Metatheria is a mammalian
clade
A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...
that includes all
mammal
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
s 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.
There are three extant subclasses of mammals, one being metatherians:
#
monotremes: egg laying mammals like the
platypus and the
echidna,
#metatheria: marsupials, which includes three American orders (
Didelphimorphia,
Paucituberculata and
Microbiotheria) and four Australasian orders (
Notoryctemorphia,
Dasyuromorphia,
Peramelemorphia and
Diprotodontia), and the
#
eutherians
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 trai ...
: 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, 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; all metatherians share derived pedal characters and
calcaneal
In humans and many other primates, the calcaneus (; from the Latin ''calcaneus'' or ''calcaneum'', meaning heel) or heel bone is a bone of the tarsus of the foot which constitutes the heel. In some other animals, it is the point of the hock.
St ...
features. The earliest known members of the group are from the latter half of the
Early Cretaceous
The Early Cretaceous ( geochronological name) or the Lower Cretaceous (chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 145 Ma to 100.5 Ma.
Geology
Pro ...
in North America. Remains of metatherians have been found on all continents.
Classification
Below is a metatherian
cladogram 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 (
monotreme
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 brain ...
s, marsupials, and
placental 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.
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.
Using this criterion, the earliest known metatherian was formerly considered to be ''
Sinodelphys szalayi'', which lived in
China
China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's most populous country, with a population exceeding 1.4 billion, slightly ahead of India. China spans the equivalent of five time zones and ...
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. 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 and Marsupialiformes. Metatherians underwent a severe decline during the
K-Pg extinction event, more severe than that suffered by contemporary eutherians and
multituberculates, and were slower to recover diversity.
Morphological and species diversity of metatherians in
Laurasia
Laurasia () was the more northern of two large landmasses that formed part of the Pangaea supercontinent from around ( Mya), the other being Gondwana. It separated from Gondwana (beginning in the late Triassic period) during the breakup of Pan ...
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 before becoming extinct, with the North American herpetotheriid ''
Herpetotherium'', the European herpetotheriid ''
Amphiperatherium
''Amphiperatherium'' is an extinct genus of metatherian mammal, closely related to marsupials. It ranged from the Early Eocene to the Middle Miocene in Europe. It is the most recent metatherian known from the continent.
Description
Like modern ...
'' and the peradectids ''
Siamoperadectes
''Siamoperadectes'' is a genus of non-marsupial metatherian from the Miocene of Thailand. A member of Peradectidae, it is the first member of its clade known from South Asia, and among the last non-marsupial metatherians.
Description
The type ...
'' and ''
Sinoperadectes'' from Asia being the youngest Laurasian metatherians.
Metatherians first arrived in Afro-Arabia during the
Paleogene
The Paleogene ( ; British English, also spelled Palaeogene or Palæogene; informally Lower Tertiary or Early Tertiary) is a geologic period, geologic period and system that spans 43 million years from the end of the Cretaceous Period million yea ...
, probably from Europe, including the possible peradectoid ''
Kasserinotherium'' from the Early Eocene of Tunisia and the herpetotheriid ''
Peratherium africanum
''Peratherium'' is a genus of metatherian mammals in the family Herpetotheriidae that lived in Europe and Africa from the Early Eocene
In the geologic timescale the Ypresian is the oldest age or lowest stratigraphic stage of the Eocene. It sp ...
'' from the Early
Oligocene
The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but the ...
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 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
The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configura ...
, up until their extinction in the
Pliocene, as well as the
Polydolopimorphia, 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
The Antarctic Peninsula, known as O'Higgins Land in Chile and Tierra de San Martín in Argentina, and originally as Graham Land in the United Kingdom and the Palmer Peninsula in the United States, is the northernmost part of mainland Antarctic ...
, 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