Pachycephalosaurs were small herbivorous Dinosaurs from the Late Cretaceous of North America and Eurasia. They are noted for the possession of thick bony domes on their heads which appear to have been sported by only one sex (probably, but not necessarily the males) and are generally thought to have been used in combat with other members of the same species.
In a paper published in the journal PLoS One on 16 July 2013, Joseph Peterson and Collin Dischler of the Department of Geology at the University of Wisconsin and Nicholas Longrich of the Department of Geology and Geophysics at Yale University, discuss the results of a study in which they examined the skulls of 109 specimens from over 14 species of Pachycephalosaurs and 30 specimens of modern Bovids (Sheep, Goats and Cattle), which are known to engage in head butting as a way to settle disputes.
Peterson et al. found that 22% of the Pachycephalosaur skulls, representing 24 individuals from nine species, had signs of cranial pathology (i.e. damage and regrowth) to the dorsal surface of their domes. Similar injuries were found in Bovids that engage in head butting behavior as a means of intraspecific conflict resolution, but at a much lower level of prevalence, suggesting that the Pachycephalosaurs had a considerably more violent lifestyle. Bone pathologies are well documented in a number of varieties of Dinosaur thought to have engaged in violent intraspecific competition, as well as in a number of modern birds where this is known to be the case, but none of these shows the level of injuries seen in the Pachycephalosaurs.
Reconstruction of Pachycephalosaurus wyomingensis with cranial lesion. Peterson et al. (2013).
The incidence of injuries on the domes varied little between Pachycephalosaur species, suggesting that this behavior was a conservative trait within the group, which varied little between species. However the distribution of pathologies did vary, suggesting that like modern Bovids, different species fought in different ways.
Selected pathological pachycephalosaurid specimens. (A) Gravitholus albertus in dorsal view of erosive lesions; (B) Stegoceras validum in dorsal view with arrows denoting dorsal lesions; (C) Stegoceras validum in dorsal view with arrow denoting dorsal lesion; (D) an unidentified pachycephalosaurid in dorsal view with arrows denoting lesions; (E) Sphaerotholus buchholtzae, in dorsal view with arrows denoting dorsal lesions; (F) Pachycephalosaurus wyomingensis in dorsal view with arrows denoting large depression features and high magnification of deep erosive lesions (G, H). Rostral portion of the frontal denoting ‘‘r’’. Peterson et al. (2013).
Hypothetical head-to-head interactions among pachycephalosaurids. (A) Bison-like head-shoving in large, broad domed specimens such as Pachycephalosaurus wyomingensis; (B) Ovis-like clashing in Prenocephale prenes; (C) Capra-style broadside butting in high-domed and large-horned specimens such as subadult Pachycephalosaurus. Peterson et al. (2013).
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