Notes on Pachycephalosaurid Dinosaurs with a Revision of Ornithopod Taxonomy

Abstract

Previously undescribed domes of pachycephalosaurid dinosaurs are described and figured. Stegoceras browni new sp. and Gravitholus albertae new genus, new sp. are proposed for two unusual specimens from the Oldman Formation (Upper Cretaceous) of Alberta. Stegoceras domes from the Judith River and Hell Creek Formations are the first record of this taxon in the United States. A large dome from the Oldman Formation of Alberta is referred to Pachycephalosaurus and is the first record of this genus in Canada. Pachycephalosaurids may have used visual clues to determine male dominance primarily with reference to the size of the dome. The endocranial cast of Yaverlandia shows a separation between the cerebrum and cerebellum, which is typical of ornithopods. However, Stegoceras and Pachyc ephalosaurus lack this separation which may be the result of head butting. A new classification of the ornithopods is presented which divides the group into five infraorders: the Hypsilophodontia, Iguanodontia, Hadrosauria Camptosauria, and Pachycephalosauria.