Palaeognathae

Palaeognathae, meaning “old jaws,” is a major clade of birds defined by a paleognathous palate, a comparatively primitive configuration of the palate bones. The group includes all living tinamous (order Tinamiformes) and the flightless or largely flightless ratites, which comprise ostriches (Struthioniformes), rheas (Rheiformes), and cassowaries and emus (Casuariiformes). Along with Neognathae, it forms the two major divisions of living birds.

Living members and distribution: Tinamous occur across Central and South America, while ratites are distributed across the southern continents and nearby regions: ostriches in Africa, rheas in South America, and cassowaries and emus in Australia and New Guinea. This geographic spread reflects a long, complex evolutionary history and ancient biogeographic patterns.

Anatomy, flight, and diversity: Paleognathous birds are often characterized by a more primitive palate and, in many lineages, reduced flight capability. Most ratites are flightless, with a reduced keel on the sternum, whereas tinamous can fly, albeit typically as weak fliers. Flightlessness has evolved independently in several paleognath lineages.

Evolution and phylogeny: The fossil record for paleognaths extends into the Cretaceous, and modern molecular studies have reshaped understanding of relationships within the group. Tinamous are generally recovered as the sister lineage to the non-tinamiform paleognaths, and the flightless ratites are not a single, monophyletic group. This indicates multiple independent losses of flight and a more complex evolutionary history than once thought, consistent with a Gondwanan-influenced biogeography and later dispersal events.