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 bone on top of their heads! (See a gallery of pachycephalosaur pictures.)
However, it's important to realize that big heads, in this case, didn't translate into big brains. Pachycephalosaurs were about as bright as other herbivorous dinosaurs of the late Cretaceous period (which is a polite way of saying "not very"); their closest relatives, the ceratopsians, weren't exactly nature's A students, either. So of all the reasons these dinosaurs evolved such thick skulls, protecting their extra-big brains wasn't one of them.
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. 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 appears to have taken off when these early genera crossed the land bridge that (back in Cretaceous times) 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 pachycephalosaur evolution because so few complete fossils have been found. 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 pachycephalosaurs could ram each other's noggins at high speed and live to tell the tale.
Not everyone is convinced, though. Some experts insist that high-speed head-butting would have produced too many casualties, and speculate that pachycephalosaurs used their heads to butt the flanks of competitors (or 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 the sides of its skull, lends some support to the head-butting-for-dominance theory.)
By the way, the evolutionary relationships among different types of pachycephalosaurs is still being sorted out. According to new research, it's likely that two supposedly separate pachycephalosaur genera--Stygimoloch and Dracorex--in fact represent earlier growth stages of Pachycephalosaurus. If the skulls of these dinosaurs changed shape as they aged, that might mean that additional genera have been misclassified, and were in fact species (or individuals) of existing dinosaurs.
Here's a list of the most notable pachycephalosaurs; just click on the links for more information.
Acrotholus North America's earliest bone-headed dinosaur.
Alaskacephale Guess what state this pachycephalosaur was found in?
Colepiocephale This thick-skulled dino's name is Greek for "knucklehead."
Dracorex The only dinosaur to be named after the Harry Potter books.
Goyocephale A primitive bonehead from Asia.
Homalocephale This herbivore had a very flat (and very thick) skull.
Micropachycephalosaurus The current record-holder for longest dinosaur name.
Pachycephalosaurus This plant-eater gave new meaning to the term "blockhead."
Prenocephale This "bonehead" had a round, thick skull.
Sphaerotholus Yet another dome-headed dinosaur from North America.
Stegoceras This herbivore was built for high-speed head-butting.
Stygimoloch Its name means "horned demon from the river of death."
Texacephale This new pachycephalosaur was recently discovered in Texas.
Tylocephale The tallest-domed of all the pachycephalosaurs.
Wannanosaurus Probably the smallest of the bone-headed dinosaurs.