Conodont Biostratigraphy And Morphologic Variation From The Middle Permian To Lower Triassic Of South China

From the termination of the Late Paleozoic Ice Age in the Early Permian,to the end of the lethally hot climate during the Early Triassic,the Earth surface system witnessed the transitions from icehouse climate,to greenhouse climate and then to normal conditions at last.During this interval,the Earth surface system experienced complex process,which promotes the great transition of marine ecosystem from Paleozoic-type to Modern-type.Hence,the way how marine organisms evolved during this interval which witnessed the transition and abrupt change of the ecosystem,has attracted much attentions among domestic and overseas for several decades.This study choose one of the most important index fossil,conodont,as topic to explore their evolutionary pattern when facing different climates and living in changing marine ecosystems,which will further tell us how they co-evolved with the environment.On the other hand,as the most important index fossils,the zonations which are established by conodonts provide us a basic way to recognize“golden spike”and contrast bio-stratigraphy.However,owing to the incompleteness of the fossil records,first occurrence?FO?of the same fossil may have different datum in different sections,which will also make further mistakes in practical work.Faced with these questions,researchers established“Unitary Association Method”,which emphasizes the minimal duration while consisting of a maximal set of intersecting ranges and reduces the reliance of a certain special fossil.Recently,some cases has been applied by using this method,and even raised some challenges to traditional interval zones.Therefore,practicability of the traditional conodont interval zones which uses FOs to correlate,has been questioned by the new quantitate method.Firstly,three sections?Motianling and Taiping sections which contains microbialites and Yanduhe section which contains siliceous shale near the Permian-Triassic boundary,PTB?which respectively represents different facies and times are chosen from this area.Systematic work are carried out in the fields and 737conodont samples,each weighting about 4 Kg,have been collected from these three sections which span about 1800 m in height.In the Motianling section,which consists of upper Permian Heshan Formation and Lower Triassic Luolou Formation,about 4000conodont elements were obtained,including 2244 well-preserved P₁ elements.15 genus and 54 species can be differentiated from these P₁ elements and eleven conodont zones can be established from this section,namely Hindeodus parvus Zone,Isarcicella lobate Zone,Isarcicella isarcica Zone,Hindeodus postparvus Zone,Hindeodus sosioensis Zone,Clarkina carinata-Clarkina planata Zone,Neospathodus dieneri Zone,Novispathodus ex.gr.waageni Zone,Novispathodus pingdingshanensis Zone,Icriospathodus collinsoni Zone and Triassospathodus homeri Zone.In the Taiping section,which spans upper Permian Heshan Formation and Lower Triassic Majiaoling Formation,about 30 conodont element were obtained.1 genus and 5species can be differentiated from these specimens and two conodont zones were established in Taiping section,namely Hindeodus changxingensis Zone and Hindeodus parvus Zone.Althouth the fact that the basal microbialites of Motianling section does not record any conodont,we suggest that the PTB among microbialites-bearing sections locates at the basal of the microbilites,according to the occurrence of Hindeodus parvus from the basal of the microbialites from the Taiping section.What's more,around 8000 conodont elements were also obtained from the Permian Qixia,Maokou,Wujiaping and Dalong Formations of the Yanduhe section,although the Triassic Daye,Jialingjiang and Badong Fomations also can be seen from this section.According to the well preserved 3140 P1 elements from the Yanduhe section,9 genus including 46species can be differentiated and 22conodont zones are established,namely,Sw.subsymmetricus Zone,Jinogondolella nankingensis Zone,Jinogondolella aserrata Zone,Jinogondolella errata Zone,Jinogondolella postserrata Zone,Jinogondolella postserrata Zone,Jinogondolella shannoni Zone,Jinogondolella altudaensis Zone,Jinogondolella prexuanhanensis Zone,Jinogondolella xuanhanensis Zone,Jinogondolella granti Zone,Clarkina postbitteri hongshuiensis Zone,Clarkina postbitteri postbitteri Zone,Clarkina dukouensis Zone,Clarkina asymmetrica Zone,Clarkina liangshanensis Zone,Clarkina guangyuanensis Zone,Clarkina transcaucasica Zone,Clarkina longicuspidata Zone,Clarkina wangi Zone,Clarkina