New data and diagnosis for the Arctic ceratopsid dinosaur Pachyrhinosaurus perotorum

The pachyrostran centrosaurine dinosaur Pachyrhinosaurus perotorum is the geologically youngest (Maastrichtian, 70–68.5 Ma) centrosaurine, and latitudinally highest distributed ceratopsid yet known. Continued preparation of material collected from the type locality, the Kikak-Tegoseak Quarry, has produced more examples of cranial material from multiple individuals, including partial skulls and incomplete parietals. The original reconstruction of the type parietal was incorrect, and the element is similar to that of other Pachyrhinosaurus species in bearing medially directed epiparietal 2 processes along its posterior margin. Pachyrhinosaurus perotorum is diagnosed by an upturned tip of the rostrum; a dorsally shifted rostral bone lacking a sharply downturned, parrot-like beak; an enlarged median ridge at the posterior end of the nasal boss; and, tentatively, a posterior sulcus on epiparietal 2 and a canal passing dorsoventrally through the base of epiparietal 2. A cladistic phylogenetic analysis incorporating new data from this and other recent studies of centrosaurine relationships recovers a monophyletic Pachyrhinosaurus clade. Pachyrhinosaurus perotorum and P. canadensis are found to be sister taxa, united by the presence of an extra ossification on the lateral surface of the rostrum between the narial fossa and nasal boss, and by enlarged supraorbital bosses that contact or nearly contact the posterior end of the nasal boss. Parietal and squamosal frill ornamentations alone do not adequately address the variables in craniofacial morphology needed to distinguish between species of Pachyrhinosaurus.