These are living and extinct ratites, a group of flightless birds with a flattened breastbone, grouped along with tinamou in the paleognaths.
The paleognaths were initially thought to be polyphyletic, (from several ancestral lines that come to resemble each other because of similar environments) as the group is scattered over four continents. Living members are the ostrich (Struthio, Africa), emu (Dromaius, Australia) cassowary (Casuarius, New Guinea and Australia), rhea (Rhea, South America), and the kiwi (Apteryx and moas, New Zealand). Extinct members are the moa (Dinornis, Euryapteryx, Emeus, Pachyornis, Megalapteryx, and Anomalopteryx) and the elephant bird of Madagascar (Aepyornis). Until recently, most researchers agreed that paleognaths represented a Gondwanan distribution, with the group having lost the power of flight well before the end of the Cretaceous. Another theory, however, argues that the group is monophyletic, with the tinamou the most primitive member, and that flightlessness had evolved only once in the group before their divergence.
One interesting point is that such ratite features as feather type, palatal structure, and the persistence of skull sutures into adulthood suggests that moas were "permanent chicks," examples of neoteny. The lesser moas formed the family Anomalopterygidae, with about two-thirds of the species in the order; the greater moas, in the family Dinornithidae, included the giants of the order.