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The subclass of Aves that contains all of the known birds other than those placed in the Archaeornithes. Comprising more than 30 orders, both fossil and living, its members are characterized by a bony, keeled sternum with fully developed powers of flapping flight (secondarily lost in a number of groups); a short tail with the caudal vertebrae fused into a single platelike pygostyle to which all tail feathers attach; a large fused pelvic girdle with a reversed pubis which is fused to a large synsacrum; and a large brain and eyes contained within a fused braincase. The jaws are specialized into a beak covered with a horny rhamphotheca; the upper jaw is kinetic, being either prokinetic or rhynchokinetic. Prokinesis refers to a bending zone at the base of the upper jaw, and rhynchokinesis to one within the upper jaw. A few fossil groups still possess teeth, but most fossil and all Recent birds have lost teeth. See Archaeornithes
The Neornithes contains two superorders, the Odontognathae and the Neognathae. The Odontognathae, alternately known as the Odontornithes, may be an artificial group. Its members, which include the Cretaceous fossil orders Hesperornithiformes and Ichthyornithiformes, are united only by the presence of teeth in all species. The Neognathae contains the remaining modern birds, which have lost the teeth, and includes 26 orders. See Aves