Bock, Walter J. Department of Biological Sciences, Columbia University, New York, New York.
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The subclass of birds (Aves) containing all of the known fossil and living birds other than those placed in the extinct Sauriurae (Archaeornithes and Enantiornithes). Comprising more than 30 orders, both fossil and living, members of the Neornithes are characterized by the nature of fusion of the bony elements making up the tarsometatarsus (the fused ankle and foot bones) and the way in which the three bones of the pectoral girdle meet and form the articulation with the humerus. The Neornithes and Enantiornithes share a number of features: a bony, keeled sternum with fully developed powers of flight (secondarily lost in a number of flightless groups); a short tail with the caudal vertebrae fused into a single platelike pygostyle to which the tail feathers attach; a large fused pelvic girdle with a reversed pubis that is fused with a large synsacrum; and large brain and eyes 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 forms possess teeth, but most fossil and all Recent birds have lost the teeth. See also: Aves; Enantiornithes
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