Study Of Permian-Triassic Boundary Brachiopod Fauna From Siliceous Rock Facies In South China

The Permian–Triassic mass extinction,which is the severest extinction event in geological history,has attracted lots of researches globally in the past decades.Among the event,not only lots of organisms were eliminated but also the marine ecosystem was fundamentally changes.Previously,most of researches on the extinction pattern were based on the database data.Based on the statistical analysis on global data,these researches built reliable extinction patterns and provided significant insights for the mass extinction processes and possible mechanisms.However,these researches may not necessarily reveal or reflect higher-resolution spatial and temporal variabilities of the extinction events at local sections.In South China,there are many well-preserved and continuous Permian–Triassic marine sections developed.Brachiopod,one of the most seriously affected organism in the Permian–Triassic mass extinction,is the topic of this study.Based on three brachiopod faunas from three sections from siliceous rock facies in South China(Xinmin section in Guizhou,Majiashan section in Anhui and Rencunping section in Hunan),this thesis discussed the characteristics and changes of the brachiopod paleocommunity in high resolution before and during the mass extinction.This thesis systematically described 19 genera and 31 species of brachiopod from Xinmin section.We mainly achieved to: 1)propose a new species Juxathyris subcircularis;2)discuss and compare the two very similar genera Fusichonetes and Tethyochonetes using bivariate analysis,multivariate analysis and cladistics analysis based on the newly collected chonetids specimens from Xinmin section and combined with data from literatures,and conclude that the two genera are synonym and suggest using the name Fusichonetes according to the zoological nomenclature;3)discuss and revise 12 local Araxathyris species from South China and provide internal structures for some species for the first time;4)illustrate the papillae in the internal surface of Orthothetina,the rarely found concentric line on the external surface of Martinia and distribution and shape of micro spine bases on the external surface of Crurithyris.Based on the brachiopod data from three sections from siliceous rock facies and combined with previous researches on brachiopod from this facies,this thesis firstly quantitatively concluded the Changhsingian brachiopod assemblage from siliceous rock facies in South China and suggested to use Crurithyris–Fusichonetes assemblage.Meanwhile,the lifestyle of this brachiopod assemblage was reconstructed,including hinge/body spine attached,pedicle attached and shell encrusting.Finally,this thesis conducted two-way cluster analysis on the brachiopod data from three studied section,and found that the structure of brachiopod paleocommunity has changed before the extinction event.There were early warning signals occurred in the paleocommunity,including: 1)diversity of brachiopod paleocommunity significantly decreased;2)the amount of the predominant genera decreased and the paleocommunity was replaced by more competitive genera;3)the body size of brachiopod significantly declined.After correlating with the published environmental data in studied sections,it was found that the declined marine primary productivity and increased volcanic event were the reasons resulting in the paleocommunity structure changes.That is to say,the decreased supply of food and increase of clastic matter in marine environment led to the failure of the genera with low resistance to the stress in environment and the body size decrease.Moreover,similar early warning signals were also found in the shallow-water carbonate rock facies and clastic rock facies in South China and even in modern ecosystem.