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 heavy skulls, which ranged from the mildly thick (in early genuses like Wannanosaurus) to the truly dense (in later genuses like Stegoceras). Some later pachycephalosaurs sported almost a foot of solid bone on top of their heads! (See a gallery of pachycephalosaur pictures.)
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However, it's important to understand that big heads didn't translate into big brains. Pachycephalosaurs were about as bright as other herbivorous dinosaurs of the late Cretaceous (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.
Pachycephalosaur Evolution
Based on the 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 boneheads were fairly small, with only slightly thickened skulls, and they may have roamed in packs as protection against carnivores.
Pachycephalosaur evolution really appears to have taken off when these early genuses crossed the land bridge that (in Cretaceous times) connected Asia 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 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 fossil record mainly by their heads, their vertebrae, femurs and other bones having long since been scattered to the winds.
Pachycephalosaur 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 pack 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 produce 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-pachycephalosaurs could easily (and safely) butt each other's flanks with their normal, non-thickened skulls.
By the way, the relationships among pachycephalosaurs is still being sorted out. According to new research, it's likely that two supposedly separate pachycephalosaur genuses--Stygimoloch and Dracorex--in fact represent earlier growth stages of Pachycephalosaurus.Here's a list of the most notable pachycephalosaur genuses; just click on the links for more information.
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."
Tylocephale The tallest-domed of all the pachycephalosaurs.
Wannanosaurus Probably the smallest of the bone-headed dinosaurs.
