An unusually diverse mollusc fauna from the earliest Triassic of South China and its implications for benthic recovery after the end-Permian biotic crisis

An unusually diverse mollusc fauna from the earliest Triassic of South China and its implications for benthic recovery after the end-Permian biotic crisis

Geobios 44 (2011) 71–85 Original article An unusually diverse mollusc fauna from the earliest Triassic of South China and its implications for benth...

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Geobios 44 (2011) 71–85

Original article

An unusually diverse mollusc fauna from the earliest Triassic of South China and its implications for benthic recovery after the end-Permian biotic crisis§ Une malacofaune exceptionnellement diverse du Trias inférieur de Chine du Sud : implications pour la récupération benthique après la crise biotique fini-permienne Michael Hautmann a,*, Hugo Bucher a, Thomas Brühwiler a, Nicolas Goudemand a, Andrzej Kaim b,c, Alexander Nützel b a

Paläontologisches Institut und Museum, Universität Zürich, Karl Schmid-Strasse 4, 8006 Zürich, Switzerland b Bayerische Staatssammlung für Paläontologie und Geologie, Ludwig-Maximilians-Universität München, Department für Geo- und Umweltwissenschaften, Sektion für Paläontologie, Richard-Wagner Strasse 10, 80333 München, Germany c Instytut Paleobiologii PAN, ul. Twarda 51/55, 00-818 Warszawa, Poland Received 22 April 2010; accepted 20 July 2010 Available online 23 December 2010

Abstract A single carbonate coquinoid lens from the Griesbachian (Early Triassic) of Shanggan, South China, yielded 11 bivalve species described in this study in addition to four gastropod and one ammonoid species reported elsewhere. This makes the Shanggan fauna one of the richest mollusc faunas from the early post-extinction interval after the end-Permian mass extinction event. Four of the present genera are long-term survivors, five are holdovers that went extinct at the end of the Griesbachian or later in the Early Triassic, and seven first appear in the Griesbachian. Three new bivalve species are described: Myalinella newelli nov. sp., Scythentolium scutigerulus nov. sp., and Eumorphotis shajingengi nov. sp. The genus Astartella, previously assumed to have vanished at the end of the Permian, is reported for the first time from the Early Triassic, which also removes Astartidae from Early Triassic Lazarus taxa. The small growth size of the Astartella specimens supports an earlier hypothesis that many of the Early Triassic Lazarus taxa did not survive in unknown refuges but were simply overlooked due to the scarcity of easily observable large-sized specimens. Ecologically, a comparatively high proportion of infaunal bivalve species (4/11) is remarkable for the early post-extinction interval, supporting the impression of a relatively advanced recovery state. Moreover, abundance-data of the bivalve-gastropod community reveal a remarkably low dominance index (D = 0.17) that is suggestive for advanced recovery and stable environmental conditions. It is proposed that the Shanggan fauna represents a late Griesbachian benthic recovery event that coincided with the appearance of similarly diverse benthic faunas in Oman and Primorye. A high proportion of genera that have previously not been reported from the Early Triassic indicate that the prevalence of poor preservation conditions is a major obstacle in identifying early phases of recovery from the greatest crisis in the history of metazoan life. The early recovery of benthic faunas reported in this study questions previous claims of a prolonged lag phase as a consequence of the extraordinary extinction magnitude or the persistence of adverse environmental conditions. # 2011 Elsevier Masson SAS. All rights reserved. Keywords: Early Triassic; China; Bivalves; Biotic recovery; Palaeoecology; Lazarus taxa

Résumé Une lentille coquillère d’âge griesbachien (Trias inférieur) découverte à Shanggan (Chine du Sud) a livré, en plus de quatre espèces de gastéropodes et d’une espèce d’ammonite décrites par ailleurs, onze espèces de bivalves qui sont décrites dans le présent travail. Cela fait de Shanggan l’une des plus riches faunes de mollusques connues pour l’intervalle succédant à l’extinction de la fin du Permien. Quatre des genres présents sont des survivants à long terme, cinq sont des rescapés qui s’éteignent à la fin du Griesbachien ou au Trias inférieur et sept apparaissent

§

Corresponding editor: Gilles Escarguel. * Corresponding author. E-mail address: [email protected] (M. Hautmann).

0016-6995/$ – see front matter # 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.geobios.2010.07.004

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nouvellement durant le Griesbachien. Trois nouvelles espèces de bivalves sont introduites : Myalinella newelli nov. sp., Scythentolium scutigerulus nov. sp. et Eumorphotis shajingengi nov. sp. Le genre Astartella, supposé disparaître à la fin du Permien, est documenté pour la première fois dans le Trias inférieur, ce qui rend caduque l’appartenance des Astartidae aux taxons Lazares du Trias inférieur. La petite taille des spécimens d’Astartella confirme le point de vue selon lequel nombre de taxons Lazares du Trias inférieur n’ont pas survécu dans d’hypothétiques refuges mais ont simplement échappé à l’échantillonnage en raison de leur petite taille. D’un point de vue écologique, la relativement forte représentation de formes infaunales parmi les bivalves (quatre espèces sur 11) est remarquable pour cette période faisant directement suite à l’extinction car elle correspondrait à un stade de récupération relativement avancé. De plus, les abondances relatives de la communauté de bivalves et de gastéropodes montrent un indice de dominance remarquablement faible (D = 0.17), ce qui suggère à nouveau un stade de récupération avancé et des conditions environnementales stables. La faune de Shanggan témoigne d’une phase de récupération benthique contemporaine de l’apparition de faunes diversifiées en Oman et au Primorye. Le nombre important de genres qui n’étaient auparavant pas signalés au Trias inférieur suggère que les aléas de la préservation constituent un obstacle majeur pour l’identification des phases précoces de la récupération biotique faisant suite à la plus grande crise du Phanérozoïque. La récupération rapide des faunes benthiques documentée dans ce travail contredit l’hypothèse d’une récupération lente résultant soit de l’intensité exceptionnelle de l’extinction, soit de la persistance de conditions environnementales défavorables. # 2011 Elsevier Masson SAS. Tous droits réservés. Mots clés : Trias inférieur ; Chine ; Bivalves ; Récupération biotique ; Paléoécologie ; Taxons Lazares

