Early Cretaceous Dinosaur And Other Tetrapod Tracks Of Southwestern China

A recent rapid increase in tracksite discovers has brought on an ichnological renaissance in Chinese Mesozoic paleontology.Over 200 Mesozoic tetrapod tracksites were known in 2015,compared with only 27 such sites in 1989.Among these records,the Lower Cretaceous red beds of southwestern China have yielded more than 20 significant dinosaur-dominated tracksites.More than half occur in the Jiaguan Formation from the Sichuan Basin with the remainder in the Feitianshan and Xiaoba formations from the Mishi-Jiangzhou Basin.All these tracksites,except Guanyuanchong and Chuanzhu sites,were found and described by the authors and colleagues since 2007.Collectively these sites from southwestern China provide evidence of at least 13 distinct dinosaurian track morphotypes(including three new ichnospecies),in addition to two avian theropod(bird)morphotypes and pterosaur and turtle tracks.The 17 morphotypes constitute a data base of 479 potential trackmakers,probably representing the same number of individuals.Such an ichnological database provides a useful proxy paleoecological census of tetrapod communities in the area during the early Cretaceous,and is especially significant given the complete absence or sparsity of skeletal remains reported from these formations.The track record demonstrates a strong similarity between the dinosaur faunal assemblages from the Sichuan Basin and the Mishi-Jiangzhou Basin.Both include ornithopod,sauropod,non-avian theropod,and pterosaur tracks,while the Jiaguan Formation also yields thyreophora and bird tracks and Feitianshan Formation yields possible turtle tracks.The record from the Xiaoba Formation is far less diverse,and consists only of theropod and sauropod tracks.None of the formations have yielded crocodilian tracks.Thus the ichnofaunas from all three formations are saurischian(theropod and/or sauropod)dominated.In this regard it should be noted that the broad category of “theropod tracks” includes at least 6–8 distinct categories,including small and large tridactyl tracks in the Grallator-Eubrontes group,two didactyl morphotypes(Velociraptorichnus and Dromaeopodus),and the distinctive morphotype Minisauripus.Thus,not only are theropod tracks dominant in these ichnofaunas,they also represent significant diversity within the theropod clade.The composition of China's Early Cretaceous ichnofaunas implies significant regional signature: Dromaeopodus–Velociraptorichnus–Minisauripus assemblage is typical of East Asia but not of other regions(southwestern China–Shandong–South Korea,Barremian–Albian).Track assemblages in the southwestern China,such as the relatively rich deinonychosaur record,suggest that its Early Cretaceous dinosaurs may be more closely related to the Jehol Biota radiation than previously thought.If this is the case there are implications indicating that local geographical isolation,thought to have been brought on by the formation of mountains,was not as significant previously suspected.Generally,this pattern of “track only” and “track dominated” assemblages appears tobe typical of Lower Cretaceous red bed deposition in southwestern China,and may prove that the utility of tracks far supersedes the skeletal record in providing paleoecological information on regional tetrapod abundance,diversity and distribution.