2. State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, China;
3. Department of Geosciences, University of Wisconsin-Milwaukee, Milwaukee 53211, USA
During the Paleozoic–Mesozoic transitional period, the life on the Earth underwent the largest mass extinction in Phanerozoic history, which resulted in a shift in marine ecosystems from the Paleozoic evolutionary fauna to Mesozoic evolutionary fauna (Song and Tong, 2016; Chen et al., 2010; Alroy et al., 2008; Fraiser and Bottjer, 2007; Erwin, 1993; Sepkoski et al., 1981; Raup, 1979). The subsequent biotic recovery was delayed to the Middle Triassic (Chen and Benton, 2012) and the long-term recovery may have resulted from severe environmental conditions, including marine anoxia and global warming during the Early Triassic (Song et al., 2014; Grasby et al., 2013; Sun et al., 2012).
Bivalves, one of the groups less affected by the end-Permian mass extinction (EPME) (Huang Y F et al., 2014; Yin, 1985), appear to have adapted to the stressed environments during and after the EPME (Petsios and Bottjer, 2016; Fraiser and Bottjer, 2007; Hallam and Wignall, 1997) and were one of the most abundant animals in the Early Triassic benthic communities (Foster et al., 2017; Hofmann et al., 2017, 2015, 2014, 2013; Petsios and Bottjer, 2016; Hautmann et al., 2015, 2013, 2011; Wasmer et al., 2012; Chen et al., 2010; Fraiser and Bottjer, 2007; Schubert and Bottjer, 1995). However, the diversity of bivalves was extremely low during the early Early Triassic, although it increased quickly in the late Early Triassic (Hofmann et al., 2017, 2015; Hautmann et al., 2013; Posenato, 2008). During this survival-recovery phase, bivalves were characterized by the proliferation of the genera Claraia, Eumorphotis, Unionites and Promyalina (Petsios and Bottjer, 2016).
The bivalve genus Claraia has long been known as an early Early Triassic index fossil, especially in the Tethys region, because of its high rates of evolution and global distribution (McRoberts, 2010; Tong and Yin, 2002; Yin, 1985, 1981; Nakazawa, 1977). The origin of Claraia has been extensively discussed because certain Claraia-like species, though they were included in different genera, such as Claraia Bittner 1900, Claraioides Fang 1993, and Pseudoclaraia Zhang 1980, were collected from the Late Permian strata in the Caucasus, Kashmir and South China (Yang T L et al., 2015; He et al., 2007; Kotlyar et al., 2004; Yang F Q et al., 2001; Newell and Boyd, 1995; Fang, 2010, 1993; Yin, 1983; Nakazawa, 1981, 1977; Kulikov and Tkachuk, 1979; Lobanova, 1979). The genera Claraia, Claraioides, and Pseudoclaraia differ mainly in the shape of the right byssal notch, and the assignment of these species has long been debated. Yang et al. (2001) and He et al. (2007) suggested that both Pseudoclaraia and Claraioides are synonyms of Claraia Bittner, a suggestion coincident with fossil assignment by others (e.g., Yin, 1983). After the EPME, Claraia dominated the survival fauna in South China, the Alps, Kashmir, Iran, North Vietnam, and the western US during the Induan Stage, but it declined quickly during the Olenekian (Chen, 2004; Yin, 1985).
However, the paleoecological and paleophysiological reasons for the bloom of Claraia in the aftermath of the EPME still remain unexplored. It should be noted that an autochthonous Claraia-dominated fauna linking both lifestyle and living environments are critical for understanding its paleoecology. Although some better-preserved benthic fossil assemblages have been reported from the Early Triassic (e.g., Hautmann et al., 2015, 2013, 2011; Hofmann et al., 2015, 2014, 2013), few autochthonous Claraia-dominated fauna have been discovered (single articulated shells reported in Komatsu et al., 2008). In this study, we describe a new mollusc fauna of the Griesbachian (Early Triassic) from the Sidazhai Section, Guizhou Province, southwestern China. Abundant articulated Claraia fossils were collected; thus, this fauna could be taken as an ideal autochthonous fauna, enabling us to explore the paleoecological features of Claraia.1 GEOLOGICAL SETTING
South China was composed of several blocks that were situated in the low-latitude eastern Paleo-Tethyan oceans during the Paleozoic–Mesozoic transition (Yin et al., 1999). The marine Late Permian to Early Triassic strata are widespread in South China and can be subdivided into several sedimentary facies (Fig. 1) (Yin et al., 2014; Tong and Yin, 2002; Feng et al., 1997). Deposits of the studied Sidazhai Section were accumulated in the Hunan-Guizhou-Guangxi deep basin environment from Late Permian to Middle Triassic (Fig. 1).