1. Introduction The alleged presence of an unusually long-lasting lag phase after the end-Permian mass extinction event that may encompass the entire Early Triassic (Hallam, 1991) is still pervasive in the literature (e.g., Pruss et al., 2006; Fraiser and Bottjer, 2007, 2009). A recently discovered diverse fauna from the late Griesbachian of Oman was assumed to reflect only a local recovery event that was enabled by palaeogeographically restricted absence of seawater anoxia (Krystyn et al., 2003; Twitchett et al., 2004). However, ammonoids rapidly diversified in the Early Triassic (Brayard et al., 2009), and complex trace fossils indicative for advanced recovery states occur in the late Griesbachian of Canada (Beatty et al., 2008), Greenland (MH, personal observation), and the Alps (Hofmann et al., in preparation), less than 1 Ma after the extinction event (Galfetti et al., 2007). Here, we report an unusually diverse and ecologically heterogeneous mollusc fauna from the late Griesbachian of South China, which further questions claims of a prolonged lag phase after the end-Permian mass extinction and provides unique palaeobiological data from a time during which well preserved fossil material is notoriously scarce. 2. Geological setting The material of the present study comes from the Early Triassic Luolou Formation near the village of Shanggan, in the Guangxi Province of South China (Fig. 1). A detailed description of the locality and section is given in Brühwiler et al. (2008). The fossil horizon is a lens-like shell accumulation (Fig. 2) labelled Sha3 in Brühwiler et al. (2008), which occurs at the top of a ca. 10 m-thick microbial limestone succession (Unit I of the Luolou Fm., see Galfetti et al., 2008) that marks the base of the Triassic (Fig. 1(C)). The presence of the ammonoid genus Ophiceras indicates a late Griesbachian age for this shell lens (Brühwiler et al., 2008). The microfacies type of fossiliferous carbonate rock is for the most part a poorly washed mollusc rudstone (Fig. 3). Originally micritic portions have been transformed to

microspar (Fig. 3(A, D, F)). Better washed intergranular portions are filled with sparry cement. Shell clasts have been entirely replaced by sparry calcite. The bioclasts are predominantly bivalve and gastropod fragments, which are usually surmounted by micrite envelopes and represent cortoids. Some bioclasts are entirely micritized (Fig. 3(E)). The micritic envelopes are probably the result of intense microboring activity by algae prior to deposition, which suggests a shallow water origin. The micritic envelopes probably cause discontinuities between matrix and fossils, which facilitate the crack out of well-preserved specimens from the rock. Similar conditions were reported for well-preserved molluscs from the Smithian (Early Triassic) Sinbad Limestone in Utah (Nützel and Schulbert, 2005) and the Induan (Early Triassic) of Abrek in South Primorye, Russia (Shigeta et al., 2009). Some of the bioclasts also show dark, micritic incrustations in transmitted light, which probably represent algal overgrowth (Fig. 3(E)). Stylolites are common and are impregnated with iron- and/or manganese oxides (Fig. 3(B)). Post-mortem modification is a virtually unavoidable fact of fossilization (Kidwell and Bosence, 1991). In the case of the present sample, cross-stratification of the coquinoid lens (Fig. 2) indicates a certain transportation of the fossil material, and the comparatively narrow size range of the shells suggests some sorting of the material. Additionally, microboring and micritic envelopes indicate a certain time lag between death and burial. However, the lack of abrasion and shell damage even in delicate shells rules out long transportation distances. Because there are no major lateral facies changes for distances of tens of kilometres around (HB, personal observation), a mixing of faunas from different habitats can be excluded. Shanggan is located on a NW-SE trending uplifted block at least 100 km in width during deposition of the microbial limestone of Griesbachian age (HB, personal observation). The material was washed in and trapped between the dome structures of the microbial limestone, thus preventing further transport and abrasion. We therefore interpret the fauna of this sample as a reasonable approximation of a former in situ palaeocommunity.

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Fig. 1. A. Palaeogeographical position of the fossil locality. B. Locality map of Shanggan. C. Stratigraphical column indicating position of sample Sha3 and other fossil horizons described in Brühwiler et al. (2008). Opb: Ophiceras beds, Pcb: Proptychites candidus beds.

[()TD$FIG] 3. Material and methods

Fig. 2. Field photograph of fossiliferous shell lens Sha3; black lines indicate lower and upper erosional limits.

The calcareous lens was mechanically disintegrated, resulting in ca. 6 kg of fossiliferous rock fragments that were quantitatively sampled. Macrofossils are represented by bivalves, gastropods, and ammonoids. The taxonomy of the ammonoids, represented by one species of Ophiceras, is discussed in Brühwiler et al. (2008), and a detailed description of the four identified gastropod species is given in Kaim et al. (2010). The present study focuses on the taxonomy and ecology of the bivalves, which represent the most diverse group in the sample. The figured specimens are deposited in the collection of the Paläontologisches Institut und Museum of the University of Zürich, Switzerland (repository numbers PIMUZ 2846328502). All fragments of the shell lens that contained fossil material have been included in the quantitative analysis, and all complete individuals and fragments representing more than

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Fig. 3. Thin sections of the Sha3 host rock, which for the most part is a poorly washed bivalve-gastropod rudstone. Bioclasts are usually surmounted by micrite envelopes (cortoids). A. Poorly washed portion dominated by micrite or microsparite. B. Stylolite impregnated with iron- and/or manganese oxides. C. Recrystallized bioclasts with micritic envelopes. D. Re-crystallized gastropod shells with micritic envelopes. E. Shell fragment in centre with dark overgrowth, probably by algae or cyanobacteria. F. Re-crystallized shell fragment with micritic envelope. Scale bar = 1 mm.

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50% of the original shell have been counted. For bivalves, the fauna has been totalled by counting only the most numerous valves (left/right) together with the articulated specimens (cf. Fürsich, 1977). In addition to bivalves, the gastropod data of Kaim et al. (2010) are included in the palaeoecological analysis of the benthic fauna. Simpson-D was calculated with PAST (Hammer et al., 2001). An important aspect in quantitative analyses is a possible preservation bias due to selective dissolution of primary aragonitic shells. Thin-section analyses have confirmed that the gastropods were primary aragonitic, the aragonite now being replaced by diagenetic calcite (Kaim et al., 2010). Shell mineralogical data are available for all bivalve genera of the sample (Carter, 1990; Cox et al., 1969). There is a certain dominance of bivalve taxa with an outer calcitic shell layer, but four species were primary completely aragonitic (Table 1). The preservation of small gastropods and tiny bivalves with completely aragonitic shells excludes a diagenetic effect on the faunal composition. 4. Systematic palaeontology Class BIVALVIA Linnaeus, 1758 Subclass PTERIOMORPHIA Beurlen, 1944 Order MYTILOIDA Férussac, 1822 Superfamily MYTILOIDEA Rafinesque-Schmaltz, 1815 Family MYTILIDAE Rafinesque-Schmaltz, 1815 Genus Modiolus Lamarck, 1799 Type species: Mytilus modiolus Linnaeus, 1758 (subsequent designation by Gray, 1847). Modiolus sp. Fig. 4(1) Material: One right valve (PIMUZ 28463). Description: Small Modiolus that is externally smooth except for growth lines. Body well inflated, unusually slender

