The second major group of dinosaurs, the Ornithischians, includes the bipedal herbivorous dinosaurs; the plated dinosaurs; the armored dinosaurs; and the horned dinosaurs.
The ornithopods. The ornithopods ("bird feet") had so many similarities to modern birds that, for years, scientists thought that the two groups were related. This diverse group became the most-widespread herbivores, thanks in part to the strong, compact, highly specialized teeth set far back in their mouths. Ornithopods had strong hind legs with a three-toed foot. Although they usually walked upright, they were able to stand on all fours.
The Iguanodon (noted earlier as the first dinosaur fossil recorded) was an ornithopod. Interestingly, its hand had four fingers and a primitive thumb. Although an Iguanodon has never been found in the United States, a close relative, the duck-billed Hadrosaurus, has been. It was about the same size as the Iguanodon, about 30 feet (9 meters) long, standing about 14 feet (4 meters) tall, and weighing about 4 tons. It is called "duck-billed" because as might be expected its head ends in what looks like the bill of a duck. Its teeth are highly compacted, and its hand has four fingers but no thumb. Some hadrosaurs, such as Corythosaurus, had a bony crest on the head that may have served as protection or as part of its sensory system.
The stegosaurs. The small-headed and small-toothed "plated dinosaurs" moved very slowly on all fours. The best known of these creatures Stegosaurus had one or two rows of upright bony plates along its back; it grew to a length of 30 feet (9 meters). Originally, the bony plates were thought to have a protective function only; recent analysis, however, has shown that they actually had many spaces through which blood could have flowed. If this is so, the plates could have played a role in a cooling system for the creature's blood, which would add further support to the theory that some dinosaurs were warm-blooded.
