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Ornithopods (or-nith-oh-PODS) are a group of ornithischian dinosaurs who started out as small, cursorial grazers, and grew in size and numbers until they became one of the most successful Cretaceous herbivores in the world, and totally dominated the North American landscape. Their major evolutionary advantage was the progressive development of a chewing apparatus that became the most sophisticated ever developed by a reptile, rivaling that of modern mammals like the domestic cow. They reached their apex in the duck-bills, before they were wiped out by the Cretaceous-Tertiary extinction event along with all other non-avian dinosaurs.
Ornithopoda means "bird feet", from the Greek ornis ("bird") and pous ("feet)"; this refers to their characteristic three-toed feet. They were also characterized by no armor, the development of a horny beak, an elongated pubis that eventually extended past the ilium, and a missing hole in the lower jaw.
The early ornithopods were only about 1 meter (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 like the bison. As they became more adapted to eating while bent over, they became semi-quadrupedal; still running on two legs, and comfortable reaching up into trees; but spending most of their time walking or grazing while on all fours.
Later ornithopods became larger, but never rivaled the incredible size of the long-necked, long-tailed sauropods that they partially supplanted; the largest, like the Edmontosaurus and Shantungosaurus, never exceeded 15 meters (50 feet).
Historically, most indeterminate ornithischians bipeds were lumped in as ornithopods. Most have since been reclassified as basal members of quadrupedal taxa like Marginocephalia; and some, like the "bone-headed" pachycephalosaurs, have been given their own taxa.
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