The Postcranial Anatomy of Pachycephalosaurs (Ornithischia: Pachycephalosauria) and its Phylogenetic and Myological Implications

Abstract

Pachycephalosaurs are a group of small-bodied, bipedal ornithischian dinosaurs.They are best known for their characteristic fusion of the frontals and parietals into a "skull dome". This feature is the most commonly preserved element of pachycephalosaurs and therefore much research has been dedicated to their cranial anatomy. The postcranial skeleton is not often preserved and does not receive the same attention. CMN 22039 at the Canadian Museum of Nature represents a rare postcranial specimen. The goal of this study is to use CMN 22039 with other comparable specimens to investigate the postcranial anatomy of pachycephalosaurs in both a functional and phylogenetic context. Examining CMN 22039 alongside other pachycephalosaurs revealed that new postcranial characters change the current phylogeny of pachycephalosaurs, although, it remains unclear if this is due to ontogenetic variations. Studying the postcranial anatomy of Stegoceras validum also facilitated the first detailed myological reconstruction of the appendicular skeleton of pachycephalosaurs. III Acknowledgements I would like to thank my supervisors, Dr. Jordan Mallon and Dr. Tim Patterson, for their support and insights throughout this project. Thank you to my other committee members, Dr. Michael Ryan and Dr. David Evans, for their guidance and helpful feedback. I would also like to thank the Canadian Museum of Nature (CMN) and the Royal Ontario Museum (ROM) for providing access to the specimens required for this study. I am grateful to Dr. Scott Ruffalo and Alan McDonald for providing me with digital specimens when in person study was impossible due to the COVID-19 pandemic. I would like to thank Mathew Roloson for teaching me how to use 3D programs such as Blender (2018) which were essential to the completion of this study. Finally, I would like to thank Cary Woodruff for teaching me proper osteohistological procedures and methods, and Dr. Kevin Seymour, and Brian Iwama for allowing access to and guiding me through the collections facilities of the ROM.