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for the genus, with prominent diagonal ridge extending from the prosogyrate umbo to the posteroventral shell margin. Beak clearly behind anterior end of shell. Anterior shell lobe not well differentiated from rest of shell. Remarks: The present specimen is remarkable for the comparatively narrow body of the shell, which distinguishes it from other Late Permian and Triassic representatives of the genus. The only other Griesbachian record of Modiolus is a single right valve described from Primorye (Far East Russia) by Kumagae and Nakazawa (2009), which differs not only in shape but also in the presence of radial ribs. It is likely that the present specimen represents a new species, but in absence of more material, we refrain from introducing a new name. Order PTERIOIDA Newell, 1965 Superfamily AMBONYCHIOIDEA Miller, 1877 Family MYALINIDAE Frech, 1891 Genus Myalinella Newell, 1942 Type species: Myalina meeki Dunbar, 1924, by original diagnosis. Myalinella newelli nov. sp. Fig. 4(2–4) 1955. Myalina (Myalinella) cf. meeki Dunbar - Newell, p. 26, pl. 5, figs. 9, 10. Derivatio nominis: In honour of N.D. Newell, who was the first to describe material of this species. Holotype: PIMUZ 28464, Fig. 4(2). Material: One right and two left valves (PIMUZ 2846428466). Diagnosis: Shell mytiliform, with subterminal beaks and slender body of ca. 408 obliquity. Valve convexity high for the genus, with left valve being slightly more convex than right valve. Description: Outline subtrigonal, elongated in posteroventral direction, with posteroventral margin broadly rounded. Shell inequivalved, with higher convexity of left valve. Anterior

Table 1 Categorization of taxa with respect to evolutionary fates, ecology, and shell mineralogy. Taxon minerology Ophiceras sp. Bellerophon abrekensis Wannerispira shangganensis Naticopsis sp. Palaeonarica guanxinensis Modiolus sp. Promyalina sp. Myalinella newelli Promysidiella sp. Bakevellia sp. Scythentolium scutigerulus Eumorphotis shajingengi Trigonodus sp. Neoschizodus laevigatus Astartella sp. Astartopis? sp. Total

Long-trem survivor genus

Holdover genus (DCW)

Newly evolved genus

Ecology

Shell mineralogy



Nekt-Pred EpiMob-Graz/Detr EpiMob-Graz/Detr EpiMob-Graz/Detr EpiMob-Graz/Detr SmIf-Susp EpiBys-Susp EpiBys-Susp EpiBys-Susp EpiBys-Susp EpiBys-Susp EpiBys-Susp InfMob-Susp InfMob-Susp InfMob-Susp InfMob-Susp

A A A A A C-A C-A C-A C-A C-A C-A C-A A A A A

               4

5

7

DCW: dead clade walking (Jablonski, 2002); Nek: nekton, Inf: infaunal, SmIf: semi-infaunal; Epi: epifaunal; Mob: mobile; Bys: byssally attached; Pred: predatory; Detr: detritus feeding; Graz: grazer; Susp: suspension feeding; A: shell completely aragonitic; C-A: shell with calcitic outer, aragonitic inner layer.

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Fig. 4. 1. Modiolus sp., PIMUZ 28463, exterior of right valve,  4. 2–4. Myalina newelli nov. sp. 2, PIMUZ 28464, holotype, exterior of right valve,  4; 3, PIMUZ 28465, interior mould with partly adherent shell of left valve,  4; 4, PIMUZ 28466, exterior of left valve,  4. 5. Promyalina cf. putiatensis (Kiparisova, 1938), PIMUZ 28467, exterior of left valve,  4. 6, Promysidiella? sp., PIMUZ 28468, exterior of right valve, dorsal margin on left side,  4. 7–9. Bakevellia sp. 7, PIMUZ 28469, exterior mould of left valve,  4; 8, PIMUZ 28470, exterior of left valve,  4; 9, PIMUZ 28471, exterior of right valve,  4. 10–19. Scythentolium scutigerulus nov. sp. 10, PIMUZ 28472, left valve,  2; 11, PIMUZ 28473, left valve,  2; 12, PIMUZ 28474, left valve,  2; 13, holotype, PIMUZ 28475, left valve,  2; 14, PIMUZ 28476, left valve,  2; 15, PIMUZ 28477, left valve,  2; 16, PIMUZ 28478, left valve,  2; 17, PIMUZ 28479, left valve,  2; 18, PIMUZ 28480, left valve,  2; 19, PIMUZ 28481, right valve,  3.

shell lobe small. Beaks subterminal, giving rise to sharp diagonal ridges. Surface of both valves smooth except for growth lines. Internal characters unknown. Remarks: The present species is morphologically indistinguishable from ‘‘Myalina (Myalinella) cf. meeki Dunbar’’, which Newell (1955) described from presumably Early Triassic allochtonous limestone blocks of the ‘‘Cape Stosch Formation’’ (= Wordie Creek Fm.?) of East Greenland. Most of these ‘‘White Blocks’’ contain Permian fossils, yet Newell’s (1955) specimens of Myalinella were associated with Promyalina groenlandica Newell, 1955, which also occurs in situ in Early Triassic beds of that area (Spath, 1930; Newell, 1955). The present finds from the Griesbachian of China is in accordance with Newell’s (1955) interpretation of an Early Triassic age of his material. According to Newell (1955), the Greenlandic specimens differ from the Permian Myalinella meeki (Dunbar, 1924) in being distinctly

more convex. The present material demonstrates that the higher valve convexity is a constant character that separates the Early Triassic species from the Permian M. meeki. No comparable species are known from the Early Triassic. Myalinella was moderately diverse in the Late Palaeozoic (Newell, 1942) and straddled the Permian-Triassic boundary. Its final extinction at the end of the Griesbachian, less than 1 Ma after the greatest Phanerozoic mass extinction event, makes it a typical example for the ‘‘Dead Clade Walking’’ phenomenon of Jablonski (2002). Genus Promyalina Kittl, 1904 Type species: Promyalina hindi Kittl, 1904 from the Bellerophonschichten (Upper Permian) near Sarajevo, BosniaHerzegovina, by original designation [a Late Permian age of Kittl’s fauna was doubted by Newell (1955: p. 26), who

