Extinction And Recovery Of Bivalves During The Permian-Triassic Transition

The end-Permian mass extinction was the largest biotic catastrophe in the Phanerozoic, and also resulted in an important transition of marine ecosystem structures from the brachiopod-dominated Paleozoic Fauna to the mollusk-dominated Modern Fauna. This study chooses the bivalves, one common group during the Permian-Triassic transition, in order to establish the bio-stratigraphic framework of bivalves, discuss the relationship of extinction pattern among bivalves with different environmental factors, analysis the recovery pattern of bivalves, and probe into the reasons for the bloom of Genus Claraia.In order to build the Lower Triassic bio-stratigraphic framework in South China using bivalve fossils, this study collected abundant bivalve fossils from nine Lower Triassic sections which were located in various palaeogeographic facies (nearshore facies:Wadu section, Panxian, Guizhou province; shallow clastic facies:Zhongzhai section, Liuzhi, Guizhou province; carbonate platform facies:Gaoqiao, Xiangkou and Xinfazhai sections, Guizhou province; deep basin facies:Sidazhai section of Guizhou province, Xiakou section of Hubei province, North Pingdingshan and West Pingdingshan section of Anhui province). We assigned the total number of4774bivalve individuals to47species in19genera.A series of bio-zones were recognized from these sections:(1) Pteria ussurica variabilis-Eumorphotis venetiana assemblage from Wadu section, with an age of earliest Griesbachian.(2) Eumorphotis venetiana acme zone (earliest Griesbachian) and Claraia wangi-Claraia radialis aeemblage zone (early Griesbachian) from Zhongzhai section.(3) Pteria ussurica variabilis-Eumorphotis venetiana and Claraia wangi-Claraia zhenanica assemblage zones from Gaoqiao section, with the age of earliest Griesbachian and early Griesbachian respectively.(4) Claraia wangi acme zone (middle Griesbachian) and Eumorphotis multiformis-Eumorphotis hinnitidea assemblage zone (late Dienerian to early Smithian) from Xiangkou section.(5) Claraia griesbachi-Claraia longyanensis assemblage zone (Griesbachian), Claraia aurita acme zone (Dienerian) and Eumorphotis multiformis-Leptochondria virgalensis assemblage zone (late Dienerian to early Smithian) from Xinfazhai section.(6) Claraia wangi-Claraia griesbachi assemblage zone from Sidazhai section, with an age of middle Griesbachian.(7) Claraia griesbachi acme zone (middle Griesbachian), Claraia stachei acme zone (late Griesbachian), Claraia aurita acme zone (end-Griesbachian to early Dienerian), Eumorphotis multiformis-Eumorphotis hinnitidea-Eumorphotis telleri assemblage zone (late Dienerian to Smithian) and Posidonia circularis acme zone (Spathian) from Xiakou section.(8) Claraia griesbachi-Claraia stachei assemblage zone (Griesbachian), Claraia aurita acme zone (end-Griesbachian to early Dienerian), Eumorphotis multiformis-Eumorphotis hinnitidea-Eumorphotis telleri assemblage zone (late Dienerian to early Smithian), and Posidonia circularis-Guichiella angulata assemblage zone (Smithian) from North Pingdingshan section.(9) Claraia stachei acme zone (late Griesbachian), Claraia aurita acme zone (late Griesbachian to early Dienerian), Eumorphotis multiformis-Eumorphotis hinnitidea-Eumorphotis telleri assemblage zone (late Dienerian to early Smithian), and Posidonia circularis-Guichiella angulata assemblage zone (middle to late Smithian) from West Pingdingshan section.The Pteria ussurica variabilis-Towapteria scythica assemblage zone is the earliest Triassic bivalve assemblage in the aftermath of the end-Permian mass extinction, which could be correlated through around the world, such as the Southern Alps, Kashmir, Oman, and northern Vietnam. The Claraia-dominated and Eumorphotis-dominated assemblages distributed globally during the Griesbachian to Smithian. The Posidonia-dominated assemblages might represent local assemblages in deeper water environments in South China.Late Permian Changhsingian bivalves in South China are very abundant and diverse in nearshore, inner shelf, outer shelf, and deep basin facies. All reported data of both bivalve genera and species from the Permian-Triassic boundary strata of South China have been systematically collected and studied for understanding the evolutionary process of various bivalve groups in different environments through the great Permian-Triassic transition. Over700species in105genera have been analyzed from63Changhsingian to Smithian sections. The age of each species is given in relation to its occurrence relative to the two extinction events during the Permian-Triassic crisis. The lifestyles of bivalves are subdivided into epifaunal attached, including epi-byssate and epi-cemented, epifaunal motile, including facultative mobile and pseudoplankton, semi-infaunal, shallow infaunal and deep infaunal habits. For comparison between infaunal and epifaunal bivalves, we assigned the epifaunal attached and epifaunal motile modes of life to epifaunal bivalves, while infaunal bivalves were assigned as semi-infaunal, shallow infaunal and deep infaunal. The feeding types of bivalves were divided into deposit feeder and suspension feeder. A z-test was used to test the significance of the extinction and origination rates between different habitats and lifestyles of bivalves. Meanwhile, the