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Stegosaurus was a large, ponderous, plant-eating dinosaur that lived in the environs of North America during the late Jurassic period (about 150 million years ago). What made this herbivore (and other stegosaurs like it) 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 evolved for defensive purposes (there were lots of hungry tyrannosaurs and other large theropods roaming the woodlands of North America), or they may have served to dissipate heat from this dinosaur's body, roughly the same function of an elephant's floppy ears. (See 10 Facts About Stegosaurus, a gallery of Stegosaurus pictures, and an article explaining how Stegosaurus was discovered.)
Besides its plates, what set Stegosaurus apart from other herbivorous dinosaurs was its unusually small, walnut-sized brain, which prompted one paleontologist to speculate that it had a supplementary brain in its butt, one of the most amusing dinosaur blunders of all time. Since Stegosaurus was one of the earliest dinosaurs ever to be discovered (the first fossils of this genus were unearthed way back 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 at least some saurischian dinosaurs (but certainly not the ornithischian Stegosaurus) may have been fairly smart, at least by the standards of the Mesozoic Era.
As a presumably gentle herbivore that preferred to mind its own business, Stegosaurus would have attracted the notice of one of the most vicious dinosaurs of late Jurassic North America, Allosaurus. While it's unlikely that even a very hungry Allosaurus would have dared to take on a full-grown Stegosaurus adult, such encounters can't be dismissed out of hand; see Allosaurus vs. Stegosaurus - Who wins? for an analysis of this earth-shaking battle.