Notes on pachycephalosaurid dinosaurs (Reptilia: Ornithischia) from North America, with comments on their status as ornithopods

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Abstract
Several biologically significant domes of pachycephalosaurid dinosaurs are described and figured. One unusual specimen from the Oldman Formation (Upper Cretaceous) of Alberta [Canada] is placed into a new genus [GRAVITHOLUS albertae gen. et sp. nov.] and another specimen from the same formation is assigned to a new species of Stegoceras [S. browni ; other species include S. validus, S. edmontonensis and Stegoceras sp. undetermined]. Domes referable to Stegoceras sp. (Judith River and Hell Creek Formations) are the 1st conclusive evidence of the presence of this genus in [Montana] USA. A large dome from the Oldman Formation of Alberta is referred to Pachycephalosaurus sp. undetermined; this specimen is the oldest described to date and is the 1st record of this genus in Canada. Endocranial casts of Yaverlandia bitholus and S. validus are described along with a discussion of endocranial trends in pachycephalosaurids. The separation between the cerebrum and cerebellum found in Yaverlandia, and typical of ornithopods in general, is lacking in Stegoceras and Pachycephalosaurus. The loss of this separation may be the result of head butting. The family Pachycephalosauridae possesses sufficient ornithopod characters to justify their retention in that suborder. It does represent an aberrant side branch of ornithopod evolution, which can best be taxonomically visualized by placing this family into a separate infraorder within the Ornithopoda.

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