Dyke, Gareth J. and Leonard, Leona
Encyclopedia of Life Sciences.
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Current evolutionary consensus, based on both molecular and morphological data, suggests that there are two broad subdivisions at the base of living birds (Neornithes), the neognaths (Neognathae) and palaeognathous birds (Palaeognathae). Palaeognaths are characterised by their primitive skull morphology and a secondarily acquired characteristic – flightlessness. The group, as traditionally defined and still recognised by phylogenetic analyses, includes flightless birds (ostrich, rhea, emu, cassowaries and kiwis) and flighted tinamous. Flightless forms are found today on the former landmasses of Gondwana, Africa, South America and Australia, whereas tinamous are restricted to South America. Traditionally, palaeognaths have been subdivided into ratites (all the large flightless groundbirds) and tinamous, although some data sets suggest that this classification might be overly simplistic: Recent molecular studies have concluded that the South American flighted taxa are nested within the traditional ratite grouping. This is interesting because it means that ratite flightlessness has evolved convergently at least four times.
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