Morphological variation and asymmetrical development in the skull of Styracosaurus albertensis

The ceratopsid Styracosaurus albertensis is an important component of the herbivorous dinosaur fauna preserved in the Campanian-aged Dinosaur Park Formation in Alberta, Canada. A new skull (UALVP55900) exhibits previously unobserved variability and asymmetry in this species. As in the type, the right lateral parietal bar bears seven epiossifications, but the left parietal bar has eight. Epiossifications p3−p6 are asymmetrical with respect to size, orientation, and position relative to the midline. A re-evaluation of variation in Styracosaurus that includes this skull expands the morphological range of the taxon to incorporate Rubeosaurus ovatus, requiring the synonymy of the latter with the former. Unlike the type, both p1 and p2 epiossifications of UALVP 55900 are large. Epiossification p1 is procurved, and p2 is medially curved. These features converge on the morphology considered diagnostic for Centrosaurus. This observation, in combination with their sequential stratigraphic relationship and geographic overlap might suggest that Centrosaurus and Styracosaurus form an anagenic series, but the stratigraphic placement of skulls exhibiting putative transitional morphologies does not support this hypothesis.