The Sidazhai Section (=Shaiwa Section of Chen et al., 2009, 2006) is situated near the Sidazhai Town, Ziyun County, Guizhou Province, Southwest China (Fig. 1). The Upper Permian to Lower Triassic succession at Sidazhai is lithologically divided in ascending order into the Upper Permian Linghao Formation (=Shaiwa Group of Chen et al., 2009, 2006), Lower Triassic Luolou, and Ziyun Formation (Fig. 2). The upper part of the Linghao Formation is mainly composed of thin-bedded siliceous mudstones, cherts and siliceous limestones, where a Bouma Sequence developed, which indicate deep water turbidity sediments (Gao et al., 2001). Abundant fossils were collected from the Linghao Formation, including ammonoids (e.g., Medlicottidae, Agathiceratidae, Paragastrioceratidae and Goniatitina), trace fossils (e.g., Nereites, Protopaleodictyon, Megagrapton, and Dendrotichnium), bivalves (e.g., Claraia primitiva, C. shabaoensis, C. zhiyunica, and Hunanopecten exilis), and brachiopods (e.g., Cathaysia chonetoides, Fanichonetes campigia, Martinia ziyunensis, Paryphella orbicularis, Transennatia gratiosa, and Tethyochonetes soochowensis) (He et al., 2014; Chen et al., 2009, 2006; Gao et al., 2004, 2001; Yang et al., 2001; Yang and Gao, 2000). The articulated shells of Late Permian Claraia (Bivalvia) specimens are commonly preserved, indicating in situ preservation. The Late Permian deep water setting have been inherited to Early Triassic (He et al., 2015). The sedimentary models of the Early and Middle Triassic successions in this section have been analyzed by He et al. (2015). The lower part of the overlying Luolou Formation is characterized by yellowish, thin-bedded silty mudstones and calcareous mudstones (=beds 1–9 of He et al., 2015), which contain abundant bivalve and ammonoid fossils, while the upper part is dominated by greyish-black, thin-bedded limestones. Beds 3, 5, 7 and 9 are composed of thin-bedded calcareous mudstones, while beds 4 and 8 are featured by thin-bedded silty mudstones. Besides, bed 6 consists of thin-bedded muddy siltstone. Laminations formed by deposition from suspension could be seen in beds 3–9, which indicates basinal facies succession. However, in the study area, the lowermost part of the Luolou Formation and the Permian-Triassic boundary is not well cropped out.2 MATERIALS AND TAPHONOMY
Over 300 bivalve and ammonoid fossil specimens were collected from yellowish, thin-bedded mudstones on a bed-by-bed basis in November 2010. All specimens are deposited in the State Key Laboratory of Biogeology and Environment Geology (BGEG), China University of Geosciences, Wuhan, Hubei Province, PR China. Most of the bivalve and ammonoid fossils are body fossils; however, the original shell minerology has changed, and only a minority of bivalve fossils were kept as external moulds. Certain shells are still articulated together, forming a butterfly preservation (Fig. 3), while most specimens are composed of isolated but complete shells, with the shells convex up. Size-frequency distributions are considered to be one of the most useful criteria for distinguishing between autochthonous and allochthonous fossil assemblages (Pan et al., 2012; Zuschin et al., 2005). The size-frequency histograms of Claraia specimens from Bed 8 indicated that there was no size sorting (Fig. 4). Also, the numbers of left valves and right valves of Claraia specimens from Bed 8 are nearly the same, with the ratio value equals one (Fig. 4). Thus, this bivalve fauna has not been transported a long distance and could be taken as forming an ideal autochthonous community.3 FOSSILS FROM THE SIDAZHAI SECTION 3.1 Bivalves
Five species in two genera, including Claraia wangi, C. griesbachi, C. stachei, C. radialis and Promyalina putiatinensis (Fig. 5), are identified in this study, and could be assigned to the Claraia wangi-C. griesbachi assemblage zone (Fig. 2). This assemblage zone is featured by Claraia wangi and C. Griesbachi, while Claraia griesbachi is the most abundant species and accounted for approximately 80% of the individuals. Claraia wangi-C. griesbachi assemblage zone in Sidazhai section could be correlated with the Claraia griesbachi-Claraia concentrica assemblage zone in the Chaohu area, South China, which is associated with the ammonoid Ophiceras-Lytophiceras assemblage and with the conodont Hindeodus typicalis, Clarkina krystyni, and C. planata zones (Tong and Zhao, 2011). Thus, the Claraia wangi-C. griesbachi assemblage zone should be of the Middle–Late Griesbachian Stage.3.2 Ammonoids
