Patterns of brachiopod faunal and body-size changes across the Permian−Triassic boundary: Evidence from the Daoduishan section in Meishan area, South China
Introduction
Numerous studies on the end-Permian mass extinction and causes have been conducted based on materials from the Meishan Section D of South China, the GSSP of the PTB (e.g., Grice et al., 2005, Xie et al., 2007, Shen et al., 2011, Joachimski et al., 2012, Sun et al., 2012, Yin et al., 2012, Song et al., 2013, Chen et al., 2015, He et al., 2015). Whether the extinction was abrupt or episodic remains debatable (Yang et al., 1991, Jin et al., 2000, Xie et al., 2005, Chen et al., 2006, Chen et al., 2010, Chen et al., 2015, Song et al., 2009, Song et al., 2013, Yin et al., 2012, Wang et al., 2014), although there is an emerging consensus that the main extinction intervals span from Beds 25 to 28 at the Meishan Section D. Brachiopods, one of the key marine benthic clades in the Permian global marine ecosystem, were severely and adversely affected by this mass extinction and, consequently, have furnished as an important source of information for the study of the end-Permian mass extinction and recovery (Chen et al., 2002, Chen et al., 2005a, Chen et al., 2005b, Chen et al., 2006, Chen et al., 2010, Chen et al., 2015, Song et al., 2013). Several brachiopod faunas have been reported or simply described from the Permian–Triassic boundary beds at the Meishan GSSP sections (Liao, 1979, Liao, 1984, Zhao et al., 1981, Sheng et al., 1984, Xu and Grant, 1994, Chen et al., 2002, Chen et al., 2006, Chen et al., 2010, Chen and Liao, 2009), but only a few papers have actually analyzed the changes of brachiopod species diversity, assemblage, and body size in the Meishan area (Jin et al., 2000, Chen et al., 2002, Chen et al., 2005b, Chen et al., 2006, He et al., 2007, He et al., 2015, Chen and Liao, 2009). Further, among these earlier studies, some have suffered either from focusing on too short a stratigraphic interval, typically just across the Permian–Triassic boundary beds, to allow a broader analysis and thus a deeper understanding of the evolution of the brachiopod assemblages over a longer timescale (e.g., Sheng et al., 1984); or from the relative small sample size of the brachiopod assemblages due to difficulty (at the time) in collecting brachiopod fossils from the dense Changxing limestone (Changxing Formation) at the Meishan sections (e.g., Chen and Liao, 2009).
Here, we present an updated and much expanded dataset of brachiopod stratigraphical occurrences and body-size changes from the upper Changhsingian to the lowest Triassic from the Daoduishan section in the Meishan area. Based on this new dataset, we studied the changing patterns of brachiopod assemblages and body sizes through time and discussed the significance of certain brachiopod morphological adaptations in aiding their survivorship across the end-Permian crisis. The primary aim was to gain a deeper understanding of the temporal dynamics of brachiopod faunal turnover and body-size changes across an expanded PTB interval in South China. The ultimate objective of this study was to provide new insights into the causes of the end-Permian mass extinction.
Geographically, the Daoduishan section is located about 2.5 km northeast of the Meishan Section D, 20 km northwest of Changxing County, Zhejiang Province, South China (Fig. 1). Palaeogeographically, the Daoduishan section was situated in the eastern part of the South China block (Fig. 2). The South China block was mainly subdivided into deep-water siliceous basins and shallow-water carbonate platforms during the Changhsingian (Fig. 2). The deep-water basins are characterized by chert and siliceous mudstone intercalated with calcareous mudstone, suggesting an outer shelf deep-sea depositional setting (He et al., 2015). The shallow-water carbonate platforms are featured by limestone, of an inner shelf origin (He et al., 2008, He et al., 2015). Similar to the Meishan Section D, the studied Daoduishan section was located on the upper part of a ramp linked to one of the carbonate platforms. In contrast, the Majiashan and Rencunping sections, both referred to in this paper for comparison with the Daoduishan section, were located in a deep-water (basinal) facies on the outer shelf (He et al., 2008, He et al., 2011, He et al., 2015). For the purpose of comparison, the Huangzhishan section was also chosen in this study to represent shallow-water carbonate platform facies (He et al., 2015).
