Bivalves from the Olenekian (Early Triassic) of south-western Utah: systematics and evolutionary...

Journal of Systematic Palaeontology, 2013Vol. 11, Issue 3, 263–293, http://dx.doi.org/10.1080/14772019.2011.637516

Bivalves from the Olenekian (Early Triassic) of south-western Utah: systematicsand evolutionary significance

Michael Hautmanna∗, Andrew B. Smithb, Alistair J. McGowanc and Hugo Buchera

aPalaontologisches Institut und Museum, Universitat Zurich, Karl Schmid-Strasse 4, 8006 Zurich, Switzerland; bDepartment ofPalaeontology, Natural History Museum, Cromwell Road, London, SW7 5BD, UK; cSchool of Geographical and Earth Sciences,

University of Glasgow, Gregory Building, Lilybank Gardens, Glasgow, G12 8QQ, UK

(Received 28 February 2011; accepted 16 August 2011; first published online 11 December 2012)

The recovery from the end-Permian mass extinction event was a key interval in the history of life, but few modern studiesprovide systematic data on benthic marine faunas from the epoch immediately following the crisis. Here, the bivalve faunafrom the early Spathian (Olenekian, late Early Triassic) Virgin Limestone Member of the Moenkopi Formation is compre-hensively documented for the first time. The new genus Sementiconcha (Myophoricardiidae), type species Sementiconcharecuperator sp. nov., and the new species Leptochondria nuetzeli, Eumorphotis ericius, E. virginensis and Pleuromya prima,are described. Leptochondriidae is placed in synonymy with Asoellidae, which is revised. With 27 species belonging to 18genera, the Virgin Limestone Member records the highest bivalve diversity reported so far from this time interval, question-ing previous claims that the recovery from the end-Permian mass extinction was delayed until the Middle Triassic. The twobivalve subclasses (Pteriomorphia and Heteroconchia) that are present in the Virgin Limestone Member clearly differ in theirevolutionary contexts. Pteriomorphs of the Virgin Limestone are nearly exclusively composed of genera that survived theend-Permian mass extinction event, whereas heteroconchs are highly dominated by genera that evolved in the Early Triassic.This contrasting evolutionary background probably reflects differential effects of the end-Permian mass extinction event andsubsequent crises on these two subclasses, possibly related to differences in filter feeding efficiency and shell mineralogy. Thehigh proportion of infaunal heteroconchs, including deep-infaunal Pholodomyoida, is an additional indicator of a relativelyadvanced recovery stage, further corroborating that recovery of benthic organisms was well underway during the late EarlyTriassic.

http://zoobank.org/urn:lsid:zoobank.org:pub:EC9802EA-F1D5-4066-A608-2B8DECE7C10D

Keywords: bivalves; Early Triassic; taxonomy; mass extinction; recovery

Introduction

The end Permian marks the greatest extinction event to haveoccurred in the last 540 Ma (Erwin 2006; Benton 2006) andresulted in a major shift in the ecological structure of marinebenthic communities (Wagner et al. 2006). Accurate taxo-nomic knowledge of shallow marine benthic faunas in theLower Triassic is critical for documenting both the severityof the latest Permian extinction event, and the rate andpattern of biological recovery that followed (Erwin 1998).Unfortunately, the marine rock record of this interval isparticularly poor (Smith 2007; Smith & McGowan 2007)and there are relatively few places in the world where goodfaunas in nearshore settings of this age are preserved. Asampling and/or preservation problem for Early Triassicfaunas is emphasized by the observation that there is apeak in the number of Lazarus taxa during this interval(Nakazawa & Runnegar 1973; Erwin 1996; Hautmann& Nutzel 2005; McGowan & Smith 2007), suggestingsome sampling or taphonomic failure. Recently described

∗Corresponding author. Email: [email protected]

well-preserved faunas from the Griesbachian (Krystynet al. 2003; Kumagae & Nakazawa 2009; Hautmannet al. 2011) confirm that the diversity of post-extinctionfaunas has been underestimated, possibly as the combinedresults of inadequate sampling and the prevalence of poorpreservational conditions at most Lower Triassic fossillocalities.

The Virgin Limestone Member of the Moenkopi Forma-tion is one of the few Early Triassic shallow-water carbonatesequences that is easily accessible and which yields abenthic fauna of late Olenekian (Spathian) age. While thepalaeoecological structure and palaeoenvironmental settingof the Virgin Limestone fauna has been extensively studied(Schubert & Bottjer 1995; Bottjer & Schubert 1997; Tang& Bottjer 1997; Pruss & Bottjer 2004; Pruss et al. 2004;Boyer et al. 2004; Fraiser & Bottjer 2004), the biologicalsignificance of this fauna is disputed. Bottjer and co-workers have interpreted the fauna of the Virgin LimestoneMember as low diversity and low heterogeneity, leadingthem to believe that an extended recovery period lasted

C© 2013 Natural History Museum

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http://dx.doi.org/10.1080/14772019.2011.637516

264 M. Hautmann et al.

throughout the Early Triassic (Bottjer et al. 2008). Faciesanalyses by Pruss et al. (2004) were interpreted as inde-pendent support for unusual, stressed marine ecosystemsduring the Early Triassic. They reported the occurrence ofwidespread ‘anachronistic’ sedimentary features, such aswrinkle structures from supposedly subtidal settings, and ahigh abundance of flat pebble conglomerates in Early Trias-sic rocks around the world, including sections of the VirginLimestone in Nevada. They interpreted these structures asevidence of reduced bioturbation and protracted environ-mental stress. However, it has not been tested whether thesesedimentary structures were significantly more abundantduring the Early Triassic than in other post-Cambrianepochs, as was suggested by Pruss et al. (2004).

A contrasting view was advanced by McGowan et al.(2009). They compared the diversity and size ranges ofVirgin Limestone Member fauna with later Triassic faunasto test whether those from the Virgin Limestone werereduced in body size (cf. Twitchett 2007; but see Brayardet al. 2010, 2011), whether the fauna is vertically andlaterally homogeneous, and whether taxonomic diversitywas significantly lower than that of comparable nearshoremarine deposits transgressive over redbed sequences fromother sequences not regarded as coming from post-extinction recovery intervals. They found no consistentbody size reduction, a strong heterogeneity of faunas bothwithin and between sections, and an alpha diversity thatis low but similar to that found in some younger Triassicfaunas from the Muschelkalk.

Despite the general interest in the nature of early Triassicbenthic faunas and their relative diversity, few taxonomicstudies have been carried out on the faunas themselves.The echinoderms of the Virgin Limestone Member wererevised relatively recently (Kier 1968; Schubert et al.1992), and Batten & Stokes (1986) and Nutzel & Schulbert(2005) analysed the gastropod fauna from the slightly older(Smithian) Sinbad Limestone of Utah, USA. However,bivalve taxonomy has been largely neglected, with only afew taxonomic works dealing with Lower Triassic bivalvesof the USA (Newell & Kummel 1942; Ciriacks 1963;Newell & Boyd 1995), and none specifically on the VirginLimestone fauna itself. Here we describe the bivalve faunafrom the Virgin Limestone Member based on new collec-tions. This fauna is much more diverse and heterogeneousthan any Smithian or earlier fauna and indeed is not so verydifferent in these respects from typical Middle Triassic(Anisian) bivalve-dominated benthic communities.

Geological setting

The Virgin Limestone Member of the Moenkopi Forma-tion is a mixed carbonate-siliciclastic deposystem that isextensively exposed in southern Utah, northern Arizona andeastern Nevada (Poborski 1953, 1954; Stewart et al. 1972;Pruss & Bottjer 2004). It was formed during a marine incur-

sion from Panthalassa into the western interior of NorthAmerica, with water depth increasing from east to west(Blakey 1972). The new find of Columbites cf. parisianusHyatt & Smith, 1905 (Fig. 1; determined by Hugo Bucher)near the middle of the Virgin Limestone Member at local-ity BM1 bed 7a (Figs 2, 3) indicates a late early Spathian(Olenkian) age (biochronologic horizon H4 of Guex et al.2010). There are no new ammonoid data from the otherinvestigated sections which, however, represent the sametransgressive episode (Stewart et al. 1972) and were thusdeposited more or less contemporaneously.

Fieldwork was undertaken by two of us (ABS and AM)together with P. D. Taylor during May 2006 in two areas inthe vicinity of St George, Utah (Fig. 2). One area stretchedfrom the type locality of Virgin (locality V1) southwardsalong Hurricane Cliffs (HC localities). The second groupof localities was located in the Beaver Dam Mountains(BM localities) (Fig. 2). Within these two areas, a numberof sections were explored before sites were selected fordetailed sampling. A third section near Kanarraville (Fig.2, K1) was also briefly explored.

Sedimentological data indicate increasing water depthfrom east to west, with the Hurricane Cliffs, Virginand Kanarraville outcrops representing a more marginal,onshore setting with rapid lateral variation in lithofacies,while the Beaver Dam Mountains sections are more basinaland laterally continuous (Figs 3, 4).

Poborski (1954) described the Beaver Dam Mountainssuccession in detail. The best exposures are towards thenorth at locality BM1. They can be followed in a south-easterly direction for several kilometres, retaining a similar,though slightly thinner, succession at the southernmostoutcrop (BM5). Marine deposition commences with arelatively thin (120 cm) carbonate-dominated succession ofbioclastic limestones with subsidiary siltstones and a faunadominated by terebratulid and rhynchonellid brachiopods.

Figure 1. Columbites cf. parisianus Hyatt & Smith, 1905 fromlocality BM1, Virgin Limestone, bed 7a, indicating a late EarlySpathian age; × 1.5.

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Bivalves from the Olenekian of south-western Utah 265

Figure 2. Map of the Hurricane region, Utah, USA showing the locations of logged and sampled sections (stars). Insert shows the positionof the map within Utah. Major highways are indicated and numbered. See Table 1 for GPS coordinates of logged sites.

Several levels near the base show well-developed mud-cracks, along with omission surfaces and small-scaletrough cross-bedded bioclastic units, indicating shallowsubtidal to intertidal setting (e.g. Poborski 1954). Therethen follows a thick (33 m) succession of siltstones withoccasional (1–2 m) calcareous shell beds deposited inshallow, protected subtidal conditions (Poborski 1954).The siltstones are largely barren, but include a fewlevels where nests of the bivalves Bakevellia, Trigonodusand Leptochondria occur together with rare Spathianammonites (Poborski 1954; McGowan et al. 2009; thisstudy). The bioclastic limestones, which are laterallycontinuous along the outcrop and were identified usingletters by Poborski (1954), all yield a similar allochthonousfauna dominated by large Eumorphotis and Promyalina,sometimes forming shell pavements, with subsidiarycrinoid debris and brachiopods. The bivalves are allsingle-valved and unaligned, and the beds presumablyrepresent winnowed storm lags. The highest part of thesuccession comprises a thin sequence of fine siliciclasticsand bioclastic limestones, often with coarse trough cross-bedding and fragmentary echinoderm and mollusc debris.This marks the return to deposition in a shoreface setting.

The Hurricane Cliffs sections are calcarenite-dominated,with abundant intraclasts in the lower beds and massivetrough cross-bedded units. Within a ridge around 1 kmlong, the thickness and relative proportions of siliciclas-tics and carbonate sediment varies considerably. Wedgingout of individual beds is conspicuous at outcrop scale. Atits thickest (at the southern extremity, HC3), a successionof bioclastic limestones is developed showing large-scaletrough cross-bedding and containing pebble- and boulder-sized intraclasts at its base, indicative of stream channeldeposits (Stewart et al. 1972). These give way upwards tomore laminar-bedded marly limestones with abundant nestsof rhynchonellids. This entire carbonate unit, however, isextremely variable laterally and at HC3 reduces from about14 m to a unit less than 1 m in thickness within a distance of300 m along an east to west transect. We interpret this as aninshore channel fill. Higher parts of the succession are wellexposed in the northern section at HC1 and comprise thick,largely unfossiliferous siltstone units with occasional thinbioclastic limestone incursions, as at the Beaver Mountainlocalities, representing a muddy tidal flat complex (Reif& Slatt 1979). Figs 3 and 4 summarize the sedimentarysuccessions in the major sections.