Although ammonoid fossils are abundant in association with bivalves from the lower part of Luolou Formation, none of the suture lines was preserved. Thus, the identification of the ammonoid fossils is very difficult and provisional. Ophiceras sp. and Ussuridiscus sp. are recognized in this study (Fig. 6). Genus Ophiceras has been reported in Griesbachian strata worldwide, including Greenland, Canada, and South China (Bai et al., 2017; Brühwiler et al., 2008; Tozer, 1994; Spath, 1935, 1930). Genus Ussuridiscus has been reported in South Primorye, Russia, indicating an age of Late Griesbachian (Shigeta et al., 2009). Thus, the occurrence of genus Ophiceras and Ussuridiscus in the Sidazhai Section might indicate the Middle–Late Griesbachian Stage of the Early Triassic.4 DISCUSSION 4.1 Claraia could Live in Dysoxic to Anoxic Conditions during the Griesbachian
In recent years, substantial progress has been made in characterizing marine redox conditions during the Permian– Triassic crisis based on the study on geochemical data and pyrite framboids, which show that pulses of shallow-marine anoxia and periodic euxinic conditions proliferated during the end-Permian Period to the Smithian Substage (Huang Y G et al., 2017; Chen et al., 2015; Tian et al., 2014; Grasby et al., 2013; Kaiho et al., 2012; Song et al., 2012; Shen et al., 2011; Bond and Wignall, 2010; Liao et al., 2010; Cao et al., 2009; Riccardi et al., 2006; Grice et al., 2005). Although the extent and duration of anoxia events differ in different areas, dysoxia-anoxia conditions occurred globally in the Griesbachian.
Genus Claraia was extremely abundant and distributed in nearly all environments in the aftermath of the EPME, especially the Tethyan region. Claraia has been proposed as typical of lower dysaerobic facies based on sedimentological and geochemical evidence in surrounding strata from Italy and Idaho (Wignall and Hallam, 1992). In addition, Claraia wangi and C. longyanensis have been collected from Bed 29 in the Meishan D Section (GSSP), classified as anoxic to dysoxic environments by pyrite framboids (Li et al., 2016; Chen et al., 2015; Shen et al., 2007). Bianyang Section, with a water depth of ~200 m during the Early Triassic (Song et al., 2013), was located on the lower slope of the northern margin of the Great Bank of Guizhou (close to the basin floor) in Hunan- Guizhou-Guangxi deep basin facies (Lehrmann et al., 2001). Although the water depth of Sidazhai Section was slightly deeper than the Biyanyang Section, the redox conditions of the Sidazhai Section in this study could be inferred from the study of the Bianyang Section. The pyrite framboid evidence showed that the seawater was anoxic-dysoxic during the Griesbachian in the Bianyang Section (Tian et al., 2014). Thus, the autochthonous Claraia fauna in the Sidazhai Section should have lived in anoxic-dysoxic environments, which means that genus Claraia could tolerate anoxic-dysoxic conditions in post- extinction oceans.4.2 Possible Explanation for the Proliferation of Claraia
The proliferation of Claraia may have been related to its physiological characteristics, which enabled it to survive the harsh environments––i.e., anoxia/euxinia (Grasby et al., 2013; Song et al., 2012), elevated weathering rates (Huang Y F et al., 2017; Song et al., 2015; Algeo and Twitchett, 2010), and high seawater temperature (Joachimski et al., 2012; Sun et al., 2012), during the Permian-Triassic crisis, as well as the subsequent prolonged deleterious environmental conditions. Although the genus Claraia went extinct by the Spathian Substage, its physiological characteristics can be inferred from its shell morphology, as well as analogous or similar bivalves in the Mesozoic (e.g., paper pectens) and modern seas (e.g., pectinids).
