Paleognathous

Paleognathous describes a group of birds characterized by a paleognathous palate, a primitive configuration of the palate bones in the skull. The term derives from Greek palaeon meaning old and gnathos meaning jaw, and it is used to distinguish this palate from the neognathous palate found in most other birds.

Anatomically, paleognaths have a relatively rigid palate with fewer mobile joints between the palatal bones, in contrast to the more flexible and complex palates of neognaths. This trait is one of several primitive features that help define this lineage, though it is not the sole criterion for classification.

Taxonomically, Paleognathae is a major clade within the class Aves. Living paleognaths include tinamous (order Tinamiformes) and the ratite lineages, traditionally grouped in various orders: Struthioniformes (ostriches), Rheiformes (rheas), Casuariformes (cassowaries and emus), and Apterygiformes (kiwis).

Geographically, paleognaths have a Gondwanan distribution. Tinamous occur in Central and South America, while ratites are found in Africa (ostrich), South America (rhea), Australia (emu), New Guinea (cassowary), and New Zealand (kiwi). Most ratites are flightless, whereas tinamous are capable of weak flight.

Evolutionarily, molecular studies support tinamous as the sister group to the other living paleognaths, with ratites forming a separate clade that lost flight in multiple lineages. Paleognathous birds therefore represent one of the oldest major splits within birds, predating most modern neognathous birds.