New World Opossums: Didelphimorphia

"NEW WORLD" MARSUPIALS?

When you hear or read the word "marsupial," you probably think of kangaroos, koalas, and Australia. Maybe you think of New Guinea, the big tropical island just north of Australia, and its hordes of tree kangaroos and other marsupial types, or the Virginia opossum, the only wild marsupial in North America north of Mexico. South and Central America might not even come to mind, but an extraordinary seventy-five species of marsupial mammals live today on those landmasses, from the deserts of northern Mexico through the forests of Central and South America, and across the grasslands of Patagonia, almost to the southern tip of South America. How did they get there, on the other side of the Pacific Ocean from Australia?

Eighty million years ago, small, early mammals, including marsupials, were flourishing. Today's southern continents were united in a supercontinent called Gondwana, which split from the northern supercontinent, Laurasia, made up of the present-day northern continents, around 160 million years ago. The two giant continents continued to split apart into the continents of the present day. The southern continents of Australia, Antarctica, and South America remained joined into a great landmass that allowed early animals to wander freely back and forth across the landmass. Ninety million years ago, Antarctica separated from South America, isolating South America (which had lost its connection with Laurasia 160 million years ago), and isolating the ancestors of the Australian marsupials and monotremes in what would become the present-day island continent of Australia and its large satellite island, New Guinea. South America, like Australasia (Australia and nearby islands), became a continent-sized refuge for early marsupial types, although these would be sharing the continent with placental mammals. By forty million years ago, marsupials had become extinct in North America, Africa, Asia, and Antarctica but flourished in Australasia and South America, where they continued to evolve and diversify.