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Figure 3. Logs of two sections in the Beaver Dam Mountains(see Fig. 2) with bed numbers. A–N = bed numbering system ofPoborski (1954).

Table 1. GPS coordinates of fossil sites (see Fig. 2).

Locality

GPS (latitude and longitudein decimal degrees, WGS84

datum)

Beaver Dam MountainsBM1 37.12660◦N 113.76424◦WBM5 37.060247◦N 113.69395◦W

Hurricane CliffsHC1 37.08494◦N 113.26954◦WHC2 37.05254◦N 113.27588◦WHC3 37.04033◦N 113.27301◦WHC5 37.10812◦N 113.25413◦WHC6 37.13865◦N 113.24702◦WHC8 37.13190◦N 113.25217◦W

VirginV1 37.19491◦N 113.22406◦W

KanarrevilleK1 37.53852◦N 113.17063◦W

We carried out surface collection on a bed-by-bed basis,picking all macrofossils well enough preserved to be identi-fiable until a large sample had been obtained or the surfaceoutcrop exhausted. Repeat collections were made fromsome selected localities and horizons. All locality details areprovided in Table 1. The collected material has previouslybeen analysed with respect to faunal diversity, heterogenityand body size by McGowan et al. (2009), but no detailedtaxonomic studies have been undertaken so far. The presentstudy provides a systematic description of the bivalve faunathat forms the basis for the interpretation of its evolution-ary context. Type and figured specimens are housed in thecollections of the Department of Paleobiology, NationalMuseum of Natural History (USNM), Smithsonian Institu-tion, Washington, DC.

Systematic palaeontology

Class Bivalvia Linnaeus, 1758Infraclass Autolamellibranchiata Grobben, 1894

Subclass Pteriomorphia Beurlen, 1944Order Arcoida Stoliczka, 1871

Superfamily Arcoidea de Lamarck, 1809Family Parallelodontidae Dall, 1898

Subfamily Parallelodontinae Dall, 1898Genus Parallelodon Meek & Worthen, 1866

Type species. Macrodon rugosus Buckman in Murchison,1845, Middle Jurassic, England.

Parallelodon? aff. beyrichii (von Strombeck, 1849)(Fig. 5A)

aff. 1849 Cucullaea beyrichii von Strombeck: 451, pl. 7A.2009 bivalve undet. A; McGowan et al.: 862, table 1.

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Figure 4. Logs of sections in the Hurricane Cliffs (HC) and Virgin (V) regions (see Fig. 2) with bed numbers. See Fig. 3 for legend.

Material. One right valve (composite mould) from BM1bed 6, and one right valve (composite mould) from BM5bed 6.

Dimensions. USNM 543476: length 18 mm; height 7.5mm.

Description. Valve elongated-trapezoidal, with shallowmedial sulcus. Acute carina delimits small, incompletelypreserved posterior area. Beak relatively close to anteriorend of valve. Shell exterior with commarginal growth rugae.Hinge and ligament area not preserved.

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Life habit. The elongated shape (length/height ratio >

1.35) and the presence of a medial sulcus indicate anepibyssate mode of life (Stanley 1970, 1972).

Remarks. In the absence of internal characters, our assign-ment of the present specimens to Parallelodon relies onexternal morphology, which is consistent with the diagno-sis of the genus given by Newell (1969a). Although Paral-lelodon occurs in the Permian (e.g. Nakazawa & Newell1968) and is diverse in the Middle and Upper Triassic(Diener 1923), it was hitherto unknown in the Lower Trias-sic. The present specimen resembles the Anisian–LadinianParallelodon beyrichii (von Strombeck, 1849) in its generalshape and the absence of a radial ornamentation, but differsfrom this species in the more anteriorly located beak andthe smaller posterior area.

Order Mytiloida de Ferussac, 1822Superfamily Mytiloidea Rafinesque-Schmaltz, 1815

Family Mytilidae Rafinesque-Schmaltz, 1815Subfamily Modiolinae Keen, 1958Genus Modiolus de Lamarck, 1799

Type species. Mytilus modiolus Linnaeus, 1758, Recent.

Modiolus sp.(Fig. 5B)

?2008 Promytilus homevalensis Waterhouse: 15, text-fig. 4.2009 Modiolus sp.; McGowan et al.: 862, table 1.

Material. One right valve with partly preserved externalshell layer from HC5 bed 6.

Dimensions. USNM 543477: length 6.5 mm; height 4.3mm.

Description. Small modioliform valve with very fine anti-marginal striations on the shell exterior. Elongated bulgebelow the dorsal margin probably represents diageneticfeature. Shell interior unknown.

Life habit. Endobyssate suspension feeder (e.g. Stanley1970, 1972).

Remarks. The present specimen is similar in shape to‘Modiolus’ sp. indet. described from the late Griesbachianof Primorye (Russia) by Kumagae & Nakazawa (2009,p. 156). However, the Russian species has well-developedradial ribs, whereas the ornamentation of the present spec-imen consists of fine, antimarginally arranged striations,which probably represent the surface expression of anantimarginal-fibrous shell microstructure rather than trueradial ribs.

Another similar species is Promytilus homevalensisWaterhouse, 2008, from the early Permian Tiverton Forma-tion of Queensland, Australia. The long time span betweenthe occurrences and the fact that only a single individualis known from the Virgin Limestone Member prevents usfrom identifying our specimen with this species. However,the slight differences in outline may result from the largersize of the Permian specimens, in which shell growth shiftedto a more ventral direction in the course of ontogeny.Early growth stages of P. homevalensis, as marked byincremental lines, are quite similar in outline to ourspecies.

Although maintaining Promytilus, Waterhouse (2008,p. 15) questioned whether separation of this genus (andthe allied Volsellina Newell, 1942) from Modiolus waspossible. In particular, Newell’s (1942) treatment ofPromytilus did not fully conform to his own diagnosis ofthe genus, which emphasized the terminal position of thebeaks. Waterhouse (2008) applied an alternative definitionthat included species with subterminal beaks (such asP. homevalensis) in Promytilus, but it is then unclearwhich morphological differences separate Modiolus fromPromytilus.

Promytilus borealis Kurushin in Dagis et al., 1989, fromthe Spathian of Siberia is apparently different in outlinefrom our specimen judging from the figures in Dagis &Kanygin (1996, pl. 7, figs 9, 10). Due to the low resolutionof these figures, however, it cannot be decided whethera generic separation from Modiolus is justified for thisspecies.

←−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−Figure 5. A, Parallelodon? aff. beyrichii (Strombeck, 1849), USNM 543476, right valve (composite mould) from BM1, bed 6, × 2. B,Modiolus sp., USNM 543477, right valve with partly preserved external shell layer from HC5, bed 6,× 3. C–H, Promyalina putiatinensis(Kiparisova, 1938): C, D, USNM 543478, left and anterior view of articulated specimen with partly eroded external shell layer from BM5(float below bed 2),× 1; E, F, USNM 543480, left and anterior view of articulated specimen (steinkern with partly adherent shell material)from BM1, bed 9,× 1; G, H, USNM 543479, left and anterior view of articulated specimen (steinkern with partly adherent shell material)from BM1, bed 9, × 1. I–Q, Bakevellia cf. exporrecta (Lepsius, 1878) from BM1, bed 7, × 1: I, J, USNM 543481, left and right view ofarticulated specimen with preserved shell material; K, L, USNM 543482, left and right view of articulated specimen with preserved shellmaterial; M–O, USNM 543483, left, right and ventral view of articulated specimen with preserved shell material; note the twisted planeof commissure; P, Q, USNM 543484, left and right view of articulated specimen with preserved shell material. R–T, Bakevellia costata(von Schlotheim, 1820) from HC1, bed 4: R, USNM 543485, exterior of left valve, with external shell layer partly preserved near shellmargin, × 1; S, USNM 543486, internal mould of left valve, showing adductor scars, × 2; T, USNM 543487, exterior of left valve withslightly abraded extenal shell layer, × 2. U, Bakevellia? cf. costata (von Schlotheim, 1820), USNM 543488, exterior of diageneticallydistorted left valve, with some microconchids on shell surface, from HC3, bed 7/8,× 2. V–A′, Leptochondria nuetzeli sp. nov. from BM1,bed 7a, all specimens with preserved external shell layer, × 2: V, USNM 543489, left valve; W, X, USNM 543490, holotype, articulatedspecimen: W, left valve; X, right valve; Y, USNM 543491, left valve; Z, USNM 543492, left valve; A′, USNM 543493, left valve.

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Order Pterioida Newell, 1965Suborder Pteriina Newell, 1965

Superfamily Ambonychioidea Miller, 1877Family Myalinidae Frech, 1891Genus Promyalina Kittl, 1904

Type species. Promyalina hindi (Kittl), Upper Permian(Bellerophonschichten), near Sarajevo, Bosnia-Herzegovina. Note that the Late Permian age of Kittl’sfauna was questioned by Newell (1955, p. 26), whosuggested that Promyalina was strictly confined to theEarly Triassic.

Promyalina putiatinensis (Kiparisova, 1938)(Fig. 5C–H)

1899 Myalina vetusta Benecke; Bittner: 17, pl. 4, figs 7–19.1938 Myalina putiatinensis Kiparisova: 261, pl. 6, figs

10–12.?1938 Myalina aff. blezingeri Philippi; Kiparisova: 264, pl.

6, fig. 9.1942 Myalina putiatinensis Kiparisova; Newell & Kummel:

957, pl. 3, figs 9, 10.1963 Promyalina putiatinensis (Kiparisova); Ciriacks: 75,

pl. 16, figs 1–5.1978 Promyalina putiatinensis (Kiparisova); Xu Jifan: 319,

pl. 104, fig. 20.2009 Promyalina putiatinensis (Kiparisova); Kumagae &

Nakazawa: 157, figs 144.4, 144.5.2009 Promyalina putiatinensis (Kiparisova); McGowan

et al.: 862, table 1.

Material. This species is widely distributed in the VirginLimestone Member, and is commonly encountered in thecalcareous shell beds of Beaver Dam Mountain localities 1and 5. Specimens have been collected from BM1 beds 5, 6,7a, 9 and 14, BM5 beds 5, 6, 8, 9, 12, HC1 bed 3, HC2 beds6, 8, HC5 beds 6, 8, and HC6 beds 9, 13. The followingdescription is based on one articulated specimen from BM5,float below bed 2, and two articulated specimens from BM1bed 9. The described specimens usually have the externalshell layer preserved but eroded to various degrees.

Dimensions. USNM 543478: length 25 mm; height 33mm; width 14 mm. USNM 543479: length 36.5 mm; height40 mm; width 18mm. USNM 543480: length 30.4 mm;height 36 mm; width 15 mm.

Description. Medium-sized, more or less equivalvedPromyalina, retrocrescent (prosocline) throughoutontogeny. Body of the shell slender for the genus. Dorsalmargin straight; anteroventral margin flattened, with narrowbyssal gape. Beaks terminal. Surface with commarginalgrowth rugae.

Life habit. The shell morphology suggests that the animalrested orthothetically on the flattened anterior shell margin,

fixed by the byssus extruding through the small gapebetween the valves.

Remarks. Promyalina was relatively diverse in the EarlyTriassic, notably in the Olenekian of Siberia (e.g. Dagis &Kanygin 1996). However, the comparatively slender bodyof the shell and the straight anterior margin distinguish P.putiatinensis from the Siberian species, which are morerounded in outline.

The largest specimen (Fig. 5E, F) is very similar to thespecimen described as Myalina aff. blezingeri Philippi byKiparisova (1938), whereas the other specimens agree verywell with the type material of Promyalina putiatinensis.Given the great morphological variability of this species(Bittner 1899, p. 18; Kiparisova 1938, p. 292; Kumagae &Nakazawa 2009, p. 158), it is likely that the slight differ-ences in shape and convexity do not justify separation ofthe larger specimen as a different species. Middle TriassicMyalina blezingeri Philippi, 1899 is much larger than P.putiatinensis, has more protruberant umbones and is verythick shelled.

The most similar species is Promyalina spathi (Newell &Kummel, 1942) from the Induan of the western USA, whichchiefly differs in being infracrescent instead of retrocres-cent. However, some specimens in our material (Fig. 5C,D) closely approach P. spathi in being only slightly oblique.Promyalina groenlandica Newell, 1955 is also infracrescentand additionally differs from P. putiatinensis (and P. spathi)in having beaks that project beyond the dorsal margin.

