Ornithopods are bird-hipped dinosaurs, although the name means 'bird-toed' because they have three toes whose impressions look like those made by birds. The first ones were small – about a metre tall – and probably ran quite quickly, as suggested by their stiff tail – more like a hunter's tail than other plant-eaters. A tail like that gives balance when moving fast. Later on they evolved and got bigger and at the end of dinosaur times, they were some of the most common medium-sized dinosaurs – up to 15 m long.

The key to their success was they way they ate and how their mouths changed to match changes in the plants that ate. Eventually they became duck-billed. Unlike most other medium-sized plant-eaters, they were not armour-plated, but they did develop a horny beak. They also evolved to become more like other grazing dinosaurs, with longer front legs and bent spines.