Afrotheria | |
---|---|
1. Aardvark 2. Dugong 3. Black and rufous elephant shrew 4. West Indian manatee 5. Cape golden mole 6. Rock hyrax 7. African bush elephant 8. Tailless tenrec | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Class: | Mammalia |
Infraclass: | Placentalia |
Superorder: | Afrotheria Stanhope MJ, Waddell VG, Madsen O, de Jong W, Hedges SB, Cleven G, Kao D, Springer MS, 1998 |
Orders | |
See below |
Afrotheria ( /æfroʊˈθɪəriə/ from Latin Afro- "of Africa" + theria "wild beast") is a superorder of mammals, the living members of which belong to groups that are either currently living in Africa or of African origin: golden moles, elephant shrews (also known as sengis), otter shrews, tenrecs, aardvarks, hyraxes, elephants, sea cows, and several extinct clades. Most groups of afrotheres share little or no superficial resemblance, and their similarities have only become known in recent times because of genetics and molecular studies. Many afrothere groups are found mostly or exclusively in Africa, reflecting the fact that Africa was an island continent from the Cretaceous until the early Miocene around 20 million years ago, when Afro-Arabia collided with Eurasia.
Because Africa was isolated by water, Laurasian groups of mammals such as insectivores, rodents, lagomorphs, carnivorans and ungulates could not reach Africa for much of the early to mid-Cenozoic. Instead, the niches occupied by those groups on the northern continents were filled by various groups of afrotheres via the process of convergent evolution. The small insectivorous afrotheres such as elephant shrews, golden moles, and tenrecs filled the niches of insectivores, the hyraxes filled the roles of rodents and lagomorphs, the aardvarks filled the roles of various medium size ant-eating mammals (anteaters, armadillos, pangolins, echidnas, numbats, etc.) found on other continents throughout the Cenozoic, and proboscideans (elephants and their relatives) filled the roles of large herbivores such as hippos, camels, rhinos, and tapirs. The sirenians developed aquatic body plans and started spreading to other parts of the world by water (evolving convergently with the other groups of marine mammals such as cetaceans and pinnipeds). In addition to their similarity with Laurasian mammals in North America, Europe, and Asia, many afrotheres also exhibit convergent evolution with groups of mammals that evolved and lived exclusively in South America, which was also an island continent for much of the Cenozoic.
The common ancestry of these animals was not recognized until the late 1990s. [1] Historically, the Paenungulata had been linked to the true ungulates (particularly the Perissodactyls); the golden mole, tenrecs, and elephant shrews with the traditional (and polyphyletic/incorrect) taxon Insectivora; and the aardvarks with the pangolins and the xenarthrans within the invalid taxon Edentata. Continuing work on the molecular [2] [3] [4] and morphological [5] [6] [7] [8] diversity of afrotherian mammals has provided ever increasing support for their common ancestry.
The afrotherian clade was originally proposed in 1998 [1] 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 [9] and there were sporadic papers in the 1980s [10] and 1990s. [11] [12] [13] 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. [14] [15] Hence, while DNA sequence data have proven essential to infer the existence of the Afrotheria as a whole, and while the Afroinsectiphilia (insectivoran-grade afrotheres including tenrecs, golden moles, sengis, and aardvarks) 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. [16]
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, [8] aspects of placental membrane formation, [17] the shape of the ankle bones, [6] [7] the relatively late eruption of the permanent dentition, [18] and undescended testicles remaining in the body near the kidneys. [19] The snout is unusually long and mobile in several Afrotherian species, and this was pointed out as a possible shared-derived character. [20] Studies of genomic data, including millions of aligned nucleotides sampled for a growing number of placental mammals, also support Afrotheria as a clade. [21] [22] Additionally, there might be some dental synapomorphies uniting afroinsectiphilians, if not afrotheres as a whole: p4 talonid and trigonid of similar breadth, a prominent p4 hypoconid, presence of a P4 metacone and absence of parastyles on M1–2. [7] [23]
Afrotheria is now recognized as one of the three major groups within the Eutheria (containing placental mammals). [24] Relations within the three cohorts, Afrotheria, Xenarthra, Boreoeutheria, and the identity of the placental root, remain somewhat controversial. [5]
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 two some 105 million years ago in the mid-Cretaceous, when the African continent was separated from other major land masses. [25] 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 [26] ).
