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Ratitae:
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 and, lacking a strong anchor for their wing muscles, could not fly even were they to develop suitable wings. The name ratite comes from the Latin word for raft (ratis), because their breastbone looks like a raft. Evolution and Systematics:
There are two taxonomic approaches to ratite classification: the one applied here combines the groups as families in the order Struthioniformes, while the other supposes that the lineages evolved mostly independently and thus elevates the families to order rank (e.g. Rheiformes, Casuariformes etc. The traditional account of ratite evolution has the group emerging in Gondwana in cretaceous times, then evolving in their separate directions as the continents drifted apart. Cladistic evidence for this is strong: ratites share too many features for their current forms to be easily explained by convergent evolution. However, recent analysis of genetic variations 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, and suggest that the kiwi are more closely related to the cassowaries than the moa. At present there is no generally accepted explanation. Also, there is the Middle Eocene fossil "proto-ostrich" Palaeotis from Central Europe, which either implies that the ancestral ratites had not yet lost flight when they were dispersing all over Gondwana - by the Middle Eocene, both Laurasia and Gondwana had separated into the continents of today - or that the "out-of-Gondwana" hypothesis is wrong. Research continues, but at present the ratites are perhaps the one group of modern birds for which no good theory of their evolution and paleobiogeography exists. Ratites are more closely related to dinosaurs than modern birds |
