Pachycephalosauria is a clade of dinosaurs that lived during the Late Cretaceous in North America. This group includes dinosaurs that were bipedal and characterized by short forelimbs, stocky and powerful hind limbs, and a short, thick neck. However, their most distinguishing feature was their unique skulls, referred to as domes, characterized by an impressive thickening of the cranial roof. In some specimens of later pachycephalosaurids, this thickened cranial roof was shown to add nearly a foot of solid bone to the head of the animal.
This addition to the skull had scientists arguing over their function for some time. The two main hypotheses were that the dome was a display structure, either for sexual selection or for species recognition, or was used in intra-specific combat. Recently, a study published in PLOS ONE by Joseph Peterson, Collin Dischler, and Nicholas Longrich, provided evidence supporting the latter hypothesis.
The authors believe that the domes did not have a display function for several reasons. First, the energy cost and amount of material needed to grow these domes was very high. It seems unlikely that the energy and material required would be worth investing for the purposes of sexual selection. Another reason that provides evidence contradictory to the display hypothesis is that based on comparisons of the domes of different species of pachycephalosaurids, there was no morphologically significant difference that would serve as a clue to aid in species recognition. Additionally, the domes of these animals underwent quite dramatic changes through the maturation process. The significant change in dome form as aging occurred makes it unlikely the structures functioned as recognition cues.
To further investigate the second hypothesis of combat, Peterson et. al. looked into the pathology of fossil skulls of pachycephalosaurids. They found that approximately 20% of the domes had lesions, called osteomyelitis. Osteomyelitis most often results from skull trauma. This trauma would also cause damage to the outer tissue of the skull that would in turn could cause 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 authors wrote.
Reconstruction of Pachycephalosaurus wyomingensis with cranial lesion.
Additionally, the authors compared results of their observations to extant species of bovids, (including various species of sheep, goats, and bison) that exhibit similar “head butting” behaviors. In their study the authors wrote, “The lesions present in the crania of… head-butting bovids are similar to the injuries observed in pachycephalosaurids, and suggest a similar behavioral origin.” This comparison to living species provides additional evidence that the domes of these animals were used in agonistic behaviors.
This research was published July 16, 2013 int he journal PLOS ONE.
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