The small-bodied ornithischian dinosaurs Micropachycephalosaurus hongtuyanensis and Wannanosaurus yansiensis from the Late Cretaceous of China

The type material of the small-bodied ornithischian dinosaurs Micropachycephalosaurus hongtuyanensis and Wannanosaurus yansiensis from the Late Cretaceous of China is reviewed. Micropachycephalosaurus, from the Wangshi Formation (Campanian) of Shandong Province, was originally referred to the clade Pachycephalosauria, but some later authors have considered it a nomen dubium. However, despite the extremely fragmentary nature of the holotype specimen, Micropachycephalosaurus can be diagnosed by the presence of prominent grooves on the ventral surface of the centra of the posterior dorsal vertebra. Unambiguous pachycephalosaur synapomorphies cannot be identified in the currently available material, and we consider this taxon Cerapoda incertae sedis. Wannanosaurus, from the Xiaoyan Formation (Campanian-?early Maastrichtian) of Anhui Province, can be diagnosed by the extreme flexure of the humerus, and is clearly referable to Pachycephalosauria on the basis of a large number of synapomorphies. Assessment of the phylogenetic placement of this Wannanosaurus within Pachycephalosauria is complicated by the probable juvenile nature of the holotype and paratype specimens.