Pachycephalosaurs have long been thought to have engaged in head-butting behavior similar to that of modern goats, due to the large bony domes on the tops of their heads, which seem unsuitable for any other function. Such behavior is likely to have resulted in injuries to the animals, but as yet no such injury has been reported.
Reconstruction of two Pachycephalosaurs engaged in head-butting combat, by dinosaur illustrator Felipe Elias.
In a paper published in the journal PLoS One on 30 April 2012, Joseph Peterson of the Department of Geology at the University of Wisconsin Oshkosh, and Christopher Vittore of the Department of Radiology at Rockford Memorial Hospital discuss the discovery of the dome of a Pachycephalosaur skull with an apparent blunt trauma injury.
The specimen is identified as having come from an adult Pachycephalosaurus wyomingensis, the largest known species of Pachycephalosaur (other several other Pachycephalosaur species have been suggested to be juvenile Pachycephalosaurus wyomingensis, in which case there is little chance of finding such fighting injuries in them), and was found in the Latest Maastrichtian (i.e. end Cretaceous) Hell Creek Formation in Carter County, Montana. It is currently located in the Burpee Museum of Natural History.
The new Pachycephalosaurus wyomingensis dome. (A) Dorsal view. Scale bar is 5 cm. (B) Close-up of the peripheral lesions external to the larger depressions. Scale bar is 10 mm. (C) Close-up of the marginal slope of the rostral-most depression. Scale bar is 10 mm. (D) Right lateral view, showing exposure of the frontoparietal suture. Abbreviations: r = rostral, c = caudal. Peterson & Vittore (2012).
The specimen shows pitting on the right side of the dome. Similar pits have been reported before in Pachycephalosaurs, but have been assumed to have happened post-mortem, the result of erosion to the fossil rather than injury to the animal. Peterson and Vittore produced computerized tomography scans of the Burpee Museum specimen (the museum was understandably reluctant to let them cut into it) which revealed denser bone tissue directly beneath the injury, which Peterson and Vittore interpreted as evidence of healing. Similar dense bone-growth is seen beneath healed injuries in modern birds that have suffered head injuries in window strikes.
Modern Greenfinch (Carduelis chloris) skulls showing recovery after window-strike injuries. Peterson & Vittore (2012).
This is the first report confirming this form of injury in a Pachycephalosaur, but since similar pits have previously been reported and regarded as signs of erosion rather than injury, Peterson and Vittore suggest that a re-examination of other such domes in museum and university collections may reveal evidence that these specimens did in fact bear similar injuries.
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