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.
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