Tuesday, November 13, 2007

>>Radiation of modern birds

Radiation of modern birds

Modern birds are classified in the subclass Neornithes, which are now known to have evolved into some basic lineages by the end of the Cretaceous . The Neornithes are split into two superorders, the Paleognathae and Neognathae. The paleognaths include the tinamous of Central and South America and the ratites. The ratites are large flightless birds, and include ostriches, rheas, cassowaries, kiwis and emus (though some scientists suspect that the ratites represent an artificial grouping of birds which have independently lost the ability to fly in a number of unrelated lineages).
The basal divergence from the remaining Neognathes was that the Galloanserae, the superorder containing the Anseriformes (ducks, geese, swans and screamers), and the Galliformes (the pheasants, grouse, and their allies, together with the mound builders, and the guans and their allies). The dates for the splits are much debated by scientists. It is agreed that the Neornithes evolved in the Cretaceous and that the split between the Galloanseri from other Neognathes occurred before the K-T extinction event, but there are different opinions about whether the radiation of the remaining Neognathes occurred before or after the extinction of the other dinosaurs.This disagreement is in part caused by a divergence in the evidence, with molecular dating suggesting a Cretaceous radiation and fossil evidence supporting a Tertiary radiation. Attempts to reconcile the molecular and fossil evidence have proved controversial.
The classification of birds is a contentious issue. Sibley and Ahlquist's Phylogeny and Classification of Birds (1990) is a landmark work on the classification of birds, although frequently debated and constantly revised. A preponderance of evidence seems to suggest that the modern bird orders constitute accurate taxa. But scientists disagree about the relationships between orders; evidence from modern bird anatomy, fossils and DNA have all been brought to bear on the problem but no strong consensus has emerged. More recently, new fossil and molecular evidence is providing an increasingly clear picture of the evolution of modern bird orders.

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