Afrotheria is a clade of mammals, the living members of which belong to groups that are
either currently living in Africa or of African origin: golden moles, sengis, 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. Historically,
the Paenungulata had been linked to other ungulates; the golden mole, tenrecs, and elephant
shrews with the traditional Insectivora; and the aardvarks with the pangolins and the xenarthrans
within the invalid taxon Edentata. Continuing work on the molecular and morphological diversity
of afrotherian mammals has provided ever increasing support for their common ancestry.
Evolutionary relationships The afrotherian clade was originally proposed
in 1998 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; there were also sporadic papers in the 1980s and 1990s. 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. Hence, while DNA sequence data
have proven essential to infer the existence of the Afrotheria as a whole, and while the
Afroinsectiphilia 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. 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, aspects of placental membrane formation, the shape of
the ankle bones, and the relatively late eruption of the permanent dentition. The snout is unusually
long and mobile in several Afrotherian species. Studies of genomic data, including millions
of aligned nucleotides sampled for a growing number of placental mammals, also support
Afrotheria as a clade. Afrotheria is now recognized as one of the
four major groups within the Eutheria. Relations within the four cohorts, Afrotheria, Xenarthra,
Laurasiatheria, and Euarchontoglires, and the identity of the placental root, remain
somewhat controversial. 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. This idea is consistent
with the fossil record of Xenarthra, which is restricted to South America.
However, Afrotheria itself does not have a fossil record restricted to Africa, although
this does seem to be true for the oldest, undisputed afrotherians. Furthermore, the
correspondence of Afrotherian origins with the Africa-South America tectonic split is
not consistent with other applications of the molecular clock or with the mammalian
fossil record. More recent, genomic-scale phylogenies favor the hypothesis that Afrotheria
and Xenarthra comprise sister taxa at the base of the placental mammal radiation.
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. These findings are compatible with the work of earlier anatomists.
Current status and distribution 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. While most extant species assigned to Afrotheria
live in Africa, some 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
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.
Afrotheria †Ocepeia:
†Hyopsodontidae: Clade Afroinsectiphilia
Order Macroscelidea: elephant shrews Order Afrosoricida: tenrecs and golden moles
Order Tubulidentata: aardvark Order †Bibymalagasia:
Order †Ptolemaiida: poorly understood carnivorous mammals.
Order †Afredentata:
Clade Paenungulata Order Hyracoidea: hyraxes or dassies
Order Proboscidea: elephants Order Sirenia: dugong and manatees
Order †Desmostylia Order †Embrithopoda
See also Evolution of mammals
Fauna of Africa Notes
References Kriegs, Jan Ole, Gennady Churakov, Martin
Kiefmann, Ursula Jordan, Juergen Brosius, Juergen Schmitz. "Retroposed Elements as Archives
for the Evolutionary History of Placental Mammals". PLoS Biol 4: e91. doi:10.1371/journal.pbio.0040091.
PMC 1395351. PMID 16515367. William J. Murphy, Eduardo Eizirik, Mark S.
Springer et al.. "Resolution of the Early Placental Mammal Radiation Using Bayesian
Phylogenetics". Science 294: 2348–2351. doi:10.1126/science.1067179. PMID 11743200.
Seiffert, Erik; Guillon, JM. "A new estimate of afrotherian phylogeny based on simultaneous
analysis of genomic, morphological, and fossil evidence". BMC Evolutionary Biology 7: 13.
doi:10.1186/1471-2148-7-224. PMC 2248600. PMID 17999766.
External links IUCN species survival commission: Afrotheria
specialist group. Information on the members of Afrotheria, with pictures.
Evolution of the mammalian placenta revealed by phylogenetic analysis