The oft-raised question of head-butting in pachycephalosaurs has been put forth once again. A new paper out last week in Palaeontologica Electronica suggests that these strange dinosaurs would have been able to butt-heads safely, based on a Finite Element Analysis. The authors, Eric Snively and Andrew Cox, suggest that both the dome and flat-headed pachycephalosaurs would have been able to butt heads without significant injury. Snively & Cox suggest that certain lineages of pachycephalosaurs show an escalation toward head-butting adaptations, while others would have been content to, at the most, flank-butt.
More interestingly, the authors endorse the idea that some pachycephalosaur taxa may represent juveniles of domed forms, and that in their flatter-headed juvenile stage, pachycephalosaurs may have flank-butted or engaged in shoving matches. When they began growing their domes, the animals may have gone through a violent head-butting stage, and injuries sustained would have been healed fairly quickly by the trabecular bone, which could be easily reworked. As adults, the bone growth changed and the dome would have been more suseptable to injury, so big males probably relied on intimidation, displays, and more flank-butting rather than head-to-head combat (HA!).
It's an interesting paper, and well worth the read. While previous studies dealing with head-butting have found negative evidence for it (Carpenter 1997, Goodwin & Horner 2004), those studies focused on adult animals. Snively & Cox suggest that head-butting did occur, but only when the animals were young adults who were actively developing their domes. The majority of the paper is, of course, a computer simulation of how the heads of pachycephalosaurs absorbed stresses induced by head-butting, and the models show that the pachycephalosaur skeleton would have been able to withstand such forces.
Even so, I am still doubtful of the head-butting hypothesis. I always have been--it's one of those cases where paleontologists feel the need to analogize an extinct organism with an extent one. In some cases, those analogies work (like with spinosaurs and crocodilians), but in other cases it probably doesn't (sauropods and elephants, ceratopsids and musk ox). There is no extent animal with a pachycephalosaur-like dome skull. The only other animal I can think of with that kind of specialization is Permian synapsid Moschops, but that creature is otherwise so unlike pachycephalosaurs that such a comparison is fundamentally flawed.
Plus, I know of no paper, this most recent foray included, which documents sustained injuries to a pachycephalosaur dome--juvenile, young adult, or adult. Snivey & Cox mention that many modern artiodactyls which do head-butt (musk ox, again) smash each other's skulls silly with no regard for injury to the overlying or underlying tissue. If we are to apply the behavior of a giant Arctic bovid to extinct bipedal archosaurs, one would expect to read about a giant healed crack across the skull of a juvenile Pachycephalosaurus one of these days...
But no such skull is, to my knoweldge, known. What's more, pachycephalosaur remains are disappointingly rare, and what most often survives is the skullcap or bits and pieces of the skull. Only Stegoceras has a good skeleton behind it, and even it is incomplete. There are so many unanswered questions about pachycephalosaur anatomy much less behavior. I suggest that we focus on figuring out what these animals looked like and how they moved before we investigate how they behaved.