Wikipedia
Palaeognathae, or paleognaths, is one of the two living clades of birds – the other being Neognathae. Together, these two clades form the clade Neornithes. Palaeognathae contains five extant branches of flightless lineages (plus two extinct clades), termed ratites, and one flying lineage, the Neotropic tinamous. There are 47 species of tinamous, 5 of kiwis (Apteryx), 3 of cassowaries (Casuarius), 1 of emus ( Dromaius) (another became extinct in historic times), 2 of rheas and 2 of ostrich. Recent research has indicated that paleognaths are monophyletic but the traditional taxonomic split between flightless and flighted forms is incorrect; tinamous are within the ratite radiation, meaning flightlessness arose independently multiple times via parallel evolution.
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.
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 retain some basal morphological characters but are by no means living fossils as their genomes continued to evolve at the DNA level under selective pressure at rates comparable to the Neognathae branch of 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).