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 more inclusive group than the marsupials; it contains all marsupials as well as many extinct non-marsupial relatives.
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.[6] Other characters include skeletal and anterior dentition, such as wrist and ankle apomorphies; all metatherians share derived pedal characters and calcaneal features.
The relationships between the three extant divisions of mammals (monotremes, marsupials, and placental mammals) was long a matter of debate among taxonomists.[8] 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.[9]
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 (including true placentals) never have more than three pairs.[10] Using this criterion, the earliest known metatherian was formerly considered to be Sinodelphys szalayi, which lived in China around 125 million years ago (mya).[11] This makes it a contemporary to some early eutherian species that have been found in the same area.[12] However, Bi et al. (2018) reinterpreted Sinodelphys as an early member of Eutheria.[3] They state that the oldest known metatherians are now the 110 million years old fossils from western North America.
The earliest definite marsupial fossil belongs to the species Peradectes minor, from the Paleocene of Montana, dated to about 65 million years ago.[1] From this point of origin in Laurasia, marsupials spread to South America, which was connected to North America until around 65 mya.[citation needed] 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.[13][14] Indeed, it appears metatherians suffered the heaviest mammalian casualties in the KT event, taking longer to recover than other groups.[15] 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.