Afrotheria (/æfroʊˈθɪəriə/ fromLatinAfro- "of Africa" +theria "wild beast") is asuperorder ofplacental mammals, the living members of which belong to groups that are either currently living inAfrica or of African origin:golden moles,elephant shrews (also known as sengis),otter shrews,tenrecs,aardvarks,hyraxes,elephants,sea cows, and several extinct clades. Most groups of afrotheres share little or no superficial resemblance, and their similarities have only become known in recent times because of genetics and molecular studies. Many afrothere groups are found mostly or exclusively in Africa, reflecting the fact that Africa was an island continent from theCretaceous until the earlyMiocene around 20 million years ago, when Afro-Arabia collided with Eurasia.
Because Africa was isolated by water,Laurasian groups of mammals such asinsectivores,rodents,lagomorphs,carnivorans andungulates could not reach Africa for much of the early to mid-Cenozoic. Instead, the niches occupied by those groups on the northern continents were filled by various groups of afrotheres via the process ofconvergent evolution. The small insectivorous afrotheres such as elephant shrews, golden moles, and tenrecs filled the niches ofinsectivores, the hyraxes filled the roles of rodents and lagomorphs, the aardvarks filled the roles of various medium size ant-eating mammals (anteaters, armadillos, pangolins, echidnas, numbats, etc.) found on other continents throughout the Cenozoic, andproboscideans (elephants and their relatives) filled the roles of large herbivores such as hippos, camels, rhinos, and tapirs. Thesirenians developed aquatic body plans and started spreading to other parts of the world by water (evolving convergently with the other groups of marine mammals such ascetaceans andpinnipeds). In addition to their similarity with Laurasian mammals in North America, Europe, and Asia, many afrotheres also exhibit convergent evolution with groups of mammals that evolved and lived exclusively in South America, which was also an island continent for much of the Cenozoic.
The common ancestry of these animals was not recognized until the late 1990s.[1] Historically, thePaenungulata had been linked to the true ungulates (particularly theperissodactyls); the golden mole, tenrecs, and elephant shrews with the traditional (and polyphyletic/incorrect) taxonInsectivora; and the aardvarks with thepangolins and thexenarthrans within the invalid taxonEdentata. Continuing work on the molecular[2][3][4] and morphological[5][6][7][8] diversity of afrotherian mammals has provided ever increasing support for their common ancestry.
The afrotherian clade was originally proposed in 1998[1] based on analyses ofDNA 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[9] and there were sporadic papers in the 1980s[10] and 1990s.[11][12][13] The core of the Afrotheria consists of thePaenungulata, i.e.,elephants,sea cows, andhyraxes, a group with a long history among comparative anatomists.[14][15] Hence, whileDNA sequence data have proven essential to infer the existence of the Afrotheria as a whole, and while theAfroinsectiphilia (insectivoran-grade afrotheres includingtenrecs,golden moles,sengis, andaardvarks) 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 aclade. ThePaleocene genusOcepeia, 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.[16]
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,[8] aspects ofplacental membrane formation,[17] the shape of the ankle bones,[6][7] the relatively late eruption of the permanent dentition,[18] and undescendedtesticles remaining in the body near the kidneys.[19] The snout is unusually long and mobile in several Afrotherian species, and this was pointed out as a possible shared-derived character.[20] Studies ofgenomic data, including millions of alignednucleotides sampled for a growing number of placental mammals, also support Afrotheria as a clade.[21][22] 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.[7][23]
Afrotheria is now recognized as one of the three major groups within theEutheria (containingplacental mammals).[24] Relations within the three cohorts, Afrotheria,Xenarthra,Boreoeutheria, and the identity of the placental root, remain somewhat controversial.[5]
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 themolecular clock, proposes that the oldest split occurred between Afrotheria and the other two some 105 million years ago in themid-Cretaceous, when the African continent was separated from other major land masses.[25] This idea is consistent with the fossil record ofXenarthra, which is restricted to South America (following recent consensus thatEurotamandua is not a xenarthran[26]).
However, Afrotheria itself does not have a fossil record restricted to Africa,[27] and appears in fact to have evolved in the continent's isolation.[28][contradictory][need quotation to verify] More recent, genomic-scale phylogenies favor the hypothesis that Afrotheria and Xenarthra comprise sister taxa at the base of the placental mammal radiation, suggesting an ancientGondwanan clade of placental mammals.[29] A 2021 morphological study also proposed to renderMeridiungulata polyphyletic and recognise most of its clades as part of a group calledSudamericungulata, closely related to hyraxes, whileLitopterna remains a sister taxon toPerissodactyla.[30]
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.[31] These findings are compatible with the work of earlier anatomists.[14][15]
Phylogenetic position of afrotherians (in red) among placentals in a genus-level molecular phylogeny of 116 extant mammals inferred from the gene tree information of 14,509coding DNA sequences.[32] The other major clades are colored:marsupials (magenta),xenarthrans (orange),laurasiatherians (green), andeuarchontoglires (blue).
Many extant members of Afrotheria appear to have a high risk of extinction (perhaps related to the large size of many). Species loss within this already small group would comprise a particularly great loss of genetic and evolutionary diversity. TheIUCN 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 of more than 1,200 mammalian species in those areas.[33]
While most extant species assigned to Afrotheria live in Africa, some (such as the Indian elephant and three of the four sirenian species) occur elsewhere; many of these are also endangered. Prior to theQuaternary extinction event, proboscideans were present on every continent of the world exceptAustralia andAntarctica. Hyraxes lived in much of Eurasia as recently as the end of thePliocene. The extinct afrotherian orders ofembrithopods anddesmostylians were also once widely distributed. However, the desmostylians have recently been viewed as possibleperissodactyls, rather than afrotheres,[34] although this is still controversial;[28] the taxonomic placement of embrithopods is also not clear.[35]
Afrotheria is a clade of placental mammals, the stem designation for which isEutheria. Based on precedent, some clades are junior synonyms and arguably should be replaced.[36][37]
^abSánchez-Villagra, Marcelo R.; Narita, Yuichi; Kuratani, Shigeru (2007). "Thoracolumbar vertebral number: The first skeletal synapomorphy for afrotherian mammals".Systematics and Biodiversity.5 (1):1–7.Bibcode:2007SyBio...5Q...1S.doi:10.1017/S1477200006002258.S2CID85675984.
^Le Gros Clark, W.E. & C.F. Sonntag (1926). "A monograph ofOrycteropus afer III, the skull, the skeleton of the trunk, and limbs".Proceedings of the Zoological Society of London.30:445–485.
^DeJong, W.W.; J.A.M. Leunissen & G.J. Wistow (1993). "Eye lens crystallins and the phylogeny of placental orders: evidence for a Macroscelid–Paenungulate clade?". In F. S. Szalay; M. J. Novacek & M.C. McKenna (eds.).Mammal Phylogeny. New York: Springer Verlag. pp. 5–12.
^abSimpson, G. G. (1945). "The principles of classification and a classification of mammals".Bulletin of the American Museum of Natural History.85:1–350.
^Rose KD, Emry RJ, Gaudin TJ, Storch G (2005). "Xenarthra and Pholidota.". In Rose KD, Archibald JD (eds.).The Rise of Placental Mammals: Origins and Relationships of the Major Extant Clades. Baltimore, MD: Johns Hopkins University Press.
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