Early Triassic Ammonoid Biostratigraphy In South Tibet,Guizhou And Guangxi And Its Evolution

The Permian-Triassic mass extinction(PTME)is the most severe mass extinction in the Phanerozoic,which killed more than 80% of marine species and triggered the transition from the Paleozoic Evolutionary Fauna to the Modern Evolutionary Fauna.In the aftermath of the PTME,some groups(e.g.,ammonoids)show rapid recovery and have taxonomic richness equal to or even higher than the pre-extinction value in less than 1 Ma,while other groups(e.g.,brachiopods)display relatively slow rediversification rates with marked diversification episodes until the Middle Triassic.Coinciding with the PTME and subsequent Early Triassic,the environment and climate show huge perturbations,including volcanic eruptions,recurrent anoxia,global warming,enhanced weathering,ocean acidification,etc.To decipher the coevolution of biotic and environmental factors during this significant interval,a high-resolution biostratigraphic framework is a prerequisite.Ammonoid is an extinct marine cephalopod animal with excellent fossil records,worldwide distribution,and a fast evolution rate.Thus,it can act as a significant index fossil for regional and global biostratigraphic correlations.However,Early Triassic ammonoid biostratigraphy in South Tibet,Guangxi and Guizhou are still poorly constrained.Ammonoids play a role as predators in marine ecosystems,and their evolutionary trajectory during the PTME and subsequent Early Triassic is a vitally important component of understanding biotic mass extinction and recovery.The diversity dynamics of ammonoids near the Permian-Triassic boundary are well understood.However,their morphological and biogeographic evolution patterns are still controversial and far to be clear.In this dissertation,Griesbachian,Dienerian,and Smithian ammonoid biozonations of Guangxi,Guizhou,and South Tibet have been reconstructed.The global correlations of ammonoid biostratigraphy in these three substages have been revised.Furthermore,based on a newly compiled global Permian-Triassic ammonoid dataset,this work quantitatively analyzed ammonoid biogeographic and morphological evolution near the Permian-Triassic boundary.Advances and novelties are summarized as follows:1.Seven ammonoid zones were recognized in the Lower Triassic of Guangxi and Guizhou.Based on bed-by-bed samplings of ammonoids from the Luolou Formation at Gujiao section in Guizhou,Motianling,Shanggang,and Nafang sections in Guangxi,a total of 32 species were identified.According to the distribution of these species,seven ammonoid zones were reconstructed.They are Griesbachian Ophiceras medium Zone and Jieshaniceras guizhouense Zone;Dienerian Ambites radiatus Zone;Smithian Flemingites rursiradiatus Zone,Owenites koeneni Zone,Anasibirites kingianus Zone,and Xenoceltites variocostatus Zone.The Ambites radiatus Zoneis is recognized in South China for the first time.The former late Smithian Anasibirites mutiformis Zone can be further subdivided into two zones: Anasibirites kingianus Zone and Xenoceltites variocostatus Zone.2.Twenty-two ammonoid zones were recognized in the Lower Triassic of South Tibet.Several years of intensive and bed-by-bed samplings of ammonoids from the Kangshare Formation at four sections(Selong,Paizi,Xialong,and Qubu)in South Tibet have yielded abundant specimens pertaining to successive Griesbachian-Smithian assemblages.A total of 140 species were identified throughout this interval.These new robust data allow the construction of a high-resolution biostratigraphy using the Unitary Association method.A total of 22 Unitary Association Zones(UAZ)were recognized,including the Griesbachian: Metophiceras cf.subdimessum and Ophiceras tibeticum UAZs;the Dienerian: Gyronites dubius,Gyronites plicosus,Gyronites frequens,Ambites bojeseni,Ambites discus,Ambites bjerageri,Vavilovites meridialis,Kingites davidsonianus,and Koninckites vetustus UAZs;the Smithian: Kashmirites armatus,Kashmirites nivalis,Brayardites compressus,Nuetzelia himalayica,Shigetaceras dunajensis,Punjiabites punjabiensis,Subvishnuites welteri,Stephanites superbus,Wasatchites distractus,Subvishnuites posterus,and Glyptophiceras sinuatum UAZs.Subvishnuites welteri UAZ is proposed for the first time,which also occurs in Oman.Based on newly obtained data in this work and previously published