Sphaerotholus
| Key | Value |
|---|---|
| Name Meaning | “spherical dome” |
| Location | United States (New Mexico, Montana), Canada (Alberta, Saskatchewan) |
| Time Period | c. 76 - 66 million years ago (Late Cretaceous) |
| Length | 6 ft (2 m) |
| Weight | 55 lb (25 kg) |
| Locomotion | Biped |
| Diet | Herbivore |
| Described | 2002 (Williamson & Carr) |
| Geological Formation(s) | Kirtland, Hell Creek, Frenchman, Horseshoe Canyon, Dinosaur Park |
| Valid Species | Sphaerotholus goodwini (type), Sphaerotholus buchholtzae, Sphaerotholus edmontensis, Sphaerotholus lyonsi, Sphaerotholus triregnum |
Phylogeny: Dinosauria > Ornithischia > Genasauria > Neornithischia > Cerapoda > Marginocephalia > Pachycephalosauria > Pachycephalosauridae > Pachycephalosaurinae
Overview: As with many other pachycephalosaurs, most of the fossils we’ve collected so far from Sphaerotholus consist of skull material, namely pieces of its thickened dome. This bone was solid in comparison to other parts of the skull or skeleton, so it’s little wonder that these domes have been so commonly preserved intact. We can fill in the gaps of this creature’s appearance with the fossils of other related animals. Sphaerotholus, like most pachycephalosaurs, was both a biped and mainly or entirely herbivorous. The tips of the jaws would’ve sported a narrow beak, while small teeth were situated farther back in the mouth. Growing to be about two meters long, Sphaerotholus was an average-sized pachycephalosaur. It was much smaller than the famous Pachycephalosaurus, a close relative, which was probably the largest known pachycephalosaur. In some regions, depending on the time period, the two genera may’ve coexisted.
The generic name of Sphaerotholus, coined in its 2002 description, is Greek for “spherical dome”, referring to its skull morphology. Paleontologists debate the exact purpose of pachycephalosaur skull domes, though it’s often assumed that they engaged in direct head butting behavior, similar to bighorn sheep or other such modern animals. Some are skeptical, pointing out that the anatomy of pachycephalosaur necks were ill suited for dealing with the stresses this would cause. It’s possible that Sphaerotholus competed with other members of its species by delivering side-to-side strikes with the head. The type specimen of this genus was found in New Mexico’s Kirtland Formation, but fossils now known to have belonged to it were actually found as early as the 1940’s, in Alberta. Sphaerotholus seems to have had a wide range over North America and across a decent amount of time. Fossils are even known from the Hell Creek Formation, so some species of Sphaerotholus may’ve had to contend with the infamous Tyrannosaurus as a potential predator.