Small-bodied ornithischian dinosaurs from the Middle Jurassic of Sichuan, China

Abstract

ABSTRACT Three taxa of small-bodied ornithischian dinosaurs have been described from the Middle Jurassic (Lower Shaximiao Formation: ?Bajocian) locality of Dashanpu, Sichuan Province, People's Republic of China. The taxonomy of these animals (Yandusaurus multidens, Xiaosaurus dashanpensis, and Agilisaurus louderbacki) has become highly confused, with many conflicting opinions regarding the validity of each species and the possible synonymies that might exist between them and other ornithischian taxa from both China and North America. Following re-examination of the relevant material, we propose that all three of the Dashanpu small ornithischians represent valid taxa, each of which can be diagnosed on the basis of at least one autapomorphy. In addition, these taxa can be distinguished from each other by a large number of cranial, dental, and postcranial character states. We demonstrate that ‘Yandusaurus’ multidens is not congeneric with either Y. hongheensis or any other taxon, and therefore propose the new generic name Hexinlusaurus for the reception of the species ‘Y.’ multidens. Consideration of the character states present in Hexinlusaurus, Agilisaurus, and Xiaosaurus shows that these taxa are not basal euornithopods (‘hypsilophodontids’), as had been suggested by the majority of previous authors, but represent more basal ornithischians. Hexinlusaurus and Agilisaurus can be regarded as Genasauria incertae sedis, whereas Xiaosaurus can only be recognized as Ornithischia incertae sedis on the basis of current data.