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suggested that Promyalina was strictly confined to the Early Triassic]. Promyalina cf. putiatensis (Kiparisova, 1938) Fig. 4(5) Material: One left valve (PIMUZ 28467). Description: Small, externally smooth Promyalina with valve obliquity of ca. 508. Remarks: The present species resembles Promyalina putiatensis (Kiparisova, 1938), which differs in being much larger and more infracrescent. However, early growth stages of P. putiatensis are more retrocrescent than later growth stages, thus the difference in shape might be due to the smaller growth size of the present specimen. Whether Promyalina is a Permian survivor or newly arose in the Early Triassic is debated (Newell, 1955: p. 26). Although Promyalina moderately diversified in the Early Triassic of Siberia (Dagys and Kanygin, 1996), it eventually shared the fate of other myalinid genera (Myalina, Myalinella, Atomodesma) to become extinct during the Early Triassic (however see Guo, 1985 for a possible occurrence of Promyalina in the Upper Triassic of Yunnan). Family MYSIDIELLIDAE Cox, 1964 Genus Promysidiella Waller, 2005 Type species: Mysidiella cordillerana Newton, 1987, by original designation. Promysidiella? sp. Fig. 4(6) Material: One right valve (PIMUZ 28468). Description: Mytiliform shell with a diagonal angle of ca. 358. Beak pointed and most likely anteriorly projecting, giving rise to comparatively sharp umbonal ridge. Remarks: Most mytiliform bivalves of the Triassic can probably be assigned to the genus Promysidiella, which is characterized (among others) by anteriorly projecting beaks (Waller, 2005). The systematic position of the genus in either Mytiloidea (Waller, 2005) or Ambonychioidea (Hautmann, 2008) is controversial. The comparatively sharp umbonal ridge distinguishes the present species form other representatives of Promysidiella. Superfamily PTERIOIDEA Gray, 1847 Family BAKEVELLIIDAE King, 1850 Genus Bakevellia King, 1848 Type species: Avicula antiqua Graf zu Münster, 1836 (in Goldfuss) [non A. antiqua Defrance] = Avicula binneyi Brown, 1841, from the Upper Permian (Zechstein) of Europe, by original designation. Bakevellia sp. Fig. 4(7–9) Material: One right and two left valves (PIMUZ 2846928471). Description: Left valve moderately inflated, with acutely angulated posterior wing and regularly spaced commarginal ribs (Fig. 4(7, 8)). Right valve less convex than left valve, with small beak not projecting above hinge line (Fig. 4(9)).

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Remarks: In absence of information on the internal shell morphology, the present specimens could be assigned to either Bakevellia keratophaga (Schlotheim, 1816) from the Zechstein (Late Permian) of Europe, or to Pteria ussurica (Kiparisova, 1938) from the Early Triassic of Siberia. The latter has been transferred from Bakevellia to Pteria on grounds of the presence of a single rather than multiple ligament pits (Kiparisova and Krishtofovich, 1954). Although this treatment is in accordance with the current definition of these genera, it should be noted that the number of ligament grooves depends on the growth stage (see also Nakazawa, 1959: p. 196), and multiple resilifers sporadically occur even in usually alivincular taxa (Hautmann, 2004a). Suborder PECTININA Waller, 1978 Superfamily PECTINOIDEA Rafinesque-Schmaltz, 1815 Family ENTOLIIDAE von Teppner, 1922 Genus Scythentolium Allasinaz, 1972 Type species: Pecten tirolicus Wittenburg, 1908, by original designation. Scythentolium scutigerulus nov. sp. Figs. 4(10–19) and 5(1, 2) Derivatio nominis: scutigerulus (Latin) = shield bearer. Holotype: PIMUZ 28475, Fig. 4(13). Material: Nine left and three right valves (PIMUZ 2847228483). Diagnosis: Shell biconvex and more or less equivalved, with straight dorsal margin and well-developed anterior auricular sinus in both valves. Right valve externally smooth, left valve with periodically raised commarginal growth lines and very faint radial ribs. Description: Valves convex, acline, nearly circular in outline, with straight hinge margins and orthogyrate umbones. Apical angle of disc ca. 858. Posterior auricles small and truncated (Figs. 4(14) and 5(2)). Right anterior auricle large, with well-developed auricular sinus (Figs. 4(19) and 5(1, 2)). Left anterior auricle similar in outline but possibly slightly larger (Fig. 4(12–14, 17)). Shell exterior of left valve with faint, widely spaced radial ribs and growth lines that periodically form minute commarginal riblets (Fig. 4(13)). Right valve externally smooth except for growth lines (Figs. 4(19) and 5(1, 2)). Remarks: Scythentolium was erected by Allasinaz (1972) in order to accommodate Early Triassic entoliids with an anterior auricular sinus. S. scutigerulus differs from other species of the genus by:  the ornamentation pattern of the left valve;  the equivalved condition;  the virtually undistinguishable shape of the anterior auricle in the left and right valve. Most hitherto described species of Scythentolium have been based on poorly preserved material, which hampers detailed comparisons with the new species. In particular, the species described by Wittenburg (1908) and subsequently assigned to Scythentolium by Allasinaz (1972: pp. 222–223, 311–314) are

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mostly not well defined. According to the descriptions of Wittenburg (1908), Scythentolium tirolicum (Wittenburg, 1908) differs from the new species in the less convex right valve and the higher number of radial ribs on the left valve [probably incorrectly separated as ‘‘var. predazzensis’’ by Wittenburg (1908)]. Scythentolium subtile (Wittenburg, 1908) has a much larger anterior auricle and more densely spaced radial ribs. Scythentolium longauris (Wittenburg, 1908) is very similar to S. subtile, differing essentially in the lack of radial ribs, which, however, might be due to poor preservation. Scythentolium rombergi (Wittenburg, 1908) differs from the new species in a flat right valve that was described as bearing radial ribs. Wittenburg’s (1908: fig. 4) figure additionally shows a posterior auricular sinus, but it is mentioned in the main text without further specification that this figure does not show all characters correctly (Wittenburg, 1908: p. 21). Scythentolium sojale (Wittenburg, 1908) has also a posterior auricular sinus and additionally differs from S. scutigerulus in a flat right valve and more pronounced incremental lines. Scythentolium eurasiaticum (Wittenburg, 1908) is poorly defined but appears to be unique in having a very narrow but relatively long anterior auricle. Scythentolium kokeni (Wittenburg, 1909) from the Smithian-Spathian of the Salt Range (Pakistan) differs in its larger size, the lack of a left anterior auricular sinus, and the coarser radial ribs on the left valve. No similar species from

