Taxonomy And Evolution Study Of New Early-Middle Triassic Eosauropterygian Materials From South China

Sauropterygia is one of the longest-lasting and most diversified Mesozoic marine reptile group.As a subclade of Sauropterygia,Eosauropterygia occurs in the Triassic worldwide with abundant fossil records.In recent years,numerous new eosauropterygian fossils have been found in South China,which are successive by time.Therefore,taxonomic research and cladistic analysis on these important materials are necessary to decipher the early radiation and evolutionary history.This study focuses on the new fossil materials of the Early-Middle Triassic eosauropterygians form Nanzhang-Yuan’an Fauna,Hubei Province and Luoping Fauna,Yunnan Province,constraining their taxonomic positions through systematic description and cladistic analysis.In this study,one new Early Triassic eosauropterygian genus and species,Chusaurus xiangensis gen.et sp.nov.,is established based on the specimen WGSC V 1901 and WGSC V 1702.The two specimens are recognized as the same new species based on the combination of following characters: oval margin of the coracoid,limited interspace of manual and pedal digits,round anterior caudal ribs.It is one of the earliest eosauropterygians.A Middle Triassic eosauropterygian specimen is recognized as a conformis of Diandongosaurus acutidentatus.It bears carnivorous teeth,anterolateral process of clavicle and flat distal end of anterior caudal ribs,which highly resembles D.acutidentatus yet with some morphological differences on their sizes,snouts and appendicular skeletons.Based on these new fossil materials,this study also established a database of Triassic marine reptiles,showing that Triassic sauropterygians mainly located in the low-latitude area of the Northern Hemisphere.Most genera had a limited distribution suggesting a weak global connectivity,due to their less-advanced swimming ability.And based on the result of our cladistic analysis,this study conducts evolutionary traits analysis,which suggests neck-elongation evolved independently in both pachypleurosaurid and pistosauroid sauropterygians in the Middle Triassic,preceding the occurrence of plesiosaurs.In addition,the fang-like teeth and comparatively large body size of the specimen WIGM SPC V 1105 reveal its role as a carnivore,while the truncated femur suggests it had been attacked by a predator.This implies the predation pressure in the Middle Triassic had already been high enough to promote the construction of modern marine ecosystem.