Early Triassic Conodont Biostratigraphy In Nanpanjiang Region And Spatio-Temporal Evolution In South China

The Permian-Triassic transition witnessed the most severe mass extinction in Phanerozoic with over 90 % marine species and 70 % terrestrial families extinct.The mass extinction was selective and the extinction rate of Paleozoic fauna was higher than that of the Modern fauna,resulting in the changeover of ecosystems from the Paleozoic type to the Modern type.In the aftermath of the extinction,extreme environment persisted throughout the whole Early Triassic.Many clades did not recover until the Middle Triassic and a stable,complex ecosystem did not re-emerge for nearly 10 million years after the extinction.In contrast to the generally delayed recovery,ammonoids,condonts and foraminifers recovered rapidly in about one million years during the early Smithian.Why did these clades flourish in the hostile environment that suppressed other fossil groups? Understanding this problem provides an important way to explore the survival and recovery mechanism during the crisis.Conodonts were widespread and rapidly evolved,which made them important index fossils in the Early Triassic.Previous studies on conodonts in South China not only played an important role in the stratigraphic correlation of different sedimentary facies,but also provide a basic foundation for understanding the extinction-recovery pattern of conodonts through the Permian-Triassic transition.However,most early-studied basinal sections are poor of Spathian fossils,making it difficult to recognize the recovery pattern in this facies.In order to establish a complete Early Triassic conodont sequence in the basinal facies and explore the survivalrecovery pattern of conodonts in South China,we studied the conodonts in two basinal sections in the Nanpanjiang Basin,namely Sidazhai section and Ganheqiao section.In addition,the Mingtang section,located in the marginal area of the isolated carbonate platform,was studied as the transition between the basin and platform.Furthermore,fourteen well-studied Permian-Triassic sections in various sedimentary facies of South China were collected and re-studied by re-identifying and making statistics of the conodont data.Besides,the sedimentary environments of these sections were combined to explore the diversification and recovery process of conodonts as well as the process and the machinism of biotic recovery and ecosystem reestablishment after the mass extinction.The Sidazhai section in the Ziyun County,southern Guizhou is composed of the Linghao Formation in Upper Permian and Luolou Formation and Xinyuan Formation in Triassic.After repeated high-resolution sample analyses of the whole section,abundant conodonts were retrieved from the Olenekian,including eight new species,though conodonts are relatively poor in the Induan.According to the distribution of conodonts,10 conodont zones are recognized in the section,including Clarkina yini zone,Hindeodus parvus zone,Neoclarkina discreta zone,Neospathodus dieneri zone,Novispathodus waageni zone,Guangxidella bransoni zone,Novispathodus pingdingshanensis zone,Icriospathodus collinsoni zone,Triassospathodus homeri zone,and Chiosella timorensis zone.Hindeodus parvus first occurs in the limestone lens of the shales in the lower Luoluo Formation,while abundant Triassic bivalves,such as Claraia wangi and C.griesbachi,were yielded in the shales below.Therefore,the Permian-Triassic boundary is defined by the bivalves in the lowermost of Luolou Formation.Novispathodus waageni,first occurring in Bed 21 at the uppermost of Luolou Formation,defines the Induan-Olenekian boundary of the Sidazhai section.The first occurrence of Novispathodus pingdingshanensis in the middle of Ziyun Formation defines the Smithian-Spathian boundary.In addition,Chiosella timorensis first occured in the limestone lens of the shales 10 m above the Ziyun Formation,suggesting the Early-Middle Triassic boundary in the lower Xinyuan Formation.Scythogondolella milleri,which was restrained in the late Smithian elsewhere,occurs in the Spathian over the Icriospathodus collinsoni zone at this section,suggesting that this species survived the late Smithian extinction event.Furthermore,a few new platform-form conodont elements as the transitional forms of the Smithian and Middle Triassic ones occurred in the upper Ziyun Formation,indicating that even though the widely distribution of platform-bearing species in the Tethys started in the Middle Triassic after the late Griesbachian extinction,some pioneers had occurred in the middle Spathian,implying that their ecological niche shift might start in the late Early Triassic.Another basinal section is the Ganheqiao section,located