Brontotholus: a pachycephalosaur known from its skull dome

Brontotholus harmoni is a pachycephalosaur named in 2025 from domed skull bones found in Montana and Alberta. The fossils come from several geological levels and include specimens of different growth stages. No vertebrae, limbs or other parts of the body have been securely assigned to the genus. The dome is therefore both its defining evidence and the main limit on reconstructing the animal.

Quick facts

Five domes from two formations

The holotype, MOR 480, and paratype, MOR 479, are frontoparietal bones from the upper Two Medicine Formation in Glacier County, Montana. They were recovered close to one another in the stratigraphic sequence, tens of metres above a dated volcanic ash bed and near the transition toward the marine Bearpaw Formation. Bob Makela discovered MOR 480 in 1987. The close geological position is useful context, but the specimens were not parts of one articulated skeleton.

The authors also assigned MOR 453 and MOR 550 from other levels of the Two Medicine Formation, along with TMP 1989.69.21 from the Oldman Formation in southern Alberta. Wendy Sloboda found the Canadian specimen in 1989. The authors treated some of the material only as Brontotholus sp. because the available characters do not support assigning every dome confidently to the species B. harmoni.

The genus name combines Greek roots for “thunder” and “dome”, referring to the Beyond Thunder Dome locality and the pachycephalosaur skull. The species honours Bob Harmon for his field and laboratory work. The formal description connected fossils that had previously been discussed under other pachycephalosaur names, so the new taxon also clarifies the history of those specimens.

How the dome is recognised

The frontoparietal dome is longer from front to back than it is wide. Its contacts with surrounding skull bones form high, vaulted margins, and the rear supraorbital contacts show a pronounced concavity. The diagnosis relies on this combination and on comparisons among specimens. No single outline alone proves that all similar domes belong to one species.

MOR 453 was studied histologically and interpreted as a late juvenile. In larger domes, a patchwork-like outer surface is reduced and the bone becomes more strongly inflated, a pattern consistent with maturation. Other specimens were assigned an age mainly from external form because suitable sampling or imaging was unavailable. Growth changes matter: domes from young animals need not have the same surface texture or proportions as those of mature individuals.

Age and relationships

The Two Medicine Formation accumulated during the Campanian, broadly around 82 to 74 million years ago. The holotype and paratype occur near the younger part of that sequence, above a volcanic ash dated to approximately 75.25 million years. The Canadian specimen comes from a different formation and locality. These placements show that the genus is represented across a region and through time, but they do not establish continuous populations or a direct ancestor-descendant series.

The published phylogenetic analysis placed Brontotholus within Pachycephalosauridae, in a position near Foraminacephale in that analysis. Relationships among dome-headed dinosaurs remain sensitive to which incomplete taxa and characters are included. Earlier suggestions that a sequence of progressively larger domes documented one straight lineage from Stegoceras toward Pachycephalosaurus are not supported as a simple chain by the new stratigraphic and phylogenetic treatment. Similarity can reflect shared ancestry without one named genus turning directly into another.

What the dome can and cannot tell us

The dome confirms that Brontotholus was a pachycephalosaur, but it does not preserve the muzzle, teeth, limbs or tail. A length estimate around 3.8 metres in the description is an extrapolation from the dome and comparisons, not a measurement of a complete skeleton. Diet and locomotion are inferred from relatives rather than direct evidence from this genus.

A thickened dome could tolerate substantial forces, but fossil structure alone does not establish how the animal used it. Display, individual recognition and physical contact have all been considered for pachycephalosaurs; none is demonstrated specifically for Brontotholus. The genus is best compared with other dome-headed dinosaurs in the dinosaur catalogue, where the amount of preserved anatomy varies widely.