Ornithopods

Ornithopods (pronounced /ɔrˈnɪθɵpɒd/) or members of the clade Ornithopoda (pronounced /ɔrnɨˈθɒpədə/) are a group of bird-hippeddinosaurs 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, and 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. Members are known from all seven continents, although the Antarctic remains are unnamed, and they are generally rare in the Southern Hemisphere.
http://en.wikipedia.org/wiki/Ornithopod