Two new species of small-bodied pachycephalosaurine (Dinosauria, Marginocephalia) from the uppermost Cretaceous of North America suggest hidden diversity in well-sampled formations

Here we report two new small-bodied pachycephalosaurines: one from the Dinosaur Park Formation of Alberta and the other from the Hell Creek Formation of Montana, each represented by an isolated squamosal. These two new specimens are approximately the same size as squamosals of Sphaerotholus buchholtzae, and possess several overlapping morphologies with the genus (such as a strongly posteroventrally projecting and laterally tapering parietosquamosal bar). Therefore, these two new specimens are identified as belonging to the genus Sphaerotholus. However, a suite of characters and combinations differentiate these two new specimens from the other three Sphaerotholus species. Most notably, they possess multiple posterior and lateral node rows on the squamosal, and lack a parietosquamosal node (a single row and a parietosquamosal node define these other species in the genus). Specifically, the Alberta specimen possesses two posterior and two lateral node rows, whereas the Montana specimen has three of each. This analysis provides phylogenetic support for the evolutionary hypothesis that a single-rowed and parietosquamosal node-less taxon underwent a cladogenetic event that gave rise to a single-rowed and parietosquamosal node-bearing lineage and a multi-rowed, parietosquamosal node-less lineage (the two taxa described herein). In turn, these two Sphaerotholus lineages possibly underwent anagenetic change throughout the latest Maastrichtian. Additionally, these new taxa suggest that while large-bodied taxa from well sampled formations are to be found with decreasing frequency, the inverse holds true for small-bodied taxa and that pachycephalosaurines were more diverse than previously recognized, and continued to diversify up until the very end of the Cretaceous.