Gongbusaurus and the early phylogeny of the Neornithischia

Abstract

Gongbusaurus shiyii (type species of the genus) is based on a premaxillary tooth and a cheek tooth from the Oxfordian of the Upper Shaximiao Formation in Rong County (Sichuan Province, P. R. China). The taxon was first considered a fabrosaurid, but it was afterwards regarded as a hypsilophodontid ornithopod. More recently, it has been suggested that it may be an ankylosaur. The dentition of G. shiyii is of a primitive type so that determining affinities on this basis is problematical. It is possibly a basal neornithischian, but it is provisionally best assigned to Ornithischia indet. “Gongbusaurus” wucaiwanensis is based on a fragmentary mandible, caudal vertebrae, and an incomplete forelimb (type specimen) from the Oxfordian of the Shishugou Formation in Fuyun County (Xinjiang Province, P. R. China). The paratype and the referred specimens described with the type include dorsal, sacral, and caudal vertebrae, an incomplete pelvis, and hind limbs. The taxon was first considered a hypsilophodontid, but it was subsequently identified as a fabrosaurid. The caudal process of the ilium with a distinct brevis shelf that first turns medially and then downwards (visible in lateral view) shows that “G.” wucaiwanensis is more primitive than Hexinlusaurus multidens (Bajocian, P. R. China). Furthermore, the degree of development of the pubic peduncle of the ilium suggests that “G.” wucaiwanensis is more basal than Agilisaurus louderbacki (Bajocian, P. R. China). Nevertheless, the caudal extension of the caudal process of the ilium in “G.” wucaiwanensis indicates that this taxon is more derived than Lesothosaurus diagnosticus (Hettangian, southern Africa). In conclusion, “Gongbusaurus” wucaiwanensis, Agilisaurus louderbacki, and Hexinlusaurus multidens appear to form a phylogenetic sequence between the most basal neornithischian (Lesothosaurus diagnosticus) and more derived taxa such as Othnielosaurus consors (Kimmeridgian, USA). This highlights the relevance of the Chinese Jurassic to improve our knowledge of the early phylogeny of the Neornithischia.