensusrepresentation reduced Adams and reduced cladistic consensustrees and profiles Systematic Biology 43 343ndash68

Wilkinson M 1995a Coping with abundant missing entries in phylo-genetic inference using parsimony Systematic Biology 44 501ndash14

Wilkinson M 1995b TAXEQ2 software and documentation BristolSchool of Biological Sciences University of Bristol

Wilkinson M 1996 On the distribution of homoplasy and choosingamong trees Taxon 45 263ndash66

Wills M A 1997 A phylogeny of Recent and fossil Crustacea derivedfrom morphological characters In Fortey R A amp ThomasR H (eds) Arthropod Relationships 189ndash209 SystematicsAssociation Special Volume 55 London Chapman and Hall

Wills M A Briggs D E G amp Fortey R A 1994 Disparity asan evolutionary index a comparison of Cambrian and Recentarthropods Paleobiology 20 93ndash130

Wills M A Briggs D E G Fortey R A amp Wilkinson M 1995The significance of fossils in understanding arthropod evolutionVerhandlungen der deutschen zoologischen Gesselschaft 88 203ndash15

Wills M A Briggs D E G Fortey R A Wilkinson M amp SneathP H A 1998a An arthropod phylogeny based on fossil andrecent taxa In Edgecombe G D (ed) Arthropod fossils andphylogeny 33ndash105 New York NY Columbia University Press

Wills M A Briggs D E G amp Fortey R A 1998b Evolutionarycorrelates of arthropod tagmosis scrambled legs In ForteyR A amp Thomas R H (eds) Arthropod Relationships 57ndash65Systematics Association Special Volume 55 London Chapmanand Hall

Winter G 1980 Beitrage zur Morphologie und Embryologie desvorderen Korperabschnitts (Cephalosoma) der PantopodaGerstaecker 1863 I Entstehung und Struktur des Zentralnerven-systems Zeitschrift fur Zoologische Systematik und Evolutionsfor-schung 18 27ndash61

Zhang W amp Hou X 1985 Preliminary notes on the occurrence of theunusual trilobite Naraoia in Asia Acta Palaeontologica Sinica 24591ndash5 [In Chinese with English summary]

TREVOR J COTTON Department of Palaeontology The Natural History Museum CromwellRoad London SW7 5BD UKe-mail trevor_cottonhotmailcom

SIMON J BRADDY Department of Earth Sciences University of Bristol Wills MemorialBuilding Queenrsquos Road Bristol BS8 1RJ UKe-mail S J Braddybrisacuk

MS received 28 July 2003 Accepted for publication 15 March 2004

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 193

Page 21: The phylogeny of arachnomorph arthropods and the origin of the

for the terrestrialization of chelicerates (Dunlop 1997 p 69Dunlop amp Webster 1999) but the model proposed heresuggests that this considerably pre-dated terrestrializationGnathobasic feeding in xiphosurans and eurypterids mayrepresent a reversal to a more primitve benthic arachnomorphlifestyle from a more pycnogonid-like ancestor

32 Evolution of the arachnomorph headThe extant arthropod classes show highly conserved patternsof head segmentation (eg Wills et al 1994) In contrast therehas been a near consensus amongst Cambrian arthropodworkers (although rarely based on an explicit phylogenetichypothesis) that patterns of head segmentation are highlyhomoplastic and hence of little systematic significanceSturmer amp Bergstrom (1978 pp 78ndash9) suggested that lsquoevenclosely related forms may have different numbers of headsegments and appendagesrsquo Bruton amp Whittington (1983pp 576ndash7) that lsquodiscussion on fossil arthropod relationshipsbased on head segmentation appears to be largely irrel-evant and at best speculativersquo and Delle Cave amp Simonetta(1991 p 191) that lsquothere are no obvious phyletic affinitiesbetween genera having the same number of cephalized seg-mentsrsquo Most recently Bergstrom amp Hou (1997 p 104) con-cluded that lsquothe segmental length of the head shield seemsto be of no relevance to the discussion and discrimination ofevolutionary lineagesrsquo It has been claimed that this view issupported by cladistic analyses in which patterns of tagmosishave been found to be rather poor at defining major arthropodclades (Briggs et al 1992 Wills et al 1994)

This suggestion is not only of relevance to systematics buthas featured prominently in recent debates surrounding thenature of the lsquoCambrian explosionrsquo (see Budd amp Jensen 2000for a recent review) Gould (1989 1991) argued that theapparent plasticity of head segmentation in Cambrian com-pared to post-Cambrian arthropod evolution suggests thatbody-plan evolution was only constrained after the Cambrianexplosion According to this view the origin of new highertaxa with distinct body-plans was only possible before theonset of these constraints (eg Valentine 1995) Taxa hererecognised as arachnomorphs clearly played a major role in thedevelopment of Gouldrsquos hypothesis This suggestion has pre-viously been assessed in terms of overall morphological diver-sity which is clearly rather distinct from Gouldrsquos concept ofdisparity Wills et al (1998b) showed that there was littlecorrelation between overall morphological disparity and de-gree of limb specialisation and that the latter was phylogeneti-cally highly plastic They remarked that (Wills et al 1998bp 64) lsquothis finding has important implications for models ofarthropod phylogeny and evolution that attribute overidingimportance to head segmentationrsquo but did not explicitlyexamine head segmentation although it is likely to have beena major component of the index of tagmosis employed

The results of the present study suggest that arguments thatarthropod head segmentation was unusually labile during theCambrian are poorly founded Rather only four major pat-terns of euarthropod head segmentation are identified Theplesiomorphic euarthropod state according to both Walossekamp Muller (1997 1998) and Scholtz (1997) which may moreproperly be the plesiomorphic state for the euarthropod crowngroup only (depending on the phylogenetic position of theMarrellomorpha) consists of four post-acronal segmentsbearing the antennae and three pairs of biramous limbs Thepresent authors suggest that the term lsquocephalonrsquo be restrictedto this kind of head which is found in stem group mandibu-lates in addition to many arachnomorphs Crown groupmandibulates share a lsquobimaxillary headrsquo in which an addi-tional pair of appendages the labium or second maxillae are

incorporated into the head but in crustaceans these are notfused to the carapace (Scholtz 1997) Finally two distinctforms of head tagmosis are found in chelicerates A head withfour pairs of appendages (but without antennae giving a totalof five segments incorporated into the head) which has beencalled the lsquocephalosomarsquo is present in pycnogonids (Walossekamp Dunlop 2002 pp 434ndash5) and probably also in some euch-elicerates (J A Dunlop pers comm 2003) The lsquoprosomarsquo ofmost euchelicerates consists of this cephalosoma and twoadditional segments

