When found more than a century ago, Stegosaurus was thought to have walked on two legs. And scientists once thought the bundle of nerves near its tail was a second brain. One scientist suggested that its plates could flap. Others thought the plates were used in defense—but they are too thin. Maybe they were used to cool or warm the animal or to attract mates. Stegosaurus’s spiked tail, however, could have been a weapon.