A bold new discovery reshapes our view of a peculiar group of plant-eating dinosaurs. In northern Montana, scientists identified a dome-headed species called Brontotholus harmoni, which roamed about 75 million years ago as shallow seas and swampy floodplains covered much of western North America.
Fossils attributed to Brontotholus harmoni were unearthed in the Two Medicine Formation of Glacier County. This dinosaur belongs to the pachycephalosaurid family, known for their unusually thick skull roofs, and lived during the Late Cretaceous as herbivores with distinctive bony domes on their heads.
Meet Brontotholus harmoni
Led by D. Cary Woodruff, Ph.D., curator of vertebrate paleontology at the Phillip and Patricia Frost Museum of Science, the research team also collaborates with another institution to study how dinosaur bones reveal growth, movement, and health across an animal’s life.
Five distinct Brontotholus harmoni fossils have been found in the western part of ancient North America’s rock formations. One study notes the herbivore measured roughly 10 feet in length, about the size of a small car.
The skull of Brontotholus features a massive frontoparietal dome, the thick, rounded bone that forms the roof of the head. New findings indicate that the dome’s size places Brontotholus as the third-largest dome-headed dinosaur known from North America.
Classified within Pachycephalosauria, Brontotholus harmoni joins other dome-headed, two-legged, herbivorous dinosaurs distinguished by thick skull roofs and short forelimbs, sometimes adorned with bony bumps and spikes around their domes.
The dome-shaped skulls are especially valuable to scientists since most dome-headed dinosaurs are known mainly from fragmentary skulls or a single thick roof bone. When a well-preserved skull is found—even if the rest of the skeleton is missing—it can dramatically reshape our understanding of how these animals looked and lived.
In many species, older individuals carry thicker domes and more elaborate surface textures, suggesting age-related changes in head armor. By comparing Brontotholus harmoni’s dome to those of other species, researchers estimate its age and infer aspects of its behavior and lifestyle.
Sea changes and family trees
During the Late Cretaceous, the Western Interior Seaway was a shallow inland sea that split North America in two, its shoreline advancing and retreating with changing sea levels and creating shifting coastal habitats for dinosaurs on the western landmass.
Cycles of isolation and reconnection likely separated some populations while bringing others back together, providing repeated opportunities for evolution, encounters, and competition over millions of years.
Before this study, some paleontologists proposed that the Two Medicine dome dinosaur represented an evolutionary intermediary between Stegoceras and Pachycephalosaurus. However, detailed phylogenetic analyses—comparing anatomical traits to reconstruct family trees—place Brontotholus on a distinct branch. This challenges the idea of a simple, linear progression and supports a scenario in which multiple dome-headed species overlapped in time and space along the seaway’s margins.
What Brontotholus harmoni tells about the past
Brontotholus lived during the Middle Campanian stage of the Late Cretaceous, indicating that large dome-headed dinosaurs appeared earlier than some records had suggested. Its Montana presence expands the known geographic range of sizable dome-headed herbivores along the western edge of the ancient seaway.
Earlier work identified Acrotholus audeti from Alberta as the oldest North American pachycephalosaur, dating to about 85 million years ago. Brontotholus harmoni helps bridge the evolutionary gap between that earlier species and later, larger dome-headed giants.
The Two Medicine Formation already yielded fossils of horned dinosaurs, hadrosaurs, and long-necked giants sharing the same ancient ecosystems. Adding Brontotholus harmoni to this community highlights how ecologically complex and crowded Late Cretaceous landscapes were for plant eaters and their predators.
This careful skull-focused study also demonstrates how museum collections—sometimes decades old—can still surprise researchers. As methods for studying skeletal growth improve, revisiting specimens can reveal new stories about how dinosaurs lived, moved, and evolved.
The study appears in the Zoological Journal of the Linnean Society.
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