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The question of whether relatives of living birds co-existed with non-bird dinosaurs has evoked controversy.
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.
However, it is known that (Proctor and Lynch 1993): A near total demise of archaic birds occurred at the K–T boundary, with a rapid reorganization and explosive early Tertiary evolution from a bottleneck of modern ornithurine morphological types, perhaps involving ‘transitional shorebirds', paleognaths and some other lineages.
The initial diversification of modern ornithurines might have taken place in the late Cretaceous (phylogenetic fuse model), but the explosive adaptive radiation followed the K–T extinction event.
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.
The distribution of some groups of birds may have been influenced by continental drift (Cracraft 1974).
Yellow areas indicate the subclass Ornithurae; blue areas indicate the subclass Sauriurae (From: Feduccia 2003). Birds of tropical rainforest: comparative biogeography and ecology. 215-228 in Biogeography and ecology of forest bird communities (A.
Currently, taxonomists recognize 40 orders, 233 families, 2229 genera, and over 10,000 species of birds.