The recent discovery of a nearly complete ceratopsid skull in the Aguja Formation of Southwest Texas supports previous conclusions that the Aguja ceratopsid represents a distinct species, Chasmosaurus mariscalensis. The diagnostic feature of C. mariscalensis include an extensive anteromedian progection of the nasal between the premaxillae, erect supraorbitl horns, and laterally rounded squamosal. Nine cranial features that vary among Chasmosaurus species, Pentaceratops sternbergii, and other chasmosaurines are analyzed. Chasmosaurus mariscalensis appears to be most closely related to northern species of Chasmosaurus (C. belli, C. russelli), which also exhibit a transversely flattened nasal horn and modifications of the anterior margin of the external naris. The genus Chasmosaurus, in turn, appears to be most closely related to the other southern chasmosaurine, Pentaceratops sternbergii. The biogeographic history inferred from these relationships suggests that the biogeographic exchange between northern and southern chasmosaurines cannot be explained by a single dispersal event to the south.
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