Neognaths (Neognathae) are birds within the subclass Neornithes of the class Aves. The Neognathae include virtually all living birds; the exception belong to their sister taxon (Palaeognathae), which contains the tinamous and the flightless ratites.
There are nearly 10,000 species of neognaths. Since the late Cretaceous, from which the earliest fossils are known, they have undergone an adaptive radiation producing the diversity of form, function, and behavior that we see today. The Passeriformes (perching birds) are the largest clade of land vertebrates, containing some 60% of living birds and being more than twice as speciose as rodents and about 5 times as speciose as Chiroptera (bats and flying foxes) which are the largest clades of mammals. On the other hand, there are some very small orders, usually birds of very unclear relationships like the puzzling Hoatzin.
The neognaths have fused metacarpals, an elongate third finger, and 13 or fewer vertebrae. They differ from the Palaeognathae in features like the structure of their jawbones. "Neognathae" means "new jaws", but ironically it seems that the supposedly "more ancient" paleognath jaws are among the few apomorphic (more derived) features of this group as compared to the neognaths.
Read more about Neognathae: Taxonomy and Systematics, See Also