The early expression of squamosal horns and parietal ornamentation confirmed by new end-stage juvenile Pachycephalosaurus fossils from the Upper Cretaceous Hell Creek Formation, Montana

ABSTRACT

New end-stage juvenile specimens of Pachycephalosaurus from the Upper Cretaceous Hell Creek Formation, Montana, confirm the earliest expression of squamosal nodes, parietal ornamentation, and jugal morphology in the smallest and presumably youngest individuals yet known. High-resolution computed tomography of the slightly thickened, undomed parietal reveals a dense cortex, a highly cancellous interior of irregularly shaped erosion cavities, and bony trabeculae indicative of primary, fast growing bone. The parietal, with its highly ornamented septum morphology and patent sutures, is nearly identical to the holotype of ‘Dracorex hogwartsia,’ and combined with these new internal histological details, supports the alternative interpretation that ‘D. hogwartsia’ is a juvenile Pachycephalosaurus wyomingensis. The squamosal nodes grow into an array of horns and secondary nodes exemplified by the pachycephalosaurin ‘Stygimoloch spinifer’ considered in this study to be a subadult P. wyomingensis. Unlike the squamosal ornamentation, the hypertrophied midline row of parietal nodes is transient as the frontoparietal dome expands later in ontogeny. We propose the term ‘ontogimorph’ as a substitute for ‘semaphoront’ to describe these taxon-specific morphological variants that grow allometrically and express extreme cranial morphology along a postnatal growth continuum ontogenetically. These juvenile-, sub-adult-, and adult-specific features in the skull of Pachycephalosaurus may have allowed the visual identification of ontogimorphs and signal their changing sociobiological status.

Citation for this article: Goodwin, M. B., and D. C. Evans. 2016. The early expression of squamosal horns and parietal ornamentation confirmed by new end-stage juvenile Pachycephalosaurus fossils from the Upper Cretaceous Hell Creek Formation, Montana. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2016.1078343.

ACKNOWLEDGMENTS

We thank D. Dufault for executing the figures. Scans of the complete parietal of ROM 53672 were completed by M. Colbert, University of Texas High-Resolution X-ray CT Facility, Austin. D. Smith provided additional graphics support. A. Farke and T. Williamson reviewed an earlier draft of this article; R. Butler provided editorial guidance, and we thank all of them for their useful comments and suggestions. The specimens described in this study were acquired by the ROM through a grant from the Louise Hawley Stone Charitable Trust. National Science Foundation grants EAR-1053370 to MBG and EAR-1258878 to UTCT supported this research. Additional research support was provided by the U.C. Museum of Paleontology to MBG and a Natural Sciences and Engineering Research Council of Canada Discovery Grant to DCE. The term, ‘ontogimorph’ was coined by D. Fowler in 2007 and used informally in the Horner Lab, Museum of the Rockies. This is UCMP contribution no. 2065.