| Metatheria | |
|---|---|
 | |
| Lycopsis longirostris, an extinctsparassodont, a relative of themarsupials | |
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| A mouse opossum (Marmosa) | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Subclass: | Theria |
| Clade: | Metatheria Huxley, 1880 |
| Subgroups | |
Metatheria is a mammalianclade that includes allmammals more closely related tomarsupials than toplacentals. First proposed byThomas 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 cladeTheria alongsideEutheria, which contains the placentals. Remains of metatherians have been found on all of Earth’s continents.
Distinctive characteristics (synapomorphies) of Metatheria include aprehensile tail, the development of a capitular tail on the humerus, the loss of tooth replacement on the second and fifthpremolars, lowercanines that outwardly diverge from each other, anangular process on the dentary bone—which additionally bears a posterior shelf in its masseteric fossa in Metatheria[4]—that is equal to or greater than the length of theramus,[4][5]: Appendix 2 and the lower fifth premolar with a "very trenchant"[4] cristid obliqua/ectolophid. The permanent, retained deciduous fifth premolars aremolar like and were historically identified as first molars, with the third premolar found in basaltherians being lost, leaving four premolars in the halves of each jaw.[4]Sinodelphys differs in its dentition and may represent a basal metatherian.[4]
The relationships between the three extant divisions of mammals (monotremes, marsupials, andplacentals) was long a matter of debate amongtaxonomists.[6] Mostmorphological evidence comparing traits, such as thenumber and arrangement of teeth and the structure of thereproductive and waste elimination systems, favors a closer evolutionary relationship between marsupials and placental mammals than either has with the monotremes, as does mostgenetic and molecular evidence.[7]
The earliest possible known metatherian isSinodelphys szalayi, which lived inChina during theEarly Cretaceous around 125 million years ago (mya).[4][8] This makes it a contemporary to some early eutherian species that have been found in the same area.[9] However, Biet al. (2018) reinterpretedSinodelphys as an early member of Eutheria. The oldest uncontested metatherians are now 110 million year old fossils from western North America.[3] Metatherians were widespread in Asia and North America during the Late Cretaceous, including bothDeltatheroida and Marsupialiformes,[10] with fossils also known from Europe during this time. During the Late Cretaceous, metatherians were more diverse than eutherians in North America.[4] Metatherians underwent a severe decline during theCretaceous–Paleogene extinction event, more severe than that suffered by contemporary eutherians andmultituberculates, and were slower to recover diversity.[10]
Morphological and species diversity of metatherians inLaurasia remained low in comparison to eutherians throughout the Cenozoic.[11] The two major groups of Cenozoic Laurasian metatherians, the opossum-likeherpetotheriids andperadectids persisted into theMiocene before becoming extinct, with the North American herpetotheriidHerpetotherium, the European herpetotheriidAmphiperatherium and the peradectidsSiamoperadectes andSinoperadectes from Asia being the youngest Laurasian non-marsupial metatherians (with marsupials invading North America during thePliocene-Pleistocene as part of theGreat American interchange).[12][10] Metatherians first arrived in Afro-Arabia during thePaleogene, probably from Europe, including the possible peradectoidKasserinotherium from the Early Eocene of Tunisia and the herpetotheriidPeratherium africanum from the EarlyOligocene of Egypt and Oman. The youngest African metatherian is the possible herpetotheriidMorotodon from the lateEarly Miocene ofUganda.[13][14]
Metatherians arrived in South America from North America during the latest Cretaceous orPaleocene and underwent a major diversificiation, with South American metatherians including both the ancestors of extant marsupials as well as the extinctSparassodonta, which were major predators in South American ecosystems during most of theCenozoic, up until their extinction in thePliocene, as well as thePolydolopimorphia, which likely had a wide range of diets.[11] 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.[11] 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 EoceneLa Meseta Formation of theAntarctic Peninsula, where they are the most diverse group of mammals, and include marsupials as well as polydolopimorphians.[11]
Below is a metatheriancladogram from Wilson et al. (2016):[15]
| Metatheria |
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Cladogram after[16]:
| Metatheria | |
Below is a listing of metatherians that do not fall readily into well-defined groups.
Basal Metatheria
Ameridelphia incertae sedis:
Marsupialia incertae sedis:
