The Palaeognathae or paleognaths are one of the two living clades of birds. The other living is Neognathae. Together these two clades form the clade Neornithes. Palaeognathae contains four extant branches of flightless ratites and one of flying tinamous. The tinamous are forty-seven living species, the kiwis five species (Apteryx), the cassowaries (Casuarius) three, the emus (Dromaius) is one living and one recently extinct, the rheas are two species and the ostriches have only one living species.
There are three extinct groups, the Lithornithiformes, the Dinornithiformes (moas) and the Aepyornithiformes (elephant birds), that are undisputed members of Palaeognathae. There are other extinct birds which have been allied with the Palaeognathae by at least one author, but their affinities are a matter of dispute (Ambiornithiformes, Gansuiformes, Paleocursornithiformes and Gobipterygidae).
The word Paleognath is derived from the ancient Greek for "old jaws" in reference to the skeletal anatomy of the palate, which is described as more primitive and reptilian than that in other birds. Paleognathous birds are uncontroversially the most primitive, or basal, living birds, though there is some controversy about the precise relationship between them and the other birds. There are also several other scientific controversies about their evolution (see below).