Superfamily Pterioidea Gray, 1847Family Bakevelliidae King, 1850

Genus Bakevellia King, 1848

Type species. Avicula antiqua Graf zu Munster in Gold-fuss, 1836 [non A. antiqua Defrance] = Avicula binneyiBrown, 1841, Upper Permian (Zechstein), Europe.

Bakevellia cf. exporrecta (Lepsius, 1878)(Fig. 5I–Q)

cf. 1878 Gervillia exporrecta Lepsius: 352, pl. 1, fig. 6a–c.cf. 1908 Gervillia exporrecta Lepsius; von Wittenburg: 279,

pl. 4, fig. 10.?cf. 1982 Gervillia exporrecta Lepsius; Shi Bingde: 25, pl.

11, fig. 9.2009 Bakevellia cf. exporrecta (Lepsius); McGowan et al.:

862, table 1.

Material. This species is relatively abundant in the thinlimestones of bed 7a at BM1 but has also occasionally beenfound at other levels, including BM1 beds 9, 10 and 15,BM5 beds 7 (base), 9, HC1 bed 4, HC5 bed 6, and HC6bed 9. The following description is based on 15 articulatedspecimens and a single left valve from BM1 bed 7a. Alldescribed specimens have the shell preserved.

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Dimensions. USNM 543481: length 22.1 mm; height 14.6mm; width 8.3 mm. USNM 543482: length 25.7 mm; height16.0 mm; width 10.0 mm. USNM 543483: length 27.5 mm;height 18.2 mm; width 10.3 mm. USNM 543484: length32.5 mm; height 19.8 mm; width 11.2 mm.

Description. Shell trapeziform, inequivalved, moderatelytwisted (i.e. commissure not in a plane; Fig. 5O). Leftvalve strongly convex, right valve moderately convex. Nobyssal gape between valves observed. Very little divergencebetween ligament areas of opposite valves.

Life habit. Shell torsion and poorly developed or absentbyssus suggest that this species lived pleurothetically withthe left valve sunken in a soft substratum (‘twisted recliner’of Seilacher 1984).

Remarks. The type material of Bakevellia exporrectacomes from the higher part of the Werfen Formation(‘Campiler Schichten’, Olenekian) of South Tirol. Thepresent specimens agree well with the figures of Lepsius(1878) and von Wittenburg (1908), but it is unclear fromthese papers whether the commissure of B. exporrecta istwisted. The assignment of our material to this species istherefore provisional.

Shi Bingde (1982) described Bakevellia exporrecta fromthe Early Triassic of north-west China, which suggests acosmopolitan distribution for this species. However, thefigure in Shi Bingde (1982, pl. 11, fig. 9) is too cloudy toverify his determination.

Bakevellia costata (von Schlotheim, 1820)(Fig. 5R–U)

1820 Mytulites costatus von Schlotheim: 298.1822 Mytulites costatus von Schlotheim; von Schlotheim:

113, pl. 37, fig. 2.1857 Bakewellia [sic] costata (von Schlotheim); von Schau-

roth: 104, pl. 5, figs 1, 3, 4.?1895 Gervillia costata Quenstedt? [sic]; Tommasi: 54, pl.

1, fig. 12.?1907 Gervilleia [sic] costata von Schlotheim sp. mut. nov;

Frech: 12, pl. 1, figs 9, 10.1923 Gervilleia costata von Schlotheim; Diener: 89 (cum

synonymis).1972 Bakevellia (Neobakevellia) costata (von Schlotheim):

Farsan: 146, pl. 38, figs 1–5 (cum synonymis).1982 Bakevellia costata (von Schlotheim); Shi Bingde: 25,

pl. 11, figs 6–8.2009 Bakevellia cf. costata (von Schlotheim); McGowan


Material. This is less commonly encountered in the VirginLimestone than Bakevellia cf. exporrecta and is most abun-dant in HC5 bed 7. It has also been found in BM1 beds 6and 9, BM5 beds 6 and 9, HC1 bed 4, HC3 bed 7/8, HC5beds 4, 6, 7, HC6 bed 10, and V bed 10b. The following

description is based on 17 left valves, mostly with preservedexternal shell layer, from HC1 bed 4.

Dimensions. USNM 543485: length > 17.4 mm; height15.9 mm. USNM 543486: length 16.2 mm; height 11 mm.USNM 543487: length 18.4 mm; height 15.2 mm. USNM543488: length > 17.4 mm; height 15.0 mm.

Description. Small Bakevellia with commarginal, mostlyirregularly spaced ribs. Anterior wing separated by shal-low sulcus from main body of the shell, possibly creatingsmall gape for byssus extrusion. Posterior wing large. Largeposterior adductor muscle scar close to dorsal margin (Fig.5S). Dark, slightly elevated patch close to umbo of an inter-nal mould (Fig. 5S) may represent impression of relativelylarge anterior adductor scar.

Life habit. The presence of a byssal gape and the lackof shell torsion suggest an endobyssate mode of life (cf.Stanley 1972; Muster 1995).

Remarks. Our collection contains only left valves, whichmight be due to hydrodynamic separation of the differ-entially vaulted valves or to weaker calcification and thuslower preservational potential of the right valve.

Bakevellia costata is abundant in the Anisian–Ladinian(uppermost Buntsandstein (Rot) and Muschelkalk) of theGermanic Basin, but it has also been reported from theLower Triassic of north-west China (Shi Bingde 1982),Hungary (as ‘new mutation’; Frech 1907), and question-ably from the Alps (Tommasi 1895). It appears that thespecimens from Hungary and the Alps are more slenderand retrocrescent than the present material and most MiddleTriassic specimens. The presence of commarginal ribs andthe lack of radial ribs distinguishes B. costata from similarEarly–Middle Triassic bakevelliids.

Nakazawa (1959, p. 200) designated Mytulites costa-tus as the type species of his new subgenus Bakevellia(Neobakevellia), which he separated from Bakevellia s.s. byits allegedly much smaller or completely reduced anterioradductor muscle. However, it appears doubtful whether thiscriterion is useful for the separation of subgenera in a familywith a general tendency towards a monomyarian condition(Geyer et al. 2005). Moreover, a reduction of the anteriormuscle scar in B. costata has never been demonstrated.The possible traces of a large anterior adductor musclefield in one of our specimens suggests that the concept ofNeobakevellia might be invalid for its type species.

Suborder Pectinina Waller, 1978Superfamily Aviculopectinoidea Meek & Hayden, 1864

Family Asoellidae Begg & Campbell, 1985

1995 Leptochondriidae Newell & Boyd: 69.

Revised diagnosis. Aviculopectinoids with absent or veryshallow left anterior auricular sinus.

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Remarks. In the original diagnosis of Asoellidae, Begg& Campbell (1985, p. 727) characterized this family by:(1) the alivincular, amphidetic ligament areas with animpressed, subumbonal triangular resilifer; (2) lack ofaccessory hinging structures which cross the hinge line; and(3) a right anterior auricle that is poorly to well developed,often almost planar. Unfortunately, all these characters areprimitive in Pectinina and thus not suitable for separationof families within this suborder.

Apart from the type genus, Begg & Campbell (1985,p. 727) included Aucellina Pompeckj, 1901, and theirnew genus Etalia Begg & Campbell, 1985 in Asoellidae.However, Sha & Fursich (1994, p. 17) removed Aucel-lina from Asoellidae and restored its traditional placementin Buchiidae. Etalia probably evolved independently ofAsoella (Waller 2005, p. 35) and should thus also be placedin a separate family close to Buchiidae.

With the removal of Aucellina and Etalia from Asoel-lidae, only the type genus remains from the originalcomposition of the family. However, Sha & Fursich(1994) added Leptochondria Bittner, 1891, OrnithopectenCox, 1962, Chaenocardia Meek & Worthen, 1869, andObliquipecten Hind, 1903 to Asoellidae. In this revisedcomposition, Asoellidae attained an aviculopectinoideanaspect, although Sha & Fursich (1994) maintained itsoriginal position in Monotoidea. In contrast to Sha &Fursich (1994), Waterhouse (2008) included Chaenocar-dia and Obliquipecten in Chaenocardiidae Miller, 1889.Ornithopecten has attracted comparatively little attentionand appears to represent a morphologically isolated cladedistally related to the ‘Antijanira group’of Hertlein (1969,p. N355) rather than to Asoellidae.

What remains to constitute Asoellidae are two morpho-logically similar genera: Asoella and Leptochondria. In factneither Tokuyama’s (1959) original diagnosis of Asoellanor the revised diagnoses of this genus and Leptochondriain the Treatise (Newell 1969b) highlight any differencesbetween the two. Tokuyama (1959) did not even refer toLeptochondria in his discussion, which may indicate that

he was not aware of this genus when erecting Asoella.Waller (2005, p. 35) pointed out that Pseudomonotis illyricaBittner, 1902, which Tokuyama (1959) included in Asoella,actually belongs to Leptochondria. This conclusion proba-bly also applies to other species in Tokuyama’s (1959, p. 2)list, such as Eumicrotis spitzbergensis Bohm, 1903. Judg-ing from Tokuyama’s figures of his three new species fromthe Late Triassic of Japan, possible differences with Lepto-chondria include a longer hinge line, a less deep byssalnotch in the right valve, and the presence of a very shallowleft anterior auricular sinus. A possibly unique character ofLeptochondria described by Newell & Boyd (1995) for L.occidanea is the lack of well-defined bourrelets in the leftvalve, but it cannot be ruled out that this is a preservationalartefact due to dissolution of the inner shell layer. Moreover,the constancy of this character in Leptochondria remainsto be demonstrated. We provisionally maintain Asoella onthe basis of the morphological differences outlined above,although it appears uncertain if they allow consistent sepa-ration from Leptochondria. It should be mentioned thataccording to IRZN Article 40, possible synonymization ofAsoella with Leptochondria would have no bearing on thevalidity of the family name Asoellidae and its priority overLeptochondriidae.

The key character that separates Asoellidae from otheraviculopectionoids is the lack or very weak development ofa left anterior auricular sinus. Newell & Boyd (1995) there-fore placed ‘Leptochondriidae’ ( = present Asoellidae)in Pseudomonotoidea, but it appears unlikely that Lepto-chondria evolved from such a highly specialized group.Leptochondria rather appears to be a derived aviculopecti-noid in which the left anterior auricular sinus has beenclosed (Hautmann 2010). A similar conclusion was reachedfor Asoella by Tokuyama (1959), for which he suggesteda derivation from Eumorphotis. Closure of the left ante-rior auricular sinus was probably advantageous becauseit provided improved protection of the byssus that passedthrough the byssal notch of the opposite valve (Hautmann2010).

←−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−Figure 6. A–D, Leptochondria nuetzeli sp. nov. from BM1, bed 7a, all specimens with preserved external shell layer, × 2: A, USNM543494, left valve; B, USNM 543495, left valve; C, USNM 543496, left valve; D, USNM 543497, left valve. E, F, Leptochondriacurtocardinalis (Hall & Whitfield, 1877) from BM1 (bed unknown), × 2: E, USNM 543498, left valve with preserved external shelllayer; F, USNM 543499, left valve with partly eroded shell. G, Eumorphotis venetiana (von Hauer, 1850), USNM 543500, left valve fromBM1 (bed unknown) with externally preserved shell, × 2. H–L, Eumorphotis ericius sp. nov., left valves with shell preservation, × 1: H,USNM 543501, from BM1 (loose); I, USNM 543502, from BM1 (bed unknown); J, USNM 543503, from HC5; K, USNM 543504, fromBM1, bed 9; L, USNM 543505, holotype, from BM1, bed 9. M–Q, Eumorphotis virginensis sp. nov. from V, bed 10, with at least partlypreserved shell material: M, USNM 543506, left valve, note encrusting microconchids,× 1; N, USNM 543507, exterior of right valve,×1; O, USNM 543508, holotype, left valve, note encrusting microconchids,× 1; P, USNM 543510, left valve,× 1; Q, USNM 543509, leftvalve, showing ligament area, lines indicate outline of shell and lateral margins of rsilifer, × 3. R, Eumorphotis cf. multiformis (Bittner,1899), USNM 543511, left valve from BM1, bed 7a, with externally preserved shell, × 2. S, T, Pernopecten? sp., USNM 543512, leftvalve from BM5, bed 9, with externally preserved shell: S, shell exterior, × 1; T, detail of dorsal margin, stippled line indicates dorsalborders of auricles, × 2. U, V, Permophorus triassicus Newell & Boyd, 1999, USNM 543513, steinkern from HC5, bed 6: U, right valve;V, left valve; note well-developed anterior adductor scar.

