Why marsupials are confined to only Australia and South America?
This is a common misconception. Their present distribution may be just limited two these southern continents, but if we crank back the geologic time a little further back, perhaps some 20-30 million years or so, we can see that marsupials were much more diverse in their geographic distribution. They existed in pretty much on all continents, including Antarctica.
The earliest marsupials (technically they are called metatherians, ancestors of modern marsupials) appear in the fossil record sometime late Jurassic (around 120 million years ago or so). The earliest record of these metatherians comes from China. This tiny little animal is called Sinodelphys (sino=china, delphys=relating to present day opossum).
Soon after we start seeing a diversification of the forms in Asia, and then North America. The earliest record of a metatherian comes from Oklahoma. Represented by a single upper tooth, and a fragment of a lower jaw, this animal is called Kokopellia. But the real diversification of the metatherians occured slightly after the infamous Cretaceous/Tertiary boundary. Paleocene (time period 65-58 million years ago) of Bolivia (and some other South American localities) a diversity of forms that are not represented by modern marsupials. We see a group called borhyaenidonts. This group of marsupials are hypercarnivorous and in Miocene (geologic time period between 26 to 6 million years ago) reached the size of large dog. In fact, they are once considered the now extinct Tasmanian Wolf. Other than the adaptation to carnivory borhyaenoids and thylacinids (Tasmanian wolf and its ancestors) are not related.
In the Paleocene and the following geologic epoch Eocene metatherians are represented by dozens of taxa everywhere from North America, Europe, North Africa, Middle East, Western and southeastern Asia.
Marsupials had a continuous presence in South America for at least last 70 million years. However, their fossil record in Australia and Antarctica does not extend that far. Current wisdom tells us that the ancestral forms of modern day marsupials were evolved in South America, and they dispersed to Antarctica sometime in the Eocene, where Antarctic had more northernly position, and wasn’t covered with ice caps. Then, eventually ancestral marsupials reached Australia through the Antarctic biogeographical link. Even though an south African link is also plausible, so far no fossil record of marsupials has been found in South Africa.
Interestingly, there is at least one back migration from Australia to South America. An interesting marsupial called Dromiciops (or locally known as Monito del Monte) is more related with the Australian marsupials then the modern South American opossums.
Hence, marsupials (and their ancestors) were not confined to southern continents only. But due to competition with placentals and local extinction their present day distribution is restricted.