In general these patterns of head segmentation are highlyphylogenetically conserved (see Fig 12) Only three homo-plastic changes namely reversal to a more primitive conditionin the Leanchoilia-Alalcomenaeus clade and the convergentorigin of a five-segmented head in Clade 3 (of Fig 8) and theXandarellida are necessary to optimise character 4 onto themost parsimonious cladogram There are also transitions toautapomorphic states in Emeraldella and Sidneyia In the caseof Sidneyia however an argument could be made (followingBergstrom amp Sturmer 1978 see discussion of Character 4) thatthe head consists of five segments as in closely related taxabut autapomorphically each segment has a separate tergiteThis would further increase the phylogenetic stability of headsegmentation Also following Bergstrom amp Sturmer (1978) thehead of Cheloniellon could be considered to consist of twotergites making it more similar to the head of Emeraldella(with six pairs of appendages)

Figure 12 Cladogram from Fig 10 showing changes in head segmen-tation Bold type shows abbreviated head segmentation formulaAbbreviations (Ant) antennae (BA) biramous appendage pairs (Ch)chelicerae (GA) great appendages (UA) uniramous appendage pairsand (X) segment without appendages Synapomorphy A is the auta-pomorphic reduction in head length to a single segment in SidneyiaAlternatively this may represent secondary division of the headshield Synapomorphy B is the increase in the length of the head inEmeraldella to six segments

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 189

4 Acknowledgements

We thank D E G Briggs J A Cotton J A Dunlop R AFortey S Harris R A Moore P Selden O E Tetlie andP Van Roy for comments on earlier drafts of this work Themanuscript was reviewed by M Wills F Schram and oneanonymous referee This research was supported by the Geo-logical Society of London Curry Research Studentshipawarded to TJC by the Department of Earth Sciences Uni-versity of Bristol and by the Department of PalaeontologyThe Natural History Museum London TJC would like tothank C Labandeira J Thompson and R Chapman fortheir hospitality at the National Museum of Natural HistoryWashington DC USA and A Olcott S Schellenberg andJ Young for making his time there so enjoyable

5 References

Abzhanov A Popadic A amp Kaufman T C 1999 Chelicerate Hoxgenes and the homology of arthropod segments Evolution andDevelopment 1 77ndash89

Akam M 2000 Arthropods developmental diversity within a (super)phylum Proceedings of the National Academy of Sciences USA97 4438ndash41

Aldridge R J Gabbott S E amp Theron J N 2001 The Soom ShaleIn Briggs D E G amp Crowther P R (eds) Palaeobiology II340ndash2 Oxford Blackwell Science

Anderson L I amp Selden P A 1997 Opisthosomal fusion andphylogeny of Palaeozoic Xiphosura Lethaia 30 19ndash31

Arango C P 2002 Morphological phylogenetics of the sea spiders(Arthropoda Pycnogonida) Organisms Diversity and Evolution2 107ndash25

Ax P 1986 Das phylogenetische System Stuttgart Gustav FischerVerlag

Babcock L E amp Zhang Wentang In press New non-mineralizedarthropods from the Chengjiang Lagerstatte Lower CambrianYunnan China Palaeontologia Cathyana

Bartels C Briggs D E G amp Brassel G 1998 The fossils ofthe Hunsruck Slate Marine life in the Devonian CambridgeCambridge University Press

Bergstrom J 1971 Paleomerus ndash merostome or merostomoid Lethaia4 393ndash401

Bergstrom J 1979 Morphology of fossil arthropods as a guideto phylogenetic relationships In Gupta A P (ed) ArthropodPhylogeny 3ndash56 New York NY Van Nostrand Reinhold Co

Bergstrom J 1992 The oldest arthropods and the origin of theCrustacea Acta Zoologica 73 287ndash91

Bergstrom J Sturmer W amp Winter G 1980 PalaeoisopusPalaeopantopodus and Palaeothea pycnogonid arthropodsfrom the Lower Devonian Hunsruck Slate West GermanyPalaontologische Zeitschrift 54 7ndash54

Bergstrom J amp Brassel G 1984 Legs in the trilobite Rhenops fromthe Lower Devonian Hunsruck Slate Lethaia 17 67ndash72

Bergstrom J amp Hou X 1998 Chengjiang arthropods and theirbearing on early arthropod evolution In Edgecombe G D(ed) Arthropod fossils and phylogeny 151ndash84 New York NYColumbia University Press

Boudreaux H B 1979 Significance of intersegmental tendon systemin arthropod phylogeny and a monophyletic classification ofArthropoda In Gupta A P (ed) Arthropod Phylogeny 551ndash86New York NY Van Nostrand Reinhold Co

Bousfield E L 1995 A contribution to the natural classification ofLower and Middle Cambrian arthropods food-gathering andfeeding mechanisms Amphipacifica 2 3ndash33

Braddy S J Aldridge R J Gabbott S E amp Theron J N 1999Lamellate book-gills in a late Ordovician eurypterid from theSoom Shale South Africa support for a eurypterid-scorpionclade Lethaia 32 72ndash4

Braddy S J amp Almond J E 1999 Eurypterid trackways from theTable Mountain Group (Ordovician) of South Africa Journal ofAfrican Earth Sciences 29 165ndash77

Bremer K 1988 The limits of amino acid sequence data in angio-sperm phylogenetic reconstruction Evolution 42 795ndash803

Bremer K 1994 Branch support and tree stability Cladistics 10295ndash304

Briggs D E G 1998 Review of Arthropods from the LowerCambrian Chengjiang fauna southwest China by HouXianguang and Jan Bergstrom Fossils and Strata 45

