Late Norian (Late Triassic) Conodont Fauna Of The Baoshan,Western Yunnan

It has been proved that the end-Triassic mass extinction was composed of multi-episodic extinction events and the Sevatian of the Norian is one of the key intervals to study the mass extinction or turnover,paleoclimate and paleoenvironment changes.Confirming the initiated mass extinction and its occurring age plays a significant role in the understanding of the relationships between the mass extinction and the changes of paleoclimate and paleoenvironment.Conodonts have good advantages in the study of biodiversity change because of its rapid evolution,wide distribution and well preservation.Conodont zoning can well constrain the age of the biotic events.In this work,we selected five sections in western Yunnan,and focus on three sections（the Potou section,the Madoupo section and the Xiquelin section）in Baoshan area according to the conodont production.On the basis of more than 10000 conodont specimens from the Dashuitang Formation and the Nanshuba Formation,detailed morphological classification,biological classification,conodont biostratigraphic division,conodont diversity changes and the morphological evolution of some conodonts were studied.First,the ages of the three sections are confirmed as the Sevatian of the late Norian based on the occurrence of marker species Mockina bidentata and the firs occurrence of marker species Parvigondolella andrusovi,and hence continuously dividing a lower Mockina bidentata Zone and an upper Parvigondolella andrusovi Zone.This is the first time to display two continuous conodont zones of the Sevatian in one section in China,which greatly improves the correlation with other Sevatian strata of the western Tethys.Second,we first illustrate abundant and diverse conodont fauna in the Sevatian.A total of 77 different morphological types from 5 known genera Ancyrogondolella,Epigondolella,Mockina,Parvigondolella and Zieglericonus and some indeterminate genera are identified here,including 21 established form species,36 new species,17indeterminate species.Among these form species,Epigondolella carinata,Epigondolella englandi,Epigondolella praespiculata,Epigondolella spiculata,Epigondolella spinosa,Mockina bidentata,Mockina hisaidaniensis,Mockina medionorica,Mockina matthewi,Mockina sakurae,Mockina slovakensis和Mockina zapfei were studied and displayed in detail,showing the ontogenetic series and intraspecific variations.We give a detailed discussion on Mockina slovakensis,showing that there were two morphological features that bearing different classification significance in Mockina slovakensis,namely,the lateral basal profile of the P₁elements varied at different growth stages of the ontogeny,while the shape of the blade remained stable at most growth stages of the ontogeny and hence plays an important role in the identification of this species.However,at the end phase of the phylogeny,the shape of the blade of Mockina slovakensis changed.The ontogenetic series of the new species were preliminarily established,e.g.Epigondolella lingi sp.nov.,Epigondolella madoupoensis sp.nov.,Epigondolella nanshubaensis sp.nov.,Epigondolella oliviformis sp.nov.,Epigondolella paraunifromis sp.no.,Epigondolella postequalis sp.nov.,Epigondolella pyriformis sp.nov.,Epigondolella xiaoi sp.nov.,Epigondolella?longienglandi sp.nov.,Mockina contracta sp.nov.,Mockina?longiobovata sp.nov.,Mockina postmultidentata sp.nov.and so on.The abundant and diverse conodonts from the three sections reveal a new situation of the Sevatian conodont fauna,namely a diversification peak exists in the Mockina bidentata Zone.Third,we have reconstructed Mockina apparatus and probable Parvigondolella apparatus,which are the youngest reconstructed conodont multielement apparatuses for the moment.The components,the morphologies of each component,the orientation of each component and the architecture of the apparatuses are reliably confirmed,revealing that Mockina and probable Parvigondolella apparatuses consist of seven different types of elements,namely a single hibbardelliform S₀element,a pair of grodelliform S₁element and enantiognathiform S₂element,paried cypridodelliform M,cypridodelliform P₂,and segminiplanate mockiniform or segminate parvigondolelliform P₁elements.The present research provides new insights into the morphology of elements occupying homological positions of P₂,M,and S.Among them,the isolated P₂elements,based on the rich conodont production from the Xiquelin section,revealed the evolutionary trend of the P₂element,that is,the shortening or disappearance of the posterior process of P₂element.The comparison among the multielement apparatuses of the three key species Mockina,Parvigondolella and Misikella reveals that the three conodont populations are closely related and have similar morphological elements in homologous locations.The reconstruction of Mockina and probable Parvigondolella multielement apparatuses and the comparisons of the P₂element reveal the evolutionary trend of the conodont apparatus from the Ordovician to the Late Triassic.Fourth,by comparing the abundant and diverse conodont fauna in the Mockina bidentata Zone with the abundant but simplified conodont fauna in the Parvigondolella andrusovi Zone,we identify an apparent conodont extinction in the transition interval between Mockina bidentata Zone and Parvigondolella andrusovi Zone.After this extinction,Ancyrogondolella conodonts totally went out,the relatively strongly ornated Epigondolella conodonts nearly all disappeared and half of the relatively weakly ornated Mockina conodonts are also extinct,however,segminate Parvigondolella P₁elements begins to occur.Hereafter,the diverse conodont fauna had never recovered.This conodont turnover in the Sevatian suggests that the protracted Late Triassic mass extinction probably initiated in the transition interval between Mockina bidentata Zone and Parvigondolella andrusovi Zone.This study also provides the precise starting age for the Late Triassic mass extinction.In addition,four evolutionary lineages of the studied conodont fauna have been established based on the abundant conodonts at the top of the Mockina bidentata zone and the Parvigondondolella andrusovi zone,including M.bidentata-M.monodentata sp.nov.-P.sp.A-P.andrusovi lineage,M.bidentata-M.monodentata sp.nov.-Mi.hernsteini lineage,Mockina bidentata-M.monodentata sp.nov.P.ciarapicae lineage and M.xiquelinensis sp.nov.-M.xiquelinensis/P.costata transitional forms-P.costata sp.nov.-P.lata lineage.