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Genus Leptochondria Bittner, 1891

Type species. Pecten (Leptochondria) aeolicus Bittner,1891, Norian, north-west Turkey.

Leptochondria nuetzeli sp. nov.(Figs 5V–A′, 6A–D)

2009 Lepidochondria [sic] sp. A; McGowan et al.: 862,table 1.

Material. This species occurs commonly in the calcareousshell beds of BM1 and BM5. It is particularly abundant inbed 7a and the lower part of bed 15 at BM1, and in beds 7and 9 at BM5. It has also been found in HC1 bed 4, HC5bed 5, and HC6 bed 9. The following description is basedon one articulated specimen and approximately 50 isolatedleft valves from BM1 bed 11. All examined specimens havetheir external shell layer preserved.

Holotype. USNM 543490 (Fig. 5W–X).

Dimensions of holotype. Length: 11.8 mm; height: 11.8mm; width: 3.3 mm.

Type locality. BM1.

Type stratum. Bed 7a.

Etymology. Named after Alexander Nutzel (Munich) inrecognition of his work on Triassic gastropods.

Diagnosis. Orbicular to slightly infracrescent Leptochon-dria with simple or rarely bifurcating radial ribs and sub-equal, comparatively well demarcated auricles in the leftvalve. Right valve ornamented with radial ribs on disc andcrescent incremental lines on anterior auricle.

Description. Shell inequivalved and nearly equilateral,with discs being round to slightly higher than long.

Left valve convex, covered with up to 60 simple, rarelybifurcating radial ribs, occasionally different in strengthand generally weaker on the auricles. Auricles subequal insize, lacking auricular sinuses but relatively well demar-cated from the disc. Umbo more or less orthogyrate andprominently projecting above hinge line.

Right valve flat, covered with comparatively weak radialribs. Anterior auricle distally rounded, leaving wide spacetowards disc for extrusion of byssus (Fig. 5X). Well-developed circular incremental lines on this auricle vestigialof earlier growth stages. Posterior auricle distally rectangu-lar, matching opposed auricle of left valve. Beak small,umbo prosogyrate, not projecting above hinge line.

Life habit. The shell morphology suggests that Leptochon-dria nuetzeli rested pleurothetically with its flat right valveon the seafloor, fixed by a strong byssus extruding belowthe anterior auricle.

Remarks. The scarcity of right valves is typical for Lepto-chondria and is possibly due to its weaker calcification

(Newell & Boyd 1995, p. 69). By analogy with RecentPropeamussium, the ventral part of the right disc might havebeen particularly weakly calcified, forming a flexible apron(Newell & Boyd 1995, p. 17). In the only available rightvalve (Fig. 5X) a serrated commarginal step that causes athinning in shell thickness towards the ventral margin mightbe due to this phenomenon. However, there is a continuationof the topologically inner shell layer towards the outermostmargin, which is a deviation from the pattern known fromPropeamussium, where the flexible margin is formed by anextension of the outermost prismatic shell layer (Newell &Boyd 1995, p. 17).

Leptochondria was widespread in the Early Triassic,but a high morphological variability within populationsand a comparatively low disparity between describedspecies complicates establishment of unequivocal crite-ria for species separation. Nearly all described species aresmall, have more or less circular discs and subequal auri-cles in the left valve, and are covered by numerous radialribs. The number of radial ribs appears to be variable withinpopulations and is thus a relatively weak taxonomic crite-rion. However, some species have their ribs intercalated intwo or more ranks whereas others have simple or bifur-cating ribs, which appears to be taxonomically significant.Other taxonomically significant differences might includethe shape of the right anterior auricle and the style of orna-mentation in the right valve, which is seldom preservedunfortunately (see above).

Leptochondria albertii (Goldfuss, 1838) differs fromthe new species chiefly in having intercalating rather thansimple or bifurcating radial ribs. Additionally, the ribs of L.albertii are sharper than those of L. nuetzeli and separatedby larger interspaces. Juvenile specimens of L. nuetzeli aresimilar in outline to ‘L. ex aff. alberti Goldfuss’ [ = L.minima (Kiparisova, 1938)], described by Bittner (1899)from far east of Russia, but the latter species is character-ized by finer and more numerous radial ribs. Similarly, L.virgalensis (von Wittenburg, 1909) has much more radialribs than L. nuetzeli and additionally differs in having abouta dozen radial ribs that are more prominent than the aver-age. In L. bittneri (Kiparisova, 1938) the ribs are interca-lated in at least three different ranks. L. occidanea (Meek,1877) differs from L. nuetzeli in having fewer radial ribsthat distally increase in number by regular intercalation ofsecond-order ribs. Other differences include the smoothdisc of the right valve and probably a more elongated rightanterior auricle in L. occidanea. Another species, L. curto-cardinalis described below, can easily be distinguished byfiner and more numerous radial ribs that are intercalated intwo ranks.

Leptochondria curtocardinalis (Hall & Whitfield, 1877)(Figs 6E, F)

1877 Aviculopecten curtocardinalis Hall & Whitfield: 273,pl. 6, fig. 4.

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1963 Monotis? thaynesiana (Girty); Ciriacks: 81, pl. 15,fig. 16.

1995 Leptochondria curtocardinalis (Hall & Whitfield,1877); Newell & Boyd: 69, fig. 51.2 (cum synonymis).

2009 Leptochondria cf. thaynesiana (Girty); McGowanet al.: 862, table 1.

Material. This species is less commonly encountered thanLeptochondria nuetzeli in the Virgin Limestone but hasbeen collected from the following localities: BM1 beds 6,9 and 14, BM 5 beds 7–9, HC1 bed 4, HC3 bed 5, HC5beds 6 and 7, and HC6 bed 9. The following descriptionis based on two left valves with (partly) preserved externalshell layer from BM1 (collected from scree).

Dimensions. USNM 543498: length 16.3 mm, height 15.9mm. USNM 543499: length 15.3 mm, height 16.8 mm.

Description. Left valve covered with numerous, relativelyweak radial ribs that distally increase in number by interca-lation. One of the left valves (Fig. 6F) is unusually inflatedfor the genus. Right valve not observed.

Life habit. Same as Leptochondria nuetzeli.

Remarks. We follow Newell & Boyd (1995, p. 69) inregarding Leptochondria thaynesiana (Girty) as a youngersynonym of L. curtocardinalis.

Leptochondria curtocardinalis is very similar to L. alber-tii Goldfuss, 1838, which is abundant in the Middle Triassic(Anisian–Ladinian) Muschelkalk of the Germanic basin,but has also been reported from Early Triassic deposits(e.g. Diener 1923; Broglio Loriga et al. 1990). The presentmaterial differs from the holotype of L. albertii figured inGoldfuss (1838, pl. 120, fig. 6 a, b) in having more andsomewhat finer radial ribs, although the number of radialribs in Leptochondria from the Muschelkalk is somewhatvariable, as observed in material housed in the Muschel-kalkmuseum Ingelfingen, Germany (Hagdorn Collection).However, the degree of intraspecific variability is stillinsufficiently known for both species, which we thereforecurrently treat separately.

Another closely related species is Leptochondriavirgalensis (von Wittenburg, 1909) from the Early Triassicof Pakistan, which differs in having more densely spacedribs and in some variability of the strength of the radial ribs,with about a dozen ribs being stronger than the average.

Family Heteropectinidae Beurlen, 1954

Remarks. Bouchet & Rocroi (2010, p. 120) recently notedthat Heteropectinidae Beurlen, 1954 has priority overEtheripectinidae Waterhouse, 1982, which is essentiallyidentical in the content of genera.

Genus Eumorphotis Bittner, 1901a

Type species. Pseudomonotis telleri Bittner, 1898 fromthe Lower Triassic Werfen Formation of the Alps (Austria,

Slovenia, Italy), and the Lower Triassic of Ravnau, Pamir(subsequent designation by Cossmann 1902). Lectotypedesignated by Ichikawa (1958): left valve figured in Bittner(1898, pl. 15, fig. 13), from Skuber Vrch, Jezersko (Ober-Seeland), Slovenia, deposited in the Geologische Bunde-sanstalt in Vienna (unnumbered).

Eumorphotis venetiana (von Hauer, 1850)(Fig. 6G)

1850 Avicula venetiana von Hauer: 110, pl. 1, figs 1–3.1898 Pseudomonotis (Avicula) venetiana von Hauer;

Bittner: 712, pl. 15, figs 2–4.1908 Pseudomonotis venetiana von Hauer; von Wittenburg:

277, pl. 3, fig. 7.1923 Eumorphotis venetiana (von Hauer); Diener: 43 (cum

synonymis).1935 Eumorphotis venetiana (von Hauer); Leonardi: 69, pl.

4, fig. 6.

Material. One left valve with preserved shell from BM1(bed unknown).

Dimensions. USNM 543500: length > 18.3 mm; height21.7 mm.

Description. Comparatively small Eumorphotis coveredwith fine, smooth radial ribs intercalated in two differentranks.

Life habit. Eumorphotis generally has a flat right valvewith a deep byssal notch below the anterior auricle (e.g.Fig. 6N), which suggests an epibyssate mode of life.

Remarks. As stated by Bittner (1898), the original mate-rial of von Hauer (1850) has been tectonically distorted,and some morphological details are not adequately reflectedin his figures. However, Eumorphotis venetiana has beendescribed and well figured by subsequent authors (seesynonymy), which allows identification of the present spec-imen as this species.

Eumorphotis ericius sp. nov.(Figs 6H–L)

?1908 Pseudomonotis beneckei Bittner; von Wittenburg:29, pl. 4, fig. 1.

2009 Eumorphotis multiformis (Bittner); McGowan et al.:862, table 1.

Material. Relatively commonly encountered, especially atBM1 beds 8/9 and 13/14, and the equivalent levels at BM5.The description is based on five left valves from BM1 beds13, 18, and loose material; one left valve from HC5. Shellpreserved in all specimens.

Holotype. USNM 543505 (Fig. 6L).

Dimensions of holotype. Length > 47 mm; height 59 mm.

Type locality. BM1.

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Type stratum. Bed 9.

Etymology. Ericius (Latin) = hedgehog, referring to thesquamose, supposedly spine-bearing radial ribs.

Diagnosis. Large Eumorphotis with squamose radial ribson left valve, intercalated in two ranks.

Description. Disc of left valve distinctly higher than long,strongly vaulted, retro- to infracrescent in small speci-mens (Fig. 6H–K), procrescent in late growth stages (Fig.6L). Anterior auricle with shallow sinus. Posterior auriclepoorly differentiated from umbonal flank. Umbo prosogy-rate, located near midpoint of dorsal margin, protrudingbeyond hinge line. Ornamentation with squamose radialribs intercalated in two ranks, with a total of up to 90ribs near ventral margin. Few third-order ribs occasionallyadded near ventral margin of large specimens. Differenti-ation of ribs in different ranks less distinct on surface ofauricles. Right valve unknown.

Life habit. As for Eumorphotis venetiana.

Remarks. The scales on the radial ribs most probablyrepresent the bases of spines that were broken off. Theornamentation pattern described above distinguishes thenew species from other species of Eumorphotis.

The new species differs from the similar species Eumor-photis inaequicostata (Benecke, 1868) in the lack of regu-larly inserted third-order ribs and in a greater height in rela-tion to length. Eumorphotis spinicosta (von Wittenburg,1908) differs in having only 15–20 spinose radial ribs ofsimilar strength, and in the lack of second order ribs. More-over, E. spinicosta has nearly circular discs. Eumorphotisvenetiana (von Hauer, 1850) is distinguished from the newspecies by its finer ribs that bear no scales. Eumorphotismultiformis (Bittner, 1899) is similar to E. ericius in shapebut has much finer scales on its ribs and generally (althoughnot invariably) bears sets of third- and fourth-order ribs.

