Study On Ontogeny And Biostratigraphy Of The Oryctocephalid Trilobites From Balang Formation Of Cambrian,Guizhou Province,china

Oryctocephalid trilobites play an important role in the division and correlation of the biostratigraphy(Stage 4,Series 2 of the Cambrian).Oryctocephalid trilobites constitute an the important part of the biological assemblage of the Balang Formation.On the basis of previous studies,through extensive work and laboratory examination,it was confirmed that Oryctocephalid trilobites were differentiated into 7 genera and 9species including a new genus and two new species as follows: Arthricocephalus chauveaui Bergeron,1899,Arthricocephalites xinzhaiheensis Chien et Lin in Lu et al.1974,Arthricocephalites intermedius Zhou in Lu et al.1974,Changaspis elongata Lee in Chien,1961,Blangia balangensis Chien,1961,Duyunaspis duyunensis Zhang et Qian in Zhou et al.,1977,Longaspis paiwuensis Peng et al.,2018,Duyunnaspis lazizhaiensis sp.nov.,Jianheaspis jiaobangensis gen.et sp.nov.However,A.Chauveaui,Ar.intermedius and C.elongata are widespread globally,facilitating the biostratigraphic division and comparison.The establishment of biostratigraphy of the Balang Formation depends on the taxonomic correctness of trilobites.The study on the ontogeny of trilobites is helpful for taxonomic determination and provides important evidence for the developmental biology of the trilobites in the early Cambrian.Based on 775 complete dorsal exoskeletons of A.chauveaui,Ar.intermedius and C.elongata,the traditional measurement method and the geometric morphological measurement method were adopted.Combined with the quantitative analyses used by foreign scholars on the development of trilobite segment,many ontogenetic changes in morphology and visualized patterns of segment growth were revealed.As a reasult,the absolute and relative growth rates of the four species showed a gradually increasing from the first segment to the last segment,that is,the absolute and relative growth rates of the newly added thoracic segments gradually increased,but the growth rates of different species showed different growth trends.The four species showed the diversity of development models.The growth control for the developing trunk of the trilobites showed that a regional control,with segment growth depending on their relativeposition along the main axis.Ar.intermedius formed from the Balang Formation is also present northern Greenland,thus it is an important species for the biostratigraphic subdivision and correlation.In order to better determine their biological attributes,with the help of results derived from quantitative analysis on ontogeny,the classification of Ar.xinzhaiheensis and Ar.intermedius was again drawn up,and the difference between the two species was confirmed as interspecific difference,while not intra-species difference caused by the ontogeny.The quantitative analysis is used to solve the problem of system classification,which provides a new approach and valuable experience for the solution of similar disputes in system classification in the future.The Balang Formation in the eastern and northeastern Guizhou Province is complete and located in the Stage 4,Series 2 of the Cambrian.The Oryctocephalid trilobites have relatively complete stratigraphic sequences,which may have the potential to become the global boundary stratotype or biostratigraphic correlatio.This study extends the biostratigraphic correlation section.Through the investigation and study of the complete xiangshuidong section,Songtao county in northeast Guizhou.The distribution and elongation of A.chauveaui,Ar.intermedius,C.elongata and D.duyunensis of the Balang Formation were summarized.The research show that C.elongata and Ar.intermedius appear later than A.chauveaui.Besides,A.c hauveaui has the widest range of extension,which has appeared at the bottom of the Balang Formation and extends to the near top of the Balang Formation.By comparing the extension and distribution of A.chauveaui,Ar.intermedius and C.elongata of Jiaopang section,Laizizhai section,Jianhe County and Geyi section,Taijiang County in northeastern Guizhou,it is considered that the extension of A.chauveaui from Xiangshuidong section,Songtao county in northeastern Guizhou is basically the same as that of three sections in eastern Guizhou.Therefore,the biostratigraphy of the Balang Formation is established as A.chauveaui zone.The biostratigraphic framework of the Balang Formation established the A.chauveaui Zone to help the correlation of global strata in the same period.