The Complex Evolution of Theria
Most scientists agree that Theria evolved from therapsids after the Permian-Triassic Extinction and the Triassic-Jurassic extinction (1)(2). After the Jurassic-Cretaceous Extinction is where the theories and debating begin.
According to Reference 3, there are to subclasses of mammals, the Theria and Prototheria, and that monotremes are Prototheria and marsupials and placentals are Theria (3). However, as stated in the previous article, monotremes might be Therians, a cross between Allotheria and Theria, or simply Prototherians (1)(2)(3). If any one of them is true, it would be significant. However, it can be agreed the either all or nearly all mammals are and have been Therians (1)(2)(3), and that 94% of mammals are Placental Therians (3).