Marsupials became extinct in North America by the end of the Oligocene, although they continued to do well in South America, which by that time had become separated from North America by an ocean channel (as a result of the ancient supercontinent of Pangea breaking up).  The mammalian carnivore niches in South America were filled entirely by marsupials of genera such as Borhyaena and Thylacosmilus, the latter being an excellent example of a "marsupial saber tooth cat".  North America's present-day opossum (Didelphis virginiana) is not descended from the continent's ancient marsupial stock, but is an immigrant from further south that arrived during the Pleistocene Epoch.  Even though the geographical and chronological relationship between the marsupials of Europe and the New World appears to now be fairly well understood, the relationship of the Australian marsupials, which are today isolated by wide ocean expanses from the other continents, was quite problematic to explain prior to the discovery that the Earth's continents had drifted over many millions of years.

. Skull of Thylacosmilus, the famous marsupial saber tooth predator from the Miocene of Argentina.

    Because the only currently convenient pathway to Australia is Asia, it was long assumed the the marsupial fauna of Australia had come from North America via Asia and then "island-hopped" through Indonesia, probably during the Cretaceous Period.  For some mysterious reason the placentals failed to follow, and the great southern land mass became a sanctuary for the marsupials, which were free to evolve in isolation from them.  A theory such as this, however, has some problems.  There is a distinct faunal break (Wallace's Line) running along the deep channel between Java and Kalimantan (Borneo) to the north-west (the Oriental realm) and Sulawesi (Celebes), Irian (western New Guinea) and Australia (the Australasian realm) to the south-east.
 

. A Tasmanian Devil (Sarcophilus harrisii) with two young.  This species is the largest of the living dasyurids.  Unlike the arboreal quolls, it is a ground dweller.  An opportunistic scavenger, Sarcophilus will eat virtually anything edible, but is primarily carnivorous.

Marsupials are present on many of the islands that lie between Wallace's line and Australia, but because there are far fewer species there than in Australia itself, it can be inferred that colonization had been from the Australian mainland to the islands, rather than vice versa.  Placental mammals, however, are present only on the western-most islands, with little overlap of the marsupial region.  This suggests that they spread from the Asian land mass.     Studies of continental drift have provided further support to suggest that Asia was not the source of the Australian marsupial fauna.

   In the mid-20th century, geological studies of plate tectonics showed that there is an Indo-Australian plate that has Wallace's Line as a boundary, resulting in a large drop in the sea floor at precisely the same point. This means that it has never been possible for a land bridge to form in the region, hence the zoological distribution.

    By the mid 1920s, L. Harrison had already put forward the idea that the Australian marsupials had reached that continent by crossing over land connections through Antarctica.  However, at the time, continental drift was such a new and controversial concept that his idea was not readily accepted by the scientific community.
 

As an understanding of forces such as ocean floor spreading and lateral movements of the continents developed, the new science of plate tectonics revolutionized the geological field, and with it came a better understanding of zoogeography through Earth's history.  Studies indicate that until Early Jurassic times (approx. 200 million years ago), the continents that we know today were unified as a single, enormous land mass - "Pangea".  Soon afterward, it divided into a northern half - Laurasia (which included what would become North America, Greenland, Europe and Asia; and a southern half - Gondwana (which included the land that is now South America, Africa, India, Antarctica and Australia.

. The Earth as it was some 210 million years ago, in the age of Pangea.