Eutheria
Encyclopedia
Eutheria is a group of mammal
s consisting of placenta
l mammals plus all extinct mammals that are more closely related to living placentals (such as humans) than to living marsupial
s (such as kangaroos). They are distinguished from noneutherians by various features of the feet, ankles, jaws and teeth. One of the major differences between placental and nonplacental eutherians is that placentals lack epipubic bones, which are present in all other fossil
and living mammals (monotreme
s and marsupial
s).
The oldest known eutherian species is Juramaia sinensis, dated at from the Jurassic
in China. The previously earliest known fossil eutherian, Eomaia scansoria, was also from China and is dated to the Early Cretaceous
period, about .
Eutheria was introduced by Thomas Henry Huxley in 1880, meant to be broader in definition than its precursor Placentalia.
s (such as kangaroos).
There are no living nonplacental eutherians, and so knowledge of their synapomorphies
("defining features") is entirely based on a few fossil
s, which means the reproductive features that distinguish modern placentals from other mammals cannot be used in defining Eutheria. The features of Eutheria that distinguish them from metatheria
ns, a group that includes modern marsupials, are:
Reproductive features are also of no use in identifying fossil placental mammals, which are distinguished from other eutherians by:
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 consisting of placenta
Placenta
The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and...
l mammals plus all extinct mammals that are more closely related to living placentals (such as humans) than to living marsupial
Marsupial
Marsupials are an infraclass of mammals, characterized by giving birth to relatively undeveloped young. Close to 70% of the 334 extant species occur in Australia, New Guinea, and nearby islands, with the remaining 100 found in the Americas, primarily in South America, but with thirteen in Central...
s (such as kangaroos). They are distinguished from noneutherians by various features of the feet, ankles, jaws and teeth. One of the major differences between placental and nonplacental eutherians is that placentals lack epipubic bones, which are present in all other fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
and living mammals (monotreme
Monotreme
Monotremes are mammals that lay eggs instead of giving birth to live young like marsupials and placental mammals...
s and marsupial
Marsupial
Marsupials are an infraclass of mammals, characterized by giving birth to relatively undeveloped young. Close to 70% of the 334 extant species occur in Australia, New Guinea, and nearby islands, with the remaining 100 found in the Americas, primarily in South America, but with thirteen in Central...
s).
The oldest known eutherian species is Juramaia sinensis, dated at from the Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
in China. The previously earliest known fossil eutherian, Eomaia scansoria, was also from China and is dated to the Early Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
period, about .
Eutheria was introduced by Thomas Henry Huxley in 1880, meant to be broader in definition than its precursor Placentalia.
Definition
Eutherians are a group of mammals consisting of placental mammals plus all extinct mammals that are more closely related to living placentals (such as humans) than to living marsupialMarsupial
Marsupials are an infraclass of mammals, characterized by giving birth to relatively undeveloped young. Close to 70% of the 334 extant species occur in Australia, New Guinea, and nearby islands, with the remaining 100 found in the Americas, primarily in South America, but with thirteen in Central...
s (such as kangaroos).
There are no living nonplacental eutherians, and so knowledge of their synapomorphies
Synapomorphy
In cladistics, a synapomorphy or synapomorphic character is a trait that is shared by two or more taxa and their most recent common ancestor, whose ancestor in turn does not possess the trait. A synapomorphy is thus an apomorphy visible in multiple taxa, where the trait in question originates in...
("defining features") is entirely based on a few fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s, which means the reproductive features that distinguish modern placentals from other mammals cannot be used in defining Eutheria. The features of Eutheria that distinguish them from metatheria
Metatheria
Metatheria is a grouping within the animal class Mammalia. First proposed by Thomas Henry Huxley in 1880, it is nearly synonymous with the earlier taxon Marsupialia though it is slightly wider since it also contains the nearest fossil relatives of marsupial mammals.The earliest known...
ns, a group that includes modern marsupials, are:
- an enlarged malleolus ("little hammer") at the bottom of the tibiaTibiaThe tibia , shinbone, or shankbone is the larger and stronger of the two bones in the leg below the knee in vertebrates , and connects the knee with the ankle bones....
, the larger of the two shin bones. - the joint between the first metatarsal bone and the entocuneiform bone in the foot is offset further back than the joint between the second metatarsal and mesocuneiform bones – in metatherians these joints are level with each other.
- various features of jaws and teeth.
Reproductive features are also of no use in identifying fossil placental mammals, which are distinguished from other eutherians by:
- the presence of a malleolus at the bottom of the fibula, the smaller of the two shin bones.
- a complete mortise and tenonMortise and tenonThe mortise and tenon joint has been used for thousands of years by woodworkers around the world to join pieces of wood, mainly when the adjoining pieces connect at an angle of 90°. In its basic form it is both simple and strong. Although there are many joint variations, the basic mortise and tenon...
upper ankle joint, where the rearmost bones of the foot fit into a socket formed by the ends of the tibia and fibula. - a wide opening at the bottom of the pelvisPelvisIn human anatomy, the pelvis is the lower part of the trunk, between the abdomen and the lower limbs .The pelvis includes several structures:...
, which allows the birth of large, well-developed offspring. Marsupials have and nonplacental eutherians had a narrower opening that allows only small, immature offspring to pass through. - the absence of epipubic bones extending forward from the pelvis, which are not found in any placental, but are found in all other mammals – nonplacental eutherians, marsupials, monotremes and mammaliformes – and even in the cynodontCynodontCynodontia or cynodonts are a taxon of therapsids which first appeared in the Late Permian and were eventually distributed throughout all seven continents by the Early Triassic . This clade includes modern mammals and their extinct close relatives. They were one of the most diverse groups of...
therapsids that are closest to mammals. Their function is to stiffen the body during locomotion. This stiffening would be harmful in pregnant placentals, whose abdomens need to expand.
