A growth series of Pachycephalosaurus wyomingensis skulls and cranial elements from youngest to oldest, plus two pachycephalosaurid skulls examined in this study.

Published name and citation, type status, element, ontogenetic series, formation and state from the Upper Cretaceous of the Western Interior, USA, are listed. All formations are Maastrichtian age except for the Campanian Judith River Formation. Abbreviations: Fm, formation; lit., literature review only; MT, Montana; ND, North Dakota; SD, South Dakota; WY, Wyoming.