New Fossil Materials From The Upper Triassic Xujiahe Formation In The Sichuan Basin And Their Palaeoenvironmental Significances

In the Phanerozoic,one of the five most significant mass extinctions occurred 201.3 Million years ago,i.e.the Triassic-Jurassic Boundary(TJB event).This event is still a controversial issue in the study of terrestrial sediments,especially the terrestrial flora,and these studies are particularly weak in East Asia.As an ideal area for studying the terrestrial Triassic-Jurassic mass extinction event,the Sichuan Basin has rich and diverse floral composition.However,update and increasing collections and systematic investigations on such fossil plants are quite necessary and will provide vital evidences for understanding the floral diversification,as well as palaeoclimatic and palaeoenvironmental significances of the Late Triassic floras in this region.Recently,a new fossil locality-Zilanba section was found in Guangyuan,northwestern Sichuan Basin in South China,with diverse and rich plant fossils.Totally 31 species of fossil plants belonging to 18 genera are systematically described in this paper.Among them,6 species of 4 genera are ascribed to Sphenopsida;5 species of 3 genera to Filicopsida;1 species of 1 genera to Pteridospermopsida;4 species of 1 genera to Bennettiopsida,8 species of 4 genera to Cycadopsida;2 species of 2 genera to Ginkgopsida;5 species of 3 genera to Coniferopsida.An exquisitely preserved taxon-Anthrophyopsis with great significance of geological age and palaeoclimatic reconstruction has been comprehensively studied in this paper.The three species of this genus:Anthrophyopsis crassinervis,A.tuberculate and A.venulosa which are widely distributed in the South China region have been investigated in details.All species of Anthrophyopsis reported around the world with their taxonomic positions have been briefly discussed.A.crassinervis was designated as the type species of genus Anthrophyopsis with essential modification of generic characteristics based on the comprehensive studies on newly found materials from Zilanba section and all the reported specimens of this genus.Based on comprehensive consideration of the overall perspective of the flora,as well as the spatial and temporal distribution pattern of the representative taxonomic type-Anthrophyopsis in the Member 3 of the Xujiahe Formation,it can be concluded that the geological age of the Member 3 of the Xujiahe Formation is the Rhaetian.In combination of the sedimentary facies analysis and plant fossil assemblage,the palaeoecological conditions of the Xujiahe Formation in Zilanba section of Guangyuan are studied.With reference to the characteristics of the floral composition,Cycadopsida and Filicopsida are the dominant plant group in the Zilanba locality,especially a mass of beautifully preserved taxon—Anthrophyopsis with large-scale leaf and complex netted venation in the statum,the plant assemblages reflected that during the Rhaetian,the Guangyuan area bear a tropical-subtropical palaeoclimatic background.This study undoubtly provides new fossil evidences for understanding the floral composition in the Sichuan Basin during the Late Triassic,as well as the palaeoclimate and palaeoenvironment reflected by the fossil plants,which are of great significance for figuring out the scientific issues such as the responses of the Late Triassic extinction event in South China area and its background mechanisms.