New data and diagnosis for the Arctic ceratopsid dinosaur Pachyrhinosaurus perotorum

Abstract

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.

Keywords:

• Cretaceous
• Alaska
• Prince Creek Formation
• Centrosaurinae
• Pachyrostra
• cladistics

Acknowledgements

We thank the numerous members of the various field crews that contributed to the data collection for this project. Particularly, we thank Thomas Adams, Christopher Strganac, Kent Newman and Jason Petula. We also thank fossil preparators Briana Smith, Rory Leahy and Thomas Diamond, as well as Olivia Fiorillo and the multitude of Perot Museum Paleo Lab volunteers who helped extract and prepare many of these specimens for study. Notwithstanding these acknowledgements, the authors take all responsibility for the conclusions drawn here. We also thank Jane and Ron Gard, Cathey and Donald Humphreys, and Virginia and Ansel Condray for their original support of this Perot Alaska dinosaur project. We thank Michael Ryan (Cleveland Museum of Natural History) and David Evans (Royal Ontario Museum) for discussion and Caleb Brown (TMP) for providing specimen photos. The Barrow Arctic Science Consortium (BASC) and CH2M Hill (formerly Veco Polar Resources) provided logistical support for this project. Funding was provided by the National Science Foundation Office of Polar Programs [OPP 0424594], as well as the National Geographic Society [W221-12]. Lastly, the Arctic Management Unit of the Bureau of Land Management provided administrative support. The specimens discussed here were collected under BLM permit number AA–86367.

Supplementary data

Supplemental material for this article can be accessed at: http://dx.doi.org/10.1080/14772019.2018.1532464.