Metatheria
Encyclopedia
Metatheria is a grouping within the animal class Mammal
ia. First proposed by Thomas Henry Huxley in 1880, it is nearly synonymous with the earlier taxon Marsupial
ia (Illiger
, 1811) though it is slightly wider since it also contains the nearest fossil relatives of marsupial mammals.
The earliest known representative, Sinodelphys
, is from the Lower Cretaceous
of China
.
The closest relatives of the metatheres are the Eutheria
(also erected by Huxley in 1880). Both are conventionally united as infraclasses within the subclass Theria
(Parker and Haswell
, 1897), which contains all living mammals except monotreme
s.
During development, metatherians produce a yolk-sac placenta and give birth to "larval-like" offspring. These offspring have under-developed posterior limbs (the pes
can be webbed), and following birth they migrate to the marsupium where they attach to a nipple. The mouth of newly-born metatherians is fused laterally, but open medially; this forms an "O" shaped mouth in which the mother's nipple fits; it then swells to secure the offspring into place for further development and growth.
The combination of the Greek elements meta- and theria in this usage roughly means the "sort-of-beasts" or " behind-beasts", in contrast with Eutheria
("true-beasts").
Period, with such forms as Deltatheridium
and Asiatherium. Some stem metatherians persisted well into the Neogene
Period before becoming extinct. Examples of these include the borhyaenids and herpetotheriids. Crown
marsupial
s, the one branch of Metatheria that survives today, diversified close to the time of extinction at the end of the Cretaceous.
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
ia. First proposed by Thomas Henry Huxley in 1880, it is nearly synonymous with the earlier taxon Marsupial
Marsupial
Marsupials are an infraclass of mammals, characterized by giving birth to relatively undeveloped young. Close to 70% of the 334 extant species occur in Australia, New Guinea, and nearby islands, with the remaining 100 found in the Americas, primarily in South America, but with thirteen in Central...
ia (Illiger
Johann Karl Wilhelm Illiger
Johann Karl Wilhelm Illiger was a German entomologist and zoologist.Illiger was the son of a merchant in Brunswick. He studied under the entomologist Johann Hellwig, and later worked on the zoological collections of Johann Centurius Hoffmannsegg...
, 1811) though it is slightly wider since it also contains the nearest fossil relatives of marsupial mammals.
The earliest known representative, Sinodelphys
Sinodelphys
Sinodelphys is an extinct mammal from the Early Cretaceous. To date, it is the oldest metatherian fossil known, estimated to be 125 million years old...
, is from the Lower Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
of China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
.
The closest relatives of the metatheres are the Eutheria
Eutheria
Eutheria is a group of mammals consisting of placental mammals plus all extinct mammals that are more closely related to living placentals than to living marsupials . They are distinguished from noneutherians by various features of the feet, ankles, jaws and teeth...
(also erected by Huxley in 1880). Both are conventionally united as infraclasses within the subclass Theria
Theria
Theria is a subclass of mammals that give birth to live young without using a shelled egg, including both eutherians and metatherians . The only omitted extant mammal group is the egg-laying monotremes....
(Parker and Haswell
William Aitcheson Haswell
William Aitcheson Haswell was a Scottish-born Australian zoologist specialising in crustaceans, winner of the 1915 Clarke Medal.-Early life:...
, 1897), which contains all living mammals except monotreme
Monotreme
Monotremes are mammals that lay eggs instead of giving birth to live young like marsupials and placental mammals...
s.
During development, metatherians produce a yolk-sac placenta and give birth to "larval-like" offspring. These offspring have under-developed posterior limbs (the pes
Pes (zoology)
The pes is the zoological term for the distal portion of the hind limb of tetrapod animals. It is the part of the pentadactyl limb that includes the metatarsals and digits . During evolution, it has taken many forms and served a variety of functions...
can be webbed), and following birth they migrate to the marsupium where they attach to a nipple. The mouth of newly-born metatherians is fused laterally, but open medially; this forms an "O" shaped mouth in which the mother's nipple fits; it then swells to secure the offspring into place for further development and growth.
The combination of the Greek elements meta- and theria in this usage roughly means the "sort-of-beasts" or " behind-beasts", in contrast with Eutheria
Eutheria
Eutheria is a group of mammals consisting of placental mammals plus all extinct mammals that are more closely related to living placentals than to living marsupials . They are distinguished from noneutherians by various features of the feet, ankles, jaws and teeth...
("true-beasts").
Evolutionary history
Metatherians first appeared in the CretaceousCretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
Period, with such forms as Deltatheridium
Deltatheridium
Deltatheridium is an extinct species of metatherian. It lived in what is now Mongolia during the Upper Cretaceous. It was a basal metatherian, which places it near start of the linage that led to the marsupials, such the kangaroo, koala, and opossum.It had a length of...
and Asiatherium. Some stem metatherians persisted well into the Neogene
Neogene
The Neogene is a geologic period and system in the International Commission on Stratigraphy Geologic Timescale starting 23.03 ± 0.05 million years ago and ending 2.588 million years ago...
Period before becoming extinct. Examples of these include the borhyaenids and herpetotheriids. Crown
Crown group
A crown group is a group consisting of living representatives, their ancestors back to the most recent common ancestor of that group, and all of that ancestor's descendants. The name was given by Willi Hennig, the formulator of phylogenetic systematics, as a way of classifying living organisms...
marsupial
Marsupial
Marsupials are an infraclass of mammals, characterized by giving birth to relatively undeveloped young. Close to 70% of the 334 extant species occur in Australia, New Guinea, and nearby islands, with the remaining 100 found in the Americas, primarily in South America, but with thirteen in Central...
s, the one branch of Metatheria that survives today, diversified close to the time of extinction at the end of the Cretaceous.