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''Ichthyornis'' (meaning "fish bird", after its fish-like vertebrae) was a genus of toothed seabird-like avialans from the late Cretaceous period of North America. Its fossil remains are known from the chalks of Alberta, Alabama, Kansas, New Mexico, Saskatchewan, and Texas, in strata that were laid down in the Western Interior Seaway during the Turonian-Campanian ages, about 95-85 mya. ''Ichthyornis'' is a common component of the Niobrara Formation fauna, and numerous specimens of both adult and immature individuals have been found.〔 ''Ichthyornis'' has been historically important in shedding light on bird evolution. It was the first known prehistoric bird relative preserved with teeth, and Charles Darwin noted its significance during the early years of the theory of evolution. ''Ichthyornis'' remains important today as it is one of the few Mesozoic era ornithurans known from more than a few specimens.〔 ==Description== It is thought that ''Ichthyornis'' was the Cretaceous ecological equivalent of modern seabirds such as gulls, petrels, and skimmers. An average specimen was the size of a pigeon, long, with a skeletal wingspan (not taking feathers into account) of around ,〔 though there is considerable size variation among known specimens, with some smaller and some much larger than the type specimen of ''I. dispar''.〔 ''Ichthyornis'' is notable primarily for its combination of vertebrae which are concave both in front and back (similar to some fish, which is where it gets its name) and several more subtle features of its skeleton which set it apart from its close relatives. ''Ichthyornis'' is perhaps most well known for its teeth. The teeth were present only in the middle portion of the upper and lower jaws. The jaw tips had no teeth and were covered in a beak. The beak of ''Ichthyornis'', like the hesperornithids, was compound and made up of several distinct plates, similar to the beak of an albatross, rather than a single sheet of keratin as in most modern birds.〔 The teeth were more flattened than the rounded teeth found in crocodilians, though they became wider towards the base of the crown. The tips of the teeth were curved backward and lacked any serrations.〔 Although the wings and breastbone were very modern in appearance (suggesting strong flight ability and placing it with modern birds in the advanced group Carinatae), the jaws had numerous small, sharp teeth. Unlike earlier avialans such as the enantiornithines, it appears to have matured to adulthood in a rather short, continuous process.〔 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「ichthyornis」の詳細全文を読む スポンサード リンク
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