CRANIAL REDESCRIPTION OF STEGOCERAS VALIDUM GIVES FIRST LOOK INTO SKULL MORPHOLOGY IN PACHYCEPHALOSAURIA (DINOSAURIA: ORNITHISCHIA) INFORMED BY X-RAY COMPUTED TOMOGRAPHY

Pachycephalosauria is a clade of small-bodied ornithischian dinosaurs from the late Cretaceous of North America and Asia, known for their cranial ornamentation and thickened skull roofs. Most pachycephalosaurian specimens consist of isolated cranial bones, with frontoparietal skull elements being the most common. Post cranial elements are rare, so much of our understanding of pachycephalosaurian phylogenetics is based on incomplete cranial material. Thus, the phylogenetic relationships of Pachycephalosauria within Ornithischia are unclear. Relationships within Pachycephalosauria are also clouded by variation in ontogenetic development of cranial ornamentation and frontoparietal domes. Although X-ray computed tomography (CT) has been applied to pachycephalosaurian specimens for histological studies and nasal cavity reconstruction, CT data has not been used to assess the complete cranial skeleton. Here, we present the first complete cranial description for Pachycephalosauria informed by CT scans and corroborated by direct osteological observation. We reconstruct the cranium, mandible, and dental morphology of a Stegoceras validum specimen (UALVP-2), one of the best-preserved examples of North American Pachycephalosauridae. We find evidence that UALVP-2 is a subadult specimen based on the presence of multiple sutures with variable patency throughout the cranium. Partial suturing exists at the medioventral contact of the frontal and parietal bones but is completely fused in the coronal region of the cranial dome. We also find asymmetrical suture patency and fusion with two supraorbital bones present on the right and a singular fused supraorbital complex present on the left side of the skull. In contrast to previous anatomical descriptions, we did not find any fusion of the prefrontal bones with the supraorbital bones on either side of the skull. Our work underscores the utility of CT scanning for investigating key anatomical and developmental characteristics that will be critical for future clarification of pachycephalosaurian ontogenetics, systematics, and taxonomy. We encourage further efforts to digitize additional pachycephalosaurian specimens to facilitate comparative studies.