Metatheria

Metatheria Temporal range:
Lycopsis longirostris,an extinctsparassodont,a relative of themarsupials
A mouse opossum (Marmosa)
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Subclass:	Theria
Clade:	Metatheria Huxley,1880
Subgroups
†?Sinodelphys †Holoclemensia †Adinodon †Deltatheroida Marsupialiformes †Adelodelphys †Alphadontidae †Asiadelphia †Sinbadelphys †Camptomus? †Archimetatheria †Stagodontidae †Iqualadelphis †Sparassodonta †Pucadelphyidae †Herpetotheriidae †Anatoliadelphyidae †Polydolopimorphia †Peradectidae †Pediomyidae Marsupialia

Metatheriais a mammaliancladethat includes allmammalsmore closely related tomarsupialsthan toplacentals.First proposed byThomas Henry Huxleyin 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 cladeTheriaalongsideEutheria,which contains the placentals. Remains of metatherians have been found on all of Earths continents.

Description

Distinctive characteristics (synapomorphies) of Metatheria include: aprehensile tail,the development of a capitular tail on the humerus, the loss of tooth replacement on the 2nd and 5thpremolarsand the retention of decidious teeth on the lower fifth premolars, the lowercaninesoutwardly diverge from each other, theangular processon the dentary is equal to or less than half the length of theramus,the dentary has a posterior masseteric shelf, and the lower 5th premolar has a "very trenchant" cristid obliqua/ectolophid. The permanent deciduous lower 5th premolars aremolarlike and were historically identified as 1st molars, with the third premolar found in basaltheriansbeing lost, leaving 4 premolars in the halves of each jaw.

Evolutionary history

The relationships between the three extant divisions of mammals (monotremes,marsupials, andplacentals) was long a matter of debate amongtaxonomists.Mostmorphologicalevidence comparing traits, such as thenumber and arrangement of teethand 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 molecularevidence.

The earliest possible known metatherian isSinodelphys szalayi,which lived inChinaduring theEarly Cretaceousaround 125 million years ago (mya).This makes it a contemporary to some early eutherian species that have been found in the same area.However, Biet al.(2018) reinterpretedSinodelphysas 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 bothDeltatheroidaand 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 theCretaceous–Paleogene extinction event,more severe than that suffered by contemporary eutherians andmultituberculates,and were slower to recover diversity.

Morphological and species diversity of metatherians inLaurasiaremained low in comparison to eutherians throughout the Cenozoic.The two major groups of Cenozoic Laurasian metatherians, the opossum-likeherpetotheriidsandperadectidspersisted into theMiocenebefore becoming extinct, with the North American herpetotheriidHerpetotherium,the European herpetotheriidAmphiperatheriumand the peradectidsSiamoperadectesandSinoperadectesfrom Asia being the youngest Laurasian non-marsupial metatherians (with marsupials invading North America during thePliocene-Pleistoceneas part of theGreat American interchange).Metatherians first arrived in Afro-Arabia during thePaleogene,probably from Europe, including the possible peradectoidKasserinotheriumfrom the Early Eocene of Tunisia and the herpetotheriidPeratherium africanumfrom the EarlyOligoceneof Egypt and Oman. The youngest African metatherian is the possible herpetotheriidMorotodonfrom the lateEarly MioceneofUganda.

Metatherians arrived in South America from North America during the latest Cretaceous orPaleoceneand 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.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 EoceneLa Meseta Formationof theAntarctic Peninsula,where they are the most diverse group of mammals, and include marsupials as well as polydolopimorphians.

Classification

Below is a metatheriancladogramfrom Wilson et al. (2016):

Metatheria

Holoclemensia Deltatheroida Pappotheriidae Pappotherium Deltatheridiidae Sulestes Oklatheridium Tsagandelta Lotheridium Deltatheroides Deltatheridium Nanocuris Atokatheridium Marsupialiformes Gurlin Tsav skull Borhyaenidae Mayulestes Jaskhadelphys Andinodelphys Pucadelphys Asiatherium Iugomortiferum Kokopellia Aenigmadelphys Anchistodelphys Glasbiidae Glasbius Pediomyidae Pediomys Stagodontidae Pariadens Eodelphis Didelphodon Turgidodon Alphadon Alphadontidae Albertatherium Marsupialia

Cladogram after:

Metatheria	Deltatheriidae Kokopellia Asiatherium Peradectidae Stagodontidae Pucadelphyidae Sparassodonta Amphiperatherium Peratherium "Herpetotheriidae" Herpetotherium Marsupialia

Below is a listing of metatherians that do not fall readily into well-defined groups.

Basal Metatheria

• †Archaeonothos henkgodthelpiBeck 2015
• †Esteslestes ensisNovacek et al. 1991
• †Ghamidtherium dimaiensisSánches-Villagra et al. 2007
• †Kasserinotherium tunisienseCrochet 1989
• †Palangania brandmayriGoin et al. 1998
• †Perrodelphys coquinenseGoin et al. 1999

Ameridelphia incertae sedis:

• †Apistodon exiguus(Fox 1971) Davis 2007
• †Cocatherium lefipanumGoin et al. 2006
• †Dakotadens morrowiEaton 1993
• †Iugomortiferum thoringtoniCifelli 1990b
• †Marambiotherium glacialisGoin et al. 1999
• †Marmosopsis juradoiPaula Couto 1962[MarmosopsiniKirsch & Palma 1995]
• †Pascualdelphys fierroensis
• †Progarzonia notostylopenseAmeghino 1904
• †ProtalphadonCifelli 1990
  • †P. lulli(Clemens 1966) Cifelli 1990a
  • †P. foxiJohnson 1996

Marsupialia incertae sedis:

• †Itaboraidelphys camposiMarshall & de Muizon 1984
• †Mizquedelphys pilpinensisMarshall & de Muizon 1988
• †Numbigilga ernielundeliusiBeck et al. 2008{NumbigilgidaeBeck et al. 2008}