However, Afrotheria itself does not have a fossil record restricted to Africa, [27] and appears in fact to have evolved in the continent's isolation. [28] More recent, genomic-scale phylogenies favor the hypothesis that Afrotheria and Xenarthra comprise sister taxa at the base of the placental mammal radiation, suggesting an ancient Gondwanan clade of placental mammals. [29] A 2021 morphological study also proposed to render Meridiungulata polyphyletic and recognise most of its clades as part of a group called Sudamericungulata, closely related to hyraxes, while Litopterna remains a sister taxon to Perissodactyla. [30]
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. [31] These findings are compatible with the work of earlier anatomists. [14] [15]
A cladogram of Afrotheria based on molecular evidence [15] |
Many extant members of Afrotheria appear to have a high risk of extinction (perhaps related to the large size of many). Species loss within this already small group would comprise a particularly great 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. [33]
While most extant species assigned to Afrotheria live in Africa, some (such as the Indian elephant and three of the four sirenian species) 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. However, the desmostylians have recently been viewed as possible perissodactyls, rather than afrotheres, [34] although this is still controversial; [28] the taxonomic placement of embrithopods is also not clear. [35]
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. [36] [37]
Ungulates are members of the diverse clade Euungulata, which primarily consists of large mammals with hooves. Once part of the clade "Ungulata" along with the clade Paenungulata, "Ungulata" has since been determined to be a polyphyletic and thereby invalid clade based on molecular data. As a result, true ungulates had since been reclassified to the newer clade Euungulata in 2001 within the clade Laurasiatheria while Paenungulata has been reclassified to a distant clade Afrotheria. Living ungulates are divided into two orders: Perissodactyla including equines, rhinoceroses, and tapirs; and Artiodactyla including cattle, antelope, pigs, giraffes, camels, sheep, deer, and hippopotamuses, among others. Cetaceans such as whales, dolphins, and porpoises are also classified as artiodactyls, although they do not have hooves. Most terrestrial ungulates use the hoofed tips of their toes to support their body weight while standing or moving. Two other orders of ungulates, Notoungulata and Litopterna, both native to South America, became extinct at the end of the Pleistocene, around 12,000 years ago.
Placental mammals are one of the three extant subdivisions of the class Mammalia, the other two being Monotremata and Marsupialia. Placentalia contains the vast majority of extant mammals, which are partly distinguished from monotremes and marsupials in that the fetus is carried in the uterus of its mother to a relatively late stage of development. The name is something of a misnomer considering that marsupials also nourish their fetuses via a placenta, though for a relatively briefer period, giving birth to less developed young which are then nurtured for a period inside the mother's pouch. Placentalia represents the only living group within Eutheria, which contains all mammals more closely related to placentals than to marsupials.
Elephant shrews, also called jumping shrews or sengis, are small insectivorous mammals native to Africa, belonging to the family Macroscelididae, in the order Macroscelidea. Their traditional common English name "elephant shrew" comes from a perceived resemblance between their long noses and the trunk of an elephant, and their superficial similarity with shrews in the order Eulipotyphla. However, phylogenetic analysis has revealed that elephant shrews are not properly classified with true shrews, but are in fact more closely related to elephants than to shrews. In 1997, the biologist Jonathan Kingdon proposed that they instead be called "sengis", a term derived from the Bantu languages of Africa, and in 1998, they were classified into the new clade Afrotheria.
The clade Afrosoricida contains the golden moles of Southern Africa, the otter shrews of equatorial Africa and the tenrecs of Madagascar. These three groups of small mammals were for most of the 19th and 20th centuries regarded as a part of the Insectivora or Lipotyphla, but both of those groups, as traditionally used, are polyphyletic.
Tenrecomorpha is the suborder of otter shrews and tenrecs, a group of afrotherian mammals indigenous to equatorial Africa and Madagascar, respectively. The two families are thought to have split about 47–53 million years ago. Potamogalid otter shrews were formerly considered a subfamily of Tenrecidae. The suborder is also presumed to contain the extinct genus Plesiorycteropus, a group of possibly fossorial insectivores similar to aardvarks, which is known to be more closely related to tenrecs of subfamily Tenrecinae than to golden moles of suborder Chrysochloridea.
A tenrec is a mammal belonging to any species within the afrotherian family Tenrecidae, which is endemic to Madagascar. Tenrecs are a very diverse group; as a result of convergent evolution, some resemble hedgehogs, shrews, opossums, rats, and mice. They occupy aquatic, arboreal, terrestrial, and fossorial environments. Some of these species, including the greater hedgehog tenrec, can be found in the Madagascar dry deciduous forests. However, the speciation rate in this group has been higher in humid forests.
Paenungulata is a clade of "sub-ungulates", which groups three extant mammal orders: Proboscidea, Sirenia, and Hyracoidea (hyraxes). At least two more possible orders are known only as fossils, namely Embrithopoda and Desmostylia.
Condylarthra is an informal group – previously considered an order – of extinct placental mammals, known primarily from the Paleocene and Eocene epochs. They are considered early, primitive ungulates. It is now largely considered to be a wastebasket taxon, having served as a dumping ground for classifying ungulates which had not been clearly established as part of either Perissodactyla or Artiodactyla, being composed thus of several unrelated lineages.
