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Afrotheria
Life
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> 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 – 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 |
