Friday, November 2, 2007

>>Dinosaurs and the origin of birds

Dinosaurs and the origin of birds

Confuciusornis, a Cretaceous bird from China

Confuciusornis, a Cretaceous bird from China
There is significant evidence that birds evolved from theropod dinosaurs, specifically, that birds are members of Maniraptora, a group of theropods which includes dromaeosaurs and oviraptorids, among others. As more non-avian theropods that are closely related to birds are discovered, the formerly clear distinction between non-birds and birds becomes blurred. Recent discoveries in Liaoning Province of northeast China, demonstrating that many small theropod dinosaurs had feathers, contribute to this ambiguity.
The oldest known bird, the Late Jurassic Archaeopteryx, is well-known as one of the first transitional fossils to be found in support of evolution in the late 19th century, though it is not considered a direct ancestor of modern birds. Protoavis texensis may be even older although the fragmentary nature of this fossil leaves it open to considerable doubt whether this was a bird ancestor.
The dromaeosaurids Cryptovolans and Microraptor may have been capable of powered flight to an extent similar to or greater than that of Archaeopteryx. Cryptovolans had a sternal keel and had ribs with uncinate processes. In fact, Cryptovolans makes a better "bird" than Archaeopteryx which is missing some of these modern bird features. Because of this, some palaeontologists have suggested that dromaeosaurs are actually basal birds, and that the larger members of the family are secondarily flightless, i.e. that dromaeosaurs evolved from birds and not the other way around. Evidence for this theory is currently inconclusive, as the exact relationship among the most advanced maniraptoran dinosaurs and the most primitive true birds are not well understood.
Although ornithischian (bird-hipped) dinosaurs share the hip structure of birds, birds actually originated from the saurischian (lizard-hipped) dinosaurs, and thus evolved their hip structure 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 Larry Martin and Alan Feduccia), states that birds (including maniraptoran "dinosaurs") evolved from early archosaurs like Longisquama, a theory which is contested by most palaeontologists and evidence based on feather development and evolution.

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