the palaeoscolecida and the evolution of the ecdysozoa · atively slowly evolving nematode complete...

The Palaeoscolecida and the evolution of the EcdysozoaAndrey Yu. Zhuravlev1, José Antonio Gámez Vintaned2 and Eladio Liñán1

1Área y Museo de Paleontología, Departamento de Ciencias de la Tierra, Facultad de Ciencias, Universidad de Zaragoza, C/ Pedro Cerbuna, 12, E-50009 Zaragoza, Spain

2Área de Paleontología, Departamento de Geologica, Facultad de Ciencias Biológicas, Univeristat de València, C/ Doctor Moliner, 50, E-46100 Burjassot, Spain

Email: [email protected]

AbstrAct

Palaeoscolecidans are a key group for understanding the ear-ly evolution of the Ecdysozoa. The Palaeoscolecida possess a terminal mouth and an anus, an invertible proboscis with pointed scalids, a thick integument of diverse plates, sensory papillae and caudal hooks. These are features that draw a secret out of these worms, indicating palaeoscolecidan af-finities with the phylum Cephalorhyncha, which embraces priapulids, kinorhynchs, loriciferans and nematomorphs. At the same time, the Palaeoscolecida share a number of char-acters with the lobopod-bearing Cambrian ecdysozoans, the Xenusia. Xenusians commonly possess a terminal mouth, a proboscis (although not retractable), and a thick integu-ment of diverse plates. However, they also have telescopic lobopods similar to those of the Onychophora and the Tar-digrada. It seems likely that cephalorhynchs are descendents of xenusians, which lost walking appendages and acquired a retractable proboscis as well as a vermifom body during adaptation for a burrowing lifestyle. The cephalorhynchs could have evolved from limb-bearing xenusian-like ec-dysozoans via intermediate forms like the semi-burrowing Facivermis, with lobopods surrounding a terminal mouth and a slim vermiform abdomen, and not vice versa. Tardi-grads, onychophorans and anomalocaridids may represent other xenusian offsprings developed through morphological and behavioural evolution for interstitial, surface-dwelling and swimming lifestyles, respectively. This great variety of Cambrian transitional forms corresponds well with the Ec-dysozoa concept, whereas no such forms are found to sup-port the Articulata hypothesis.

rÉsUMÉ

Les Paléoscolécides sont un groupe clé pour la compréhen-sion des débuts de l’évolution des Ecdysozoa. Les Pal-aeoscolecida possèdent une bouche terminale et un anus, un proboscis inversible aux scalides pointues, un tégument épais de plaques diverses, des papilles sensorielles et des crochets caudaux. Ceux-ci sont des traits qui tirent un secret de ces vers, ce qui indique des affinités paléoscolecides avec le phylum des Cephalorhyncha qui inclut les priapulides, les kinorhynches, les loricifères et les nématomorphes. Cepen-dant, les Palaeoscolecida ont aussi quelques-uns des mêmes caractères que les Xenusia, ces écdysozaires cambriens qui portaient des lobopodes. Les xénusiens possèdent générale-ment une bouche terminale, un proboscis (quoique non re-tractile) et un tégument épais de plaques diverses. Cependant, ils ont aussi des lobopodes téléscopiques qui ressemblent à ceux des Onychophora et des Tardigrada. Il est très probable que les céphalorhynches sont les descendants de xénusiens qui ont perdu des appendices locomotaires et ont acquis un proboscis retractile de même qu’un corps vermiforme pen-dant l’adaptation à un mode de vie fouisseur. Les céphalor-hynches auriaent pu évoluer d’écdysozaires xénusimorphes qui avaient des membres, via des formes intermédiaires comme Facivermis semi-fouisseur, qui a une bouche ter-minale entourée de lobopodes et un abdomen vermiforme mince, et pas vice versa. Les tardigrades, les onychophores et les anomalocaridides pourraient représenter d’autres de-scendants xénusiens évolus au moins d’une évolution mor-phologique et comportementale pour la vie interstitielle, la vie sur la surface et la vie nageante respectivement. Cette grande variété de formes transitionnelles du Cambrien cor-respond bien à l’idée d’Ecdysozoa, tandis qu’on n’a trouvé aucune forme qui appuie la proposition d’Articulata.

From: Johnston, P.A., and Johnston K.J. (eds.), 2011. International Conference on the Cambrian Explosion, Proceedings. Palaeontographica Canadiana No. 31: 177–204. © 2011 Joint Committee on Paleontological Monographs for CSPG/GAC

PALAEONTOGRAPHICA CANADIANA, No. 31, 2011178

IntrodUctIon

For a long time, the Articulata paradigm, which assumes a unity of the Annelida and the Arthropoda, and the origin of the latter from annelid worms, led phylogenetic analyses (e.g., Nielsen, 1995). However, when Aguinaldo et al. (1997) assessed the phylogenetic position of nematodes using a rel-atively slowly evolving nematode complete 18S ribosomal (r) DNA gene sequence—also known as the small subunit rRNA gene, or SSU rRNA—this paradigm was questioned because the Nematoda turned out to be closer relatives of the Arthropoda than were the Annelida. In this molecular research, the nematode Trichinella spiralis, in addition to Caenorhabdites elegans, which was common in many stud-ies, had been used to eliminate the potential analytic problem of long-branch attraction. According to comparative data on partial gene sequences of small (18S) and large (28S) nu-clear subunits, which contain conservative areas permitting establishment of a degree of relationships between phylo-genetically remote groups (such as classes and phyla), as well as highly variable areas allowing clarification of affini-ties of close taxa (such as species), molluscs are united with brachiopods, other lophophorates, nemertines, sipunculids, echiurids, pogonophorans and annelids. In turn, arthropods are related to onychophorans, tardigrades, pentastomids, pri-apulids, kinorhynchs, nematomorphs and nematodes. These two groups represent major clades of protostomian animals: the Lophotrochozoa and the Ecdysozoa, respectively (Agui-naldo et al., 1997; Aleshin et al., 1998; Giribet, 1999; Garey, 2001).

This basic subdivision of the protostomian bilaterians has been subsequently supported by independent data sets, most notably Hox genes among which Ubx, abd-A, and Abd-B have ecdysozoan-specific peptides (de Rosa et al., 1999). Additionally, Haase et al. (2001) reported that the Ecdysozoa show common neural expression of horseradish peroxidase (HRP) immunoreactivity that is absent in other animals, and they suggested that the presence of anti-HRP-reactive glycoprotein(s) is a synapomorphy for this clade.

Further, Anderson et al. (2004) published data from the sodium-potassium ATPase α-subunit gene, which also fitted in with the Ecdysozoa hypothesis. Unfortunately, this re-search did not take annelids into consideration. Combined multigene studies of metazoan relationships from taxonomi-

cally well sampled molecular data sets including 18S rRNA and 28S rRNA (Mallatt and Giribet, 2006), 18S rRNA, myo-sin II, histone H3 and elongation factor 1-α (Giribet, 2003), and 18S rRNA, two 28S rRNA segments, cytochrome c oxidase subunit I, histone H3, and U2 small nuclear RNA (Colgan et al., 2008) also confirmed the Ecdysozoa unity. Further support comes from investigations of complete mi-tochondrial (mt) genome sequences (Webster et al., 2006; Podsiadlowski et al., 2008); of small non-coding regulatory genes, microRNAs, showing very low homoplasy, rare sec-ondary losses and unessential convergence (Sempere et al., 2007); and of multiple nuclear protein-encoding genes (Ba-guñà et al., 2008; Bourlat et al., 2008). The data on mtDNA are especially important because this molecule is remark-ably uniform across different groups of bilaterians, while genes encoded in mtDNA are compactly arrayed, separated without—or by only a few—nucleotides and contain neither introns nor regulatory sequences (Lavrov, 2007). Finally, the Ecdysozoa were validated by expressed sequence tags (EST), which permitted a reconstruction of evolutionary re-lationships based on short fragments of hundreds of thou-sands of genes (Roeding et al., 2007; Dunn et al., 2008). These strongly expressed fragments, obtaining by sequenc-ing random clones drawn from a complementary DNA li-brary, allow 150 genes to be identified as equivalent across a sample of 71 metazoans.

However, the critics of the Ecdysozoa concept continued to deny it as they found that the Ecdysozoa were merely a molecular clade (e.g., Wägele and Misof, 2001; Pilato et al., 2005). Also, some genome-scale analyses had claimed to refute the Ecdysozoa hypothesis (e.g., Philip et al., 2005; Rogozin et al., 2007), but these analyses were flawed ow-ing to limited taxon sampling, which included only three or four taxa in total, and an inability to correct phylogenies adequately for highly derived C. elegans sequences, i.e., a long-branch attraction artefact (Copley et al., 2004; Philippe et al., 2005; Irimia et al., 2007).

Etymologically, the name Ecdysozoa is derived from Greek εκ-δυω (which means, “to take off one’s cloth”), hinting at a property of all members of this clade, and only members of this clade to undergo a full ecdysis (to take off their complete cuticle), at least once during their life cycles. (Incomplete shedding, gradual shedding, or a regeneration of

text-fig. 1. Anatomy of Cambrian cephalorhynchs replaced by chlorite, Murero Lagerstätte, Aragón, Spain (A, E–upper Bilbilian Stage, Lower Cambrian. d, G–Caesaraugustan Stage, Middle Cambrian) and reconstructions of developmental stages of palaeoscolecidans (b, c, F). A–body cavity in palaeoscolecidan gen., and sp. indet., MPZ 2009/1235; b–Markuelia-type embryo (based on Dong et al., 2005); c–Shergoldana-type larva (based on Maas et al., 2007c); d, E–imprints of intestine and retractors onto integument (arrowed) in palaescolecidan Schistoscolex sp.: d–MPZ 2009/1237; E–MPZ 2009/1238; F–adult palaeoscolecid (based on Müller and Hinz-Schallreuter, 1993; Han et al., 2007): 1–mouth cone bearing pharyngeal sensory-masticatory teeth, 2–introvert bearing sensory-locomotory scalids, 3–abdomen (for the greatest part omitted), 4–plates, 5–microplates, 6–platelets, 7–sensory papillae, 8–sensory tubules, 9–pore canals, 10–caudal hooks; G–mid-gut diverticulae (arrowed) in cephalorhynch gen., and sp. indet., MPZ 2009/1239. Scale bar = 5 mm (A, D, E, G), 20 μm (B, C), 2 mm (F).

ZHURAVLEV et al.—Palaeoscolecida and evolution of Ecdysozoa 179


arthropod ground pattern and is revealed in metanaupli of branchiopod crustaceans, embryos of malacostracan crus-taceans, and in the development of Hexapoda and Xipho-sura (Harzsch, 2006).

The ring-type brain organization in the majority ecdyso-zoan groups is probably related to the mouth structure, which is represented by a more or less well expressed anterior pro-boscis, commonly by a retractable one or an introvert. The introvert incorporates brain and inner and outer retractor muscles and is covered with numerous concentric rows of generally hollow sensory-locomotory spines called scalids. The introvert, which is used for locomotion, is present—at least at a larval stage—in the Priapulida, the Loricifera, the Kinorhyncha and the Nematomorpha, which Malakhov (1980; Malakhov and Adrianov, 1995) included in the phylum Cephalorhyncha; Nielsen (1995) into the Introverta (includ-ing the Nematoda); and Lemburg (1999) into the Scalidopho-ra (excluding the Nematomorpha). Some nematodes possess an introvert also (Nielsen, 1995), while pycnogonids have a non-retractile proboscis and a triradiate pharynx typical of some cephalorhynchs (Eriksson and Budd, 2001b; Miyazaki, 2002). Tardigrades have a prominent telescoping mouth cone surrounded by a ring of plate-like peribuccal lamellae and a tri-radial bucco-pharyngeal apparatus armed with teeth that resembles a loriciferan mouth cone with its closing apparatus (collar) (Dewel and Dewel, 1997; Kristensen, 2002; Dewel and Eibye-Jacobsen, 2006). If the name “Proboscidea” were not affixed to a few mammalian species, it would be suitable for the Ecdysozoa—not including euarthropods.

Recent investigations on the embryogenesis of ecdysozo-ans have revealed an absence of any traces of spiral cleavage as well as ciliated larvae and a larval apical organ, which characterizes the other principal clade of protostomians, the Lophotrochozoa (Nielsen, 2003; Ungerer and Scholz, 2009). The same similarities in early embryogenesis are also ob-served among the Euarthropoda, the Tardigrada, the Pycno-gonida, the Nematoda and the Priapulida. Such an early em-bryogenesis is characterized by a holoblastic, irregular radial, equal to subequal cleavage pattern and gastrulation specified by large, division-retarded, immigrating blastomeres fol-lowed by smaller immigrating blastomeres (Hejnol and Sch-label, 2005; Schulze and Schierenberg, 2008; Ungerer and Scholz, 2009). In addition, the Ecdysozoa are direct-devel-opers, although this is not a unique feature of this clade (Sly et al., 2003).

Many more morphological features in common charac-terize stem-lineage ecdysozoans, whose fossil remains crowd Cambrian strata (542–490 Ma) especially in Lagerstätten such as the Burgess Shale in Canada, Chengjiang in China, the Sirius Passet in Greenland, Sinsk in Yakutia, Russia, Murero in Spain, and some others.

the cuticle is observed in some leeches, polychaetes and gas-trotrichs [Pilato et al., 2005].) In addition to this key-feature, which is relied upon presence of certain proteins responsible for moulting of the cuticle, the Ecdysozoa share other com-mon morphological features that have not been treated as important until the present time.

These principal features common to ecdysozoan body anatomy and embryology include: (1) the same type of cuticle; (2) a similar basic organisation of the nervous system; and (3) the same pathway of early embryogen-esis.

The ecdysozoan cuticle is differentiated into a lamel-lar epicuticle secreted by the tips of epidermal microvilli followed by a protein-containing exocuticle and a chitin-ous (α-chitin) fibrillary endocuticle. Its moulting is in-duced by ecdysone steroid hormones, as has been shown by euarthropods, onychophorans, tardigrades, priapulids, kinorhynchs, loriciferans and nematomorphs (Robson, 1964; Crowe et al., 1970; Plotnick, 1990; Lemburg, 1995; Schmidt-Rhaesa et al., 1998; Nielsen, 2001; Schmidt-Rhaesa, 2006). Among the Nematomorpha, adults appar-ently lack cuticular chitin, but the larval cuticle is the same as in loriciferan and priapulid larvae, and the posi-tion of chitin within the basal fibrillar cuticle layer cor-responds exactly to the location of cuticular chitin in the Loricifera and the Priapulida (Neuhaus et al., 1996). A chitinous cuticle is almost lost in the Nematoda but is still preserved in the structure of their pharynx (Neuhaus et al., 1997). By comparison, annelid cuticle is composed principally of layers of thick oriented collagen fibers, penetrated by numerous processes from the epithelial cells and, thus, differs radically from that of ecdysozo-ans (Robson, 1964). Chitin is apparently absent in nearly all annelid cuticles, and it is detected in annelid chaetae in the form of β-chitin only; in molluscs, bryozoans and brachiopods, γ-chitin is present (Plotnick, 1990). None of the ecdysozoans possesses either locomotory cilia as adults or larval cilia, but this is merely a consequence of the presence of moulting cuticle that prevents the devel-opment of any external soft tissues or cells.

A similar basic organization of the cerebrum among the majority of priapulids, loriciferans, kinorhynchs, nematomorphs, nematodes, tardigrades, onychophorans and pycnogonids is expressed in the presence of a cir-cumpharyngeal brain consisting of three successive rings and a ventral nerve cord (Eernisse, 1997; Dewel et al., 1999; Eriksson et al., 2003; Maxmen et al., 2005; Telford et al., 2008). Judging by this feature, the worm-like ec-dysozoans (+Gastrotricha) were even separated into the phylum Cycloneuralia (Nielsen, 1995). Regardless of the ancestral position of the mouth, an embryonic circum-oral nerve ring is thought to be most likely a part of an


lack key synapomorphies of crown-group priapulids. For in-stance, Middle Cambrian Ancalagon and Fieldia do not pos-sess a retractable proboscis (introvert), while all the extant priapulids have introverts (Conway Morris, 1977; Malakhov and Adrianov, 1995). The proboscis of Ancalagon bears non-differentiated scalids that very much resemble trunk spines. Nonetheless, the basic features of such worms as the Pal-aeoscolecida, Ancalagon and Fieldia still satisfy the defini-tion of the phylum Cephalorhyncha.

Thus, the representatives of the Class Palaeoscolecida Conway Morris and Robison (1986) possess such key fea-tures of the Cephalorhyncha as an introvert bearing a termi-nal mouth as well as sensory-locomotory scalids, a straight

cAMbrIAn VErMIForM EcdYsoZoAns

cAMbrIAn cEphALorhYnchs

These are the ecdysozoan cuticles that compose a significant part, in terms of both biovolume and biodiversity, of Cambri-an fossils including so-called “soft-bodied” ones (e.g., Con-way Morris, 1986; Ivantsov et al., 2005; Dornbos and Chen, 2008). Aside from arthropods, palaeoscolecidans, priapulids, other cephalorhynch worms, xenusians and anomalocari-dids represent principal groups of Cambrian ecdysozoans. It should be emphasized that many Cambrian “priapulids”

text-fig. 2. Muscle system of fossil and extant cephalorhynchs. A–scheme of longitudinal body section of female priapulid Tubiluchus arcticus Adrianov, Malakhov, Tchesunov and Tzetlin (modified from Malakhov and Adrianov, 1995); b–palaeoscolecidan Schistoscolex sp., chlorite, MPZ 2006/372 (retractors of introvert are arrowed); c–spiny plates from posterior end of palaeoscolecidan Wronascolex lubovae (Ivantsov and Wrona), phosphate, PIN 4349/850-1; d–circular muscle fibers in abdomen of louisellian Vladipriapulus malakhovi Ivantsov and A. Zhuravlev, phosphate, PIN 4349/819-1; E–transversely arranged plates on abdomen of palaeoscolecidan Piloscolex platum Ivantsov and A. Zhuravlev, phosphate, PIN 4349/670; F–scheme of abdomen cross section of priapulid Priapulus caudatus Lamarck (modified from Malakhov and Adrianov, 1995); G–possible palaeoscolecidan burrow, MPZ 2009/1240. 1–introvert, 2–scalids, 3–circumpharyngeal nerve ring, 4–circular muscles, 5–pharynx, 6–retractor muscules of introvert, 7–abdomen, 8–longitidinal muscles, 9–ventral nerve cord, 10–cuticle. A, F–extant; b, G–Caesaraugustan Stage, middle Cambrian, Murero Lagerstätte, Aragón, Spain; c–E, Botoman Stage, Lower Cambrian, Sinsk Lagerstätte, Sakha-Yakutia, Russia. Scale bar = 1 cm (B, G), 500 μm (A), 100 μm (D, F), 10 μm (C, E).


of 20 to 50 μm in size (Text-fig. 2C). A primary phosphatic composition of palaeoscolecidan sclerites is highly probable because even plates diagenetically altered into different clay minerals still keep traces of calcium and phosphate, while there is a lack of evidence for diagenetic phosphatic over-printing (Wrona, 2004; Han et al., 2007; Gámez Vintaned et al., 2009). In longitudinal thin sections, the cuticular plates display a three-layered microstructure of an extremely thin (2 to 3 μm), dense, finely laminated, outer layer characterized by: strong birefringence of apatite crystal and C-axis orien-tation perpendicular to the surface; a somewhat thicker ho-mogenous middle layer without pronounced birefringence; and a thick basal layer penetrated by vertical canals, and with only faint birefringence of apatite crystals indicating a weak-ly preferred orientation of C-axes parallel to the surfaces of the plate (Bengtson, 1977, text-fig. 4; Wrona, 1982, pl. 3, figs 1a, 5.; Bendix-Almgreen and Peel, 1988, fig. 7; Märss, 1988, pl. IV; Müller and Hinz, 1992, figs 14A, B, E, F; Brock and Cooper, 1993, fig. 10; Text-fig. 3B). The basal surface of a plate bears a regular cross-hatching system of canals (Müller, 1973, taf. 34, fig. 8a, b; Dzik, 1986, fig. 7B, D; Kraft and Mergle, 1989, pl. 6, fig. 4; van den Boogard, 1989, pl. 4, fig. 4; Müller and Hinz, 1992, fig. 14C, D).

A cuticle of similar tree-layered structure is observed in a rigid lorica of larval priapulids consisting of a thin lami-nated epicuticle, a middle homogenous layer and a thick basal layer penetrated by vertical cuticular canals; the lat-ter is underlain by a lamina pierced by oblique longitudinal canals housing cortical bandages consisting of collagen fi-bers (Malakhov and Adrianov, 1995, fig. 2.9B; Text-fig. 3A). The same type of cross-hatching collagenous fibers is also observed in the basal layer of nematomorph cuticle (Poinar, 1999, fig. 4). However, the thickness of palaeoscolecid fibers is two to five times greater than that of nematomorphs (mea-sured from Müller, 1973; Dzik, 1996; Poinar, 1999, respec-tively). It should be noted that ornamentation of Houscolex from the Lower Cambrian of South China (Zhang and Pratt, 1996, figs 2.5, 2.6) resembles that of extant adult nemato-morphs, which is built of minute platelets or areolae (Malak-hov and Adrianov, 1995, fig. 5.2A, B).

An absence of growth lines on palaeoscolecidan sclerites is indicative of animal growth by periodic moulting of the cuticle rather by an accretion of its elements. Furthermore, within a single taxon derived from the same sample unit, there are remarkable differences in abdomen diameter, and the smaller individuals possess the same ornamentation and arrangement of plates as the larger ones, but the interspace between the single plates is narrower due to an incomplete development of platelets. An older palaeoscolecidan cuticle was moulted during the life of the worm and replaced by a new one formed underneath the previous cover. Exactly such a pattern is known in three-dimensionally preserved palaeoscolecidans from the Middle Cambrian of Australia (Müller and Hinz-Schallreuter, 1993, text-figs 12B, 14C).

intestine, a terminal anus, caudal hooks, an abdomen with sensory spines and secretory papillae (Text-figs. 1A, D–F, 2B). Over the past 100 years, the Palaeoscolecida from Cam-brian to Silurian strata were compared to the Annelida due to a superficial resemblance of external annulation. From time to time, palaeoscolecidan cuticular plates, due to their multilayered structure, have been interpreted as scales of the oldest vertebrates (Dzik, 1996; Young et al., 1996). Since the 1990s, this group is assigned either to priapulids (Dzik and Krumbiegel, 1989; Han et al., 2007) or to nematomorphs (Hou and Bergström, 1994; Hou et al., 2004a)—despite sig-nificant differences in both of these taxa—or to basal Cepha-lorhyncha/Scalidophora (Ivantsov and Zhuravlev, 2005; Maas et al., 2007a; Harvey et al., 2010).

AnAtoMY And AFFInItIEs oF thE pALAEoscoLEcIdA

General Morphology and IntegumentsThe long, slender vermiform transversally circular body of palaeoscolecidans is subdivided into two principal parts: an introvert and an annulated abdomen (Text-figs. 1E, F, 2B). The annulation is homonomous. The introvert differ-entiation is not well expressed. It bears a mouth cone with papillae and a central field with posteriorly directed senso-ry-locomotory scalids. Both these types of appendages are arranged in circles and stand alternately between adjacent rings (Han et al., 2003; Maas et al., 2007a; Conway Morris and Peel, 2010). In papers on Cambrian palaeoscolecidans, these projections are coined variably as proboscis spines and proboscis hooks (Han et al., 2003, text-fig. 1), spines and scalids (Maas et al., 2007a, fig. 1B, C), stout spines and scalids (Conway Morris and Peel, 2010, figs. 4A, 5), re-spectively. A bursa, which is a short smooth posterior abdo-men extension, is recognizable on some palaeoscolecidans from the Lower Cambrian of the Sirius Passet Lagerstätte (Conway Morris and Peel, 2010; fig. 2C). In some pal-aeoscolecidans from the Middle Cambrian of central Aus-tralia and the Lower Cambrian of Chengjiang, the posterior end of the abdomen is terminated with paired caudal hooks (Müller and Hinz-Schallreuter, 1993, text-fig. 11B, G; Hou and Bergström, 1994, figs 1B, 2B, 4B) as in some extant priapulids (Malakhov and Adrianov, 1995, fig. 2.3B) and like the latero-terminal spines of kinorhynchs (Neuhaus and Higgins, 2002, fig. 3D; Text-fig. 1F).

The cuticle is built of various tessellating sclerites in-cluding plates, microplates and platelets in descending order (Text-figs. 1F, 2B, C, E). These structures form a repeatable transverse pattern imparting an annulated appearance to the entire abdomen. Cambrian strata beginning from the latest Atdabanian level (Cambrian Series 2, Stage 3) are abundant with these elements known as Milaculum, Lenargyrion, Had-imopanella and Kaimenella, as well as some other sclerites


(e.g., Tubiluchus corallicola). Finally, such a combination of the muscle system and rigid ornamented cuticle hardly al-lowed palaeoscolecidans easy surface-dwelling excursions.

nervous system A ventral nerve cord seems to be present in palaeoscolecidans because their cuticles preserve a narrow longitudinal struc-ture following the entire length of the abdomen (Müller and Hinz-Schallreuter, 1993, text-fig. 4B–D). Similarly, extant priapulids display a distinct deviation of cuticular ornamenta-tion along the ventral nerve cord, which is usually expressed by a paired row of scalids and other surface structures that are crowded together (Malakhov and Adrianov, 1995, fig. 2.2B).

sensory organs and Glands Sensory organs of palaeoscolecidans are represented by structures comparable in morphology and size with floscu-lae, tubules, sensory spines and papillae of extant priapulids (Text-fig. 1F). Flosculae are microscopic rounded plates with a central pore. Such microelements were figured by Müller and Hinz (1992, fig. 9C, F, I) from the Middle Cambrian of the Georgina Basin in Australia, although at that time, the authors interpreted them as individual skeletal organisms and compared them to Ediacaran Dickinsonia. In fossils, sensory-glandular tubules are difficult to distinguish from sensory spines because the former differ from the later by their function only. Both are elongated cones with a central opening as in some Australian palaeoscolecidans. However, certain smaller cones (Müller and Hinz-Schallreuter, 1993, text-fig. 5E) are possible to assign to either sensory spines or to tubules, while larger annulated nipple-like structures are probably papillae. Such structures are well known in a num-ber of phosphatized palaeoscolecidan cuticles where they are mostly restricted to the ventral side (Müller and Hinz-Schall-reuter, 1993, text-fig. 13; Zhang and Pratt, 1996, figs. 2.7, 2.8, 2.13). Even in non-phosphatized palaeoscolecidans, whose integuments have been replaced by clay minerals, the openings of papillae are still recognizable on the cuticle. Be-

Musculature The presence of a definite introvert in palaeoscolecidans from Murero is indicated by traces of powerful, long, retrac-tor muscles overprinted onto the integument (Text-figs. 1E, 2B). Probably, these imprints are preserved due to a thick collagen cover of retractors like that of extant priapulids; the cover ties individual muscle fibers together into thick bunches.

In palaeoscolecidans, introvert scalids are pointed back and abdominal plates, which construct the cuticle, are or-ganized in transverse bands to resist a back movement of the trunk when the worm burrows through sediment (Text-figs. 2C, E). To better fix the worm body, plates are arranged by their sharp ribs normally affixed to the body axis or else bear longer spines oriented rearward, thus, lowering the bur-rowing load of the introvert (Müller and Hinz-Schallreuter, 1993, text-fig. 10H; Han et al., 2003, pl. I, fig. 1a–c; Ivantsov and Zhuravlev, 2005, pl. XXI). This observation is supported by finds of palaeoscolecidans in their burrows (Zhang et al., 2006) as well as of some burrows bearing imprints of pal-aeoscolecidan cuticular bands (Text-fig. 2G). Furthermore, an absence of microborings, which are common in phosphat-ic shells of coeval epibenthic animals, indicates a burrowing lifestyle for palaeoscolecidans (Zhang and Pratt, 2008).

Thus, the palaeoscolecidans must have had the same type of hydrostatic skeleton allowing a peristaltic locomotion style and the same abdominal muscle system consisting of longitudinal and circular muscles as do large extant priapu-lids (Text-fig. 2A, F). In the absence of longitudinal muscles, the worm could not burrow completely, while an absence of circular muscles would not allow constriction of different parts of its body to regulate the motion of liquid within the body to create enough pressure for burrowing.

A differentiation of the abdomen ornamentation into dorsal and ventral sides (Kraft and Mergl, 1989; Han et al., 2003; Ivantsov and Zhuravlev, 2005; Conway Morris and Peel, 2010) does not contradict this lifestyle, as even in ex-tant burrowing priapulids such a differentiation is developed

text-fig. 3. Cuticle structure of extant and extinct ecdysozoans. A–extant larval priapulid Halicryptus spinulosus von Seibold (modified after Malakhov and Adrianov, 1995), b–extinct palaeoscolecidan (based on Bengtson, 1977; Dzik, 1986; Brock and Cooper, 1993); c–extinct xenusian (based on Bengtson et al., 1986; Dzik, 2003). 1–exocuticle, 2–second layer, 3–basal layer with vertical canals, 4–underlying mem-brane with horizontal canals after collagen threads. Scale bar = 20 μm.


2007a, fig. 1B, C). The scalids are simple and taper to a fine point from an expanded base (Conway Morris and Peel, 2010, figs 4B, 6G).

digestive system and body cavityIn the palaeoscolecidan digestive system, the presence of a terminal mouth, a straight intestine and a terminal anus is established (Conway Morris and Robison, 1986; Barskov and Zhuravlev, 1988; Müller and Hinz-Schallreuter, 1993). The mouth leads to a retractable pharynx armed with pha-

sides, circular pores are observed in some specimens (Brock and Cooper, 1993, fig. 9.14). Such pores penetrating the cuticle are known in nematodes (Wright and Hope, 1968), kinorhynchs (Malakhov and Adrianov, 1995, fig. 4.3a) and tardigrades (Crowe et al., 1970).

Spine-like scalids representing sensory-locomotory ap-pendages, papillae resembling abdominal tubules, and pha-ryngeal teeth are distinguishable in introverts of some Cam-brian palaeoscolecidans from Chengjiang (Han et al., 2003, text-fig. 1; Hou et al., 2004a, figs 11.1a, 11.5b; Maas et al.,

text-fig. 4. Cambrian xenusians. A–xenusian gen., and sp. indet. showing proboscis and stubby lobopods, chlorite, MPZ 2009/1241, upper-most Bilbilian Stage, Lower Cambrian, Murero Lagerstätte, Aragón, Spain; b, c–xenusian gen., and sp. indet., phosphate, Botoman Stage, Lower Cambrian, Sinsk Lagerstätte, Sakha-Yakutia, Russia: b–annulated abdomen with lobopods, PIN 4349/820-1, c–circular muscle fibers of a lobopod (a single muscle bundle is arrowed), PIN 4349/820-2. Scale bar = 1 cm (A, B), 100 μm (C).


and profusely annulated vermiform organism tightly coiled in an S-shaped or inverted S-shaped loop into a sphere, with anterior and posterior poles juxtaposed laterally. The ante-rior region exhibits a presumed terminal orifice proceeded by a smooth tapering field, the outer rim of which is adorned with intercalating rows of posteriorly directed conical spines, resembling larval scalids of extant priapulids and nemato-morphs. Markuelia lauriei (Haug et al., 2009), by its minute size (less than 320 μm), is an especially plausible candidate for a palaeoscolecidan pre-hatching larva. It possesses caudal hooks, anterior backward pointing hollow spines of a scalid-like shape, and an abdomen bearing tubules like those of lar-val priapulids. The elongated, vermiform body is folded in a globule inside an egg-shell, which is slightly compressed. In general such an embryo resembles those of both priapulids and nematomorphs.

If such Cambrian embryos belonged to palaeosco-lecidans, they were indeed direct developers like extant cephalorhynchs (Malakhov and Andrianov, 1995; Janssen et al., 2009).

Affinities of palaeoscolecidans and other Ancient cepha-lorhynchsThe morphological and anatomical features of the Palaeosco-lecida listed above allow us to place them within the Cepha-lorhyncha, close to the crown-group of this clade. In addition to a large number of fossils described as palaeoscolecidans, this class includes some other plate-bearing worms such as Tabelliscolex, Cricocosmia, Tylotites and, possibly, Mao-tianshania from the Lower Cambrian of South China (Text-fig. 5.5). The more definite position of the Palaeoscolecida within the Cephalorhyncha is indicated by a rigid trilaminate cuticle somewhat like that of larval priapulids, by a relatively primitive structure of introvert, and by a number of features shared by palaeoscolecidans with nematomorphs (some cu-ticular structures, straight intestine, larval development), xe-nusians (cuticular microstructure and structure; see below) and even with extant onychophorans (cuticular structure and sensory papillae; e.g., V.M.St.J. Read, University College of North Wales, School Animal Biology, unpublished data, 1985, pls 6.23b, 6.25d), tardigrades, kinorhynchs and nema-todes (cuticular pore canals). An especially striking similar-ity is observed between palaeoscolecidan and onychophoran integuments in both their ornamentation and size range of cuticular elements, despite an unhardened state of the latter (e.g., Manitouscolex in Lehnert and Kraft, 2006, fig. 4.6, 4.8; and Peripatopsis in Robson, 1964, fig. 2).

Such a mosaic character pattern demonstrated by pal-aeoscolecidans emphasizes their affinities with stem-lineage cephalorhynchs, abutting the crown-group, which includes priapulids, loriciferans and nematomorphs. Hou and Berg-ström (1994) drew attention to certain similarities of pal-aeoscolecidans with adult nematomorphs, which is expressed in a slim elongated body, a rigid cuticle with cross-hatching

ryngeal, usually simple, spine-like teeth (Hou et al., 2004a, fig. 11.1a; Maas et al., 2007a, fig. 2B, C; Conway Morris and Peel, 2010, fig. 4E).

Palaeoscolecidans from Murero demonstrate the pres-ence of a voluminous body cavity occupying the entire space between the body wall and the intestine (Text-fig. 1A). In a single extant priapulid (Meiopriapulus), the body cavity is represented by a true coelom, while other priapulids pos-sess a haemocoel (Malakhov and Adrianov, 1995; Park et al., 2006), which is impossible to distinguish in fossil material. However, a fixed axial position of the intestine in the major-ity of palaeoscolecidans (Text-fig. 1D) hints at the presence in their body of supportive dorso-ventral mesenteries, which in turn may be indicative of a coelom.

