Final answer:
Marsupials diversified in Australia due to the continent's long isolation following continental drift, which began with Pangea's separation. Lack of placental mammal competition allowed marsupial diversification in unique ways in Australia, with fossil records like Sinodelphys and Eomaia providing insights into their ancient evolution.
Step-by-step explanation:
The development and dispersal of marsupials are intimately linked to the historical process of continental drift. Originally, marsupials evolved about 130 million years ago when the supercontinent Pangea began to break apart. As the continents drifted, marsupials spread across the world, with different lineages forming in isolation.
In particular, the marsupials in Australia radiated into the vast array of species we see today due to the continent's long geographical isolation after it separated from Antarctica. This isolation led to a lack of competition from placental mammals, allowing marsupials to diversify into many different ecological niches. As endemic species, marsupials exhibit a great variety of forms that closely relate to the environmental features of Australia.
The closest relatives to Australian marsupials are found in South America, which also was part of the ancient supercontinent. The fossil record, including the discovery of species like Sinodelphys and Eomaia, helps in understanding the early evolution and diversity of marsupial and placental mammals. Research like the mammalian supertree provides evidence that marsupial evolution is much more ancient than the diversification seen in Australia, which only became prominent after the dinosaurs went extinct and marsupials found a refuge from placental competitors on the isolated continent.