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Afrotheria

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Afrotheria (/æfroʊˈθɪəriə/ from Latin Afro- "of Africa" + theria "wild beast") is a superorder of placental 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), 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 the Cretaceous until the early Miocene around 20 million years ago, when Afro-Arabia collided with Eurasia.

Afrotheria
Temporal range: Paleocene – Holocene, 65–0 Ma
1. Aardvark 2. Dugong 3. Black and rufous elephant shrew 4. West Indian manatee 5. Cape golden mole 6. Rock hyrax 7. African bush elephant 8. Tailless tenrec
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Infraclass: Placentalia
Superorder: Afrotheria
Stanhope MJ, Waddell VG, Madsen O, de Jong W, Hedges SB, Cleven G, Kao D, Springer MS, 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, camels, rhinos, and tapirs. The sirenians 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 as cetaceans and pinnipeds). 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 the true ungulates (particularly the perissodactyls); the golden mole, tenrecs, and elephant shrews with the traditional (and polyphyletic/incorrect) taxon 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

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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] the relatively late eruption of the permanent dentition,[18] and undescended testicles 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 of genomic data, including millions of aligned nucleotides 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 the Eutheria (containing placental 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 the molecular clock, proposes that the oldest split occurred between Afrotheria and the other two some 105 million years ago in the mid-Cretaceous, when the African continent was separated from other major land masses.[25] 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[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 ancient Gondwanan clade of placental mammals.[29] A 2021 morphological study also proposed to render Meridiungulata polyphyletic and recognise most of its clades as part of a group called Sudamericungulata, closely related to hyraxes, while Litopterna remains a sister taxon to Perissodactyla.[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]

Phylogeny

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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 14,509 coding DNA sequences.[32] The other major clades are colored: marsupials (magenta), xenarthrans (orange), laurasiatherians (green), and euarchontoglires (blue).
Afrotheria
A cladogram of Afrotheria based on molecular evidence[15]

Current status and distribution

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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.[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 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,[34] although this is still controversial;[28] the taxonomic placement of embrithopods is also not clear.[35]

Classification

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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.[36][37]