subcarinata Zone,Clarkina changxingensis Zone,Clarkina yini Zone.According to the bio-sequences of the studied three section,we have some new understandings.In the Taiping section,the FO of Hindeodus parvus locates at the basal of the microbialites,which further suggests that the microbialites around the Permian-Triassic boundary firstly developed in the Triassic.In the Yanduhe section,the completeness of Middle-Upper Permian conodont sequence suggest that this sections was not affected largely by the late Middle Permian regression.Secondly,to explore the Lower Triassic conodont Unitary Associated Zones?UAZs?of South China and their different with traditional interval zones,28 well-studied sections?including Motianling section?with good records of conodont,were chosen to be analyzed via software PAST.At last,Based on 72 conodont species,26 UAZs are established for the latest Permian to earliest Middle Triassic of South China.These UAZs provide quantitative high-resolution tools to correlate sequences in the Early Triassic of South China and to compare and test high-resolution conodont biostratigraphy based on interval conodont zones developed over the past threedecades.Our quantitative analyses provide insights on ongoing debates relating to the First Appearance Datum?FAD?of the conodont Hindeodus parvus which is used to place and subsequently correlate the“Golden Spike”defining the base of the Triassic?PTB?at the base of Bed 27c at the GSSP Meishan D Section.Previous proposals that suggested potential earlier occurrences of Hindeodus parvus below its FAD at Meishan section are not supported by our results.In deep water sections,the First Occurrence?FO?of Hineodus parvus lies at the base of UAZ 5 at the Bianyang section while it lies within UAZ 6 at the Meishan section.This indicates that the earliest occurrence of Hindeodus parvus in South China is in the Bianyang section but this conclusion needs further testing due to the reliance on“spot”data by the UAM.Anomalously high occurrences of Hindeodus in the Mingtang section?close to the Bianyang section?further suggests that the Bianyang-Mingtang area may have provided a temporally extended habitable zone for Anchignathodontid conodonts.Stage boundaries currently proposed or established using interval conodont zones locate within or between UAZs and are difficult to correlate with carbon isotope curves.UAZs are useful in helping to define GSSPs by recognizing which correlations are most robust and in the selection of the most appropriate species and level for GSSPs.Thirdly,according to the analysis of conodont elements'body evolution from the Yanduhe and Motianling sections,we found that conodont elements experienced dynamic evolution during this interval,including their size and morphology.Results from the Motianling section suggest that conodont elements endured“Lilliput effect”in early Griesbachian,late Griesbachian and at the Smithian-Spathian boundary,and episodes of size increasing?rebounds?in the middle Griesbachian,Induan-Olenekian boundary and middle-late Spathian.Results from the Yanduhe section indicate that they went through“Lilliput effect”in the latest Early Permian and Middle Capitanian?late Middle Permian?,and“Brobdingnag effect”in early Middle Permian and at Captianian-Wuchiapingian boundary during the amelioration of the environment following these two ecological crisis.The mentioned two effects are all the result of disappearance of larger elements and appearance of more smaller elements.Moreover,results from the Yanduhe section suggest that conodont elements also experienced several other size changes,including size increases in late Wuchiapingian,early-middle Changhsingian,and size decreases at ealy-middle Wuchiapingian,Wuchiapingian-Changhsingian boundary,late Changhsingian.However,these changes are mainly caused by the increment or decrement of the ratio of smaller elements,and elements with larger size kept thriving during these changes.Combining the phenomenon that the episodes occurrences of Sweetognathus and Iranognathus during the Permian and Clarkina during the Early Triassic,we conclude that conodont animals have quick response when facing the changing of the environment.But warming or cooling seemingly are not the real reasons why conodont elements become smaller or larger,these are indeed caused by suddenly warming or cooling after warming.This also means that conodont sizes are mainly influenced by the stability of the environment.Further,we conclude that the instability of the Earth surface system are the mainly reason causing the delayed recovery of the ecosystem.