Afrotheria is a clade of mammals, the living members of which include golden moles, sengis (also known as elephant shrews), tenrecs, aardvarks, hyraxes, elephants and sea cows.
Afrotheria was originally proposed in 1998 based on analyses of DNA sequence data. However, previous studies hinting at the close interrelationships among subsets of endemic African mammals date to the 1920s, and much later sporadic papers in the 1980s and 1990s. The core of 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 Afrotheria as a whole, and while the insectivoran-grade afrotheres (tenrecs, golden moles, sengis) would probably not have been recognized as part of Afrotheria without DNA data, there is some precedent in the comparative anatomical literature for the idea that at least part of this group forms a clade.
Since the 1990s, increasing amounts of molecular and anatomical data have been applied that 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 or all afrotherians include a high vertebral count, features of placentation, shape of the ankle bones, and relatively late eruption of the permanent dentition. Studies of genomic data, including millions of aligned nucleotides sampled for a growing number of placental mammals, also support Afrotheria as a clade.
Afrotheria is now recognized as one of four major groups within 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 is a clade usually discussed without a Linnaean rank, but has been assigned the rank of cohort or magnorder, and superorder. One reconstruction that applies the molecular clock proposes that the oldest split occurred between Afrotheria and the other three some 105 million years ago 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.
Current status and distribution
Many members of Afrotheria appear to be at high risk of extinction. Species loss within this group would therefore comprise a particularly devastating loss of genetic and evolutionary diversity. The Afrotheria Specialist Group notes that Afrotheria as currently reconstructed includes nearly a third of all mammalian orders currently found in Africa and Madagascar, but only 75 out of more than 1200 mammalian species in those areas.
While most extant species assigned to the cohort Afrotheria live in Africa, some (such as the Indian elephant and three out of four species of sirenian) occur elsewhere; many of these are endangered as well. Prior to the Quaternary extinction event, proboscideans were present on every continent of the world except Australia and Antarctica. Hyraxes lived in much of Eurasia as recently as the end of the Pliocene; the extinct afrotherian orders of embrithopods and desmostylians were also once widely distributed.
Afrotheria is a clade of placental mammals, the stem designation for which is Eutheria. Note that ranks in Linnean taxonomy are arbitrary and without intrinsic biological meaning. While this recognition does not mean that the Linnean system must be completely replaced, it does have implications for high-level taxon names that, traditionally, are associated with certain suffixes (e.g., "oidea" need not be reserved for superfamily). Based on precedent, some clades listed below are junior synonyms and arguably should be replaced (e.g., Tenrecoidea McDowell 1958 instead of "Afrosoricida" Stanhope et al. 1998).
* Kriegs, Jan Ole, Gennady Churakov, Martin Kiefmann, Ursula Jordan, Juergen Brosius, Juergen Schmitz (2006). "Retroposed Elements as Archives for the Evolutionary History of Placental Mammals". PLoS Biol 4 (4): e91. doi:10.1371/journal.pbio.0040091. (pdf version)
1. ^ Stanhope, M.J. et al. (1998) Molecular evidence for multiple origins of Insectivora and for a new order of endemic African insectivore mammals. Proc. Natl. Acad. Sci. U. S. A. 95, 9967–9972
Source: Wikispecies, Wikipedia: All text is available under the terms of the GNU Free Documentation License