Ornithopoda

Ornithopoda (/ˌɔːrnəˈθɒpədə/) is a clade of ornithischian dinosaurs, called ornithopods (/ˈɔːrnəθəˌpɒdz, ɔːrˈnɪθ-/), that started out as small, bipedal running grazers and grew in size and numbers until they became one of the most successful groups of herbivores in the Cretaceous world, dominating the North American land. Their major evolutionary advantage was the progressive development of a chewing apparatus that became the most sophisticated ever developed by a non-avian dinosaur, rivaling that of modern mammals such as the domestic cow. They reached their apex of diversity and ecological dominance in the hadrosaurids (colloquially known as 'duck-bills'), before they were wiped out by the Cretaceous–Paleogene extinction event along with all other non-avian dinosaurs. Members are known from all seven continents, though they are generally rare in the Southern Hemisphere.