Here's a breakdown of what happened:
* Early Separation: Marsupials and placentals diverged from a common ancestor around 160 million years ago (mya). At that time, the supercontinent Pangaea was breaking apart. Marsupials, originating in what is now North America, were pushed to the southern continents (South America, Antarctica, and Australia). Placentals, on the other hand, remained in North America and eventually spread to Eurasia and Africa.
* Island Hopping: About 50-30 mya, the continents of South America and Australia separated, leaving marsupials in isolation in these regions. This separation allowed marsupials to diversify in isolation and develop their unique characteristics, such as pouches for raising young.
* Collision & Competition: South America, once isolated, eventually collided with North America around 3 mya. This brought placental mammals from North America into contact with South American marsupials. This contact led to a phenomenon known as the Great American Interchange, where a massive exchange of species occurred between the two continents. Some marsupials migrated northward while placental mammals spread southward.
* The Winners: The Great American Interchange had a dramatic impact on both continents. While some marsupials like the opossum made it to North America, many South American marsupials faced competition from the more diverse and often larger placentals. Placentals, having evolved in a wider variety of environments, were often better equipped to compete for resources and territory, leading to a decline in many South American marsupial species.
* Continuing Competition: In Australia, marsupials faced no such competition and continued to flourish, becoming the dominant mammalian group in the region. This isolation allowed them to evolve into a remarkable array of species, from kangaroos and koalas to wombats and Tasmanian devils.
In summary: The interaction between marsupials and placentals was a complex and competitive story. The separation of continents played a key role in shaping the evolution of both groups, and their subsequent collision led to a dynamic exchange of species that resulted in the success of placentals in South America and the remarkable diversity of marsupials in Australia.