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Skull ornamentation in juvenile Pachycephalosaurus fossils from Upper Cretaceous Hell Creek Formation

Ben Creisler

A new paper:

Mark B. Goodwin & David 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 (advance online publication)
DOI: 10.1080/02724634.2016.1078343

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.