A Taxonomic Review of the Pachycephalosauridae (dinosauria: Ornithischia)

The order “Pachycephalosauria” and the family Pachycephalosauridae are re-evaluated and determined to include the same taxa. They are thus of equal rank and redundant, so only the latter taxon is recognized, the former one being redundant. The family Pachycephalosauridae includes the so-called “flat-headed” and “dome-headed” morphotypes. Stenopelix valdensis is removed from the “Pachycephalosauria” and is considered to be an indeterminate ornithischian dinosaur. The North American taxa Colepiocephale lambei, Stegoceras validum, Prenocephale brevis, P. goodwini, P. edmontonensis, and Hanssuesia sternbergi are valid taxa. All species of the genus Prenocephale are distinguished by a single row of nodes on the lateral and posterior side of the squamosals. Alaskacephale gangloffi, n. gen. and sp., is a distinct taxon based on a squamosal characterized by two diverging row of nodes. The flat-headed Asian taxa Goyocephale lattimorei, Tylocephale gilmorei and Homalocephale calathocercos are considered valid pachycephalosaurids, although the validity of G. lattimorei is somewhat problematic. The node-based taxa Goyocephalia, Homalocephaloidea and Tholocephalidae are rejected as unsubstantiated typological conventions. Wannanosaurus yansiensis is considered Pachycephalosauridae incertae sedis. The distinctive corner node seen in such taxa as Prenocephale and Homalocephale is homologous with the large, hypertrophied central spike emanating from the squamosal in Stygimoloch and Dracorex. The holotype of Tylosteus ornatus, a small squamosal with broken spikelike nodes and low bulbous nodes, is not referable to Pachycephalosaurus wyomingensis, but rather is consistent with the ornamentation of Dracorex hogwartsia. The taxon Ferganocephale adenticulatum from the Middle Jurassic of Central Asia is not a “pachycephalosaur” and is considered to be a nomen dubium. A robust cladistic analysis of the Pachycephalosauridae is not presently possible due to uncertainty in the polarities of characters.