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The evolution of birds is thought to have begun in the Jurassic Period, with the earliest birds derived from a clade of theropoda dinosaurs named Paraves. Birds are categorized as a biological class, Aves. The earliest known is ''Archaeopteryx lithographica'', from the Late Jurassic period, though ''Archaeopteryx'' is not commonly considered to have been a true bird. Modern phylogenies place birds in the dinosaur clade Theropoda. According to the current consensus, Aves and a sister group, the order Crocodilia, together are the sole living members of an unranked "reptile" clade, the Archosauria. Phylogenetically, Aves is usually defined as all descendants of the most recent common ancestor of a specific modern bird species (such as the house sparrow, ''Passer domesticus''), and either ''Archaeopteryx'', or some prehistoric species closer to Neornithes (to avoid the problems caused by the unclear relationships of ''Archaeopteryx'' to other theropods). If the latter classification is used then the larger group is termed Avialae. Currently, the relationship between dinosaurs, ''Archaeopteryx'', and modern birds is still under debate. ==Origins== (詳細はsignificant evidence that birds emerged within 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 less so. This was noted already in the 19th century, with Thomas Huxley writing: We have had to stretch the definition of the class of birds so as to include birds with teeth and birds with paw-like fore limbs and long tails. There is no evidence that ''Compsognathus'' possessed feathers; but, if it did, it would be hard indeed to say whether it should be called a reptilian bird or an avian reptile.''〔Huxley, T.H. (1876): Lectures on Evolution. ''New York Tribune''. Extra. no 36. In Collected Essays IV: pp 46-138 (original text w/ figures )〕 Discoveries in northeast China (Liaoning Province) demonstrate that many small theropod dinosaurs did indeed have feathers, among them the compsognathid ''Sinosauropteryx'' and the microraptorian dromaeosaurid ''Sinornithosaurus''. This has contributed to this ambiguity of where to draw the line between birds and reptiles.〔Norell, M & Ellison M (2005) ''Unearthing the Dragon, The Great Feathered Dinosaur Discovery'' Pi Press, New York, ISBN 0-13-186266-9〕 ''Cryptovolans'', a dromaeosaurid found in 2002 (which may be a junior synonym of ''Microraptor'') was capable of powered flight, possessed a sternal keel and had ribs with uncinate processes. ''Cryptovolans'' seems to make a better "bird" than ''Archaeopteryx'' which lacks some of these modern bird features. Because of this, some paleontologists have suggested that dromaeosaurs are actually basal birds whose larger members are secondarily flightless, i.e. that dromaeosaurs evolved from birds and not the other way around. Evidence for this theory is currently inconclusive, but digs continue to unearth fossils (especially in China) of feathered dromaeosaurs. At any rate, it is fairly certain that flight utilizing feathered wings existed in the mid-Jurassic theropods. The Cretaceous unenlagiine ''Rahonavis'' also possesses features suggesting it was at least partially capable of powered flight. 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. They thus arrived at their hip structure condition independently. In fact, a 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, notably Larry Martin and Alan Feduccia, states that birds (including maniraptoran "dinosaurs") evolved from early archosaurs like ''Longisquama''. This theory is contested by most other paleontologists and experts in feather development and evolution. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「evolution of birds」の詳細全文を読む スポンサード リンク
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