Ivantsov et al. (2005) suggested, judging by an ability of palaeoscolecidans to inhabit dysaerobic environments, that these worms relied on hemerythrin as an oxygen-transport-ing pigment similar to Recent priapulids. Without a doubt, the body cavity was filled with a liquid necessary for hydro-static locomotion of these extinct worms.

EmbryogenesisA probable juvenile palaeoscolecidan, named Shergoldana, was described from the Middle Cambrian of the Georgina Basin (Maas et al., 2007c; Text-fig. 1C). By its size range, cuticular ornamentation and precise restriction to the same horizon of the same locality where palaeoscolecidans of the genus Schistoscolex were discovered (Müller and Hinz-Schallreuter, 1993), it seems that Shergoldana is a juvenile form of one of them (cf. Müller and Hinz-Schallreuter, 1993, figs 10B, 11E and Maas et al., 2007c, fig. 3). Shergoldana is subdivided into a frontal region with a terminal depression surrounded by plicae of cuticular tissue, an annulated region covered with spiny plates of Schistoscolex-type, a posterior region bearing structures resembling sensory papillae of adult palaeoscolecidans, and a caudal region with a pair of caudal hooks. Besides, microhairs are recognisable on papil-lae, and the entire papilla is like a kinorhynch sensory spot (cf. Maas et al., 2007c, fig. 4C and Malakhov and Adrianov, 1995, fig. 4.18). In general, this juvenile form resembles, to a certain extent, extant larval cephalorhynchs, especially priapulids, loriciferans and nematomorphs, each of which possess similarly morphologically subdivided larvae. Prob-ably, Shergoldana is preserved with its introvert being fully retracted (cf. priapulid larva in Janssen et al., 2009, fig. 2F; and nematomorph larva in Malakhov and Adrianov, 1995, fig. 5.12A).

Some Cambrian fossil embryos assigned to Markuelia may be related to palaeoscolecidans also (Text-fig. 1B). A number of such embryos co-occur with palaeoscolecidans in Lower Cambrian lagerstätten of the Siberian Platform (Ivantsov and Zhuravlev, 2005, pl. XIV, fig. 13) and Middle Cambrian strata of the Georgina Basin (Donoghue et al., 2006; Haug et al., 2009). These are embryos of an elongate


vanced pentaradial arrangement of the pharyngeal teeth and 25-radial pattern of scalids, which is typical of many Recent priapulids (Adrianov and Malakhov, 2001), nonetheless pos-sess rather primitive abdomens and do display a different pattern of scalid series that occupy the anterior of the intro-vert only (Huang et al., 2004a, figs 2, 4c, d).

Larval Cambrian cephalorhynchs, having been described recently, are of special interest because they commonly com-bine features of different clades. Thus, Orstenoloricus from the Middle Cambrian of Australia possesses an annulated neck and an elongated sack-like abdomen formed by longi-tudinal plate-like structures and bearing several paired spine-like outgrowths (Maas et al., 2009). All these features are typical of both priapulid and loriciferan larvae (Malakhov and Adrianov, 1995, figs. 2.38 and 3.3). However, by their number, position and size, these spines are closer to larval tubules of priapulids rather than to locomotory spines of the Loricifera. Sirilorica from the Lower Cambrian of Sirius Passet is by now the earliest representative of larval cepha-lorhynchs (Peel, 2010). It has a lorica comparable to loricas of larval priapulids and loriciferans but 500 times longer than that of typical loriciferans and 150 times the length of priapulid larvae. Multicuspidate denticules preserved in a circlet just beyond the lorica can be compared in shape to the pharyngeal teeth of larval priapulids (e.g., Malakhov and Adrianov, 1995, fig. 2.37b) and to scalids in the seventh row of loriciferan Higgins larvae (e.g., Kristensen et al., 2007, fig. 5), while the sixfold symmetry of their arrangement is more like that of the Nematomorpha.

Cambrian fossil embryos of the ecdysozoans, known as Markuelia (Dong et al., 2005; Donoghue et al., 2006), prob-ably belong to vermiform ecdysozoans. The oral part of em-bryos display a radial folding pattern very much resembling the oral cone of extant cephalorhynchs (priapulids, kino-rhynchs, loriciferans) and that of their larvae, particularly the larvae of nematomorphs, which pass through the same phases of embryogenesis (Malakhov and Adrianov, 1995). These embryos demonstrate a direct development, and their post-embryonic forms, sometimes called larvae, are built according to the same body plan as the adults (Donoghue et al., 2006). Also, these microfossils reveal that the mouth of the most ancient ecdysozoans developed in a terminal position. It should be noted that even Recent crown-group lobopodians (the Onychophora) preserve this primitive and strikingly similar feature at early stages of their embryonic development; at later stages only, the mouth is displaced to a ventral position as in adult onychophorans and arthropods (Eriksson et al., 2003). Another interesting feature of Cam-brian ecdysozoan embryos preserved at various successive stages (from cleavage to pre-hatching) is a central terminal depression surrounded by a radial array of spines, which oc-curs on both ends of the embryo (Dong et al., 2005), thus demonstrating a slight differentiation of oral and aboral ends only. Some Cambrian adult cephalorhynchs also possess ap-

canals in the innermost layer, and caudal appendages. How-ever, adult hairworms lack any kind of pseudosegments, and their caudal hooks, which are present at the larval stage only, are directed ventrally or posteriorly to anchor the body as the animal forces its way into host tissue (Poinar, 1999). On the contrary, palaeoscolecidans always bear prominent pseudosegments, while their caudal hooks are recurved and pointed anteriorly to compensate the burrowing load of the introvert (Text-fig. 1F). Probably, the closest extant rela-tives of palaeoscolecidans are priapulids. Both classes share a similar structure of the muscle and digestive systems, of sensory-locomotory organs, and of glands.

Many other Cambrian cephalorhynchs, commonly treat-ed as either priapulids or nematomorphs, also lack a num-ber of crown-group synapomorphies. The most primitive Cambrian cephalorhynchs, Ancalagon and Fieldia, probably lack a retractable proboscis because they never have been found preserved with an inverted proboscis, and they lack imprints of retractor muscles (Conway Morris, 1977). Thus, they should be placed at the very base of the cephalorhynch stem-lineage (Text-figs. 5.3, 5.4).

Enormous in size, Omnidens displays a mosaic set of features shared with larval priapulids (pharyngeal teeth morphology), as well as with kinorhynchs (placoids of the closing apparatus of the mouth cone; cf. Malakhov and Adrianov, 1995, fig. 4.5B and Hou et al., 2006, figs 4C, 6). Many other Cambrian worms possess scalids and pharyngeal teeth similar to those of larval priapulids rather than to adult ones (cf. Malakhov and Adrianov, 1995, fig. 2.37C and Hou et al., 2006, figs. 4A, 5; Maas et al., 2007a, fig. 7A), and some ancient cephalorhynchs display even more primitive features such as a trunk covered with scalid-like spines (e.g., Anningvermis in Huang et al., 2004b, fig. 3). Besides, a num-ber of fossil cephalorhynchs (Louisella in Conway Morris, 1977; Corynetis and Anningvermis in Huang et al., 2004b) bear long spines at the anterior end that very much resemble oral stylets of the Kinorhyncha, and Louisella demonstrates a double longitudinal row of nematomorph-type natatory bristles (Text-fig. 5.6). Cambrian stem-lineage priapulids (Acosmia in Chen and Zhou, 1997; Palaeopriapulites and Protopriapulites in Hou et al., 1999; Vladipriapulus in Ivantsov and Zhuravlev, 2005), although having a true intro-vert, lack a distinct differentiation of appendages into senso-ry-locomotory scalids and sensory-masticatory teeth, while others probably have an abdomen encased in a larval-type lorica (Palaeopriapulites, Protopriapulites) (Text-fig. 2D).

Finally, even cephalorhynchs approaching the priapulid crown-group, such as Selkirkia and Ottoia (Conway Morris, 1977), have pharyngeal teeth of a rather larval priapulid mor-phology, while Ottoia, Xiaoheiqingella and Yunnanpriapulus have a bursa at the posterior end (Conway Morris, 1977, pl. 8, figs 5–7; caudal appendage in Huang et al., 2004a, figs 2b, 4a), which is typical of larval priapulids as well. Besides, both Xiaoheiqingella and Yunnanpriapulus, showing an ad-


ervation and a chemical composition of fossilized midgut glands characterized by elevated calcium and phosphate in sharp taphonomic contrast from immediately adjacent body structures (in the Murero case, clay replicas; Gámez Vin-taned et al., 2011). The preferential and rapid permineraliza-tion of these organs in phosphate is probably related to their secretory function, to their enhanced chemical reactivity, and to an internal source of phosphate (Butterfield, 2002). Thus, contrary to Edgecombe’s (2009) conclusion, the presence of midgut glands certainly is not a derived feature of the Panar-thropoda. The latter is the clade that includes the Arthropoda, the Onychophora and the Tardigrada (Nielsen, 1995).

The trunks of the majority of xenusians are covered with three-layered cuticular plates of absolutely similar chemical composition, morphologies and microstructure, like those of the palaeoscolecidans (Bengtson et al., 1986, fig. 3, 13; Text-fig. 3C). The diversity of xenusian cuticular plates shows a perfect parallelism with those of the palaeoscolecidans (ar-eolae in Houscolex of Zhang and Pratt, 1995, fig. 2.5 and Orstenotubulus of Maas et al., 2007b, fig. 4c; flat knobs in Piloscolex of Ivantsov and Zhuravlev, 2005, pl. XXI and Quadratopora of Hou and Aldridge, 2007, text-fig. 5P; claw-like spines in Tylotites of Han et al., 2003, pl. I, fig. 1 and Luolishania of Ma et al., 2008, fig. 4E; net-like cones in Cri-cocosmia of Han et al., 2007, fig. 2 and Onychomicrodictyon of Demidenko, 2006, pl. 2; rectangular plates in Tabelliscolex of Han et al., 2007, fig. 2 and Cardiodictyon of Hou et al., 2004a, fig. 14.5; palaeoscolecidan genera are listed first).

In turn, the similar cuticular structure in both groups in-dicates an alike muscular system of the body and hydrostatic pressure-type locomotion, although with the use of walking appendages instead of an introvert. This is not surprising, as telescopic appendages of xenusians require a hydrostatic skeleton to bear a load because the vascular system provides hydrostatic pressure, which imparts a transverse inflated an-nulated appearance to the cuticle of the body and appendages (Text-fig. 4B, C). A presence or, vice versa, an absence of telescopic appendages indicates different ways of substrate occupation rather than profound morphological differences between xenusians and stem-group cephalorhynchs. Such in-termediate forms as Early Cambrian Facivermis and Cardio-dictyon from Chengjiang and Early Cambrian Mureropodia from Murero further bridge the morphological gap between these two groups (Gámez Vintaned et al., 2011). Facivermis possesses an anterior body part bearing spiny telescopic ap-pendages like those of xenusians (e.g., Luolishania in Hou et al., 2004a, fig. 14.1) and a posterior part devoid of limbs but covered with plates and hooks like those of palaeosco-lecidans; the mouth and the anus both are terminal (Delle Cave and Simonetta, 1991; Zhuravlev, 1995; Liu et al., 2006; Text-fig. 5.2b). Its intestine is straight, but not U-shaped as in typical sedentary animals, and with a terminal anus.

Cardiodictyon as well as many other Chinese xenusians and Mureropodia, although bearing appendages along the

parently similar anterior and posterior ends (Huang et al., 2004a; Ivantsov and Zhuravlev, 2005). Of course, this is a morphological similarity but not a physiological one. Later on, in the evolution of cephalorhynchs, the oral spines devel-oped into scalids and teeth of the proboscis, while those of the aboral end turned into caudal hooks.

The phosphatized embryos extend the fossil record of the Ecdysozoa two stages down, up to the very beginning of the Cambrian (ca. 15 m.y. older) and confirm the hypothesis that the latest common ancestor of the crown-group ecdysozoans was a direct developer.

othEr cAMbrIAn VErMIForM EcdYsoZoAns And thEIr rELAtIonshIps

None of the extant cephalorhynchs lacks an introvert, at least at a larval stage. Only some their Cambrian relatives, namely Ancalagon and Fieldia, have a non-retractable pro-boscis among vermiform fossils. However, Cambrian strata are relatively rich in ecdysozoan fossils possessing a non-re-tractable proboscis and legs (Text-fig. 5.2a). The later group of extinct ecdysozoans is coined either the class Xenusia (Dzik and Krumbiegel, 1989) or the phylum Tardypolypoda (Chen and Zhou, 1997) and is usually shoehorned into the Onychophora and even into the Panarthropoda, despite an absence of key panarthropod features such as a ventrally located mouth and pre-oral appendages including antennae (Zhuravlev, 2005). Only Luolishania (=Miraluolishania) possesses a head with putative antenniform limbs and eyes, but it still has a terminal rather than an antero-ventral mouth and a proboscis-like extension of the gut (Liu et al., 2004; Ma et al., 2009). Interestingly, antenniform limbs are expressed as a highly mutable feature of xenusians, being present in Onychodictyon ferox but absent in closely related O. gracilis (Liu et al., 2008). All other xenusians, which are preserved as complete body fossils, except for Megadictyon, posses a terminal mouth located on a proboscis-like extension (Whit-tington, 1978; Ma et al., 2009). In some of them, the pro-boscis is as prominent as in Cambrian cephalorhynchs (e.g., Xenusion in Dzik and Krumbiegel, 1989).

Both palaeoscolecidans and xenusians usually share a straight gut, a terminal anus and a spacious body cavity (Hou et al., 2004b; Ma et al., 2009). Additionally, three-dimension-ally preserved, serially repeated, paired, axial structures are observed in a Middle Cambrian cephalorhynch worm from Murero (Text-fig. 1G). These structures very much resemble midgut glands of some Cambrian stem-lineage euarthro-pods and anomalocaridids (e.g., Leanchoilia, Laggania and Opabinia from the Burgess Shale; Butterfield, 2002, fig. 4; Zhang and Briggs, 2007, fig. 1), as well as diverticulae of xenusians (Kerygmachela and Pambdelurion in Budd, 1997, fig. 11.9; Jianshanopodia in Liu et al., 2006b, fig. 3B2, C1; Magadictyon [sic] in Liu et al., 2007, Text-fig. 2L, M). Also, all of these fossils are characterized by similar relief pres-


related to a transition from a life in an open space to cryp-tic fossorial habitats, such as burrows and organic litter, and usually was accompanied by axial elongation of the trunk and polymerization of body segments. These are not occa-sional coincidences as an anterior expansion of domains of expression of midbody Hox genes may simultaneously ac-count for all three morphogenic processes, namely limb loss, trunk elongation and polymerization of segments (Cohn and Tickle, 1999; Greene and Cundall, 2000). It is noteworthy, that fossorial amphisbaenians or worm lizards even obtained an integument ornamentation superficially resembling that of palaeoscolecidans, including transverse annuli and an in-ner fibrous layer, probably due to a similar type of burrowing by rapid movement of shorting and widening of transverse annuli (Lee, 2000). (An incomplete helminthization, without a loss of appendages, is observed among myriapods; Shcher-bakov, 1999.)

However, what is in common between the Vertebrata and the Ecdysozoa? Of course, the same groups of homeobox regulatory genes, which are responsible for the development of appendages in such now remote clades as deuterostomes and protostomes (the same genes are expressed in the devel-opment of ampullae in tunicates, of tube feet in echinoderms and of parapodia in annelids). In all the bilaterians, the de-velopment of limbs is a result of the expression within the same orthogonal axis of an integrated three dimensional pat-terning system of homologous Hox genes (Distal-less/Dlx, hedgehog/Sonic hedgehog, decapentaplegic/bone morpho-genetic proteins, and fringe/Radical fringe in euarthropods/vertebrates, respectively) (Tabin et al., 1999). Among primi-tive chordates, the same family of the Hox genes controls the development of branchial arches. Thus, it is not surpris-ing that branchial arches are discovered in the earliest stem-lineage chordates from the Lower Cambrian of Chengjiang (Hou et al., 2002; Shu et al., 2003).

As a result, the cephalorhynchs were the ones that origi-nated from the xenusians via such intermediate forms as Facivermis, and not vice versa. This morphologic and ge-netic observations are supported by the palaeoichnological record, in which the first arthropod-type trace fossils (Mono-morphichnus lineatus) are recorded in lowermost Cambrian strata together with Phycodes pedum and apparently pre-date the increase in tiering complexity and burrowing depth of infaunal communities, in which burrowing vermiform animals played a main role (Droser and Li, 2001; Jensen, 2003; Gámez Vintaned and Liñán, 2007). Ediacaran and pre-Ediacaran trace fossils were hardly made by any group of Phanerozoic-type multicellular animals but rather by diverse unicellular and non-Metazoan multicellular organisms (Matz et al., 2008; Zhuravlev et al., 2009). An exploitation of bur-rowing, which is certainly not a primitive lifestyle, caused a development of such a complicated organ as the introvert and contemporary loss of appendages, some of which turned into sensory papillae. Sensory papillae on the palaeoscolecidan

entire trunk, were hardly able to use them for locomotion on the sediment surface, as the appendages of the first of them were disproportionately long and thin (Hou et al., 1991, 1999), while those of the latter were too short (Text-fig. 4A). It should be noted that no fossil xenusians were found with retracted lobopods, while in extant velvet worms compressed and flattened in taphonomic experiments, lobo-pods always preserve a stubby shape (Monge-Nájera and Hou, 2002). Worm-like crawling rather than walking would be more suitable for animals possessing such a ratio of body width to appendage length. This suggestion is confirmed by a restudy of Chinese Lower Cambrian Luolishania, which possessed slim appendages spread horizontally (Ma et al., 2009), and by the discovery of the three-dimensionally pre-served, Orsten-type, phosphatized xenusian Orstenotubulus from the Middle Cambrian of Sweden, whose lobopods were pointed laterally (Maas et al., 2007b). Besides, the latter dis-plays a striking mosaic character pattern shared with fossil palaeoscolecidans (areolae and sensory papillae), as well as with extant priapulids (sensory papillae), nematomorphs (areolae and single gonopore), onychophorans (areolae and sensory papillae) and tardigrades (single gonopore). Well de-veloped peripheral circular and longitudinal trunk muscula-ture in some xenusians may indicate a preference for using a sinuous vermiform-type movement by these animals (Budd, 1998; Zhuravlev, 2005). Judging by the trace fossil record, possible walking lobopod-bearing animals appeared only in the Middle Cambrian (Lane et al., 2003).

