CharacteristicsTherian mammals give birth to live young without a shelled egg (biology), egg. This is possible thanks to key proteins called Syncytin-1, syncytins which allow exchanges between the mother and its offspring through a placenta, even Marsupial#Reproductive system, rudimental ones such as in marsupials. Genetic studies have suggested a viral origin of syncytins through the Endogenous retrovirus, 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. While the marsupials continued to give birth to an underdeveloped fetus 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. Pinna (anatomy), 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.
EvolutionThe earliest known therian mammal fossil is ''Juramaia'', from the Late Jurassic (Oxfordian (stage), Oxfordian stage) of China. However, molecular data suggests that therians may have originated even earlier, during the Early Jurassic.
TaxonomyThe 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 Cladistics, cladistic system instead.
See also* Marsupialia, Marsupials * Marsupionta * Monotremes * Placentalia, Placental mammals