Afrotheria

Life > Eukaryotes > Opisthokonta > Metazoa (animals) > Bilateria > Deuterostomia > Chordata > Craniata > Vertebrata (vertebrates)  > Gnathostomata (jawed vertebrates) > Teleostomi (teleost fish) > Osteichthyes (bony fish) > Class: Sarcopterygii (lobe-finned fish) > Stegocephalia (terrestrial vertebrates) > Reptiliomorpha > Amniota > Synapsida (mammal-like reptiles) > Therapsida > Theriodontia >  Cynodontia > Mammalia (mammals) > Placentalia (placental mammals)

A group of placental mammals including golden moles (Chrysochloridae), tenrecs (Tenrecidae), elephant-shrews (order Macroscelidea), aardvarks (order Tubulidentata) and a clade containing hyraxes (order Hyracoidea), elephants (order Proboscidea) and the dugongs and manatees (order Sirenia). It is defined on the basis of strong molecular evidence that all these groups originated from a single common ancestor and are more closely related to one another than to any other groups of mammals.

Evolutionary relationships

Afrotheria is a remarkable assemblage of mammalian species that includes forms as morphologically diverse as golden moles (Chrysochloridae), tenrecs (Tenrecidae), elephant-shrews (order Macroscelidea), aardvarks (order Tubulidentata) and a clade containing hyraxes (order Hyracoidea), elephants (order Proboscidea) and the dugongs and manatees (order Sirenia). They are thought to have descended from a common ancestor that gave rise to ecologically divergent adaptive types that evolved in Africa when the continent was isolated through tectonic movement. Unlike other large evolutionary assemblages of mammals, afrotherian recognition has rested exclusively on molecular DNA sequence and other genomic data Ė the lack of fossils, evidence from morphology or soft anatomy, has made it highly unlikely that an afrotherian clade that includes the golden moles and tenrecs would have ever have been recognized without genetic evidence. This makes Afrotheria one of the most remarkable hypotheses in mammalian evolution.

However, although DNA data show unequivocally that paenungulates (that is, elephants, hyraxes, and sea cows), elephant shrews, aardvark, golden moles, and tenrecs are all more closely related to each other than they are to any other extant placental mammal, the relationships among the species are obscure. Morphological analysis of anatomical and fossil evidence favours the association of Sirenia and Proboscidea (Tethytheria) to the exclusion of Hyracoidea. Although results from mitochondrial DNA sequence data favour Tethytheria, there is no consensus support for this clade from nuclear DNA although in many instances Hyracoidea + Proboscidea is suggested in analysis large molecular data sets.

Biogeographic history

The Afrotheria concept underscores the importance of geographic isolation and continental break-up in the early diversification of placental mammals. Molecular clock calibrations suggest their origin and radiation into distinct clades in the mid-Cretaceous (105-90 mya) thus predating the CT boundary and the demise of the dinosaurs. There is good evidence to suggest that Africa was completely isolated between ~105-25 mya and it is during this time that the afrotherians were evolving and adapting to different ecological niches. It is only after this window of isolation that tenuous connections developed between northern Africa and Europe allowing faunal exchange among these continents.

Classification

See under placental mammals.

Links

Text by Professor Terence J. Robinson, Department of Botany & Zoology, Evolutionary Genomics Group, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa


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