12 June 2005

Garbage can zoo: Virginia opossum

We found this opossum (Didelphis virginiana) sleeping in our garbage can one day last March. Even after I tipped the can over it stayed in it until after it got dark.

The Virginia opossum is the only marsupial mammal in North America. The female opossum has a pouch like that of a kangaroo. After the baby opossums are born, they climb into their mother's pouch and spend about two months feeding on her milk. Then they move out of the pouch to their mother's back where they spend four to six weeks.

In contrast to North America's only one marsupial, Australia has more than 200 marsupial species. Among these are the kangaroo, koala, Tasmanian devil and the wombat. Marsupials also live in South America and in New Guinea, north of Australia.

Map was from here.

Despite the fact that Australia today has more marsupial species than any other continent, the oldest marsupial fossils have been found in North America and China. This suggests that marsupials first originated on a northern continent and then spread to the southern continents.

Until about 120 million years ago there were two supercontinents. The one in the north, called Laurasia, consisted of what are now North America, Greenland, Europe, and Asia. The one in the south, called Gondwana, consisted of what are now South America, Africa, Australia, India, and Antarctica. The first marsupials must have evolved on Laurasia and spread south to Gondwana as the two supercontinets were separating from each other and breaking apart into smaller continents. This theory predicts that marsupials once lived on Antarctica when that continent wasn't covered with ice as it is now and when South America, Antarctica and Australia were directly connected to each other. Marsupial fossils have indeed been found in Antarctica.

Interestingly, all the original marsupials in North America became extinct about 40 million years ago. The Virginia opossum migrated to North America from South America about 4 million years ago.

Follow these links for more information on opossums and other marsupials:
Virginia opossum
Virginia opossum
marsupial evolution and continental drift
marsupial evolution and continental drift

Cross-posted at Transitions: The Evolution of Life

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