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Theria ( or ; ) is a subclass of amongst the Theriiformes. Theria contains the (which includes ) and the (which includes ) but excludes the egg-laying and various extinct mammals evolving prior to the common ancestor of placentals and marsupials.
The marsupials and the placentals evolved from a common therian ancestor that gave live birth by suppressing the mother's immune system. While the marsupials continued to give birth to an underdeveloped fetus after a short pregnancy, the ancestors of placentals gradually evolved a prolonged pregnancy.
The exit openings of the urogenital system and the rectal opening (anus) are separated. The lead to the . Unlike , have no coracoid bone.
Pinnae (external ears) are also a distinctive trait that is a therian exclusivity, though some therians, such as the , have lost them secondarily. The flexible and protruding nose in therians is not found in any other vertebrates, and is the product of modified cells involved in the development of the upper jaw in other tetrapods. Almost all therians have whiskers.
The SRY gene is a protein in therians that helps initiate male sex determination.
A 2022 review of the Southern Hemisphere Mesozoic mammal fossil record has argued that Tribosphenida mammals arose in the Southern Hemisphere during the Early Jurassic, around 50 million years prior to the clade's earliest undisputed appearance in the Northern Hemisphere.
Molecular data suggests that therians may have originated even earlier, during the Early Jurassic. Hugall, A.F. et al. (2007) Calibration choice, rate smoothing, and the pattern of tetrapod diversification according to the long nuclear gene RAG-1. Syst Biol. 56(4):543–63. Therian mammals began to diversify 10–20 million years before the dinosaur extinction.
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In the above system, Theria is a subclass. Alternatively, in the system proposed by McKenna and Bell (1997)McKenna, Malcolm C., and Bell, Susan K. 1997. Classification of Mammals Above the Species Level. Columbia University Press, New York, 631 pp. it is ranked as a supercohort under the subclass Theriiformes:
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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.
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