Metatheria is a mammalian

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

that includes all

mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

s more closely related to

marsupial Marsupials are a diverse group of mammals belonging to the infraclass Marsupialia. They are natively found in Australasia, Wallacea, and the Americas. One of marsupials' unique features is their reproductive strategy: the young are born in a r ...

s than to

placental Placental mammals (infraclass Placentalia ) 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 ...

s. First proposed by Thomas Henry Huxley in 1880, it is a more inclusive group than the marsupials; it contains all marsupials as well as many extinct non-marsupial relatives. It is one of two groups placed in the clade

Theria Theria ( or ; ) is a scientific classification, subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the Placentalia, placental mammals) and the metatherians (including the marsupials) but excludes the egg-lay ...

alongside

Eutheria Eutheria (from Greek , 'good, right' and , 'beast'; ), also called Pan-Placentalia, is the clade consisting of Placentalia, placental mammals and all therian mammals that are more closely related to placentals than to marsupials. Eutherians ...

, which contains the placentals. Remains of metatherians have been found on all of Earth’s continents.

Description

Distinctive characteristics ( synapomorphies) of Metatheria include a prehensile tail, the development of a capitular tail on the humerus, the loss of tooth replacement on the second and fifth premolars, lower canines that outwardly diverge from each other, an angular process on the dentary bone—which additionally bears a posterior shelf in its masseteric fossa in Metatheria—that is equal to or greater than the length of the ramus, and the lower fifth premolar with a "very trenchant" cristid obliqua/ectolophid. The permanent, retained deciduous fifth premolars are molar like and were historically identified as first molars, with the third premolar found in basal therians being lost, leaving four premolars in the halves of each jaw. ''

Sinodelphys ''Sinodelphys'' is an extinct mammal from the Early Cretaceous, estimated to be 125 million years old. It was discovered and described in 2003 in rocks of the Yixian Formation in Liaoning Province, China, by a team of scientists including Zhe-X ...

'' differs in its dentition and may represent a basal metatherian.

Evolutionary history

The relationships between the three extant divisions of mammals ( monotremes, marsupials, and placentals) was long a matter of debate among taxonomists. Most morphological evidence comparing traits, such as the number and arrangement of teeth and the structure of the reproductive and waste elimination systems, favors a closer evolutionary relationship between marsupials and placental mammals than either has with the monotremes, as does most genetic and molecular evidence. The earliest possible known metatherian is '' Sinodelphys szalayi'', which lived in

China China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...

during the

Early Cretaceous The Early Cretaceous (geochronology, geochronological name) or the Lower Cretaceous (chronostratigraphy, chronostratigraphic name) is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 143.1 ...

around 125 million years ago (mya). This makes it a contemporary to some early eutherian species that have been found in the same area. However, Bi ''et al.'' (2018) reinterpreted ''Sinodelphys'' as an early member of Eutheria. The oldest uncontested metatherians are now 110 million year old fossils from western North America. Metatherians were widespread in Asia and North America during the Late Cretaceous, including both

Deltatheroida Deltatheroida is an extinct group of basal metatherians that were distantly related to modern marsupials. The majority of known members of the group lived in the Cretaceous; one species, '' Gurbanodelta kara'', is known from the late Paleocene ( ...

and Marsupialiformes, with fossils also known from Europe during this time. During the Late Cretaceous, metatherians were more diverse than eutherians in North America. Metatherians underwent a severe decline during the

Cretaceous–Paleogene extinction event The Cretaceous–Paleogene (K–Pg) extinction event, also known as the K–T extinction, was the extinction event, mass extinction of three-quarters of the plant and animal species on Earth approximately 66 million years ago. The event cau ...

, more severe than that suffered by contemporary eutherians and multituberculates, and were slower to recover diversity. Morphological and species diversity of metatherians in

Laurasia Laurasia () was the more northern of two large landmasses that formed part of the Pangaea supercontinent from around ( Mya), the other being Gondwana. It separated from Gondwana (beginning in the late Triassic period) during the breakup of Pa ...

remained low in comparison to eutherians throughout the Cenozoic. The two major groups of Cenozoic Laurasian metatherians, the opossum-like herpetotheriids and peradectids persisted into the

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

before becoming extinct, with the North American herpetotheriid '' Herpetotherium'', the European herpetotheriid '' Amphiperatherium'' and the peradectids '' Siamoperadectes'' and '' Sinoperadectes'' from Asia being the youngest Laurasian non-marsupial metatherians (with marsupials invading North America during the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58Pleistocene The Pleistocene ( ; referred to colloquially as the ''ice age, Ice Age'') is the geological epoch (geology), epoch that lasted from to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fin ...

as part of the

Great American interchange The Great American Biotic Interchange (commonly abbreviated as GABI), also known as the Great American Interchange and the Great American Faunal Interchange, was an important late Cenozoic paleozoogeographic biotic interchange event in which land ...

