First pachycephalosaurid (Dinosauria: Ornithischia) from the Frenchman Formation (upper Maastrichtian) of Saskatchewan, Canada

• We report on the first occurrence of a pachycephalosaurid from Saskatchewan, Canada.
• This marks a probable northern range extension for Sphaerotholus buchholtzae.
• S. buchholtzae occurs within the final 300,000 years of the Cretaceous.
• Our findings support the hypothesis of low beta diversity in North America during the late Maastrichtian.

Compared to earlier representatives of the family, pachycephalosaurids are less well known from upper Maastrichtian deposits around the world. Here, we report on a nearly complete left postorbital attributable to the pachycephalosaurid Sphaerotholus cf. Sphaerotholus buchholtzae from the upper Maastrichtian Frenchman Formation of Saskatchewan. This marks a probable northern range extension for the species into Canada, and the first occurrence of a pachycephalosaurid from this formation. We further demonstrate the taxonomic distinction between Stegoceras edmontonense and S. buchholtzae, which has been debated, based on postorbital sutural proportions. The northerly occurrence of Sphaerotholus cf. S. buchholtzae is consistent with the hypothesis of low beta diversity during the late Maastrichtian of North America, and its high stratigraphic incidence documents the persistence of non-pachycephalosaurin pachycephalosaurines in a critical interval for understanding the end-Cretaceous mass extinction event.