Page "Cretaceous–Paleogene extinction event" Paragraph 38

Most paleontologists regard birds as the only surviving dinosaurs ( see Origin of birds ).

However, all non-neornithean birds became extinct, including flourishing groups like enantiornithines and hesperornithiforms.

Several analyses of bird fossils show divergence of species prior to the K – T boundary, and that duck, chicken and ratite bird relatives coexisted with non-avian dinosaurs.

Large collections of bird fossils representing a range of different species provides definitive evidence for the persistence of archaic birds to within 300, 000 years of the K – Pg boundary.

None of them are known to survive into the Paleogene, and their persistence into the latest Maastrichtian therefore provides strong evidence for a mass extinction of archaic birds coinciding with the Chicxulub asteroid impact.

A small fraction of the Cretaceous bird species survived the impact, giving rise to today's birds.

So far, only a single bird species, which has not been named, has been confidently identified from both above and below the K-Pg boundary ( it is present in the Maastrichtian Hell Creek Formation and Danian Fort Union Formation ).

The only bird group known for certain to have survived the K – T boundary is the Neornithines ( though one Paleogene species, Qinornis paleocenica, may represent a surviving non-neornithine bird ).

Neornithines may have been able to survive the extinction as a result of their abilities to dive, swim, or seek shelter in water and marshlands.

Many species of neornithines can build burrows, or nest in tree holes or termite nests, all of which provided shelter from the environmental effects at the K – T boundary.

Long-term survival past the boundary was assured as a result of filling ecological niches left empty by extinction of non-avian dinosaurs.