premise: In the Cretaceous non-avian dogs died out.
hypothesis: Cretaceous . The Cretaceous ended with a large mass extinction , the Cretaceous -- Paleogene extinction event , in which many groups , including non-avian dinosaurs , pterosaurs and large marine reptiles died out . Edmontosaurus ( [ ɛdˌmɒntəˈsɔːrəs ] ) is a genus of hadrosaurid ( duck-billed ) dinosaur . Ornithopods ( [ ɔːrˈnɪθəˌpɒdz , _ ˈɔːrnɪ - ] ) or members of the clade Ornithopoda ( [ ˌɔːrnɪˈθɒpədə ] or [ ɔːrˌnɪθəˈpoʊdə , _ ˌɔːrnɪ - ] ) are a group of ornithischian dinosaurs 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 .