The first pachycephalosaurid from the Late Cretaceous Two Medicine Formation: effects of the Western Interior Seaway on North American pachycephalosaurid evolution

Abstract

During the latest Cretaceous, western North America experienced several regressive and transgressive cycles of the Western Interior Seaway (WIS). Closely related, time-successive taxa of tyrannosaurids, ceratopsids, hadrosaurids, and pachycephalosaurids have been proposed to have evolved via anagenesis driven by habitat area fluctuations related to sea level change. Previous examinations into these anagenetic hypotheses have resulted in equivocal results. However, evolution related to this WIS hypothesis has yet to be tested for Pachycephalosauria. Originally, it was hypothesized that an undescribed taxon from the Two Medicine Formation constituted an anagenetic intermediate between the Campanian Stegoceras validum and the Maastrichtian Pachycephalosaurus wyomingensis. Here we describe this Two Medicine Formation pachycephalosaurid and test the proposed anagenetic lineage. This taxon is the first pachycephalosaurid from the Two Medicine Formation, and the massive frontoparietal dome indicates that it was the third largest North American pachycephalosaurid. Phylogenetic analyses recover this new taxon distant from both Stegoceras and Pachycephalosaurus; thus, refuting the hypothesis that this taxon constitutes any part of an ancestor–descent series between Stegoceras and Pachycephalosaurus. However, the new taxon not only increases understanding of pachycephalosaurid morphology and diversity, but shows that this clade contained relatively large body-sized taxa as early as the Middle Campanian.