The upper Changhsingian to the basal Triassic sequences at the Daoduishan section include the upper part of the Changxing Formation and the basal part of the Yinkeng Formation (Fig. 3). The component of the upper Changxing Formation consists of gray to light gray thin- to medium-bedded bioclastic limestone, occasionally intercalated with gray thick-bedded bioclastic limestone and black cherty nodules, with hummocky cross-stratification, parallel stratification, or wavy cross-bedding in Beds 14, 15, 23, and 24 (Fig. 3, Fig. 4A, B). The basal Yinkeng Formation is composed of dark gray thin-bedded calcareous mudstone and dark gray thin- to medium-bedded argillaceous limestone, intercalated with pale volcanic ash, with fine laminations in calcareous mudstone (Fig. 3).
At Daoduishan, the upper part of Bed 21 to Bed 24b yield the conodont Clarkina yini Zone, equal to the counterpart from the top part of Bed 22 to Bed 24d of the Meishan Section D (for the latest conodont zonation of the Meishan Section D, see Yuan et al., 2014). Beds 24c to 26 yield the Clarkina meishanensis Zone, roughly equivalent to Beds 24e to 25 of the Meishan Section D (Yuan et al., 2014). Bed 27 at Daoduishan yields conodont Clarkina taylorae, suggesting correlation with Beds 27a–b of the Meishan Section D (Zhang et al., 2009). Additionally, Bed 26 at Daoduishan contains ammonoid Hypophiceras sp., typical for the uppermost Changhsingian, thus allowing alignment with Bed 26 of the Meishan Section D where Hypophiceras sp. is also present (Yin et al., 2001). Bed 29 at Daoduishan yields ammonoid Ophiceras sp., typical for the basal Triassic. Following these correlations, the PTB at the Daoduishan section is therefore placed between Beds 28 and 29 (Fig. 3), although the typical fossil Hindeodus parvus marking the base of the Triassic has not been found at this section.
Section snippets
Methods of brachiopod collection and body-size measurement
Brachiopods were collected from selected beds of the upper part of the Changhsingian and the basal Triassic strata at Daoduishan. These beds were chosen because they had relatively abundant brachiopods. Meanwhile, to reduce sampling bias, an area of about 0.5–0.6 m2 was sampled in each studied bed. All brachiopods found during excavating were collected, including both complete individuals and fragments. The number of specimens was counted for each species in each bed. When counting, if both a
Stratigrapical ranges of brachiopod species and temporal fluctuations of brachiopod diversity
The stratigrapical ranges of brachiopod species along the upper Changxing Formation to the basal Yinkeng Formation show that most species (10 species) survived into the lowest Triassic (Beds 29–30) except some larger species (such as Araxathyris sinensis, linoproductids, ?Prelissorhynchia sp., Prelissorhynchia xui, ?Squamularia sp., and Haydenella sp.) which disappeared in Bed 26 or older beds (Fig. 3).
Turnover of brachiopod assemblages
The stratigraphic variation of the abundance of brachiopod species, measured by the counts of
Discussion
Why did the turnover of brachiopod assemblages take place at the Permian–Triassic transition? A number of factors could each have played a role, or collectively inter-played to reinforce the turnover of brachiopod assemblages through time.
Firstly, the change of substrate, from the lime-rich sediment in Beds 14–24e to the mud-rich substrate in Beds 26–29, could explain the turnover, at least in part. As already demonstrated in several recent studies (He et al., 2015, Zhang et al., 2016),
Acknowledgments
We thank L. Y. Zhu, X. Chen, and B. X. Su for help with field work. We also wish to express thanks to S. Z. Shen for his helpful suggestion on the identification of brachiopods, sincere thanks to E. A. Weldon for her further improving the manuscript and thanks to three anonymous reviewers for offering good comments and suggestions for the revision of this manuscript. This paper has been supported by NSFC (grant nos. 41372030, 40872008) and the Ministry of Education of China (B08030 of 111
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