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What is Theria?
Theria are all modern mammals and could possibly include monotremes.
There were once a few groups of mammals; most notably Theria and Allotheria. After quite a few mass extinctions, Allotheria and all the other groups went extinct. The theria were the only ones that survive. Theria include placentals and marsupials. There is debate over whether monotremes are true Theria, or a cross between Theria and Allotheria.