Corrections and thought-provoking suggestions from two anonymous reviewers greatly improved the manuscript. We thank G. Paul for pachycephalosaur mass estimates, and M. Vickaryous, D. Evans, N. Longrich, R. Fox, and R. Sissons for discussions. C. Kriegner and T. Miyashita donated photographic services, and M. Manabe provided specimen numbers. A. Bauer, R. Gardner, R. Curry, J. Johnson, and J. Prazenika (Villanova University) generously facilitated the research. A. Russell provided use of phylogenetic inference software. Funding for this study came from the Department of Biology at Villanova University, a Jurassic Foundation grant to E.S., field assessments by A.P. Russell and Associates through University of Calgary Research Services, and an NSERC Discovery grant to A. Russell. This report is Publication No. 1 of TGICEA, Villanova University BIO 7970.