Pachycephalosaurs - The Bone-Headed Dinosaurs

The Evolution and Behavior of Pachycephalosaur Dinosaurs

pachycephalosaurus
Like others of its kind, Pachycephalosaurus had an unusually thick skull (Wikimedia Commons).

Pachycephalosaurs (Greek for "thick-headed lizards") were an unusually small family of dinosaurs with an unusually high entertainment value. As you can guess from their name, these two-legged herbivores were distinguished by their skulls, which ranged from the mildly thick (in early genera like Wannanosaurus) to the truly dense (in later genera like Stegoceras). Some later pachycephalosaurs sported almost a foot of solid, albeit slightly porous, bone on top of their heads!

(See a gallery of bone-headed dinosaur pictures and profiles.)

However, it's important to understand that big heads, in this case, didn't translate into equally big brains. Pachycephalosaurs were about as bright as the other plant-eating dinosaurs of the late Cretaceous period (which is a polite way of saying "not very"); their closest relatives, the ceratopsians, or horned, frilled dinosaurs, weren't exactly nature's A students, either. So of all the possible reasons pachycephalosaurs evolved such thick skulls, protecting their extra-big brains certainly wasn't one of them.

Pachycephalosaur Evolution

Based on the available fossil evidence, paleontologists believe that the very first pachycephalosaurs--such as Wannanosaurus and Goyocephale--arose in Asia about 85 million years ago, only 20 million years before the dinosaurs went extinct. As is the case with most progenitor species, these early bone-headed dinosaurs were fairly small, with only slightly thickened skulls, and they may have roamed in herds as protection against hungry raptors and tyrannosaurs.

Pachycephalosaur evolution really seems to have taken off when these early genera crossed the land bridge that (back during the late Cretaceous period) connected Eurasia and North America. The largest boneheads with the thickest skulls--Stegoceras, Stygimoloch and Sphaerotholus--all roamed the woodlands of western North America, as did Dracorex hogwartsia, the only dinosaur ever to be named after the Harry Potter books.

By the way, it's especially difficult for experts to untangle the details of pachycephalosaur evolution, for the simple reason that so few complete fossil specimens have ever been discovered. As you might expect, these thick-skulled dinosaurs tend to be represented in the geological record mainly by their heads, their less-robust vertebrae, femurs and other bones having long since been scattered to the winds.

Pachycephalosaur Behavior and Lifestyles

Now we get to the million-dollar question: why did pachycephalosaurs have such thick skulls? Most paleontologists believe male boneheads head-butted each other for dominance in the herd and the right to mate with females, a behavior that can be seen in (for example) modern-day bighorn sheep. Some enterprising researchers have even conducted computer simulations, showing that two moderately sized pachycephalosaurs could ram each other's noggins at high speed and live to tell the tale.

Not everyone is convinced, though. Some people insist that high-speed head-butting would have produced too many casualties, and speculate that pachycephalosaurs instead used their heads to butt the flanks of competitors within the herd (or even smaller predators).

However, it does seem odd that nature would evolve extra-thick skulls for this purpose, since non-pachycephalosaur dinosaurs could easily (and safely) butt each others' flanks with their normal, non-thickened skulls. (The recent discovery of Texacephale, a small North American pachycephalosaur with shock-absorbing "grooves" on either side of its skull, lends some support to the head-butting-for-dominance theory.)

By the way, the evolutionary relationships among different genera of pachycephalosaurs are still being sorted out, as are the growth stages of these strange dinosaurs. According to new research, it's likely that two supposedly separate pachycephalosaur genera--Stygimoloch and Dracorex--in fact represent earlier growth stages of the much bigger Pachycephalosaurus. If the skulls of these dinosaurs changed shape as they aged, that may mean that additional genera have been classified improperly, and were in fact species (or individuals) of existing dinosaurs.