Except an increase in size and the evolution of the most remarkable chewing arrangement of all dinosaurs, different ornithopod families varied from one another in minor details. All were bipedal plant-eaters.
Family: Heterodontosauridae:
Except an increase in size and the evolution of the most remarkable chewing arrangement of all dinosaurs, different ornithopod families varied from one another in minor details. All were bipedal plant-eaters.
Family: Heterodontosauridae: These small, nimble bipedal plant-eaters have been found mainly in Early Jurassic rocks of southern Africa.
Their teeth were sharp and tusklike in front, but the teeth at the sides of the jaw were built for chewing and slicing plants. They had large caninelike teeth (cone-shaped, pointed teeth) at the corners of the upper and lower jaws.
Family: Hypsilophodontidae: This was the most widespread and longest-lived ornithopod family. It flourished almost worldwide from the Middle Jurassic until the end of the Cretaceous.
Hypsilophodontids were small but had relatively large heads. Their feet were primitive with four functional toes. The first toe was smaller than the others; the fifth toe was only a splint. Hypsilophodontids had small front limbs with tiny hands. Bony tendons strengthened the back and stiffened the tail. Most were small dinosaurs about six to ten feet long, but some Tenontosaurus species were as long as 22 feet. The Late Cretaceous Thescelosaurus grew to about 18 feet long.
Family: Dryosauridae: This short-lived family arose about the same time as the Hypsilophodontidae. The earliest dryosaurid was Dryosaurus from the Late Jurassic of western North America and eastern Africa. Valdosaurus from the Early Cretaceous of Europe and northern Africa and Kangnasaurus from Africa are the other two genera in this family. Dryosaurids, with small front limbs and heads, were larger and more powerful than hypsilophodontids. They lacked teeth at the front of the snout and instead had a well-developed beak that may have had a horny covering.
Family: Camptosauridae: The Late Jurassic genus Camptosaurus from western North America was a chubby, medium-size ornithopod about 15 feet long. It had specialized feet and skull.
Family: Iguanodontidae: Iguanodon is one of the best-known dinosaurs. This bulky, 35-foot-long ornithopod had a deep, narrow skull; a strong, well-developed pelvis; rows of bony tendons running along its back; a hand in which the thumb had become a sharp spike; and three broad toes plus an inner toe