). Metatherians first arrived in Afro-Arabia during the

Paleogene The Paleogene Period ( ; also spelled Palaeogene or Palæogene) is a geologic period and system that spans 43 million years from the end of the Cretaceous Period Ma (million years ago) to the beginning of the Neogene Period Ma. It is the fir ...

, probably from Europe, including the possible peradectoid '' Kasserinotherium'' from the Early Eocene of Tunisia and the herpetotheriid '' Peratherium africanum'' from the Early

Oligocene The Oligocene ( ) is a geologic epoch (geology), epoch of the Paleogene Geologic time scale, Period that extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that defin ...

of Egypt and Oman. The youngest African metatherian is the possible herpetotheriid '' Morotodon'' from the late

Early Miocene The Early Miocene (also known as Lower Miocene) is a sub-epoch of the Miocene epoch (geology), Epoch made up of two faunal stage, stages: the Aquitanian age, Aquitanian and Burdigalian stages. The sub-epoch lasted from 23.03 ± 0.05 annum, Ma to ...

Uganda Uganda, officially the Republic of Uganda, is a landlocked country in East Africa. It is bordered to the east by Kenya, to the north by South Sudan, to the west by the Democratic Republic of the Congo, to the south-west by Rwanda, and to the ...

. Metatherians arrived in South America from North America during the latest Cretaceous or

Paleocene The Paleocene ( ), or Palaeocene, is a geological epoch (geology), epoch that lasted from about 66 to 56 mya (unit), million years ago (mya). It is the first epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), ...

and underwent a major diversificiation, with South American metatherians including both the ancestors of extant marsupials as well as the extinct

Sparassodonta Sparassodonta (from Ancient Greek, Greek to tear, rend; and , gen. , ' tooth) is an extinct order (biology), order of carnivore, carnivorous metatherian mammals native to South America, related to modern marsupials. They were once cons ...

, which were major predators in South American ecosystems during most of the

Cenozoic The Cenozoic Era ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterized by the dominance of mammals, insects, birds and angiosperms (flowering plants). It is the latest of three g ...

, up until their extinction in the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58Polydolopimorphia, which likely had a wide range of diets. Metatherians then declined in diversity in South America during the Late Eocene as well as the later Oligocene epoch. The oldest known Australian marsupials are from the early Eocene, and are thought to have arrived in the region after having dispersed via Antarctica from South America. During the Oligocene epoch, Australian metatherians radiated rapidly, which contributed most to the global peak in metatherian diversity during the Early Miocene. The only known Antarctic metatherians are from the Early Eocene La Meseta Formation of the

Antarctic Peninsula The Antarctic Peninsula, known as O'Higgins Land in Chile and Tierra de San Martin in Argentina, and originally as Graham Land in the United Kingdom and the Palmer Peninsula in the United States, is the northernmost part of mainland Antarctica. ...

, where they are the most diverse group of mammals, and include marsupials as well as polydolopimorphians.

Classification

Below is a metatherian

cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...

from Wilson et al. (2016): Cladogram after: Below is a listing of metatherians that do not fall readily into well-defined groups. Basal Metatheria *†'' Archaeonothos henkgodthelpi'' Beck 2015 *†'' Esteslestes ensis'' Novacek et al. 1991 *†'' Ghamidtherium dimaiensis'' Sánches-Villagra et al. 2007 *†'' Kasserinotherium tunisiense'' Crochet 1989 *†'' Palangania brandmayri'' Goin et al. 1998 *†'' Perrodelphys coquinense'' Goin et al. 1999 Ameridelphia incertae sedis: *†'' Apistodon exiguus'' (Fox 1971) Davis 2007 *†'' Cocatherium lefipanum'' Goin et al. 2006 *†'' Dakotadens morrowi'' Eaton 1993 *†'' Iugomortiferum thoringtoni'' Cifelli 1990b *†'' Marambiotherium glacialis'' Goin et al. 1999 *†'' Marmosopsis juradoi'' Paula Couto 1962 armosopsini Kirsch & Palma 1995*†'' Pascualdelphys fierroensis'' *†'' Progarzonia notostylopense'' Ameghino 1904 *†'' Protalphadon'' Cifelli 1990 **†'' P. lulli'' (Clemens 1966) Cifelli 1990a **†'' P. foxi'' Johnson 1996 Marsupialia incertae sedis: *†'' Itaboraidelphys camposi'' Marshall & de Muizon 1984 *†'' Mizquedelphys pilpinensis'' Marshall & de Muizon 1988 *†'' Numbigilga ernielundeliusi'' Beck et al. 2008

References

{{Taxonbar, from=Q1096960 Early Cretaceous mammals Cretaceous mammals Paleogene mammals Neogene mammals Quaternary mammals Prehistoric marsupials Extant Early Cretaceous first appearances Mammal unranked clades