Fighting in Pachycephalosaurs.

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).

See also Head injury in a pachycephalosaur and Head-butting in a Pachycephalosaur.

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Head injury in a pachycephalosaur.

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.

See also Head-butting in a Pachycephalosaur and Dinosaurs on Sciency Thoughts YouTube.

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Head-butting in a Pachycephalosaur.

The pachycephalosaurs were a group of ornithischian (bird-hipped) dinosaurs most closely related to the horned and duck-billed dinosaurs. They were bipedal, herbivorous dinosaurs with distinctive thickened skulls. For many years palaeontologists have argued over wether these creatures fought by head-butting in a manor similar to modern goats or musk-ox; the thick skull looks immediately like an adaptation to this sort of combat, but some biologists have suggested that it had a porous structure unsuitable for this role and that it may have been a heat exchange mechanism, and that it may have been impossible for pachycephalosaurs to lower their heads in a way suitable for head-butting. Pachycephalosaurs ranged in size from under a meter to about four and a half meters in length, though the smaller forms lacked the thickened skulls and it has been argued that they may have been juveniles of the larger forms. They are best know from the Late Cretaceous, earlier forms have been found, but these are on the whole not good specimens, so the origins of the group are not well documented.

A reconstruction of two pachycephalosaurs fighting at Plzeň Dinopark in the Czech Republic.

This month (June 2011) a paper appeared in the Journal PLoS ONE by Eric Snively of the Department of Mechanical Engineering at Ohio University and Jessica M. Theodor of the Department of Biological Sciences at the University of Calgary in which they study the structure of the skull of the pachycephalosaur Stegoceras validum using a computerized tomographic scanner (CT scanner), and compare it to various modern mammals, then developed a computer model of how the skull would have dealt with the stress of impacts.

Stegoceras validum was a two meter pachycephalosaur from the Late Cretaceous (83-70 million years ago) of what is now North America. It has a good fossil record, with several known specimens and is the best studied of the pachycephalosaurs. Stegoceras had a 7.5 cm thick, rounded skull, which, it has been argued, would have been unsuitable for direct forehead-to-forehead butting as seen in goats or musk oxen, but more suited to side swiping as in giraffes or horses. Furthermore it has been suggested that Stegoceras could not have held its head down with its neck behind it as a goat does, but was obliged to hold its neck in an 'S' shape, as in a duck.

Snively and Theodor found that Stegoceras had a layered structure to its skull, with dense, rigid boney layers and spongey vascularized layers. They found similar structures in goats and musk oxen, but not in giraffes or pronghorn antelopes (which clash horns, but do not head-butt), suggesting that pachycephalosaurs did indeed head-butt. The computer simulations also suggested that Stegoceras could withstand considerable impacts to its skull, further supporting the head-butt hypothesis.

A CT scan of the skull of Stegoceras, showing areas of high and low density bone.

All this suggests that pachycephalosaurs, and Stegoceras in particular, did indeed engage in head-butting behavior. It does not settle the debate - it is more-or-less impossible to completely determine the behavior of an animal that has been extinct for tens of millions of years - but it does counter the argument that the skull structure was wrong for head-butting. In fact the skull appears to be so well adapted to head-butting that any other behavioral theory would need to suggest an alternative reason for the structure. It does not counter the argument that the 'S' shaped neck of Stegoceras would be unsuitable for head-butting in the manor of a goat or musk ox; but Stegoceras was neither of those creatures. Woodpeckers also have 'S' shaped necks, and engage in impressive head-butting behavior (though it is unlikely that Stegoceras closely mimicked this behaviour either).

See also An Australian Spinosaurid,
The Ashdown Maniraptoran
and Dinosaurs on Sciency Thoughts YouTube.