Stegoceras. Sergey Krasovskiy


Stegoceras (Greek for "roof horn"); pronounced STEG-oh-SEH-rass


Forests of western North America

Historical Period:

Late Cretaceous (75 million years ago)

Size and Weight:

Up to six feet long and 100 pounds



Distinguishing Characteristics:

Light build; bipedal posture; extremely thick skull in males

About Stegoceras

Stegoceras was the prime example of a pachycephalosaur ("thick-headed lizard")--a family of ornithischian, plant-eating, two-legged dinosaurs of the late Cretaceous period, characterized by their extremely thick skulls. This otherwise sleekly built herbivore had a noticeable dome on its head made of almost-solid bone; paleontologists speculate that Stegoceras males held their heads and necks parallel to the ground, build up a head of speed, and rammed each other on the noggins as hard as they could. (They may also, secondarily, have used their heads to butt away the flanks of encroaching tyrannosaurs, though we have no objective proof of this behavior.)

The sensible question is: What was the point of this Three Stooges routine? Extrapolating from the behavior of present-day animals, it's likely that Stegoceras males head-butted each other for the right to mate with females. This theory is supported by the fact that researchers have discovered two distinct varieties of Stegoceras skulls, one of which is thicker than the other and presumably belonged to the males of the species. (However, some paleontologists dispute this theory, noting that such high-speed collisions would tend to be disadvantageous from an evolutionary perspective--for example, a dizzy, concussed Stegoceras could easily be picked off by a hungry raptor!)

The "type specimen" of Stegoceras was named by the famous Canadian paleontologist Lawrence Lambe in 1902, following its discovery in the Dinosaur Provincial Park formation of Alberta, Canada. For a few decades, this unusual dinosaur was believed to be a close relative of Troodon (which was actually a saurischian rather than an ornithischian dinosaur, and thus resided on an entirely different branch of the dinosaur family tree), until the discovery of further pachycephalosaur genera made its provenance clear.

For better or for worse, Stegoceras is the standard by which all subsequent pachycephalosaurs have been judged--which is not necessarily a good thing, considering how much confusion still exists about the behavior and growth stages of these dinosaurs. For example, the presumed pachycephalosaurs Dracorex and Stygimoloch may have been either juvenile, or unusually aged adults, of the well-known genus Pachycephalosaurus-- and at least two fossil specimens that were initially assigned to Stegoceras have since been promoted to their own genera, Colepiocephale (Greek for "knucklehead") and Hanssuesia (named after the Austrian scientist Hans Suess).