Evolution Of The Quaternary Cambrian Trilobites In Eastern Guizhou And Its Stratigraphic Significanc

Oryctocephalid trilobites are globally widespread in Cambrian period and are of great importance in Cambrian chronostratigraphy.The common base of the Cambrian Miaolingian(Series 3)and Wuliuan(Stage 5)are defined by the FAD(first appearance datum)of the oryctocephalid trilobite Oryctocephalus indicus.Until now,researchers suggest that the oryctocephalid trilobites Oryctocarella duyunensis or Arthricocephalus chauveaui could be potential marks of the Cambrian Stage 4,but this idea is still in debate.The underlying cause is that the lowest horizon of a fossil species or genus in the different regions is usually considered as isochronous,without taking temporal differences caused by factors such as biogeographic dispersal into account.to Carrying out a comprehensive study including phylogeny and biogeography on all oryctocephalid trilobites preserving complete evolutionary sequence provides opportunities to solve this problem.Oryctocephalid trilobites reach a high level in taxonomic diversity,abundance,and preserving quality in eastern Guizhou Province,China.There are three subfamilies,24 genera and 46 species in total.Two subfamilies,12 genera and 16 species are from Cambrian Stage 4.Therefore,Jiaobang section and Songshan section from Jianhe County and Huanglian section from Songtao County were ideal places for carrying quantitative morphological studies and phylogenetic analysis of the oryctocephalid trilobites(quantification of intraspecific and interspecific variation and intergeneric differences are important for the selection and coding of morphological characters in phylogenetic analysis).However,quantitative studies require a large number of samples as well as related genera and species.The only trilobites of the oryctocephalid trilobite(Cambrian Stage 4)that fulfill both conditions are Balangia and Duyunaspis.A typical case for quantifying intraspecific and interspecific variation and intergeneric differences of oryctocephalid trilobites was established through geometric morphometric analysis of Balangia and Duyunaspis.Based on this,Arthricocephalus chauveaui,Oryctocarella duyunensis and Protoryctocephalus wuxunensis,which have a large number of specimens,were analyzed for intraspecific or interspecific variation.In conjunction with the quantitative morphological results described above,20 morphological traits were selected for phylogenetic analysis of 17 species of 14 genera of oryctocephalid trilobites.The phylogenetic results show that Panguaspis yuanlongensis n.gen.n.sp.is a transitional species in the evolution history of the oryctocarid trilobites to the oryctocephalid trilobites,and thus the two subfamilies can be considered as a monophyletic group.In addition,the evolutionary relationship between Balangia and Duyunaspis was verified through evolutionary changes in the life cycle.The taxonomic diversity of the oryctocarin trilobites in Jianhe County,Guizhou Province reaches the highest level in the lower part of the Balang Formation(lower part of the Arthricocephalus chauveaui Zone)and decreases significantly in the Tsinghsutung Formation(upper part of the Arthricocephalus chauveaui Zone),which may be related to the fact that the environment is not suitable for the survival of the oryctocarin trilobites due to the decline in sea level.The species taxonomic diversity of the oryctocephalin trilobites remains low in the Balang Formation and the Tsinghsutung Formation(Oryctocarella duyunensis Zone – Protoryctocephalus wuxunensis Zone)and begins to increase(Bathynotus kueichouensis – Ovatoryctocara sinensis Assemblage Zone)and peaks(Oryctocephalus indicus Zone)in the early Wuliuan Age(Kaili Formation).Based on global paleogeographic distribution of oryctocephalid trilobites,this thesis finds that changes in taxonomic diversity in species level of oryctocephalin and oryctocarin trilobites from the early Cambrian Age 4 to the early Wuliuan age are consistent with that of Jianhe area,with the former gradually increased and the latter gradually decreased.Based on the evolutionary trends in biodiversity,in this thesis,the stratigraphic extension of the oryctocephalid trilobites(except for the subfamily Cheiruroidinae)is divided into three stages:(1)lower part of the Stage 4(Oryctocarella duyunensis Zone – Arthricocephalus chauveaui Zone);(2)upper part of the Stage 4(Protoryctocephalus wuxunensis Zone – Bathynotus kueichouensis-Ovatoryctocara sinensis Assemblage Zone);(3)lower part of the Wuliuan Stage(Oryctocephalus indicus belt).Combining the palaeogeographic distribution,stratigraphic occurrences of fossils,biodiversity and oceanic flow,it can be assumed that oryctocephalid trilobites(except for the subfamily Cheiruroidinae)originated in southern China(early Age 4,Cambrian)and spread along the South China – Siberia and Greenland – North America(or Newfoundland and Morocco)– North China and Australia – India and Antarctica line from the late Age 4 to the Wuliuan Age.Based on the occurrence horizon and intraspecific variation of A.chauveaui and O.duyunensis in eastern Guizhou,here we suggest that their earliest global fossil records are located from the Balang Formation of South China,and that their fossil records in the Henson Gletscher Formation of Greenland are isochronous with Tsinghsutung Formation which is the overlying strata of Balang Formationin Jianhe,confirming that it is not as high as previously thought in appearance horizon.In addition,the analysis showed some exchanges between A.chauveaui from the Henson Gletscher(Greenland)and Tsinghsutung(Jianhe)formations,and relatively little exchange in O.duyunensis.By comparing the biostratigraphy of oryctocephalid trilobites from South China and Greenland,the Eoagnostus roddyi-Oryctocarella duyunensis Zone(Greenland)corresponds to the top of the Arthricocephalus chauveaui Zone to the Protoryctocephalus wuxunensis Zone(South China).Due to the lack of single species with global distribution,international correlation of the Cambrian Stage 4 has been difficult.The accurate stratigraphic correlation between South China and Greenland,combined with Redlichia and archaeocyatha,which can provide information for the establishment of a global accurate stratigraphic correlation framework.In summary,this thesis suggests that the FAD of Oryctocarella duyunensis is suitable and feasible to define the base of the Cambrian Stage 4,and proposes that the FAD of Protoryctocephalus wuxunensis can be used as the base of the substages of the Cambrian Stage 4,which can not only enhance the global comparison of the Cambrian Stage 4,but also provides insights into the evolution of the oryctocephalid trilobites.