Subgroups
- BoreoeutheriaBoreoeutheriaBoreoeutheria is a clade of placental mammals that is composed of the sister taxa Laurasiatheria and Euarchontoglires...
, e.g. badgerBadgerBadgers are short-legged omnivores in the weasel family, Mustelidae. There are nine species of badger, in three subfamilies : Melinae , Mellivorinae , and Taxideinae...
s, rabbits, guinea pigGuinea pigThe guinea pig , also called the cavy, is a species of rodent belonging to the family Caviidae and the genus Cavia. Despite their common name, these animals are not in the pig family, nor are they from Guinea...
s, dogDogThe domestic dog is a domesticated form of the gray wolf, a member of the Canidae family of the order Carnivora. The term is used for both feral and pet varieties. The dog may have been the first animal to be domesticated, and has been the most widely kept working, hunting, and companion animal in...
s, dolphinDolphinDolphins are marine mammals that are closely related to whales and porpoises. There are almost forty species of dolphin in 17 genera. They vary in size from and , up to and . They are found worldwide, mostly in the shallower seas of the continental shelves, and are carnivores, mostly eating...
s- EuarchontogliresEuarchontogliresEuarchontoglires is a clade of mammals, the living members of which are rodents, lagomorphs, treeshrews, colugos and primates .-Evolutionary relationships:...
, e.g. humanHumanHumans are the only living species in the Homo genus...
s, monkeys, rats, hares - 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....
, e.g. cattle, whales, bats, cats
- Euarchontoglires
- 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...
, e.g. armadillos, anteaters - AfrotheriaAfrotheriaAfrotheria is a clade of mammals, the living members of which belong to groups from Africa or of African origin: golden moles, sengis , tenrecs, aardvarks, hyraxes, elephants and sea cows. The common ancestry of these animals was not recognized until the late 1990s...
, e.g. elephantElephantElephants are large land mammals in two extant genera of the family Elephantidae: Elephas and Loxodonta, with the third genus Mammuthus extinct...
s, manateeManateeManatees are large, fully aquatic, mostly herbivorous marine mammals sometimes known as sea cows...
s
These groups together make up the crown group
Crown group
A crown group is a group consisting of living representatives, their ancestors back to the most recent common ancestor of that group, and all of that ancestor's descendants. The name was given by Willi Hennig, the formulator of phylogenetic systematics, as a way of classifying living organisms...
Placentalia (placental mammals). Eutheria also includes now extinct lineages that lie outside of Placentalia (see below).
Analysis of transposable element insertions around the time of divergence of Boreoeutheria, Afrotheria, and Xenarthra strongly support a near-concomitant origin (trifurcation) of these three superorders, making further subdivision impractical and meaningless. These observations eliminate the need to choose between the previously proposed groupings of Boreoeutheria and Xenarthra (Exafroplacentalia
Exafroplacentalia
Exafroplacentalia or Notolegia is a subcohort selected in 2001 on the basis of molecular researches.-Exafroplacentalia:Exafroplacentalia places Xenarthra as a sister group to the magnorder Boreoeutheria , thus making Afrotheria a primitive group of placental mammals...
), Afrotheria and Xenarthra (Atlantogenata
Atlantogenata
Atlantogenata is a proposed clade of mammals containing the cohorts or super-orders Afrotheria, Xenarthra and Meridiungulata. These groups originated and radiated in the South American and African continents, presumably in the Cretaceous. Together with Boreoeutheria it makes up Eutheria...
), Afrotheria and Boreoeutheria (Epitheria
Epitheria
Epitherians comprise all the placental mammals except the Xenarthra. They are primarily characterized by having a stirrup-shaped stapes in the middle ear, which allows for passage of a blood vessel. This is in contrast to the column-shaped stapes found in marsupials, monotremes, and xenarthrans...
).
Evolutionary history
| The fossil eutherian species believed to be the oldest known is Juramaia Juramaia Juramaia is an extinct genus of very basal eutherian mammal from late Middle Jurassic deposits of western Liaoning, China. Juramaia is known from the holotype BMNH PM1343, an articulated and nearly complete skeleton including incomplete skull preserved with full dentition... sinensis, said to be about . Another early eutherian mammal is the nonplacental Eomaia scansoria Eomaia Eomaia is an extinct fossil mammal, discovered in rocks that were found in the Yixian Formation, Liaoning Province, China, and dated to the Barremian Age of the Lower Cretaceous about . The fossil is in length and virtually complete. An estimate of the body weight is between . It is exceptionally... from the Lower Cretaceous Cretaceous The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the... of China China Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture... , dated to about . Some of its fossil Fossil Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past... s show thick fur Fur Fur is a synonym for hair, used more in reference to non-human animals, usually mammals; particularly those with extensives body hair coverage. The term is sometimes used to refer to the body hair of an animal as a complete coat, also known as the "pelage". Fur is also used to refer to animal... . Montanalestes Montanalestes Montanalestes is an extinct mammal known from the Cretaceous in North America.... was found in North America, while all other nonplacental eutherian fossils have been found in Asia. The earliest known placental fossils have also been found in Asia. |
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Further reading
- Goloboff, P.A.; Catalano, S.A.; Mirande, J.M.; Szumik, C.A.; Arias, J.S.; Källersjö, M & Farris, J.S. 2009. Phylogenetic analysis of 73 060 taxa corroborates major eukaryotic groups. CladisticsCladistics (journal)Cladistics is a bimonthly peer-reviewed scientific journal publishing research in cladistics. It is published by Wiley-Blackwell on behalf of the Willi Hennig Society. Cladistics publishes papers relevant to evolution, systematics, and integrative biology...
25 (3): 211–230