Quantifying vascularity in the frontoparietal dome of Stegoceras validum (Dinosauria: Pachycephalosauridae) from high resolution CT scans

ABSTRACT

The thickened frontoparietal in pachycephalosaurid dinosaurs expands dramatically during ontogeny from a flat-headed to a domed state. This expansion results in the formation of zonal tissue characterized largely by differences in vascularity and bony tissue structure that changes through ontogeny. Void space identified in CT scans of the frontoparietal is a suitable proxy for relative vascularity. An increase in relative vascularity occurs with the development of the dome in the pachycephalosaur Stegoceras validum, followed by a significant decrease in late stage ontogeny. We employ a script adapted from an algorithm for human cortical bone imaging to: (1) determine the percent vascularity in any given CT slice; (2) quantify these observed changes in relative vascularity within a complete frontoparietal; and (3) identify ontogenetic changes in vascularity from a cranial growth series of Stegoceras. Morphological landmarks identified in the CT scans facilitate an accurate slice-by-slice comparison of homologous regions of the dome between skulls. This new tool enables: (1) a complete assessment of bone vascularity from CT scans; (2) is applicable to any fossil or modern bone in the vertebrate skeleton; and (3) provides an alternative measure to pixel-by-pixel manual thresholding, a time intensive and subjective process.

ACKNOWLEDGMENTS

The National Science Foundation grants EAR-1053370 to M.B.G. and EAR-1561622 to UTCT provided financial support for this research. The Doris O. and Samuel P. Welles Fund and the UCMP contributed funding and facilities support to M.B.G. This research was also supported by an NSERC Discovery Grant (File Number: RGPIN-2018-06788) to D.C.E. Funding for uploading the CT data to MorphoSource was provided by NSF grant DBI-1902242 to J. Maisano and D. Blackburn. Discussions with C. Marshall, P. Holroyd, and R. Full were beneficial. P. Currie granted access to the CT scans of UAVLP 2. D. Strauss provided photographs of AMNH 5450, TMP 1984.005.0001, and UCMP 130051. W. Dunaway assisted with computer and software support. L. Weaver and an anonymous reviewer reviewed an earlier draft of this article; M. D’Emic provided editorial guidance, and we thank all of them for their useful comments and suggestions. We thank C. Mehling (AMNH) and B. Strilisky (TMP) for facilitating loans of AMNH 5450 and TMP 1984.005.0001 respectively. This is University of California Museum of Paleontology contribution 3011.