Study On The Bivalve Faunas In Southwestern China During The Permian-Triassic Transitional Time

The Permian-Triassic(P-Tr)mass extinction event has not only resulted in the largest decline in marine biodiversity through the Phanerozoic history,but also an abrupt ecological switch of marine ecosystem,which laid the foundation for modern marine biodiversity and ecosystems.The major turnover of biota and marine ecosystem is mainly reflected in the transformation of dominant organisms from brachiopods into mollusk bivalves in the marine benthic ecosystem.Therefore,systematical study on the switch from brachiopods to bivalves during the P-Tr transitional time is a key to fully reveal this event,especially the evolution and rising of bivalves opened the prelude to the establishment of the newly evolved marine biota and ecosystem.Hence,this study focuses on bivalves from the P-Tr transitional beds as well as the Lower Triassic,and tries to understand the evolutionary pattern of the bivalve faunas in both taxonomic and ecological composition during the critical time.Previous researches have shown that the eastern margin of the Kangdian Oldland on the Upper Yangtze Block,southwestern China yields rich bivalve fossils in the P-Tr transitional beds and Lower Triassic,with relatively successive evolutionary sequences,which could reveal the evolution of marine benthic communities and ecosystems during the great transitional time.Therefore,this study has chosen some typical P-Tr sections from the region and tried to study their bivalve fossils,including the temperal and spatial evolution of bivalves during the P-Tr transitional time(transitional beds)and the early Early Triassic(Feixianguan Formation).In meantime,31 species from 10 genera of bivalves have been checked and redescribed systematically.A total of 5,351 bivalve specimens are collected from the transitional beds of five P-Tr boundary sections from the marine-terrestrial transitional facies and littoral-neritic facies in the southwestern China.13 species and 20 genera have been identified,including two new species,i.e.Gujocardita kelensis sp.nov.,Astartella guizhouensis sp.nov.,and one species have been revised as Pteria variabilis.According to the locations on the paleogeographic map,the five sections can be subdivided into three types,which are landward marine-terrestrial transitional facies(Chinahe section and Jinzhong section),paralic marine-terrestrial transitional facies(Jinjibang section)and the littoral-neritic facies(Kele section and Wadu section).According to the distribution of the bivalve fossils in the sections,a bivalve assemblage zone could be established from the transitional beds in the studied region,namely Pteria variabilis-Promyalina schamarae-Neoschizodus orbicularis assemblage zone,which is consistent with the bivalve assemblage zones widespread in South China.However,the bivalves in the assemblage zone also show a certain variation of vertical distributions in each section.Further analyse using Cluster analysis indicates that bivalves in the assemblage zone could be clearly distinguished into two groups in each section,and the lower and the upper groups of each section have high similarities respectively.Thus,they are grouped into two sub-assemblage zones statistically.Since these bivalve fossils were preserved nearly in situ,the bivalves in the two sub-zones could be regarded as two different bivalve communities.Therefore,the P-Tr transitional bivalve fauna can be divided into two communities,namely Promyalina–Neoschizodus community(early community)and Pteria community(late community).The F test suggests that the two communities are significantly different.The Promyalina– Neoschizodus community has Permian holdover taxa,long-term survivors and newly evolved taxa,high taxonomic diversity and evenness,and more abundant mobile infaunal guilds,while the Pteria community lacks the Permian holdover taxa but has lower diversity and evenness,more abundant stationary epifaunal guilds.From the Promyalina–Neoschizodus community to the Pteria community,the diversity and evenness declined,the ecological guilds decreased,the stationary epifaunal bivalves increased and the mobile infaunal reduced.Both Promyalina–Neoschizodus community and Pteria community can be subdivided into three(geographic)subcommunities according to the depostional facies of the five sections.Cluster analysis suggests that the three subcommunities of the Promyalina–Neoschizodus community differed greatly,while the similarity among the three subcommunities of the Pteria community was extremely high,suggesting that the Promyalina–Neoschizodus community develops a strong spatial differentiation,while the Pteria community has no distinct spatial variation.The composition,diversity and ecological guilds of the Promyalina-Neoschizodus community are similar to those of the Paleozoic bivalve communites prior to the the end Permian mass extinction,so it should belong to the Late Permian,while the characteristics of the Pteria community are analogous to the bivalve community dominated by eurytopic "disaster taxa" in the Early Triassic after the earliest Triassic mass extinction,and it is highly homogeneous to the Eumorphotis venetiana-Towapteria scythica-Pteria ussurica variabilis assemblage of an earliest Triassic age in the upper part of transitional beds in Huangzhishan section from Huzhou,Zhejiang Province.Therefore,the Pteria community should be the earliest Triassic in age.In summary,the Permian-Triassic boundary of the clastic facies in the southwestern China should be located between the