In phylogenetic reconstructions, Cambrian cephalor-hynchs are placed unanimously at the base of a phylogenetic tree leading independently to arthropods via xenusians, ony-chophorans and anomalocaridids, depending on a taxonomic treatment of these worms as either annelids or as priapulids, and on the evolution of tubules into walking lobopods on the ventral side and into defensive spines on the dorsal side (e.g., Dzik and Krumbiegel, 1989; Conway Morris, 2000; Budd, 2001b, 2008; Zhang and Briggs, 2007). Such a unity of views clearly reflects an ignorance of proper features of these fossils rather than good agreement of different scientif-ic approaches. Is it really possible to breed an arthropod-like creature moving on a substrate with the help of its pivot-joint appendages (even an onychophoran-like one with lobopods) from a worm moving through the substrate by the hydro-static action of its introvert, which, due to its musculature arrangement and rigid cuticular armour, would be absolutely helpless on the sediment surface?

On the contrary, during the evolution of life, animals lost their appendages many times. For instance, among various families of extant squamates (lizards and snakes), limb re-duction and limb loss are observed at least 62 times in 53 different lineages, while in all the vertebrates being taken to-gether—from ancient amphibians to cetaceans—the append-ages were lost more than 100 times (Greer, 1991; Caldwell, 2003). Generally, the phenomenon of “helminthization” was


blastopore (Malakhov, 2004). This hypothesis is consistent with the analysis of transcription factors in larval develop-ment, which also indicates that primitive bilaterians were holobenthic animals, while complex life cycles including a planktotrophic larva evolved independently multiple times (Dunn et al., 2007; Raff, 2008). Combined sources of avail-able information from classical morphology, developmen-tal biology, various molecular phylogenetic analyses, and whole genomic organization make it possible to hypothesize that ancestors of the Bilateria were bilaterally symmetrical (this is clear enough), triploblastic, probably serially iter-ated animals, with both anterior/posterior and dorsal/ventral polarities, and possessing anterior nerve cell concentrations and muscle cells of mesodermal affinities (Balavoine and Adoutte, 2003; Budd and Jensen, 2004; Malakhov, 2004; Finnerty et al., 2004; Boero et al., 2007; Gabriel and Gold-stein, 2007). Furthermore, it is suggested that this Urbilat-erian ancestor may have even borne appendages (Panganiban et al., 1997; Shubin et al., 1997; Balavoine and Adoutte, 2003; Jacobs et al., 2007; Prpic, 2008). These may be the type of appendages resembling, to a certain extent, cnidarian tentacles that are observed in some xenusians.

As already emphasized above, data from developmental genetics hint at the presence of appendages in a common an-cestor of all the bilaterians (Panganiban et al., 1997; Tabin et al., 1999; Prpic, 2008). At first glance, these data contra-dict widespread hypotheses that root the bilaterian phyloge-netic tree at microscopic multicellular precursors of the size of planktic or meiobenthic animals. However, principal body systems of bilaterians, such as musculature, a blood vascular system, excretory nephridia and a coelom could hardly have evolved in microscopic metazoans (Budd and Jensen, 2004). Indeed, evidence from molecular biology, comparative anat-omy and embryology indicates that interstitial animals such as tardigrades, kinorhynchs, loriciferans and others acquired their “primitive” characters as a result of secondary reduction and loss of a number of features during adaptations for life in microspace (Lorenzen, 1985; Westheide, 1987; Higgins and Thiel, 1988; Dewel and Dewel, 1997; Valentine et al., 1999; Aleshin and Petrov, 2002) and a pseudocoelomate appear-ance could be the result of coelom reduction (Rieger et al., 1991).

Also, developmental genetics data clarify that larval de-velopment in distant clades such as molluscs and echino-derms is not expressed by Hox orthologs (Dunn et al., 2007). These larvae, despite a certain morphological resemblance (topologically similar apical turf and ciliated band), were late convergent innovations intercalated multiple times indepen-dently into already existing direct-developing strategies even within phylogenetically remote lineages (Sly et al., 2003; Dunn et al., 2007). Thus, the common Urbilaterian ancestor hardly was a microscopic planktic animal.

In turn, it is difficult to exclude that the first cnidarians could be descendents of the bilaterians, which were adapted

ventral side differ from lobopods by a smaller size only. Even in Recent priapulids, papillae still preserve the mor-phology of lobopods (e.g., Malakhov and Adrianov, 1995, fig. 2.17A). It is not surprisipaung that both these organs are expressed by the same set of regulatory genes (Jacobs et al., 2007). In turn, the cephalorhynch scalids can be derivates of xenusian lobopod claws with which they are morphological-ly and, probably, structurally similar (e.g., Lemburg, 1995, fig. 3C). In many xenusians, such claws were used for an-choring either to substrate or to a host animal (Whittington, 1978; Chen and Zhou, 1997).

The data on the stratigraphic distribution of vermiform ecdysozoans do not contradict the idea of their origin from xenusians. The first definite xenusian plates (Quadrato-pora; Microdictyon tenuiporatum in Bengtson et al., 1986) appeared in the fossil record by the end of the Tommotian Stage (Cambrian Series 1, Stage 2), one stage earlier than the first palaeoscolecidan plates and body fossils. More-over, phosphatized xenusian claws are well known under the names Mongolodus and Maldeotaia from the very base of the Cambrian Stage 1 (the Nemakit-Daldynian Stage; Roza-nov and Zhuravlev, 1992, fig. 11). Also, these microfossils, although compared to chaetognathan grasping spines (Van-nier et al., 2007), very much resemble in size and morphol-ogy the claws and jaws of extant onychophorans (Robson, 1964, fig. 3E, F).

phYLoGEnEtIc rELAtIonshIps AMonG thE EcdYsoZoA

Text-figure 5 summarizes morphological, anatomical, em-bryological and molecular data on ecdysozoans discussed in this paper with some backward extrapolations. This is not a strict, formal cladistic assessment because broad evolution-ary innovations hardly develop in accordance with cladistic rules.

Node (0) further extrapolates the idea on the origin of the Bilateria from a bilateral holobenthic ancestor and places this clade within the Eumycetozoa—social amoebas that already obtained regulatory homeobox genes for anterior-posterior partitioning (Han and Firtel, 1998). Such common epibenthic mobile antecedents probably made some traces in sediments of ca. 1,700 Ma (Bengtson et al., 2007). Actu-ally, the origin of the appendages is merely explicated by a duplicate expression of the family of regulatory homeobox genes already involved in growth and in anterior-posterior patterning, when these genes are expressed along multiple axes divergent from the main body axis (Minelli, 2003).

Node (1) shows the position of the Urbilaterian ancestor. Classical comparative anatomy and embryology criteria in-dicate that the ventral side of all the adult bilaterians, exclud-ing the chordates, originated from the blastoporal surface of their predecessors, while the mouth and the anus developed from the anterior and posterior extremities of an elongated


Node (4), the Ancalagonia clade, shows further develop-ment of the cephalorhynch Bauplan as indicated by differ-entiation of sensory-locomotory papillae into anterior and abdominal areas.

Node (5) fixes the Palaeoscolecida clade, which displays the entire set of basic cephalorhynch characters, including a retractable proboscis (introvert) with scalids, retractor and protractor(?) muscles, caudal hooks/bursa, and probably a septum separating the introvert and the abdomen; sensory-locomotory structures are represented by scalids, papillae, spines, tubules and flosculae; larvae possessed placoids on the closing apparatus of the mouth cone.

Node (6), represented by the Louselliai clade, indicates some simplification of the cuticle, although palaeoscolecid-type plates cover the abdomen of these worms, and a regular pattern of introvert sensory-locomotory structures (scalids, etc.) is expressed.

Node (7) exposes the first cephalorhynch crown-group, the Priapulida, the majority of which have lost the coelom and preserve a rigid, but not biomineralized, cuticle (lorica) at the larval stage only; the body acquired a distinct sub-division into an introvert, a neck and an abdomen (includ-ing larva; Janssen et al., 2009); the pentaradially arranged pharyngeal teeth are highly differentiated; the primary (an-teriormost) scalids, innervated directly from the circumpha-ryngeal brain, display an octaradial symmetry; and scalids are organized into a complicated 25-radial pattern along the body axis, which actually is a combination of nana-, octa-, and octaradial (9+8+8) symmetries (Adrianov and Malak-hov, 2001). The cephalic nervous system of priapulids is interpreted by Eriksson and Budd (2001) as derived from the circumpharyngeal ring that characterizes the ecdysozoan clade. Molecular studies have shown that priapulids are basal within the crown-group cephalorhynchs but at the same time can be polyphyletic with coelomate (Meiopriapulus) and pseudocoelomate clades (Garey, 2001; Webster et al., 2006). Judging by molecular analysis, Meiopriapulus always forms a clade with the Onychophora (Park et al., 2006), and it is difficult to exclude the coelom as a symplesiomorphic char-acter of this priapulid, inherited from xenusian ancestors.

Node (8) denotes the Loricifera, whose morphology and anatomy, including the presence of a lorica, very much re-semble those of larvae of a dwarfed priapulid (Tubiluchus), thus indicating a possibility of a neotenic origin of loricifer-ans from priapulids (Malakhov and Adrianov, 1995). Finds of Cambrian larval cephalorhynchs such as Orstenoloricus and Sirilorica, which possess features of both priapulids and loriciferans, further support this suggestion (Maas et al., 2009; Peel, 2010). Loriciferans being interstitial predators obtained a buccal canal armed with oral stylets supported by muscles, a hexaradial suctorial pharynx and a further differ-entiated system of retractor muscles, while circular-longitu-dinal peripheral musculature is reduced and protonephridia moved inside the gonads (Kristensen, 2002).

to a passive sedentary lifestyle that turned their ventral side up (Jägersten, 1955, 1959). The presently available data on the cnidarian Hox gene expression pattern, although being equivocal, do not contradict at least such a possibility (Ball et al., 2007). Fossil ecdysozoans allow a further reconstruc-tion of some deep basic features of the Bilateria, including serially repeated (but not necessarily metameric) body units with blocks of circular muscles, a body cavity (possibly a coelom, relics of which are preserved in such remote clades as some priapulids and euarthropods; Störch et al., 1989) with an open circulatory system, a nervous system that con-sisted of a pre-esophageal structure and a ventral cord, and a tubular intestine with a terminal mouth and an anus. Ex-tant onychophorans and tardigrades still keep many of these primitive (=symplesiomorphic) features.

Node (2) is the Ur-Ecdysozoa, which are represented in the fossil record by the Early to Middle Cambrian Xenusia (as cuticular derivates from the very base of the Cambrian and as body fossils from the late Atdabanian Stage/Cambrian Series 2, Stage 3). The xenusian ground plan includes: a seg-mented vermiform body lacking tagmosis; a non-retractable proboscis; paired lobopod appendages pointed laterally and equipped with terminal claws; a three-laminated chitinous-phosphate cuticle displaying a repeated anatomical pattern-ing; a straight digestive tract with both a terminal mouth and an anus and axial paired midgut diverticulae; a nervous sys-tem, probably consisting of a circumpharyngeal brain ring and a ventral cord with paired ganglia (the latter, possibly, has been documented in Paucipodia; Hou et al., 2004b), a voluminous body cavity of a probably coelomic type sur-rounded by peripheral circular and longitudinal cross-stri-ated musculature in addition to which diagonally oriented fibers might be developed (evident in Pambdelurion; Budd, 1998 and in Paucipodia; Hou et al., 2004b); and a direct em-bryonic development. It is highly probable that a coelom is a plesiomorphic state of xenusians because it is preserved in onychophorans and euarthropods (during embryogenesis and as sacculi in adults) and in the priapulid Meiopriapu-lis (around the foregut) (Störch et al., 1989; Eriksson et al., 2003; Schmidt-Rhaesa, 2006). Extant lobopodian and vermi-form ecdysozoans still share a number of fine details in both functional and morphological appearances, such as sensory papillae (Robson, 1964, figs 3D, 4D; Malakhov and Adri-anov, 1995, figs 2.15E, 2.17), which very likely are derivates of similar structures in their common ancestors, the xenu-sians.

Node (3) points to the most primitive cephalorhynchs of the Fieldiai clade typified by a loss of lobopod appendages, some of which are preserved as sensory-locomotory papil-lae, along with lobopod claws, possibly preserved as teeth, a loss of the circulatory system and appearance of the retract-able mouth cone. (The Fieldiai class as well as the Ancala-gonia and the Louselliai were established by Malakhov and Adrianov, 1995.)


vergently as a consequence of sclerotization and miniaturiza-tion. Besides, embryologic studies have revealed neither a euarthropod-type sequential development of trunk segments including the somites and limb buds nor the presence of a growth zone (Hejnol and Schnabel, 2005). Tardigrades share a number of symplesiomorphic characters, including the ventral nerve cord of paired ganglia, the circumpharyngeal brain, and sensory papillae with kinorhynchs; and the cu-ticular columns with nematomorphs and nematodes (Crowe et al., 1970; Dewel and Dewel, 1997); their spermatozoa do possess some similarity in ultrastructure to those of the Kinorhyncha, the Priapulida and the Loricifera (Heiner et al., 2009). Besides, tardigrades have a prominent telescoping mouth cone surrounded by a ring of plate-like peribuccal lamellae and a tri-radial bucco-pharyngeal apparatus armed with teeth (Dewel and Eibye-Jacobsen, 2006). This structure was compared to the mouth cone and the closing apparatus of loriciferans (Kristensen, 2002) and considered to be ho-mologous to the circumoral ring of onychophorans and to the ring of radiating plates of anomalocaridids (Dewel and Eibye-Jacobsen, 2006). Although the later authors suggested that this is a synapomorphic feature of the Panarthropoda, the possible presence of a similar bucco-pharyngeal apparatus in the xenusian Kerygmachela and in the stem-lineage cepha-lorhynch Omnidens (Budd, 2001b, fig. 7; Hou et al., 2006, figs. 4C, 6) may be indicative of its symplesiomorphic rather than synapomorphic character. It is interesting to note that in advanced herbivorous tardigrades, only the mouth occupies an antero-ventral position, but not as extremely so as in adult onychophorans, while carnivorous and omnivorous tardi-grades possess a terminal mouth opening (Morgan, 1982). A presumed internalization of the former’s anterior limbs into a buccal cavity to form a stylet apparatus (Dewel and Eib-ye-Jacobsen, 2006) overprinted bilateral symmetry onto the probably primary tri-radial symmetry of the mouth apparatus in the early evolution of the Tardigrada. As a result, a pair of unique mouth stylets appeared during internalization of an appendage-bearing segment into a buccal cavity, like those of the Onychophora (Mayer and Koch, 2005; Dewel and Eibye-Jacobsen, 2006), and is probably homologous with lobopod terminal claws.

There is no evidence for a tripartite/three-segmented brain organization among the Tardigrada, which commonly is cited as a principal euarthropod/tardigrade synapomorphy; on the contrary, their brain anatomy data indicate a compact, complete, unsegmented, circumpharyngeal ring composed of fibers and nerve cell perikaria resembling the conditions in cephalorhynchs and nematodes (Zantke et al., 2008). The median part of the brain is directly connected to the first pair of trunk ganglia via circumesophageal connectives. The four-paired ventral ganglia do not show segmental commissures typical of the ladder-like nervous system of euarthropods, but instead represent a character state resembling the condition of priapulids, kinorhynchs and nematodes where, apart from

Node (9) shows the Nematomorpha which, due to a para-sitic lifestyle, preserved a typical set of cephalorhynch char-acters, including a three-layered chitin-containing cuticle and a septum separating the introvert and abdomen, at the larval and juvenile stages only. In adults, circular muscu-lature and the respiratory and circulatory systems are com-pletely reduced, and the digestive and excretory (protone-phridia) systems are degenerated, while reproductive organs are hypertrophied; the body cavity is filled with a spacious parenchyme; the two-layered cuticle of adult forms is built of spiral collagenous fibers and lacks chitin; and the nervous system comprises, in addition to the ventral nerve cord, an ectodermal dorsal one (Nielsen, 2001, 2003). It is difficult to exclude the possibility that the horsehair worms reca-pitulate some features of stem-lineage cephalorhynchs, such as an annulated cuticle and a slim vermiform body lacking a distinct proboscis, a neck and an abdomen. Analyses of 18S rRNA and histone 3 sequences indicate a sister-group relationship between loriciferans and nematomorths, which might have evolved through progenesis (sexual maturation of larva) (Park, 2006; Sørensen et al., 2008). This idea is supported by some morphological features of the larva and of the pharynx including a hexaradial arrangement of epithe-lial muscle cells and a triradiate lumen enclosed in myoepi-thelium, albeit the pharynx similarity can be a convergence (Nielsen, 2001; Sørensen et al., 2008).

