Stegosaurs were a group of large, herbivorous dinosaurs that lived during the late Jurassic period, around 155 to 150 million years ago. They are best known for their distinctive double row of bony plates along their backs and the spikes on their tails, which were likely used for defense against predators. These dinosaurs walked on all fours and had a small head compared to their massive bodies. The most famous species is Stegosaurus, which could grow up to 30 feet long. Stegosaurs primarily fed on low-lying plants, using their flat teeth to grind leaves and ferns.