[()TD$FIG]

China are known so far. ‘‘Entolium discites microtis (Bittner)’’ figured by Gu et al. (1976: pl. 34, figs. 12, 13) remotely resembles S. scutigerulus in the presence of radial ribs but differs in the absence of an auricular sinus and does therefore not belong to Scythentolium. Given its archetypical morphology and the stratigraphic appearance near the base of the Early Triassic, the new species is likely to be near the root of the genus, which moderately radiated in the course of the Early Triassic before it went extinct at the end of this epoch (Allasinaz, 1972). The presence of auricular sinuses hints at adult byssal attachment, possibly brought about by paedomorphic retention of the byssus, which is a remarkable atavism because Palaeozoic entoliids were unattached and possibly able to swim (Stanley, 1972). The return to a sessile or hemisessile mode of life ca. 125 Ma after the first appearance of entoliids (Waller, 2006) may reflect unusual ecological conditions in the earliest Triassic such as reduced primary production and high seawater temperatures with reduced ability to dissolve oxygen, which favoured passive lifestyles with low metabolic rates (Hautmann and Nützel, 2005). Superfamily AVICULOPECTINOIDEA Meek and Hayden, 1864 Family ETHERIPECTINIDAE Waterhouse, 1982 Genus Eumorphotis Bittner, 1901.

Fig. 5. 1, 2. Scythentolium scutigerulus nov. sp. 1, PIMUZ 28482, right valve,  3; 2, PIMUZ 28483, right valve,  3. 3–13. Eumorphotis shajingengi nov. sp. 3, PIMUZ 28484, left valve,  2; 4, PIMUZ 28485, left valve,  2; 5, PIMUZ 28486, left valve,  2; 6, PIMUZ 28487, left valve,  2; 7, PIMUZ 28488, left valve,  2; 8, PIMUZ 28489, left valve,  2; 9, PIMUZ 28490, left valve,  2; 10, PIMUZ 28491, left valve,  2; 11, holotype, PIMUZ 28492a, left valve,  2; 12, PIMUZ 28492b, exterior mould of left valve,  2; 13, PIMUZ 28493, right valve,  2. 14. Neoschizodus laevigatus (Zieten, 1830), PIMUZ 28494, exterior of right valve,  3. 15. Trigonodus? sp., PIMUZ 28495, exterior of left valve,  3. 16–19. Astartella sp. 16, PIMUZ 28496, exterior of right valve,  4; 17, PIMUZ 28497, exterior of left valve,  3; 18, PIMUZ 28498, exterior of right valve,  4; 19, PIMUZ 28499, exterior of right valve,  4. 20–22. Astartopis? sp. 20, PIMUZ 28500, exterior of left valve,  4; 21, PIMUZ 28501, exterior of right valve,  3; 22, PIMUZ 28502, exterior of left valve,  4.

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Type species: Pseudomonotis telleri Bittner, 1898, by subsequent designation (Cossmann, 1902). Eumorphotis shajingengi nov. sp. Fig. 5(3–13) Derivatio nominis: Named after Prof. Sha Jingeng (Nanjing) in recognition of his work on Triassic bivalves. Holotype: PIMUZ 28493, Fig. 5(11–12). Material: Nine left and one right valve (PIMUZ 2848428493). Diagnosis: Comparatively small Eumorphotis with pro- to retrocrescent valves and two to three orders of radial ribs bearing fine knobs where intersecting growth lines. Description: Growth direction of valves changing during ontogeny, being procrescent in early stages but infra- to retrocrescent in later stages. Umbo orthogyrate, located near midpoint of dorsal margin. Ornamentation of left valve with radial ribs intercalating in two or three ranks, plus periodically enhanced incremental lines forming small knobs where they intersect the radial costae (Fig. 5(3–12)). Auricles of left valve feebly demarcated, with unusually week anterior auricular sinus (Fig. 5(11)). Right valve only slightly convex and externally smooth, with deep byssal notch below anterior auricle (Fig. 5(13)). Remarks: The scarcity of right valves is probably due to weaker calcification, which is a frequent phenomenon in anatomically lower valves that are tightly levelling with the substrate (Newell and Boyd, 1995). In Eumorphotis, separation of species is therefore chiefly based on the characters of the left valve, which is also morphologically more distinctive than the flat right valve. The most similar species is Eumorphotis inaequicostata (Benecke, 1868) from the Early Triassic of the Dolomites (Alps), which differs in more widely spaced first order ribs, ontogenetically earlier appearance of third order ribs, and a more distinct anterior auricular sinus. The specimens described as E. inaequicostata by Gu et al. (1976): pl. 30, figs. 31, 32) differ in coarser radial ribs and possibly the absence of thirdorder costellae. Subclass HETEROCONCHIA Hertwig, 1895 Superorder PALAEOHETERODONTA Newell, 1965 Order UNIONOIDA Stoliczka, 1871 Superfamily UNIONOIDEA Fleming, 1828 Family TRIGONODIDAE Modell, 1942 Genus Trigonodus Sandberger in Alberti, 1864 Type species: Trigonodus sandbergi Alberti, 1864, by subsequent designation (Stoliczka, 1871). Trigonodus? sp. Fig. 5(15) Material: One left valve (PIMUZ 28495). Description: Shell trapezoidal. Dorsal and ventral margin subparallel. Umbo strongly prosogyrate, located on anterior 15% of dorsal margin, giving rise to comparatively sharp diagonal ridge. Shell exterior devoid of ornamentation. Remarks: Although the external morphology of the present specimen is suggestive for Trigonodus, a definite assignment is