Node (10) crowns the vermiform ecdysozoan branch with the Nematoda characterized by a low number of cells (eu-tely); a mostly collagenous cuticle, except for a pharyngeal area; and an absence of a proboscis even at the larval stages. The morphological similarity of the adult Nematomorpha to the Nematoda, including hexagonal symmetry of cephalic structures, can be interpreted as a heterochronic shift to the latter group, reducing their possible nematomorph-like juve-nile stages and thus supporting the Nematoida clade (Zrzavý, 2001; Sørensen et al., 2008). This suggestion has some sup-port from molecular data (Bleidorn et al., 2002). Sometimes roundworms are cited as an example of organisation that rejects the Ecdysozoa hypothesis due to the impossibility of reaching such a simplification from a relatively complex ancestor which possessed appendages, segmentation, and excretory and nervous systems (e.g., Fitch, 2005). However, nodes (1) to (10) reveal that such a degeneration of the body plan was achieved step by step in a very long row of nema-tode ancestors.

Node (11) places the Tardigrada acquiring: tagmosis; dorsal and ventral sclerotized plates and flanges; non-meta-meric nephridia of a reduced number; suppression of circular body-wall musculature; muscles in bundles, with each leg having a different compliment of muscles; and segmented ganglia lacking commissures in the nerve cord (Dewel and Dewel, 1997; Kristensen, 2002; Zantke et al., 2008; Edge-combe, 2009). Although some of these features resemble those of the Euarthropoda, they probably were obtained con-


ies are randomly scattered throughout the entire length of each nerve cord, resembling the organization in nematodes and platyhelminthes (Mayer and Harsch, 2007). Besides, velvet worms preserve a primitive neuroanatomy of their pre-oral appendages (antennae), which are innervated by the protocerebrum (Eriksson et al., 2003), while first append-ages of all living euarthropods are deutrocerebral. Ontogenic studies indicate: that the onychophoran ventral mouth is em-bryologically terminal (Eriksson et al., 2003); that the anten-nae are modified legs retaining the original arrangement of transient nephridia anlagen at their bases despite the evolu-tionary change in position and function of these appendages (Mayer and Koch, 2005); and that the embryonic jaws and the slime papillae are lobopod-like outgrowths (Strausfeld et al., 2006). The jaws in adult onychophorans are structures like terminal claws of walking lobopods and thus can be derivates of lobopod terminal claws of ancestral xenusians. Budd (2001a) even suggested that onychophorans, as well as tardigrades, and perhaps pentastomids, are merely extant representatives of paraphyletic xenusians.

Node (14) indicates the position of the Euarthropoda (excluding the Tardigrada, the Onychophora and Pycno-gonida), which are characterized by: sclerotization; dis-tinct tergites; jointed segments and appendages; significant cephalization expressed in a distinct head tagma anteriorly subdivided into three elements, namely a protocerebral or ocular region also bearing a labrum and a mouth, deutro-cerebral or antennal (cheliceral) region, and tritocerebral or second antennal (pedipalpal) region; biramous post-antennal appendages comprising an endopod and an exopod; a lever system of muscles and a reduction of peripheral muscula-ture; segmented bilaterally arranged ganglia that are longi-tudinally linked by connectives and transversely connected by commissures in the ladder-like nervous system; saccu-late nephridia; and compound eyes. The latter show neither morphogenetic nor ultrastructural similarities with those of onychophorans (Mayer, 2006). On the contrary, the pattern of some Hox gene expression in onychophorans supports the idea that the development of arthropod joint limbs is derived

lateral commissures, none connecting the midventral nerve cords have been reported.

In aggregate molecular studies using mostly EST data (Roeding et al., 2007; Dunn et al., 2008), and statistically well supported phylogenies grouping the Tardigrada with the Nematoida (Lartillot and Philippe, 2008), together with ana-tomical and morphological evidence, reveal that water bears hardly can represent an arthropod stem-lineage. Tardigrades might originate from xenusians via Hadranax-type forms possessing a heteronomous annulation (Budd, 2001b). It is noteworthy that a Middle Cambrian fossil tardigrade from Siberia bears cuticular plates (Maas and Waloszek, 2001, fig. 9B) resembling those of some palaeoscolecidans (Pi-loscolex in Ivantsov and Zhuravlev, 2005, pl. XXI, fig. 1).

Node (12) depicts the Kinorhyncha as a derivation of the Tardigrada, thus revealing possible polyphyly for the Cepha-lorhyncha clade. Indeed, the kinorhynch autapomorphies in-clude metamery, enclosing of the integument, musculature and nervous systems, sensory papillae, sclerotized dorsal and ventral plates, and a retractable mouth cone equipped with stylets (Nielsen, 2001; Neuhaus and Higgins, 2002), which equally can be treated as tardigrade/kinorhynch syn-apomorphies. On the other hand, it could have been scleroti-zation that led to transformation of the circular-longitudinal muscle system into serially arranged dorso-ventral and di-agonal bunches and to segmentation of the nervous system.

Node (13) features Onychophora autapomorphies of which there are: tagmosis; a “ventral” mouth, specialized an-tennae, jaws, slime papillae; segmented, diagonally oriented, leg muscles; an open circulatory system formed by fusion of both the coelomic cavities and the primary body cavity into a haemocoel; segmented nephridia with functional cilia; and a dorsal heart with characteristic openings (ostia) into the circulatory system (Nielsen, 2001; Eriksson et al., 2003; Edgecombe, 2009). Unlike the Euarthropoda, the bilaterally paired ventral nerve cords are longitudinal structures con-nected to each other by numerous commissures but lacking segmental thickenings as well as any recognisable serial or segmental organization of neuron cell bodies; such cell bod-

text-figure 5. General phylogeny of the Ecdysozoa. 0–migration slag of amoebozoan Dictyostelium discoideum Raper, extant; 1–Urbilat-erian, hypothetic (modified from van Beneden, 1891); 2a–xenusian Microdictyon sinicum Chen, Hou and Lu, Lower Cambrian (modified from Hou and Bergström, 1995); 2b–xenusian Facivermis yunnanicus Hou and Chen, Lower Cambrian (modified from Delle Cave and Simonetta, 1991); 3–Fieldia lanceolata Walcott, Middle Cambrian (modified from Conway Morris, 1977); 4–Ancalagon minor Walcott, Middle Cambrian (modified from Conway Morris, 1977); 5–palaeoscolecid Cricocosmia jinningensis Hou and Sun, Lower Cambrian (modi-fied from Han et al., 2007); 6–Louisella pedunculata Walcott, Middle Cambrian (modified from Conway Morris, 1977); 7–priapulid larva Halicryptus spinulosus von Seibold, extant (modified from Malakhov and Adrianov, 1995); 8–loriciferan larva Pliciloricus ornatus Higgins and Kristensen, extant (modified from Malakhov and Adrianov, 1995); 9–nematomorph larva Gordionus senkovi Malakhov and Spiridonov, extant (modified from Malakhov and Adrianov, 1995); 10–nematodes Greeffiella (left) and Criconema (right), extant (modified from Brusca and Brusca, 2003); 11–tardigrade Stygarctus abornatus McKirdy, Schmidt and McGinty-Bayly, extant (modified from McKirdy et al., 1976); 12–kinorhynch Centroderes eisigii Zelinka, extant (modified from Malakhov and Adrianov, 1995); 13–onychophoran Peripatopsis moseleyi (Wood-Mason), extant (modified from Ruhberg in Monge-Nájera and Hou, 1999); 14–larval euarthropod Ascalaphus sp., extant; 15–larval pentastomid Boeckelericambria pelturae Walossek and Müller, Upper Cambrian (modified from Maas and Waloszek, 2001); 16–anomalo-caridid Anomalocaris saron Hou, Bergström and Ahlberg, Lower Cambrian (modified from Hou et al., 1995); 17–protonymphon larva of pycnogonid Anoplodactylus sp., extant (modified from Maxmen et al., 2005).


plate-like teeth; ventro-lateral, stalked, compound eyes; ven-trally pointed, imbricating lobes, possibly with gill blades; and a “tail fan” (in the majority of forms) (Bergström, 1987; Hou et al., 1995, 2006; Daley et al., 2009). Budd and Telford (2009) added to this list midgut diverticulae and suggested that all the above-mentioned characters are synapomorphies of the Euarthropoda. However, midgut diverticulae are pres-ent in vermiform ecdysozoans and xenusians, and thus repre-sent a symplesiomorphic feature, while the pre-oral position of the only pair of jointed appendages can hardly be con-sidered an anomalocaridid-euarthropod synapomorphy. On the contrary, the mouth cone structure is completely incon-sistent with the euarthropod ground plan while pivot-joint pre-oral appendages, compound eyes and a frontal carapace, which is developed in Hurdia, rather resulted from a conver-gence due to a sclerotization of the cuticle at the head region. Even within the Euarthropoda, compound eyes may not even be phylogenetically homologous to each other as the same developmental programme allows for multiple origins of these organs through regulatory gene mutations (Oakley et al., 2007). Certainly, euarthropod-type features were not accumulated within the anomalocaridid lineage but merely display a mosaic pattern acquired by them in a different tem-poral order (e.g., Cucumericrus possesses appendages inter-mediate between lobopods and arthropodia and an abdomen covered by wrinkled skin rather than by tergites; Bergström and Hou, 2003). From a biomechanical point of view, pad-dle-like, lateral, overlapping lobes, plexus on lateral sides, skin-like cuticle, and large ventro-lateral eyes evident at least in Anomalocaris and Amplectobelua are a logic conse-quence of pursuing swimming ability, if their lobes operated as a continuous single flap, which is optimal for swimming motion as in a manta ray (Usami, 2006). Thus, the origin of anomalocaridids is rooted in Kerygmachela-like swim-ming xenusians and in Tamisiocaris-like anomalocaridids possessing unsclerotized soft appendages (Daley and Peel, 2010). Another xenusian, Magadictyon cf. haikouensis, also accumulated a number of anomalocaridid features including Peytoia-like antero-ventral mouth cone with three circles of cuneiform plates, and frontal appendages with terminal spines (Liu et al., 2007).

Possible offspring of the anomalocaridids are the Vetuli-colida, which are not indicated in Text-figure 5. These Cam-brian animals were interpreted as early deuterostomes due to the presence of gill pouches (Shu et al., 2001; Aldridge et al., 2007). However, vetulicolians resemble stem-lineage ecdysozoans by displaying: a segmented cuticularized body; a terminal mouth with a mouth cone consisting of 25 plates; and a straight alimentary canal with a terminal anus. This set of features including priapulid-type symmetry closely matches that of ecdysozoans. In turn, so-called gill pouches can be interpreted as midgut diverticulae like those found in the Middle Cambrian arthropod Leanchoilia superlata and

from mechanisms present in this group (Panganiban et al., 1997), while a biramous limb comprising an endopod and an exopod evolved as a result of a split of the initial limb bud within euarthropods (Wolff and Scholtz, 2008). The later hy-pothesis is now supported by redescription of the Early Cam-brian primitive arthropod Fuxianhuia, which lacks a special-ization between and within ventral appendages (Chen et al., 1995; Bergström et al., 2009). Some other, mostly Cambrian, stem-lineage arthropods possibly possessed a pair of proto-cerebral sensory organs, known as “frontal appendages” or “primary antennae”, in front of the deutrocerebral appendage pair; thus a rotation of the anterior end of the body is implied to obtain the present euarthropod ground plan (Scholz and Edgecombe, 2006; Hughes et al., 2008). Indeed, based on joint study of Hox gene expression in embryologic develop-ment and of ontogeny, the euarthropod labrum is suggested to be derived evolutionarily from paired limb-bud-like pri-mordia by rotation and fusion; this process is recapitulated ontogenetically to a different extent in different arthropods; the labrum of at least arachnids and hexapods includes rem-nants of these ancestral appendages (Kimm and Prpic, 2006; Scholz and Edgecombe, 2006). The “frontal appendages” themselves were fairly simple limbs that could conceivably evolve in various directions.

The placement of Node (15), the Pentastomida, is en-tirely based on molecular and spermatological analyses ac-cording to which pentastomids are highly derived parasitic branchiuran crustaceans (Lavrov et al., 2004). However, a convergence of spermatological features is thought to be widespread among invertebrates, especially among parasites (Zrzavý, 2001), while a more recent treatment of pentasto-mid morphological characters indicates a position of pentas-tomids within the tardigrade/vermiform ecdysozoan clade (Almeida et al., 2008). In striking contrast to extant pentas-tomid larvae, late Cambrian fossils have a frontally placed mouth, two pairs of vestigial anterior appendages and up to two pairs of rudimentary limbs on the tail part that are inter-mediate between a lobopod with claw-like terminations and a multi-segmented arthropodium of more advanced euar-thropods (Maas and Waloszek, 2001; Waloszek et al., 2006). Similarity of pentastomid claws with those of tardigrades was noted by von Haffner (1977). At the same time, these three-dimensionally preserved larvae already demonstrate a high degree of adaptation to parasitism. Thus, a direct root-ing of pentastomids within the Xenusia cannot be excluded at the moment (but see Lavrov et al., 2004, who exclude Cambrian putative pentastomids from tongue worms). The pentastomid autapomorphies include an unusual cuticular β-chitin and a suctorial mouth flanked by two pairs of hooks.

Node (16) points to the most unusual ecdysozoan stem-lineage, the Anomalocaridida, autapomorphies of which include: sclerotized and articulated pre-oral grasping ap-pendages; an antero-ventral mouth cone with a circlet of


intermediate forms, supporting the Articulata hypothesis, thus rejecting the critics of the Ecdysozoa concept. Highly vari-able morphologically xenusians, which crawled with their lobopods along the sediment surface, may have given rise to four morphofunctional lineages, namely to: stem-lineage cephalorhynchs (via Facivermis-type forms) by adaptation for burrowing with introvert; tardigrades (via Hadranax-like forms) by adaptation for an interstitial habitat; stem-lineage panarthropods (via Luolishania-type forms) by adaptation to a walking lifestyle with joint appendages; and anomalocari-dids (via Kerygmachela-type forms) by adaptation to swim-ming with lateral flaps in the pelagic realm. The finds of such forms as Pambdelurion with diagonal muscles, Paucipodia with structures resembling paired ventral ganglia, Jiansha-nopodia with a some kind of biramous appendages bearing a flapping exopod, and Luolishania with advanced cephaliza-tion (Budd, 1998; Hou et al., 2004b; Liu et al., 2006b; Ma et al., 2009) may indicate that even the euarthropod ground plan can be developed independently, as all these features are absent in onychophorans. Besides, neither a ventral mouth, nor antennae, nor, probably, eyes in onychophorans are ho-mologous to those structures in euarthropods. However, many attempts to create an euarthropod creature on the xenu-sian body ground might be blind branches of the ecdysozoan phylotree, similar to multiplicity of mammal-like reptiles, only one clade of which finally turned to be “true” mammals (Tatarinov, 1976; Ponomarenko, 2008). Thus, “arthropodiza-tion” can be a trend independent of “arthrodization”, and both of these trends could go in all directions, only one of which was “correct enough”. The xenusians fired one of three major volleys in the Cambrian Explosion and the rapid diver-sification of body plans among ecdysozoans is supported by molecular phylogenetics.

AcknowLEdGEMEnts

The authors are grateful to Jan Bergström and Brian Pratt for their comprehensive and constructive reviews, to Françoise Debrenne and Carole Pizzolante for French translation, and to Peter Spinella and the volume editors for English editing. This is a contribution to the projects: Consolíder CGL2006–12975/BTE (“MURERO”; Ministerio de Educación y Cien-cia, [(MEC) Spain, and FEDER, European Union), CGL 2011-24516 and ACI2009-1037 (Ministereo de Ciencia e Innovación, MICINN, Spain), Multidisciplinar PM067/2006 (Gobierno de Aragón), Grupo Consolidado E–17 (“Pale-ontología del Neoproterozoico-Mesozoico. Patrimonio y Museo Paleontológico”; Gobierno de Aragón), and IGCP 587 (“Identity, Facies and Time, the Ediacaran (Vendian) Puzzle”). AZ benefited from the grants MI042/2006, Depar-tamento de Ciencia, Tecnología y Universidad (Gobierno de Aragón) and CB 3/08 Programa Europa XXI de Estancias de Investigación (CAI–CONAI+D) 2008. JAGV received fi-

the anomalocaridid Opabinia regalis (Butterfield, 2002). Both axial structures are three-dimensionally preserved and are characterized by sub-millimetric arrangement of planar elements. These are the ones that were taken for gill slits in vetulicolians.

