Marsupials

Perhaps because we have so only one species of marsupial in North America, the opossum being our sole representative of this interesting group of animals, I have always been fascinated by marsupials. The recent Olympics in Sydney, Australia have piqued my interest in learning more about these pouched mammals.

The earliest fossil records of marsupials go back to the Middle Cretaceous Period (140 to 65 million years ago). In the Cenozoic Era (65 million years ago to present), they radiated into a wide range of ecological niches. Marsupials developed insectivorous (insect eating), herbivorous (plant eating), and carnivorous (meat eating) forms similar to that of placental animals. The kangaroos, which have no equivalent in the placental world, also appeared during this time.

The marsupial first appeared in North or South America, and one group migrated to Australia via Antarctica (which was much warmer then than it is today). Another group of the American marsupials migrated to Europe by way of North America during the early Eocene Epoch (55 to 36 million years ago), then into Africa and Asia during the Oligocene Epoch (36 to 22.5 million years ago). See for more information about the relative positions of the continents.

The North American marsupial that moved into the Old World (Europe, Africa, and Asia) are known as the Didelphids, and today are represented by the opossums. They are the earliest known marsupials. The didelphids became extinct in North America during the Early Miocene Epoch (22.5 to 5 million years ago), and had become extinct in Europe by the Middle Miocene Epoch. About three million years ago during the Pliocene Epoch (5 million years ago to present), the Virginia Opossum returned to North America from South America via the newly-formed Panama land bridge.

The return of the opossum to North America was, however, unfortunately accompanied by the invasion of South America by placental mammals from North America. The reproductive techniques employed by placental mammals allowed them to out-compete the placental mammals.

In contrast, the relative isolation of Australia reduced the ammount of competition from placental mammals (although this is still a factor in the reduction in numbers of marsupial mammals). In addition, Australia experienced climate changes during the Cenozoic Era, caused by the continent gradually moving northward through Continental Drift. The movement and accompanying climate changes were relatively rapid, changing from temperate to tropical in a few tens of millions of years. This forced the marsupials to change in order to keep up with the climate changes, and many biologists believe that this kept the Australian marsupials genetically "strong". While the relatively stationary position and climate of South America did not force the South American marsupials to change as rapidly.

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