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uncertain without information on the hinge structure. The most similar species is Trigonodus orientalis Bittner, 1899, from the Early Triassic of Siberia (see also Kumagae and Nakazawa, 2009), which differs in its blunter diagonal ridge. Order TRIGONIOIDA Dall, 1899 Superfamily MYOPHORIOIDEA Bronn, 1849 Family MYOPHORIIDAE Bronn, 1849 Genus Neoschizodus Giebel, 1855 Type species: Lyrodon laevigatum Goldfuss, 1837, by subsequently designation (Stoliczka, 1871). Neoschizodus laevigatus (Zieten, 1830) Fig. 5(14) Material: One right valve (PIMUZ 28494). Description: Outline subtrigonal, posteriorly truncated, with sharp posterior ridge. Beak mid of shell length. Remarks: The present specimen agrees in all observable characters with the widespread species N. laevigatus. Superorder HETERODONTA Neumayr, 1884 Order VENEROIDA Adams and Adams, 1856 Superfamily CRASSATELLOIDEA Férussac, 1822 Family ASTARTIDAE d’Orbigny, 1844 Genus Astartella Hall, 1858 Astartella sp. Fig. 5(16–19) Material: Three right valves and one left valve (PIMUZ 28496-28499). Description: Minute Astartella with alternating commarginal furrows and ribs. Umbo prosogyrate, slightly in front of midpoint of dorsal margin. Ventral margin slightly curved. Remarks: Because of the diagnostic shape and ornamentation pattern, assignment of the present specimens to Astartella is quite certain even in absence of information on internal shell characters. Morphologically, the present species is intermediate between the Late Permian Astartella tunstallensis King, 1850 and Astartella vallisneriana King, 1848 in sharing the minute size with the former and the scarcely rounded ventral margin and less tumid beaks with the latter. Newell (1955) suggested that A. tunstallensis and A. vallisneriana might be conspecific, not at least because both taxa commonly co-occur. The intermediate morphology of the present specimens supports Newell’s (1955) suggestion, although Logan (1967) maintained both names in his comprehensive monograph on Zechstein bivalves. This is the first record of the genus Astartella and the family Astartidae from the Early Triassic. Astartella was previously assumed to have vanished at the end of the Permian. Like Myalinella sp. described above, its protracted extinction makes it a ‘‘Dead Clade Walking’’ taxon. The Astartidae is an extant family with a well-documented fossil record since the Devonian (Chavan in Cox et al., 1969), except for the Early Triassic, from which astartids have previously not been reported. Genus Astartopis von Wöhrmann, 1889 Type species: Myophoria richthofeni Stur, 1868, by monotypy.

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Astartopis? sp. Fig. 5(20–22) Material: Two left and one right valve (PIMUZ 2850028502). Description: Valves higher than long, with tumid, prosogyrate umbo located near midpoint of dorsal margin, giving rise to blunt umbonal ridge. Lunula broad and shallow. Remarks: The present specimens are tentatively assigned to Astartopis on base of the overall shape, which is suggestive for this genus. 5. Discussion Well-preserved bottom-level faunas from the Early Triassic are rare, which is a major obstacle in reconstructing evolutionary pathways during the ecological vacuum that followed the greatest biotic crisis in Earth’s history. The present fauna represents one of the best preserved benthic associations from the Griesbachian and is virtually unbiased by a selective loss of taxa with aragonitic shells (see above). It thus provides a unique snapshot of a palaeocommunity that was established relatively soon after the end-Permian mass extinction. In the following sections, the palaeobiological data of the Shanggan fauna are discussed in the context of post-extinction palaeoecological conditions and scenarios of recovery from the endPermian crisis. 5.1. Evolutionary context of taxa The genera of the present sample represent a mixture of long-term survivors (4), holdovers (5), and newly evolved genera (7; Table 1). Although the overall character of the fauna is thus somewhat transitional between late Palaeozoic and early Mesozoic, the high proportion of newly evolved genera (44%) clearly indicates ongoing recovery soon after the end-Permian mass extinction. Two of the four gastropod genera in the present sample (Bellerophon and Wannerispira) represent gastropod families that were well diversified in the Permian and survived the crisis at the Permian-Triassic boundary, but became extinct by the end of the Early Triassic (Kaim et al., 2010) Naticopsis belongs to a group that was diverse in the Permian and rebounded rapidly after the end-Permian extinction, maintaining a high diversity and abundance throughout the Triassic. The find of Palaeonarica is the earliest record of the genus, which is otherwise known from numerous reports from the Late Triassic. The dominant bivalve species in the present sample belong to genera that evolved from Permian ancestors during or in the immediate wake of the crisis: Scythentolium [descending from Pernopecten (Allasinaz, 1972)] and Eumorphotis [descending from Heteropecten (Newell and Boyd, 1995)]. Surprisingly, neither of these genera rapidly diversified into the largely vacated Early Triassic ecospace. Scythentolium vanished at the end of the Early Triassic and was probably not ancestral to other entoliids, which were directly linked to the Pernopecten-Entolium lineage (Allasinaz, 1972: p. 281). Although Eumorphotis possibly extended until the Carnian

(Ichikawa, 1958: p. 153), its diversity abruptly dropped after the Early Triassic, and it is unclear whether it ever gave rise to other genera. Similarly, the newly evolved genera Promysidiella and Trigonodus form the stock of only moderately diverse Triassic bivalve families (Mysidiellidae and Trigonodidae, respectively; Cox et al., 1969; Geyer et al., 2005; Hautmann, 2008). By contrast, Permian survivor Bakevellia was ancestral to more than a dozen Mesozoic genera united within the family Bakevelliidae, and Neoschizodus was an important element in the rapid diversification of Triassic myophoriids. Apparently, taxa that evolved in background times were in advantage when environmental conditions return to ‘‘normal’’, provided that they were able to pass the evolutionary bottleneck imposed by the crisis. A possible exception to this pattern is Astartopis?, which could potentially be ancestral to post-Palaeozoic astartids. Unfortunately, the present material shows no hinge details that would allow testing this hypothesis. Modiolus is a long-ranging genus with Palaeozoic roots that is ubiquitous in Mesozoic and Cenozoic benthic communities but seldom dominant or particularly diverse. The reminder bivalve species belong to Permian holdover genera that vanished in the course of the Griesbachian (Myalinella, Astartella) or later in the Early Triassic (Promyalina). 5.2. ‘‘Lilliput effect’’ and Lazarus-taxa The ‘‘Lilliput effect’’, i.e. a reduction of mean body size in the wake of mass extinction events (Harris and Knorr, 2009), has attracted considerable interest in the past decade. Although first recognized in Late Silurian graptoloids (Urbanek, 1993), mostly Early Triassic faunas have been cited as examples for this phenomenon (e.g., Twitchett, 2001; Fraiser et al., 2005; Payne, 2005). The ‘‘Lilliput effect’’ has been attributed to harsh environmental conditions (Urbanek, 1993; Fraiser and Bottjer, 2004; Fraiser et al., 2005), reduced primary production (Twitchett, 2001), or preferred survival of small species (cf. Payne, 2005). A major problem for demonstrating body size reduction within particular clades are possible effects of mechanical size sorting. For instance, the alleged dwarfism of Early Triassic gastropods (e.g., Fraiser and Bottjer, 2004; Payne, 2005) turned out to be largely an artefact of size-sorting in the two most important Early Triassic gastropod Lagerstätten (Nützel and Schulbert, 2005; Brayard et al., 2010, 2011). Excluding such effects requires demonstrating either:  an adult stage for the specimens under consideration;  the global absence of large representatives of the taxon in question. Based on the finds of a minute lucinid (Sinbadiella pygmaea) in the Smithian Sinbad Limestone of Utah, which was the first Early Triassic record of the former Lazarus taxon Heterodonta, Hautmann and Nützel (2005) suggested that dwarfism might be responsible for the Lazarus phenomenon, i.e. the temporal disappearance of taxa from the fossil record of the post-extinction interval. Accordingly, Lazarus taxa might