Holm-Bonferroni method was performed to correct type1errors when performing multiple pairwise comparisons. The selectivity of survivorship is also evaluated by using multiple logistic regression.At the genus level, bivalves show an extinction rate of50%in the first phase of the Permian-Triassic crisis, though only9.1%in the second phase. Although bivalves seemed to have suffered moderately at the genus level during the Permian-Triassic crisis, high species extinction and origination rates might explain why bivalves in the Early Triassic were so different in appearance from those of the Late Permian. In comparison with the high extinction rates of brachiopods and foraminifers, the second crisis should not be a significant catastrophe for bivalves at least in South China. It seems that the genus extinction rate of bivalves in the second crisis is characteristic of the background extinction. This might be due to the big extinction of bivalves during the first phase of the end-Permian mass extinction and the physiological advantages of surviving bivalves in the aftermath of the first crisis. There appears to be no selection of extinction in life-style, feeding type or habitat among bivalves from South China through the crisis. There is no significant difference in the extinction rates between epifaunal and infaunal bivalve genera in all environments. Suspension feeders showed moderate extinction rates, while no deposit feeders went extinct at the genus level. In addition, infaunal bivalves were not at more risk of extinction than epifaunal forms. Among all those environmental factors, only ocean anoxia and high seawater temperature could have led to the selective extinction among bivalves with different ecological traits. However, the anoxia and high seawater temperature likely had opposite effects on selectivity of survivorship. Infaunal bivalves and the bivalves in deeper water could have been more affected under anoxia environments, while epifaunal bivalves and the bivalves in shallower water would have been worse off under high seawater temperatures. In this study, there was no significant ecological selective extinction among bivalves during the Permian-Triassic mass extinction, which implies that this overall lack of selectivity of extinction perhaps reflects the combination of different environmental factors. Consequently, we propose that a combination of marine anoxia and high seawater temperatures could have been contributed to the extinction among bivalves during the Permian-Triassic crisis.After the quantitative analysis of all reported Late Permian to Middle Triassic bivalve genera all around the world, we found that there were105genera of bivalves globally during the Changhsingian age prior to the end-Permian mass extinction,22genera in the mixed bed,40genera during the Griesbachian after the second phase of Permian-Triassic crisis,30genera during the Dienerian,43genera during the Smithian,58genera during the Spathian, and95genera during the Anisian (Middle Triassic). All the bivalves belong to three subclasses and five lifestyles showed this similar pattern. The diversity curves of bivalves infer that the recovery of bivalves was delayed to Anisian, during when the genus diversity nearly increased to that of the Changhsingian prior to the end-Permian mass extinction. In addition, Permian survivors including long-term survivors, holdover species, and Lazarus taxa dominated the bivalves during the Early Triassic, but not in the Anisian (Middle Triassic). The extinction rates of bivalves during the second phase of Permian-Triassic crisis, Smithian-Spathian event, Spathian-Anisian event were13.6%,14%, and20.7%, respectively at the genus level, and all equal to the background value, indicating that bivalves did not suffer greatly during these minor extinctions. Also, there was no difference between the extinction rates of epifaunal and infaunal bivalves. This may have been resulted from the higher tolerance of bivalves to severe environments due to their physiological advantages, or caused by multiple environmental factors.After the end-Permian mass extinction, the Early Triassic bivalve Genus Claraia diversified rapidly into more than80new species and subspecies in South China in～2My. Quantitative analysis of all reported Late Permian and Early Triassic Claraia species in South China show six species in Changhsingian age strata,20species in the mixed bed,47species in Griesbachian age strata,59species in Dienerian age strata,11species in Smithian age strata, and no reliably reported species in Spathian and later age strata. An examination of the environmental distribution of Claraia indicates that the genus moved from offshore to onshore environments during the Permian-Triassic crisis, and then retreated offshore during the Smithian. Claraia was a significant crisis progenitor during Early Triassic in South China, and its flourishing was likely due to its physiological characteristics. Claraia lived an epibyssate mode of life, and usually lay on soft substrates with its right valve. The global distribution of Claraia was probably related to its planktonic larval stage. Claraia might also have hosted chemosymbionts and could have survived in dysoxic to anoxic waters. The retreat of the expanded oxygen minimum zone (OMZ) may have contributed to the fall of Claraia.