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Of Marsupials and Placentals

ISSUE:  Summer 2016

Why are there no giant tree-climbing, carnivorous kangaroos in the stately magnolias and maples of Central Park?

It’s a question of what might have been. To be fair, those lethal hoppers—called marsupial lions, less for their gross anatomy than for their sharp teeth—have been absent from Australia for 40,000 years, too. Gone are their giraffe-like cousins, which browsed on the branches of eucalyptus trees. Gone are the giant-fanged sparassodonts of South America. Gone is Thylacosmilus, the saber-toothed opossum, which weighed about as much as the average American male today—a lot, in other words.

In Latin, a marsupium is a large purse, like the messenger bag that a well-equipped hipster carries. Applied to a class of animals, it refers to a flap of skin that covers the nipples. Marsupials give birth to their young prematurely, with the newborn continuing to develop outside the womb, nursing within this pouch. This period of development corresponds to the last period of gestation in the placental mammals, which deliver their young fully formed thanks to the placenta, an organ that delivers oxygen and nutrients to the growing embryo.

There are other anatomical distinctions between marsupial and placental mammals. For one thing, female marsupials have twin vaginas that share a cloaca with the intestinal tracts. There are differences in dentition and metabolic rate. Marsupials lack the corpus callosum of placentals, the brain structure that joins the two halves of the cerebral cortex, and they are, well, less intelligent than placentals overall.

Marsupials were once thought to be different enough from placental mammals to warrant distinct classification, but now that cladistic approaches are common—using behavior and ecological setting to guide taxonomy—marsupials are considered an infraclass within Mammalia. Of the 335-odd known marsupial species, all but one is confined to the Southern Hemisphere—and therein lies a Darwinian tale.

Marsupials developed about 120 million years ago, in the southern supercontinent called Gondwana, which took in modern Australia, Antarctica, and South America. As those continents separated to take their present positions, marsupials went with them. They continued to develop in Australia, where the first Europeans found such bewildering animals as koalas, wallabies, and Tasmanian devils. The marsupials had tougher luck in South America, where, 12 million or so years ago, a land bridge rose to connect it to North America, opening the door to an invasion of carnivorous placental mammals that preyed on the marsupial fauna. No saber-toothed opossum could hope to stand up next to a real saber-toothed placental cat, and so they died. And as for arboreal kangaroos, forget about it.

One marsupial, however, headed in the opposite direction from the stream of predatory newcomers, the only one of its kind—its infraclass—to make its way to the Northern Hemisphere. Didelphis virginiana, the Virginia opossum, may not be very bright, but it outwitted extinction.


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