Metatheria is a mammalian clade that includes all mammals more closely related to marsupials than to placentals. First proposed by Thomas Henry Huxley in 1880, it is a slightly more inclusive group than the marsupials; it contains all marsupials as well as many extinct non-marsupial relatives. There are three extant subclasses of mammals, one being metatherians:
Metatherians belong to a subgroup of the northern tribosphenic mammal clade or Boreosphenida. They differ from all other mammals in certain morphologies like their dental formula, which includes about five upper and four lower incisors, a canine, three premolars, and four molars. Other morphologies include skeletal and anterior dentition, such as wrist and ankle apomorphies; all metatherians share derived pedal characters and calcaneal features.
Classification
Below is a metatherian cladogram from Wilson et al. : 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 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
†Pascualdelphys fierroensis
†Progarzonia notostylopense Ameghino 1904
†Protalphadon Cifelli 1990
*†P. lulli 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
Evolutionary history
The relationships between the three extant divisions of mammals 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. Around the end of the Triassic period, a Therapsid developed traits or characteristics that are diagnostic of the class Mammalia. This class gave rise to Multituberculata, Triconodonta and Symmetrodonta. However, these are not seen after the end of the Early Cretaceous and by the Late Cretaceous marsupials and placentals had evolved from a common eupantotherian ancestor. The Mammalia class probably saw its first eutherian in the early CretaceousJehol biota in China called Acristatherium yanesis. This eutherian was determined to be the most basal based on a phylogenetic analysis that used a data matrix of many other species. Metatherians probably evolved to take advantage of open arboreal niches. Adaptive radiation of marsupials excluded competition with their terrestrial placental counterparts. Fossil metatherians are distinguished from eutherians by the form of their teeth: metatherians possess four pairs of molar teeth in each jaw, whereas eutherian mammals never have more than three pairs. Using this criterion, the earliest known metatherian is Sinodelphys szalayi, which lived in China around 125 million years ago. This 2003 study presents a new fossil from the early Cretaceous Yixian formation in China called Sinodelphys szalayi that gives enough morphological data to possibly be a basal metatherian in its didelphid-like morphology; it shares derived traits, such as dental formation and wrist and ankle structures. The fossil is about 125 million years old, making it one of the oldest metatherian fossils found and gives support to the claim that Asia was probably the center for diversification during the early Cretaceous. The researchers hypothesize that the divergence of Metatheria from Eutheria occurred in Asia no later than 125 million years ago, followed by the evolution of deltatheroidian-like taxa in Asia and North America about 120–100 million years ago and then the Paleocene diversification of relatives to the crown marsupials in South America. This makes it a contemporary to some early eutherian species that have been found in the same area. However, Bi et al. reinterpreted Sinodelphys as an early member of Eutheria. They state that the oldest known metatherians are now the 110 million years old fossils from western North America. The oldest metatherian fossils are found in present-day China. About 100 mya, the supercontinent Pangaea was in the process of splitting into the northern continent Laurasia and the southern continent Gondwana, with what would become China and Australia already separated by the Tethys Ocean. From there, metatherians spread westward into modern North America, where the earliest true marsupials are found. It is difficult to identify which fossils are marsupials, as they are characterized by aspects of the reproductive system that do not normally fossilize and by subtle changes in the bone and tooth structure that show a metatherian is part of the marsupialcrown group. The earliest definite marsupial fossil belongs to the species Peradectes minor, from the Paleocene of Montana, dated to about 65 million years ago. From this point of origin in Laurasia, marsupials spread to South America, which was connected to North America until around 65 mya. Laurasian marsupials eventually died off; traditionally this has been assumed to be due to competition with placental mammals, but generally this is no longer considered to be the case, as metatherian diversity doesn't seem to be correlated to placental diversity. Indeed, it appears metatherians suffered the heaviest mammalian casualties in the KT event, taking longer to recover than other groups. In Laurasian landmasses, herpetotheriids and peradectids remained alive until the mid to late Miocene, with the peradectids Siamoperadectes and Sinoperadectes being the youngest Laurasian metatherians.
