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265FRY ´ DA AND ROHR—SHELL HETEROSTROPHY IN EARLY ORDOVICIAN MACLURITELLA

FIGURE 1—Schematic diagrams showing four possibilities of the relationship between shell coiling and body asymmetry in shell-bearing gastropods(1–4) and the relationship between the coiling of larval (protoconch II) and postlarval (teleoconch) shells ( 5, 6 ). Orthostrophy (1, 2) means thatanatomically dextral (or sinistral) animals occupy dextrally (or sinistrally) coiled shells. Conversely, hyperstrophy (3, 4) means that handedness of the shell and soft body is different. Coiling of all shell whorls in the same direction (e.g., Caenogastropoda and Neritimorpha) is termed homeos-trophic (5). If handedness of some whorls is opposite (e.g., Heterobranchia), the coiling is termed heterostrophic (6 ).

in Williams Canyon as well as from the Ramparts Road sectionabout 4 km north-northeast of the Williams Canyon section. Thetype locality corresponds to our locality R01–01 and is in theMiddle Cherty Member of the Manitou Formation, along the oldroad to the visitor center at Cave of the Winds, UTM Zone 13,506860E, 4302191N, NAD 27.

The second locality contains sponges, and the gastropods seemto be concentrated locally. The beds at this locality are withinTrilobite Zone F (Loche in Rigby and Myrow, 1999). In this re-gion, the entire Manitou is Early Ordovician in age ( Rossodusmanitouensis Zone) (Myrow et al., 1999). The gastropods hereare larger than those that occur at Williams Canyon. Gastropodsare from locality R01–03D in the Ptarmigan Chert Member of the Manitou Formation (Myrow et al., 2003). This corresponds to

Rigby and Myrow’s (1999) sponge locality. The reddish cherty,dolomitic limestone is exposed on the Ramparts Forest Serviceroad north of the Garden of the Gods at UTM zone 13, 508877E,4304961N, NAD 27. The depositional and stratigraphic setting of the Lower Ordovician Manitou Limestone has been reviewed byRigby and Myrow (1999) and Myrow et al. (2003).

SYSTEMATIC PALEONTOLOGY

Class GASTROPODA Cuvier, 1797Subclass UNCERTAIN

Superfamily MACLURITOIDEA Carpenter, 1861

Discussion. Macluritoidea are a well-known group of Ordo-vician gastropods (Knight, 1952; Rohr, 1979, 1994; Rohr andGubanov, 1997; Fryda and Rohr, 2004). However, little is knownabout their phylogenetic position. Wenz (1938) placed the family

Macluritidae together with the Euomphalidae, Omphalocirridae,Platyacridae, Cirridae, Oriostomatidae, and Poleumitidae withinthe superfamily Euomphaloidea (Euomphalacea). This conceptwas followed by Yochelson (1956), who interpreted the Euom-phaloidea to have been derived from the Macluritoidea duringEarly Ordovician time. Later Cox and Knight (1960) placed theMacluritoidea Carpenter, 1861, together with the Euomphaloideade Koninck, 1881, in the suborder Macluritina of the Archaeo-gastropoda (Knight et al., 1960). Their concept of the Macluritinawas later criticized by several authors (Morris and Cleevely, 1981;Dzik, 1983; Linsley and Kier, 1984; Yochelson, 1984). Accordingto these workers, the Macluritoidea and Euomphaloidea should

not be linked together in a common taxon. Similarly, opinions onthe generic composition of the Macluritoidea Carpenter, 1861were changed several times (Linsley and Kier, 1984; Yochelson,1984; Rohr et al., 1992; Rohr, 1994; Wagner, 2002).

Family MACLURITIDAE Carpenter, 1861Genus MACLURITELLA Kirk, 1927

Type species. Macluritella stantoni Kirk, 1927, p. 288, figs.1–12; Knight, 1941, p. 184, pl. 66, fig. 2a–d.

Included species. Beside the type species, Macluritella stan-toni, only a few poorly known species were placed into this genus.Sando (1957) established Macluritella marylandicus from theLower Ordovician Rockdale Run Formation [ Rossodus mani-touensis through Oepikodus communis conodont zones of Harris

and Harris (1978)] in Maryland. Macluritella may also be rep-resented by internal molds from the Lower Oslobreen Limestones(Lower Ordovician) in Spitsbergen (Hallam, 1958). The shellidentified as Macluritella by Gubanov (in Sokolov, 1992) fromthe Silurian (Llandoverian) of the Siberian Platform is poorly pre-served, but does not appear to belong to the genus because itlacks diagnostic features.

