Theria

Theria (/ˈθɪəriə/; Greek: θηρίον, wild beast) is a subclass of mammals that give birth to live young without using a shelled egg, consisting of the eutherians (including the placental mammals) and the metatherians (including the marsupials). The only omitted extant mammal group is the egg-laying monotremes.

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.

Taxonomy

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:

Another classification proposed by Luo et al. (2002) does not assign any rank to the taxonomic levels, but uses a purely cladistic system instead.