It is generally agreed that the Neornithes evolved in the Cretaceous and that the split between the Palaeognathae and Neognathae, and then the split between fowl and the other Neognathae, occurred before the K/T extinction event. The earliest fossil remains of the Neornithes might be about ten million years after the start of the Cretaceous. Gallornis may be the first fossil, but its remains are only a worn partial femur and a fragment of the humerus.
There are different opinions about whether the radiation of the remaining neognathes occurred before or after the extinction of the other dinosaurs. Molecular dating suggests a Cretaceous radiation, while fossil evidence suggest an early Tertiary radiation. Attempts made to bring together the molecular and fossil evidence have proved controversial.
The classification of modern birds is difficult. Sibley & Ahlquist's Phylogeny and classification of birds (1990) is a landmark work on the classification of birds, though it is often debated and revised. Most evidence suggests that the modern bird orders are accurate taxa. More recently, new fossil and molecular evidence is providing an increasingly clear picture of the evolution of modern bird orders.
- Hope, Sylvia 2002. The Mesozoic radiation of Neornithes. In Chiappe, Luis M. & Witmer, Lawrence M. (eds) Mesozoic birds: above the heads of dinosaurs. pp. 339–388 ISBN 0-520-20094-2
- Ericson P.G.P.; et al. (22 December 2006). "Diversification of Neoaves: integration of molecular sequence data and fossils". Biol Lett. 2 (4): 543–547. doi:10.1098/rsbl.2006.0523. PMC 1834003. PMID 17148284. Explicit use of et al. in:
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- McDowell, Sam (1948). "The bony palate of birds" (PDF). The Auk. 65 (4): 520–549.