Theria (; Greek: , wild beast) is a subclass of mammals amongst the Theriiformes. Theria includes 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 tr ...

(including the placental mammals) and the metatherians (including the

marsupials 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 po ...

) but excludes the egg-laying monotremes.

Characteristics

Therian mammals give birth (''see

viviparity Among animals, viviparity is development of the embryo inside the body of the parent. This is opposed to oviparity which is a reproductive mode in which females lay developing eggs that complete their development and hatch externally from the ...

'') to live young without a shelled egg. This is possible thanks to key proteins called syncytins which allow exchanges between the mother and its offspring through a

placenta The placenta is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas and waste exchange between the physically separate mate ...

, even rudimental ones such as in marsupials. Genetic studies have suggested a viral origin of syncytins through the endogenization process. The marsupials and the placental mammals evolved from a common therian ancestor that gave live birth by suppressing the mother's

immune system The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as cancer cells and objects such as wood splinte ...

. While the marsupials continued to give birth to an underdeveloped

fetus A fetus or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring that develops from an animal embryo. Following embryonic development the fetal stage of development takes place. In human prenatal development, fetal dev ...

after a short pregnancy, the ancestors of placental mammals gradually evolved a prolonged pregnancy. Therian mammals no longer have the coracoid bone, contrary to their cousins, monotremes. Pinnae (external ears) are also a distinctive trait that is a therian exclusivity, though some therians, such as the earless seals, have lost them secondarily.

Evolution

The earliest known therian mammal fossil is '' Juramaia'', from the Late Jurassic ( Oxfordian stage) of China. However, molecular data suggests that therians may have originated even earlier, during the Early Jurassic. Therian mammals began to diversify 10-20 million years before dinosaur extinction.

Taxonomy

The rank of "Theria" may vary depending on the classification system used. The textbook classification system by Vaughan et al. (2000) gives the following: In the above system Theria is a subclass. Alternatively, in the system proposed by McKenna and Bell (1997) it is ranked as a supercohort under the subclass Theriiformes: Another classification proposed by Luo et al. (2002)Luo, Z.-X., Z. Kielan-Jaworowska, and R. L. Cifelli. 2002. In quest for a phylogeny of Mesozoic mammals. Acta Palaeontologica Polonica, 47:1-78. does not assign any rank to the taxonomic levels, but uses a purely cladistic system instead.

References

External links

Theria — supercohort — Tree of Life
Mammal taxonomy Extant Middle Jurassic first appearances Taxa described in 1897 Taxa named by William Aitcheson Haswell {{mammal-stub

Characteristics

Evolution

Taxonomy

See also

References

External links