Afrotheria
Encyclopedia
Afrotheria is a clade
of mammal
s, the living members of which belong to groups from Africa
or of African origin: golden mole
s, sengis (also known as elephant shrew
s), tenrecs, aardvark
s, hyrax
es, elephant
s and sea cows. 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 (and paraphyletic) Insectivora
; and the aardvarks with the pangolin
s and the xenarthrans within the invalid taxon, the Edentata. Continuing work on the molecular and morphological diversity of afrotherian mammals has provided ever increasing support for their common ancestry.
sequence data. However, previous studies hinting at the close interrelationships among subsets of endemic African mammals date to the 1920s, and much later sporadic papers in the 1980s and 1990s. The core of Afrotheria consists of the Paenungulata
, i.e., elephant
s, sea cows, and hyrax
es, a group with a long history among comparative anatomists. Hence, while DNA
sequence data have proven essential to infer the existence of Afrotheria as a whole, and while the insectivora
n-grade afrotheres (tenrecs, golden mole
s, 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, features of placentation, shape of the ankle bones, and relatively late eruption of the permanent dentition.
Studies of genomic
data, including millions of aligned nucleotide
s sampled for a growing number of placental mammals, also support Afrotheria as a clade
.
Afrotheria is now recognized as one of four major groups within Eutheria
(containing placental mammals). Relations within the four cohorts, Afrotheria, Xenarthra
, Laurasiatheria
, and Euarchontoglires
, and the identity of the placental root, remain somewhat controversial. 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. 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). 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.
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.
of placental mammals, the stem designation for which is Eutheria
. Note that ranks in Linnean taxonomy are arbitrary and without intrinsic biological meaning. While this recognition does not mean that the Linnean system must be completely replaced, 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 listed below are junior synonyms and arguably should be replaced (e.g., Tenrecoidea McDowell 1958 instead of "Afrosoricida" Stanhope et al. 1998).
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
of mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s, the living members of which belong to groups from Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
or of African origin: golden mole
Golden mole
Golden moles are small, insectivorous burrowing mammals native to southern Africa. They form the family Chrysochloridae, and are taxonomically distinct from the true moles which they resemble due to convergence...
s, sengis (also known as elephant shrew
Elephant shrew
Elephant shrews or jumping shrews are small insectivorous mammals native to Africa, belonging to the family Macroscelididae, in the order Macroscelidea...
s), tenrecs, aardvark
Aardvark
The aardvark is a medium-sized, burrowing, nocturnal mammal native to Africa...
s, hyrax
Hyrax
A hyrax is any of four species of fairly small, thickset, herbivorous mammals in the order Hyracoidea. The rock hyrax Procavia capensis, the yellow-spotted rock hyrax Heterohyrax brucei, the western tree hyrax Dendrohyrax dorsalis, and the southern tree hyrax, Dendrohyrax arboreus live in Africa...
es, elephant
Elephant
Elephants are large land mammals in two extant genera of the family Elephantidae: Elephas and Loxodonta, with the third genus Mammuthus extinct...
s and sea cows. The common ancestry of these animals was not recognized until the late 1990s. Historically, the Paenungulata
Paenungulata
Paenungulata is a taxon that groups some remarkable mammals, including three orders that are extant: Proboscidea , Sirenia , and Hyracoidea . At least two more orders are known only as fossils, namely Embrithopoda and Desmostylia...
had been linked to other ungulates; the golden mole, tenrecs, and elephant shrews with the traditional (and paraphyletic) Insectivora
Insectivora
The order Insectivora is a now-abandoned biological grouping within the class of mammals...
; and the aardvarks with the pangolin
Pangolin
A pangolin , also scaly anteater or Trenggiling, is a mammal of the order Pholidota. There is only one extant family and one genus of pangolins, comprising eight species. There are also a number of extinct taxa. Pangolins have large keratin scales covering their skin and are the only mammals with...
s and the xenarthrans within the invalid taxon, the Edentata. Continuing work on the molecular and morphological diversity of afrotherian mammals has provided ever increasing support for their common ancestry.
Evolutionary relationships
Afrotheria was originally proposed in 1998 based on analyses of DNADNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
sequence data. However, previous studies hinting at the close interrelationships among subsets of endemic African mammals date to the 1920s, and much later sporadic papers in the 1980s and 1990s. The core of Afrotheria consists of the Paenungulata
Paenungulata
Paenungulata is a taxon that groups some remarkable mammals, including three orders that are extant: Proboscidea , Sirenia , and Hyracoidea . At least two more orders are known only as fossils, namely Embrithopoda and Desmostylia...
