The First Record Of Freshwater Plesiosaurian From The Jurassic Of Gansu,China,with Its Study Of Tooth Morphology

The plesiosaurs are one of the common groups in the Mesozoic aquatic reptiles,which mainly lived in the marine environments.The freshwater plesiosaurs are rarely found in the world,especially from the Jurassic.There are two major types of plesiosaurs,the Plesiosauroidea and the Pliosauroidea,which are distributed all over the world.There are few plesiosaurs found in China,and they are all relatively rare freshwater plesiosaurs.This present paper reports the first record composed of four isolated teeth of freshwater plesiosaur from the Middle Jurassic fluviolacustrine strata of Qingtujing area,Jinchang City,Gansu Province.These materials are important for understanding the paleogeography and paleoecology of this group.The main research results of these Jinchang specimens are as follows:1.The shape and ornamentation of tooth are reliable taxonomic features that can be used for genus or species identification.The Jinchang specimens are congruent with the characters of plesiosaurian teeth.Compared with the marine and non-marine plesiosaurian teeth,Jinchang specimens are similar to the teeth of some members of Pliosauridae.However,Jinchang specimens still have unique characteristics and can only be tentatively classified as plesiosaurian,and more likely to be members of the pliosaurs.The discovery of Jinchang specimens has enriched the contents of Jurassic freshwater plesiosaurs in China.2.These teeth from the same fossil site are thought to be from one individual,and have similar crown shapes and ornamentation:slender and slightly curved;circular cross-section;the enamel surface of the crown is smooth;slight wear near the top;surface of the crown with distinct and wide spaced enamel ridges;the enamel ridges on the concave surface of the crown extend almost to the top,while the part of enamel ridges on the convex surface,So these teeth should come from the same individual.The teeth position of specimens from Jinchang was estimated based on the partial tooth position characteristics of the other existing plesiosaurs.These characteristics of Jinchang teeth enrich the information of the tooth morphology of the plesiosaurs of Middle Jurassic in China.3.The plesiosaurs occurred in the non-marine environments are typically dominated by small individuals or juveniles.The comparative analysis of the corresponding relationship between the body and tooth sizes of the known freshwater plesiosaurus shows that Jinchang teeth should be from a small pliosaurid individual.4.Based on the megafossil plants and palynoflora from the lower part,as well as the lacustrine invertebrate assemblage from the upper part of the Xinhe Formation,the plesiosaur-bearing horizon is constrained to the late Bathonian in age.Based on the discussion on the adaptive radiation of the plesiosaurs and the background of palaeobiogeography,this paper proposes a scenario that there may be a river leading to the Meso-Tethys in the late Middle Jurassic in Jinchang area,which provided a channel for the seasonal migration of plesiosaurs.