A new pachycephalosaurid from the Hell Creek Formation, Garfield County, Montana, U.S.A.

ABSTRACT

A partial skull of a pachycephalosaurid from the Upper Cretaceous Hell Creek Formation, Montana, is interpreted as a new taxon, Platytholus clemensi gen. et sp. nov. MOR 2915 does not fit into an ontogenetic continuum of known pachycephalosaurids from the Hell Creek Formation, Montana, and contemporaneous sediments from the Western Interior. Comparisons to known ontogimorphs of Sphaerotholus and Pachycephalosaurus preclude including this specimen into an ontogenetic series of either taxon. We hypothesize that MOR 2915 is a new species based on a relatively low, broad dome at this advanced ontogenetic age that is neither round nor oval in dorsal view, distinct but fused lateral cranial elements fully incorporated into the dome without any dorsal lobe differentiation, and individual tab-like tubercle ornamentation dorsolaterally. Phylogenetic analysis posits that Platytholus clemensi is a Prenocephale-grade taxon deeply nested within Pachycephalosaurinae, but it is not a member of Pachycephalosaurini. Platytholus clemensi is intermediate in size between the other contemporaneous pachycephalosaurids in the Hell Creek Formation and suggests a diverse set of taxa-partitioned ecological niches by body size. We confirm a well organized, major internal vascular network using high resolution computed tomography. Foramina present on the orbital roofs indicate these canals penetrated the entire ceiling of the orbits within the frontal and supraorbital bones. Abundant neurovascular canals passing through the dome to the ectocranial surface indicate a keratinous structure of some kind, possibly with a vertical structural framework, was present on the dome. We review the history of the head-butting hypothesis and associated behavioral implications.

ACKNOWLEDGMENTS

The authors thank the Charles M. Russell National Wildlife Refuge for permission to collect the specimen and reposit it with the Museum of the Rockies vertebrate collection. We also thank E-T. Lamm, MOR, for the histological sectioning; M. Colbert, UTCT, for CT scanning and post processing and J. Maisano, UTCT, for assistance uploading digital files to MorphoSource; and from the UCMP, T. Huynh for 3D animations; D. Smith, for graphics support and figures; D. Strauss for photography; P. Holroyd for helpful discussions; and W. Dunaway for computer and software assistance. A Smithsonian Institution grant to J.R.H. provided funding for this research. NSF grants EAR-1053370 to M.B.G. and EAR-1561622 to UTCT provided additional financial support for this research. D.C.E. was supported by an NSERC Discovery Grant (File Number: RGPIN-2018-06788). The Doris O. and Samuel P. Welles Fund and the UCMP contributed research funds and facilities support to M.B.G. NSF grant DBI-1902242 to J. Maisano and D. Blackburn, UTCT, provided funding for uploading the CT data to MorphoSource. J. Mallon, J. Peterson, and C. Woodruff reviewed the manuscript; T. Lyson, M. D'Emic, P. Godoy, and J. Harris provided editorial and technical guidance, and we thank all of them for their comments and suggestions that improved the quality of the manuscript. We thank J. Scannella and E. Metz, MOR, for facilitating the loan of MOR 2915.

AUTHOR CONTRIBUTIONS

JRH discovered MOR 2915 and posed the question, “Could it be a new species?” All authors participated in answering the question by gathering and analyzing data, then drafting and editing the manuscript for publication.

SUPPLEMENTARY INFORMATION

Supplementary Information_S1.docx : character matrix

Supplementary Information_S2.nex : final data matrix

Supplementary Information_S3.tre : most parsimonious trees