Everything about Struthioniformes totally explained
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 couldn't 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
Image:Strauss m Tanzania.jpg|Ostrich
Image:Rhea side profile.jpg|American Rhea
Image:Darwin-Nandus.jpg|Darwin's Rhea
Image:Double-wattled Cassowary.jpg|Southern Cassowary
Image:Mooruk-Wolf.jpg|Dwarf Cassowary
Image:Emu02 - melbourne zoo.jpg|Emu head
Image:Apteryx haastii.jpg|Great Spotted Kiwi
Image:Apteryx owenii 0.jpg|Little Spotted Kiwi
Image:Apteryx australis.jpg|Tokoeka
Image:Kiwifugl.jpg|North Island Brown Kiwi
Further Information
Get more info on 'Struthioniformes'.
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