Afrotheria

Afrotheria (from Latin Afro- "of Africa" + theria "wild beast") 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, elephant shrews (also known as sengis), 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 the early Cenozoic until around 25 million years ago, when the Tethys Sea shrank.

Afrotheria
Temporal range: Paleocene - Holocene, 65–0 Ma
1. Aardvark 2. Dugong 3. Black and rufous elephant shrew 4. West Indian manatee 5. Golden mole 6. Rock hyrax 7. African bush elephant 8. Tailless tenrec
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Infraclass: Placentalia
Superorder: Afrotheria
Stanhope et al., 1998
Orders

See below

Because Africa was isolated by water, Laurasian groups of mammals such as insectivores, rodents, lagomorphs, carnivorans and ungulates 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 of convergent evolution. The small insectivorous afrotheres such as elephant shrews, golden moles, and tenrecs filled the niches of insectivores, 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, and proboscideans (elephants and their relatives) filled the roles of large herbivores such as hippos and rhinos. The sirenians quickly became aquatic and started spreading to other parts of the world by water, evolving convergently with the other groups of marine mammals. 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, 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[2][3][4] and morphological[5][6][7][8] diversity of afrotherian mammals has provided ever increasing support for their common ancestry.

Evolutionary relationships

The afrotherian clade was originally proposed in 1998[1] 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[9] and there were sporadic papers in the 1980s[10] and 1990s.[11][12][13] 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.[14][15] 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.[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 of placental membrane formation,[17] the shape of the ankle bones,[6][7] and the relatively late eruption of the permanent dentition.[18] The snout is unusually long and mobile in several Afrotherian species.[19] Studies of genomic data, including millions of aligned nucleotides sampled for a growing number of placental mammals, also support Afrotheria as a clade.[20][21] 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][22]

Afrotheria is now recognized as one of the three major groups within the Eutheria (containing placental mammals).[23] 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 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.[24] 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[25]).

However, Afrotheria itself does not have a fossil record restricted to Africa,[26] and appears in fact to have evolved in the continent's isolation.[27] 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 ancient Gondwanan clade of placental mammals.[28]

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.[29] These findings are compatible with the work of earlier anatomists.[14][15]

Phylogeny
Afrotheria
Afroinsectiphilia
Tubulidentata

Orycteropodidae

 
Afroinsectivora
Macroscelidea

Macroscelididae

 
Afrosoricida
 

Chrysochloridae

 
Tenrecomorpha
 

Potamogalidae

 
 

Tenrecidae

 
 
 
 
 
Paenungulata
Hyracoidea

Procaviidae

 
Tethytheria
Proboscidea

Elephantidae

 
Sirenia
 

Dugongidae

 
 

Trichechidae

 
 
 
 
 

Current status and distribution

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. The IUCN 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.[30]

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 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. However, the desmostylians have recently been viewed as possible perissodactyls, rather than afrotheres,[31] although this is still controversial;[27] the taxonomic placement of embrithopods is also not clear.[32]

Classification

Afrotheria is a clade of placental mammals, the stem designation for which is Eutheria. Based on precedent, some clades are junior synonyms and arguably should be replaced.[33][34]

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See also

Notes
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References
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