Cranial morphology and systematics ofChasmosaurus(Dinosauria: Ceratopsidae) from the Upper Cretaceous of western Canada

Abstract

ABSTRACT The ceratopsid dinosaur genus Chasmosaurus from the Judith River Formation (Campanian; Judithian) of western Canada shows considerable variability in cranial morphology due in part to an unusually long ontogenetic trajectory. Nevertheless, two more or less distinct morphs can be identified. Allometry and sexual dimorphism cannot be invoked to account for this variability, leaving the presence of two species as the most likely explanation. In C. belli, the posterior margin of the parietal frill shows little or no emargination and bears one large triangular epoccipital on each posterolateral corner. Additional epoccipitals may be present, but are much less prominent. Chasmosaurus Canadensis, Chasmosaurus kaiseni, and Chasmosaurus brevirostris also show this morphology, and are here considered junior synonyms of Chasmosaurus belli. Chasmosaurus russelli is characterized by a deeply emarginated posterior parietal margin. On each side, the posterior margin of the frill forms a broad arch, bearing thre...