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It is clear that modern birds from at least the latest Cretaceous lived at the same time as archaic birds including Hesperornis, Ichthyornis, and the diverse Enantiornithiformes. Additional fossils and molecular data are still required to help understand the role of biotic interactions in the evolution of Late Cretaceous birds and thus to test that the mechanisms of microevolution are sufficient to explain macroevolution.

The fossil record also indicates that some groups once occurred in areas well outside their present range (e.g., parrots in Wyoming), while others have apparently always been limited to certain areas: The past history of bird distribution can, to varying degrees, be inferred based on present distribution & the geological history of changes in climate, sea level, and the location of land masses.

A birdwatcher 65 million years ago could have seen relatives of today's loons, geese and ducks, albatrosses and petrels, and gulls and shorebirds, and possibly other familiar birds as well.

What does the fossil record reveal concerning the present-day distribution of birds?

Some investigators, using “molecular clock” models and DNA sequence data as well as the distribution of living birds, have concluded that relatives of living birds must have existed alongside non-avian dinosaurs and survived the mass extinction of dinosaurs at the K/T boundary.

Others believe such data are unreliable, that the fossil record shows no evidence of living bird lineages in the Cretaceous, and that relatives of today’s birds evolved after the K/T boundary.

Prominent and well-known Cretaceous bird taxa included the Enantiornithes, a fairly diverse group of birds, mostly flying forms; Hesperornithiformes, toothed birds (see drawing below) which were mostly flightless swimmers; and Ichthyornithiformes, toothed flying birds that probably fed on fish.

These taxa are extinct today, but by the close of the Cretaceous, representatives of several modern bird taxa were sharing the skies with these extinct birds. Phenomena that can mislead phylogenetic analyses (e.g., long branch attraction, base compositional bias, discordance between gene trees and species trees, and sequence alignment errors) were eliminated as explanations for this result. The most plausible hypothesis requires at least three losses of flight and explains the many morphological and behavioral similarities among ratites by parallel or convergent evolution. Credibility intervals (95%) are indicated by grey bars at numbered internal nodes. Periods when Antarctica was ice-covered (black continuous bars) are indicated by shaded grey rectangles. Birds occupy all continents and habitats but, of course, some continents & some habitats have more species than others. How are birds currently distributed throughout the planet and what factors have contributed to this distribution? Dashed lines show postulated phylogeny compiled from the literature (Slack et al. The penguin fossils were from a Paleocene (early Tertiary) formation just above a well-known Cretaceous/Tertiary boundary site. Single origin of a pan-Pacific bird group and upstream colonization of Australia.