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, which was reduced to a splint. The teeth were thick and were always being replaced. It ate tough plants, Iguanodon walked on two legs, but it could also use its hands to walk on all fours.
Iguanodontids have been found in Early and Late Cretaceous formations in North America, northern Africa, Europe, and Asia. The most advanced were Probactrosaurus from central Asia and Ouranosaurus from northern Africa. The teeth of Probactrosaurus had a more complicated pattern of replacement than those of Iguanodon. The vertebrae of Ouranosaurus had very long spines, creating a sail. With the rise of duckbilled dinosaurs, iguanodontids faded away.
Family: Hadrosauridae: This group consists of two groups, the Hadrosaurinae and the Lambeosaurinae. They were both duckbilled dinosaurs that were closely related. They were large to very large plant-eaters of the Late Cretaceous in North and South America and Eurasia. The largest were Shantungosaurus from China and Lambeosaurus laticaudus from Mexico, both of which may have grown over 50 feet long and weighed over 20 tons. They may have been the largest known animals able to walk on two feet. Most duckbilled dinosaurs were about 30 feet long.
The teeth were in thirty to forty vertical rows like steps on a moving escalator. As each tooth wore away, it was replaced by the tooth directly below it. This process started when (or even before) the animal hatched and continued as long as it lived. The skulls of hadrosaurines were generally longer and not as deep as those of lambeosaurines, and their ducklike beaks were flatter and broader.
Some hadrosaurines lacked cranial crests (Edmontosaurus); others had arched nasal bones (Kritosaurus); others had solid cranial crests (Prosaurolophus and Maiasaura). But all lambeosaurines in which the skulls are well known (Hypacrosaurus, Corythosaurus, Lambeosaurus, and Parasaurolophus) had hollow crests with looping nasal passages that may have been used to make sounds.
The duckbilled dinosaurs were perhaps the most advanced of all the dinosaurs. They had excellent hearing, eyesight, voice, and sense of smell. They lived in huge herds and may have migrated seasonally, returning to the same place each year to mate and lay their eggs.