According to a study by scientists at the University of Wisconsin and Yale University, small, herbivorous, dome-headed dinosaurs from the late Cretaceous period known as pachycephalosaurids used domes atop their heads to fight for territory and mates.

Paleontologists have heavily debated for several decades the adaptive significance of pachycephalosaurid domes. Some suggested that they are the product of sexual selection as an adaptation for species recognition, others that they are weapon employed in intraspecific combat.

“There are two primary hypotheses proposed to explain dome function,” study lead author Dr Joseph Peterson from the University of Wisconsin and his colleagues explained in a paper published in the open-access journal PLoS ONE.

“The first suggests the dome was a display structure, and acted either as a sexually selected display or for species recognition. These explanations are problematic because the dome requires a very high investment of material for a display structure, and because the gross similarity in dome shape between different species, as well as extraordinary changes in shape between juveniles and adults, would have made the dome relatively ineffective for species recognition.”

“The second hypothesis suggests the dome’s structure served a mechanical function, specifically, that the thickened dome was used in intraspecific agonistic bouts, with pachycephalosaurids butting flanks or heads. The hypothesis that the dome functioned as a weapon is supported by a number of lines of evidence.”

To test the second hypothesis, Dr Peterson’s team examined the distribution and frequency of pathologies in skulls and skull domes of pachycephalosaurids.

The examination of 109 domes from over 14 dinosaur species revealed an extraordinary high incidence of pathology.

“Approximately one-fifth of all domes have lesions that are consistent with osteomyelitis. Osteomyelitis can result from a number of different processes, but the most likely one in this context is trauma to the skull, with damage to the tissue overlying the skull leading to an infection of the bone tissue. The high frequency of pathology seen in pachycephalosaurids is, therefore, consistent with the hypothesis that the dome was employed in intraspecific combat. It is also difficult to explain in any other context.”

The scientists found more support for the head-butting hypothesis by looking at extant head-butting mammals.

They examined 30 skeletons of bovid species such as domestic goats, bighorn and dall sheep, American bisons, to explore the distribution of healed fractures and lesions.

“Comparisons with injuries in extant bovids illustrate the variation in injury and lesion distribution related to behavior and suggest that the distribution of injuries in extinct animals can therefore be similarly used to infer behavior in extinct taxa,” they wrote.

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Bibliographic information: Peterson JE, Dischler C, Longrich NR. 2013. Distributions of Cranial Pathologies Provide Evidence for Head-Butting in Dome-Headed Dinosaurs (Pachycephalosauridae). PLoS ONE 8 (7): e68620; doi: 10.1371/journal.pone.0068620