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Globidonta is a clade of alligatoroids that includes alligators, caimans, and closely related extinct forms. It is defined as a stem-based clade including ''Alligator mississippiensis'' (the American Alligator) and all forms more closely related to it than to ''Diplocynodon''. The group's fossil range extends back into the Late Cretaceous with early alligatoroids such as ''Albertochampsa'' and ''Brachychampsa''. Extinct globidontans were particularly common in North America and Eurasia, and their modern range also includes South America. The oldest known globidontan is ''Acynodon'' from France, which is also the most basal. Basal globidontans are characterized by their blunt snouts and bulbous teeth. Modern globidontans have flattened snouts and more conical teeth, and are seen as more generalized than earlier globidontans. Generalized forms are usually expected to be ancestral to more specialized forms rather than descendants of them, so it is unusual for basal members of the group to appear specialized. This seems to conflict with the "Law of the Unspecialized" first proposed by Edward Drinker Cope in 1894. Under the Law of the Unspecialized, morphological change is always directed toward specialization, and specialized forms can never become "unspecialized" again. This pattern of change, while not seen in globidontans, can be observed in basal members of Alligatoroidea and Crocodyloidea. Flat-snouted globidontans occurred two times in the evolution of the clade: once in caimans and once in alligators. ''Alligator sinensis'', the Chinese Alligator, has a snout that is somewhat blunt and could be considered specialized. However, its snout is not nearly as blunt as those of more basal globidontans such as ''Albertachampsa''.〔 If the last common ancestor of ''Diplocynodon'' and globidontans was more like ''Diplocynodon'', it would have had a generalized snout shape. It is also possible that the generalized form of ''Diplocynodon'' may also have arisen from a specialized blunt-snouted ancestor. ''Baryphracta'', a diplocynodontine closely related to ''Diplocynondon'', has a blunt snout and may have been similar in appearance to such an ancestor.〔 ==Phylogenetics== Below is a cladogram modified from Martin (2007):〔 |label2= Globidonta |2= |2= }} |2= }} }} |label2= Caimaninae |2= }} |2= }} |2= }} }} }} }} }} }} 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「globidonta」の詳細全文を読む スポンサード リンク
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