Ornithopoda means "bird feet", from the Greek ornithos ("bird") and pous ("feet"); this refers to their characteristic three-toed feet, although many early forms retained four toes. They were also characterized by having no armour, the development of a horny beak, an elongated pubis that eventually extended past the ilium, and a missing hole in the lower jaw. A variety of ornithopods and related cerapods had thin cartilaginous plates along the outside of the ribs; in some cases, these plates mineralized and were fossilized. The function of these intercostal plates is unknown. They have been found with Hypsilophodon, Othnielosaurus, Parksosaurus, Talenkauen, Thescelosaurus, and Macrogryphosaurus to date.
Size of a variety of iguanodontian ornithopods
The early ornithopods were only about 1 metre (3 feet) long, but probably very fast. They had a stiff tail, like the theropods, to help them balance as they ran on their hind legs. Later ornithopods became more adapted to grazing on all fours; their spines curved, and came to resemble the spines of modern ground-feeders such as the bison. As they became more adapted to eating while bent over, they became facultative quadrupeds; still running on two legs, and comfortable reaching up into trees; but spending most of their time walking or grazing while on all fours. The taxonomy of dinosaurs previously ascribed to the Hypsilophodontidae is problematic. The group previously consisted of all non-iguanodontian bipedal ornithischians, but a phylogenetic reappraisal has shown such species to be paraphyletic. As such, the hypsilophodont family is currently represented only by Hypsilophodon.
Later ornithopods became larger, but never rivalled the incredible size of the long-necked, long-tailed sauropods that they partially supplanted. The very largest, such as Shantungosaurus, were as heavy as medium-sized sauropods at up to 23 metric tons (25 short tons) but never grew much beyond 15 metres (50 feet).