Late Ordovician Brachiopods From Northern South China And Northwestern Tarim Basin

Two Late Ordovician shallow water brachiopod faunas were found in Xichuan County of Henan Province,along the northern margin of the South China paleoplate,and Mengdaleke of Wushi County,northwestern part of the Tarim Basin.Through detailed laboratory preparation and serial sectioning,28 species are recognized that belong to 25 genera,16 families,15 superfamilies,9 orders,and 4 classes.They are Acrosaccus sp.,Paracraniops sp.,Strophomena sp.,Strophomena?sp.,Strophomenoidea gen.et sp.indet.,Christiania sp.,Leptellina?sp.,Ishimia sp.,Sowerbyella(Sowerbyella)sinensis Wang 1964,Plectambonitoidea gen.et.sp.indet.,Triplesia zhejiangensis Liang in Liu et al.1983,Skenidioides sp.,Dolerorthis sp.,Dinorthis kassini Rukavishnikova 1956,Pionorthis?sp.,Bokotorthis cf.minuta Popov and Cocks 2014,Zhejiangorthis shiyanheensis sp.nov.Zhang 2021,Wangyuella lineata sp.nov.Zhang 2021,Rostricellula xichuanensis Xu 1996a,Altaethyrella inflata(Xu 1996a),Altaethyrella seletensis(Nikitin et al.2003),Rongatrypa xichuanensis(Xu 1996a),Xichuanatrypa sigangensis(Xu 1996a),Eospirigerina cf.pennata(Rukavishnikova 1956),Qilianotryma suspectum(Popov in Nikiforova et al.,1982),Schachriomonia dichotoma(Fu 1982),Schachriomonia wushiensis sp.nov.Zhang 2021,and Eospirifer(Protospirifer)sp.Among these fossils,a new genus,Xichuanatrypa under the subfamily of Clintonellinae Poulsen 1943,is established with the type species Nalivkinia(Nalivkinia)sigangensis Xu 1996.Three species(i.e.,Zhejiangorthis shiyanheensis sp.nov.Zhang 2021,Wangyuella lineata sp.nov.Zhang 2021,and Schachriomonia wushiensis sp.nov.Zhang 2021)are described as new taxa.Brachiopod faunas in Xhichuan of Henan can be subdivided into the Sowerbyella-Xichuanatrypa assemblage,the Rongatrypa-Altaethyrella assemblage,and the Qilianotryma-Schachriomonia assemblage.These faunas indicate that these regions had the paleotropical,shallow marine environment(corresponding mainly to BA3)during the Late Ordovician.Brachiopod fauna from Wushi of Xinjiang is dominated by atrypid Schachriomonia wushiensis sp.nov.and rhynchonellid Altaethyrella seletensis(Nikitin et al.,2003)with several orthid and strophomenid taxa,indicating a shallow marine environment(corresponding mainly to BA3).In taxonomic composition,these faunas are similar to those from JCY area of Zhejiang and Jiangxi,Beiguoshan of North China,the Qilian Mountain of Qinghai,the Kazakhstan terranes,and eastern Australia.A newly defined growth metric is proposed here as a quantitative index for brachiopod allometry study,calculated by a newly developed equation herein.Through linear regression,Rongatrypa xichuanensis shows an allometric growth pattern through ontogeny.The shell length shows a negative allometry(a=0.8840),the shell width an isometry(a=1.0113),and the shell depth a positive allometry(a=1.1623).The internal structures of this species also show a considerable variation through its growth.The linear regression of morphometric measurements in Sowerbyella(Sowerbyella)sinensis shows a slightly negative allometry of the shell length(a=0.9583),and a weakly positive allometry of the shell width(a=1.0762).Parasitic drilling has been recognized on a brachiopod dorsal valve of Rongatrypa xichuanensis by serial sectioning.Detailed acetate peel replication and 3-D modeling indicate that the drilling may have been created by a parasitic platyceratid gastropod,providing information on the complex marine ecosystem during the Great Ordovician Biodiversification Event(GOBE).Paleozoic brachiopod parasitic traces are suggested to be subdivided into two types:a constructive association and a destructive association.Brachiopod faunas from Xichuan of Henan and Mengdaleke of Wushi show that few endemic genera(or specises)survived there,while the number of cosmopolitan-distributed genera is a little more than that of regional-distributed genera.Regional geological evidence indicates that these regions had limited spatial and temporal differentiation for brachiopods to survive during the Late Ordovician.The environments of these regions are very similar to the "island-like marine environment".Brachiopod faunas in this study are characteristic of high immigration rates,low speciation rates,and the high competition stress among species.The macroevolution pattern here is highly affected by the paleogeography and the paleoenvironment,which is worthy of further study in the future.