Woodruff, D. C. Horner, J. R. Goodwin, M. B. Evans, D. C. 2025 The first pachycephalosaurid from the Late Cretaceous Two Medicine Formation: effects of the Western Interior Seaway on North American pachycephalosaurid evolution Zoological Journal of the Linnean Society 205 zlaf087 Psittacosaurus_mongoliensis Yinlong_downsi Wannanosaurus_yanshiensis Goyocephale_lattimorei Homalocephale_calathocercos Tylocephale_gilmorei Prenocephale_prenes Foraminocephale_brevis Hanssuesia_sternbergi Colepiocephale_lambei Stegoceras_validum Dracorex_hogwartsia Stygimoloch_spinifer Pachycephalosaurus_wyomingensis Alaskacephale_gangloffi Sphaerotholus_goodwini Sphaerotholus_buchholtzae Sphaerotholus_edmontonensis Sphaerotholus_triregnum Sphaerotholus_lyonsi Acrotholus_audeti Stegoceras_novomexicanum Amtocephale_gobienses Platytholus_clemnsi Brontotholus_harmoni Osteology Maximum Parsimony Based on reanalysis of the original matrix. Woodruff_etal_2025a