A new mamenchisaurid from the Upper Jurassic Suining Formation of the Sichuan Basin in China and its implication on sauropod gigantism.

The Sichuan Basin has yielded abundant sauropod dinosaurs from the Middle-Late Jurassic, and Mamenchisauridae had predominated the dinosaur faunae during the Late Jurassic in the Sichuan Basin. Here, we describe a new sauropod dinosaur, Tongnanlong zhimingi gen. et sp. nov. from the Upper Jurassic Suining Formation in the Sichuan Basin, southwestern China. The holotype includes three dorsal and six caudal vertebrae, scapula, coracoid, and some hindlimb bones. It is diagnosed by the complex structures of the dorsal and anterior caudal vertebrae compared to other mamenchisaurids. Phylogenetic analysis shows that it is more closely related to Mamenchisaurus than to Omeisaurus. The huge-sized scapula and coracoid also indicate that the specimen belongs to an extremely gigantic individual with a body length approaching about 25 ~ 26 m. The new specimen enriches the diversity of Mamenchisauridae and provides additional information for understanding the evolution and diversity of eusauropod dinosaurs.