Discussion. Wenz (1938) placed Macluritella in the familyMacluritidae Carpenter, 1861, and this concept was followed byKnight et al. (1960). Later Yochelson and Stinchcomb (1987)moved the genus to the Euomphaloidea de Koninck, 1881. Wag-ner (2002), based on a phylogenetic analysis of numerous teleo-conch features, placed Macluritella back in the Macluritoidea.

Macluritella is generally considered to be the oldest representativeof the family (Knight et al., 1960; Wagner, 2002). Knight et al.

(1960) placed two subgenera under Macluritella: Macluritella and Euomphalopsis Ulrich and Bridge, 1931. Yochelson and Stinch-comb (1987) restored Euomphalopsis to full generic status, andthey extended the range of Macluritella down from the LowerOrdovician into the Upper Cambrian with the addition of Ma-cluritella? walcotti (Howell, 1946), a species with an acutely tri-angular profile and unknown early whorls. Yochelson and Stinch-comb (1987) interpreted the type lot of M. stantoni to be rounded, juvenile forms that during later ontogeny would evolve whorlswith a triangular profile. We exclude M.? walcotti from the genus

Macluritella because of its distinctly triangular shape and shallowsinus. Thus, Macluritella has no certain Cambrian record.



266 JOURNAL OF PALEONTOLOGY, V. 80, NO. 2, 2006

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FIGURE 2— Macluritella stantoniKirk, 1927 from the Manitou Limestone, Williams Canyon area, near Manitou Springs, Colorado. Specimen numbersare those of the U.S. National Museum, Museum of Natural History, Washington, DC. 1–7, Oblique lateral, oblique apical, apical, apertural, apical,oblique apertural, and oblique basal views of specimen USNM 528374 from locality R01-01, all 5; 8, 10–12, 14, 15, oblique apical, apical,detail of figure 8, oblique, lateral, and oblique lateral views of the same specimen showing distinct dextral coiling in early whorls, 8, 18; 10,15; 11, 25; 12, 14; 14, 20; 15, 15; 9, dextrally coiled juvenile specimen USNM 528375 from locality R01-01, 12; 13, detail of dextrallycoiled early whorls in specimen USNM 528376 from locality R01-03, 25.

MACLURITELLA STANTONI Kirk, 1927Figure 2

Macluritella stantoni KIRK, 1927, p. 289, figs.10–12; KNIGHT, 1941, p.184, pl. 66, fig. 2a–d; YOCHELSON AND STINCHCOMB, 1987, p. 57;WAGNER, 2002, p. 35.

Emended description. Discoidal shells with rounded, sinis-trally coiled whorls in adult shells; juvenile whorls distinctly dex-

trally coiled (Fig. 2); all whorls openly coiled; width of teleo-conch about double its height; whorl profile roughly subcircularbut somewhat flattened at upper whorl surface (Fig. 2.1, 2.4, 2.6);slightly concave, nearly flat upper shell surface bearing centraldepression formed by umbilicus of dextrally coiled initial part;umbilicus of sinistrally coiled teleoconch wide, its diameter widerthan that of aperture (Fig. 2.4, 2.6); umbilical and lateral whorlsurfaces separated by rounded keel (Fig. 2.4, 2.7); diameter of subcircular aperture about one-third of maximum shell diameter(Fig. 2.4, 2.6, 2.7); shell wall moderately thick; ornamentationconsisting of rather widely spaced, irregular transverse lirae (Fig.2.1–2.3); initial part of shell not well preserved, but at least twofirst whorls distinctly dextrally coiled (Fig. 2.8–2.15); protoconch/ teleoconch transition not preserved due to silicification of shell;diameter of first preserved whorl about 0.5 mm; first preserved

(probably juvenile teleoconch) whorl openly coiled; gap separat-ing first and second whorls much wider than that in other whorls(Fig. 2.8–2.15).

Aperture tangential with no reentrants; closely spaced growthlamella developed near aperture of larger specimens indicatesadult stage; thick, internally tapering aperture on some specimenssuggests an operculum may have been retracted a short distanceinside shell, but no opercula were found with shells.

Material examined . Knight (1941) designated the specimenillustrated by Kirk, 1927 as figures 10–12 on page 289 as theholotype (one of six original syntypes under No. 71710 in theU.S. National Museum, Washington, DC). The remaining fivesyntypes represent paratypes.