Node (17) places the Pycnogonida, their mouth appa-ratus—unusual for the Euarthropoda—comprising the pro-boscis, followed by paired chelifores, parapalps and ovigers. Other characters associated with this node: the absence of nephridia; extreme reduction of the abdomen and resulting displacement of the organ system into the legs; and genital openings involving multiple gonopores on preabdominal so-mites of the legs (Dunlop and Arango, 2005). It was suggest-ed that appendages on the first head segment had not been lost in the Pycnogonida, unlike all extant euarthropods, but had been retained as chelifores innervated from the protoce-rebrum as indicated by neuroanatomical analysis of larvae from Anoplodactylus sp. (Maxmen et al., 2005). However, further examination of Hox gene expression in another sea spider, Eudeis spinosus, refuted such an interpretation and revealed that the anterior boundaries of Hox gene expression domains placed the chelifores as clearly belonging to the sec-ond head segment and innervated from the deutrocerebrum like those of the Chelicerata and the Mandibulata (Jager et al., 2006). This contradiction can be reconciled if the loca-tion of chelifore ganglia—just in front of, and in close prox-imity to, the first part of brain—is a derived character, thus implying that pycnogonid ancestors underwent both anterior condensation of the central nervous system and a rotation of the brain, which displaced the deutrocerebrum toward the protocerebrum (Jager et al., 2006). In such a case, pycno-gonids can be descendents of anomalocaridids with which they share common head anatomy, including pre-oral joint appendages and a circular mouth, as well as highly devel-oped midgut diverticulae. Jurassic sea spiders, despite close similarities with extant pycnogonid families, bear very long and subcylindrical proboscises (Charbonnier et al., 2005) that resemble those of xenusians. The origin of sea spiders could be neotenic, as the body of their protonymphon larva effectively becomes the head of the adult to which a trunk of joint segments is added during ontogeny (Vilpoux and Walo-szek, 2003). Such phosphatized Orsten-type larvae, bearing vestigial pre-chelifore, pre-oral appendages, existed already as early as the latest Cambrian of Sweden (Waloszek and Dunlop, 2002). Molecular data are equivocal concerning the position of pycnogonids within the Ecdysozoa (e.g., Rieger et al., 2005; Mallatt and Giribet, 2006; Park et al., 2007). In turn, if pycnogonids represent the euchelicerate stem-lineage (Dunlop, 2006), the Euarthropoda would be a paraphyletic group.

In summary, Cambrian Lagerstätten yield a large variety of species representing intermediate sets of features between vermiform and appendage-bearing ecdysozoans but lack any


Bengtson, S. 1977. Early Cambrian button-shaped phosphat-ic microfossils from the Siberian Platform. Palaeontol-ogy, 20: 751–762.

Bengtson, S., Matthews, S.C., and Missarzhevsky, V.V. 1986. The Cambrian netlike fossil Microdictyon. In Problem-atic Fossil Taxa. Edited by A. Hoffman and M.H. Nitecki. Oxford Monographs on Geology and Geophysics, v. 5. Oxford University Press, New York; Clarendon Press, Oxford. pp. 97–115.

Bengtson, S., Rasmussen, B., and Krapez, B., 2007. The Pa-leoproterozoic megascopic Stirling biota. Paleobiology, 33: 351–381.

Bergström, J. 1987. The Cambrian Opabinia and Anomalo-caris. Lethaia, 20: 187–188.

Bergström, J., and Hou, X.-G., 2003. Arthropod origins. Bul-letin of Geosciences, 78: 323–334.

Bergström, J., Hou, X.-g., Zhang, X.-g., and Clausen, S., 2009. A new view of the Cambrian arthropod Fuxian-huia. GFF, 130 (for 2008): 189–201.

Bleidorn, C., Schmidt-Raesa, A., and Garey, J.R., 2002. Sys-tematic relationships of Nematomorpha based on molec-ular and morphological data. Invertebrate Biology, 121: 357–364.

Boero, F., Schierwater, B., and Piraino, S., 2007. Cnidarian milestones in metazoan evolution. Integrative and Com-parative Biology, 47: 693–700.

Bourlat, S.J., Nielsen, C., Economou, A.D., and Telford, M.J., 2008. Testing the new animal phylogeny: A phylum level molecular analysis of the animal kingdom. Molecu-lar Phylogenetics and Evolution, 49: 23–31.

Brock, G.A., and Cooper, B.J. 1993. Shelly fossils from the Early Cambrian (Toyonian) Wirrealpa, Aroona Creek, and Ramsay limestones of South Australia. Journal of Paleontology, 67: 758–787.

Brusca, R.C., and Brusca, G.J., 2003. Invertebrates, 2nd edn. Sinauer Associates, Sunderland, Massachusetts, 936 pp.

Budd, G.E. 1997. Stem group arthropods from the Lower Cambrian Sirius Passet fauna of North Greenland. In Ar-thropod Relationships. Edited by R.A. Fortey and R.H. Thomas. The Systematic Association Special Volume, Series 55. Chapman and Hall, London; Weinheim; New York; Tokyo; Melbourne; Madras. pp. 125–138.

Budd, G.E. 1998. Arthropod body-plan evolution in the Cambrian with an example of anomalocaridid muscle. Lethaia, 31: 197–210.

Budd, G.E., 2001a, Ecology of nontrilobite arthropods and lobopods in the Cambrian. In The Ecology of the Cam-brian Radiation. Edited by A.Yu. Zhuravlev and R. Rid-ing. Columbia University Press, New York. pp. 404–427.

Budd, G.E., 2001b, Tardigrades as ‘stem-group arthropods’: The evidence from the Cambrian fauna. Zoologischer Anzeiger, 240: 265–279.

Budd, G.E., and Jensen, S., 2004. The limitations of the fos-sil record and the dating of the origin of the Bilateria.

nancial support from MICINN (Juan de la Cierva” contract, Ref. JCI-2009-05319). We also thank Isabel Pérez-Urresti (funded by Universidad de Zaragoza-MEC-European Social Fund) and Alevtina Sukhorukova for the preparation of some of the text-figures.

rEFErEncEs

Adrianov, A.V., and Malakhov, V.V. 2001. Symmetry of pri-apulids (Priapulida). 1. Symmetry of adults. Journal of Morphology, 247(2): 99–110.

Aguinaldo, A.M.A., Turbeville, J.M., Linford, L.S., Rivera, M.C., Garey, J.R., Raff, R.A., and Lake, J.A. 1997. Ev-idence for a clade of nematodes, arthropods and other moulting animals. Nature, 387: 489–493.

Aldridge, R.J., Hou, X.-G., Siveter, D.J., Siveter, D.J., and Gabbott, S.E., 2007. The systematics and phylogenetic relationships of vetulicolians. Palaeontology, 50: 131–168.

Aleshin, V.V., Milyutina, I.A., Kedrova, O.S., Vladychens-kaya, N.S., and Petrov, N.B. 1998. Phylogeny of Nema-toda and Cephalorhyncha derived from 18S rDNA. Jour-nal of Molecular Evolution, 47: 597–605.

Aleshin, V.V., and Petrov, N.B., 2002. (Molecular evidence of the regress in the evolution of multicellular animals). Zhurnal obshchey biologii, 63(3): 195–208 [in Russian].

Almeida, W.O., Christoffersen, M.L., Amorim, D.S., and Eloy, E.C.C., 2008. Morphological support for the phy-logenetic positioning of Pentastomida and related fossils. Biotemas, 21: 81–90.

Anderson, F.E., Córdoba, A.J., and Thollesson, M., 2004. Bilaterian phylogeny based on analysis of a region of the sodium-potassium ATPase α-subunit gene. Journal of Molecular Evolution, 58: 252–268.

Baguñà, J., Martinez, P., Paps, J., and Riutort, M., 2008. Back in time: a new systematic proposal for the Bilat-eria. Philosophical Transactions of the Royal Society B: Biological Sciences, 363: 1481–1491.

Balavoine, G., and Adoutte, A., 2003. The segmented Urbi-lateria: A testable scenario. Integrative and Comparative Biology, 43: 137–147.

Ball, E.E., de Jong, D.M., Schierwater, B., Shinzato, C., Hayward, D.C., and Miller, D.J., 2007. Implications of cnidarian gene expression patterns for the origins of bi-laterality—is the glass half full or half empty? Integrative and Comparative Biology, 47: 701–711.

Barskov, I.S., and Zhuravlev, A.Yu. 1988. (Soft-bodied or-ganisms from the Cambrian of the Siberian Platform). Paleontologicheskiy zhurnal, 1988(1): 3–9 [in Russian].

Bendix-Almgreen, S.E., and Peel, J.S. 1988. Hadimopanella from the Lower Cambrian of North Greenland: structure and affinities. Geological Society of Denmark, Bulletin, 37: 83–103.


for Ecdysozoa after accounting for the idiosyncrasies of Caenorhabditis elegans. Evolution & Development, 6: 164–169.

Crowe, J.H., Newell, I.M., and Thomson, W.W. 1970. Echiniscus viridus (Tardigrada): Fine structure of the cu-ticle. American Microscopical Society, Transactions, 89: 316–325.

Daley, A.C., Budd, G.E., Caron, J.-B., Edgecombe, G.D., and Collins, D., 2009. The Burgess Shale anomalocaridid Hurdia and its significance for early euarthropod evolu-tion. Science, 323: 1597–1600.

Daley, A.C., and Peel, J.S., 2010. A possible anomalocari-did from the Cambrian Sirius Passet Lagerstätte, North Greenland. Journal of Paleontology, 84: 352–355.

Delle Cave, L., and Simonetta, A.M. 1991. Early Palaeozoic arthropods and problem of arthropod phylogeny; with some notes on taxa of doubtful affinities. In The Early Evolution of Metazoa and the Significance of Problem-atic Taxa. Edited by A.M. Simonetta and S. Conway Mor-ris. Cambridge University Press, Cambridge; New York; Port Chester; Melbourne; Sydney. pp. 189–244.

Demidenko, Yu.E., 2006. New Cambrian lobopods and chae-tognaths of the Siberian Platform. Paleontologicheskii Zhurnal, 2006(3): 6–14.

De Rosa, R., Grenier, J.K., Andreeva, T., Cook, C.E., Ad-outte, A., Akam, M., Carroll, S.B., and Balavoine, G. 1999. Hox genes in brachiopods and priapulids and pro-tostome evolution. Nature, 399: 772–776.

Dewel, R.A., Budd, G.E., Castano, D.F., and Dewel, W.C. 1999. The organization of the subesophageal nervous system in tardigrades: insights into the evolution of the arthropod hypostome and tritocerebrum. Zoologischer Anzeiger, 238: 191–203.

Dewel, R.A., and Dewel, W.C. 1997. The place of tardigrades in arthropod evolution. In Arthropod Relationships. Ed-ited by R.A. Fortey and R.H. Thomas. The Systematic Association Special Volume, Series 55. Chapman and Hall, London; Weinheim; New York; Tokyo; Melbourne; Madras. pp. 109–123.

Dewel, R.A., and Eibye-Jacobsen, J., 2006. The mouth cone and mouth ring of Echiniscus viridissimus Peterfi, 1956 (Heterotardigrada) with comparisons to corresponding structures in other tardigrades. Hydrobiologia, 558: 41–51.

Dong, X.-p., Donoghue, P.C.J., Cunningham, J.A., Liu, J.-b., and Cheng, H., 2005. The anatomy, affinity, and phylo-genetic significance of Markuelia. Evolution & Develop-ment, 7: 468–482.

Donoghue, P.C.J., Kouchinsky, A., Waloszek, D., Bengtson, S., Dong, X.-p., Val´kov, A.K., Cunningham, J.A., and Repetski, J.E., 2006. Fossilized embryos are widespread but the record is temporally and taxonomically biased. Evolution & Development, 8: 232–238.

In Telling the Evolutionary Time, Molecular Clocks and the Fossil Record. Edited by P.C.J. Donoghue and M.P. Smith. Systematics Association Publication, 66. CRC Press (Taylor and Francis), Boca Raton; London; New York; Washington, D.C. pp. 166–189.

Budd, G.E., and Telford, M.J., 2009. The origin and evolu-tion of arthropods. Nature, 457: 812–817.

Butterfield, N.J., 2002. Leanchoilia guts and the interpreta-tion of three-dimensional structures in Burgess Shale-type fossils. Paleobiology, 28: 155–171.

Caldwell, M.W., 2003. “Without a leg to stand on”: On the evolution and development of axial elongation and limb-lessness in tetrapods. Canadian Journal of Earth Scienc-es, 40: 573–588.

Charbonnier, S., Vannier, J., and Riou, B., 2007. New sea spi-ders from the Jurassic La Voulte-sur-Rhône Lagerstätte. Proceedings of the Royal Society, B, 274: 2555–2561.

Chen, J.Y., Edgecombe, G.D., Ramsköld, L., and Zhou, G.-Q. 1995. Head segmentation in Early Cambrian Fuxian-huia: implications for arthropod evolution. Science, 268. 1339–1343.

Chen, J., and Zhou, G. 1997. Biology of the Chengjiang fau-na. In The Cambrian Explosion and the Fossil Record. Edited by J. Chen, Y.-n. Cheng, and H.V. Iten. National Museum of Natural Science, Taichung, Bulletin, 10: 11–105.

Cohn, M.J., and Tickle, C. 1999. Developmental basis of limblessness and axial patterning in snakes. Nature, 399: 474–479.

Colgan, D.J., Hutchings, P.A., and Beacham, E., 2008. Multi-gene analyses of the phylogenetic relationships among the Mollusca, Annelida, and Arthropoda. Zoo-logical Studies, 47: 338–351.

Conway Morris, S. 1977. Fossil priapulid worms. Special Papers in Palaeontology, 20: 1–155.

Conway Morris, S. 1986. The community structure of the Middle Cambrian Phyllopod Bed (Burgess Shale). Pal-aeontology, 29: 423–467.

Conway Morris, S., 2000. The Cambrian “explosion”: Slow-fuse or megatonnage? National Academy of Sciences of the United States of America, Proceedings, 97: 4426–4429.

Conway Morris, S., and Peel, J.S., 2010. New palaeosco-lecidan worms from the Lower Cambrian Sirius Passet, Latham Shale and Kinzers Shale. Acta Palaeontologica Polonica, 55: 141–156.

Conway Morris, S., and Robison, R.A. 1986. Middle Cam-brian priapulids and other soft-bodied fossils from Utah and Spain. The University of Kansas Paleontological Contributions, 117 (for 1985): 1–22.

Copley, R.R., Aloy, P., Russell, R.B., and Telford, M.J., 2004. Systematic searches for molecular synapomor-phies in model metazoan genomes give some support


Eriksson, B.J., Tait, N.N., and Budd, G.E., 2003. Head de-velopment in the onychophoran Euperipatoides kanan-grensis with particular reference to the central nervous system. Journal of Morphology, 255: 1–23.

Finnerty, J.R., Pang, K., Burton, P., Paulson, D., and Mar-tindale, M.Q., 2004. Origins of bilateral symmetry: Hox and Dpp expression in a sea anemone. Science, 304: 1335–1337.

Fitch, D.H.A., 2005. Introduction to nematode evolution and ecology. In WormBook. Edited by The C. elegans Research Community. doi/10.1895/wormbook.1.19.1, http:www.wormbook.org.

Gabriel, W.N., and Goldstein, B., 2007. Segmental expres-sion of Pax3/7 and Engrailed homologs in tardigrade development. Development Genes and Evolution, 217: 421–433.

Gámez Vintaned, J.A. and Liñán, E., 2007. The Precambrian/Cambrian boundary in Spain: ichnofossil palaeobiology and zonation. In The Rise and Fall of the Vendian (Edia-caran) Biota. Origin of the Modern Biosphere. Edited by M.A. Semikhatov. Transactions of the International Con-ference on the IGCP Project 493, August 20–31, 2007, Moscow. GEOS, Moscow. pp. 54–57.

Gámez Vintaned, J.A., Liñán, E., and Zhuravlev, A. Yu, 2011. A new Early Cambrian lopodod-bearing animal (Murero, Spain) and the problem of the ecdysozoan early diversification. In Evolutionary Biology—Concepts, Biodiversity, Macroevolution and Genome Evolution. Edited by P. Pontarotti. Springer-Verlag, Berlin; Heidel-berg. pp. 193–219.

Garey, J.R., 2001. Ecdysozoa: The relationship between Cy-cloneuralia and Panarthropoda. Zoologischer Anzeiger, 240: 321–330.

Giribet, G. 1999. Ecdysozoa versus Articulata, dos hipóte-sis alternativas sobre la posición de los artrópodos en el reino animal. In Evolución y Filogenia de Arthropoda. Edited by A. Melic, J.J. de Haro, M. Méndez, and I. Ri-bera. Sociedad Entomológica Aragonesa, Boletín, 26: 145–160.

Giribet, G., 2003. Molecules, development and fossils in the study of metazoan evolution; Articulata versus Ecdyso-zoa revisited. Zoology, 106: 303–326.

Greene, H.W., and Cundall, D., 2000. Limbless tetrapods and snakes with legs. Science, 287: 1939–1941.

Greer, A.E. 1991. Limb reduction in squamates: Identifica-tion of the lineages and discussion of the trends. Journal of Herpetology, 25: 166–173.

Haase, A., Stern, M., Wächtler, K., and Bicker, G., 2001. A tissue-specific marker of Ecdysozoa. Development Genes and Evolution, 211: 428–433.

Haffner, K., von, 1977. Über die systematische Stellung und die Vorfahren der Pentastomida auf Grund neuer verglei-chenden Untersuchungen. Zoologischer Anzeiger, 199: 353–370.

Dornbos, S.Q., and Chen, J.-Y., 2008. Community palaeo-ecology of the Early Cambrian Maotianshan Shale biota: Ecological dominance of priapulid worms. Palaeogeog-raphy, Palaeoclimatology, Palaeoecology, 258: 200–212.

Droser, M.L., and Li, X., 2001. The Cambrian radiation and the diversification of sedimentary fabrics. In The Ecology of the Cambrian Radiation. Edited by A.Yu. Zhuravlev and R. Rid-ing. Columbia University Press, New York. pp. 137–169.