A possibly related species from the Anisian is Pseu-domonotis beneckei Bittner, 1901a. This species is similarto E. ericius in the large shell size and in the presenceof spinose first-order ribs, but differs in having sets of5–7 weak third-order ribs intercalated between eachpair of second-order ribs. Posenato (2008a) tentativelysynonymized P. beneckei with Ostrea compta Goldfuss,1838, which he transferred to Neomorphotis Yin & Yin,1983. Bittner (1901a, p. 575) recognized the virtually iden-tical ornamentation of the left valve of these two species,but he separated them on the basis of Philippi (1898a,p. 618), who assigned Ostrea compta to Prospondylus,implying that the right valve was cemented. Separation ofNeomorphotis from Eumorphotis is somewhat nebulous inthe original diagnosis of Yin & Yin (1983; see Fang et al.2009 for an English translation), apparently based chieflyon the large size and Spondylus-like ornamentation of thosespecies that Yin & Yin (1983) assigned to Neomorphotis.Posenato (2008a) suggested that the relatively largersize of ‘Neomorphotis’ in comparison with Eumorphotismight reflect an Anisian recovery signal. The large sizeof our specimen indicates that such a trend, if confirmed,might have started already in the Spathian. The report ofP. beneckei from the upper Werfen Formation (Spathian)by von Wittenburg (1908, pl. 4, fig. 1) is clearly based ona misidentification, because the first-order ribs are closelyspaced in von Wittenburg’s specimen, leaving no space forsets of 5–7 third-order ribs, as present in the type materialof P. beneckei. Judging from von Wittenburg’s (1908)figure, his specimen might belong to E. ericius.

Eumorphotis virginensis sp. nov.(Fig. 6M–Q)

2009 Eumorphotis sp. A; McGowan et al.: 862, table 1.

Material. This species occurs at V1, bed 10, and at HC1bed 4, HC2 beds 6 and 8, HC 3 beds 7/8, 10 and 11, and

←−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−Figure 7. A, Permophorus triassicus Newell & Boyd, 1999, USNM 543514, dorsal margin of steinkern from HC5, bed 6; note embaymentof commissure in front of the beak, indicating large cardinal tooth 2, × 2. B, Protopis? aff. waageni (Schnetzer, 1934), USNM 543515,internal mould of left valve from HC1, bed 4,× 2. C, Myoconcha cf. plana Kiparisova, 1938, USNM 543516, steinkern from HC5 (loose),left valve, × 2. D–G, Trigonodus cf. orientalis Bittner, 1899, from BM1, bed 9, × 1: D, E, USNM 543517, steinkern of articulatedspecimen, left valve and dorsal view; F, USNM 543518, internal mould of left valve; G, USNM 543519, internal mould of left valve.H–J, Trigonodus cf. sandbergeri von Alberti, 1864, USNM 543520, steinkern from BM1, bed 13, × 1: H, right valve; I, left valve; J,dorsal view. K, Trigonodus sp., USNM 543521, internal mould with partly adherent shell, right valve from BM1, bed 13, × 1. L–O,Unionites? fassaensis (Wissmann, in Munster 1841) from BM1, bed 7a, steinkerns, × 2; L–M, USNM 543522: L, right valve; M, dorsalview, showing beaks, lunule, escutcheon and opisthodetic ligament; N, USNM 543523, right valve of steinkern; O, USNM 543524, rightvalve of steinkern. P–W, Unionites? canalensis (Catullo, 1846) from BM5, bed 5/6, steinkerns, × 2: P–Q, USNM 543528: P, left valve;Q, right valve; R, S, USNM 543526: R, left valve; S, right valve; T, U, USNM 543527: T, left valve; U, right valve; V, W, USNM 543525:V, left valve; W, dorsal view, showing position of possible pedal muscle scar (arrow). X, Y, Unionites? cf. borealis (Spath, 1935), steinkernfrom BM1, bed 7a, USNM 543529, × 2: X, left valve; Y, dorsal view. Z–B′, Heminajas? cf. balatonis (Frech, 1905), USNM 543530,steinkern from BM1, bed 7a, × 1.5: Z, left valve; A′, right valve; B′, dorsal view. C′, Neoschizodus praeorbicularis (Bittner, 1901b),USNM 543532 from K1, steinkern, view of right valve, × 1.5. D′, Neoschizodus laevigatus (von Zieten, 1830), USNM 543531 fromBM5, bed 11, steinkern, view of posterior part of left valve, × 1.5.

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HC5 beds 7 and 8. The type series comprises one right andseven left valves from V1, bed 10. The shell is at least partlypreserved in these specimens.

Holotype. USNM 543508 (Fig. 6O).

Dimensions of holotype. Length > 37 mm; height > 42mm.

Type locality. V1.


Etymology. After the type locality Virgin in Utah.

Diagnosis. Left valve with simple radial ribs that fade awaytowards ventral margin.

Description. Comparatively large Eumorphotis withprocrescent valves.

Left valve with simple radial ribs that fade away towardsventral margin. Beak pointed, projecting beyond hinge line.Anterior auricle with shallow sinus. Ligament area slightlyinclined towards the opposite valve, with subparallel dorsaland ventral margins; resilifer broad, with slightly divergentmargins (Fig. 6Q).

Right valve flat, with straight dorsal margin and deeplyincised byssal notch. Surface of right valve devoid of orna-mentation apart from incremental lines.


Remarks. Although our material is fragmentary, it allowsan almost complete characterization of the new species.Radial ribs that fade away ventrally have not been observedin any other Eumorphotis species.

Newell & Boyd (1995, p. 39) mentioned that a tetragonalresilifer might be distinctive for Eumorphotis. The resiliferof E. virginensis is not strictly tetragonal but has slightlydivergent margins widening the resilifer towards the ventralborder of the ligament area (Fig. 6Q). However, the resiliferlacks the triangular shape of closely related Heteropecten(see Newell & Boyd 1995, fig. 27) and thus might still be auseful criterion for separating these genera.

Eumorphotis cf. multiformis (Bittner, 1899)(Fig. 6R)

cf. 1899 Pseudomonotis multiformis Bittner: 10, pl. 2, figs11–22.

cf. 1938 Eumorphotis multiformis (Bittner); Kiparisova:224, pl. 2, figs 4, 9, 12, pl. 3, figs 2–4.

cf. 1963 Eumorphotis multiformis (Bittner); Ciriacks: 77,pl. 15, figs 13, 15.

cf. 1995 Eumorphotis multiformis (Bittner); Newell &Boyd: 39, fig. 27.

cf. 2009 Eumorphotis multiformis (Bittner, 1899); Kuma-gae & Nakazawa: 162, fig. 144.17.

Material. One left valve with externally preserved shellfrom BM1 bed 7a.

Dimensions. USNM 543511: length > 10 mm; height 13.3mm.

Description. Incompletely preserved left valve withprocrescent disc, externally covered with at least sevenfirst-order radial ribs, which are intercalated with weakersecond-order ribs and still weaker third-order ribs, the latteroccasionally multiple.


Remarks. We refer this incompletely preserved specimententatively to Eumorphotis multiformis, based on the char-acteristic ornamentation pattern.

Superfamily Entolioidea Korobkov, 1960Family Entoliidae Korobkov, 1960

Remarks. This group has recently been split into severalfamilies and subfamilies that differ morphologically incharacters apparently of generic significance only. The mostrecent revision of Waller (2006) emphasized the impor-tance of shell microstructural differences but, unfortunately,the underlying observations have not been illustrated, andit has not been indicated which species and how manyspecimens were examined. It is thus unclear how variablethe shell microstructural characters were and whether theyactually reflect phylogenetic relationships or rather ecolog-ical conditions and seawater chemistry. With the presentstate of knowledge, all described genera of Entolioidea caneasily be accommodated in Entoliidae. However, it is neces-sary to place Entoliidae in its own superfamily in order toaccount for its independent origin from Pectinoidea (Haut-mann 2010; Carter & Hautmann 2011).

Genus Pernopecten Winchell, 1865

Type species. Aviculopecten limaformis White & Whit-field, 1862, Mississippian, Iowa.

Pernopecten? sp.(Fig. 6S, T)

2009 Entolium sp.; McGowan et al.: 862, table 1.

Material. One left valve from BM5 bed 8.

Dimensions. USNM 543512: length 27.0 mm; height 27.6mm.

Description. Left valve moderately convex for the genus,virtually equilateral, with circular disc covered with numer-ous commarginal riblets. Two broad sulci extending frombeak to anterior and posterior margin of disc. Auriclessubequal, relatively small, with blunt angles towards disc,distally projecting above hinge line. Incremental lines onanterior auricle slightly prosocline, suggesting the presenceof a shallow byssal sinus in juvenile stage (not observed).Incremental lines on posterior auricle inclined towardsbeak.

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Life habit. The circular discs, smooth shell exterior, lowvalve convexity, and an obsolete byssal notch suggest thatPernopecten was a free-living genus with the ability to swimby rapidly clapping the valves and pressing water out of themantle cavity on either side of the dorsal margin (Stanley1972; Hautmann 2004 and references therein; Waller 2006).

Remarks. We identified our specimen as a left valve basedon the prosocline inclination of the incremental lines on the(inferred) anterior auricle. Generic assignment then followsfrom the presence of an angulated dorsal margin that occursin the left valve in Pernopecten (Newell 1938, p. 110),in contrast to all other entoliids where projecting auriclesoccur in the right valve (e.g. Waller 2006). The presence orabsence of an anterior and posterior gape between the valves(see Yin 1983) cannot be confirmed because of diageneticdistortion. Pernopecten is a chiefly Palaeozoic genus that,however, has also been reported from the Early Triassic(e.g. Posenato et al. 2005).

Subclass Heteroconchia Hertwig, 1895Superorder Palaeoheterodonta Newell, 1965

Order Modiomorphoida Newell, 1969cSuperfamily Modiomorphoidea Miller, 1877

Family Kalenteridae Marwick, 1953

Remarks. As noted by Damborenea (2004), Chavan(1969) has incorrectly referred to IRZN Article 40 whenassuming priority of Permophoridae van de Poel, 1959 overKalenteridae Marwick, 1953. This applies to cases wherethe type genus is a junior synonym, in which instance areplacement name proposed before 1961 would be valid,retaining its own author but taking the priority of thereplaced name. However, Permophoridae was suggested toreplace Pleurophoridae Dall, 1895, which was invalidatedby homonymy (not synonymy) of its type genus. In that caseIRZN Article 39 applies, and accordingly, the invalidatedname must be replaced by the next oldest available namefrom among its synonyms, which is Kalenteridae Marwick,1953 (Damborenea 2004).

Genus Permophorus Chavan, 1954

Type species. Arca costata Brown, 1841, Permian (Zech-stein), Yorkshire (UK).

Permophorus triassicus Newell & Boyd, 1999(Figs 6U, V, 7A)

1999 Permophorus triassicus Newell & Boyd: 2, figs 2–4.2009 Permophorus triassicus Newell & Boyd; McGowan


Material. Rather uncommon except in HC5 beds 6 and 7.It has also been found in BM1 bed 6, BM5 beds 6 and 8/9,and HC1 bed 4. The following description is based on fivesteinkerns from HC5 bed 6.

Dimensions. USNM 543513: length > 28.2 mm, height19.3 mm.

Description. Outline suboval, valves posteriorly higherthan anteriorly. Anterior adductor muscle scar reflected byconspicuous elevation on internal moulds. Hinge with welldeveloped cardinal tooth 2, reflected by a deep subumbonalembayment of the commissural line in internal moulds (Fig.7A).

Life habit. Outline and cross section of valves suggests ashallow burrowing mode of life (cf. Stanley 1970).

Remarks. The observable characters of our specimensagree quite well with Permophorus triassicus, describedfrom the upper Thaynes Formation of the central RockyMountains, particularly in having a deeply impressed ante-rior adductor (reflected by a suboval elevation in oursteinkern material), and in the presence of a ‘bulbous’ tooth2. In our specimens, the valves are posteriorly slightly moreexpanded than those figured by Newell & Boyd (1999), butaccording to these authors the shape of P. triassicus is highlyvariable; thus it is likely that our specimens fall within therange of intraspecific variation.

