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The horned dinosaur  Chasmosaurus  from the late Campanian Dinosaur Park Formation of Alberta, is known from numerous skulls and skeletons, but over a century after its description, the taxonomy of the genus is controversial. Two species,  Chasmosaurus belli  and  C. russelli , are currently recognized, with a third species,  C. irvinensis , recently placed in a new genus,  Vagaceratops . Here, the Yale  Chasmosaurus  skull is described, and implications for  Chasmosaurus  systematics are explored. The Yale skull is intermediate between typical  C. belli  and  C. irvinensis .  C.   belli -like features include large, triangular lateral epiparietals, large parietal fenestrae, and an emarginate parietal. Yet the skull also exhibits derived features of  C. irvinensis , including a posteriorly inclined narial strut, brow horns replaced by rugose bosses, reduced parietal emargination, five pairs of epiparietals, and epiparietals that fuse at their bases and hook forward over the frill. Specimen-level phylogenetic analysis provides a hypothesis of relationships upon which to base the taxonomy of  Chasmosaurus .  C. belli  is paraphyletic with respect to  C. irvinensis , and the Peabody skull is closer to  C. irvinensis  than to other  C. belli . The holotype of  C. russelli  clusters with  C. belli , making  C. russelli  a junior synonym of  C .  belli . Accordingly,  Chasmosaurus  can be divided into three species:  C. belli ,  C. irvinensis , and  C. priscus  sp. nov, including specimens previously referred to  C .  russelli . The systematics of  Chasmosaurus  show how specimen level phylogeny can provide an evolutionary framework upon which to establish taxonomies. However, the resulting phylogenies may lead to paraphyletic species and genera.

Systematics of Chasmosaurus - new information from the Peabody Museum skull, and the use of phylogenetic analysis for dinosaur alpha taxonomy