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.

[1] 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.

[4] Williamson and Carr considered the species to be invalid,[1] but in 2010 Nicholas Longrich et al. created the novel combination Sphaerotholus edmontonensis that could be distinguished from S. goodwini by the paired hornlets on the back of the dome, and from S. buchholtzae by the elongate parietals.

[5] It was once considered a species of Stegoceras and was about the same size, reaching 2 m (6.6 ft) in length and 40 kg (88 lb) in body mass.

[10] It is named for the resemblance of its domed skull with a triple "crown"-like appearance of nodes to the Papal tiara or "Triregnum".

[10] In their 2023 description of S. lyonsi and S. triregnum, Woodruff, Schott & Evans analyzed the placement of the genus Sphaerotholus within the Pachycephalosauria.