|A human (placental) holding two koalas, (marsupial), representing the two extant infraclasses of therian mammals.|
Parker & Haswell, 1897
Theria (; Greek: theríon, wild beast) is a subclass of mammals amongst the Theriiformes (the sister taxon to Yinotheria.) Theria includes the eutherians (including the placental mammals) and the metatherians (including the marsupials.) Therian mammals are the dominant group of terrestrial amniotes on Earth since replacing the non-avian dinosaurs due to the K-Pg extinction event.
Therian mammals give birth 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, 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. 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.
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.
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: