Neornithes

Neornithes, more commonly referred to as modern birds, are the most recent common ancestor of all living birds (class Aves) and all its descendants.

Taxonomy
Modern birds are divided into two superorders; the Paleognathae (tinamous and flightless ratites like 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 Paleognathae and Neognathae, and then the split between Galloanserae (fowl) and the other Neognathae, occurred before the Cretaceous-Tertiary (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 made to reconcile the molecular and fossil evidence have proved controversial.

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.

Classification
Basal bird phylogeny showing Neornithes and extinct ancient groups (simplified after Chiappe, 2007 )

Phylogeny
Basal divergences of modern birds based on the Sibley-Ahlquist taxonomy.