Original articleAn unusually diverse mollusc fauna from the earliest Triassic of South China and its implications for benthic recovery after the end-Permian biotic crisisUne malacofaune exceptionnellement diverse du Trias inférieur de Chine du Sud : implications pour la récupération benthique après la crise biotique fini-permienne☆
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, Fraiser and Bottjer, 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.
Section snippets
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
Material and methods
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
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 for the genus, with
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
Conclusions
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 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
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.
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Corresponding editor: Gilles Escarguel.