Therians give birth(seeViviparity) to live young without a shelledegg. This is possible thanks to key proteins calledsyncytins which allow exchanges between the mother and its offspring through aplacenta, evenrudimental ones such as in marsupials. Genetic studies have suggested a viral origin of syncytins through theendogenization process.[3]
The marsupials and the placentals evolved from a common therian ancestor that gave live birth by suppressing the mother'simmune system. While the marsupials continued to give birth to an underdevelopedfetus after a shortpregnancy, the ancestors of placentals gradually evolved a prolonged pregnancy.[4]
The exit openings of theurogenital system and the rectal opening (anus) are separated.
Therians no longer have thecoracoid bone, unlike their cousins,monotremes.
Pinnae (external ears) are also a distinctive trait that is a therian exclusivity, though some therians, such as theearless seals, have lost them secondarily.[5]
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.[6]
The earliest known therian mammal fossil isJuramaia, from China's Late Jurassic (Oxfordian stage). However, the age estimates of the site are disputed based on the geological complexity and the geographically widespread nature of the Tiaojishan Formations.[9][10] Further, King and Beck in 2020 argue for an Early Cretaceous age forJuramaia sinensis, in line with similar early mammaliaformes.[11]
A 2022 eview of the Southern Hemisphere Mesozoic mammal fossil record has argued thattribosphenic 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.[12]
Molecular data suggests that therians may have originated even earlier, during the Early Jurassic.[13] Therian mammals began to diversify 10–20 million years before the dinosaur extinction.[14]
The rank of "Theria" may vary depending on the classification system used. The textbook classification system by Vaughan et al. (2000)[15] gives the following:
In the above system, Theria is a subclass. Alternatively, in the system proposed byMcKenna and Bell (1997)[16] it is ranked as a supercohort under the subclass Theriiformes:
^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.
^Vaughan, Terry A., James M. Ryan, and Nicholas J. Czaplewski. 2000.Mammalogy: Fourth Edition. Saunders College Publishing, 565 pp.ISBN0-03-025034-X
^McKenna, Malcolm C., and Bell, Susan K. 1997.Classification of Mammals Above the Species Level. Columbia University Press, New York, 631 pp. ISBN0-231-11013-8
^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.