Family Healeyidae Hautmann, 2008Genus Protopis Kittl, 1904

Type species. Protopis triptycha Kittl, 1904, Anisian,Bosnia-Herzegovina.

Protopis? aff. waageni (Schnetzer, 1934)(Fig. 7B)

aff. 1934 Joannina waageni Schnetzer: 28, pl. 1, figs. 2–5.2009 Joannina sp.; McGowan et al.: 862, table 1.

Material. One internal mould of a left valve from HC1bed 4.

Dimensions. USNM 543515: length > 10 mm; height 10.0mm.

Description. Valve with strongly prosogyrate umbo,nearly terminal beak, faint anterior radial ridge andpronounced posterior keel. No anterior shell lobe observed.

Life habit. The incoiled, nearly terminal beaks togetherwith the flattened anterior shell margins are interpretedas a base on which the animal rested orthothetically onthe substratum, possibly fixed by byssal threads extrudingbetween the valves.

Remarks. We place this incompletely preserved specimenin Protopis rather than in Joannina because of the absenceof a pronounced anterior shell lobe; see revised diagnosesof both genera in Hautmann (2008). The type material ofJoannina waageni Schnetzer, 1934 is incomplete in thisrespect, which further complicates comparison of our mate-rial with this species. However, the general morphology isquite similar except for the probably less elongated shape of

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our specimen. The systematic position of Protopis is uncer-tain due to the lack of knowledge on internal characters.Its placement in Healeyidae is thus tentative (Hautmann2008).

Family Myoconchidae Newell, 1957Genus Myoconcha de Sowerby, 1824

Type species. Myoconcha crassa de Sowerby, 1824,Middle Jurassic, England.

Myoconcha cf. plana Kiparisova, 1938(Fig. 7C)

cf. 1938 Myoconcha plana Kiparisova: 266, pl. 6, figs 14,15.

2009 Pseudomyoconcha cf. gastrochaena; McGowan et al.:862, table 1.

Material. This species was only common in HC5 bed 7,although it was also found in BM1 beds 6 and 9, andBM5 bed 6. The description is based on one well-preservedsteinkern from HC5 (loose but presumably derived frombed 7).

Dimensions. USNM 543516: length 20 mm; height 10.0mm.

Description. Beaks close to anterior end but not terminal;dorsal and ventral margin slightly divergent, thus height ofshell increasing towards posterior end; anterior adductormuscle buttressed.

Life habit. The shell morphology is analogous to that ofRecent Modiolus, which suggests an endobyssate mode oflife (e.g. Stanley 1970).

Remarks. Our specimen is similar to Myoconcha plana insize, length/height ratio, and position of the umbo. It differsfrom the specimens figured by Kiparisova (1938, pl. 6, figs14, 15) in a slightly greater divergence of the dorsal andventral margins, but it appears likely that this falls withinthe intraspecific variation. Myoconcha sp. indet. figured byKumagae & Nakazawa (2009, fig. 145.26) is more similarin this respect and might be conspecific. However, the latterhas prominent radial ribs that cannot be confirmed in ourspecimen and in the types of M. plana due to steinkernpreservation.

Order Unionoida Stoliczka, 1871Superfamily Unionoidea Fleming, 1828

Family Trigonodidae Modell, 1942Genus Trigonodus Sandberger in von Alberti, 1864

Type species. Trigonodus sandbergeri von Alberti, 1864,Ladinian, Germany.

Trigonodus cf. orientalis Bittner, 1899(Fig. 7D–G)

cf. 1899 Trigonodus orientalis Bittner: 266, pl. 3, fig. 27.

1938 Trigonodus orientalis Bittner; Kiparisova: 217, pl. 2,figs 1, 2.

cf. 2009 Trigonodus orientalis Bittner; Kumagae &Nakazawa: 165, figs 144.18–144.20.

2009 Trigonodus sp. B; McGowan et al.: 862, table 1.

Material. This is most common in bed 9 at BM1 but hasalso been found at BM1 in beds 6 and 14, at BM5 in bed8/9, HC5 in beds 6 and 7, and HC6 in bed 11. The followingdescription is based on internal moulds of four left valvesand of one articulated specimen from BM1 bed 13.

Dimensions. USNM 543517: length 25.7 mm, height 19.8mm, width 13.7 mm. USNM 543518: length 27 mm, height19.9 mm.

Description. Shell well inflated, trapezoidal in outline,posteriorly truncated, with beaks located on anterior 25%of dorsal margin. Valves slightly bent along line connectingumbones with posteroventral shell edge, causing a bluntposterior ridge. Anterior adductor buttress corresponds tofaint grooves on internal moulds. Details of hinge notobserved.

Life habit. The truncated posterior shell region, whereinhalent and exhalent currents were located, probably layparallel to and leveled with the surface of the substratum (cf.Stanley 1970, 1977), suggesting a shallow-infaunal habit.The relatively high inflation of the shell is indicative forsluggish burrowing species, although some exceptions areknown from Recent bivalves (Stanley 1970, p. 59).

Remarks. Trigonodus orientalis Bittner, 1899 was erectedon the basis of a single left valve from the Early Triassicof the far east of Russia. The type specimen agrees quitewell with our material in general shape except for havinga more anteriorly located umbo. Later, Kiparisova (1938)described specimens of T. orientalis from the same area,in which the position of the umbo is comparable to thepresent specimens. However, the umbo of the specimensrecently described by Kumagae & Nakazawa (2009) is evenmore anteriorly located than in the holotype, from which itadditionally differs in being longer in relation to height. Itappears likely that all these differences reflect intraspecificvariations.

Trigonodus cf. sandbergeri von Alberti, 1864(Fig. 7H–J)

cf. 1864 Trigonodus sandbergeri von Alberti: 126, pl. 2,fig. 10.

cf. 1923 Trigonodus sandbergeri von Alberti; Diener: 192(cum synonymis).

2009 Trigonodus sp. A; McGowan et al.: 862, table 1.

Material. This species co-occurs with T. cf. orientalis inBM1 beds 7a and 9 where it is relatively common. It also isfound in BM1 beds 14–15, BM5 beds 6–12, and HC5 bed6. One steinkern from BM1 bed 13 is described below.

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Dimensions. USNM 543520: length 28.3 mm, height 18.3mm, width 11.6 mm.

Description. Shell elliptical in outline, with strongly pros-ogyrate umbones, distinct posterior carina and pointedposterior shell edge. Beaks located on anterior 16% ofdorsal margin. Anterior adductor muscle relatively largeand buttressed. One well-developed subumbonal tooth ineach valve, reflected by deep subumbonal embayments ofthe commissural line in the internal moulds.

Life habit. The more streamlined shape of the shellsuggests that this species had slightly higher burrowingrates than Trigonodus cf. orientalis described above.

Remarks. The present taxon agrees well with the LadinianTrigonodus sandbergeri von Alberti, 1864 in general shapebut differs in having a larger anterior adductor muscle. Themorphological differences with T. cf. orientalis describedabove, which co-occurs in some beds, include more stronglyprosogyrate umbones, more anteriorly located beaks, amore pronounced posterior carina, a pointed posterior shelledge, and a smaller height/length ratio. However, onespecimen co-occurring with T. cf. sandbergeri (Fig. 7K)approaches T. cf. orientalis in some of these aspects.

Superfamily Anthracosioidea Amalitsky, 1892Family Anthracosiidae Amalitsky, 1892

Genus Unionites Wissmann in Graf zu Munster, 1841

Type species. Unionites muensteri Wissmann in Graf zuMunster, 1841, Carnian, South Tyrol.

Remarks. Based on well-preserved material from theLadinian of Germany, Unionites was revised by Hautmann(in Geyer et al. 2005), who re-assigned this genus to Anthra-cosiidae Amalitsky, 1892. Unionites has a peculiar hingewith the anterior hinge margin of the right valve project-ing beyond the plane of commissure, a variably developedanterior lateral tooth, and a well-developed posterior lateraltooth in the right valve. The left valve has a small subum-bonal tooth, a slightly projecting posterior hinge margin,and recesses corresponding to shell projections of the oppo-site valve. Unionites has frequently been reported from theEarly Triassic (see Diener 1923), but in none of these reportshas the hinge morphology been given, which makes thesegeneric assignments uncertain.

Unionites? fassaensis (Wissmann in Graf zu Munster,1841)

(Fig. 7L–O)

1841 Myacites fassaensis Wissmann in Graf zu Munster:9, pl. 16, fig. 2a–c.

1923 Anodontophora fassaensis (Wissmann); Diener: 231(cum synonymis).

1935 Homomya fassaensis (Wissmann); Leonardi: 32, pl.1, fig. 5.

1963 Unionites fassaensis (Wissmann); Ciriacks: 82, pl.16, fig. 13.2009 Unionites fassaensis (Wissmann); Kumagae &Nakazawa: 167, figs 144.5–144.9 (cum synonymis).

2009 Unionites fassaensis (Wissmann); McGowan et al.:862, table 1.

Material. Three steinkerns from BM1 bed 7a.

Dimensions. USNM 543522: length > 19.0 mm, height12.8 mm, width 6.6 mm. USNM 543523: length 16.0 mm,height 11.6 mm, width 6.2 mm. USNM 543524: length >

14.4 mm, height 10.0 mm, width 5.7 mm.

Description. Valves elliptical in outline, with prosogyrateumbones and beaks located approximately on anterior 35%of dorsal margin. Small lunule and narrow escutcheondelimited by blunt ridges. Ligament external, opisthode-tic, and probably parivincular. Anterior hinge margin notprojecting beyond plane of commissure.

Life habit. The elliptical outline of the valves and the formand position of the umbones is analogous to Recent burrow-ing bivalves such as Mercenaria, as described by Stanley(1975). The lack of siphons in Unionites (Geyer et al. 2005)indicates a shallow infaunal mode of life.

Remarks. The present species differs from Unionites s.s.in the absence of a projection of the anterior hinge margin.Due to the lack of knowledge of the shell interior, an alter-native assignment cannot presently be proposed.

Unionites? fassaensis is one of the most frequentlyreported bivalve species from the Early Triassic and amongthe first to recolonize marine ecosystems after the end-Permian mass extinction. It shows a high variability ingeneral shape and position of the beak (e.g. Kumagae &Nakazawa 2009).

In one of our specimens, the ligament is partly preservedas limonite, showing the typical appearance of a parivincu-lar ligament system (Fig. 7M), although the presence of anymph cannot be verified in the absence of shell preserva-tion.

Unionites? canalensis (Catullo, 1846)(Fig. 7P–W)

1846 Tellina canalensis Catullo: 56, pl. 4, fig. 4.1859 Tellina (Myacites) canalensis Catullo; von Schauroth:

327, pl. 2, fig. 17.1923 Anodontophora canalensis (Catullo); Diener: 230(cum synonymis).

1935 Homomya canalensis (Catullo); Leonardi: 35, pl. 1,figs 13–15.2009 Unionites canalensis (Catullo); Kumagae &Nakazawa: 166, figs 145.1–145.4 (cum synonymis).

2009 Unionites sp. C; McGowan et al.: 862, table 1.

Material. Four steinkerns, partly with adherent shell mate-rial, from BM5 bed 6.

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Dimensions. USNM 543525: length 22.0 mm, height 12.3mm, width 7.2 mm. USNM 543526: length 18.5 mm, height11.3 mm, width 7.0 mm. USNM 543527: length > 21.0 mm,height 13.8 mm, width 7.6 mm. USNM 543528: length 14.6mm, height 8.8 mm, width 4 mm.

Description. Valves subrectangular in outline, with nearlystraight ventral margin. Umbones prosogyrate, beaks small,located approximately on anterior 25–30% of dorsalmargin. Posterior transverse ridge blunt. Small elevationsnear anterior dorsal margin in one specimen (Fig. 7W)possibly represent pedal muscle attachment scars.

Life habit. The elongated shape of the shell and the trun-cated posterior shell margin suggest a shallow-burrowingmode of life.

Remarks. The original figure and description of Catullo(1846) is somewhat indistinct and possibly the sourceof subsequent misidentifications. Von Schauroth (1859)published a more accurate description of Unionitescanalensis, apparently based on his own collections fromthe type locality at Canale d’Agordo (Veneto, Italy). Ourmaterial agrees quite well with this description, and alsowith material from own collections in other parts of theDolomites.