, i.e., elephant
Elephant
Elephants are large land mammals in two extant genera of the family Elephantidae: Elephas and Loxodonta, with the third genus Mammuthus extinct...
s, sea cows, and hyrax
Hyrax
A hyrax is any of four species of fairly small, thickset, herbivorous mammals in the order Hyracoidea. The rock hyrax Procavia capensis, the yellow-spotted rock hyrax Heterohyrax brucei, the western tree hyrax Dendrohyrax dorsalis, and the southern tree hyrax, Dendrohyrax arboreus live in Africa...
es, a group with a long history among comparative anatomists. Hence, while DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
sequence data have proven essential to infer the existence of Afrotheria as a whole, and while the insectivora
Insectivora
The order Insectivora is a now-abandoned biological grouping within the class of mammals...
n-grade afrotheres (tenrecs, golden mole
Golden mole
Golden moles are small, insectivorous burrowing mammals native to southern Africa. They form the family Chrysochloridae, and are taxonomically distinct from the true moles which they resemble due to convergence...
s, 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
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
.
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, features of placentation, shape of the ankle bones, and relatively late eruption of the permanent dentition.
Studies of genomic
Genomics
Genomics is a discipline in genetics concerning the study of the genomes of organisms. The field includes intensive efforts to determine the entire DNA sequence of organisms and fine-scale genetic mapping efforts. The field also includes studies of intragenomic phenomena such as heterosis,...
data, including millions of aligned nucleotide
Nucleotide
Nucleotides are molecules that, when joined together, make up the structural units of RNA and DNA. In addition, nucleotides participate in cellular signaling , and are incorporated into important cofactors of enzymatic reactions...
s sampled for a growing number of placental mammals, also support Afrotheria as a clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
.
Afrotheria is now recognized as one of four major groups within Eutheria
Eutheria
Eutheria is a group of mammals consisting of placental mammals plus all extinct mammals that are more closely related to living placentals than to living marsupials . They are distinguished from noneutherians by various features of the feet, ankles, jaws and teeth...
(containing placental mammals). Relations within the four cohorts, Afrotheria, Xenarthra
Xenarthra
The superorder Xenarthra is a group of placental mammals , existent today only in the Americas and represented by anteaters, tree sloths, and armadillos. The origins of the order can be traced back as far as the Paleogene in South America...
, Laurasiatheria
Laurasiatheria
Laurasiatheria is a large group of placental mammals believed to have originated on the northern supercontinent of Laurasia. It includes shrews, hedgehogs, pangolins, bats, whales, most hoofed mammals, and carnivorans, among others....
, and Euarchontoglires
Euarchontoglires
Euarchontoglires is a clade of mammals, the living members of which are rodents, lagomorphs, treeshrews, colugos and primates .-Evolutionary relationships:...
, and the identity of the placental root, remain somewhat controversial. 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
Molecular clock
The molecular clock is a technique in molecular evolution that uses fossil constraints and rates of molecular change to deduce the time in geologic history when two species or other taxa diverged. It is used to estimate the time of occurrence of events called speciation or radiation...
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. This idea is consistent with the fossil record of Xenarthra
Xenarthra
The superorder Xenarthra is a group of placental mammals , existent today only in the Americas and represented by anteaters, tree sloths, and armadillos. The origins of the order can be traced back as far as the Paleogene in South America...
, which is restricted to South America (following recent consensus that Eurotamandua
Eurotamandua
Eurotamandua is an extinct mammal that lived some 49 million years ago, during the early Eocene.A single fossil is known, coming from the Messel Pit. It was about 90 cm long. It is often classified as a pangolin...
is not a xenarthran). 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
Molecular clock
The molecular clock is a technique in molecular evolution that uses fossil constraints and rates of molecular change to deduce the time in geologic history when two species or other taxa diverged. It is used to estimate the time of occurrence of events called speciation or radiation...
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.
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
Quaternary extinction event
The Quaternary period saw the extinctions of numerous predominantly larger, especially megafaunal, species, many of which occurred during the transition from the Pleistocene to the Holocene epoch. However, the extinction wave did not stop at the end of the Pleistocene, but continued especially on...
, proboscideans were present on every continent of the world except Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
and Antarctica. Hyraxes lived in much of Eurasia as recently as the end of the Pliocene
Pliocene
The Pliocene Epoch is the period in the geologic timescale that extends from 5.332 million to 2.588 million years before present. It is the second and youngest epoch of the Neogene Period in the Cenozoic Era. The Pliocene follows the Miocene Epoch and is followed by the Pleistocene Epoch...
; the extinct afrotherian orders of embrithopods
Embrithopoda
Embrithopoda is an extinct order of mammals which first appeared in the fossil record during the late Eocene and then died out during the Oligocene....
and desmostylians
Desmostylia
Desmostylia is an extinct order of marine mammals which existed from the Arikareean age of the late Oligocene epoch to the Tortonian age of the late Miocene epoch . Their dental and skeletal form suggests desmostylians were amphibious herbivores dependent on littoral habitats...
were also once widely distributed.
