Afrotheria

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Afrotheria is a clade of mammals with the rank of cohort or superorder, that has been proposed based on DNA analysis. Genetic analyses since the 1990s have identified Afrotheria as one of four major groups within the infraclass Eutheria (containing placental mammals). Exact relations among the four cohorts, Afrotheria, Xenarthra, Laurasiatheria, and Euarchontoglires (aka Supraprimates) remain somewhat controversial. One reconstruction proposes that the oldest split was between Afrotheria and the other three some 105 million years ago when the African continent was separate from other major landmasses. (The name Afrotheria was coined from two roots, Afro- for 'Africa' and -theria meaning 'animal' in Greek.) Genetic analysis and the fossil record suggests that Xenarthra developed in South America and diverged from the remaining two somewhat later. Laurasiatheria and Euarchontoglires are more closely related than the other two cohorts and may be grouped together within the taxon Boreoeutheria.

Some researchers consider these classifications based on recent comparative DNA analysis to be preliminary or controversial, as they often cut across previous groupings of mammalian relationships that were based on morphological considerations. For example, the order Insectivora, which comprised many genera and species of mostly small, insect-eating mammals, now appear to be only distantly related, and apparently share similar anatomy and behaviors primarily as the result of convergent evolution. [1] As another example, distinctive morphological features of the Xenarthra (which includes anteaters, sloths, and armadillos) previously led taxonomists group all other Eutherian mammals into the taxon Epitheria, with Xenarthra as the most distantly related grouping. Yet another reconstruction would place Xenarthra and Afrotheria together within the clade Atlantogenata as a sister clade to the Boreoeutheria.

Many members of Afrotheria appear to be at high risk of extinction; if the grouping is accurate, this would be 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 are found in Africa, some (such as the Indian elephant and the three species of manatee) are found elsewhere; many of these are endangered as well.

Contents

[edit] Organization

Afrotheria is a division of the infraclass Eutheria or Placentalia and groups together six living orders of mammals:

[edit] Classification Problems

Afrotheria are believed to have originated in Africa at a time when the continent was isolated from other continents. Their only externally visible common characteristic is the movable snout, although there is no convincing evidence that this structure is in fact homologous across all members of this group.

The biggest problem with considering Afrotherians as an originally African clade is the fossil record. The earliest fossil evidence for African ungulates and elephant shrews are found outside Africa. The Afrotheres are part of the proposed clade Atlantogenata.

Afrotherian monophyly is not universally accepted, and morphological evidence places the elephants and their relatives as true ungulates. This may also be the case for the aardvarks and the elephant shrews, although not the tenrecs and golden moles, and the elephant shrews may be related to gnawing mammals (within Glires). A mammal known from Madagascar (Plesiorycteropus) is of unknown affinities but may also be an ungulate perhaps related to the mainland aardvark. Some morphological evidence does support the affinity of the tenrecs and golden moles to other Lipotyphlan insectivores, especially to Solenodon in the Caribbean region. This is a more traditional interpretaton of Tenrecomorph relationships.

[edit] References

  • William J. Murphy, Eduardo Eizirik, Mark S. Springer et al. (14 December 2001). "Resolution of the Early Placental Mammal Radiation Using Bayesian Phylogenetics". Science 294 (5550): 2348-2351. DOI:10.1126/science.1067179. 
  • Kriegs, Jan Ole, Gennady Churakov, Martin Kiefmann, Ursula Jordan, Juergen Brosius, Juergen Schmitz (2006). "Retroposed Elements as Archives for the Evolutionary History of Placental Mammals". PLoS Biol 4 (4): e91. DOI:10.1371/journal.pbio.0040091.  (pdf version)

[edit] Notes

  1. ^ See the Kriegs et al. (2001) paper for discussion of a number of possible reconstructions of cohorts and other clades based on different forms of genetic and morphological evidence. They believe that the Retrotransposon presence/absence data produces fewer errors than other existing methods of taxonomic reconstruction.

[edit] External links

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