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Afrotheria

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Afrotheria
Temporal range: Paleocene - Recent
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. Tail-less Tenrec
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Subclass: Theria
Infraclass: Eutheria
Superorder: Afrotheria
Stanhope et al., 1998
Orders

See below

Afrotheria is a clade of mammals, the living members of which belong to groups from Africa or of African origin: golden moles, sengis (also known as elephant shrews), tenrecs, aardvarks, hyraxes, elephants and sea cows. 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, the 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.

Contents

Evolutionary relationships

Afrotheria was originally proposed in 1998[1] 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,[8] and much later sporadic papers in the 1980s[9] and 1990s.[10][11] The core of 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 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,[7] features of placentation,[14] shape of the ankle bones,[5][6] and relatively late eruption of the permanent dentition.[15] Studies of genomic data, including millions of aligned nucleotides sampled for a growing number of placental mammals, also support Afrotheria as a clade.[16][17]

Afrotheria is now recognized as one of four major groups within Eutheria (containing placental mammals).[18] Relations within the four cohorts, Afrotheria, Xenarthra, Laurasiatheria, and Euarchontoglires, and the identity of the placental root, remain somewhat controversial.[19] 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.[20] 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[21]). However, Afrotheria itself does not have a fossil record restricted to Africa,[22] although this does seem to be true for the oldest, undisputed afrotherians.[23] Furthermore, the correspondence of Afrotherian origins with the Africa-South America tectonic split is not consistent with other applications of the molecular clock[24] or with the mammalian fossil record.[25] More recent, genomic-scale phylogenies favor the hypothesis that Afrotheria and Xenarthra comprise sister taxa at the base of the placental mammal radiation.[26]

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.

Organization

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.[27] While this recognition does not mean that the Linnean system must be completely replaced,[28] 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 are junior synonyms and arguably should be replaced (e.g., Tenrecoidea McDowell 1958[29] instead of "Afrosoricida" Stanhope et al. 1998).

References

Notes

  1. ^ a b 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
  2. ^ Springer MS, Stanhope MJ, Madsen O, de Jong WW: Molecules consolidate the placental mammal tree. Trends Ecol Evol 2004, 19(8):430-438.
  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- ROYAL SOCIETY OF LONDON B. 271(1547): 1477-1484
  4. ^ Asher RJ, Bennet N, Lehmann T. 2009. The new framework for understanding placental mammal evolution. BioEssays 31:853–864
  5. ^ a b Tabuce, R., Marivaux, L., Adaci,M., Bensalah,M., Hartenberger, J. L., et al. Early tertiary mammals from north Africa reinforce the molecular afrotheria clade. Proc Royal Soc B-Biol Sci 2007. 274: 1159–1166.
  6. ^ a b Seiffert, E., A new estimate of afrotherian phylogeny based on simultaneous analysis of genomic, morphological, and fossil evidence. BMC Evol Biol 2007. 7: 13.
  7. ^ a b Sanchez-Villagra, M. R., Narita, Y. and Kuratani, S., Thoracolumbar vertebral number: the first skeletal synapomorphy for afrotherian mammals. Syst Biodivers 2007. 5: 1–17.
  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., A: Zweers, and M. Goodman. 1981. Relationship of aardvark to elephants, hyraxes and sea cows from alpha-crystallin sequences. Nature 292: 538-540.
  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? pp. 5–12 in F. S. Szalay, M. J. Novacek, and M.C. McKenna (eds.), Mammal Phylogeny. Springer Verlag, New York.
  11. ^ Springer, M.S. and Cleven, G.C. and Madsen, O. and De Jong,W.W. and Waddell, V.G. and Amrine, H.M. and Stanhope, M.J. 1997. Endemic African mammals shake the phylogenetic tree. Nature 388: 61--64.
  12. ^ 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. ^ Tabuce R, Asher RJ, Lehmann T. 2008. Afrotherian mammals: a review of current data. Mammalia 72: 2-14.
  14. ^ Mess, A. and A.M. Carter. 2006. Evolutionary Transformations of Fetal Membrane Characters in Eutheria with Special Reference to Afrotheria. Journal of Experimental Zoology 306B:140–163.
  15. ^ Asher, R. J. and Lehmann, T. 2008. Dental eruption in afrotherian mammals. BMC Biol 6: 14.
  16. ^ Murphy, W. J., Pringle, T. H., Crider, T. A., Springer, M. S. and Miller, W., Using genomic data to unravel the root of the placental mammal phylogeny. Genome Res 2007. 17: 413–421.
  17. ^ Nikolaev, S., Montoya-Burgos, J. I., Margulies, E. H., Program, N. C. S., Rougemont, J., et al. Early history of mammals is elucidated with the ENCODE multiple species sequencing data. PLoS Genet 2007.3: e2.
  18. ^ Murphy, W. J., Eizirik, E., O’Brien, S. J., Madsen, O., Scally, M., et al. Resolution of the early placental mammal radiation using Bayesian phylogenetics. Science 2001. 294: 2348–2351.
  19. ^ Asher RJ, Bennett N, Lehmann T. 2009. The new framework for understanding placental mammal evolution. Bioessays 31(8): 853-864.
  20. ^ Springer MS, Murphy WJ, Eizirik E, O'Brien SJ: Placental mammal diversification and the Cretaceous-Tertiary boundary. Proc Natl Acad Sci USA 2003, 100(3):1056-1061.8.
  21. ^ Rose KD, Emry RJ, Gaudin TJ, Storch G. 2005. Chapter 8, 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, Baltimore, MD.
  22. ^ Zack, S.P., T.A. Penkrot, J.I. Bloch and K.D. Rose. 2005. Affinities of ‘‘hyopsodontids’’ to elephant shrews and a Holarctic origin of Afrotheria. Nature 434: 497–501.
  23. ^ Robinson, T.J. and E.R. Seiffert. 2004. Afrotherian origins and interrelationships: new views and future prospects. Curr. Top. Dev. Biol. 63: 37–60.
  24. ^ Kitazoe Y et al. 2007. PloS ONE 2,e384
  25. ^ Archibald JD, Deutschmann DH. 2001. J. Mamm. Evol. 8:107-124.
  26. ^ Prasad, A. B. Allard, M. W., NISC Comparative Sequencing Program and Green, E. D., Confirming the phylogeny of mammals by use of large comparative sequence data sets. Mol Biol Evol 2008. 25: 1795–1808
  27. ^ Kuntner M, Agnarsson I. 2006. Are the Linnean and Phylogenetic Nomenclatural Systems Combinable? Recommendations for Biological Nomenclature. Systematic Biology 55:774-784
  28. ^ Forey PL. 2002. PhyloCode: Pain, No Gain. Taxon 51:43-54.
  29. ^ McDowell, S. B. 1958. The Greater Antillean insectivores. Bulletin of the American Museum of Natural History 115: 115--213.

External links

Extant mammal orders by infraclass
Australosphenida
Metatheria
(Marsupial inclusive)
Eutheria
(Placental inclusive)
Afrotheria

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