Sphaerotholus (Wiiliamson and Carr, 2002) is a genus of pachycephalosaurid dinosaur from the Upper Cretaceous of the western United States. To date, two species have been described: the type species, S. goodwini, from the Den-na-zin Member of the Kirtland Formation (Late Campanian) of San Juan County, New Mexico, and a second species, S. buchholtzae, from the Hell Creek Formation (Late Maastrichtian) of western Carter County, Montana. The etymology of Sphaerotholus is a combination of the Greek sphaira, meaning "ball," and tholos, meaning "dome," and is a reference the characteristically dome-shaped pachycephalosaurian skull. The survival of Sphaerotholus from the Campanian of New Mexico to the end of the Maastrichtian of Montana demonstrates that this taxon had both a relatively long duration (approximately 7-8 million years) and a widespread distribution. Williamson and Carr (2002) diagnose this genus as follows: "Differs from all other pachycephalosaurids where known in the possession of a parietosquamosal bar that decreases in depth laterally as seen in caudal view and is bordered by a single row of nodes and one lateroventral corner node." Sphaerotholus is considered a highly derived pachycephalosaur.
The holotype of the type species (NMMNH P-27403, New Mexico Museum of Natural History) consists of an incomplete skull lacking the facial and palatal elements. The species is diagnosed by Carr and Williamson (2002) as follows: "Sphaerotholus which in caudal view possesses a parietosquamosal bar that reduces in depth laterally to a lesser extent than in S. buchholtzae and the parietal is reduced to a thin slip between the squamosals." The species name honors paleontologist Mark Goodwin for his work with pachycephalosaurian dinosaurs. A second specimen was referred to cf. S. goodwini (NMMNH P-30068), from the younger Farmington Member of the Kirtland Formation. It consists of a fragmentary squamosal and almost complete dentary, and another unidentified skull fragment.
The holotype of S. buchholtzae (TMP 87.113.3) consists of an incomplete skull. The species is diagnosed by Carr and Williamson (2002) as follows: "Sphaerotholus which in caudal view possesses a parietal that is widely exposed between the squamosals that is wide enough to bear parietosquamosal nodes , the caudal margin of the parietosquamosal shelf shallows to a greater degree than in S. goodwini, the lateral corner node is reduced in size relative to S. goodwini and located above the ventral margin of the parietosquamosal bar, and the nodes in the lateral margin of the parietosquamosal shelf are reduced on the squamosal and coalesce into a ridge on the postorbital." The species name honors Emily A. Buchholtz for her extensive work with pachycephalosaurians.
Its been proposed that this species could be a subjective junior synonym of P. edmontonensis.