Scandinavian University Press Oslo Norway 1997 Palaeo-geography Palaeoclimatology Palaeoecology 143 193ndash4

Briggs D E G Bruton D L amp Whittington H B 1979 Append-ages of the arthropod Aglaspis spinifer (Upper CambrianWisconsin) and their significance Palaeontology 22 167ndash80

Briggs D E G Fortey R A amp Wills M A 1992 Morphologicaldisparity in the Cambrian Science 256 1670ndash3

Briggs D E G Fortey R A amp Wills MA 1993 How big was theCambrian explosion A taxonomic and morphologic comparisonof Cambrian and Recent arthropods In Lees D R amp EdwardsD (eds) Evolutionary patterns and processes 33ndash44 LondonLinnean Society of London Linnean Society Symposium Series

Briggs D E G Erwin D H amp Collier F J 1994 The fossils of theBurgess Shale Washington DC Smithsonian Institution Press

Briggs D E G amp Bartels C 2001 New arthropods from the LowerDevonian Hunsruck Slate (Lower Emsian Rhenish Massifwestern Germany) Palaeontology 44 275ndash304

Briggs D E G amp Bartels C 2002 Magnoculocaris a new name forthe arthropod Magnoculus Briggs and Bartels 2001 from theLower Devonian Hunsruck Slate Germany Palaeontology 45419

Briggs D E G amp Collins D 1988 A Middle Cambrian cheliceratefrom Mount Stephen British Columbia Palaeontology 31 779ndash98

Briggs D E G amp Collins D 1999 The arthropod Alalcomenaeuscambricus Simonetta from the Middle Cambrian Burgess Shale ofBritish Columbia Paleontology 42 953ndash78

Briggs D E G amp Fortey R A 1982 The cuticle of aglaspididarthropods a red-herring in the early history of vertebratesLethaia 15 25ndash9

Briggs D E G amp Fortey R A 1989 The early radiation andrelationships of the major arthropod groups Science 246 241ndash3

Briggs D E G amp Fortey R A 1992 The early Cambrian radiationof arthropods In Lipps J H amp Signor P W (eds) Origin andEarly Evolution of the Metazoa 335ndash373 New York NY PlenumPress

Briggs D E G amp Robison R A 1984 Exceptionally preservednontrilobite arthropods and Anomalocaris from the MiddleCambrian of Utah University of Kansas PaleontologicalContributions Paper 111 1ndash23

Broili F 1932 Ein neuer Crustacee aus dem rheinischen UnterdevonSitzungsberichte der Mathematisch-Naturwissenschaftlichen(Abteilung) Klasse der Bayerischen Akademie der Wissenschaftenzu Munchen 1932 27ndash38

Broili F 1933 Ein zweites Exemplar von Cheloniellon Sitzungsber-ichte der Bayerischen Akademie der Wissenschaften 1933 11ndash32

Bruton D L 1981 The arthropod Sidneyia inexpectans MiddleCambrian Burgess Shale British Columbia Philosophical Trans-actions of the Royal Society of London Series B 295 619ndash56

Bruton D L amp Whittington H B 1983 Emeraldella and Leanchoiliatwo arthropods from the Burgess Shale British Columbia Philo-sophical Transactions of the Royal Society of London Series B 300553ndash85

Budd G E 1995 Kleptothule rasmusseni gen et sp nov anolenellid-like trilobite from the Sirius Passet fauna (BuenFormation Lower Cambrian North Greenland) Transactions ofthe Royal Society of Edinburgh Earth Sciences 86 1ndash12

Budd G E 1996a Progress and problems in arthropod phylogenyTrends in Ecology and Evolution 11 356ndash8

Budd G E 1996b The morphology of Opabinia regalis and thereconstruction of the arthropod stem-group Lethaia 29 1ndash14

Budd G E 1997 Stem-group arthropods from the Lower CambrianSirius Passet fauna of North Greenland In Fortey R A ampThomas R H (eds) Arthropod Relationships 125ndash38 LondonChapman and Hall Systematics Association Special Volume 55

Budd G E 1999a A nektaspid arthropod from the Early CambrianSirius Passet fauna with a description of retrodeformation basedon functional morphology Palaeontology 42 99ndash122

Budd G E 1999b The morphology and phylogenetic significance ofKerygmachela kiergkegaardi Budd (Buen Formation LowerCambrian N Greenland) Transactions of the Royal Society ofEdinburgh Earth Sciences 89 (for 1998) 249ndash290

Budd G E 2002 A palaeontological solution to the arthropod headproblem Nature 417 271ndash5

Budd G E amp Jensen S 2000 A critical reappraisal of the fossilrecord of the bilaterian phyla Biological Reviews 75 253ndash95

Chen J Zhou G Zhu M amp Yeh K 1996 The Chengjiang biota ndasha unique window of the Cambrian explosion Taichung NationalMuseum of Natural Science [In Chinese]

Chen J Edgecombe G D amp Ramskold L 1997 Morphological andecological disparity in naraoiids (Arthropoda) from the Early

190 TREVOR J COTTON AND SIMON J BRADDY

Cambrian Chengjiang fauna China Records of the AustralianMuseum 49 1ndash24

Chen J amp Zhou G 1997 Biology of the Chengjiang fauna In ChenJ Cheng Y amp Iten H V (eds) The Cambrian explosion and thefossil record Bulletin of the National Museum of Natural ScienceTaiwan 10 11ndash106

Child C A 1997 Some deep-sea Pycnogonida from Argentine slopeand basin Proceedings of the Biological Society of Washington110 128ndash42

Chlupac I 1988 The enigmatic arthropod Duslia from the Ordovicianof Czechslovakia Palaeontology 31 611ndash20

Chlupac I 1995 Lower Cambrian arthropods from the PasekyShale (Barrandian area Czech Republic) Journal of the CzechGeological Society 40 9ndash36

Chlupac I amp Havlıcek V 1965 Kodymirus n g a new aglaspidmerostome of the Cambrian of Bohemia Sbornık GeologickechVid Paleontologie 6 7ndash20

Cisne J L 1975 Anatomy of Triarthrus and the relationships of theTrilobita Fossils and Strata 4 45ndash63