Occurrence. Macluritella stantoni was described from a ho-rizon about 125 ft below the top of the Manitou Limestone (‘‘Tre-madocian,’’ Lower Ordovician), in Williams Canyon, near Man-

itou Springs, Colorado. In addition to the type area, Macluritellastantoni has been reported from the upper part of the RoubidouxFormation in Missouri (Heller, 1954).

EVIDENCE OF HYPERSTROPHY IN MACLURITOIDEA

The Ordovician genus Maclurites LeSueur, 1817 belongs to alimited group of Paleozoic gastropods which developed a thick,calcified operculum. This type of operculum had a high proba-bility of being fossilized, and in some specimens it has been foundin situ within the aperture. The spiral operculum of Maclurites issinistrally (counterclockwise) coiled. As noted by Knight (1952),this feature was interpreted by Woodward (1854) as evidence fordextral hyperstrophic coiling. Detailed discussion on dextral hy-perstrophy in the Macluritoidea may be found in several papers(Knight, 1952; Knight et al., 1960; Yochelson, 1984, 1990; Lin-

sley and Kier, 1984; Peel and Horny, 1996). Yochelson (1990)suggested that the operculum in Maclurites grew differently thanspiral opercula in more advanced gastropods. This interpretation

is based on the polygonal aperture of Maclurites. According toYochelson, the macluritid opercula grew by accretion around theentire margin and thus not just at a narrow growth zone as inrounded spiral opercula. Nevertheless, counterclockwise curva-ture of the exterior surface of the operculum in Maclurites sup-ports its interpretation as dextrally hyperstrophic (Yochelson,1990; Peel and Horny, 1996).

The Macluritoidea are one of the oldest gastropod groups and,

in contrast to vast majority of fossil and living gastropods, theyhave been interpreted as dextrally hyperstrophic. Thus, the doc-umentation of sinistral heterostrophy in the oldest macluritid ge-nus Macluritella (see below) has significant implications for in-terpretation of shell ontogeny in the Macluritoidea. Macluritellacan be interpreted as dextrally orthostrophic during its early on-togeny when its shell was dextrally coiled (Fig. 2.8–2.15). Thus,at this ontogenetic stage, the Macluritoidea had the same type of soft-body-shell arrangement (anatomically dextral body in a dex-trally coiled shell) as the vast majority of living and fossil gas-tropods. During later ontogeny the shell of Macluritella changedthe coiling of its shell to sinistral, and thus became dextrally hy-perstrophic. This interpretation of the oldest macluritid genus sug-gests that the Macluritoidea evolved from dextrally orthostrophicgastropods (Haeckel’s biogenetic law), and their dextral hyper-strophy is a derived rather than a primary shell feature. This facthas significant implications for evaluation of their phylogeny andrelationships to other gastropod groups (see below).

HETEROSTROPHIC COILING IN MACLURITELLA AND WITHIN THE

CLASS GASTROPODA

Heterostrophy is defined as the condition where the protoconchand/or early whorls of the teleoconch appear to be coiled in anopposite direction to the later teleoconch whorls. A more detaileddescription and different usage of the term heterostrophy may befound in Knight et al. (1960), Dzik (1983), Hadfield and Strath-mann (1990), and Robertson (1993). The present study of Ma-cluritella stantoni has yielded evidence for heterostrophic coilingof its shell. At least the first two shell whorls in Macluritellastantoni are distinctly dextrally coiled (Fig. 2.8–2.15). During lat-er ontogeny the shell whorls become planispirally and subse-

quently sinistrally coiled (Fig. 2.1–2.7). This represents the firstevidence of heterostrophic coiling in the Macluritoidea. Shell het-erostrophy has been considered by many neontologists to be lim-ited to the gastropod subclass Heterobranchia. However, this shellfeature was later also documented in some archaeogastropods(Hadfield and Strathmann, 1990; Fryda and Blodgett, 1998,2001). The distribution of heterostrophy within the class Gastro-poda is discussed in the following paragraphs.

