Studies of large-scale diversity changes and patterns of evolution can be adversely affected by a lack of understanding of alpha-taxonomy and systematics, and pachycephalosaur dinosaurs have significantly contributed to this problem. This is primarily a result of the relatively incomplete nature of the pachycephalosaur fossil record and the lack of understanding of specific and ontogenetic variation within the group. The taxon previously known as ‘Prenocephale’ brevis has been particularly problematic and has been placed in several different genera in addition to being synonymized with the species Stegoceras validum multiple times. Here we evaluate the validity of this taxon based on old and new material and using multiple, independent lines of evidence. We identify and describe the first peripheral skull elements, as well as the first juvenile and adult specimens, assignable to this taxon. These provide information on key parts of the morphology that have previously been lacking. Comparative anatomy, bivariate and multivariate morphometrics, osteohistology, and systematic analyses were used to test the hypothesis that this material is representative of a distinct taxon and to evaluate the differences between the other pachycephalosaur species of the closely related Belly River Group assemblage. No support was found for the assignment of ‘P.’ brevis to Stegoceras or Prenocephale, and thus we have erected a new genus, Foraminacephale gen. nov., to contain this species. Our data support the conclusion that the Foraminacephale brevis material represents an ontogenetic growth series of a single species distinct from St. validum. Evaluation of the two other members of the Belly River Group assemblage (Hanssuesia sternbergi and Colepiocephale lambei) was hindered by the small number of known specimens, but our results suggest that specimens currently referred to H. sternbergi may represent at least two species. We find that both ontogeny and osteohistology can vary greatly amongst even closely related species. It is therefore critical that multiple, independent lines of evidence be used to establish the ontogenetic trajectories of species prior to delineation, and that results of these analyses are used to produce robust and phylogenetically informative characters for use in phylogenetic analyses.