Cisne J L 1981 Triarthrus eatoni (Trilobita) anatomy of its exo-skeletal skeletomuscular and digestive systems Palaeonto-graphica Americana 9 95ndash142

Conway Morris S 1982 Atlas of the Burgess Shale London ThePalaeontological Association

Conway Morris S 1989 The persistence of Burgess Shale-type faunasimplications for the evolution of deeper-water faunas Trans-actions of the Royal Society of Edinburgh Earth Sciences 80271ndash83

Conway Morris S 1998 The crucible of creation Oxford OxfordUniversity Press

Cotton T J 1999 Review of Arthropod fossils and phylogenyPalaeogeography Palaeoclimatology Palaeoecology 151 341ndash3

Cotton T J 2002 The phylogeny and morphological evolution ofCambrian trilobites and their relatives Unpublished PhD ThesisUniversity of Bristol

Damen W G M Hausdorf M Seyfarth E A amp Tautz D 1998 Aconserved mode of head segmentation in arthropods revealed bythe expression pattern of Hox genes in a spider Proceedings of theNational Academy of Sciences USA 95 10665ndash70

Delle Cave L amp Simonetta A M 1991 Early Palaeozoic arthropodsand problems of arthropod phylogeny with some notes on taxa ofdoubtful affinities In Simonetta A M amp Conway Morris S(eds) The early evolution of Metazoa and the significance ofproblematic taxa 189ndash244 Cambridge Cambridge UniversityPress

Dewel R A amp Dewel W C 1997 The place of tardigrades inarthropod evolution In Fortey R A amp Thomas R H (eds)Arthropod Relationships 109ndash23 London Chapman and HallSystematics Association Special Volume 55

Dunlop J A 1997 Palaeozoic arachnids and their significance forarachnid phylogeny Proceedings of the 16th European Colloquiumof Arachnology 65ndash82 Siedlce Poland

Dunlop J A 1998 The origins of tetrapulmonate booklungs and theirsignificance for chelicerate phylogeny In Selden P A (ed)Proceedings of the 17th European Colloquium of ArachnologyEdinburgh 1997 pp 9ndash16

Dunlop J A 1999 Pasando revista a la evolucion de los quelicerados[A review of chelicerate evolution] In Melic A DeHaro J JMedez M amp Ribera I (eds) Volumen Monografico Evolucion yFilogenia de Arthropoda Boletin de la Sociedad EntolomologicaAragonesa 26 255ndash272 [In Spanish and English]

Dunlop J A amp Selden P A 1997 The early history and phylogenyof the chelicerates In Fortey R A amp Thomas R H (eds)Arthropod relationships Systematics Association Special Volume55 221ndash35 London Chapman and Hall

Dunlop J A amp Webster M 1999 Fossil evidence terrestrializationand arachnid phylogeny Journal of Arachnology 27 86ndash93

Dzik J 1993 Early metazoan evolution and the meaning of its fossilrecord Evolutionary Biology 27 339ndash86

Dzik J amp Lendzion K 1988 The oldest arthropods of the EastEuropean Platform Lethaia 21 29ndash38

Edgecombe G D Wilson G D F Colgan D J Gray M R ampCassis G 2000 Arthropod cladistics combined analysisof Histone H3 and U2 snRNA sequences and morphologyCladistics 16 155ndash203

Edgecombe G D amp Ramskold L 1999 Relationships of CambrianArachnata and the systematic position of Trilobita Journal ofPaleontology 73 263ndash87

Emerson M J amp Schram F R 1997 Theories patterns and realitygame plan for arthropod phylogeny In Fortey R A amp Thomas

R H (eds) Arthropod Relationships Systematics AssociationSpecial Volume 55 67ndash87 London Chapman and Hall

Farris J S 1989 The Retention Index and the Rescaled ConsistencyIndex Cladistics 5 417ndash19

Felsenstein J 1985 Confidence limits on phylogenies an approachusing the bootstrap Evolution 39 783ndash91

Flower R H 1968 Merostomes from a Cotter Horizon of the El PasoGroup New Mexico Bureau of Mines and Mineral ResourcesMemoir 22 35ndash44

Fortey R A 1990a Trilobite evolution and systematics In MikulicD G (ed) Arthropod paleobiology 44ndash65 Short Courses inPaleontology 3 Knoxville Tenessee The Paleontological Society

Fortey R A 1990b Ontogeny hypostome attachment and trilobiteclassification Palaeontology 33 529ndash76

Fortey R A 1997 Classification In Kaesler R L (ed) Treatise oninvertebrate paleontology Part O Arthropoda 1 (revised) 289ndash302Boulder Colorado amp Lawrence Kansas Geological Society ofAmerica and University of Kansas Press

Fortey R A amp Theron J N 1994 A new Ordovician arthropodSoomaspis and the agnostid problem Palaeontology 37 841ndash61

Fortey R A amp Whittington H B 1989 The Trilobita as a naturalgroup Historical Biology 2 125ndash38

Fry W G 1978 A classification within the pycnogonids ZoologicalJournal of the Linnean Society of London 63 35ndash58

Giribet G Edgecombe G D amp Wheeler W C 2001 Arthropodphylogeny based on eight molecular loci and morphology Nature413 157ndash61

Gould S J 1989 Wonderful life The Burgess Shale and the nature ofhistory New York NY Norton

Gould S J 1991 The disparity of the Burgess Shale arthropod faunaand the limits of cladistic analysis why we must strive to quantifymorphospace Paleobiology 17 411ndash23

Gurich G 1931 Mimetaster hexagonalis ein neuer Kruster aus demunterdevonischen Bundenbacher Dachschiefer PalaontologischeZeitschrift 12 204ndash38

Hammann W Laske R amp Pillola G L 1990 Tariccoia arrusensisngnsp an unusual trilobite-like arthropod Rediscovery of thelsquophylocaridrsquo beds of Taricco (1922) in the Ordovician lsquoPuddingarsquosequence of Sardinia Bolletino della Societa PaleontologicaItaliana 29 163ndash78

Haas M S Brown S J amp Beeman R W 2001 Pondering theprocephalon the segmental origin of the labrum DevelopmentGenes and Evolution 211 89ndash95