The majority of marine, shell-bearing Heterobranchia has de-veloped heterostrophically coiled shells. In these gastropods thetiming of the change of the shell coiling (from sinistral to dextralor vice versa) is always connected with an ontogenetic stage whena true larval shell (protoconch II) is completed and the teleoconchstarts to form. The latter type of shell heterostrophy is consideredto be an apomorphy of the Heterobranchia and it occurs in all

heterobranchs where planktotrophic larvae have been developed.In heterobranch gastropods with a lecitotrophic or direct devel-opment (some marine, freshwater, and terrestrial Heterobranchia),




this shell feature occurs rarely because the morphology of theearly shell is simplified. Modern Heterobranchia represent a verysuccessful gastropod group which unites many thousands of ex-tant species. Also, the fossil record of shell-bearing heterobranchsis relatively rich in Tertiary and Mesozoic strata. Unfortunately,our knowledge of early members of the Heterobranchia, whichundoubtedly evolved in the Paleozoic, is still very poor. Hetero-branchia have been well documented from late Paleozoic strata,

the vast majority of which belong to the superfamily Streptaci-doidea Knight, 1931. These gastropods which have slender, high-spired, multiwhorled shells bearing a sinistral protoconch wererelatively common in Carboniferous faunal communities (Donald,1898; Knight, 1931; Batten, 1966; Anderson et al., 1985, 1990;Yoo, 1994; Bandel, 1996, 2002). The Givetian (late Middle De-vonian) genus Heteroloxonema Fryda, 2000 represents probablythe oldest-known streptacidoidean genus. In addition to the Strep-tacidoidea, several additional Paleozoic groups of Heterobranchiawith heterostrophic shells have been documented (Fryda andBlodgett, 2001, 2004; Bandel and Heidelberger, 2002; Bandel,2002; Nutzel et al., 2002). Taken together, in Paleozoic time therewere at least three distinct groups belonging to the subclass Het-erobranchia Gray, 1840: the Streptacidoidea Knight, 1931 (?Mid-dle Devonian, Mississippian–Late Permian, Allogastropoda Hasz-prunar, 1985), Stuoraxidae Bandel, 1996 (Late Permian,Architectonicoidea Gray, 1850), and Kuskokwimiidae Fryda andBlodgett, 2001 (Devonian). The phylogenetic relationships of theDevonian taxa within the Heterobranchia are still uncertain.

In contrast to the Heterobranchia, the development of shell het-erostrophy in members of the Archaeogastropoda is very rare andis limited to several small groups. Hadfield and Strathmann (1990)described a slight, but distinct sinistral coiling in the first whorlof three members of extant Trochoidea (Vetigastropoda, Archaeo-gastropoda) with dextrally coiled teleoconchs. They interpretedtheir findings as evidence for heterostrophic shell coiling. Heter-ostrophy also has been found in some fossil archaeogastropods(Knight, 1941; Knight et al., 1960; Batten, 1966; Bandel, 1993;Fryda, 1997; Fryda and Blodgett, 1998; Fryda et al., 2002; Frydaand Farrell, 2005). These taxa belong to the Paleozoic AgnesiinaeKnight, 1956 (Porcelliidae Koken in Zittel, 1895) and the Me-

sozoic Cirridae Cossmann, 1916. At present it is difficult to solvethe question of whether sinistral heterostrophy is a characterwhich is apomorphic for the Porcellioidea (thus, uniting the Me-sozoic Cirridae and the Paleozoic Porcelliidae into one naturalgroup), or if this character originated independently in these twoarchaeogastropod groups. Nevertheless, both groups representgood examples of the occurrence of sinistral heterostrophy in fos-sil archaeogastropods. Fryda and Blodgett (1998) showed thatboth types of shell heterostrophy (anastrophic as well as inclinedheterostrophic coiling) were present in the Archaeogastropoda.Later Fryda and Blodgett (2001, 2004) discussed the origin of shell heterostrophy in the Agnesiinae and concluded that the de-velopment of shell heterostrophy in the Archaeogastropoda andHeterobranchia is not homologous. Discovery of shell heterostro-phy in Macluritella stantoni represents the first evidence for the

occurrence of this remarkable shell feature in the Macluritoidea.Significantly, it also represents the oldest evidence (Early Ordo-vician) for shell heterostrophy in the class Gastropoda.