data from Spiti(India)and Salt Range(Pakistan),integrated Dienerian and Smithian ammonoid zonations,12 for the Dienerian and 16 for the Smithian,were constructed using the Unitary Association method.The correlation of these high-resolution ammonoid UAZs and conodont biostratigraphy is also summarized.Finally,a time calibration of Early Triassic climatic changes was updated according to these high-resolution ammonoid UAZs.3.Thanks to the bed-rock-controlled extensive samplings,detailed taxonomic revisions and quantitative stratigraphic method,the resolution of the Early Triassic ammonoid biostratigraphical scales has been significantly improved,and their correlations have been strengthened.However,due to the heterogeneous sampling effort and biotic endemism among different regions,there are still many uncertainties in the correlation.Based on the newly obtained data from Guizhou,Guangxi,and South Tibet,as well as published works,the global correlation of the Griesbahcian,Dienerian,and Smithian ammonoid biostratigraphy were summarized and revised.New revisions include the following: 1)the former early early Smithian Hedenstroemia hedenstroemi Zone is correlated with the late early Smithian in Tethys,and 2)most of the early Smithian ammonoid assemblages found in the western US basin are revised to correspond to the middle Smithian.4.Revealed the ammonoid morphological evolution near the PTME and refuted the former ammonoid morphological nonselective model.Previous works documented that the PTME caused a diversity bottleneck but had little effect on ammonoid disparity.Ammonoids show morphological nonselective extinction during the PTME.However,these results are not congruent with newly found fossil records in recent years.I compiled a comprehensive morphological dataset containing 127 ammonoid species and used the nonmetric multidimensional scaling method to reveal the impact of the PTME on the morphological selectivity of ammonoids.New results show that post-extinction taxa occupied a quite different morphospace when compared with the pre-extinction assemblages.The survivors were mainly smooth and weakly ornamented forms,while the late Permian species were dominated by coarsely ornamented forms.Contrary to previously recognized nonselective patterns,these results suggest a morphological selectivity of the Permian-Triassic crisis.Newcomers in the Griesbachian were mainly compressed and smooth forms.This morphological shift from the coarsely ornamented ammonoids dominating the Changhsingian to the smooth ammonoids dominating the Griesbachian possibly suggests an ecological turnover of ammonoids during the PTME.5.Confirmed ammonoid cosmopolitanism caused by the PTME and revealed the mechanisms leading to cosmopolitanism.Biotic mass extinctions can not only cause biodiversity bottlenecks and affect ecological structure but also change biogeographic structures.Based on a newly compiled global middle Permian to Middle Triassic ammonoid occurrence dataset and using two kinds of network methods,two episodes of ammonoid cosmopolitanism were documented during the Early Triassic.They occurred during the Permian-Triassic mass extinction and the late Smithian crisis.Three theoretical models,not mutually exclusive,can lead to cosmopolitanism:(1)selective extinction in endemic taxa,(2)endemic taxa becoming cosmopolitan after the extinction,and(3)an increase in the number of newly originated cosmopolitan taxa after extinction.Partitioned analyses of survivors and newcomers revealed that the immediate cosmopolitanism event(Griesbachian)after the Permian–Triassic event can be attributed to endemic genera becoming cosmopolitan and an increase in the number of newly originated cosmopolitan genera after the extinction.Late Smithian cosmopolitanism is caused by selective extinction in endemic taxa and an increase in the number of newly originated cosmopolitan genera.The survivors of the PTME did not show a wider geographic range,suggesting that this mass extinction is nonselective among the biogeographic ranges,while late Smithian survivors exhibit a wide geographic range,indicating selective survivorship among cosmopolitan genera.The difference in biogeographic selective extinction among these two extinction events indicates that the PTME has a wide geographic impact and effect on both endemic and cosmopolitan taxa.