Heider K 1913 Entwicklungsgeschichte und Morphologie derWirbellosen Die Kultur der Gegenwart Theil 3 Abt 4 2 176ndash332

Henriksen K 1928 Critical notes upon Cambrian arthropodsdescribed by Charles D Walcott Videnskabelige Meddelelser fraDansk Naturhistorisk Forening 86 1ndash20

Hesselbo S P 1988 Trace fossils of Cambrian aglaspidid arthropodsLethaia 21 139ndash46

Hesselbo S P 1989 The aglaspidid arthropod Beckwithia from theCambrian of Utah and Wisconsin Journal of Paleontology 63636ndash42

Hesselbo S P 1992 Aglaspidida (Arthropoda) from the UpperCambrian of Wisconsin Journal of Paleontology 66 885ndash923

Hou X 1987a Three new large arthropods from Lower CambrianChengjiang fauna eastern Yunnan Acta Palaeontologica Sinica26 272ndash85 [In Chinese with English summary]

Hou X 1987b Two new arthropods from Lower CambrianChengjiang eastern Yunnan Acta Palaeontologica Sinica 26236ndash56 [In Chinese with English summary]

Hou X Chen J amp Lu H 1989 Early Cambrian new arthropodsfrom Chengjiang Yunnan Acta Palaeontologica Sinica 28 42ndash57[In Chinese with English summary]

Hou X Ramskold L amp Bergstrom J 1991 Composition andpreservation of the Chengjiang fauna a Lower Cambrian soft-bodied biota Zoologica Scripta 20 395ndash411

Hou X Bergstrom J amp Ahlberg P 1995 Anomalocaris and otherlarge animals in the Lower Cambrian Chengjiang fauna ofsouthwest China GFF 117 163ndash83

Hou X amp Bergstrom J 1997 Arthropods of the Lower CambrianChengjiang fauna southwest China Fossils and Strata 45 1ndash116

Hughes C P 1975 Redescription of Burgessia bella from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 4415ndash36

Ivantsov A Y 1999 Trilobite-like arthropod from the LowerCambrian of the Siberian Platform Acta PalaeontologicaPolonica 44 455ndash66

King P E 1973 Pycnogonids London St Martin Press

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 191

Lauterbach K E 1973 Schlussereignisse in der Evolution derStammgruppe der Euarthropoda Zoologische Beitrage 19259ndash99

Lauterbach K E 1980 Schlussereignisse in der Evolution derGrundplans der Arachnata (Arthropoda) Abhandlungen desNaturwissenschaftlichen Vereins in Hamburg 23 163ndash327

Lauterbach K E 1983 Synapomorphien zwischen Trilobiten- undCheliceraten-zweig der Arachnata Zoologischer Anzeiger 210213ndash38

Lendzion K 1975 Fauna of the Mobergella Zone in the Polish LowerCambrian Kwartalnik Geologiczny 19 237ndash42

Lieberman B S 1998 Cladistic analysis of the Early Cambrianolenelloid trilobites Journal of Paleontology 72 59ndash78

Lu Y 1940 On the ontogeny and phylogeny of Redlichia intermediaLu (sp nov) Bulletin of the Geological Society of China 20333ndash42 [In Chinese]

Maddison W P amp Maddison D R 1997 MacClade analysis ofphylogeny and character evolution Version 307 Sunderland MASinauer Associates

Mittmann B amp Scholtz G 2003 Development of the nervous systemin the lsquoheadrsquo of Limulus polyphemus (Chelicerata Xiphosura)morphological evidence for the correspondence between the seg-ments of the chelicerae and the (first) antennae of MandibulataDevelopment Genes and Evolution 213 9ndash17

Moore R C (ed) 1959 Treatise on invertebrate paleontology Part OArthropoda 1 Boulder Colorado Geological Society of America

Muller K J amp Walossek D 1986 Arthropod larvae from the UpperCambrian of Sweden Transactions of the Royal Society ofEdinburgh Earth Sciences 77 157ndash79

Muller K J amp Walossek D 1987 Morphology ontogeny and lifehabit of Agnostus pisiformis from the Upper Cambrian of SwedenFossils and Strata 19 1ndash124

Munilla T 1999 Evolucion y filogenia de los Picnogonidos InMelicA DeHaro J J Medez M amp Ribera I (eds) VolumenMonografico Evolucion y Filogenia de Arthropoda Boletin de laSociedad Entolomologica Aragonesa 26 273ndash91 [In Spanish andEnglish]

Orr P J Siveter D J Briggs D E G Siveter D J amp Sutton M D2000 A new arthropod from the Silurian Konservat-Lagerstatteof Herefordshire Proceedings of the Royal Society of LondonSeries B 267 1497ndash504

Page R D M 1996 On consensus confidence and lsquototal evidencersquoCladistics 12 83ndash92

Palmer A R amp Repina L N 1993 Through a glass darklytaxonomy phylogeny and biostratigraphy of the OlenellinaUniversity of Kansas Paleontological Contributions New Series 31ndash35

Platnick N I Griswold C E amp Coddington J A 1991 On missingentries in cladistic analysis Cladistics 7 337ndash43

Pleijel F 1995 On character coding for phylogeny reconstructionCladistics 11 309ndash15

Raasch G O 1939 Cambrian Merostomata Special Papers of theGeological Society of America 19 1ndash146

Ramskold L Chen J Edgecombe G D amp Zhou G 1996Preservational folds simulating tergite junctions in tegopeltid andnaraoiid arthropods Lethaia 29 15ndash20

Ramskold L Chen J Edgecombe G D amp Zhou G 1997Cindarella and the arachnate clade Xandarellida (ArthropodaEarly Cambrian) from China Transactions of the Royal Society ofEdinburgh Earth Sciences 88 19ndash38

Ramskold L amp Edgecombe G D 1991 Trilobite monophylyrevisited Historical Biology 4 267ndash83

Ramskold L amp Edgecombe G D 1996 Trilobite appendagestructure ndash Eoredlichia revisited Alcheringa 20 269ndash76

Raw F 1957 Origin of chelicerates Journal of Paleontology 31139ndash193

Repina L N amp Okuneva O G 1969 Cambrian arthropods of themaritime territory Paleontological Journal 1969 95ndash103

