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.