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Palaeognathae, or paleognaths, is one of the two living clades of birds – the other being Neognathae. Together, these two clades form the clade Neornithes. Palaeognathae contains five extant branches of flightless lineages (plus two extinct clades), termed ratites, and one flying lineage, the Neotropic tinamous.〔 *Wetmore, A. (1960)〕〔 There are 47 species of tinamous, 5 of kiwis (''Apteryx''), 3 of cassowaries (''Casuarius''), 1 of emus (''Dromaius'') (another became extinct in historic times), 2 of rheas and 2 of ostrich.〔Clements, J. C. ''et al.'' (2010)〕 Recent research has indicated that paleognaths are monophyletic but the traditional taxonomic split between flightless and flighted forms is incorrect; tinamous are within the ratite radiation, meaning flightlessness arose independently multiple times via parallel evolution.〔 There are three extinct groups, the Lithornithiformes, the Dinornithiformes (moas) and the Aepyornithiformes (elephant birds), that are undisputed members of Palaeognathae. There are other extinct birds which have been allied with the Palaeognathae by at least one author, but their affinities are a matter of dispute. The word ''Paleognath'' is derived from the ancient Greek for "old jaws" in reference to the skeletal anatomy of the palate, which is described as more primitive and reptilian than that in other birds.〔Houde, P. T. (1988)〕 Paleognathous birds are uncontroversially the most primitive, or basal, living birds, though there is some controversy about the precise relationship between them and the other birds. There are also several other scientific controversies about their evolution (see below).〔Leonard, L. ''et al.'' (2005)〕 == Evolution == No unambiguously paleognathous fossil birds are known until the Cenozoic (though birds occasionally interpreted as lithornithids occur in Maastrichtian appalachian sites〔(Palaeogene Fossil Birds )〕〔(A lithornithid (Aves: Palaeognathae) from the Paleocene (Tiffanian) of southern California )〕 ), but there have been many reports of putative paleognathes, and it has long been inferred that they may have evolved in the Cretaceous. One study of molecular and paleontological data found that modern bird orders, including the paleognathous ones, began diverging from one another in the Early Cretaceous.〔Cooper, Alan & Penny, David (1997)〕 Benton (2005) summarized this and other molecular studies as implying that paleognaths should have arisen 110 to 120 million years ago in the Early Cretaceous. He points out, however, that there is no fossil record until 70 million years ago, leaving a 45 million year gap. He asks whether the paleognath fossils will be found one day, or whether the estimated rates of molecular evolution are too slow, and that bird evolution actually accelerated during an adaptive radiation after the Cretaceous–Paleogene boundary (K–Pg boundary).〔Benton, Michael J. (2005)〕 Other authors questioned the monophyly of the Palaeognathae on various grounds, suggesting that they could be a hodgepodge of unrelated birds that have come to be grouped together because they are coincidentally flightless. Unrelated birds might have developed ratite-like anatomies multiple times around the world through convergent evolution. McDowell (1948) asserted that the similarities in the palate anatomy of paleognathes might actually be neoteny, or retained embryonic features. He noted that there were other feature of the skull, such as the retention of sutures into adulthood, that were like those of juvenile birds. Thus, perhaps the characteristic palate was actually a frozen stage that many carinate bird embryos passed through during development. The retention of early developmental stages, then, may have been a mechanism by which various birds became flightless and came to look similar to one another.〔McDowell, Sam (1948)〕 Hope (2002) reviewed all known bird fossils from the Mesozoic looking for evidence of the origin of the evolutionary radiation of the Neornithes. That radiation would also signal that the paleognaths had already diverged. She notes five Early Cretaceous taxa that have been assigned to the Palaeognathae. She finds that none of them can be clearly assigned as such. However, she does find evidence that the Neognathae and, therefore, also the Palaeognathae had diverged no later than the Early Campanian age of the Cretaceous period.〔Hope, Sylvia (2002)〕 ''Vegavis'' is a fossil bird from the Maastrichtian period of Late Cretaceous Antarctica. ''Vegavis'' is most closely related to true ducks. Because virtually all phylogenetic analyses predict that ducks diverged after paleognathes, this is evidence that paleognathes had already arisen well before that time.〔Clarke, J. A. ''et al.'' (2005)〕 An exceptionally preserved specimen of the extinct flying paleognathe ''Lithornis'' was published by Leonard et al. in 2005. It is an articulated and nearly complete fossil from the early Eocene of Denmark, and thought to have the best preserved lithornithiform skull ever found. The authors concluded that ''Lithornis'' was a close sister taxon to tinamous, rather than ostriches, and that the lithorniforms + tinamous were the most basal paleognaths. They concluded that all ratites, therefore, were monophyletic, descending from one common ancestor that became flightless. They also interpret the paleognath-like ''Limenavis'', from Late Cretaceous Patagonia, as possible evidence of a Cretaceous and monophyletic origin for paleognathes.〔 An ambitious genomic analysis of the living birds was performed in 2007, and it contradicted Leonard et al. (2005). It found that tinamous are not primitive within the paleognathes, but among the most advanced. This requires multiple events of flightlessness within the paleognathes and partially refutes the Gondwana vicariance hypothesis (see below). The study looked at DNA sequences from 19 loci in 169 species. It recovered evidence that the paleognathes are one natural group (monophyletic), and that their divergence from other birds is the oldest divergence of any extant bird groups. It also placed the tinamous within the ratites, more derived than ostriches, or rheas and as a sister group to emus and kiwis, and this makes ratites paraphyletic.〔Hackett, S. J. ''et al.'' (2008)〕 A related study addressed the issue of paleognath phylogeny exclusively. It used molecular analysis and looked at twenty unlinked nuclear genes. This study concluded that there were at least three events of flightlessness that produced the different ratite orders, that the similarities between the ratite orders are partly due to convergent evolution, and that the Palaeognathae are monophyletic, but the ratites are not.〔Harshman, J. ''et al.'' (2008)〕 Beginning in 2010, DNA analysis studies have shown that tinamous are the sister group to extinct moa of New Zealand. 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「Palaeognathae」の詳細全文を読む スポンサード リンク
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