The chasmosaurine ceratopsid Chasmosaurus is known from the Upper Cretaceous Dinosaur Park Formation (DPF) of southern Alberta and Saskatchewan. The two species, Chasmosaurus belli and Chasmosaurus russelli, are distinguished by differences in cranial ornamentation, and their purported stratigraphic segregation in the DPF, with C. russelli replaced by C. belli over time. In the DPF, Chasmosaurus is replaced by a third chasmosaurine, Vagaceratops irvinensis. The results of the analyses conducted indicate that 1, C. belli and C. russelli lack phylogenetic, stratigraphic or ontogenetic separation, but can be distinguished by the relative embayment of their posterior frill margin; 2, Vagaceratops is the sister taxon to Chasmosaurus; 3, the Chasmosaurus specimens AMNH 5402 and YPM 2016 are referable to Vagaceratops, and may represent directed morphological change over time; 4, no stratigraphic separation exists between Chasmosaurus and Vagaceratops. C. belli and C. russelli are considered to be synonymous, with C. belli having taxonomic priority.