The placental mammals dominate
The mammalian body is a very complicated machine that takes a long time to develop. Even as an embryo it is warm blooded and burns up fuel very quickly. Both these characters demand that the developing young should be supplied with considerable quantities of food. All mammals have found methods of providing far more than could ever be packed within the confines of a shelled egg. We do not know whether the early mammals in the northern supercontinent ever passed through a marsupial stage before developing the placenta. It could be that they sprang from a branch of the mammal like reptiles that never acquired pouches. The placental and marsupial forms probably arose independently from a common ancestor, and they evolved side by side. Certainly the fossil record of the placentals is as ancient as that of the marsupials, and they arose sometime during the Cretaceous period. During the early stages of their evolutionary histories they were probably well matched, so that marsupial adaptations were about as efficient in evolutionary terms as placental adaptations. However, during the Cenozoic, the placental animals came to dominate in all areas of the world except the large island of Australia, which until the advent of many had never witnessed placental mammals. In Australia the marsupial animals achieved the sophisticated levels of efficiency occurring in the Red Kangaroo.