Ornithopod
The ornithopods became one of the most successful groups of herbivores in the Cretaceous world, and dominated the landscape. They started out as small, bipedal running browsers, and grew in size and numbers. Hypsilophodon from the Lower Cretaceous of the Isle of Wight, England, is a typical example. As they grew in size, they could move on four legs but still kept the habit of standing up on their hind legs when needed to reach food or perhaps to defend themselves or to run.