Vertebrate Zoology - V. M. Konstantinov 2011

Chordates
Terrestrial, or Four-Legged, Vertebrates — Vertebrates with Embryonic Membranes
Class Birds — Subclass Fan-tailed, or True, Birds — Superorder Ratites, or Ostriches

Ratites are a highly distinct group characterized, on the one hand, by primitive Organization and, on the other, by a high degree of specialization associated with running or walking on the ground rather than flying. They rely on long, powerful hindlimbs for locomotion. The number of toes (except in kiwis) is reduced to three or two, which is an adaptation for speed. Primitive Anatomical Features include the absence of apteria, a lack of clear differentiation of feathers into groups, and feathers lacking interlocking barbs, meaning the plumage serves primarily for thermal insulation. Much like some dinosaurs, they possess a sternal callous on the Skin. The Sternum is small and lacks a keel. The scapulae and coracoids are fused into a single bone, and the clavicles are vestigial. The pelvic and Skull bones fuse late and often incompletely. Bone pneumatization is poorly developed, and the uropygial gland is absent. Males possess a functioning copulatory organ.

The current distribution of ratites is restricted to Africa, South America, Australia, and New Zealand. In the past, their range was much broader. For instance, remains of birds belonging to a distinct order, the elephant birds (Aepyornithiformes), have been found in Paleogene-Neogene and even Quaternary deposits in Madagascar, while moas (Dinornithiformes) inhabited New Zealand well into historical times. True ostriches are known from Paleogene-Neogene deposits across various locations in Asia and Southern Europe.