Paleognathae

Paleognathae, or paleognathous birds, is a major clade within the class Aves that includes the ratites and tinamous. The name refers to the paleognathous palate, a relatively primitive arrangement of the palatal bones that distinguishes paleognaths from most other birds.

Most paleognaths are large or medium-sized, and many are flightless. Among the ratites, species are typically ground-dwelling with a reduced or absent keel on the sternum, reflecting limited flight capability. Tinamous, in contrast, are smaller and capable of flight, though generally not strong fliers and often difficult to detect in dense habitats.

Taxonomy and major groups: Paleognathae comprises five orders. Struthioniformes includes ostriches; Rheiformes comprises rheas; Casuariiformes contains cassowaries and emus; Apterygiformes covers kiwis; and Tinamiformes encompasses tinamous. The ratites represent the mostly flightless lineages within Paleognathae, while tinamous are the only paleognaths with reliable sustained flight.

Distribution and ecology: Paleognaths have a predominantly southern-hemisphere distribution. Ostriches are native to Africa; rheas to South America; cassowaries and emus to parts of Australia and New Guinea; kiwis to New Zealand; tinamous to Central and South America. Ecological roles range from large terrestrial herbivores to forest-dwelling insectivores.

Evolution and fossil record: Paleognathae first appear in the Cretaceous, with diversification continuing into the Cenozoic. The group is generally associated with the ancient geography of Gondwana, and modern diversity reflects a long history of continental drift and lineage-specific adaptations.