Flexi Says: All modern birds lie within the subclass Neornithes, which has two subdivisions: the Paleognathae, containing mostly flightless birds like ostriches, and the diverse Neognathae, which contains all other birds. Depending on the taxonomic viewpoint, the number of known living bird species varies anywhere from 9,800 to 10,050. The Neognathae, or neognaths (“new jaws”), include virtually all living birds. There are about 10,000 species of living neognaths. Since the late Cretaceous period (from about 145 million years ago to 66 million years ago), from when the earliest fossils are known, they have developed into very different body forms (especially the bills and feet) and a diverse range of behaviors.