Research On Conodont Fauna And Its Biostratigraphy Of The Middle Triassic In Yunnan And Guizhou Provinces

The End-Permian Mass Extinction（EPME）and the extreme lethally environment of the Early Triassic restricted the recovery of ecosystem and biotic.Until the ecosystem in the Anisian stage of the Middle Triassic,when the ecosystems have re-merged and the biotics have flourished again.Therefore,the Middle Triassic has become a hot topic in scientific research in recent years,as it was a key period that recorded the fully recovery of ecosystem after the EPME.The marine strata from Permian to Triassic are well developed in South China with various sedimentary facies from platform to basin,being one of the best regions for the comprehensive recognition of the nature of the full recovery of the complex ecosystems during the Early-Middle Triassic.Conodont has been long considered as an effective index in biochronology,due to its wide distribution,and rapid evaluation,which has been applied here to establish a high-resolution conodont biostratigraphy framework of the Middle Triassic.In this research,our studied sections in Guizhou and Yunnan provinces of South China were selected from different sedimentary environments,including the slope-basin facies of Shaiwa section（Ziyun county,Guizhou province）,the platform margin facies of Guandao section（Luodian county,Guizhou province）,the platform facies of Nanpenghe section（Gengma county,Yunan province）and the Xuanlai section（Zhenkang county,Yunan province）.After systematic analyses of conodont samples,a total of 7,994 P₁specimens were obtained,and 56 species（including 4 new species）in12 genera were identified.The Shaiwa section is located on the northwest flank of Nanpanjiang Basin with well-exposed strata from Permian to Middle Triassic,and the Middle Triassic Xinyuan Formation was analyzed through the high-resolution and large conodont biostratigraphic samples.The section is 371m in thickness,and 131 rock samples were sampled.A total of 2,792 P₁elements were obtained,and 26 species in 10 genera were identified.According to the distribution of conodonts,six conodont zones were recognized from Anisian,in ascending order,they are:the Nicoraella germanica Zone,the Nicoraella kockeli Zone,the Paragondolella bulgarica Zone,the Neogondolella constricta Zone,the Neogondolella cornuta Zone and the Paragondolella excelsa Zone.The Bithynian-Pelsonian substage boundary is placed at the lower part of Bed 1,based on the first occurrence of Nicoraella kockeli.The first occurrence of Neogondolella constricta at the middle part of Bed 19,defines the Pelsonian-Illyrian substage boundary of the Shaiwasection.Moreover,our conodont occurrences suggested that the uppermost strata could be very close to the Anisian-Ladinian Boundary.The variation in relative abundance segminate and segminiplanate elements has been recognized in three assemblages:the first assemblage is dominated by segminate elements,and the second assemblage is characterized by the co-existence of the segminate and segminiplanate elements,and the third assemblage is dominated by the segminiplanate elements.The replacement from segminate to segminiplanate elements is indicated the gradual normalization of the marine environment through the early Anisian.The abrasion or worn of the conodont surface and sedimentary facies provides valid evidences for demonstrating reworking at the Shaiwa section,which makes some conodonts possess a longer distribution than previously reported.The previous study had demonstrated detailed conodont sequences through the Triassic at the Guandao section.In this study,we enable high-resolution conodont biostratigraphic study focused on the Middle to lower Upper Triassic based on previously reported.In this study,86 rock samples were resampled from the 139.1m thickness of the Guandao section.A total of 1,981 P₁elements were obtained at this section,24 species in 6 genera were identified,and a total of eight conodont zones from late Anisian to early Carnian were recognized,in ascending order,they are:the Neogondolella cornuta Zone,the Budurovignathus truempyi Zone,the Budurovignathus hungaricus Zone,the Budurovignathus mungoensis Zone,the Paragondolella inclinata Zone,the Paragondolella foliata Zone,the Quadralella polygnathiformis Zone and the Quadralella intermedius Zone.Based on the updated conodont distributions,the relatively（sub）stage boundaries of Ladinian stage at Guandao are refined.The Anisian-Ladinian stage boundary is placed at the horizon 0.4m lower than the base of Bed 23,on the basis of the first occurrence of Budurovignathus truempyi,it is about14.8m lower than the