The paper pectens are epifaunal, thin-shelled, flat-valved bivalves, although they vary in taxonomic classifications (Wignall, 1994). Three distinct morphotypes have been recognized: Dunbarelliform, Posidoniform, and Mytiliform (Wignall, 1994). Different modes of life, including pseudoplanktonic, nektonic, epibyssate, and epibenthic free-lying, have been proposed for paper pectens (Schatz, 2005; Tong, 1997; Wignall, 1994), but recent research has shown that a portion of the paper pectens were epibenthic free-lying or epibyssate bivalves (McRoberts, 2010; Schatz, 2005). Similar to the other paper pectens, various modes of life have been suggested for Claraia, including pseudoplanktonic, epifaunal byssate, swimming, and facultative epibenthic (Tong and Xiong, 2006; Yang et al., 2001; Yin, 1985, 1981).
A pseudoplanktonic mode of life was proposed for Claraia (Yin et al., 1995; Yin, 1981), based on the widespread distribution of Claraia species, especially for those living in dysoxic to anoxic shelf-basin environments. Pseudoplanktonic bivalves are known in the fossil record, and they typically have been observed with their byssus attached on floating objects, such as driftwood, pumice, vesicular algae, or cephalopod shells (Wignall, 1994; Wignall and Simms, 1990). However, Claraia has never been found attached to any object or in an explicit spatial relationship to floats, including ammonoid shells. Byssally attached pseudoplanktonic bivalves typically have very short hinge lines, equivalved shells, and small body sizes (Wignall, 1994; Wignall and Simms, 1990). However, Claraia was mostly inequivalved, with a long hinge line, and even reached larger sizes than coeval ammonoids. Though vesicular algae are difficult to recognize in the fossil records due to poor preservation, pseudoplanktonic bivalves that attached on pelagic or benthic algae should have been limited to the photic zone, which is in contrast with the occurrence of Claraia in deep-basin environments (more than 200 m depth). Thus, it is not very likely that Claraia lived a pseudoplanktonic life.
The strong development of a deep byssal notch indicates that Claraia lived an epifaunal byssate mode of life, and muscle scars suggest that it might have attached to substrates with the right valve (Ichikawa, 1958). It has been suggested that Early Triassic Claraia had greater mobility than Late Permian Claraia, as indicated by the narrowing of the byssal notch (He et al., 2007), or that Early Triassic Claraia could be free-lying on the substrate.
Although knowledge of the larval phase of Claraia is poor, it can be proposed that Claraia may have gone through a planktonic larval phase based on the observation of modern marine pectinids (Pennec et al., 2003). The duration of the larval stage could span a couple of weeks––although the time varies among different genera, such as 25 days (Robert and Gerard, 1999) for Pecten maximus, 20–24 days (Uriarte et al., 2001) for Aequipecten purpuratus, 20–26 days (Uriarte et al., 2001) for Argopecten ventricosus, 25–27 days (Uriarte et al., 2001) for Nodipecten nodosus, and 30–40 days (Yamamoto, 1960) for Mizuhopecten yessoensis. During these several weeks, the larvae of bivalves have the potential to travel for thousands of kilometers along the ocean current; this finding may explain the worldwide distribution of Claraia species.
Claraia specimens show preferential preservation in mudstone and siltstone relative to limestone, indicating their stronger preference for softer substrates. This finding is consistent with the thin-shelled bivalves paper pectens, which may represent snow-shoe strategies in soft substrates (Schatz, 2005; Wignall, 1994, 1993). Additionally, it has been proposed that the sediment fluxes in marine environments increased by ~7-fold in South China due to the extensive loss of plants on land and acid rain in the latest Permian, carrying more clay into shelf and basin environments (Algeo et al., 2011; Algeo and Twitchett, 2010). Consequently, seawater may have become more turbid and the substrates dominated by mud and silt, further facilitating the proliferation of Claraia.
