First record of a eusauropod (Dinosauria: Sauropoda) from the Upper Jurassic Qigu-Formation (southern Junggar Basin, China), and a reconsideration of Late Jurassic sauropod diversity in Xinjiang

An isolated tooth-crown from the middle Qigu Formation (Late Jurassic, ?Oxfordian-Kimmeridgian) of Liuhuanggou near Urumqi, southern Junggar Basin, Xinjiang Uygur Autonomous Region, Peoples Republic of China, is described. Its morphological features, including shape and cross-section of the crown and morphology of the wear facet closely approach the well-known Chinese sauropod genus Mamenchisaurus. The morphology of the lingual depression and the lack of denticles (unlikely to be caused by wear) even more approaches Euhelopus. The tooth is referred to ?Mamenchisauridae gen. et sp. indet., but it may possibly represent an early euhelopodid, a family not yet known from the Jurassic. It is the first record of a eusauropod from the Qigu-Formation of the southern Junggar Basin. If indeed a mamenchisaurid, a family well-known from the northern Junggar Basin and the adjacent Turpan Basin, it might fill a palaeobiogeographic gap and underlines faunistic similarity between the Qigu Formation of the southern Junggar Basin and the Shishugou Formation of the northern Junggar Basin. If it is a euhelopodid, it would be the earliest known representative of that family. New data on the age of the Qigu Formation indicate that Xinjiang shows the highest diversity of Late Jurassic sauropods currently known from Asia. This may in part be the result of taxonomic oversplitting, as there is little, if any, overlap in the type specimens of several named taxa.