The holotype partial skull of Sphaerotholus buchholtzae (TMP 87.113.3) was found in the upper Hell Creek Formation of Montana's Carter County by Emily A. Buchholtz née Giffen who assigned it, in 1989, to Sternberg's Stegoceras edmontonense which Brown and Schlaikjer had initially named Troödon edmontonensis in 1943. Sullivan referred Stegoceras edmontonense in its entirety to Prenocephale (as Prenocephale edmontonensis) in 2000, then Williamson and Carr removed TMP 87.113.3 and named it Sphaerotholus buchholtzae, in honour of Buchholtz, in 2003. Mallon et al. referred a skull here in 2015 which made Sphaerotholus buchholtzae the first pachycephalosaurid known from the Frenchman Formation of Saskatchewan and thus expanded its range into Canada. Three specimens of Sphaerotholus buchholtzae (ROM 53582, ROM 53584, and ROM 53585) were found at the main "Natali’s Channel site" less than 500 m away from the Sphaerotholus triregnum holotype locality. The holotype of Sphaerotholus lyonsi—a skull bone (a nearly complete right squamosal) catalogued as TMP 2002.12.63—was collected from the Upper Dinosaur Park Formation (75 mya), on the north side of the Red Deer River, in the northeast region of Dinosaur Provincial Park, Alberta, Canada. The species epithet honours Randy Lyons, a juggler and long-time TMP Field Experience Program participant who discovered the specimen in 2002. ZooBank registry: urn:lsid:zoobank.org:act:3A090588-955C-4E41-ACC1-F27CFD5AFE66. The holotype of Sphaerotholus triregnum—a skull bone (left squamosal) catalogued as ROM 53583—was discovered in 2009 by CK Preparations, in the lower half of the Hell Creek Formation (67 mya), around 45 km southwest of the town of Jordan in Garfield County, Montana, USA, from the same channel complex and no more than 500 m from the holotype locality of the dromaeosaurid Acheroraptor temertyorum. Three specimens of Sphaerotholus buchholtzae (ROM 53582, ROM 53584, and ROM 53585) were found at the main Natali’s Channel site less than 500 m away. The species epithet refers to the Triregnum, a large, domed, three-crowned ceremonial head piece (also known as the Papal tiara), in reference to the domed skull roof of pachycephalosaurids in general, and to the three dome-encircling "crowns" of nobbly nodes on this pachycephalosaurid inparticular. ZooBank registry: urn:lsid:zoobank.org:act:00B76290-735F-426D-A037-FCF5E785B706.