Stegosaurus, (genus Stegosaurus), genus various plated dinosaurs of the Late Jurassic Period (159 million to 144 million years ago) recognizable by its spiked tail and series of large triangular bony plates along the back. Stegosaurus usually grew to a length of about 6.5 metres, but some reached 9 metres. The skull and brain were very small for such a large animal. The forelimbs were much shorter than the hind limbs, which gave the back a characteristically arched appearance. The feet were short and broad.