A revision of the late campanian centrosaurine ceratopsid genus Styracosaurus from the Western Interior of North America

Abstract

ABSTRACT The centrosaurine ceratopsid genus Styracosaurus is known from multiple specimens and a multigeneric bone bed in the upper 30 m of the Late Cretaceous Dinosaur Park Formation of southern Alberta, and a single specimen (S. ovatus) from approximately time equivalent sediments of the Two Medicine Formation of Montana. Key cranial elements (nasals and postorbitals) of Styracosaurus appear to undergo similar ontogenetic changes as documented in Centrosaurus. Although all adult-sized centrosaurines except Centrosaurus apertus are known to possess spike-like parietal ornamentation at the P3 position on the parietal, only Styracosaurus has the P4 ornamentation expressed as a well-developed spike. Styracosaurus shows intraspecific variation in the shape of the more anteriorly placed P5–P7 ornamentation that are either the typical unmodified crest-shaped epoccipitals of other centrosaurines or are developed as short spikes. S ovatus from Montana is retained as a valid species based on the autapomorphic convergence of the P3 spikes toward the midline.