ratite n : flightless birds having flat breastbones lacking a keel for attachment of flight muscles: ostriches; cassowaries; emus; moas; rheas; kiwis; elephant birds [syn: ratite bird, flightless bird] [ant: carinate]

English

Noun

1. The order of Struthioniformes, a diverse group of large running, flightless birds, mostly extinct, but including the cassowary, elephant bird, emu, kiwi, moa, ostrich, rhea and tinamou

A ratite is any of a diverse group of large, flightless birds of Gondwanan origin, most of them now extinct. Unlike other flightless birds, the ratites have no keel on their sternum - hence their name which comes from the Latin (ratis) for raft. Without this to anchor their wing muscles they could not fly even if they were to develop suitable wings.

Most parts of the former Gondwana have ratites, or have had until the fairly recent past. Their closest living relatives are the tinamous of South America. The traditional account of ratite evolution has the group emerging in flightless form in Gondwana in the Cretaceous, then evolving in their separate directions as the continents drifted apart. However, recent analysis of genetic variation between the ratites conflicts with this: DNA analysis appears to show that the ratites diverged from one another too recently to share a common Gondwanian ancestor. Also, the Middle Eocene fossil "proto-ostrich" Palaeotis from Central Europe may imply that the "out-of-Gondwana" hypothesis is wrong.

A comparative study of the full mitochondrial DNA sequences of living ratites plus two moas places moas in the basal position, followed by rheas, followed by ostriches, followed by kiwis, with emus and cassowaries being closest relatives. What these studies have in common is branching dates that imply that while the ancestors of moas may have been present in New Zealand since it split off from other parts of Gondwana, the ancestors of kiwis appear to have somehow dispersed there from Australia more recently, perhaps via a land bridge or by island-hopping. Similarly, ostriches seem to have arrived in Africa by some route after it detached from South America. Other aspects of ratite paleobiogeography were found to be consistent with the vicariance (plate tectonic split-up of Gondwana) hypothesis.

References and Notes

Gallery of Living Species