5 Facts About Pachycephalosaurus

Monday, May 22, 2017

Reconstruction of Pachycephalosaurus wyomingensis with cranial lesion.

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!

Size comparison of an adult P. wyomingensis (green), potential growth stages, and a human by Matt Martyniuk

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.

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.

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.

Source: thoughtco.com, wikipedia.org