Size and Weight:
Stegoceras was a prime example of a pachycephalosaur--a family of ornithischian, plant-eating dinosaurs characterized by their extremely thick skulls. This otherwise sleekly built herbivore had a noticeable dome on its head made of solid bone; paleontologists believe 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.
The sensible question is: Why? 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 types of Stegoceras skulls, one of which is thicker than the other and presumably belonged to the males of the species. (Some experts dispute this theory, noting that such high-speed collisions would tend to be evolutionarily disadvantageous--for example, a concussed Stegoceras could easily be picked off by a hungry raptor!)
The "type specimen" of Stegoceras was named by the famous paleontologist Lawrence Lambe in 1902, following its discovery in the Dinosaur Provincial Park formation in Alberta, Canada. For a few decades, this unusual dinosaur was believed to be a close relative of Troodon, until the discovery of further pachycephalosaur genera convinced scientists otherwise. In fact, Stegoceras is the standard by which all subsequent pachycephalosaurs were judged; not necessarily a good thing, considering how much confusion exists about the growth stages of these dinosaurs.