Response Of Ostracod Assemblages From Microbialites In South China To The End-Permian Mass Extinction

The end-Permian mass extinction was the largest biotic crisis in the geological history,which caused the disappearance of more than 80% of marine species on the earth.The causes of this extinction event are multifaceted and comprehensive.It is generally accepted that the Siberian Large Igneous Province triggered forest wildfires,and the massive emission of greenhouse gases(like methane)and toxic gases,which firstly led to the collapse of the terrestrial ecosystem.And then,global warming was accelerating,and ocean acidification,poisoning,and the expansion of oxygen minimum zone have led to the collapse of marine ecosystems.Although there have been relatively approved causes and mechanisms of extinction events based on a great number of evidences,there may be significant differences in the responses of different taxa,and even the same taxon under different environments,to mass extinctions.Not only the environmental control factors,we should also focus on the co-evolution between specific taxa and the environment,so that more details of the adaptive evolution of organisms in the extinction and following survival can be revealed.As the Permian mass extinction began,the biomass reduced sharply,and most of the taxa disappeared in the fossil record which makes it difficult to obtain the details of the extinction and survival process.Fortunately,a large number of ostracod fossils are found in the Permian-Triassic microbialite(PTBM)section.Ostracoda has so unique advantages in abundance and distribution that it is the most common metazoan in the microbialdominated ecosystem after the mass extinction,which provides sufficient and important study materials for paleontological and paleoecological analyses during the PermianTriassic transition.The materials in this paper are mainly collected from 4 PTBM sections in different paleogeographical locations in South China(Zuodeng and Pojue in Guangxi Province,Zhaixia in Hubei Province and Panjiazhuang in Zhejiang Province),and totally282 samples,3167 well-preserved ostracod specimens were collected,which were identified as 27 genera and 139 species.In addition to the traditional systematic taxonomy description,several common species with stable traits have been described by the Right Triangle Method.Combined with previous reported taxonomic data,the Permian-Triassic ostracod faunas in South China are dominated by smooth-shell ostracods,especially specimens of Bairdiidae,which have a distinct advantage in different paleogeographical locations and indicate that these microbialites were developed on the shallow open platforms with normal oxygen content.Although the ostracods in the survival interval still maintained a high abundance and diversity,from the perspective of evolution,the faunas have not changed substantially,because of which mainly are dominated by Paleozoic affinities who survived the mass extinction and Meso-Cenozoic affinities have not evolved significantly.Traditional ostracod paleoecological groups at the level of family/superfamily can indicate large-scale sea level changes.For example,PTBM generally formed in shallower water than its underlying bioclastic limestones.However,the traditional paleoecological group is difficult to reflect detailed eustatic changes in the sedimentary environment within the microbialite interval.But dominant species groups have this advantage.From the research sections,the variations in the groups of dominant species has been identified,and the general change is from the groups of Bairdia or typical Paleozoic genera in the lower part of microbialite interval to the groups of multiple dominant species coexisting in the middle part of the microbialite interval,and finally to a group of Liuzhinia antalyaensis and Bairdiacypris ottomanensis,which indicates that the water environment gradually tends to stabilize from the turbulence after the mass extinction.Four survival species widely distributed in the study section and crossing the mass extinction boundary were selected for a comprehensive analysis of the body size.Based on discrimination of ontogenetic series,data on their instar structures,juvenile growth rate and body size of each instar were obtained.The study found that most of the ostracod species were affected by the rapid warming during the Permian-Triassic transition,which is reflected by the abnormal change of the body size and growth rate.Nevertheless,the direction and amplitude of the changes were not the same,and the responses of the same species from different lithofacies might be different.This shows that different species or aquatic environment are not only affected by the mass extinction to different degrees,but also ostracods may have different active adaptation capabilities and survival strategies in the face of disasters.Therefore,the study on the body size changes of organisms during the extinction should be limited to the intraspecific changes in the size as much as possible.The response of organisms to the environment is not completely passive,adapting or even modifying the environment through self-regulation is necessary for long-term reproduction of species.A total of 410 species of ostracod profile data were collected for geometric morphometric analysis from the ostracod faunas from our study sections and other contemporary microbialite sections with published SEM photos of ostracods.The morphospace and morphological disparity of ostracods during Permian-Triassic was presented for the first time,and the decoupling in morphological disparity and taxonomic diversity during the end-Permian mass extinction,and their differences between varied lithofacies in the survival interval were detected.At the beginning of the mass extinction,the morphological disparity of the ostracod faunas from microbialite lithofacies decreased significantly in the Paleo-Tethys.The morphological convergence in ostracods may indicate the restriction of environmental pressure.Since then,low morphological disparity of the ostracod faunas continued until it fell again during the second pulse of extinction in the Early Triassic,but the magnitude was significantly reduced,probably because the ecosystems in these areas had been collapsed at the beginning of the endPermian mass extinction.Persistent environmental pressure not only delayed the recovery of organisms and the reconstruction of the ecosystem,but also resulted in an ecosystem with not much room for further deterioration in the earliest Triassic.However,the disparity changes were just the opposite in the Panjiazhuang section in Zhejiang Province,which was affected by the current from Panthalassa.At the end of the Permian,the ostracod disparity declined slightly during the first pulse of extinction,but a greater reduction of disparity occurred during the second pulse extinction.At the beginning of the mass extinction,the environmental deterioration,like hypoxia and acidification,was possibly weakened by fresh ocean currents.However,the paleoecological crisis became obvious by the continuous environmental pressure in the Earliest Triassic.Comparing the taxonomic and morphological information of ostracods from the microbialite layer and non-microbialite interlayer in the same section,the taxonomic diversity in the microbialite fabric is significantly higher than that of the non-microbialite fabric.However,the difference in disparity between the two is very subtle,and the ostracods outside of the non-microbialite morphospace do not uniformly expand in a certain morphological direction.In addition,the proportion of Paleozoic affinities is obviously lower than that of the whole fauna.If it is an in-situ community,it may indicate that the internal environment of the microbial mat has nothing to do with the prosperity of ostracods during the mass extinction.Moreover,the ostracods exclusively occurring in microbialite morphospace account for a higher proportion of filter-feeding species,which contradicts the idea of eutrophication led by microbial flourish.Therefore,it indicates that these ostracods were probably not living in the microbial mats but transported from nearby.Even though some deposit-feeding ostracods might be fed by microbial mats,there is still not related trace fossil found in the microbialites until now.For all these reasons,we argue that microbial mats might simply have enhanced fossil preservation,rather than providing an essential oxygen and food source for benthic metazoans like ostracods.