What is important about plate tectonics? Australia? South America?
Australia was completely isolated 135 million years ago. All it had were monotremes and marsupials. Eutherians evolved later, and never made their way to Australia (Pangea split), until humans came and brought dogs, and rats. Also, notice that in the Cretaceous, North and South America were separated, and only joined when Panama connected them, about 5 million years ago. This resulted in the movement of Eutherians into S. America, and a mass extinction of marsupials. But there are still a lot of marsupials in S. America, and the opossum is one of the marsupials that migrated North, when Panama formed.