Birds

Birds evolved from earlier theropods, and thus constitute the only known living dinosaurs. Whilst traditional Linnaean taxonomy placed birds in their own class (owing to their unique morphology and being the only group of animals other than mammals where all its species are warm-blooded), birds are considered reptiles in the modern cladistic sense of the term. Their closest living relatives are the crocodilians, both forming a clade known as archosaurs. Birds are descendants of the primitive avialans (whose members include Archaeopteryx) which first appeared during the Late Jurassic. According to some estimates, modern birds (Neornithes) evolved in the Late Cretaceous or between the Early and Late Cretaceous (100 Ma) and diversified dramatically around the time of the Cretaceous–Paleogene extinction event 66 million years ago, which killed off the pterosaurs and all non-ornithuran dinosaurs.