The evolution of the Australian marsupials.

Tne oldest marsupial fossils have been found in North America and the marsupials are thought to have originated there in the early Cretaceous. The marsupial eutherian ancestor could have had either a marsupial type of viviparity or may have been oviparous. Fossil evidence of early eutherians indicates that the common ancestor may have had a primitive marsupial type of reproduction . The marsupials are thought to have come to Australia from South America via Antarctica and to have radiated in Australia in isolation, with no eutherian competition. The Australian radiation was monophyletic and followed three broad lines of evolution; the polyprotodont, didactylous Marsupicarnivore; the polyprotodont, syndactylous Peramelina; the diprotodont; syndatctylous Diprotodonta.