Dunlop, J.A., 2006. New ideas about the euchelicerate stem-lineage. In European Arachnology 2005. Edited by C. Deltshev and P. Stoev. Acta zoologica bulgarica, Suppl. no. 1: 9–23.

Dunlop, J.A., and Arango, C.P., 2005. Pycnogonid affinities: a review. Journal of Zoological Systematics and Evolu-tionar Research, 43: 8–21.

Dunn, C.W., Hejnol, A., Matus, D.Q., Pang, K., Browne, W.E., Smith, S.A., Seaver, E., Rouse, G.W., Obst, M., Edgecombe, G.D., Sørensen, M.V., Haddock, S.H.D., Schmidt-Rhaesa, A., Okusu, A., Kristensen, R.M., Wheeler, W.C., Martindale, M.Q., and Giribet, G., 2008. Broad phylogenomic sampling improves resolution of the animal tree of life. Nature, 452: 745–749.

Dunn, E.F., Moy, V.N., Angerer, L.M., Angerer, R.C., Morris, R.L., and Peterson, K.J., 2007. Molecular paleoecology: Using gene regulatory analysis to address the origins of complex life cycles in the late Precambrian. Evolution & Development, 9: 10–24.

Dzik, J. 1986. Chordate affinities of the conodonts. In Prob-lematic Fossil Taxa. Edited by A. Hoffman and M.H. Nitecki. Oxford Monographs on Geology and Geophys-ics, v. 5. Oxford University Press, New York; Clarendon Press, Oxford. pp. 240–254.

Dzik, J., 2003. Early Cambrian lobopodian sclerites and associated fossils from Kazakhstan. Palaeontology, 46: 93–112.

Dzik, J., and Krumbiegel, G. 1989. The oldest ‘onychopho-ran’ Xenusion: A link connecting phyla? Lethaia, 22: 169–181.

Edgecombe, G.D., 2009. Palaeontological and molecular evidence linking arthropods, onychophorans, and other Ecdysozoa. Evolution: Education and Outreach, 2: 178–190.

Eernisse, D.J. 1997. Arthropod and annelid relationship re-examined. In Arthropod Relationships. Edited by R.A. Fortey and R.H. Thomas. The Systematic Association Special Volume, Series 55. Chapman and Hall, London; Weinheim; New York; Tokyo; Melbourne; Madras. pp. 43–56.

Eriksson, B.J., and Budd, G.E., 2001. The cephalic nerves of the Onychophora and their bearing on our understanding of head segmentation and stem-group evolution of Ar-thropoda. Arthropod Structure & Development, 29 (for 2000): 197–209.


Hou, X.-G., Bergström, J., and Jie, Y., 2006. Distinguishing anomalocaridids from arthropods and priapulids. Geo-logical Journal, 41: 259–269.

Hou, X, Bergström, J., Wang, H., Feng, X., and Chen, A. 1999. The Chengjiang Fauna. Exceptionally well-pre-served animals from 530 million years ago. Yunnan Sci-ence and Technology Press, 170 pp.

Hou, X.-G., Ma, X.-Y., Zhao, J., and Bergström, J., 2004b, The lobopodian Paucipodia inermis from the Lower Cambrian Chengjiang fauna, Yunnan, China. Lethaia, 37: 235–244.

Hou, X., Ramsköld, L., and Bergström, J. 1991. Composition and preservation of the Chengjiang fauna—A Lower Cam-brian soft-bodied biota. Zoologica Scripta, 20: 395–411.

Huang, D., Vannier, J., and Chen, J.Y., 2004a, Recent Priapu-lidae and their Early Cambrian ancestors: Comparisons and evolutionary significance. Geobios, 37: 217–228.

Huang, D.-Y., Vannier, J., and Chen, J.-Y., 2004b, Anatomy and lifestyles of Early Cambrian priapulid worms exem-plified by Corynetis and Anningvermis from the Maotian-shan Shale (SW China). Lethaia, 37: 21–33.

Irimia, M., Maeso, I., Penny, D., Garcia-Fernàndez, J., and Roy, S.W., 2007. Rare coding sequence changes are con-sistent with Ecdysozoa, not Coelomata. Molecular Biol-ogy and Evolution, 24: 1604–1607.

Ivantsov, A.Yu., and Zhuravlev, A.Yu., 2005. (Cephalor-hynchs). In [Unique Sinsk Localities of Early Cambrian Organisms (Siberian Platform)]. Edited by A.G. Ponoma-renko. Palaeontological Institute, Russian Academy of Sciences, Transactions, 284: 61–73.

Ivantsov, A.Yu., Zhuravlev, A.Yu., Leguta, A.V., Krassilov, V.A., Melnikova, L.M., and Ushatinskaya, G.T., 2005. Palaeoecology of the Early Cambrian Sinsk biota from the Siberian Platform. Palaeogeography, Palaeoclimatol-ogy, Palaeoecology, 220: 69–88.

Jacobs, D.K., Nakanishi, N., Yuan, D., Camara, A., Nichols, S.A., and Hartenstein, V., 2007. Evolution of sensory structures in basal metazoa. Integrative and Comparative Biology, 47: 712–723.

Jager, M., Murienne, J., Clabaut, C., Deutsch, J., Le Guyader, H., and Manuel, M., 2006. Homology of arthropod ante-rior appendages revealed by Hox gene expression in a sea spider. Nature, 441: 506–508.

Jägersten, G. 1955. On the early phylogeny of the metazoa. The Bilaterogastraea theory. Zoologiska Bidrag från Up-psala, 30: 321–354.

Jägersten, G. 1959. Further remarks on the early phylogeny of metazoa. Zoologiska Bidrag från Uppsala, 33: 79–108.

Janssen, R., Wennberg, S.A., and Budd G.E., 2009. The hatching larva of the priapulid worm Halicryptus spinu-losus. Frontiers in Zoology, 6: 8–14.

Jensen, S., 2003. The Proterozoic and earliest Cambrian trace fossil record; patterns, problems and perspectives. Inte-grative and Comparative Biology, 43: 219–228.

Han, J., Liu, J., Zhang, Z., Zhang, X., and Shu, D., 2007. Trunk ornament on the palaeoscolecid worms Cricocos-mia and Tabelliscolex from the Early Cambrian Chengji-ang deposits of China. Acta Palaeontologica Polonica, 52: 423–431.

Han, J., Zhang, Z.-f., and Shu, D.-g., 2003. Discovery of the proboscis on Tylotites petiolaris. Northwestern Geology, 36: 87–93.

Han, Z., and Firtel, R.A. 1998. The homeobox-containing gene Wariai regulates anterior-posterior patterning and cell-type homeostasis in Dictyostelium. Development, 125: 313–325.

Harvey, T.H.P., Dong, X., and Donoghue P.C.J., 2010. Are palaeoscolecids ancestral ecdysozoans? Evolution & De-velopment, 12: 177–200.

Harzsch, S., 2006. Neurophylogeny: Architecture of the ner-vous system and a fresh view on arthropod phylogeny. Integrative and Comparative Biology, 46: 162–194.

Haug, J.T., Maas, A., Waloszek, D., Donoghue, P.C.J., and Bengtson, S., 2009. A new species of Markuelia from the Middle Cambrian of Australia. Association of Austral-asian Palaeontologists, Memoirs, 37: 303–313.

Heiner, I., Sørensen, K.J.K., and Kristensen, R.M., 2009. The spermiogenesis and the early spermatozoa of Ar-morloricus elegans (Loricifera, Nanaloricidae). Zoomor-phology, 128: 285–304.

Hejnol, A., and Schnabel, R., 2005. The eutardigade Thulin-ia stephaniae has an indeterminate development and the potential to regulate blastomere ablations. Development, 132: 1349–1361.

Higgins, R.P., and Thiel, H., eds. 1988. Introduction to the Study of Meiofauna. Smithsonian Institution Press, Washington, D.C. 488 pp.

Hou, X.G., and Aldridge, J., 2007. Development and diversi-fication of trunk plates of the Lower Cambrian lobopodi-ans. Palaeontology, 50: 401–415.

Hou, X.-g., Aldridge, R.J., Bergström J., Siveter, D.J., Sivet-er, D.J., and Feng, X.-h. 2004a. The Cambrian Fossils of Chengjiang, China: The Flowering of Early Animal Life. Blackwell Publishing, Malden, MA; Oxford; Carlton, Victoria. 233 p.

Hou, X.-g., Aldridge, R.J., Siveter, D.J., Siveter, D.J., and Feng, X.-h. 2002. New evidence on the anatomy and phylogeny of the earliest vertebrates. Proceedings of the Royal Society, B, 269: 1865–1869.

Hou, X., and Bergström, J. 1994. Palaeoscolecid worms may be nematomorphs rather than annelids. Lethaia, 27: 11–17.

Hou, X., and Bergström, J. 1995. Cambrian lobopodians—Ancestors of extant onychophorans? Linnean Society, Zoological Journal, 114: 3–19.

Hou, X., Bergström, J., and Ahlberg, P. 1995. Anomalocaris and other large animals in the Lower Cambrian Chengji-ang biota of southwest China. Geologiska Föreningens i Stockholm Förhandlingar, 117: 163–183.


Liu, J., Shu, D., Han, J., Zhang, Z., and Zhang, X., 2006b, A large xenusiid lobopod with complex appendages from the Lower Cambrian Chengjiang Lagerstätte. Acta Pal-aeontologica Polonica, 51: 215–222.

Liu, J., Shu, D., Han, J., Zhang, Z., and Zhang, X., 2007. Morpho-anatomy of the lobopod Magadictyon cf. haik-ouensis from the Early Cambrian Chengjiang Lagerstätte, South China. Acta Zoologica (Stockholm), 88: 279–288.

Liu, J., Shu, D., Han, J., Zhang, Z., and Zhang, X., 2008. The lobopod Onychodictyon from the Lower Cambrian Chengjiang Lagerstätte revisited. Acta Palaeontologica Polonica, 53: 285–292.

Lorenzen, S. 1985. Phylogenetic aspects of pseudocoelomate evolution. In The Origin and Relationships of Lower In-vertebrates. Edited by S. Conway Morris, J.D. George, R. Gibson and H.M. Platt. Systematics Association Special Volume 28. Clarendon Press, Oxford. pp. 210–223.

Ma, X., Hou, X., and Bergström, J., 2009. Morphology of Luolishania longicruris (Lower Cambrian, Chengjiang Lagerstätte, SW China) and the phylogenetic relationships within lobopodians. Arthropod Structure & Development, 38: 271–291.

Maas, A., Huang, D., Chen, J., Waloszek, D., and Braun, A., 2007a. Maotianshan-Shale nemathelminths—Morphol-ogy, biology, and the phylogeny of Nemathelminthes. Palaeogeography, Palaeoclimatology, Palaeoecology, 254: 288–306.

Maas, A., Mayer, G., Kristensen, R.M., and Waloszek, D., 2007b. A Cambrian micro-lobopodian and the evolution of arthropod locomotion and reproduction. Chinese Sci-ence Bulletin, 52: 3385–3392.

Maas, A., and Waloszek, D., 2001. Cambrian derivatives of the early arthropod stem lineage, pentastomids, tardigrades and lobopodians—An ‘Orsten’ perspective. Zoologischer Anzeiger, 240: 451–459.

Maas, A., Waloszek, D., Haug, J.T., and Müller, K.J., 2007c. A possible larval roundworm from the Cambrian ‘Orsten’ and its bearing on the phylogeny of Cycloneuralia. As-sociation of Australasian Palaeontologists, Memoirs, 34: 499–519.

Maas, A., Waloszek, D., Haug, J.T., and Müller, K.J., 2009. Loricate larvae (Scalidophora) from the Middle Cambrian of Australia. Association of Australasian Palaeontologists, Memoirs, 37: 281–302.

Malakhov, V.V. 1980. (Cephalorhyncha—A new phylum of the Animalia Kingdom united the Priapulida, Kino-rhyncha, Gordiacea and the system of pseudocoelomate worms). Zoologicheskiy zhurnal, 59(4): 485–499 [in Russian].

Malakhov, V.V., 2004. New ideas on the origin of bilateral ani-mals. Russian Journal of Marine Biology, 30 (supplement 1): S22–S33.

Malakhov, V.V., and Adrianov, A.V. 1995. Cephalorhyncha—A New Phylum of the Animal Kingdom. KMK Scientific Press, Moscow, 200 pp. [in Russian; English summary].

Kimm, M.A., and Prpic, N.-M., 2006. Formation of the ar-thropod labrum by fusion of paired and rotated limb-bud-like primordia. Zoomorphology, 125: 147–155.

Kraft, P., and Mergl, M. 1989. Worm-like fossils (Palaeosco-lecida; ?Chaetognatha) from the Lower Ordovician of Bohemia. Sbornik geologických vĕd, Paleontologie, 30: 9–36.

Kristensen, R.M., 2002. An introduction to Loricifera, Cy-cliophora, and Micrognathozoa. Integrative and Com-parative Biology, 42: 641–651.

Kristensen, R.M., Heiner, I., and Higgins, R.P., 2007. Mor-phology and life cycle of a new loriciferan from the At-lantic coast of Florida with emended diagnosis and life cycle of Nanaloricidae (Loricifera). Invertebrate Biol-ogy, 126: 120–137.

Lane, A.A., Braddy, S.J., Briggs, D.E.G., and Elliott, D.K., 2003. A new trace fossil from the Middle Cambrian of the Grand Canyon, Arizona, USA. Palaeontology, 46: 987–997.

Lartillot, N., and Philippe, H., 2008. Improvement of mo-lecular phylogenetic inference and the phylogeny of Bi-lateria. Philosophical Transactions of the Royal Society B: Biological Sciences, 363: 1463–1472.

Lavrov, D.V., 2007. Key transitions in animal evolution: a mitochonrial DNA perspective. Integrative and Compar-ative Biology, 47: 734–743.

Lavrov, D.V., Brown, W.M., and Boore, J.L., 2004. Phylo-genetic position of the Pentastomida and (pan)crustacean relationships. Proceedings of the Royal Society, B, 271: 537–544.

Lee, M.S.Y., 2000. Convergent evolution and character cor-relation in burrowing reptiles: Towards a resolution of squamate relationships. Linnean Society, Biological Journal, 65: 369–453.

Lehnert, O., and Kraft, P., 2006. Manitouscolex, a new pal-aeoscolecidan genus from the Lower Ordovician of Col-orado. Journal of Paleontology, 80: 386–391.

Lemburg, C. 1995. Ultrastructure of the introvert and asso-ciated structures of the larvae of Halicryptus spinulosus (Priapulida). Zoomorphology, 115: 11–29.

Lemburg, C. 1999. Ultrastrukturelle Untersuchungen an den Larven von Halicryptus spinulosus und Priapulus cau-datus. Hypothesen zur Phylogenie der Priapulida und deren Bedeutung für die Evolution der Nemathelmin-thes. Cuviller, Göttingen, 393 pp.

Liu, J., Han, J., Simonetta, A.M., Hu, S., Zhang, Z., Yao, Y., and Shu, D., 2006a, New observations of the lobopod-like worm Facivermis from the Early Cambrian Chengji-ang Lagerstätte. Chinese Science Bulletin, 51: 358–363.

Liu, J.H., Shu, D.G., Han, J., and Zhang, Z.F., 2004. A rare lobopod with well-preserved eyes from Chengjiang La-gerstätte and its implications for the origin of arthropods. Chinese Science Bulletin, 49: 1063–1071.


Müller, K.J., and Hinz-Schallreuter, I. 1993. Palaeoscolecid worms from the Middle Cambrian of Australia. Palaeon-tology, 36: 549–592.

Neuhaus, B., Bresciani, J., and Peters, W. 1997. Ultrastruc-ture of the pharyngeal cuticle and lectin labelling with wheat germ agglutinin-gold conjugate indicating chitin in the pharyngeal cuticle of Oesophagostomum dentatum (Strongylida, Nematoda). Acta Zoologica (Stockholm), 78: 205–213.

Neuhaus, B., Kristensen, R.M., and Lemburg, C. 1996. Ultra-structure of the cuticle of the Nemathelminthes and elec-tron miscroscopical localization of chitin. Verhandlungen der Deutschen Zoologischen Gesellschaft, 89: 221.

Neuhaus, B., and Higgins, R.P., 2002. Ultrastructure, biol-ogy, and phylogenetic relationships of Kinorhyncha. Inte-grative and Comparative Biology, 42: 619–632.

Nielsen, C. 1995. Animal Evolution. Interrelationships of the Living Phyla, 1st edn. Oxford University Press, Oxford, 467 pp.

Nielsen, C., 2001. Animal Evolution. Interrelationships of the Living Phyla, 2nd edn. Oxford University Press, Ox-ford, 563 pp.

Nielsen, C., 2003. Proposing a solution to the Articulata—Ecdysozoa controversy. Zoologica Scripta, 32: 475–482.

Oakley, T.H., Plachetzki, D.C., and Rivera, A.S. 2007. Furca-tion, field-splitting, and the evolutionary origins of nov-elty in arthropod photoreceptors. Arthropod Structure & Development, 36: 386–400.

Panganiban, G., Irvine, S.M, Lowe, C., Roehl, H., Corley, L.S., Sherbon, B., Grenier, J.K., Fallon, J.F., Kimble, J., Walker, M., Wray, G.A., Swalla, B.J., Martindale, M.Q., and Carroll, S.B. 1997. The origin and evolution of ani-mal appendages. National Academy of Sciences of the United States of America, Proceedings, 94: 5162–5166.