PromyalinNeoschizodus and the Pteria communities.In addition,the diversity of the trantional bivalve communities in the clastic facies in southwestern China is quite high by comparing with other early Triassic bivalve faunas,suggesting that bivalve recovery occurred immediately after the end Permian mass extinction.In order to understand the development history of the Early Triassic bivalves,this study selected five Lower Triassic sections in the eastern margin of Kangdian Oldland,which yield rich bivalve fossils.7,350 bivalve fossils belonging to 29 species in 13 genera were collected from the Feixianguan Formation(excluding the transitional beds).These five sections were deposited in the clastic facies(Zhonghe section)and the carbonate platform facies(Changzheng section,Shikanzi section,Shiba section and Mayigou section)on the paleogeographic map,respectively.According to the distribution of bivalve fossils,several bivalve zones can be recognized at each section: Claraia wangi zone in Zhonghe section;Claraia wangi zone,C.stachei zone,C.aurita zone,Eumorphotis multiformis zone and Entolium discites zone in Changzheng section;Claraia wangi zone,C.cf.yunnanensis zone,C.clarai zone,C.stachei zone,C.aurita zone and Bakevellia subpannonica zone in Shikanzi section;Claraia wangi zone and C.stachei zone in Shiba section;Claraia wangi zone,C.anulata zone,Eumorphotis multiformis zone,Leptochondria albertii zone and Austrotindaria antiqua zone in Mayigou section.All the five sections have Claraia wangi zone in the lowest Triassic,so the bivalves from the Claraia wangi zone can be correlated among thses sections.In order to further understand the eco-geographical differentiation of the earliest Triassic bivalves,the bivalves of the Claraia wangi zone from the five sections are clustered,and the results show that they are clearly grouped into two types corresponding to the depostional facies perfectly.Combined with the Claraia wangi zones widespread in South China in different facies,it can also be noted that the bivalve assemblages in Claraia wangi zones do have significant differences in various facies.Among which,the Claraia wangi zone in the carbonate platform facies has the most species,highest diversity,lowest dominance,most diversified guilds,most stationary epifaunal and moderate mobile infaunal.Correspondingly,the basin facies have the lowest diversity,fewest species,highest dominance,stationary epifaunal guilds with highest dominance and lowest diversity,and mobile infaunal disappeared.Previous studies indicated that the second episode of the P-Tr mass extinction in the earliest Triassic had no significant effect on the diversity of bivalves.However,from the Pteria variabilis community in the upper part of the transitional beds to the communities dominated by Claraia wangi after the second epidode of the mass extinction,the taxonomical and ecological compositions of bivalve faunas changed greatly: the dominant species changed from Pteria variabilis to Claraia wangi,many new species occurred,effective diversity increased distinctly,dominance decreased greatly,and the most diverse habitat was transferred from the clastic facies to the carbonate platform facies.Based on the bivalve distributions,the bivalve biostratigraphic successions of the Feixianguan Formation in the studied region can be summarized as Claraia wangi zone,C.stachei zone,C.aurita zone and Eumorphotis multiformis zone in ascending order,which is the same as the bivalve succession previously established in South China.Therefore,these bivalve zones should belong to late Griesbachian(Claraia wangi zone and Claraia stachei zone),end Griesbachian-early Dienerian(Claraia aurita zone),late Dienerian-early Smithian(Eumorphotis multiformis zone).Among them,the Claraia wangi association has abundant species,high diversity and diverse ecological groups,suggesting that bivalves started to recover in the late Griesbachian.The diversity of Claraia stachei association declined greatly,which indicates that the recovery was restrained.During Griesbachian-Dienerian,the diversity of bivalves increased significantly again,indicating that the bivalve recovery resumed at this time.The significantly reduced diversity in Dienerian indicates that the recovery was again inhibited.In conclusion,the latest Permian Promyalin-Neoschizodus community from the lower transitional bed has high diversity,suggesting that bivalves began to recover immediately after the first episode of the P-Tr mass extinction.Moreover,bivalve faunas changed greatly into the communities dominated by Claraia wangi from Pteria community after the the second episode of extinction.Besides,the diversity increased remarkably to the new community,suggesting that the initial recovery did not stop by this event.However,the initial recovery was inhibited in late Griesbachian.During Griesbachian-Dienerian the diversity of bivalves increased again,sussgesting that the recovery started again.However,the recovery was depressed in Dienerian again.The two stagnation of the recovery are most likely related to the harsh and unstable environment in the Early Triassic.In addition,both of the the Griesbachian-Dienerian recovery event and the Dienerian crisis of bivalves are also recorded in Far East of Russia,western United States and northern Italy.These events indicate that although bivalves are more tolerant to the variation of seawater temperature and redox condition,the diversity and ecology of the Early Triassic bivalve faunas expressed a remarkable reaction to the violent variations in seawater temperature and redox condition.