See also

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Notes

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  1. ^ a b Stanhope, M. J.; Waddell, V. G.; Madsen, O.; de Jong, W.; Hedges, S. B.; Cleven, G. C.; Kao, D.; Springer, M. S. (1998). "Molecular evidence for multiple origins of Insectivora and for a new order of endemic African insectivore mammals". Proceedings of the National Academy of Sciences. 95 (17): 9967–9972. Bibcode:1998PNAS...95.9967S. doi:10.1073/pnas.95.17.9967. PMC 21445. PMID 9707584.
  2. ^ Springer, Mark S.; Michael J. Stanhope; Ole Madsen; Wilfried W. de Jong (2004). "Molecules consolidate the placental mammal tree" (PDF). Trends in Ecology & Evolution. 19 (8): 430–438. doi:10.1016/j.tree.2004.05.006. PMID 16701301. S2CID 1508898.
  3. ^ Robinson, T. J.; Fu, B.; Ferguson-Smith, M. A.; Yang, F. (2004). "Cross-species chromosome painting in the golden mole and elephant-shrew: support for the mammalian clades Afrotheria and Afroinsectiphillia but not Afroinsectivora". Proceedings of the Royal Society B: Biological Sciences. 271 (1547): 1477–1484. doi:10.1098/rspb.2004.2754. PMC 1691750. PMID 15306319.
  4. ^ Nishihara, H.; Satta, Y.; Nikaido, M.; Thewissen, J.G.M.; Stanhope, M.J.; Okada, N. (2005). "A retroposon analysis of Afrotherian phylogeny". Molecular Biology and Evolution. 22 (9): 1823–1833. doi:10.1093/molbev/msi179. PMID 15930154.
  5. ^ a b Asher RJ, Bennett N, Lehmann T (2009). "The new framework for understanding placental mammal evolution". BioEssays. 31 (8): 853–864. doi:10.1002/bies.200900053. PMID 19582725.
  6. ^ a b c Tabuce, R.; Marivaux, L.; Adaci, M.; Bensalah, M.; Hartenberger, J.-L.; Mahboubi, M.; Mebrouk, F.; Tafforeau, P.; Jaeger, J.-J. (2007). "Early Tertiary mammals from North Africa reinforce the molecular Afrotheria clade". Proceedings of the Royal Society B: Biological Sciences. 274 (1614): 1159–1166. doi:10.1098/rspb.2006.0229. PMC 2189562. PMID 17329227.
  7. ^ a b c d Seiffert, Erik R (2007). "A new estimate of afrotherian phylogeny based on simultaneous analysis of genomic, morphological, and fossil evidence". BMC Evolutionary Biology. 7 (1): 224. Bibcode:2007BMCEE...7..224S. doi:10.1186/1471-2148-7-224. PMC 2248600. PMID 17999766.
  8. ^ a b Sánchez-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. S2CID 85675984.
  9. ^ Le Gros Clark, W.E. & C.F. Sonntag (1926). "A monograph of Orycteropus afer III, the skull, the skeleton of the trunk, and limbs". Proceedings of the Zoological Society of London. 30: 445–485.
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  16. ^ a b Gheerbrant, Emmanuel; Amaghzaz, Mbarek; Bouya, Baadi; Goussard, Florent; Letenneur, Charlène (2014). "Ocepeia (Middle Paleocene of Morocco): The Oldest Skull of an Afrotherian Mammal". PLOS ONE. 9 (2): e89739. Bibcode:2014PLoSO...989739G. doi:10.1371/journal.pone.0089739. PMC 3935939. PMID 24587000.
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  18. ^ Asher, Robert J; Lehmann, Thomas (2008). "Dental eruption in afrotherian mammals". BMC Biology. 6 (1): 14. doi:10.1186/1741-7007-6-14. PMC 2292681. PMID 18366669.
  19. ^ Sharma, Virag; Lehmann, Thomas; Stuckas, Heiko; Funke, Liane; Hiller, Michael (2018). "Loss of RXFP2 and INSL3 genes in Afrotheria shows that testicular descent is the ancestral condition in placental mammals". PLOS Biology. 16 (6): e2005293. doi:10.1371/journal.pbio.2005293. ISSN 1545-7885. PMC 6023123. PMID 29953435.
  20. ^ Hedges, SB (2001). "Afrotheria: Plate tectonics meets genomics". Proc Natl Acad Sci U S A. 98 (1): 1–2. Bibcode:2001PNAS...98....1H. doi:10.1073/pnas.98.1.1. PMC 33345. PMID 11136239.
  21. ^ Murphy, W. J.; Pringle, T. H.; Crider, T. A.; Springer, M. S.; Miller, W. (2007). "Using genomic data to unravel the root of the placental mammal phylogeny". Genome Research. 17 (4): 413–421. doi:10.1101/gr.5918807. PMC 1832088. PMID 17322288.
  22. ^ Nikolaev, Sergey; Montoya-Burgos, Juan I.; Margulies, Elliott H.; NISC Comparative Sequencing Program; Rougemont, Jacques; Nyffeler, Bruno; Antonarakis, Stylianos E. (2007). "Early History of Mammals Is Elucidated with the ENCODE Multiple Species Sequencing Data". PLOS Genetics. 3 (1): e2. doi:10.1371/journal.pgen.0030002. PMC 1761045. PMID 17206863.
  23. ^ Cote S, Werdelin L, Seiffert ER, Barry JC (March 2007). "Additional material of the enigmatic Early Miocene mammal Kelba and its relationship to the order Ptolemaiida". Proc Natl Acad Sci USA. 104 (13): 5510–5. Bibcode:2007PNAS..104.5510C. doi:10.1073/pnas.0700441104. PMC 1838468. PMID 17372202.
  24. ^ William J. Murphy; Eduardo Eizirik; Mark S. Springer; et al. (14 December 2001). "Resolution of the Early Placental Mammal Radiation Using Bayesian Phylogenetics" (PDF). Science. 294 (5550): 2348–2351. Bibcode:2001Sci...294.2348M. doi:10.1126/science.1067179. PMID 11743200. S2CID 34367609. Archived from the original (PDF) on 27 March 2016. Retrieved 15 May 2014.
  25. ^ Springer, M. S.; Murphy, W. J.; Eizirik, E.; O'Brien, S. J. (2003). "Placental mammal diversification and the Cretaceous-Tertiary boundary". Proceedings of the National Academy of Sciences. 100 (3): 1056–1061. Bibcode:2003PNAS..100.1056S. doi:10.1073/pnas.0334222100. PMC 298725. PMID 12552136.
  26. ^ 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.
  27. ^ Zack S.P.; Penkrot T.A.; Bloch J.I.; Rose K.D. (2005). "Affinities of 'hyopsodontids' to elephant shrews and a Holarctic origin of Afrotheria" (PDF). Nature. 434 (7032): 497–501. Bibcode:2005Natur.434..497Z. doi:10.1038/nature03351. PMID 15791254. S2CID 4428738.
  28. ^ a b Gheerbrant, Emmanuel; Filippo, Andrea; Schmitt, Arnaud (2016). "Convergence of Afrotherian and Laurasiatherian Ungulate-Like Mammals: First Morphological Evidence from the Paleocene of Morocco". PLOS ONE. 11 (7): e0157556. Bibcode:2016PLoSO..1157556G. doi:10.1371/journal.pone.0157556. PMC 4934866. PMID 27384169.
  29. ^ Prasad, A. B.; Allard, M. W.; Green, E. D. (2008). "Confirming the Phylogeny of Mammals by Use of Large Comparative Sequence Data Sets". Molecular Biology and Evolution. 25 (9): 1795–1808. doi:10.1093/molbev/msn104. PMC 2515873. PMID 18453548.
  30. ^ Avilla, Leonardo S.; Mothé, Dimila (2021). "Out of Africa: A New Afrotheria Lineage Rises From Extinct South American Mammals". Frontiers in Ecology and Evolution. 9. doi:10.3389/fevo.2021.654302. ISSN 2296-701X.
  31. ^ Svartman, M.; Stanyon, R. (2012). "The Chromosomes of Afrotheria and Their Bearing on Mammalian Genome Evolution". Cytogenetic and Genome Research. 137 (2–4): 144–153. doi:10.1159/000341387. PMID 22868637. S2CID 24353318.
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  35. ^ Erdal, O.; Antoine, P.-O.; Sen, S.; Smith, A. (2016). "New material of Palaeoamasia kansui (Embrithopoda, Mammalia) from the Eocene of Turkey and a phylogenetic analysis of Embrithopoda at the species level". Palaeontology. 59 (5): 631–655. Bibcode:2016Palgy..59..631E. doi:10.1111/pala.12247. S2CID 89418652.
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  38. ^ A Anthony Ravel; Maeva Orliac (2014). "The inner ear morphology of the 'condylarthran' Hyopsodus lepidus". Historical Biology. 27 (8): 957–969. doi:10.1080/08912963.2014.915823. S2CID 84391276.
  39. ^ a b Avilla, Leonardo S.; Mothé, Dimila (2021). "Out of Africa: A New Afrotheria Lineage Rises from Extinct South American Mammals". Frontiers in Ecology and Evolution. 9. doi:10.3389/fevo.2021.654302.

References

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