Richter R amp Richter E 1929 Weinbergina opitz ng n sp einSchwerttrager (Merost Xiphos) aus dem Devon (Rheinland)Senkenbergiana 11 193ndash209

Robison R A 1984 New occurences of the unusual trilobite Naraoiafrom the Cambrian of Idaho and Utah University of KansasPaleontological Contributions Paper 112 1ndash8

Rominger C 1887 Description of primordial fossils from MtStephens NW Territory of Canada Proceedings of the Academyof Natural Sciences of Philadelphia 1887 12ndash19

Scholtz G 1997 Cleavage germ band formation and head segmen-tation the ground pattern of the Euarthropoda In Fortey R Aamp Thomas R H (eds) Arthropod Relationships 317ndash32 LondonChapman and Hall Systematics Association Special Volume 55

Schram F R 1978 Arthropods a convergent phenomenonFieldiana Geology 39 61ndash108

Schram F R 1993 Review of Boxshall Stromberg and Dahl TheCrustacea Origin and Evolution Journal of Crustacean Biology13 820ndash2

Schram F R Vonk R amp Hof C H J 1997 Mazon CreekCycloidea Journal of Paleontology 71 261ndash84

Schram F R amp Emerson M J 1991 Arthropod Pattern Theorya new approach to arthropod phylogeny Memoirs of theQueensland Museum 31 1ndash18

Schram F R amp Hof C H J 1998 Fossils and the interrelationshipsof major crustacean groups In Edgecombe G D (ed) Arthropodfossils and phylogeny pp 233ndash302 New York NY ColumbiaUniversity Press

Shergold J H 1991 Protaspis and early meraspis growth stages of theeodiscoid trilobite Pagetia ocellata Jell and their implications forclassification Alcheringa 15 65ndash86

Shu D Geyer G Ling C amp Zhang Z 1995 Redlichiacean trilobiteswith preserved soft parts from the Lower Cambrian Chengjiangfauna (South China) Beringeria (Special Issue) 2 203ndash41

Shultz J W 1990 Evolutionary morphology and phylogeny ofArachnida Cladistics 6 1ndash38

Shultz J W 2000 Skeletomuscular anatomy of the harvestmanLeiobunum aldrichi (Weed 1893) (Arachnida OpilionesPalpatores) and its evolutionary significance Zoological Journalof the Linnean Society 128 401ndash38

Shultz J W 2001 Gross muscular anatomy of Limulus polyphemus(Xiphosura Chelicerata) and its bearing on evolution in the theArachnida Journal of Arachnology 29 283ndash303

Siewing R (ed) 1985 Lehrbuch der Zoologie II Systematik 3rd ednStuttgart Fischer

Simonetta A M 1970 Studies on the non trilobite arthropods ofthe Burgess Shale (Middle Cambrian) the genera LeanchoiliaAlalcomenaeus Opabinia Burgessia Yohoia and ActaeusPalaeontographica Italica 66 35ndash45

Simonetta A M amp Delle Cave L 1975 The Cambrian non trilobitearthropods from the Burgess Shale of British Columbia A studyof their comparative morphology taxonomy and evolutionarysignificance Palaeontographica Italica 69 1ndash37

Simonetta A M amp Delle Cave L 1981 An essay in the compara-tive and evolutionary morphology of Palaeozoic ArthropodsAccademia Nazionale dei Lincei Atti dei Convegni Lincei 49389ndash439

Starobogatov Y I 1990 Sistema i filogeniya nizshikh khelitserovykh(analiz morfologii paleozoysikh grupp) [The systematics andphylogeny of the lower chelicerates (a morphological analysis ofthe Palaeozoic groups)] Paleontologicheskii Zhurnal 1990 4ndash17

Stoslashrmer L 1944 On the relationships and phylogeny of fossil andrecent Arachnomorpha Skrifter utgitt ac Det Norske Videnskaps-Akademi i Olso I Matematisk-naturvitenskaplig Klasse 5 1ndash158

Stoslashrmer L 1951 Studies on trilobite morphology Part III Theventral cephalic structures with remarks on the zoologicalposition of the trilobites Norsk geologisk tidsskrift 29 108ndash58

Stoslashrmer L 1956 A Lower Cambrian merostome from Sweden Arkivfor Zoologi Ser 2 9 507ndash14

Stoslashrmer L 1959 Trilobitoidea In Moore R C (ed) Treatise onInvertebrate Paleontology part O Arthropoda 1 23ndash37 BoulderColorado Geological Society of America

Strong E E amp Lipscomb D 1999 Character coding and inapplicabledata Cladistics 15 363ndash71

Sturmer W amp Bergstrom J 1976 The arthropods Mimetaster andVachonisia from the Devonian Hunsruck Shale PalaontologischeZeitschrift 50 78ndash111

Sturmer W amp Bergstrom J 1978 The arthropod Cheloniellon fromthe Devonian Hunsruck Shale Palaontologische Zeitschrift 5257ndash81

Sturmer W amp Bergstrom J 1981 Weinbergina a xiphosuran fromthe Devonian Hunsruck Slate Palaontologische Zeitschrift 55237ndash55

Sutton M D Briggs D E G Siveter D J Siveter D J amp Orr PJ 2002 The arthropod Offacolus kingi (Chelicerata) from theSilurian of Herefordshire England computer based morphologi-cal reconstructions and phylogenetic affinity Proceedings of theRoyal Society of London Series B 269 1195ndash1203

Swofford D L 1999 PAUP Phylogenetic Analysis Using Parsimony(and other methods) Version 40b2a Sunderland MA SinauerAssociates

Telford M J amp Thomas R H 1998 Expression of homeobox genesshows chelicerate arthropods retain their deutocerebral segmentProceedings of the National Academy of Sciences USA 9510671ndash5

192 TREVOR J COTTON AND SIMON J BRADDY

Thorley J L amp Page R D M 2000 RadCon phylogenetic treecomparison and consensus Bioinformatics 16 486ndash7

Valentine J W 1995 Why no new phyla after the Cambrian ndashgenome and ecospace hypotheses revisited Palaios 10 190ndash194

