Dinosaurs are a diverse group of reptiles[note 1] of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago,[1][2] although the exact origin and timing of the evolution of dinosaurs is the subject of active research.[3] They became the dominant terrestrial vertebrates after the Triassic–Jurassic extinction event 201 million years ago; their dominance continued through the Jurassic and Cretaceous periods. Reverse genetic engineering[4] and the fossil record both demonstrate that birds are modern feathered dinosaurs,[5] having evolved from earlier theropods during the late Jurassic Period.[6] As such, birds were the only dinosaur lineage to survive the Cretaceous–Paleogene extinction event 66 million years ago.[7] Dinosaurs can therefore be divided into avian dinosaurs, or birds; and non-avian dinosaurs, which are all dinosaurs other than birds. This article deals primarily with non-avian dinosaurs.