Metatheria is a mammalian
clade that includes all
mammals more closely related to
marsupial
Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsupials are endemic to Australasia, Wallacea and the Americas. A distinctive characteristic common to most of these species is that the young are carried in a ...
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 distinguishe ...
s. First proposed by
Thomas Henry Huxley
Thomas Henry Huxley (4 May 1825 – 29 June 1895) was an English biologist and anthropologist specialising in comparative anatomy. He has become known as "Darwin's Bulldog" for his advocacy of Charles Darwin's theory of evolution.
The stori ...
in 1880, it is a 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:
#
monotremes
Monotremes () are prototherian mammals of the order Monotremata. They are one of the three groups of living mammals, along with placentals (Eutheria), and marsupials ( Metatheria). Monotremes are typified by structural differences in their br ...
: egg laying mammals like the
platypus
The platypus (''Ornithorhynchus anatinus''), sometimes referred to as the duck-billed platypus, is a semiaquatic, egg-laying mammal endemic to eastern Australia, including Tasmania. The platypus is the sole living representative or mono ...
and the
echidna
Echidnas (), sometimes known as spiny anteaters, are quill-covered monotremes (egg-laying mammals) belonging to the family Tachyglossidae . The four extant species of echidnas and the platypus are the only living mammals that lay eggs and the ...
,
#metatheria: marsupials, which includes three American orders (
Didelphimorphia,
Paucituberculata
Paucituberculata is an order of South American marsupials. Although currently represented only by the seven living species of shrew opossums, this order was formerly much more diverse, with more than 60 extinct species named from the fossil rec ...
and
Microbiotheria
Microbiotheria is an australidelphian marsupial order that encompasses two families, Microbiotheriidae and Woodburnodontidae, and is represented by only one extant species, the monito del monte, and a number of extinct species known from fossi ...
) and four Australasian orders (
Notoryctemorphia,
Dasyuromorphia
Dasyuromorphia (, meaning "hairy tail" in Greek) is an order comprising most of the Australian carnivorous marsupials, including quolls, dunnarts, the numbat, the Tasmanian devil, and the thylacine. In Australia, the exceptions include the omn ...
,
Peramelemorphia
The order Peramelemorphia includes the bandicoots and bilbies; it equates approximately to the mainstream of marsupial omnivores. All members of the order are endemic to the twin land masses of Australia-New Guinea and most have the character ...
and
Diprotodontia
Diprotodontia (, from Greek "two forward teeth") is the largest extant order of marsupials, with about 155 species, including the kangaroos, wallabies, possums, koala, wombats, and many others. Extinct diprotodonts include the hippopotamus-sized ...
), and the
#
eutherians: placental mammals, consisting of four superorders divided into 21 orders.
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
Dentition pertains to the development of teeth and their arrangement in the mouth. In particular, it is the characteristic arrangement, kind, and number of teeth in a given species at a given age. That is, the number, type, and morpho-physiolog ...
, which includes about five upper and four lower incisors, a canine, three premolars, and four molars.
Other characters include skeletal and anterior dentition, such as wrist and ankle
apomorphies
In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to have ...
; all metatherians share derived pedal characters and
calcaneal features. The earliest known members of the group are from the latter half of the
Early Cretaceous in North America. Remains of metatherians have been found on all continents.
Classification
Below is a metatherian
cladogram
A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to ...
from Wilson et al. (2016):
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 Kirsch & Palma 1995">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
Evolutionary history
The relationships between the three extant divisions of mammals (
monotremes, marsupials, and
placental mammal
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 distinguishe ...
s) 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.
Fossil metatherians are distinguished from
eutheria
Eutheria (; from Greek , 'good, right' and , 'beast'; ) is the clade consisting of all therian mammals that are more closely related to placentals than to marsupials.
Eutherians are distinguished from noneutherians by various phenotypic tra ...
ns 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.
Using this criterion, the earliest known metatherian was formerly considered to be ''
Sinodelphys szalayi
''Sinodelphys'' is an extinct eutherian 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 ...
'', which lived in
China 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
Eutheria (; from Greek , 'good, right' and , 'beast'; ) is the clade consisting of all therian mammals that are more closely related to placentals than to marsupials.
Eutherians are distinguished from noneutherians by various phenotypic tra ...
. 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. Metatherians underwent a severe decline during the
K-Pg extinction event, more severe than that suffered by contemporary eutherians and
multituberculates
Multituberculata (commonly known as multituberculates, named for the multiple tubercles of their teeth) is an extinct order of rodent-like mammals with a fossil record spanning over 130 million years. They first appeared in the Middle Jurassic, a ...
, and were slower to recover diversity.
Morphological and species diversity of metatherians in
Laurasia 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
''Herpetotherium'' is an extinct genus of metatherian mammal, belonging to the possibly paraphyletic family Herpetotheriidae. Native to North America from the Eocene to Early Miocene, fossils have been found in California, Oregon, Texas, Florida ...
'', the European herpetotheriid ''
Amphiperatherium'' and the peradectids ''
Siamoperadectes'' and ''
Sinoperadectes'' from Asia being the youngest Laurasian metatherians.
Metatherians first arrived in Afro-Arabia during the
Paleogene, probably from Europe, including the possible peradectoid ''
Kasserinotherium'' from the Early Eocene of Tunisia and the herpetotheriid ''
Peratherium africanum'' from the Early
Oligocene of Egypt and Oman. The youngest African metatherian is the possible herpetotheriid ''
Morotodon'' from the late Early Miocene of Uganda.
Metatherians arrived in South America from North America during the
Paleocene
The Paleocene, ( ) or Palaeocene, is a geological epoch that lasted from about 66 to 56 million years ago (mya). It is the first epoch of the Paleogene Period in the modern Cenozoic Era. The name is a combination of the Ancient Greek ''pal ...
and underwent a major diversificiation, with South American metatherians including both the ancestors of extant marsupials as well as the extinct
Sparassodonta, which were major predators in South American ecosystems during most of the
Cenozoic, up until their extinction in the
Pliocene
The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58[Polydolopimorphia
Polydolopimorphia is an extinct order of metatherians, more closely related to extant marsupials than other extinct mammals. Known from the Paleocene-Pliocene of South America and the Eocene of Antarctica, they were a diverse group during the P ...](_blank)
, which likely had a wide range of diets.
The oldest known Australian marsupials are from the early Eocene, and are thought to have arrived in the region after having dispersed from Antarctica. The only known Antarctic metatherians are from the Early Eocene
La Meseta Formation of the
Antarctic Peninsula, where they are the most diverse group of mammals, and include marsupials as well as polydolopimorphians.
References
{{-
Marsupials of Central America
Marsupials of South America
Mammals of North America
Early Cretaceous mammals
Cretaceous mammals
Paleogene mammals
Neogene mammals
Quaternary mammals
Prehistoric marsupials
Extant Early Cretaceous first appearances
Mammal unranked clades