Wagele J W 1993 Rejection of the lsquoUniramiarsquo hypothesis andimplications of the Mandibulata concept Zoologische JahrbucherAbteilung fur Systematik Geographie und Biologie der Tiere 120253ndash88

Walcott C D 1911 Middle Cambrian Merostomata Cambriangeology and palaeontology II Smithsonian MiscellaneousCollections 57 17ndash40

Walcott C D 1912 Middle Cambrian Branchiopoda MalacostracaTrilobita and Merostomata Cambrian geology and palaeontol-ogy II Smithsonian Miscellaneous Collections 57 145ndash228

Walcott C D 1918 Geological explorations in the CanadianRockies Explorations and fieldwork of the SmithsonianInstitution in 1917 Smithsonian Miscellaneous Collections 684ndash20

Walossek D 1993 The Upper Cambrian Rehbachiella kinnekullensisand the phylogeny of Branchiopoda and Crustacea Fossils andStrata 32 1ndash202

Walossek D amp Dunlop J A 2002 A larval sea spider (ArthropodaPycnogonida) from the Upper Cambrian lsquoOrstenrsquo of Swedenand the phylogenetic position of pycnogonids Palaeontology 45421ndash46

Walossek D amp Muller K J 1990 Upper Cambrian stem-lineagecrustaceans and their bearing upon the monophyletic origin ofCrustacea and the position of Agnostus Lethaia 23 409ndash27

Walossek D amp Muller K J 1997 Cambrian lsquoOrstenrsquo-type arthro-pods and the phylogeny of Crustacea In Fortey R A amp ThomasR H (eds) Arthropod Relationships 139ndash53 SystematicsAssociation Special Volume 55 London Chapman and Hall

Walossek D amp Muller K J 1998 Early arthropod phylogeny in lightof the Cambrian lsquoOrstenrsquo fossils In Edgecombe G D (ed)Arthropod fossils and phylogeny 185ndash231 New York NYColumbia University Press

Weygoldt P 1979 Significance of later embryonic stages and headdevelopment in arthropod phylogeny In Gupta A P (ed)Arthropod Phylogeny 107ndash35 New York NY Van NostrandReinhold Co

Weygoldt P 1998 Evolution and systematics of the ChelicerataExperimental and Applied Acarology 22 63ndash79

Weygoldt P amp Paulus H F 1979 Unterzuchungen aur Morpholo-gie Taxonomie und Phylogenie der Chelicerata Zeitschriftfur Zoologische Systematik und Evolutionsforschung 17 85ndash116177ndash200

Wheeler W C Cartwright P amp Hayashi C Y 1993 Arthropodphylogeny a combined approach Cladistics 9 1ndash39

Wheeler W C amp Hayashi C Y 1998 The phylogeny of extantchelicerate orders Cladistics 14 173ndash92

Whittington H B 1971 Redescription of Marrella splendens(Trilobitoidea) from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 209 1ndash24

Whittington H B 1974 Yohoia Walcott and Plenocaris n genarthropods from the Burgess Shale Middle Cambrian BritishColumbia Geological Survey of Canada Bulletin 231 1ndash21

Whittington H B 1975a Trilobites with appendages from the MiddleCambrian Burgess Shale British Columbia Fossils and Strata 497ndash136

Whittington H B 1975b The enigmatic animal Opabinia regalisMiddle Cambrian Burgess Shale British Columbia PhilosophicalTransactions of the Royal Society of London Series B 271 1ndash43

Whittington H B 1977 The Middle Cambrian trilobite NaraoiaBurgess Shale British Columbia Philosophical Transactions of theRoyal Society Series B 280 409ndash43

Whittington H B 1979 Early arthropods their appendages andrelationships In House M R (ed) The origin of majorinvertebrate groups 253ndash68 Systematics Association SpecialVolume 12 New York NY Academic Press

Whittington H B 1980 Exoskeleton moult stage appendage mor-phology and habits of the Middle Cambrian trilobite Olenoidesserratus Palaeontology 23 171ndash204

Whittington H B 1981 Rare arthropods from the Burgess ShaleMiddle Cambrian British Columbia Philosophical Transactionsof the Royal Society of London Series B 292 329ndash57

Whittington H B 1985 Tegopelte gigas a second soft-bodied trilo-bite from the Burgess Shale Middle Cambrian British ColumbiaJournal of Palaeontology 23 171ndash204

Whittington H B 1989 Olenelloid trilobites type species functionalmorphology and higher classification Philosophical Transactionsof the Royal Society of London Series B 324 111ndash47

Whittington H B amp Almond J E 1987 Appendages and habits ofthe Upper Ordovician trilobite Triarthus eatoni PhilosophicalTransactions of the Royal Society of London Series B 317 1ndash46

Wilkinson M 1992 Ordered versus unordered characters Cladistics8 375ndash85

Wilkinson M 1994 Common cladistic information and its consensusrepresentation reduced Adams and reduced cladistic consensustrees and profiles Systematic Biology 43 343ndash68

Wilkinson M 1995a Coping with abundant missing entries in phylo-genetic inference using parsimony Systematic Biology 44 501ndash14

Wilkinson M 1995b TAXEQ2 software and documentation BristolSchool of Biological Sciences University of Bristol

Wilkinson M 1996 On the distribution of homoplasy and choosingamong trees Taxon 45 263ndash66

Wills M A 1997 A phylogeny of Recent and fossil Crustacea derivedfrom morphological characters In Fortey R A amp ThomasR H (eds) Arthropod Relationships 189ndash209 SystematicsAssociation Special Volume 55 London Chapman and Hall

Wills M A Briggs D E G amp Fortey R A 1994 Disparity asan evolutionary index a comparison of Cambrian and Recentarthropods Paleobiology 20 93ndash130

Wills M A Briggs D E G Fortey R A amp Wilkinson M 1995The significance of fossils in understanding arthropod evolutionVerhandlungen der deutschen zoologischen Gesselschaft 88 203ndash15

Wills M A Briggs D E G Fortey R A Wilkinson M amp SneathP H A 1998a An arthropod phylogeny based on fossil andrecent taxa In Edgecombe G D (ed) Arthropod fossils andphylogeny 33ndash105 New York NY Columbia University Press

