How did mammalian evolution occur; did marsupials evolve before placental mammals?
According to the fossil record, mammals evolved at around the same time as the dinosaurs, in the late Triassic, about 220 million years ago. The primitive mode of reproduction in mammals is of course egg laying, so egg laying mammals evolved at around the same time as dinosaurs in the late Triassic. Both marsupials and placentals evolved in the late Cretaceous, about 80-90 million years ago, not long before the end of Cretaceous extinction of the dinosaur. Marsupials and placentals are closely related. Since marsupials do not have a true placenta, they merely retain the eggs within the uterus so the embryo can absorb the yolk sac while it develops. Placentals probably evolved from a species of marsupial, although it is possible that living marsupials and placentals both evolved at around the same time from a primitive mammal that retained the eggs within the body. Living marsupials have many specializations, such as double vaginas and a dual chambered uterus, that are not found in plac
