Theria
| Therians Temporal range: Middle Jurassic – Holocene, 160–0 Ma |
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| A human (placental) holding Koalas (marsupials), representing the two surviving groups of therian mammals. | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Clade: | Tribosphenida |
| Subclass: |
Theria Parker & Haswell, 1897 |
| Infraclasses | |
Theria (/ˈθɪəriə/; Greek: θηρίον, wild beast) is a subclass of mammals amongst the Theriiformes (the sister taxa to Yinotheria). Theria includes the eutherians (including the placental mammals) and the metatherians (including the marsupials).
Therian mammals give birth to live young without using a shelled egg. It is possible thanks to key proteins called syncytins, which allow exchanges between the mother and its offspring through a placenta even rudimental such as the marsupial ones. Genetic studies have enlighted the viral origin of syncytins through endogenization process.
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.
The earliest known therian mammal fossil is Juramaia, from the Middle Jurassic 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:
Class Mammalia
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:
Class Mammalia
Another classification proposed by Luo et al. (2002) does not assign any rank to the taxonomic levels, but uses a purely cladistic system instead.
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