Researchers have examined the musculature of a bone-headed dinosaur in a bid to better understand hypothesised intraspecific, head-butting combat. Pachycephalosaurs are a group of Late Cretaceous, bipedal ornithischian dinosaurs known from Asia and North America. They are characterised by their thickened skulls, which are sometimes adorned with lumps, bumps and spikes.

The skulls, some of which can be up to 20 cm thick have been the focus of a lot of research. It has been suggested that these thickened skull domes evolved as these dinosaurs indulged in intraspecific head-butting contests, either head-to-head impacts or using their heads to butt the flanks of their opponents.

A replica skull of Pachycephalosaurus wyomingensis.
Pachycephalosaurus wyomingensis replica skull. The thickened skull domes, which in some specimens of P. wyomingensis were up to 20 cm thick are thought to have evolved as these dinosaurs participated in intraspecific head-butting combat. Picture credit: Everything Dinosaur.

To read a blog post from 2011 looking at the evidence for head-butting combat in pachycephalosaurs: Study Supports Theory of Pachycephalosaur Intraspecific Combat (Head-butting).

Stegoceras Muscle Study

Writing in the open-access, on-line journal PLoS One, researchers from Carleton University, Ottawa in collaboration with Professor Phil Currie (University of Alberta) have examined the postcranial skeleton of a specimen of the pachycephalosaur Stegoceras validum to gain a better understanding of the musculature of the limbs, hips and the base of the tail. The specimen (UALVP 2) is one of the best preserved pachycephalosaur postcranial skeletons discovered to date and the best preserved pachycephalosaur discovered in Canada. The limb bones preserve muscle scars and other surface textures which enabled the research team to accurately construct the muscles associated with the forelimbs, hindlimbs and the pelvic region.

Pachycepahlosaurus indulging in head-butting combat.  Reporting upon a Stegoceras muscle study.
A charging Pachycephalosaurus.

Focusing on Muscles Not Bones

Unlike most studies relating to the Dinosauria, the fossil bones were not the central focus of this research. The scientists who include Professor Phil Currie (University of Alberta) and PhD student Bryan Moore (Carleton University), examined the bones to determine the layout, shape and size of the muscles that were attached to them. The team were interested in mapping the *myology of the back end of a pachycephalosaur so that they could assess how the postcranial skeleton would have assisted with the hypothesised head-butting contests.

The term *myology refers to the study of the shape, structure and arrangement of muscles.

Stegoceras muscle study.
Pelvic and hind limb muscular reconstruction of Stegoceras validum. Superficial musculature in lateral view (A). Deep musculature in lateral view (B). Superficial musculature in anterodorsolateral view (C) and (D) deep musculature in anterodorsolateral view. Picture credit: Moore et al/PLoS One.

Strong Legs and a Wide Pelvis

The study of specimen number UALVP 2 demonstrated that the forelimbs of Stegoceras validum were not especially robust and strong, particularly in comparison to early, lizard-hipped bipeds such as the Triassic theropod Tawa hallae. However, in contrast, in Stegoceras the hind limbs and pelvic area were more robust with large, powerful muscles associated with the pelvis, the thighs and the base of the tail. These larger muscles, in combination with the wide pelvis and stout hind limbs, produced a stronger, more stable pelvic structure that would have proved advantageous during hypothesised intraspecific head-butting contests.

The new for 2020 Wild Safari Prehistoric World Pachycephalosaurus dinosaur model.
The new for 2020 Wild Safari Prehistoric World Pachycephalosaurus model. The model has a large dent in its skull dome, suggesting damage resulting from a headbutting contest with a rival.

The picture above shows a Pachycephalosaurus dinosaur model from the Wild Safari Prehistoric World range, to view this range of figures in stock at Everything Dinosaur: Safari Ltd Dinosaur Models.

The research team concludes that the hind quarters of Stegoceras evolved to help this small dinosaur deliver and absorb impact forces associated with the proposed head-butting behaviour. The scientists suggest that more research is needed to examine the potential velocity at which the thickened skull could be propelled forward during such contests. They propose additional research assessing the postcranial properties of other pachycephalosaurs and comparing their bauplan with similar sized dinosaurs such as Thescelosaurus (T. neglectus).

The scientific paper: “The appendicular myology of Stegoceras validum (Ornithischia: Pachycephalosauridae) and implications for the head-butting hypothesis” by Bryan R. S. Moore, Mathew J. Roloson, Philip J. Currie, Michael J. Ryan, R. Timothy Patterson and Jordan C. Mallon published in PLoS One.

The Everything Dinosaur website: Everything Dinosaur.