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not have survived in unknown refuges (the prevalent hypothesis; e.g., Vermeij, 1986; Hallam, 1991), but simply persisted as very small species that have been overlooked in previous studies. The Heterodonta, a taxon of superorder rank, was probably the most prominent Early Triassic Lazarus taxon, because it included at least seven families with a pre- and postEarly Triassic evolutionary history (Hautmann and Nützel, 2005). After demonstrating the presence of a minute lucinid species in the Smithian (Hautmann and Nützel, 2005), the new finds of extremely small astartids from the Griesbachian add another former Lazarus family of the Heterodonta to the Early Triassic fossil record. With a maximum size of 3.5 mm, the present astartids are even smaller than the lucinid S. pygmaea, which does not exceed 8 mm at maximum (Hautmann and Nützel, 2005). The global lack of large heterodonts in the Early Triassic indicates that the present specimens are not just juveniles that have been mechanically separated from the rest of the community. Growth lines are barely visible in the present Astartella and Astartopis? specimens, but we note that the periodically enhanced growth lines of Scythentolium scutigerulus become densely spaced towards the shell margin (Fig. 4(13)), indicating that small specimens of other members of the palaeocommunity represent adult individuals. The new finds of previously unrecognized small-sized heterodont genera in the Griesbachian of Shanggan therefore lends further credence to the Lilliput scenario as an explanation for the Lazarus phenomenon. 5.3. Diversity, palaeoecology, and state of recovery Taxonomical and ecological pauperization is an inevitable outcome of mass extinction events. However, the decline of diversity in local ecosystems (a-diversity) does not necessarily reflect the global loss of taxa, because the latter also includes losses in ß- and g-diversity. Moreover, ecosystem recovery may be decoupled from and proceed much faster than the taxonomical differentiation between ecosystems and palaeogeographic regions. As shown for the end-Triassic mass extinction event, a-diversity may indeed be virtually undistinguishable from that of comparable communities in background times as soon as environmental stress has ceased (Hautmann et al., 2008b). In the case of the end-Permian mass extinction event, the early view that Early Triassic bottom-level faunas are generally paucispecific communities with overwhelming dominance of opportunistic or progenitor taxa such as Lingula, Claraia, and Unionites (e.g., Hallam and Wignall, 1997) has been challenged by recent finds of a comparatively diverse benthic fauna from the Griesbachian of Oman (Krystyn et al., 2003). With 11 bivalve species, plus one ammonoid species (Brühwiler et al., 2008) and four gastropod species (Kaim et al., 2010), the present fauna is similarly diverse, but differs considerably in its taxonomic composition by a clear prevalence of bivalves. A comparably diverse late Griesbachian mollusc fauna has recently been described by Kumagae and Nakazawa (2009) and Kaim (2009) from Primorye (Far East Russia), which is also quite similar in the composition of genera.

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Relatively high species diversity soon after a mass extinction event can reflect local proliferation of opportunistic and/or disaster and progenitor taxa (Harris et al., 1996; Looy et al., 2001), but we note that the typical Induan examples of such taxa (Lingula, Claraia, Unionites) are absent in the present fauna, and played only minor roles in the Oman and Primorye faunas. The relatively high diversity of these faunas therefore indicates an interregional late Griesbachian recovery pulse for benthic communities, which has not been recognized before. Apart from the evidence of skeletonized faunas, a surprisingly fast recovery of benthic communities is also confirmed by recent finds of complex late Griesbachian trace fossil communities from Canada (Beatty et al., 2008), East Greenland (MH, personal observation), and the Alps (Hofmann et al., in prep.). In the case of the Shanggan fauna, the comparatively high portion of infaunal taxa (4/15; Table 1 and Fig. 6(B)) is an additional indicator for an advanced recovery state (cf. Twitchett et al., 2004: Table 1). However, the most conclusive evidence that the Shanggan fauna represents an advanced recovery state is its low dominance index (Fig. 6(A)). Dominance (Simpson D) is only D = 0.17 (bootstrapped 95% confidence interval 0.14–0.22), which is the lowest value reported so far for the Early Triassic (cf. Twitchett et al., 2004). The most abundant species in the present sample are two pectinoids (E. shajingengi and S. scutigerulus; Fig. 6(A)), but their dominance is still low and not comparable with typical disaster taxa or opportunists such those forming ‘‘clam spikes’’ (McRoberts, 2003; Ward et al., 2007). In addition to these two pectinoids, three gastropod species belong to the trophic nucleus of the fauna (Fig. 6(A)), which thus comprises epifaunal filter feeders as well as epifaunal grazers or detritivores (Table 1). This heterogeneity brings further evidence for ecological normalization that reflects fast recovery. 5.4. Assessment of environmental stress Hallam’s (1991) suggestion that ongoing shallow sea anoxia caused a several million years delay of recovery after the endPermian mass extinction has been challenged in the recent years by demonstrating that:  ammonoids and conodonts recovered notably fast (Brayard et al., 2006, 2009; Orchard, 2007);  high stages of benthic recovery occurred at least locally already in the Griesbachian (Krystyn et al., 2003);  evidence for global shallow sea anoxia in the Griesbachian is ambiguous (Heydari et al., 2008; Hermann et al., in press). The small average growth size of the Shanggan fauna is not a sufficient indicator for ongoing environmental stress such as seawater anoxia, because it could simply result from preferred extinction of large-sized species during the end-Permian crisis (Payne, 2005). By contrast, the high diversity, low dominance, and ecological complexity (Fig. 6) of the Shanggan fauna are incompatible with the effects of anoxia in the late Griesbachian at this locality. A similar conclusion was reached by Krystyn