Ciriacks (1963, p. 81) emphasized the relatively medianposition of the umbones as a diagnostic feature of U.canalensis, probably influenced by the inaccurate figuresof Catullo (1846). In our material, the beak is clearly moreanteriorly located than in many other species of Early Trias-sic ‘Unionites’, including U .? fassaensis described above.The identity of the specimens figured by Ciriacks (1963, pl.16, figs 11, 12) is currently uncertain.

Unionites? cf. borealis (Spath, 1935)(Fig. 7X, Y)

cf. 1930 Anodontophora sp. n. cf. subrecta (Bittner); Spath:53, pl. 10, fig. 12.

cf. 1935 Anodontophora borealis Spath: 76.

Material. This species occurs abundantly at BM1 in bed7a but has not been found elsewhere. Two steinkerns fromthis level form the basis for the following description.


Description. Valves subrectangular in outline, posteriorlytruncated, with prosogyrate umbones and beaks located onanterior 18% of dorsal margin. Posterior transverse ridgewell defined.

Life habit. Shallow burrowing (see above).

Remarks. This species differs from Unionites? canalen-sis described above in being higher relative to its length,having a sharper posterior transverse ridge, more anteriorlylocated beaks, and a slightly curved ventral shell margin.

We therefore separate it from U .? canalensis, although alarger sample might reveal morphological overlap betweenthe two species.

Our specimens agree quite well with U. borealis (Spath,1935) from East Greenland, described from loose blocksallocated to the Griesbachian. However, the comparativelypoor preservation of the type material complicates definiteidentification with our material.

Order Trigonioida Dall, 1899Superfamily Myophorioidea Bronn, 1849

Family Myophoriidae Bronn, 1849Genus Heminajas Neumayr, 1891

Type species. Myophoria fissidentata von Wohrmann,1889, Carnian, Austria.

Heminajas? cf. balatonis (Frech, 1905)(Fig. 7Z–B′)

cf. 1905 Myophoria balatonis Frech: 7, fig. 9.cf. 1906 Myophoria balatonis Frech; von Arthaber: pl. 34,

fig. 8.cf. 1907 Heminajas balatonis Frech; Waagen: 143.2009 Neoschizodus cf. laevigatus; McGowan et al.: 862,table 1.

Material. Uncommon, being occasionally found in BM1beds 7a and 14, HC3 bed 10, HC5 bed 6, and V bed 9. Onesteinkern from BM1 bed 7a is illustrated here.


Description. Outline elongate-ovate, posteriorly trun-cated, with nearly straight ventral margin. Beaks locatedat anterior 34% of shell length. Blunt ridge extending fromumbo to posteroventral shell edge. Anterior myophorousbuttress reflected by sharp groove on steinkerns. Hingedetails not observed.

Life habit. Probably all Mesozoic Trigonioida were shal-low burrowers (Stanley 1977).

Remarks. We allocate this species to Heminajas? cf. bala-tonis on the basis of the very good agreement in thegeneral morphology. The present specimen differs fromNeoschizodus thaynesianus Newell & Boyd, 1975 in havinga more elongated shape and a nearly straight ventral margin.Because hinge details and possible morphological varia-tions within the population are unknown, our assignmentis, however, tentative.

Genus Neoschizodus Giebel, 1855

Type species. Lyrodon laevigatum Goldfuss, 1838 ( =Trigonia laevigata von Zieten, 1830), Middle Triassic,Germany.

Neoschizodus laevigatus (von Zieten, 1830)(Fig. 7D′)

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Figure 8. A–L, Sementiconcha recuperator gen. et sp. nov. from BM5, bed 7, steinkerns: A, B, holotype, USNM 543533: A, left valve;B, dorsal view, × 3; C, D, USNM 543534, × 2: C, left valve; D, dorsal view; E, F, USNM 543535, × 2: E, left valve; F, dorsal view;G, H, USNM 543536, × 2: G, left valve; H, dorsal view; I, J, USNM 543537, × 2: I, left valve; J, dorsal view; K, L, USNM 543538,× 2: K, left valve; L, dorsal view. M, Unicardium? sp., USNM 543539, mould of left valve from V bed 7, ×1. N, O, S, Arcomya? sp.,× 1.5: N, O, USNM 543540, steinkern (composite mould) from BM5, bed 6: N, left valve; O, right valve; S, USNM 543541, steinkernfrom BM5, bed 8/9, left valve. P–R, Pleuromya prima sp. nov., USNM 543542 from BM1, bed 9: P, left valve, × 1; Q, dorsal view, × 1;R, detail of anterior dorsal margin, showing possible pedal retractor scar (pr) and subumbonal shell protuberance (sp).

1830 Trigonia laevigata von Zieten: 94, pl. 71, figs 2, 6.1856 Neoschizodus laevigatus (von Zieten, 1830); Giebel:

40, pl. 3, figs 1, 9, 10.1923 Myophoria laevigata (von Zieten, 1830); Diener: 174

(cum synonymis).

Material. One internal mould of left valve, posterior part,from BM5 bed 11.

Description. Posterior part of ventral margin nearlystraight, posterior margin truncated. Sharp diagonal carinaextending from umbo to posteroventral shell corner. Poste-rior adductor muscle reflected by oval elevation on corselet.

Life habit. As for Heminajas? cf. balatonis.

Remarks. The present specimen agrees in all observedcharacters with the widespread Neoschizodus laevigatus.

Neoschizodus praeorbicularis (Bittner, 1901b)(Fig. 7C′)

1901 Myophoria praeorbicularis Bittner: 86, pl. 9, figs3–9.

2009 Neoschizodus sp. B; McGowan et al.: 862, table 1.

Material. One steinkern from K1.

Description. Shell ovate, with blunt diagonal ridge.Imprint of the anterior myophorous buttress in front of beak.

Life habit. As for Heminajas? cf. balatonis.

Remarks. The incompletely preserved steinkern shows thecharacteristic outline of this species, as described by Bittner(1901b).

Superorder Heterodonta Neumayr, 1884Order Veneroida Adams & Adams, 1856

Superfamily Crassatelloidea de Ferussac, 1822Family Myophoricardiidae Chavan in Vokes, 1967

Genus Sementiconcha gen. nov.

Type species. Sementiconcha recuperator sp. nov.

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Diagnosis. Shell subrectangular in outline, elongated,posteriorly truncated, with acute posterior ridge and well-developed anterior cardinal tooth in the right valve. Orna-mentation with irregularly spaced growth rugae.

Etymology. Combination of sementis (Latin) = seed, andconcha (Latin)= shell, referring to the kernel-like morphol-ogy of this small bivalve and its mass occurrence withinmarly beds.

Remarks. We assign this new genus to Myophoricardiidaebecause its shape agrees very well with other genera unitedin this family (Cox & Chavan 1969). The new genus canbe distinguished from other genera of Myophoricardiidaechiefly by the position of the right anterior cardinal tooth,which is reflected on steinkerns by a pronounced embay-ment of the commissure in front of the beak (Fig. 8B, F).More specifically, the right valves of Myophoricardium vonWohrmann, 1889 and Pseudocorbula Philippi, 1898b havea cardinal tooth (interpreted as 3b in Hautmann 2001) in acentral position, in contrast to the anteriorly placed cardinaltooth in the right valve of Sementiconcha. The hinge denti-tion of Myophoriopis is variable (Cox & Chavan 1969) andthus more difficult to compare with that of the new genus,but Myophoriopis differs from Sementiconcha in havingregular commarginal ribs on its flank, being much shorterin relation to height, and having its beak in a more posteriorposition.

Sementiconcha recuperator sp. nov.(Fig. 8A–L)

Material. This species occurs in abundance in BM5 bed 7(lower part) but has not been found at other localities. Thefollowing description is based on 119 steinkerns (mostlycomposite moulds) from this level.

Holotype. USNM 543533 (Fig. 8A, B).

Paratypes. USNM 543534–543538.

Type locality. BM5.

Type stratum. Lower part of bed 7.

Diagnosis. As for genus.

Etymology. Recuperator (Latin)= re-conqueror, referringto the reoccupation of a vacant niche after the end-Permianmass extinction.

Dimensions. USNM 543533: length 9.8 mm, height 6.4mm, width 4.7 mm. USNM 543534: length 11 mm, height6.3 mm, width 4.4 mm. USNM 543535: length 13.6 mm,height 8.5 mm, width 5.5 mm. USNM 543536: length 13.3mm, height 8.3 mm, width 5.0 mm. USNM 543537: length13.1 mm, height 7.4 mm, width 5.4 mm. USNM 543538:length 13.1 mm, height 7.8 mm, width 5.6 mm.

Description. Outline of valves subrectangular, posteriorlytruncated, with nearly straight ventral margin. Beak located

at anterior 30% of dorsal margin, umbo prosogyrate, givingrise to sharp diagonal carina. Posterior dorsal marginstraight, probably fitted with nymph. Well-developedanterior cardinal tooth 3a in the right valve, indicatedby corresponding embayment of commissure in internalmoulds. Posterior limits of anterior adductor muscleimprints reflected on steinkerns by small ridge in front ofbeaks. Minute additional muscle impression dorsally toanterior adductor muscle may represent trace of anteriorpedal retractor. Ornamentation with irregularly spacedincremental lines on flank of composite moulds.

Life habit. The elongated shape and truncated posteriormargin of the shell suggest a shallow burrowing mode oflife.

Remarks. Occasionally, parts of the external ligament arepreserved by limonitic replacement material. We concludefrom the semicircular cross section that this represents aparivincular ligament system supported by nymphs.

Superfamily Lucinoidea Fleming, 1828Family Mactromyidae Cox, 1929

Genus Unicaridum d’Orbigny, 1850

Type species. Corbula cardioides Phillips, 1829, EarlyJurassic, England.

Unicardium? sp.(Fig. 8M)

Material. Mould of one left valve from V bed 7.

Description. Valve well inflated, subelliptical in outline,with blunt posterior diagonal ridge and truncated posteriormargin. Umbo broad, slightly prosogyrate to nearly orth-ogyrate, and placed in central position.

Life habit. This species is interpreted as a sluggishburrower, based on the mode of life of Recent represen-tatives of its family and on the general shape of the shell,which is not suitable for rapid burrowing (cf. Stanley 1970).

Remarks. We tentatively assign this specimen toUnicardium based on the observed external characterssuch as the subelliptical shape and the broad, centrallyplaced and only slightly incurved umbo. The latter featuredifferentiates Unicardium from Unionites, which usuallyhas a smaller, more anteriorly placed and more proso-gyrate umbo. The most similar species is Unicardiumschmidi (Geinitz, 1842) from the Rot and Muschelkalk(Anisian–Ladinian) of Germany.

Superorder Anomalodesmata Dall, 1889Order Pholadomyoida Newell, 1965

Superfamily Pholadomyoidea Gray, 1847Family Pholadomyidae Gray, 1847

Genus Arcomya Agassiz, 1843

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Type species. Solen helveticus Thurmann in Roemer,1839, Late Jurassic, Switzerland.

Arcomya? sp.(Fig. 8N, O, S)

2009 Arcomya sp. A; McGowan et al.: 862, table 1.2009 Homomya sp. McGowan et al.: 862, table 1.

Material. Rare in the Virgin Limestone, with only oneslightly compressed steinkern (composite mould) fromBM5 bed 5/6, and one steinkern from BM5 bed 8/9.

Description. Outline subelliptical to subrectangular, withblunt posterior ridge and small umbones placed well anteri-orly. Shell posteriorly damaged in the available materialbut presence of small posterior gape between valves issuggested by curvature of shell near the posterior margin.Ornamentation with irregularly spaced growth rugae only.

Life habit. The possible presence of a posterior gapebetween the valves hints at the presence of mantle siphonsand thus at a relatively deep-infaunal mode of life.

Remarks. Diagenetic distortion of one specimen (Fig. 8N,O) has led to slight differences in its shape compared tothe other available specimen (Fig. 8S). More specifically,the area above the posterior ridge appears enlarged in theleft valve of the distorted specimen (Fig. 8N), and theposteroventral part of the right valve slightly expanded.Given the relatively poor state of preservation, a definitedetermination of the present specimens is difficult, butthe observed characters agree well with an assignment toArcomya.

