The stegosaurs (Stegosauria) were one of the two groups comprising the thyreophorans (although some classifications group them with the anklyosaurs in the thyreophorans division). These quadruped, “plated” dinosaurs first appeared in the Middle Jurassic. They were medium in size, from 13 to 26 feet (4 to 9 meters) long, with a small, long skull with simple teeth, and a long toothless beak. They also had armor consisting of two rows of vertical bony plates or spines along their tail, back, and neck. Fossils of stegosaurs—each species with a unique arrangement of spines and plates—are found in Jurassic and Cretaceous periods rock layers around the world, but they were most abundant in the Jurassic.