RELATIONSHIPS OF MACLURITOIDEA TO EUOMPHALOIDEA

Yochelson (1956) considered the Early Ordovician Maclurito-idea to be the ancestral group of the Euomphaloidea. This conceptwas subsequently followed by Knight et al. (1960) and manyother paleontologists. However, Yochelson (1984) concluded thatthe concept of the Macluritina uniting the superfamilies Maclur-itoidea and Euomphaloidea should be abandoned. He interpretedthe Euomphaloidea as an independent superfamily derived from

Lesueurilla-like forms of the pleurotomarioidean stock. On theother hand, McLean (1981) united members of the modern Neom-phaloidea McLean, 1981 with the Paleozoic Euomphaloidea andplaced them in the Archaeogastropoda. Later Bandel and Fryda(1998) showed that an unusual protoconch morphology of De-vonian and Carboniferous euomphaloidean genera distinguishesthem from members of extant gastropod groups (Patellogastro-poda, Archaeogastropoda, Neritimorpha, Caenogastropoda, and

Heterobranchia). Consequently, Bandel and Fryda (1998) consid-ered euomphaloideans to represent an independent, exclusivelyPaleozoic (Cambrian–Permian) gastropod group. Recently, Nutzel(2002) confirmed earlier observations (Yoo, 1994; Bandel andFryda, 1998) on the nature of the protoconch and the shape of the boundary between the protoconch and teleoconch. He alsonoted that the protoconch of some euomphalids is not fundamen-tally different from that of Docoglossa, Cocculiniformia, andNeomphalidae. Thus, recent models of gastropod evolution haveoffered the following possibilities for the relationships of the Ma-cluritoidea and Euomphaloidea: 1) both groups are closely relatedand belong to the Archaeogastropoda (Knight, 1952; Knight etal., 1960; Wagner, 2001, 2002); 2) both groups are not closelyrelated and belong to the Archaeogastropoda (Yochelson, 1984);or 3) both groups are closely related but they are not gastropods(Linsley and Kier, 1984).

The protoconch morphology of the Macluritoidea is still un-known; however, the morphology of the openly coiled earlywhorls in Macluritella stantoni (Fig. 2.10–2.12) resembles that inthe Euomphaloidea (Yoo, 1994; Bandel and Fryda, 1998; Hei-delberger and Bandel, 1999; Nutzel, 2002). In addition, recentanalysis of the biodiversity of Ordovician gastropods (Fryda andRohr, 2004) has shown a similarity in the diversity and turnoverpatterns of the Ordovician Euomphaloidea and Macluritoidea.This fact may suggest similarities in some of their life strategies(Fryda and Rohr, 2004). In summary, although an integration of the Macluritoidea and Euomphaloidea into one higher taxon (e.g.,Macluritina) was criticized by several authors (Morris and Cleev-ely, 1981; Linsley and Kier, 1984; Yochelson, 1984), the similar-ity in early shell morphology suggests a dextrally coiled commonancestor and seems to support the old concept uniting both of

these superfamilies in a common higher taxon. Nevertheless, ab-sence of knowledge on the protoconch morphology in the Ma-cluritoidea prevents more detailed analysis of relationships be-tween the Macluritoidea and Euomphaloidea.

RELATIONSHIPS OF MACLURITOIDEA TO ONYCHOCHILOIDEA

Knight et al. (1960) considered members of the OnychochilidaeKoken, 1925 to be closely related to macluritid gastropods. Thisapproach was followed by several authors (Wangberg-Eriksson,1979; Linsley and Kier, 1984). Knight et al. (1960), in contrastto Wenz (1938), considered onychochylids to be dextral hyper-strophic gastropods like the macluritids. Clisospirids were inter-preted as sinistral orthostrophic gastropods with an uncertainhigher taxonomic position. However, analysis of the shell mor-phologies in later-discovered clisospirid and onychochilid taxa

suggests that both the families Clisospiridae and Onychochilidaeare closely related (Horny, 1964; Golikov and Starobogatov,1975; Wangberg-Eriksson, 1979; Peel, 1986). McLean (1981)suggested that the members of the superfamilies Macluritoideaand Onychochiloidea do not belong to his suborder Euomphalinabut represent a separate lineage. In addition, Dzik (1983), on thebasis of his study of the early shell ontogeny of the onychochi-loidean genus Mimospira Koken, 1925, separated both the Cli-sospiridae and the Onychochilidae from the suborder Macluritinaand established a new suborder Mimospirina for them. The lattertaxon was interpreted as belonging to the Archaeogastropoda.Linsley and Kier (1984) proposed to unite the Onychochiloidea




