A new study suggests dome-headed dinos did not head butt each other

Bone-headed living battery ram dinosaurs, the pachycephalosaurs, may have had more extravagant heads than once thought, a new study finds.
Christopher McFadden
Casts of three pachycephalosaurid skulls from the Museum of the Rockies.

Tim Evanson/Wikimedia Commons 

A new study by researchers, including the legendary John "Jack" Horner, suggests that pachycephalosaurs, those famous living battering rams, may have had far more ostentatious features than previously thought. This could also overturn years of the assertion that their iconic domed skulls were used as a form of a biological crash helmet during male dominance displays.

The new hypothesis comes from discovering and studying a partially unearthed pachycephalosaur skull featuring a distinctively round dome shape. It was found in 2011 within the Hell Creek Formation in Montana. This area has been a well-known source of dinosaur fossils for paleontologists for many years due to its Upper Cretaceous rock layers.

If you need your memory refreshed, Pachycephalosaurs were a group of small-to-medium herbivorous dinosaurs that lived during the Late Cretaceous period, distinguished by their domed, thick skulls, believed to have been used for head-butting during mating competitions or territorial disputes. They were bipedal, with short arms and long legs, showcasing fast running capabilities. Despite their unique cranial features, overall body size remained modest, typically ranging from 6.6 feet (2 meters) to 16.5 feet (5 meters) in length. Their fossils are predominantly found in North America and Asia.

When sectioning and computerized tomography (CT) scanning the skull, Horner and his team found some exciting features that would seem to indicate it likely had some interesting (now lost) vertical structures on top of its dome. They also found that the creature had suffered some form of severe head injury in the past but had managed to survive it.

"We don't know the exact shape of what was covering the dome, but it had this vertical component that we interpret as covered with keratin," Goodwin said. He also noted that the hypothesized bristly, flat-topped covering "biologically makes sense. Animals change or use certain features, particularly on the skull, for multiple functions — it could be for display or social and biological interactions involving visual communication," he added.

"I would guess that there was something pretty elaborate up there," said dinosaur legend Horner, a lecturer and presidential fellow at Chapman, professor emeritus at Montana State University in Bozeman, and emeritus curator at the Museum of the Rockies. Regarding the injury, Goodwin explained that "we see probably the first unequivocal evidence of trauma in the head of any pachycephalosaur, where the bone was actually ejected from the dome somehow and healed partially in life."

"We don't know how that was caused. It could be head-butting — we don't dispute that," he said. The use of ornamentation is a common trait among the reptile ancestors of dinosaurs and their bird descendants. It serves the purpose of attracting mates and intimidating rivals. However, researchers Horner and Goodwin have put forward a different perspective.

They argue that the internal structure of pachycephalosaur skulls does not provide enough cushioning to support head-butting without causing severe brain damage. Additionally, they assert that head-butting is a behavior that is more commonly observed in mammals than in reptiles or birds. This is because pachycephalosaur skulls lack the specialized features, such as a pneumatic chamber above the braincase, present in mammals that engage in violent head-butting behavior, like bighorn sheep.

A new study suggests dome-headed dinos did not head butt each other
Image showing the damage to the skull.

"I don't see any reason to turn dinosaurs into mammals rather than just trying to figure out what they might be doing as bird-like reptiles," Horner said.

You can review the study for yourself in the journal Taylor and Francis.

Study abstract:

"A partial skull of a pachycephalosaurid from the Upper Cretaceous Hell Creek Formation, Montana, is interpreted as a new taxon, Platytholus clemensi gen. et sp. nov. MOR 2915 does not fit into an ontogenetic continuum of known pachycephalosaurids from the Hell Creek Formation, Montana, and contemporaneous sediments from the Western Interior. Comparisons to known ontogimorphs of Sphaerotholus and Pachycephalosaurus preclude including this specimen into an ontogenetic series of either taxon. We hypothesize that MOR 2915 is a new species based on a relatively low, broad dome at this advanced ontogenetic age that is neither round nor oval in dorsal view, distinct but fused lateral cranial elements fully incorporated into the dome without any dorsal lobe differentiation, and individual tab-like tubercle ornamentation dorsolaterally. Phylogenetic analysis posits that Platytholus clemensi is a Prenocephale-grade taxon deeply nested within Pachycephalosaurinae, but it is not a member of Pachycephalosaurini. Platytholus clemensi is intermediate in size between the other contemporaneous pachycephalosaurids in the Hell Creek Formation and suggests a diverse set of taxa-partitioned ecological niches by body size. We confirm a well organized, major internal vascular network using high resolution computed tomography. Foramina present on the orbital roofs indicate these canals penetrated the entire ceiling of the orbits within the frontal and supraorbital bones. Abundant neurovascular canals passing through the dome to the ectocranial surface indicate a keratinous structure of some kind, possibly with a vertical structural framework, was present on the dome. We review the history of the head-butting hypothesis and associated behavioral implications."

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