Dinosaurs as ancestors of birds, and birds as descendants of dinosaurs

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  • 鳥類の起源としての恐竜と，恐竜の子孫としての鳥類
  • チョウルイ ノ キゲン ト シテ ノ キョウリュウ ト 、 キョウリュウ ノ シソン ト シテ ノ チョウルイ

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Abstract

The origin of birds has been a fundamental and challenging subject in ornithology since the discovery of Archaeopteryx. Although the phylogenetic relationship between birds and crocodiles has been recognized since the beginning of discussion, researchers were unable to reached a consensus as to which was the ancestor of modern birds. Pterosauria, Crocodilia, Thecodont, Ornithischia, and Theropoda have all been put forward as the ancestors of birds. Recent paleontological studies have clarified that birds are derived from a clade of Maniraptora in the clade Coelurosauria among the Theropod dinosaurs. Fossil evidence has demonstrated that birds share several features with Coelurosauria dinosaurs, such as bipedal locomotion, fork-shaped furcula, air sacs, and laterally flexing wrists. Embryological and molecular biological evidence correspond with this view. Today, birds are phylogenetically defined as avian dinosaurs, and other dinosaurs are defined as non-avian dinosaurs. This implies that dinosaurs did not become extinct at the end of the Cretaceous period, 66 million years ago. Feathered dinosaurs represent one of the most remarkable topics of recent paleontology. Various dinosaurs with contour feathers have been found among the Coelurosauria, while simple filament-like protofeathers have been detected in an even wider range of taxa, including the Ornithischia. Furthermore, flightless Ornithomimus edmontonicus has been shown to have possessed pennaceous wings. These findings imply that non-avian dinosaurs evolved feathers and wings for reasons unrelated to flight, perhaps for display. While toothless beaks, the pygostyle, shortened tails, and keeled sternums are considered to have evolved for flight, bipedalism, air sacs, and furcula are considered to be cases of exaptation. The fields of ornithology and paleontology may contribute to each other based on the close relationships between birds and dinosaurs. Collaboration between these two fields is essential for their future development.

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