Palaeontology,Palaeoecology And Palaeobiogeography Of The Late Devonian And Early Carboniferous Ostracoda From The Western Junggar,NW China And South China

Ostracoda constitute one of the most widespread and diverse groups of crustaceans since the Early Ordovician with abundant fossil species.Devonian and Carboniferous are prosperous periods of ostracods,most of which have been discovered from coastal lagoons,neritic carbonate platform to the deep sea.Ostracods are important in stratigraphic division and correlation and offer special insights into palaeoenvironmental and palaeogeographical reconstruction.This study focuses on biostratigraphy,palaeoecology,ontogeny and generic evolution and responses to the F-F(Frasnian-Famennian)event of Late Devonian-Early Carboniferous ostracods based on abundant materials collected from seven sections in the western Junggar and South China.Quantitative data on global ostracods from the Frasnian to Famennian,have been reviewed as to their geographic distribution,migration and evolution at a generic level,to find out how ostracod faunas responded to plate tectonics,climate changes and bio-environmental events in the Late Devonian.The main conclusions and progressions are noted as follows.In total,816 ostracods samples with a total weight of more than 1000 Kg were collected from the studied sections in western Junggar and South China.About 15000 specimens were obtained in this work and about 9000 carapaces were well-preserved.145 species belonging to 59 genera were recognized.Among them,one genus(Wangshangkia n.gen.)and 10 species are new,namely,Pribylites wulankeshunensis sp.nov.,P.junggarensis sp.nov.,Coelonella xizhunensis sp.nov.,Bairdia shaerbuertiensis sp.nov.,Wangshangkia dushanensis n.gen.n.sp.,Wangshangkia bailouensis n.gen.n.sp.,Microchelinella bulongourensis sp.nov.,M.hoxtolgaiensis sp.nov.,Arcuaria hebukesarensis sp.nov.and Cribroconcha honggulelengensis sp.nov.The age implications of some typical pelagic ostracods from South China were discussed.Two ostracod assemblage zones and two subzones were recognized from Lower Member of Hongguleleng Formation in western Junggar,i.e.,Hollinella valentinae –Samarella coumiacensis Zone and Microcheilinella hoxtolgaiensis –Praepllatina adamczaki Zone,the latter can be divided into lower and upper subzones.A few Frasnian ostracods were found in the base of the Hongguleleng Formation and suggest that the F-F boundary might be present in the Lower Member.The ostracod faunas from the Yangdi section and Nandong section are indicative of environments of carbonate platform to slope,and basin,respectively.Based on ostracod assemblages from the Lower Member of the Hongguleleng Formation,three ecological assemblages of ostracods in the open oceanic island arc are summarized,i.e.,OA1(Hollinoidea-dominated assemblages),OA2(Bairdioidea-dominated assemblages)and OA3(Bairdiocypridoidea-dominated assemblages),which suggest sedimentary environments of foreshore,nearshore and offshore,and offshore transition,respectively.The ecological assemblages of ostracods from island arc are complex and diverse,which is different from those in the cratonic blocks(such as in South China).Ostracods in different environments respond to the F-F event differently.For example,58 percent of the benthic ostracod species disappear in the Yangdi section,whereas most benthic ostracods species actually survive the F-F crisis in the Nandong section with the extinction of pelagic entomozoacean.The rate of extinction of benthic ostracods decreased with increasing water depth in the F-F event,which caused substantial losses among pelagic entomozoacean lineages.The abundance and diversity of ostracods decreased in anoxic environment and where they were rapid changes of paleotemperature during the F-F boundary in South China.The rate of filter-feeding ostracod faunas increased in the F-F boundary(i.e.,filter-feeders signal,FFS),which implies that the filter-feeders have a strong ability to survive the abnormal environment.Therefore,it is hypothesized that assemblages dominated by filter-feeders are primarily a signal of sea-level changes.This paper applied quantitative multivariate analysis to study the ontogeny and functional morphology of Podocopina Cribroconcha honggulelengensis sp.nov.Shape change of the carapace during ontogeny includes fast height development,less overlaps and extended pit-field.Qualitative and quantitative methods were used to study the evolution of the genus Cribroconcha and two evolutionary lineages(i.e.Lineage I and Lineage II)were summarized.Lineage I originated from the Euramerican plate with rapid evolution,which can be divided into three evolutionary patterns in the Tournaisian.Lineage II arose from the Kazakhstan plate with lower evolutionary speed than Lineage I.Overall Cribroconcha evolved from spine-absent to spine-developed at the valves.Cribroconcha broadens environmental distribution from shallow water to deep basin during ontogeny.The morphological progression and ecological development of Cribroconcha in the Devonian-Carboniferous Paleotethys may be related to an invasion of cold deep-water currents that originated in the glacial north-Gondwanian area.A global Late Devonian ostracod database is constructed here incorporating new materials from the western Junggar and South China.Four palaeobiogeographical units(Cathaysia,North America,Europe and peri-Gondwana)are recognized for the Frasnian and five palaeobiogeographical units(Cathaysia,North America,Europe,Siberia and Australia)for the Famennian.Three controlling factors(climatic zonation,geographical isolation and global sea-level changes)are identified to have played roles in shaping the palaeogeographical regionalization of ostracod faunas in the Late Devonian.The ostracod palaeobiogeography in the Frasnian was mainly influenced by climatic zonation while rapid changes in tectonic configuration in the Famennian drastically altered the global palaeobiogeography of ostracods.The palaeobiogeographical regionalization of ostracod faunas suggests that Laurussia and Gondwana continued to draw near during the Late Devonian,with the first collision occurring in Southern Central Europe in the Famennian.The South China plate drifted northward to the Kazakhstan plate away from the Australian plate,which gradually became isolated during the Famennian.For the South China plate,one ostracod assemblage migrated northward through the Kazakhstan plate and spread along the Ural Ocean margin,with a few ecological generalists continuing to move westward to the Laurussia;the other assemblage migrated southward to the Australian plate.Some ostracods in North America moved northward via the Ural Ocean to Siberia while a larger group headed southward across the Rheic Ocean to North Africa and Europe in the Frasnian.However,in the Famennian the main migration paths changed for the narrowed Rheic Ocean,and most ostracods migrated northward and dispersed around the Ural Ocean.