Early Triassic trace fossils from the Three Gorges area of South China: Implications for the recovery of benthic ecosystems following the Permian–Triassic extinction
Introduction
The largest mass extinction in geological history happened near the Permian–Triassic (P–Tr) boundary and killed over 90% of marine species (Erwin, 1993, Song et al., 2013). Biotic recovery following this event has attracted much attention in recent years and remains the topic of much debate. It has long been considered that the P–Tr mass extinction was followed by a period of delayed biotic recovery that lasted until the Middle Triassic (Stanley, 1990). The Early Triassic is an interval characterised by continued low biodiversity (Erwin, 1993), blooms of opportunistic and disaster taxa (Bottjer et al., 2008), reduction in the size of both metazoans (Twitchett, 2007) and protozoans (Song et al., 2011a), and the absence of metazoan reefs (Flügel, 2002) and of calcareous algae (Flügel, 1985). Carbon and sulphur isotopic data illustrate a series of large fluctuations throughout the Early Triassic (Payne et al., 2004, Tong et al., 2007, Song et al., 2014a), reflecting stressed marine environments. Recent geochemical data show that the stressed environments of the Early Triassic are characterised by a long-term hot climate (Sun et al., 2012, Romano et al., 2013) as well as severe oceanic anoxia (Wignall et al., 2010, Song et al., 2012, Grasby et al., 2013). However, recent palaeontological data show that a fitful recovery of many marine organisms began in the Smithian and Spathian, including nektons, such as ammonoids and conodonts (Brayard et al., 2009, Stanley, 2009), benthic foraminifers and calcareous algae (Song et al., 2011b), and even metazoan reefs (Brayard et al., 2011). As a result, the mechanisms that caused biotic recovery in the Early Triassic are still a subject of debate.
Trace fossils are good indicators of palaeoenvironmental conditions and the behaviours of benthic invertebrates. They have been used to reveal the tempo and pattern of ecologic recovery following the P–Tr mass extinction (e.g., Twitchett and Wignall, 1996, Wignall et al., 1998, Twitchett, 1999, Twitchett, 2006, Pruss and Bottjer, 2004, Twitchett and Barras, 2004, Zonneveld, 2004, Beatty et al., 2005, Beatty et al., 2008, Fraiser and Bottjer, 2009, Zonneveld et al., 2010a, Zonneveld et al., 2010b, Chen et al., 2011, Chen et al., 2012, Hofmann et al., 2011, Baucon et al., 2014). These ichnofossil data, derived from global localities, show that benthic invertebrate trace-makers at boreal palaeolatitudes, e.g., Spitsbergen (Wignall et al., 1998) and western Canada (Zonneveld, 2004, Beatty et al., 2005, Beatty et al., 2008, Zonneveld et al., 2010a, Zonneveld et al., 2010b), recovered before those at low and middle palaeolatitudes, e.g., the western United States (Pruss and Bottjer, 2004, Twitchett and Barras, 2004, Fraiser and Bottjer, 2009), northern Italy (Twitchett and Wignall, 1996, Twitchett, 1999, Twitchett and Barras, 2004), the Lower Yangtze region of South China (Chen et al., 2011), and Western Australia (Chen et al., 2012). However, recent studies showed that a diverse and complex ichnofauna began to occur in the late Griesbachian in Italy (Hofmann et al., 2011). Thus, the environmental factors that structure the temporal and spatial patterns of Early Triassic ichnofossil assemblages are still unclear and additional quantitative analyses are required (Fraiser and Bottjer, 2009).
In this study, abundant and diverse trace fossils from the Lower Triassic successions in the Three Gorges area were described and analysed. Five proxies, including ichnodiversity, forms and complexity, bioturbation index, burrow sizes, and tiering levels, were used to evaluate the tempo and pattern of benthic ecosystem recovery in the aftermath of the P–Tr mass extinction in the Middle Yangtze region of South China. Additionally, the latest geochemical data combined with a hypothesised recovery model (Song et al., 2014b) were used to explain the temporal and spatial patterns of Early Triassic ichnofossil assemblages.
Section snippets
Studied sections and stratigraphic setting
The South and North Xiakou sections are situated 3 km west of Jianyangping, Xiakou Town, Xingshan County of Hubei Province and approximately 80 km away from Yichang City (Fig. 1). These two sections are located along both banks of the Xiangxi River, a branch of the Yangtze River. The present study made use of a previously established site along the south bank (starting at 110°48′13″ E, 31°06′52″ N, referred to as South Xiakou section) as well as a newly surveyed section on the north bank of the
Materials and methods
In this study, trace fossils in the Lower Triassic succession at Xiakou have been investigated bed-by-bed. A total of 17 ichnospecies in 14 ichnogenera were identified based on field observations and descriptions of specimens collected from the Daye and Jialingjiang formations (Fig. 5, Fig. 6, Fig. 7, Fig. 8). In addition, taxonomic revision of the Lower Triassic trace fossils previously reported from the Middle Yangtze region (Yang et al., 1992, Ma et al., 2008) was undertaken based upon the
Arenicolites isp.
The trace fossil Arenicolites, a common ichnotaxon in the Lower Triassic of Three Gorges region, occurs in Members III and IV of the Daye Formation and the base of Member II of the Jialingjiang Formation (Fig. 3, Fig. 4). Arenicolites isp. is preserved as vertical to slightly oblique U-tubes without spreite. The penetration depth of burrows is difficult to measure. On bedding planes, this trace fossil occurs as pairs of holes (Fig. 5A, B). The burrow diameters of tubes range from 1 to 8 mm and
Trace fossil diversity
Only one ichnospecies, P. montanus, was uncovered from Member II of the Daye Formation in the Three Gorges area. At present, no trace fossil has been found in this member in any other area of the Middle Yangtze region, including the Guangji and Huangshi areas (Table 1; Yang et al., 1992, Ma et al., 2008). The two ichnospecies, D. lyelli and P. montanus, are present in the lower part of Member III of the Daye Formation (Fig. 3, Fig. 4).
Trace fossils in the middle and upper parts of Member III of
Conclusion
The Lower Triassic Daye and Jialingjiang formations of the Three Gorges area record the recovery interval of marine organisms in the aftermath of the P–Tr mass extinction. These strata include abundant and diverse trace fossil assemblages. A total of 17 ichnospecies in 14 ichnogenera are documented from the Smithian and Spathian strata. These trace fossils are interpreted to represent activities of a diverse invertebrate infauna and epifauna, including anemones, crustaceans, molluscs,
Acknowledgements
This study was supported by the 973 Program (2011CB808800), the National Natural Science Foundation of China (nos. 41240016, 41272372, 41172312, 41302010), and the 111 Project (B08030). It is a contribution to the IGCP-630 program “Permian–Triassic climatic and environmental extremes and biotic response”.
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