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Fig. 6. A. Frequency distribution and trophic nucleus of benthic taxa. B. Ecological categorization of benthic taxa.

et al. (2003) and Twitchett et al. (2004) for a late Griesbachian fauna of Oman, and can also be inferred for the late Griesbachian mollusc fauna of South Primorye, as described by Kaim (2009) and Kumagae and Nakazawa (2009). Given the large palaeogeographic distances between these regions, plus the presence of complex late Griesbachian trace fossil communities in Canada (Beatty et al., 2008), East Greenland (MH, personal observation), and the Alps (Hofmann et al., ongoing work), any late Griesbachian shallow marine anoxia must have been the exception rather than the rule. Whereas there is no evidence for direct effects of environmental stress on the Shanggan fauna, the prevalence of bivalves with a calcitic outer shell layer (Table 1) could reflect preferential extinction of taxa with completely aragonitic shells during preceding extinction events, caused by short-time declines of calcium carbonate saturation (Knoll et al., 2007; Payne, 2007; Heydari et al., 2008) due to SO2 and CO2 emissions from the Siberian trap volcanism (Reichenow et al., 2008; Svensen et al., 2009). In this respect, the present fauna resembles patterns associated with the end-Triassic mass extinction event (Hautmann, 2004b, 2006; Hautmann et al., 2008a, 2008b).

population sizes (Wignall and Benton, 1999), the relatively low amount of exposed Early Triassic rocks that is available for research (Peters and Foote, 2002), or simply poor preservation (Erwin, 1996). The present fauna contains minute taxa with originally aragonitic shell mineralogy (Table 1), which excludes diagenetic distortion of the faunal composition of shelly taxa. It therefore offers a unique opportunity to test preservation effects on the completeness of the Early Triassic fossil record. Five out of 16 genera (31%) that are observed in the Shanggan mollusc fauna have previously not been reported from the Griesbachian or other substages of the Early Triassic: Wannerispira, Palaeonarica, Promysidiella, Astartopis?, and Astartella. Given the low provincialism during the Griesbachian (e.g., Hallam and Wignall, 1997: p. 116; Brayard et al., 2006, 2007), this is a comparatively high portion of previously unknown genera, which indicates that the incompleteness of the Early Triassic fossil record may at least partly be linked with the scarcity of suitable preservation conditions. Potential consequences of this Early Triassic fossilization low include artificial concentration of recorded extinctions of taxa at the Permian-Triassic boundary and masking of early recovery signals.

5.5. Preservation effects on the completeness of the Early Triassic fossil record

6. Conclusions

A notable incompleteness of the Early Triassic fossil record has been inferred from the high number of Lazarus taxa in that time interval (Erwin, 1996). Possible causes include reduced

Benthic life started to recover from the greatest mass extinction event in Earth’s history much earlier than previously assumed. Difficulties to reconstruct the extent of this early recovery impulse include the prevalence of poor preservation

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conditions in the Griesbachian and at least to some extent the relatively small growth size of many taxa that were involved in ecosystem rebuilding, because these are easily overlooked during routine field surveys. The unexpectedly early recovery impulse suggests that:  the extraordinary extinction magnitude of the end-Permian event had surprisingly little effect on the duration of the lag phase;  there were no adverse environmental conditions in the late Griesbachian that inhibited benthic diversification and ecosystem restoration. Preliminary data based on literature studies and field surveys suggest that benthic recovery was slowed down or reset in the Dienerian, but details on the fate of the late Griesbachian recovered faunas need to be clarified in future studies. Acknowledgements Thomas Galfetti (Zürich) and Arnaud Brayard (Dijon) are acknowledged for their support in the field. This paper is a contribution to the Swiss National Science Foundation projects 200020-113554 and 200021-121774. The research of A.K. has been supported by the Alexander von Humboldt Stiftung. A.N. acknowledges funding by the Deutsche Forschungsgemeinschaft (NU 96/6-1, 6-2). We thank F. Stiller and C.A. McRoberts for helpful reviews. References Adams, H., Adams, A., 1856. The genera of Recent Mollusca, arranged according to their organisation. Volume 2 (1854–1858) John van Vorst, London. Alberti, F.von, 1864. Überblick über die Trias, mit Berücksichtigung ihres Vorkommens in den Alpen. J. G. Cottasche Buchhandlung, Stuttgart. Allasinaz, A., 1972. Revisione dei Pettinidi Triassici. Rivista Italiana di Paleontologia e Stratigrafia 78, 189–428. Beatty, T.W., Zonneveld, J.P., Henderson, C.M., 2008. Anomalously diverse Early Triassic ichnofossil assemblages in northwest Pangea: a case for a shallow-marine habitable zone. Geology 36, 771–774. Benecke, E.W., 1868. Über einige Muschelkalkablagerungen der Alpen. Geognostisch-paläontologische Beiträge 2, 1–67. Beurlen, K., 1944. Beiträge zur Stammesgeschichte der Muscheln. Sitzungsberichte der Bayerischen Akademie der Wissenschaften 1/2, 133–145. Bittner, A., 1898. Beiträge zur Paläontologie, insbesondere der triadischen Ablagerungen centralasiatischer Hochgebirge. Jahrbuch der kaiserlichköniglichen geologischen Reichsanstalt 48, 689–718. Bittner, A., 1899. Trias-Ablagerungen des Süd-Ussuri-Gebietes in der ostsibirischen Küstenprovinz. Mémoires du Comité Géologique 7, 1–35. Bittner, A., 1901. Ueber Pseudomonotis telleri und verwandte Arten der unteren Trias. Jahrbuch der kaiserlich-königlichen Geologischen Reichsanstalt 50, 559–592. Brayard, A., Bucher, H., Escarguel, G., Fluteau, F., Bourquin, S., 2006. The Early Triassic ammonoid recovery: paleoclimatic significance of diversity gradients. Palaeogeography, Palaeoclimatology, Palaeoecology 239, 374– 395. Brayard, A., Escarguel, G., Bucher, H., 2007. The biogeography of Early Triassic Ammonoid Faunas: clusters, gradients, and networks. Geobios 40, 749–765. Brayard, A., Escarguel, G., Bucher, H., Monnet, C., Brühwiler, T., Goudemand, N., Galfetti, T., Guex, J., 2009. Good genes and good luck:

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