Paper pectens have been known to harbor chemoautotrophic bacteria, enabling them to survive in anoxic/euxinic environments (Waller and Stanley, 2005; Seilacher, 1990). It has been suggested that the anterior byssal tube of Halobia, a Late Triassic paper pecten, may have functioned as a sulfur pump (Seilacher, 1990), thus transferring oxygen directly to the mantle cavity (Waller and Stanley, 2005). Similarly, the modern chemosymbiotic bivalves from hydrothermal vent and methane seep faunas (e.g., Lucinid, Thyasirid, and Solemyid) usually host symbionts in their gills, which enable them to live in anoxic/euxinic environments. Although questions still exist (McRoberts, 2010; Wignall, 1994), it is likely that certain organs of Claraia may have hosted endosymbionts.4.3 Claraia as a Disaster Taxon
The repopulation interval in the aftermath of mass extinction can be subdivided into survival and recovery phases (Kauffman and Harries, 1996). Some specific taxa, such as disasters, opportunists, progenitors, ecological generalists, and preadapted survivors, are able to survive the harsh environments during the survival phase, thus the survival faunas are usually dominated by progenitors, opportunistics, and disasters (Chen and Benton, 2012; Hallam and Wignall, 1997; Harries et al., 1996; Kauffman and Harries, 1996; Levinton, 1970).
After the EPME, the inarticulate brachiopod Lingula, microbialites, some foraminiferal genera, and calcareous sclerobionts have been proposed as disaster taxa (Song et al., 2016; He et al., 2012; Rodland and Bottjer, 2001; Schubert and Bottjer, 1992), while microgastropods and some marine invertebrate tracemakers have been interpreted as opportunists (Fraiser and Bottjer, 2009, 2004). Genus Claraia has been proposed as a progenitor taxon (Hallam and Wignall, 1997) or a potential disaster taxon (Petsios and Bottjer, 2016; Tong and Xiong, 2006; Schubert and Bottjer, 1995) for its great abundance after the EPME. All of these taxa experienced greatly increases in numbers following the extinction and usually signal elevated environmental stress (Fraiser and Bottjer, 2009, 2004; Hallam and Wignall, 1997; Schubert and Bottjer, 1992). Genus Claraia began to diversify during the Changhsingian, before the EPME, and increased rapidly in richness and abundance due to its higher ability to live in the deleterious environments at and above the extinction level, becoming extinct by the Smithian-Spathian boundary, when the ecosystems became more robust (Chen, 2004; Hallam and Wignall, 1997). Thus, we propose that Claraia was a disaster and opportunistic taxon. The nature of Claraia may be similar to the bivalve genus Mytiloides, which originated in the Late Cenomanian before the Cenomanian-Turonian mass extinction event and showed rapid evolution just above the Cenomanian-Turonian boundary (Kauffmann and Harries, 1996).5 CONCLUSION
We reported a new Griesbachian (Early Triassic) mollusc fauna, dominated and characterized by Claraia fossils, from deep water settings in South China. The butterfly-shaped preserved Claraia fossils could indicate an ideal in situ preserved fauna, which linked both the lifestyle and living environments of Claraia. Claraia had an epibyssate mode of life, and usually lay on soft substrates with its right valve. Meanwhile, the shallow- and deep-marine environments became dysoxic to anoxic globally during the early Early Triassic, supported by evidence from the geochemical and pyrite framboids. Thus, genus Claraia could have lived in dysoxic/anoxic waters, and its flourishing was likely due to its physiological characteristics. Claraia might have hosted chemosymbionts and thus survived in dysoxic to anoxic waters. The global distribution of Claraia was probably related to its planktonic larval stage. Additionally, Claraia is characteristic of a significant disaster taxon during the Early Triassic in South China.ACKNOWLEDGMENTS
We thank Xinqi Xiong, Wenting Ji and Hui Shi for help in collecting the fossils in the field, and Xu Dai for laboratory assistance. This study was supported by the National Natural Science Foundation of China (No. 41502012) and the Yangtze Youth Fund of Yangtze University China (No. 2015cqn27). This is a contribution to IGCP 630. The final publication is available at Springer via https://doi.org/10.1007/s12583-017-0966-7.
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