Park, J.-K., Rho, H.S., Kristensen, R.M., Kim, W., and Giri-bet, G., 2006. First molecular data on the Phylum Loric-ifera—An investigation into the phylogeny of Ecdysozoa with emphasis on the positions of Loricifera and Priapu-lida. Zoological Science, 23: 943–954.

Park, S.-J., Lee, Y.-S., and Hwang, U.W., 2007. The complete mitochondrial genome of the sea spider Achelia bituber-culata (Pycnogonida, Ammotheidae): arthropod ground pattern of gene arrangement. BioMed Central Genomics, 8: 343–358.

Peel, J.S., 2010. A corset-like fossil from the Cambrian Sirius Passet Lagerstätte of North Greenland and its implica-tions for cycloneuralian evolution. Journal of Paleontol-ogy, 84: 332–340.

Philip, G.K., Creevey, C.J., and McInerney, J.O., 2005. The Opisthokonta and the Ecdysozoa may not be clades: Stronger support for the grouping of plant and animal than for animal and fungi and stronger support for the Coelomata than Ecdysozoa. Molecular Biology and Evo-lution, 22: 1175–1184.

Mallatt, J.M., and Giribet, G., 2006. Further use of nearly complete 28S and 18S rRNA genes to classify Ecdysozoa: 37 more arthropods and a kinorhynch. Molecular Phyloge-netics and Evolution, 40: 772–794.

Märss, T. 1988. Early Palaeozoic hadimopanellids of Estonia and Kirgizia (USSR). Eesti NSV Teaduste Akadeemia Toimetised, Geoloogia, 37(1): 10–17.

Matz, M.V., Frank, T.M., Marshall, N.J., Widder, E.A., and Johnsen, S., 2008. Giant deep-sea protist produces bilate-rian-like traces. Current Biology, 18: 1849–1854.

Mayer, G., 2006. Structure and development of onychopho-ran eyes: What is the ancestral visual organ in arthropods? Arthropod Structure & Development, 35: 231–245.

Mayer, G., and Harzsch, S., 2007. Immunolocalization of serotonin in Onychophora argues against segmented ganglia being an ancestral feature of arthropods. BioMed Central Evolutionary Biology, 7: 118–125.

Mayer, G., and Koch, M., 2005. Ultrastructure and fate of nephridial anlagen in the antennal segment of Epiperipa-tus biolleyi (Onychophora, Peripatidae)—evidence for the onychophoran antennae being modified legs. Arthro-pod Structure & Development, 34: 471–480.

Maxmen, A., Browne, W.E., Martindale, M.Q., and Giribet, G., 2005. Neuroanatomy of sea spiders implies an ap-pendicular origin of the protocerebral segment. Nature, 437: 1144–1148.

McKirdy, D., Schmidt, P., and McGinty-Bayly, M. 1976. In-terstitielle Fauna von Galapagos XVI. Tardigrada. Mik-rofauna des Meeresbodens, 58: 409–448.

Minelli, A., 2003. The origin and evolution of appendages. Integrative Journal of Developmental Biology, 47: 573–581.

Miyazaki, K., 2002. On the shape of the foregut lumen in sea spiders (Arthropoda: Pycnogonida). Marine Bio-logical Association of the United Kingdom, Journal, 82: 1037–1038.

Monge-Nájera, J., and Hou, X. 1999. 500 millones de años de evolución: Onicóforos, los primeros animales que caminaron (Onychophora). In Evolución y Filogenia de Arthropoda. Edited by A. Melic, J.J. de Haro, M. Méndez and I. Ribera. Boletín de la Sociedad Entomológica Ara-gonesa, 26: 171–176.

Monge-Nájera, J., and Hou, X., 2002. Experimental tapho-nomy of velvet worms (Onychophora) and implications for the Cambrian “explosion, disparity and decimation” model. Revista de Biologia Tropical, 50: 1133–1138.

Morgan, C.I. 1982. Tardigrada. In Synopsis and Classifica-tion of Living Organisms. Edited by S.P. Parker. Mc-Graw-Hill, Inc., New York. pp. 731–739.

Müller, K.J. 1973. Milaculum n.g., ein phosphatisches Mi-krofossil aus dem Altpaläozoikum: Paläontologische Zeitschrift, 47: 217–228.

Müller, K.J., and Hinz, I. 1992. Camrogeorginidae fam. nov., soft-integumented Problematica from the Middle Cam-brian of Australia. Alcheringa, 16: 333–353.


Rogozin, I.B., Wolf, Y.I., Carmel, L., and Koonin, E.V. 2007. Ecdysozoan clade rejected by genome-wide analy-sis of rare amino acid replacements. Molecular Biology and Evolution, 24: 1080–1090.

Rozanov, A.Yu., and Zhuravlev, A.Yu. 1992. The Lower Cambrian fossil record of the Soviet Union. In Origin and Early Evolution of the Metazoa. Edited by J.H. Lipps and P.W. Signor. Plenum Press, New York. pp. 205–282.

Schmidt-Rhaesa, A., 2006. Perplexites concerning the Ec-dysozoa: A reply to Pilato et al.. Zoologischer Anzeiger, 244: 205–208.

Schmidt-Rhaesa, A., Bartolomaeus, T., Lemburg, C., Ehlers, U., and Garey, J.R. 1998. The position of the Arthropoda in the phylogenetic system. Jounal of Morphology, 238: 263–285.

Schulze, J., and Schierenberg, E., 2008. Cellular pattern for-mation, establishment of polarity and segregation of col-ored cytoplasm in embryos of the nematode Romanomer-mis culicivorax. Developmental Biology, 315: 426–436.

Sempere, L.F., Martínez, P., Cole, C., Baguñà, J., and Peter-son, K.J., 2007. Phylogenetic distribution of microRNAs supports the basal position of acoel flatworms and the polyphyly of Platyhelminthes. Evolution & Develop-ment, 9: 409–415.

Shcherbakov, D.E. 1999. Controversies over the insect ori-gin revisited. First Palaeoentomological Conference, Proceedings, Moscow 1998. AMBA projects, Bratislava. pp. 141–148.

Shu, D.-G., Conway Morris, S., Han, J., Chen, L., Zhang, X.-L., Zhang, Z.-F., Liu, H.-Q., Li, Y., and Liu, J.-N., 2001. Primitive deuterostomes from the Chengjiang Lager-stätte (Lower Cambrian, China). Nature, 414: 419–424.

Shu, D.-G., Conway Morris, S., Han, J., Zhang, Z., Yasui, K., Janvier, P., Chen, L., Zhang, X., Liu, J., Li, Y., and Liu, H., 2003. Head and backbone of the Early Cambrian vertebrate Haikouichthys. Nature, 421: 526–529.

Shubin, N., Tabin, C., and Carroll, S. 1997. Fossils, genes and the evolution of animal limbs. Nature, 388: 639–648.

Sly, B.J., Snoke, M.S., and Raff, R.A., 2003. Who came first—larvae or adults? Origins of bilaterian metazoan larvae. International Journal of Developmental Biology, 47: 623–632.

Sørensen, M.V., Hebsgaard, M.B., Heiner, I., Glenner, H., Willerslev, E., and Kristensen, R.M., 2008. New data from an enigmatic phylum: evidence from molecular se-quence data supports a sister-group relationship between Loricifera and Nematomorpha. Journal of Zoological Systematics and Evolutionary Research, 46: 231–239.

Störch, V., Higgins R.P., and Morse, P. 1989. Ultrastructure of the body wall of Meiopriapulus fijiensis (Priapulida). American Microscopical Society, Transactions, 108: 319–331.

Philippe, H., Lartillot, N. and Brinkmann, H., 2005. Multi-gene analyses of bilaterian animals corroborate the mono-phyly of Ecdysozoa, Lophotrochozoa, and Protostomia. Molecular Biology and Evolution, 22: 1246–1253.

Pilato, G., Binda, M.G., Biondi, O., D’Urso, V., Lisi, O., Marletta, A., Maugeri, S., Nobile, V., Rappazzo, G., Sa-bella, G., Sammartano, F., Turrisi, G., and Viglianisi, F., 2005. The clade Ecdysozoa, perplexites and questions. Zoologischer Anzeiger, 244: 43–50.

Plotnick, R.E. 1990. Paleobiology of the arthropod cuticle. In Arthropod Paleobiology: Short Courses in Paleon-tology, 3. Edited by D.G. Mikulic. The Paleontological Society Publication, University of Tennessee, Knoxville. pp. 177–196.

Podsiadlowski, L., Braband, A., and Mayer, G., 2008. The complete mitochondrial genome of the onychophoran Epiperipatus biolleyi reveals a unique transfer RNA set and provides further support for the Ecdysozoa hypoth-esis. Molecular Biology and Evolution, 25: 42–51.

Poinar, G., Jr. 1999. Palaeochordodes protus n.g., n.sp. (Nematomorpha, Chordodidae), parasites of a fossil co-chroach, with a critical examination of other fossil hair-worms and helminths of extant cochroaches (Insecta: Blattaria). Invertebrate Biology, 118: 109–115.

Ponomarenko, A.G., 2008. (Early stages of arthropod evolu-tion). In (Introduction to the Palaeoentomology). Edited by V.V. Zherikhin, A.G. Ponomarenko, and A.P. Rasnit-syn. Tovarishchestvo nauchnykh izdaniy KMK, Mos-cow. pp. 254–279 [in Russian].

Prpic, N.-M., 2008. Parasegmental appendage allocation in annelids and arthropods and the homology of parapodia and arthropodia. Frontiers in Zoology, 5: 17–21.

Raff, R.A., 2008. Origins of the other metazoan body plans: The evolution of larval forms. Philosophical Transac-tions of the Royal Society B: Biological Sciences, 363: 1473–1479.

Rieger, R.M., Haszprunar, G., and Schuchert, P. 1991. On the origin of the Bilateria: Traditional views and recent alternative concepts. In The Early Evolution of Metazoa and the Significance of Problematic Taxa. Edited by A.M. Simonetta and S. Conway Morris. Cambridge University Press, Cambridge; New York; Port Chester; Melbourne; Sydney. pp. 107–112.

Rieger, R.M., Ladurner, P., and Hobmayer, B., 2005. A clue to the origin of the Bilateria? Science, 307: 353–355.

Robson, E.A. 1964. The cuticle of Peripatopsis moseleyi. The Quarterly Journal of Microscopic Science, 105: 281–299.

Roeding, F., Hagner-Holler, S., Ruhberg, H., Ebersberger, I., von Haeseler, A., Kube, M., Reinhardt, R., and Bur-mester, T., 2007. EST sequencing of Onychophora and phylogenomic analysis of Metazoa. Molecular Phyloge-netics and Evolution, 45: 942–951.


Webster, B.L., Copley, R.R., Jenner, R.A., Mackenzie-Dodds, J.A., Bourlat, S.J., Rota-Stabelli, O., Littlewood, D.T.J., and Telford, M.J., 2006. Mitogenomics and phylogenom-ics reveal priapulid worms as extant models of the ances-tral Ecdysozoa. Evolution & Development, 8: 502–510.

Westheide, W. 1987. Progenesis as a principle in meiofauna evolution. Journal of Natural History, 21: 843–854.

Whittington, H.B. 1978. The lobopod animal Aysheaia pe-dunculata Walcott, Middle Cambrian, Burgess Shale, British Columbia. Philosophical Transactions of the Roy-al Society B: Biological Sciences, 284: 165–97.

Wolff, C., and Scholtz, G., 2008. The clonal composition of biramous and uniramous arthropod limbs. Proceedings of the Royal Society, B, 275: 1023–1028.

Wright, K.A., and Hope, W.D. 1968. Elaborations of the cuti-cle of Acanthonchus duplicatus Wieser, 1959 (Nematode: Cyatholaimidae) as revealed by light and electron mi-croscopy. Canadian Journal of Zoology, 46: 1005–1011.

Wrona, R. 1982. Early Cambrian phosphatic microfossils from southern Spitsbergen (Hornsund Region). Palaeon-tologia Polonica, 43: 9–16.

Wrona, R., 2004. Cambrian microfossils from glacial erratics of King George Island, Antarctica. Acta Palaeontologia Polonica, 49: 13–56.

Young, G.C., Karatajute-Talimaa, V.N., and Smith, M.M. 1996. A possible Late Cambrian vertebrate from Austra-lia. Nature, 383: 810–812.

Zantke, J., Wolff, C., and Scholtz, G., 2008. Three-dimen-sional reconstruction of the central nervous system of Macrobiotus hufelandi (Eutardigrada, Parachela): impli-cations for the phylogenetic position of Tardigrada. Zoo-morphology, 127: 21–36.

Zhang, X., and Briggs, D.E.G., 2007. The nature and signifi-cance of the appendages of Opabinia from the Middle Cambrian Burgess Shale. Lethaia, 40: 161–173.

Zhang, X.-G., Hou, X.-G., and Bergström, J., 2006. Early Cambrian priapulid worms buried with their lined bur-rows. Geological Magazine, 143: 743–748.

Zhang, X.-G., and Pratt, B.R. 1996. Early Cambrian pal-aeoscolecid cuticles from Shaanxi, China. Journal of Pa-leontology, 70: 275–279.

Zhang, X.-g., and Pratt, B.R., 2008. Microborings in Early Cam-brian phosphatic and phosphatized fossils. Palaeogeography, Palaeoclimatology, Palaeoecology, 267: 185–195.

Zhuravlev, A.Yu. 1995. (The world that can not be). Priroda, 1995(12): 21–28 [in Russian].

Zhuravlev, A.Yu., 2005. (Tardipolypodians). In [Unique Sinsk Localities of Early Cambrian Organisms (Siberian Platform)]. Edited by A.G. Ponomarenko. Palaeontologi-cal Institute, Russian Academy of Sciences, Transactions, 284: 56–61 [in Russian].

Zhuravlev, A.Yu., Gámez Vintaned, J.A., and Ivantsov, A.Yu., 2009. First finds of problematic Ediacaran fossil Gaojiashania in Siberia and its origin. Geological Maga-zine, 146: 775–780.

Strausfeld, N.J., Strausfeld, C.M., Stowe, S., Rowell, D., and Loesel, R., 2006. The organization and evolutionary implications of neuropils and their neurons in the brain of the onychophoran Euperipatoides rowelli. Arthropod Structure & Development, 35: 169–196.

Tabin, C.J., Carroll, S.B., and Panganiban, G. 1999. Out on a limb: Parallels in vertebrate and invertebrate limb pat-terning and the origin of appendages. American Zoolo-gist, 39: 650–663.

Tatarinov, L.P. 1976. (Morphological Evolution of Theri-odons and General Problems of the Phylogenetics). Nau-ka, Moscow, 258 pp. [in Russian].

Telford, M.J., Bourlat, S.J., Economou, A., Papillon, D., and Rota-Stabelli, O., 2008. The evolution of the Ecdysozoa. Philosophical Transactions of the Royal Society B: Bio-logical Sciences, 363: 1529–1537.

Ungerer, P., and Scholtz, G., 2009. Cleavage and gastrula-tion in Pycnogonum litorale (Arthropoda, Pycnogonida): morphological support for the Ecdysozoa? Zoomorphol-ogy, 128: 263–274.

Usami, Y., 2006. Theoretical study on the body form and swimming pattern of Anomalocaris based on hydrody-namic simulation. Journal of Theoretical Biology, 238: 11–17.

Valentine, J.W., Jablonski, D., and Erwin, D.H. 1999. Fos-sils, molecules and embryos: New perspectives on the Cambrian explosion. Development, 126: 851–859.

van Beneden, E. 1891. Recherches sur le dèveloppment des Arachnactis. Contribution à la morphologie des Cèrian-thides. Archives de Biologie, Liège, 11: 115–146.

van den Boogaard, M. 1989. Isolated tubercles of some Pal-aeoscolecida. Scripta Geologica, 90: 1–12.

Vannier, J., Steiner, M., Renvoisé, E., Hu, S.-X., and Casa-nova, J.-P., 2007. Early Cambrian origin of modern food webs: evidence from predator arrow worms. Proceedings of the Royal Society, B, 274: 627–633.

Vilpoux, K., and Waloszek, D., 2003. Larval development and morphogenesis of the sea spider Pycnogonum lito-rale (Ström, 1972) and the tagmosis of the body of Pan-topoda. Arthropod Structure & Development, 32: 349–383.

Wägele, J.-W., and Misof, B., 2001. On quality of evidence in phylogeny reconstruction: a reply to Zrzavý’s defence of the ‘Ecdysozoa’ hypothesis. Journal of Zoological Systematics and Evolutionary Research, 39: 165–176.

Waloszek, D., and Dunlop, J.A., 2002. A larval sea spider (Arthropoda: Pycnogonida) from the Upper Cambrian ‘Orsten’ of Sweden, and the phylogenetic position of pycnogonids. Palaeontology, 45: 421–446.

Waloszek, D., Repetski, J.E., and Maas, A., 2006. A new Late Cambrian pentastomid and a review of relationships of this parasitic group. Transactions of the Royal Society of Edinburgh, Earth Sciences, 96 (for 2005): 163–176.


Zrzavý, J., 2001. The interrelationships of metazoan para-sites: a review of phylum and higher-level hypotheses from recent morphological and molecular phylogenetic analyses. Folia Parazitologica, 48: 81–103.

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