Birds evolved from certain
feathered theropod dinosaurs, and there is no real dividing line between birds and
dinosaurs, except of course that some of the former survived the
Cretaceous–Paleogene extinction event while the latter did not. For the purposes of this article, a 'bird' is considered to be any member of the clade
Aves sensu lato. Some dinosaur groups which may or may not be true birds are listed below under 'Proto-birds'.