Organization
Afrotheria is a cladeClade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
of placental mammals, the stem designation for which is Eutheria
Eutheria
Eutheria is a group of mammals consisting of placental mammals plus all extinct mammals that are more closely related to living placentals than to living marsupials . They are distinguished from noneutherians by various features of the feet, ankles, jaws and teeth...
. Note that ranks in Linnean taxonomy are arbitrary and without intrinsic biological meaning. While this recognition does not mean that the Linnean system must be completely replaced, 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 listed below are junior synonyms and arguably should be replaced (e.g., Tenrecoidea McDowell 1958 instead of "Afrosoricida" Stanhope et al. 1998).
- INFRACLASS EUTHERIAEutheriaEutheria is a group of mammals consisting of placental mammals plus all extinct mammals that are more closely related to living placentals than to living marsupials . They are distinguished from noneutherians by various features of the feet, ankles, jaws and teeth...
: placental mammals- Superorder Afrotheria
- Clade AfroinsectiphiliaAfroinsectiphiliaThe Afroinsectiphilia is a clade that has been proposed based on the results of recent molecular studies. Many of its orders were once regarded as part of the order Insectivora, but Insectivora is now considered to be polyphyletic and is, as a result, possibly obsolete...
?- Order AfrosoricidaAfrosoricidaThe order Afrosoricida contains the golden moles of southern Africa and the tenrecs of Madagascar and Africa, two families of small mammals that have traditionally been considered to be a part of the order Insectivora.Some biologists use Tenrecomorpha as the name for the tenrec-golden mole clade,...
- Suborder Chrysochloridea: golden moles (21 species)
- Suborder Tenrecomorpha: tenrecs and otter shrews (32 species)
- Order Macroscelidea: elephant shrews or sengis (17 species)
- Order Tubulidentata: aardvark (1 species)
- Order †Bibymalagasia?: extinct
- Order †Afredentata?: extinct
- Order †PtolemaiidaPtolemaiidaPtolemaiida is a taxon of wolf-sized mammals presumed to be descended from pantolestid cimolestids that lived in Northern and Eastern Africa during the Paleogene...
: extinct
- Order Afrosoricida
- Clade PaenungulataPaenungulataPaenungulata is a taxon that groups some remarkable mammals, including three orders that are extant: Proboscidea , Sirenia , and Hyracoidea . At least two more orders are known only as fossils, namely Embrithopoda and Desmostylia...
- Order Hyracoidea: hyraxes (4-6 species, depending on classification)
- Order ProboscideaProboscideaProboscidea is a taxonomic order containing one living family, Elephantidae, and several extinct families. This order was first described by J. Illiger in 1881 and encompasses the trunked mammals...
: elephants (3 species) - Order SireniaSireniaSirenia is an order of fully aquatic, herbivorous mammals that inhabit swamps, rivers, estuaries, marine wetlands, and coastal marine waters. Four species are living, in two families and genera. These are the dugong and manatees...
: manatees (3 species); dugong (1 species) - Order †EmbrithopodaEmbrithopodaEmbrithopoda is an extinct order of mammals which first appeared in the fossil record during the late Eocene and then died out during the Oligocene....
: extinct - Order †DesmostyliaDesmostyliaDesmostylia is an extinct order of marine mammals which existed from the Arikareean age of the late Oligocene epoch to the Tortonian age of the late Miocene epoch . Their dental and skeletal form suggests desmostylians were amphibious herbivores dependent on littoral habitats...
: extinct
- Clade Afroinsectiphilia
- Superorder XenarthraXenarthraThe superorder Xenarthra is a group of placental mammals , existent today only in the Americas and represented by anteaters, tree sloths, and armadillos. The origins of the order can be traced back as far as the Paleogene in South America...
- taxonTaxon|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...
BoreoeutheriaBoreoeutheriaBoreoeutheria is a clade of placental mammals that is composed of the sister taxa Laurasiatheria and Euarchontoglires...
- Superorder LaurasiatheriaLaurasiatheriaLaurasiatheria is a large group of placental mammals believed to have originated on the northern supercontinent of Laurasia. It includes shrews, hedgehogs, pangolins, bats, whales, most hoofed mammals, and carnivorans, among others....
- Superorder EuarchontogliresEuarchontogliresEuarchontoglires is a clade of mammals, the living members of which are rodents, lagomorphs, treeshrews, colugos and primates .-Evolutionary relationships:...
(Supraprimates)
- Superorder Laurasiatheria
- Superorder Afrotheria