A novel genus and species of pachycephalosaurid dinosaur has been elucidated by paleontologists, derived from five fossilized specimens discovered within the Two Medicine Formation in Montana, USA, dating back to the Late Cretaceous period.
Life reconstruction of Brontotholus harmoni. Image credit: Connor Ashbridge / CC BY 4.0.
This prehistoric herbivore roamed North America approximately 75 million years in the past, during the latter stages of the Cretaceous era.
Designated as Brontotholus harmoni, this ancient creature is estimated to have measured about 3 meters (10 feet) in length.
Its taxonomic classification places it within Pachycephalosauridae, a family characterized by bipedal, dome-headed dinosaurs belonging to the ornithischian clade Pachycephalosauria.
“The Pachycephalosauria, an ornithischian clade distinguished by its dome-headed morphology, exhibits a unique array of anatomical traits,” stated Dr. D. Cary Woodruff, a paleontologist affiliated with the Phillip and Patricia Frost Museum of Science and the Museum of the Rockies, along with his colleagues.
“These include a bipedal locomotion, varied dentition, ossified myosepta in the tail, and notably, their characteristic thickened frontoparietal domes and elaborately ornamented craniums.”
“The majority of pachycephalosaurid species are known solely from cranial fossil evidence. Consequently, the anatomical structure, functional characteristics, and systematic significance of the frontoparietal domes have garnered the most substantial attention from the paleontological community.”
“This clade is exclusively documented from the Late Cretaceous epochs of Asia and western North America, with the latter region presenting a more varied assortment of pachycephalosaurids.”
“The earliest North American representative is Acrotholus audeti, a species dating from the Santonian age. The diversification of this group appears to have occurred between the Middle and Late Campanian stages, yielding a profusion of presently identified species.”
A collection of five specimens, now attributed to Brontotholus harmoni, were exhumed from the Two Medicine Formation in Glacier County, Montana.
“This newly identified species represents the inaugural pachycephalosaurid discovery from the sedimentary layers of the Two Medicine Formation,” the paleontological team announced.
The considerable cranial dome of Brontotholus harmoni suggests it was the third largest pachycephalosaurid dinosaur discovered in North America.
“Phylogenetic investigations indicate that this novel species is evolutionarily distinct from both Stegoceras and Pachycephalosaurus,” the researchers reported.
“Therefore, this observation refutes the hypothesis suggesting that this species might represent an intermediate stage in an ancestral lineage connecting Stegoceras and Pachycephalosaurus.”
“Nevertheless, this new species not only enhances our comprehension of pachycephalosaurid morphology and their evolutionary breadth but also demonstrates that this clade encompassed species of relatively large body size as early as the Middle Campanian epoch.”
The unveiling of Brontotholus harmoni is detailed in a publication released on October 9, 2025, within the pages of the Zoological Journal of the Linnean Society.
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D. Cary Woodruff et al. 2025. The first pachycephalosaurid from the Late Cretaceous Two Medicine Formation: effects of the Western Interior Seaway on North American pachycephalosaurid evolution. Zoological Journal of the Linnean Society 205 (2): zlaf087; doi: 10.1093/zoolinnean/zlaf087
