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Afrotheria is a clade of mammals, the living members of which belong to groups that are either currently living in Africa or of African origin: golden moles, sengis (also known as elephant shrews), tenrecs, aardvarks, hyraxes, elephants, sea cows, and several extinct clades. They share few anatomical features but many are partly or entirely African in their distribution. This probably reflects the fact that Africa was an island continent through the early Cenozoic. Because the continent was isolated by water, Laurasian groups such as insectivores, rabbits, carnivorans and ungulates could not become established. Instead, the niches occupied by those groups were filled by tenrecs, hyraxes and elephants that evolved from the ancestral afrothere. This adaptive radiation may have occurred in response to the Cretaceous–Paleogene mass extinction. The common ancestry of these animals was not recognized until the late 1990s. Historically, the Paenungulata had been linked to other ungulates; the golden mole, tenrecs, and elephant shrews with the traditional (and polyphyletic) Insectivora; and the aardvarks with the pangolins and the xenarthrans within the invalid taxon Edentata. Continuing work on the molecular and morphological〔〔 diversity of afrotherian mammals has provided ever increasing support for their common ancestry. ==Evolutionary relationships== The afrotherian clade was originally proposed in 1998〔 based on analyses of DNA sequence data. However, previous studies had hinted at the close interrelationships among subsets of endemic African mammals, some of these studies date to the 1920s; there were also sporadic papers in the 1980s and 1990s. The core of the Afrotheria consists of the Paenungulata, i.e., elephants, sea cows, and hyraxes, a group with a long history among comparative anatomists. Hence, while DNA sequence data have proven essential to infer the existence of the Afrotheria as a whole, and while the Afroinsectiphilia (insectivoran-grade afrotheres including tenrecs, golden moles, sengis, and aardvarks) were not recognized as part of Afrotheria without DNA data, some precedent is found in the comparative anatomical literature for the idea that at least part of this group forms a clade. The Paleocene genus ''Ocepeia'', which is the most completely-known Paleocene African mammal and the oldest afrotherian known from a complete skull, shares similarities with both Paenungulata and Afroinsectiphilia, and may help to characterize the ancestral body type of afrotherians. Since the 1990s, increasing molecular and anatomical data have been applied to the classification of animals; both types of data support the idea that afrotherian mammals are descended from a single common ancestor to the exclusion of other mammals. On the anatomical side, features shared by most, if not all, afrotheres include high vertebral counts,〔 aspects of placental membrane formation, the shape of the ankle bones,〔〔 and the relatively late eruption of the permanent dentition. The snout is unusually long and mobile in several Afrotherian species. Studies of genomic data, including millions of aligned nucleotides sampled for a growing number of placental mammals, also support Afrotheria as a clade. Additionally, there might be some dental synapomorphies uniting afroinsectiphilians, if not afrotheres as a whole: p4 talonid and trigonid of similar breadth, a prominent p4 hypoconid, presence of a P4 metacone and absence of parastyles on M1–2. Afrotheria is now recognized as one of the four major groups within the Eutheria (containing placental mammals). Relations within the four cohorts, Afrotheria, Xenarthra, Laurasiatheria, and Euarchontoglires, and the identity of the placental root, remain somewhat controversial. Afrotheria as a clade has usually been discussed without a Linnaean rank, but has been assigned the rank of cohort, magnorder, and superorder. One reconstruction, which applies the molecular clock, proposes that the oldest split occurred between Afrotheria and the other three some 105 million years ago in the mid-Cretaceous, when the African continent was separated from other major land masses. This idea is consistent with the fossil record of Xenarthra, which is restricted to South America (following recent consensus that ''Eurotamandua'' is not a xenarthran). However, Afrotheria itself does not have a fossil record restricted to Africa, although this does seem to be true for the oldest, undisputed afrotherians. Furthermore, the correspondence of Afrotherian origins with the Africa-South America tectonic split is not consistent with other applications of the molecular clock or with the mammalian fossil record. More recent, genomic-scale phylogenies favor the hypothesis that Afrotheria and Xenarthra comprise sister taxa at the base of the placental mammal radiation. Relations between the various afrotherian orders are still being studied. On the basis of molecular studies, elephants and manatees appear to be related, and likewise elephant shrews and aardvarks. These findings are compatible with the work of earlier anatomists.〔〔 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「Afrotheria」の詳細全文を読む スポンサード リンク
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