Afrotheria

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Afrotheria
Temporal range: Paleocene - Recent
Afrotheria.jpg
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 e
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Clade: Atlantogenata
Superorder: Afrotheria
Stanhope et al., 1998
Orders

See below

Afrotheria is a clade of mammals, the living members of which belong to groups currently living in Africa or that are of African origin: golden moles, sengis (also known as elephant shrews), tenrecs, aardvarks, hyraxes, elephants, sea cows, and the extinct Desmostylia. 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, 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.[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] and morphological [4][5][6][7] diversity of afrotherian mammals has provided ever increasing support for their common ancestry.

Evolutionary relationships[edit]

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;[8] there were also sporadic papers in the 1980s[9] and 1990s.[10][11] 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.[12][13] 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.[14]

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,[7] aspects of placental membrane formation,[15] the shape of the ankle bones,[5][6] and the relatively late eruption of the permanent dentition.[16] The snout is unusually long and mobile in several Afrotherian species.[17] Studies of genomic data, including millions of aligned nucleotides sampled for a growing number of placental mammals, also support Afrotheria as a clade.[18][19]

Afrotheria is now recognized as one of the four major groups within the Eutheria (containing placental mammals).[20] Relations within the four cohorts, Afrotheria, Xenarthra, Laurasiatheria, and Euarchontoglires, and the identity of the placental root, remain somewhat controversial.[4]

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.[21] 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[22]).

However, Afrotheria itself does not have a fossil record restricted to Africa,[23] although this does seem to be true for the oldest, undisputed afrotherians.[24] Furthermore, the correspondence of Afrotherian origins with the Africa-South America tectonic split is not consistent with other applications of the molecular clock[25] or with the mammalian fossil record.[26] More recent, genomic-scale phylogenies favor the hypothesis that Afrotheria and Xenarthra comprise sister taxa at the base of the placental mammal radiation.[27]

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.[28] These findings are compatible with the work of earlier anatomists.[12][13]

Current status and distribution[edit]

Many members of Afrotheria appear to have a high risk of extinction. Species loss within this group would therefore comprise a particularly devastating 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.[29]

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.

Organization[edit]

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.[30][31]

See also[edit]

Notes[edit]

  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. 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". Trends in Ecology & Evolution 19 (8): 430–438. doi:10.1016/j.tree.2004.05.006. 
  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. 
  4. ^ 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. 
  5. ^ a b 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. 
  6. ^ a b c 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. doi:10.1186/1471-2148-7-224. PMC 2248600. PMID 17999766. 
  7. ^ 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. doi:10.1017/S1477200006002258. 
  8. ^ 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 London 30: 445–485. 
  9. ^ DeJong W.W., Goodman M. (1981). "Relationship of aardvark to elephants, hyraxes and sea cows from alpha-crystallin sequences". Nature 292 (5823): 538–540. doi:10.1038/292538a0. PMID 7254349. 
  10. ^ DeJong, W.W., J.A.M. Leunissen, and 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, and M.C. McKenna (eds.). Mammal Phylogeny. Springer Verlag. pp. 5–12. 
  11. ^ Springer, M.S.; Cleven G.C.; Madsen, O.; De Jong, W.W.; Waddell, V.G.; Amrine, H.M.; and Stanhope, M.J. (1997). "Endemic African mammals shake the phylogenetic tree". Nature 388: 61–64. 
  12. ^ a b Simpson, G. G. 1945. The principles of classification and a classification of mammals. Bulletin of the American Museum of Natural History 85: 1-350.
  13. ^ a b Tabuce, Rodolphe; Asher, Robert J.; Lehmann, Thomas (2008). "Afrotherian mammals: a review of current data". Mammalia 72 (1): 2–14. doi:10.1515/MAMM.2008.004. 
  14. ^ 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. doi:10.1371/journal.pone.0089739. 
  15. ^ Mess, Andrea; Carter, Anthony M. (2006). "Evolutionary transformations of fetal membrane characters in Eutheria with special reference to Afrotheria". Journal of Experimental Zoology Part B: Molecular and Developmental Evolution 306B (2): 140–163. doi:10.1002/jez.b.21079. 
  16. ^ Asher, Robert J; Lehmann, Thomas (2008). "Dental eruption in afrotherian mammals". BMC Biology 6 (1): 14. doi:10.1186/1741-7007-6-14. 
  17. ^ Christine M. Dengler-Crish et al. "Organization of the somatosensory cortex in elephant shrews (E. edwardii)". Anat. Rec. 288A. pp. 859–866. doi:10.1002/ar.a.20357. 
  18. ^ 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. 
  19. ^ 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. 
  20. ^ Murphy, W. J.; et al. (2001). "Resolution of the Early Placental Mammal Radiation Using Bayesian Phylogenetics". Science 294 (5550): 2348–2351. doi:10.1126/science.1067179. 
  21. ^ 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. doi:10.1073/pnas.0334222100. 
  22. ^ Rose KD, Emry RJ, Gaudin TJ, Storch G. (2005). "Xenarthra and Pholidota.". In Rose KD and Archibald JD (eds.). The Rise of Placental Mammals: Origins and Relationships of the Major Extant Clades. Johns Hopkins University Press. 
  23. ^ 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". Nature 434 (7032): 497–501. Bibcode:2005Natur.434..497Z. doi:10.1038/nature03351. PMID 15791254. 
  24. ^ Robinson T.J., Seiffert E.R. (2004). "Afrotherian origins and interrelationships: new views and future prospects. Curr. Top". Dev. Biol 63: 37–60. doi:10.1016/s0070-2153(04)63002-x. 
  25. ^ Kitazoe, Yasuhiro; Kishino, Hirohisa; Waddell, Peter J.; Nakajima, Noriaki; Okabayashi, Takahisa; Watabe, Teruaki; Okuhara, Yoshiyasu; Hahn, Matthew (2007). "Robust Time Estimation Reconciles Views of the Antiquity of Placental Mammals". PLoS ONE 2 (4): e384. doi:10.1371/journal.pone.0000384. PMC 1849890. PMID 17440620. 
  26. ^ Archibald, J. David; Deutschman, Douglas H. (2001). "Quantitative Analysis of the Timing of the Origin and Diversification of Extant Placental Orders". Journal of Mammalian Evolution 8 (2): 107–124. doi:10.1023/A:1011317930838. 
  27. ^ 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. 
  28. ^ 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. 
  29. ^ "What is Afrotheria?". IUCN Afrotheria Specialist Group. Retrieved 10 December 2013. 
  30. ^ "Afrosoricida". Mammal Species of the World, 3rd edition. Retrieved 17 March 2014. 
  31. ^ McDowell, S. B. 1958. The Greater Antillean insectivores. Bulletin of the American Museum of Natural History 115: 115--213.
  32. ^ Rodolphe Tabuce et al. (2007) Early Tertiary mammals from North Africa reinforce the molecular Afrotheria clade. Proc. R. Soc. B 2007 274, doi: 10.1098/rspb.2006.0229
  33. ^ Horovitz, Inés, Gerhard Storch, and Thomas Martin (2005). "Ankle structure in Eocene pholidotan mammal Eomanis krebsi and its taxonomic implications". Acta Palaeontol. Pol. 50 (3): 545–548. 

References[edit]

External links[edit]