Pachycephalosaurian specimens from the Upper Cretaceous (upper Campanian) Kirtland Formation of northwestern New Mexico include a partial skull that preserves much of the basicranium. It represents a new genus and species, Sphaerotholus goodwini. A new species, S. buchholtzae, from the upper Maastrichtian Hell Creek Formation of Montana is referred to this genus. Additional New Mexico pachycephalosaurians include a partial dentary with associated skull material that is tentatively referred to S. goodwini and a partial frontoparietal that is referred to Pachycephalosauridae incertae sedis.

On the basis of a hypothetical growth series of Stegoceras, we excluded characters based on dome development and suture closure from a cladistic analysis of pachycephalosaurian relationships. Ornatotholus browni, the only putative “flat-headed” pachycephalosaurian from North America, is considered a nomen dubium and may represent a juvenile Stegoceras. Gravitholus albertae is an adult Stegoceras sp.; Stegoceras edmontonense is a nomen dubium and is referred to cf. Sphaerotholus sp.

Based on the results of a quantitative cladistic analysis, Stegoceras (including Stegoceras breve, S. lambei, S. sternbergi, and UCMP 130051) is the sister taxon to all other domed pachycephalosaurians. Derived pachycephalosaurids consist of two principal clades: a lineage that includes Stygimoloch, Pachycephalosaurus, and Sphaerotholus and a lineage represented by the Asian taxa, Tylocephale and Prenocephale.

With biogeographic occurrences mapped onto the phylogeny, a single dispersal event from Asia into North America, followed by dispersal of Prenocephale and Tylocephale into Asia prior to the late Campanian is indicated (ACCTRAN) or two independent dispersals into North America prior to the late Campanian is indicated (DELTRAN). Pachycephalosaurian phylogeny does not support Asian-North American contiguity throughout the Campanian and Maastrichtian.