VISUALIZING CRANIAL AND SUTURAL MORPHOLOGY OF PACHYCEPHALOSAUR DOMES USING COMPUTED TOMOGRAPHY

The frontoparietal dome in pachycephalosaurs, or “dome-headed” dinosaurs, expands dramatically from a flat-headed to a domed-morph during ontogeny. This expansion results in the formation of zonal bone tissue characterized by differences in relative vascularity or void space, variable osteohistology, and a micron to sub-micron neurovascular network. Previous studies demonstrated that void space identified in CT scans is a suitable proxy for relative vascularity. A decrease in relative vascularity correlates with the development of the dome during ontogeny. We use a novel script written in MATLAB adapted from an algorithm for human cortical bone imaging to quantify relative vascularity in a cranial growth series of Stegoceras validum. Precise volume renderings of dome growth, suture morphology, and relative vascularity are obtained using high resolution CT and synchrotron micro-CT scans. Morphological landmarks enable a precise slice-by-slice comparison between skulls.

Dome growth proceeds anteroposteriorly in Stegoceras validum; the frontals inflate first, followed by the parietal. MATLAB analysis of the CT scans indicates relative vascularity decreases in a growth series (n=5) of S. validum from 14.9% in a “flat-headed” juvenile (AMNH 5450) to 2.6% in an adult ontogimorph (UCMP 130051). We confirm the presence of three osteohistological zones within the frontoparietal dome. The percent vascularity plotted against the CT slice number(s) shows a clear distinction between these zones in subadults. Conversely, other specimens have poorly defined zonal boundaries with more consistent vascularity. Lines of arrested growth appear to preserve the silhouette of pre- and post-expansion of the frontals and parietal into a dome. Cranial suture morphology and the degree of suture obliteration are highly variable ontogenetically and not a reliable indicator of skeletal age in S. validum.

This new method provides an accurate assessment of relative vascularity in bone tissue. Landmark standardization allows the analysis and comparison of multiple specimens simultaneously. Preliminary visualization of cranial sutures and vascular network architecture using computed tomography shows even more promise for modeling growth dynamics in these dinosaurs and in other fossil vertebrates.