Revision of the dinosaur Stegoceras Lambe (Ornithischia, Pachycephalosauridae)

Abstract

ABSTRACT Stegoceras (sensu lato) has been the recipient of a number of species. Most have been placed into synonymy with S. validum, some have been transferred to other genera (i.e., Gravitholus, Ornatotholus), whereas others have been interpreted as being sexual dimorphs. A parsimonious phylogenetic analysis using 49 characters, which include 9 new cranial characters based on pachycephalosaur frontoparietal domes, now permits a revision of the genus Stegoceras. This analysis concludes that Stegoceras validum (sensu stricto) is a primitive, incipiently-domed pachycephalosaur that is characterized by a well-developed squamosal shelf and open supratemporal fossae. It is the sister taxon to the fully-domed Pachycephalosaurinae. The taxon Ornatotholus browni has a parietal and displays features identical to S. validum and is therefore a subjective junior synonym. Specimens consisting of flat, paired frontals are immature individuals of S. validum. The holotype of “Stegoceras” lambei, and all morphotypes of this species, are unique in the construction of the posterior squamosal region and are placed in a new genus, Colepiocephale. Colepiocephale lambei is known solely from the Foremost Formation and is the oldest diagnostic pachycephalosaur from North America. An incomplete, and indeterminate, skull from the upper part of the Milk River Formation (upper Santonian) has the distinction of being the oldest known North American pachycephalosaur. “S.” sternbergi lacks a posterior squamosal shelf and has squamosals directed laterally and is placed in a new genus, Hanssuesia, as H. sternbergi. Although the taxon Gravitholus albertae displays some characters (i.e., retention of the posterior squamosals separated by a distinct median parietal) and is similar to H. sternbergi in some respects, the holotype is too incomplete for any definitive diagnosis and thus it is considered a nomen dubium. Sphaerotholus is a subjective junior of Prenocephale and the species Sphaerotholus buchholtzae is a subjective junior synonym of Prenocephale edmontonensis. The taxa Prenocephale brevis, P. edmontonensis, and P. goodwini, form a monophyletic clade with monotypic Asian taxa Prenocephale prenes and Tylocephale gilmorei as an unresolved sister group. They are united by the possession of a distinct row of nodes on the squamosal and parietale. Tylocephale, if valid, is interpreted as the sister taxon to the Prenocephale clade. Stenotholus kohleri is formally recognized as a junior synonym of Stygimoloch spinifer. Based on the presence of hypertrophied nodes Stygimoloch spinifier and Pachycephalosaurus wyomingensis are united in the clade Pachycephalosaurini new taxon. Small-to-medium size Late Cretaceous pachycephalosaurs were rather diverse, especially during the late Campanian (ludithian), and many species coexisted during Campanian and Maastrictian times. Ancestry and directionality of dispersal (in part) of the North American and Asian taxa remains uncertain and certainly antedates Campanian time.