Throughout most of the Permian, Earth's land masses had merged into the mega-continent of Pangea. Then, in the Triassic (about 200 million years ago), the Tethys Sea began to form, rifting the northern Continents (Laurasia) from the southern Continents (Gondwana); the latter included South America, Africa, Antarctica and Australia. About 160 million years ago, the northern Atlantic started to open, dividing future North America from Eurasia, and Madagascar broke from Africa, drifting south to join the Antarctica-India land mass.
Marsupials arose in North America during the Cretaceous Period, some 100 million years ago; other mammals had become eutherians (placental mammals) and were beginning to colonize the northern Continents. Just before marsupials spread southward and eastward through Gondwana, the southern Atlantic split Africa from South America and Madagascar rifted from Antarctica, eventually docking off the coast of Africa. About 90 million years ago, India broke from Antarctica, heading toward its collision with southern Asia; South America drifted into isolation 75 million years ago and Australia followed suite 20 million years later.
Marsupials had spread through South America, Antarctica, India and Australia before these latter rifts developed. Those that remained in North America were rapidly displaced by eutherians and, when India merged with Asia, 60 million years ago, the same occurred in that region. South America's marsupials managed to hold on during its prolonged isolation but those in Antarctica succumbed to the climate as that Continent drifted to the South Pole; only those in Australia, free from the competition of eutherians, thrived and diversified. Finally, after Panama lodged between North and South America, some 3 million years ago, the Virginia opossum, our Continent's lone marsupial, spread back from the south.