in the Wangmo County,southern Guizhou.This section consists of the Early Triassic Luolou Formation and Ziyun Formation,and the Middle Triassic Xinyuan Formation.After systematic analyses of conodont samples,20 species in 9 genera were identified at this section.Conodonts are poor in the Dienerian and Smithian rocks,but abundant in the Spathian.According to the distribution of conodonts,6 conodont zones are recognized,including Neospathodus dieneri zone,Novispathodus waageni zone,Guangxidella bransoni zone,Novispathodus pingdingshanensis zone,Triassospthodus homeri zone,and Chiosella timorensis zone in ascending order.The Induan-Olenekian boundary is identified by the first occurrence of Novispathodus waageni in the lower part of Luolou Formation.Novispathodus pingdingshanensis,as the proposed index fossil of Smithian-Spathian boundary,first occurred in the limestone lens of upper Luolou Formation.The first occurrence of Chiosella timorensis in the lower Xinyuan above the tuff,marks the Lower-Middle Triassic boundary at the Ganheqiao section.The Mingtang section in the Luodian County of South China is located in the marginal area of an isolated platform,the Great Bank of Guizhou.This section is comprised of the Dalong Formation in late Permian and the Luolou Formation and Poduan Formation in Triassic.In total,82 samples were collected for the construction of conodont sequence.More than 2000 specimens were obtained from this section,and most of these specimens were yielded in the middle Spathian rocks.Based on the distribution of conodonts,8 conodont zones are recognized at this section.They are Hindeodus parvus zone,Neoclarkina discreta zone,Neospathodus dieneri zone,Novispathodus waageni zone,Neospathodus triangularis-Triassospathodus homeri zone,Chiosella timorensis zone,Nicoraella germanica zone,and Nicoraella kockeli zone.Besides,abundant macrofossils of the Permian-Triassic mixed fauna occurred in the lower Luolou Formation below the Hindeodus parvus Zone.The lower part of the interval is dominated by Permian-type brachiopods,while the upper part is characterized by the Triassic bivalves,Claraia.Accordingly,the Permian-Triassic boundary is positioned in the middle part of the interval yielding the mixed fauna.The Induan-Olenekian boundary can be well defined by the first occurrence of Novispathodus waageni in the lower-middle part of the Luolou Formation,while the Lower-Middle Triassic boundary is placed at 0.2 m below the top of the Luolou Formation according to the first occurrence of Chiosella timorensis.In order to explore the extinction-recovery pattern and ecological diversification of conodonts and their influencing environmental factors through the Permian-Triassic transition in South China,we collected and re-studied the conodont data from 14 wellstudied Permian-Triassic sections in addition to the three sections above and sorted them according to sedimentary paleogeography.In total,these sections could be sorted into four sedimentary facies,i.e.Huangzhishan,Yangou,Dajiang,Taiping section,Guanziba,Gaimao sections in the platform interior facies;Qingyan,Mingtang and Guandao sections in the upper slope facies;Sidazhai,Meishan,Jiarong and Bianyang sections in the lower slope facies;and West Pingdingshan,Zuodeng and Ganheqiao sections in the basinal facies.After a careful taxonomic revision of the conodonts from these sections,including the fossils collected by the P-T research groups at China University of Geosciences,113 species of 28 genera are determinated totally.On this basis,the diversity,extinction rate and origination rate were calculated as well as the number of species in the four sedimentary facies.The results show that conodont fauna was uniformly distributed in all studied facies during the Permian-Triassic boundary interval,but concentrated in the waters of moderate depth(lower slope facies)during the two extinction events later in the late Griesbachian and late Smithian.The diversification event between these two extinctions was marked by a rapid diversification of Neospathodus that happened in the deeper water(basin/deep shelf)facies.Constrained by the secular environmental changes in South China during the Early Triassic,high-temperature and low oxygen might be responsible for the two extinctions(late Griesbachian,late Smithian),while oxygenation in the late Dienerian-early Smithian might be the major contributor to the diversification in deeper water.Moreover,the uniform diversification of Hindeodus and Clarkina during the Permian-Triassic boundary interval,associated with morphological diversification in these and descendant genera,might result from their successful rapid response to the environmental stress caused by the extinction event.