previously definition.Based on the first occurrence of Budurovignathus mungoensis,the Fassanian-Longobardian substage boundary is placed at 8.7m above the base of Bed 26.The Ladinian-Carnian stage boundary is replaced at the horizon 1.3m above the base of Bed 30 instead of defined by the first occurrence of Quadralella intermedius,it is about 10.2m lower than the level that was defined previously definition by the first occurrence of the Quadralella polygnathiformis.Additionally,the correlation between Quadralella polygnathiformis and Quadralella intermedius,and indexes for the Ladinian-Carnian stage boundary have been discussed,via the multiple stratigraphiescorrelation from different regions.The Nanpenghe section is belong to the Gengma stratigraphic area,which is situated at the south of Baoshan Block,which was located the south margin of Paleo-Tethys during the Triassic.The strata exposed at this section is represented by the Hewanjie Formation,which is composed of light-dark grey medium to thick bedded limestone containing bioclastic and chert nodules.The section is 148m in thickness,and63 rock samples were sampled.A total of 1,631 P₁elements were obtained,15 species in 3 genera were identified（including four new species:Neogondolella comma sp.nov.,Neogondolella mengdingensis sp.nov.,Neogondolella nanpengheensis sp.nov.and Paragondolella hongyanjiaoensis sp.nov.）at the Nanpenghe section.There are three conodont zones that were established,in ascending order,they are:the Budurovignathus truempyi Zone,the Budurovignathus hungaricus Zone and the Budurovignathus mungoensis Zone.The Fassanian-Longobardian substage boundary can be defined at the middle part of Bed 6,based on the first occurrence of Budurovignathus mungoensis.The first report of Budurovignathus mungoensis except for the Guandao section in South China indicated that the late Ladinian（Longobardian substage）marine strata are still developed in Baoshan block,western Yunnan province,whereas were absent over much area of south and north China that was caused of the Great Ladinian Regression.The Xuanlai section is located in the Shidian stratigraphic area in the south of Baoshan Block.Although the strata at this section also represented by the Hewanjie Formation,however,it is composed of dark grey-black thin to medium bedded limestone,which is different from that of the Nanpenghe section.A total of 103 rock samples were sampled from the 239m thickness of this section.A total of 1,585 P₁elements were obtained,22 species in 5 genera were identified,and a total of six conodont zones from Anisian to Ladinian stage were recognized,in ascending order,they are:the Paragondolella bulgarica Zone,the Neogondolella constricta Zone,the Paragondolella excelsa Zone,the Budurovignathus truempyi Zone,the Budurovignathus hungaricus Zone and the Paragondolella trammeri Zone.Based on the first occurrence of Neogondolella constricta,the Pelsonian-Illyrian substage boundary is defined at the top of Bed 6.The Anisian-Ladinian boundary is placed at the middle part of Bed 10,on the basis of the first occurrence of Budurovignathus truempyi.In this study,in order to investigate the systematic classification and evolutionary relationship of Middle Triassic conodonts,a phylogenetic analysis was undertaken basedon a data matrix of 55 characters for 24 P₁elements species belonging to the Middle Triassic genera Budurovignathus,Paragondolella and Quadralella.According to this proposed phylogeny,Budurovignathus can be divided into two types:absent or present ornamentation occurring on the platform margins.Genus Budurovignathus is probably evolved from Paragondolella trammeri,and two evolution lineages can be recognized:the first one is Paragondolella trammeri-Budurovignathus praehungaricus-Budurovignathustruempyi-Budurovignathus hungaricus-Budurovignathus japonicus;and the second one is Paragondolella trammeri-Budurovignathuspraehungaricus-Budurovignathus truempyi-Budurovignathus mirautae-Budurovignathus mungoensis-Budurovignathus diebeli（or to Budurovignathus longobardicus-Budurovignathus mostleri）.Based on the collection of the conodont specimens in this study,these studied sections could be sorted from Early Anisian to Early Carian.On this basis,the conodont diversity increased from early Anisian to early Ladinian,and reached a peak in Fassanian,then decreases rapidly in Longobardian（late Ladinian）.Considering the diversity of different faunas drop in Ladinian,and the fluctuation of inorganic carbon isotope near the Ladinian-Carnian boundary,a minor“biotic crisis”probably occurred in the late Ladinian,which may be closely related to the Great Ladinian Regression.