(including the Clisospiridae and Onychochilidae), Macluritoidea,and possibly the Euomphaloidea in the new order Hyperstrophinaof the new class Paragastropoda. The Paragastropoda has beenconsidered to unite untorted mollusks. However, Fryda (1999,2001) and Fryda and Rohr (1999, 2004) pointed out that membersof the Onychochiloidea and Euomphaloidea have quite differentprotoconch morphologies. The Paleozoic (Cambrian–Devonian)Mimospirina have a sinistrally coiled teleoconch with a large,

smooth, sinistrally coiled protoconch consisting of several whorls(Dzik, 1983; Fryda, 1989, 1992, 1999). This protoconch type isunlike that of the Archaeogastropoda and it has been interpretedas a true larval shell (protoconch II; Fryda, 1995, 2001; Frydaand Rohr, 2004). The Paleozoic (Cambrian–Permian) Euomphal-oidea had a cyrtoconic, openly coiled early whorl changing to ashort, curved, tubular shell (Yoo, 1994; Bandel and Fryda, 1998;Heidelberger and Bandel, 1999; Nutzel, 2002). The latter shellhas been interpreted as an embryonic shell (protoconch I; Fryda,1999, 2001; Fryda and Rohr, 2004). Dissimilarity in protoconchmorphologies of the Onychochiloidea and Euomphaloidea wasinterpreted as a basis for suggesting that the class Paragastropodais an artificial group (Fryda, 1999, 2001; Fryda and Rohr, 2004).Moreover, the discovery of dextral coiling of the early shell in

Macluritella stantoni (Fig. 2) as well as its shell heterostrophyprovides evidence that the Macluritoidea and Onychochiloidea arenot closely related taxa.

Ponder and Lindberg (1997, p. 203) speculated that the ances-tors of the Heterobranchia may have arisen from a lineage withhyperstrophic dextral (i.e., sinistrally coiled) shells. They sug-gested that such an ancestor might be found among the Maclur-itoidea, and the Ordovician ‘‘sinistral hyperstrophic’’ Mimospirais mentioned as their possible example. However, there is no ev-idence that Mimospira and related genera belonging to the su-perfamily Onychochiloidea were hyperstrophic. As noted above,the Onychochiloidea have homeostrophic, sinistrally coiled shellsand most probably were not closely related to the Macluritoidea(Dzik, 1983; Fryda, 1999, 2001; Wagner, 2001, 2002; Fryda andRohr, 2004). Our discovery of sinistral heterostrophy in the oldestmacluritid genus, Macluritella, also does not support the specu-lation that the dextral hyperstrophic Macluritoidea are ancestral

to the Heterobranchia.

CONCLUSIONS

New data on the early whorl morphology of the Early Ordo-vician macluritid gastropod Macluritella stantoni significantly ad-vances our knowledge of the early phylogeny of the superfamilyMacluritoidea as well as of the phylogenetic relationships of ma-cluritoideans to other Paleozoic gastropods:

1. Early whorls in Macluritella stantoni are openly and dextrallycoiled. The adult teleoconch of this species is sinistrallycoiled as are those in all macluritoidean gastropods. The sin-istrally coiled teleoconch of the Macluritoidea has been in-terpreted as dextrally hyperstrophic. Thus juvenile Macluri-tella stantoni, with a dextrally coiled shell, was dextral

orthostrophic at this ontogenetic stage and so it had the sametype of soft-body-shell arrangement (anatomically dextralbody in dextrally coiled shell) as in the vast majority of gas-tropods.

2. Interpretation of juvenile Macluritella, which is the oldestmacluritid gastropod, as dextrally orthostrophic suggests thatthe Macluritoidea evolved from the dextrally orthostrophicgastropods (Haeckels biogenetic law). Thus the dextral hy-perstrophy of the Macluritoidea is a derived and not a pri-mary shell feature.

3. Occurrence of dextral coiling of the early whorls as well asshell heterostrophy in Macluritella stantoni suggests that the

Macluritoidea and Onychochiloidea are not closely relatedtaxa. However, some similarities in early shell morphologybetween the Macluritoidea and Euomphaloidea may supportthe old concept uniting both of these superfamilies in a com-mon higher taxon.

ACKNOWLEDGMENTS

P. M. Myrow, Colorado College, provided invaluable assistancewith his knowledge of the Manitou Formation. This study wassupported by a Faculty Research Enhancement Grant from SulRoss State University to Rohr and by the Alexander von Hum-boldt-Stiftung and grants 206/04/0599 and 206/04/0600 from theGrant Agency of the Czech Republic to Fryda. We also thank K.Bandel (Germany), A. Nutzel (Germany), and D. M. Work (Maine) for their thoughtful reviews of the manuscript.

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