SPECIES RECOGNITION IN CHASMOSAURUS (CERATOPSIA:CHASMOSAURINAE) FROM THE DINOSAUR PARK FORMATION OF WESTERN CANADA

Ceratopsid dinosaurs are diagnosed primarily on cranial ornamentation characters. Some taxa, such as the valid species of Chasmosaurus, C. belli and C. russelli, have very similar cranial ornamentations that have been traditionally difficult to quantify. The validity of these taxa have been supported, in part, by their reported stratigraphic segregation in the Dinosaur Park Formation. A new analysis of taxonomically informative Chasmosaurus specimens from the Dinosaur Park Formation indicates that C. belli and C. russelli have overlapping stratigraphic intervals, but they can be diagnosed by a single phylogenetically informative character—the embayment angle formed by the posterior parietal bars relative to the parietal midline. Neither the length of the postorbital horncores or the morphologies of the epiparietals can be used to consistently distinguish every specimen of C. belli from C. russelli. The data from the suite of skulls examined, including information from a new Chasmosaurus baby (1.5 m long, total length) skeleton, were used to define the ontogenetic growth stages of Chasmosaurus and revealed that C. belli and C. russelli have indistinguishable ontogenetic histories.