Family Pleuromyidae Zittel, 1895Genus Pleuromya Agassiz, 1842

Type species. Mya gibbosa de Sowerby, 1823 [ =Donacites alduini Brongniart, 1821], Late Jurassic,England.

Pleuromya prima sp. nov.(Fig. 8P–R)

Material. One steinkern from BM1 bed 9.

Holotype. USNM 543542 (Fig. 8P–R).

Dimensions of holotype. Length 36.8 mm; height 20.2mm, width 13.5 mm.

Type locality. BM1.


Etymology. Primus (Latin) = the first, because this is thegeologically oldest known species of Pleuromya.

Diagnosis. Subrectangular-elongated Pleuromya withumbones located relatively far behind anterior shell end.Subumbonal protuberance well developed.

Description. Shell moderately large, well inflated, withumbones located at anterior 38% of shell length.Height/length ratio low for the genus. Ventral margin nearlystraight. Subumbonal protuberance of shell margin promi-nent, reflected by embayment of commissure on steinkerns(Fig. 8R). Anterior adductor muscles posteriorly sunken.Additional small muscle impressions located dorsally of theanterior adductor muscles probably represent pedal retrac-tors (Fig. 8R). Pallial line and anterior or posterior gapebetween valves not observed.

Life habit. Although not visible in our material, the pres-ence of a pallial sinus and thus a deep-infaunal mode of lifeis well established for Pleuromya (e.g. Newell 1969d).

Remarks. The generic assignment is assured by thegeneral shape and the presence of a subumbonal protu-berance. The present specimen represents the earliest defi-nite occurrence of Pleuromya. The only other doubtfulrecord of Pleuromya from the Early Triassic is that of‘Pleuromya’ elongata (von Schlotheim, 1822) mentionedby Broglio Loriga et al. (1990) from the basal Olenekian ofthe Werfen Formation in South Tyrol. However, the internalcharacters of this taxon, alternatively placed in Unionitesor Pholadomya by other authors, are insufficiently known(Neri & Posenato 1985). The type stratum of P. elongata isthe Upper Muschelkalk (Ladinian) of the Germanic basin,and it appears questionable that the poorly preserved mate-rial from the Werfen Formation can be attributed to thisspecies with certainty.

Although only one specimen is available, diagnos-tic features are sufficiently known to characterize thisnew species in detail. Pleuromya musculoides and relatedspecies from the Muschelkalk (Anisian–Ladinian) havegenerally more anteriorly placed umbones that are nearlytouching (Bender 1921). However, generic assignment ofall these species is uncertain because the internal charac-ters including the presence or absence of the subumbonalprotuberance are generally unknown.

Evolutionary significance of the fauna

Previous studies of the Virgin Limestone Member listed two(Boyer et al. 2004, table 2) to 10 (Schubert & Bottjer 1995,appendix 1) bivalve genera, with no reference to the numberof species. This apparently low diversity, in combinationwith ecological assumptions, led the authors of these stud-ies to conclude that biotic recovery from the end-Permianmass extinction was slow and incomplete during the entireEarly Triassic, either as a consequence of the extraordinaryextinction magnitude (Schubert & Bottjer 1995, p. 28) ordue to long-lasting environmental stress (Boyer et al. 2004,p. 379). Our study documents 27 bivalve species belongingto 18 genera from the relatively short-lived marine ingres-sion of late early Spathian age represented by the Virgin

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Limestone Member. This is the highest reported bivalvediversity of all geological units from this time interval andthe entire Early Triassic, documented in a relatively limitedset of intra- to shallow subtidal habitats.

Detailed comparisons of alpha diversity with pre-Spathian bivalve faunas will be an important future task,but differences in bulk diversity provide at least an impres-sion of the relative state of taxonomic recovery. Ciriacks(1963) documented 15 species representing eight generafrom the Griesbachian Dinwoody Formation of the MiddleRocky Mountains, which is comparable to Griesbachianbivalve faunas from the Alps (eight genera/16 species; vonWittenburg 1908; Broglio Loriga et al. 1983) and the fareast of Russia (10 genera/12 species; Kumagae & Nakazawa2009). Although not directly comparable, a single carbonatelens from the late Griesbachian of South China yielded 11bivalve genera, each represented by one species (Hautmannet al. 2011). Dienerian bivalve faunas are poorly known, butit appears that the diversity is not significantly different fromthe Griesbachian (Hautmann et al. 2011). The Smithianwitnessed at least local diversity increases in the WerfenFormation of the Alps (Broglio Loriga et al. 1990; Posenato2008b) and the Yakuno Group of Japan (Nakazawa 1961;for stratigraphy see Nakazawa et al. 1994). However, taxo-nomically revised species-level data from biostratigraphi-cally well-dated sections are not yet available in the WerfenFormation, and separation between Smithian and Spathianin the Yakuno Group is not always possible (Nakazawa etal. 1994). Nakazawa (1961, table 1) reported 13 genera and29 species from the basal part (‘T1a’) of the Yakuno Group,which he subsequently referred to the Olenekian (Nakazawaet al. 1994). However, it remains unclear which stratigraph-ical interval exactly is represented by the basal part of theYakuno Group, and the number of taxa appears overesti-mated, with several uncertain species e.g. in Promyalina,Bakevellia, Eumorphotis and ‘Dacryomya’.

In comparison with pre-Spathian bivalve faunas, theVirgin Limestone Member fauna thus reflects a consid-erable increase in bivalve diversity. Due to the relativelyshort time interval and limited set of habitats representedby the Virgin Limestone Member, and possible taxonomicoversplitting in the descriptions of pre-Spathian faunas, thenominal increase in the number of taxa probably underes-timates the real diversity increase.

Little data on other Spathian bivalve faunas are availablefor comparison. Neri & Posenato (1985) reported 13bivalve genera containing 25 species from the SpathianCencenighe and St. Lucano members of the Werfen Forma-tion of the Alps. This is less diverse but not dramaticallydifferent from the Virgin Limestone fauna, suggesting thatthere was an interregional increase in bivalve diversityduring the Spathian.

The unexpectedly high diversity of the Virgin Limestonebivalve fauna is at variance with claims of a long-termdelay in recovery, as is the degree of faunal heterogeneity

and average body size of the Virgin Limestone Memberfauna in comparison to Permian and post Early Triassicfaunas (McGowan et al. 2009). Recent studies haveshown that incipient recovery from the end-Permian massextinction already occurred in the Griesbachian (Krystynet al. 2003; Brayard et al. 2009; Hautmann et al. 2011),and that additional extinction events rather than persistingenvironmental stress were responsible for the repeatedoccurrences of low diversity intervals in the Early Triassic(e.g. Galfetti et al. 2007a, b, c, 2008; Hautmann et al.2011; Hofmann et al. 2011; Orchard 2007). The earlySpathian increase in bivalve diversity documented in theVirgin Limestone Member represents a major progressin recovery in comparison to pre-Spathian faunas, withimportant implications for future reconstructions ofextinction-diversification dynamics at the dawn of theMesozoic and for addressing how quickly ecosystems canrecover after major extinction events (cf. Hautmann et al.2008b).

The diversity of the Virgin Limestone bivalve faunais remarkably high for the Early Triassic, but it doesnot indicate that taxonomic recovery was completed inthe Spathian. Bivalve alpha-diversity was higher in theMiddle Triassic Muschelkalk, although there is some over-lap with the Virgin Limestone fauna (McGowan et al.2009), and the extraordinary high diversity in the LateAnisian Leidapo Member of the Qingyan Formation inGuizhou, China (49 bivalve genera with 103 species; Stiller2001) provides a glimpse of fully recovered faunas fromlater times. However, the diversity of the Virgin Limestonebivalve fauna is the highest reported so far from the entireEarly Triassic, indicating a much more advanced state inthe process of recovery than previously assumed.

The Virgin Limestone bivalve fauna is composed of twosubclasses, Pteriomorphia and Heteroconchia. Pteriomor-phian bivalves identified in this study include seven generawith 12 species. Most of these were sessile, except forthe facultatively swimming Pernopecten. Most of the pteri-omorphian genera are survivors from the Palaeozoic, withEumorphotis being one possible exception, although sepa-ration from morphologically similar Permian Heteropectenmay be arbitrary (Newell & Boyd 1995). A second possibleexception is Promyalina, a genus that was first describedfrom the Upper Permian Bellerophonschichten near Sara-jevo, Bosnia-Herzegovina. However, the Late Permian ageof the type material was doubted by Newell (1955, p. 26),who suggested that Promyalina was strictly confined to theEarly Triassic. Whenever Promyalina first appeared, thisgenus represents a family that flourished in the Late Palaeo-zoic (Newell 1942) and became extinct by the end of theEarly Triassic (Hautmann et al. 2011).

In contrast to the Pteriomorphia, which nearly exclu-sively comprise Permian survivor genera, only two generaof Heteroconchia (Permophorus and Neoschizodus) werePermian survivors, with the remaining nine genera from

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the Virgin Limestone Member newly evolved in the EarlyTriassic. This remarkable contrast indicates a differentialeffect of the end-Permian mass extinction and subsequentenvironmental disturbances on these two major subclassesof Bivalvia. Similar effects have been observed in the end-Triassic mass extinction, where higher extinction levels ofheteroconchs have been attributed to less efficient filterfeeding (McRoberts & Newton 1995; McRoberts et al.1995) and their completely aragonitic shell mineralogy(Hautmann 2004, 2006; Hautmann et al. 2008a, b).

As stated by various authors (Nakazawa & Runnegar1973; Hautmann & Nutzel 2005; Hautmann 2007), EarlyTriassic bivalve faunas were usually dominated by sessilefilter feeders, most of which belong to Pteriomorphia. Thefauna from the Virgin Limestone Member is thus not onlyremarkable for its high diversity but also for its high propor-tion of heteroconchs, which were mostly burrowers, withthe exception of the endobyssate species Myoconcha cf.plana and the epibyssate species Protopis? aff. waageni.The high proportion of burrowing species, including deep-infaunal anomalodesmatans, is another indicator of a rela-tively advanced recovery state (cf. Twitchett et al. 2004,table 1).

Among the Heteroconchia, the superorders Heterodontaand Anomalodesmata were previously recognized as EarlyTriassic Lazarus clades (Nakazawa & Runnegar 1973).Heterodont bivalve genera have subsequently been iden-tified from the Griesbachian (Hautmann et al. 2011) andSmithian (Hautmann & Nutzel 2005), but the occurrenceof Arcomya? sp. and Pleuromya prima sp. nov. providesthe first definite record of the superorder Anomalodes-mata from the Early Triassic. Previous records of anoma-lodesmatans from the Early Triassic mostly refer to ‘Pleu-romya’ elongata (von Schlotheim, 1822), but the affili-ation of the poorly preserved Early Triassic specimensassigned to this species appears uncertain (Neri & Posenato1985).

In summary, the evolutionary character of the bivalvefauna described here is transitional between typical EarlyTriassic post-extinction faunas, represented by chieflysessile, epifaunal to semi-infaunal pteriomorphs belongingto Permian survivor genera, and Mesozoic-style faunas withnewly evolved burrowing taxa including representatives ofthe deep-infaunal guild. The relatively high diversity of thefauna and the distinctly ‘modern’ aspect imparted by thehigh number of newly evolved heteroconch genera chal-lenges previous claims that benthic ecosystems recoveredsignificantly slower than nekton (e.g. Stanley 2009). Withrespect to the end-Smithian mass extinction event (Galfettiet al. 2007a, b, c, 2008; Orchard 2007), the high diversityof the present fauna indicates either a negligible effect ofthis event on bivalves, or a nearly instantaneous recovery, asobserved after the end-Triassic mass extinction (Hautmannet al. 2008b).

Acknowledgements

This paper is a contribution to the Swiss National ScienceFoundation project 200021–121774. Funding for fieldworkcame from the Natural History Museum and benefited frominput from P. D. Taylor and M. A. Wilson. Helpful reviewsof T. Kumagae and C. McRoberts and the careful editorialwork of P. D. Taylor are gratefully acknowledged.

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Bivalves from the Olenekian (Early Triassic) of south-western Utah: systematics and evolutionary...

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