Cranial Morphology, Taxonomy, and Systematics of Pachycephalosaurids (Dinosauria, Ornithischia)
Creators: Dyer, Aaron David
Publisher: name: University of Alberta Library
Year: 2022

<p>Pachycephalosauridae (pachcycephalosaurids) were small to medium sized bipedal ornithischians, known solely from the Late Cretaceous of North America and Asia. These dinosaurs are characterised by thick, often domed frontals and parietals (frontoparietal dome), which are thought to have been used in intraspecific head-butting combat rituals. The dome, along with their thickened skull bones have a high preservation potential, particularly compared to the rest of the skeleton. Thus, the vast majority of pachycephalosaurid fossils consist of their unique skull bones, and so much research on the group has focused on these elements. The morphology of the thickened skull roof is included in the diagnosis of every species, yet the skull roof changes dramatically through ontogeny, and numerous putative species have been reidentified as juvenile or subadult representatives of other species.<br>
In this thesis, I focus on exploring morphological variation amongst pachycephalosaurid skulls, particularly the frontoparietal dome, and address its utility in pachycephalosaurid taxonomy and phylogenetics. I begin with describing the anatomy of the only known specimen of the controversial Gravitholus albertae, from the Late Cretaceous of Alberta, Canada. The specimen represents a heavily fused partial skull roof, and the degree of fusion has prevented a detailed description of the specimen, thus the validity of the species has been repeatedly justified and challenged. Alternative taxonomic hypotheses suggest Gravitholus albertae may be synonymous with Hanssuesia sternbergi or Stegoceras validum). I utilise synchrotron µCT imaging to identify the locations of the holotype’s fused contacts, which facilitates a detailed description of the specimen. Gravitholus albertae was then included with Hanssuesia sternbergi and Stegoceras validum in morphometric analyses, to evaluate the distinctness of these three species. There are no ontogenetic-independent features of “Gravitholus albertae” that would justify a unique species. Instead, Gravitholus albertae and Hanssuesia sternbergi are morphologically consistent with mature Stegoceras validum. Thus, Gravitholus albertae and Hanssuesia sternbergi are synonymous with Stegoceras validum. Interestingly, Stegoceras validum appears to possess adult dimorphism in the thickness of the frontonasal boss, which is not explained by previous taxonomic hypotheses. Instances of post-traumatic injuries, consistent with head-butting, appear restricted to individuals with thicker frontonasal bosses. The dimorphism in Stegoceras validum is interpreted as sexual dimorphism, with the thicker bossed sex engaging in ritualistic intraspecific combat.<br>
    I then move onto assessing morphological variation in pachycephalosaurid frontoparietals by statistically testing previously proposed discrete character states used in phylogenetic analyses, attempt to identify new characters and states, and comment on the validity of Stegoceras novomexicanum. The use of several features previously used as structures for phylogenetic characters are supported, and the distinction of their character stares are statistically demonstrated. These states are broadly consistent with previous taxonomic assessments, although a few species in each revised character are reassessed. There is no morphological support for “Stegoceras novomexicanum” and is regarded as Pachycephalosauridae indeteterminate. “Stegoceras novomexicanum “, along with other invalidated pachycephalosaurids, were removed from a phylogenetic analysis based on a revised morphological character matrix. This phylogenetic tree of Pachycephalosauria (pachycephalosaurians) is broadly similar to pervious analyses. The main differences include recovering Colepiocephale lambei as a basal Pachycephalosaurinae (pachycephalosaurines), and a polyphyletic Sphaerotholus. Derived pachycephalosaurines appear to be united by a cranial dome that initially develops on the parietals (as opposed to initially developing on the frontals). This distinction deserves further investigation with histological and CT methodologies to determine the developmental pathways that different pachycephalosaurid species took to grow their domes. </p>