Ratite - Evolution and Classification

Evolution and Classification

The earliest known ratite fossils date to the Paleocene epoch about 56 million years ago (e.g. Diogenornis, a possible early relative of the Rhea). However, more primitive paleognaths are known from several million years earlier, and the classification and membership of the Ratitae itself is uncertain.

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.).

Some studies based on morphology, immunology and DNA sequencing had indicated that ratites are monophyletic. 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. Furthermore, recent analysis of twenty nuclear genes has drawn into question the monophyly of the group, suggesting that the flighted tinamous cluster within the ratite lineage. The authors say the data "unequivocally places tinamous within ratites".

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. Another study has reversed the relative positions of moas and rheas, and indicated that elephant birds are not close relatives of ostriches or other ratites, while a study of nuclear genes shows ostriches branching first, followed by rheas and tinamous, then kiwis splitting from emus and cassowaries. These studies share branching dates which suggest that, while ancestral moas may have been present in New Zealand since it split off from Gondwana, the ancestors of kiwis appear to have arrived there from Australia more recently, perhaps via a land bridge or by island-hopping. A recent extensive morphological comparison makes South American and Australian ratites a clade, with three successively more distant sister groups consisting of ostriches, elephant birds, and a clade of New Zealand ratites.

All analyses but the most recent morphological study show that rheas and extant Australo-Pacific ratites are monophyletic. The DNA data showing the ostriches branching first would match the sequence of Gondwana's plate tectonic breakup. Other, but not all, aspects of ratite paleobiogeography were found to be consistent with the vicariance (plate tectonic split-up of Gondwana) hypothesis. The sister-group relationship of New Zealand ratities to other ratites proposed on morphological grounds would not be consistent with vicariance, while the proposed South American-Australian clade would be.

Recent phylogenomic studies suggest that tinamous may in fact belong to this group. If so, this would make 'ratites' paraphyletic rather than monophyletic. Since tinamous are weak fliers, this raises interesting questions about the evolution of flightlessness in this group. While the ratites were traditionally thought of as an ancestrally flightless, monophyletic group, the branching of the tinamous within the ratite lineage suggests that ratites evolved flightlessness at least three times. Re-evolution of flight in the tinamous would be an alternative explanation, but such a development is without precedent in avian history, while loss of flight is commonplace.