Dinosaurs (Owen, 1842) are a clade of vertebrate animals that dominated terrestrial ecosystems for approximately 170 million years, first appearing roughly 240 million years ago. At the end of the Cretaceous Period, 66 million years ago, non-avian dinosaurs suffered a catastrophic extinction, which ended their dominance on land. Taxonomists state that birds are the only remaining dinosaurs today, since birds are direct descendants of theropod dinosaurs, birds retain some dinosaurian features.