Abstract

Only the type specimen of Arrhinoceratops brachyops Parks can be assigned without question to the ceratopsid genus Arrhinoceratops. The original description, however, requires revision. Contrary to published accounts, a nasal horn core is present, the jugal cannot be characterized by an unusually long anterior process, and there is no nasal–rostral contact in this or any other known ceratopsian. Evidence for an interparietal is ambiguous. These and other observations necessitate a redefinition of the genus.Arrhinoceratops belongs to the long-squamosaled ceratopsids but is not closely related to any other genus in this lineage. With respect to characters of the frill, Torosaurus is the closest relative of Arrhinoceratops. Since the frills of Arrhinoceratops and Torosaurus form a graded series, the practice of automatically assigning isolated, fenestrated frills from Lance equivalent sediments to Torosaurus and from Horseshoe Canyon equivalent sediments to Arrhinoceratops should be abandoned.

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