Wills M A Briggs D E G amp Fortey R A 1998b Evolutionarycorrelates of arthropod tagmosis scrambled legs In ForteyR A amp Thomas R H (eds) Arthropod Relationships 57ndash65Systematics Association Special Volume 55 London Chapmanand Hall

Winter G 1980 Beitrage zur Morphologie und Embryologie desvorderen Korperabschnitts (Cephalosoma) der PantopodaGerstaecker 1863 I Entstehung und Struktur des Zentralnerven-systems Zeitschrift fur Zoologische Systematik und Evolutionsfor-schung 18 27ndash61

Zhang W amp Hou X 1985 Preliminary notes on the occurrence of theunusual trilobite Naraoia in Asia Acta Palaeontologica Sinica 24591ndash5 [In Chinese with English summary]

TREVOR J COTTON Department of Palaeontology The Natural History Museum CromwellRoad London SW7 5BD UKe-mail trevor_cottonhotmailcom

SIMON J BRADDY Department of Earth Sciences University of Bristol Wills MemorialBuilding Queenrsquos Road Bristol BS8 1RJ UKe-mail S J Braddybrisacuk

MS received 28 July 2003 Accepted for publication 15 March 2004

PHYLOGENY OF ARACHNOMORPH ARTHROPODS 193

Page 22: The phylogeny of arachnomorph arthropods and the origin of the
Page 23: The phylogeny of arachnomorph arthropods and the origin of the
Page 24: The phylogeny of arachnomorph arthropods and the origin of the
Page 25: The phylogeny of arachnomorph arthropods and the origin of the

Phylogeny of Elatinaceae and the Tropical Gondwanan Origin

Arthropods & echinoderms

AP Biology 2/28/13 Chp.25 ~ Phylogeny & Systematics (& Chp.26~Review the Origin of Life, Dating Fossils)

Evans 1999. Origin of the Sulawesi Macaques (Cercopithecidae- Macaco) as Suggested by Mitochondrial DNA Phylogeny

Venomous Arthropods

The origin and evolution of arthropods - nhmThe origin and evolution of arthropods. The arthropods are one of the most familiar and ubiquitous of all ani- mal groups. They have far

OTHER ARTHROPODS

Stem Arthropods

Origin and phylogeny of velatid asteroids (Echinodermata

Arthropods Review

Ancient Neotropical origin and recent recolonisation: Phylogeny

Origin of vertebrates - Indiana University Bloomingtong404/Lectures/Lecture 2 - Origin of vertebrates... · Phylogeny and classification Origin of vertebrates. ... semicircular canals

Phylogeny, systematics, and origin of the … · MAISCH, PHYLOGENY, SYSTEMATICS, AND ORIGIN OF THE ICHTHYOSAURIA 153 Switzerland (early and late Lower Jurassic with Ichthyo- saurus,

TALL TALES FROM SMALL ANIMALS: DIVERSITY, PHYLOGENY … · TALL TALES FROM SMALL ANIMALS: DIVERSITY, PHYLOGENY AND BIOGEOGRAPHY OF NEGLECTED SOUTHERN AFRICAN ARTHROPODS INTRODUCTION

MORPHOLOGICAL PHYLOGENY OF ALPHEID · PDF fileevolution, 60(12), 2006, pp. 2507-2528 morphological phylogeny of alpheid shrimps: parallel preadaptation and the origin of a key morphological

Classical Biological Control of Arthropods Arthropod index in ......arthropods in Australia up to 2000, including (when available) their origin, year(s) of liberation, whether they

16.2 Arthropods. Objectives Describe the characteristics of arthropods. Identify the major kinds of arthropods. Explain how arthropods are classified

Origin, phylogeny and evolution of pineapple species · 2015-08-10 · Fruits - vol. 47, n°l, 1992 25 Origin, phylogeny and evolution of pineapple species . ORIGIN, PHYLOGENY AND

Arthropods evelyn

Origin of the Sulawesi macaques (Cercopithecidae: Macaca ... · Origin of the Sulawesi macaques (Cercopithecidae: Macaca) as suggested by mitochondrial DNA phylogeny BEN J. EVANS1,2,

Phylogeny, systematics, and origin of the Ichthyosauria – the

The Arthropod Assemblage of the Upper Devonian Strud ...799035/FULLTEXT01.pdf · 9 1. Introduction to the Arthropoda Morphology and phylogeny Arthropods are the largest living animal

Arthropods Chapter 28. Arthropods include: Insects, Arachnids, Crustaceans, Centipedes

The Early Cambrian origin of thylacocephalan arthropods · The Early Cambrian origin of thylacocephalan arthropods JEAN VANNIER, JUN−YUAN CHEN, DI−YING HUANG, SYLVAIN CHARBONNIER,

Molecular phylogeny of cuckoos supports a polyphyletic ...hera.ugr.es/doi/15011112.pdf · Molecular phylogeny of cuckoos supports a polyphyletic origin of brood parasitism S. ARAGO´N,*A.P.MØLLER,*J.J.SOLER€&

ARTHROPODS. Insects, spiders, crabs, and lobsters are all arthropods, so there are a lot of arthropods on Earth. The earliest arthropods on Earth developed

Phylogeny of Arthropods Arthropoda Worm-like Ancestor Trilobites (extinct) Annelids (worms) Onychophorans (worms w/legs) Chelicerates (spiders) Crustaceans

Phylum: Arthropods Arthropods are segmented coelomates with exoskeletons and jointed appendages Arthropods are segmented coelomates with exoskeletons and

Arthropods Biology

Arthropods By: Jeremy Maes. Arthropods All arthropods have jointed legs All arthropods have exoskeletons The 5 classes Insect Arachnids Diplopoda Chilipoda

7 Arthropods. Features of Arthropods Arthropod Diversity

Tracing Phylogeny. Phylogeny u Phylon = tribe, geny = genesis or origin u The evolutionary history of a species or a group of related species

Arthropods Notes

Arthropods 2013

Phylogeny of the Trichogramma endosymbiont Wolbachia an ... · Wolbachia infects the reproductive tissues of arthropods, are transmitted through the egg cytoplasm and cause several