Stegosaurus was a large, ponderous plant-eater that lived in the environs of North America during the late Jurassic era (about 150 million years ago). What made this herbivore especially striking were the double rows of large, bony plates jutting out of its back. No one is quite sure why Stegosaurus had these plates: they may have been intended for defense (there were lots of larger, carnivorous dinosaurs roaming around in the Late Jurassic), or they may have been used to dissipate heat from its body, via the same mechanism as an elephant's floppy ears. (See more facts, figures and news about Stegosaurus and 10 Facts About Stegosaurus.)
Besides its plates, what set Stegosaurus apart from other dinosaurs was its unusually small, walnut-sized brain. Since stegosaurs were some of the earliest dinosaurs to be discovered (the first fossils of this genus were unearthed in 1877), this led to the popular misconception that all dinosaurs were nature's D students. Recently, though, scientists have come to the conclusion that some dinosaurs (but not Stegosauruses) may have been fairly smart, at least by Jurassic standards.
