are divided into two non-taxonomic, functional groups: the ratites, or
flightless birds, and the carinates, flying forms with keeled sternums) make
their appearance in the fossil record about 150 million years ago,
most likely having evolved from small, bipedal dinosaurs. With over 9,000 species (more than any other vertebrate
group except for the
ray-finned fishes), they are found in just about every habitat on earth.
In terms of their phylogeny, birds and crocodilians form a monophyletic sister
group and are therefore placed in the clade Archosauria, a group that also
contains the extinct dinosaurs and pterosaurs (flying reptiles).