South American native ungulates, commonly abbreviated as SANUs, are extinct ungulate-like mammals of controversial affinities that were indigenous to South America prior to the Great American Biotic Interchange. They comprise five major groups conventionally ranked as orders—Astrapotheria, Litopterna, Notoungulata, Pyrotheria, and Xenungulata—as well as the primitive "condylarth" groups Didolodontidae and Kollpaniinae. It has been proposed that some or all of the members of this group form a clade, named Meridiungulata, though the relationships of South American ungulates remain largely unresolved. The two largest groups of South American ungulates, the notoungulates and the litopterns, were the only groups to persist beyond the mid Miocene. Only a few of the largest species of notoungulates and litopterns survived until the end-Pleistocene extinction event around 12,000 years ago where they became extinct with most other large mammals in the Americas, shortly after the first arrival of humans into the region.
Cimolesta is an extinct order of non-placental eutherian mammals. Cimolestans had a wide variety of body shapes, dentition and lifestyles, though the majority of them were small to medium-sized general mammals that bore superficial resemblances to rodents, lagomorphs, mustelids, and marsupials.
The web-footed tenrec, Malagasy otter shrew, or aquatic tenrec is the only known semiaquatic tenrec, and is found in eastern Madagascar, especially in and around Ranomafana National Park. It grows to between 25 and 39 cm, and was once thought to be extinct. It feeds on crabs, aquatic insects, and crayfish. The population is considered vulnerable. It was formerly placed in the monotypic genus Limnogale, but has been moved to Microgale based on molecular data showing it to be deeply nested within the latter.
The Afroinsectiphilia is a clade that has been proposed based on the results of recent molecular phylogenetic studies. Many of the taxa within it were once regarded as part of the order Insectivora, but Insectivora is now considered to be polyphyletic and obsolete. This proposed classification is based on molecular studies only, and there is no morphological evidence for it.
Ferungulata is a grandorder of placental mammals that groups together mirorder Ferae and clade Pan-Euungulata. It has existed in two guises, a traditional one based on morphological analysis and a revised one taking into account more recent molecular analyses. The Fereungulata is a sister group to the order Chiroptera (bats) and together they make clade Scrotifera.
Pseudoungulata, or "false hoofed mammals", is a possible clade made up of two subgroups, aardvarks and paenungulates. Before this group was proposed, it was thought that aardvarks were more closely related to xenarthrans. However, all of these mammals are now considered to be part of Afrotheria, which also includes elephant shrews and afrosoricidans. Other positions of aardvarks within Afrotheria are possible, such as being closest relatives of elephant shrews and/or afrosoricidans.
Amblysomus is a genus of the golden mole family, Chrysochloridae, comprising five species of the small, insect-eating, burrowing mammals endemic to Southern Africa. All five species can be found in South Africa and some are also found in Eswatini and Lesotho.
Potamogalidae is the family of "otter shrews", a group of semiaquatic riverine afrotherian mammals indigenous to sub-Saharan Africa. They are most closely related to the tenrecs of Madagascar, from which they are thought to have split about 47–53 million years ago. They were formerly considered a subfamily of Tenrecidae.
Ptolemaiida is a taxon of wolf-sized afrothere mammals that lived in northern and eastern Africa during the Paleogene. The oldest fossils are from the latest Eocene strata of the Jebel Qatrani Formation, near the Fayum oasis in Egypt. A tooth is known from an Oligocene-aged stratum in Angola, and Miocene specimens are known from Kenya and Uganda.
Phenacodontidae is an extinct family of large herbivorous mammals traditionally placed in the “wastebasket taxon” Condylarthra, which may instead represent early-stage perissodactyls. They lived from the late early Paleocene to early middle Eocene and their fossil remains have been found in North America and Europe. The only unequivocal Asian phenacodontid is Lophocion asiaticus.
Ocepeia is an extinct genus of afrotherian mammal that lived in present-day Morocco during the middle Paleocene epoch, approximately 60 million years ago. First named and described in 2001, the type species is O. daouiensis from the Selandian stage of Morocco's Ouled Abdoun Basin. A second, larger species, O. grandis, is known from the Thanetian, a slightly younger stage in the same area. In life, the two species are estimated to have weighed about 3.5 kg (7.7 lb) and 10 kg (22 lb), respectively, and are believed to have been specialized leaf-eaters. The fossil skulls of Ocepeia are the oldest known afrotherian skulls, and the best-known of any Paleocene mammal in Africa.
Paenungulatomorpha is a clade of afrotherian mammals that can be characterized according to Gheerbrant et al. (2016):
by a mandibular retromolar fossa, the absence of hypocone, an ectoloph selenodont and linked to strong styles such as mesostyle in basal taxa, and a more or less developed pseudohypocone.