Modern Birds - Taxonomy

Taxonomy

Modern birds are divided into two superorders; the Palaeognathae (tinamous and flightless ratites such as ostriches), and the wildly diverse Neognathae, containing all other birds. Depending on the taxonomic viewpoint, the number of species cited varies anywhere from 8,800 to 10,200 known living bird species in the world. It is generally agreed that the Neornithes evolved in the Cretaceous Period and that the split between the Palaeognathae and Neognathae, and then the split between Galloanserae (fowl) and the other Neognathae, occurred before the Cretaceous–Paleogene extinction event (the earliest fossil remains confidently referred to the Neornithes come from the possible galliform Austinornis, dated to about 85 mya, though the 130 million year old Hauterivian age Gallornis straeleni may also be a neornithine), 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 made to reconcile the molecular and fossil evidence have proved controversial.

The classification of modern birds is a contentious issue. Sibley & 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. However, scientists are not in agreement as to the relationships between the 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. See also: Sibley-Ahlquist taxonomy and dinosaur classification.

In traditional classification, the Neornithes also included a third superorder, the Odontognathae, containing advanced toothed birds from the Cretaceous, like Hesperornis and Ichthyornis. This superorder is likely paraphyletic, and fall outside the crown group birds. It is not entirely clear whether the Palaeognathae too are paraphyletic, or represent a primitive grade of birds.