Although ornithischian (bird-hipped) dinosaurs share the same hip structure as birds, birds actually originated from the saurischian (lizard-hipped) dinosaurs (if the dinosaurian origin theory is correct), and thus arrived at their hip structure condition independently. In fact, the bird-like hip structure also developed a third time among a peculiar group of theropods, the Therizinosauridae.
An alternate theory to the dinosaurian origin of birds, espoused by a few scientists (most notably Lary Martin and Alan Feduccia), states that birds (including maniraptoran "dinosaurs") evolved from early archosaurs like Longisquama, a theory which is contested by most other scientists in paleontology, and by experts in feather development and evolution such as R.O. Prum. See the Longisquama article for more on this alternative.
Modern birds are classified in Neornithes, which are now known to have evolved into some basic lineages by the end of the Cretaceous (see Vegavis). The Neornithes are split into the Paleognathae and Neognathae. The paleognaths include the tinamous (found only in Central and South America) and the ratites. The ratites are large flightless birds, and include ostriches, cassowaries, kiwis and emus (though some scientists suspect that the ratites represent an artificial grouping of birds which have independently lost the ability to fly in a number of unrelated lineages). The basal divergence from the remaining Neognathes was that of the Galloanseri, the superorder containing the Anseriformes (ducks, geese and swans), and the Galliformes (the pheasants, grouse, and their allies). See the chart for more information.
The classification of birds is a contentious issue. Sibley & Ahlquist's Phylogeny and Classification of Birds (1990) is a landmark work on the classification of birds (although frequently debated and constantly revised). A preponderance of evidence seems to suggest that the modern bird orders constitute accurate taxa. However, scientists are not in agreement as to the relationships between the orders; evidence from modern bird anatomy, fossils and DNA have all been brought to bear on the problem but no strong consensus has emerged. More recently, new fossil and molecular evidence is providing an increasingly clear picture of the evolution of modern bird orders. See also: Sibley-Ahlquist taxonomy.
An alternate theory to the dinosaurian origin of birds, espoused by a few scientists (most notably Lary Martin and Alan Feduccia), states that birds (including maniraptoran "dinosaurs") evolved from early archosaurs like Longisquama, a theory which is contested by most other scientists in paleontology, and by experts in feather development and evolution such as R.O. Prum. See the Longisquama article for more on this alternative.
Modern birds are classified in Neornithes, which are now known to have evolved into some basic lineages by the end of the Cretaceous (see Vegavis). The Neornithes are split into the Paleognathae and Neognathae. The paleognaths include the tinamous (found only in Central and South America) and the ratites. The ratites are large flightless birds, and include ostriches, cassowaries, kiwis and emus (though some scientists suspect that the ratites represent an artificial grouping of birds which have independently lost the ability to fly in a number of unrelated lineages). The basal divergence from the remaining Neognathes was that of the Galloanseri, the superorder containing the Anseriformes (ducks, geese and swans), and the Galliformes (the pheasants, grouse, and their allies). See the chart for more information.
The classification of birds is a contentious issue. Sibley & Ahlquist's Phylogeny and Classification of Birds (1990) is a landmark work on the classification of birds (although frequently debated and constantly revised). A preponderance of evidence seems to suggest that the modern bird orders constitute accurate taxa. However, scientists are not in agreement as to the relationships between the orders; evidence from modern bird anatomy, fossils and DNA have all been brought to bear on the problem but no strong consensus has emerged. More recently, new fossil and molecular evidence is providing an increasingly clear picture of the evolution of modern bird orders. See also: Sibley-Ahlquist taxonomy.
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