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Nearly Half of South America’s Mammals Came from North America, New Research May Explain Why | Smart News

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North and South America haven’t always been connected. South America functioned as a continent-sized island for millions of years following the extinction of the dinosaurs, incubating its own strange assembly of animals such as giant ground sloths, massive armored mammals akin to armadillos and saber-toothed marsupial carnivores. Meanwhile, North America was exchanging animals with Asia, populating it with the ancestors of modern horses, camels and cats, writes Asher Elbein for the New York Times.

Finally, when tectonic activity formed the Isthmus of Panama roughly ten million years ago, a massive biological exchange took place. The many species that had been evolving in isolation from one another on both continents began migrating across the narrow new land bridge. Llamas, raccoons, wolves and bears trekked south, while armadillos, possums and porcupines went north.

It would be reasonable to expect this grand biological and geological event, known to paleontologists as the Great American Biotic Interchange, resulted in equal numbers of northern and southern species spreading across the two land masses; but that’s not what happened.

Instead, many more North American mammal species made homes down south than the other way around. Almost half of living South American mammals have North American evolutionary roots, whereas only around ten percent of North American mammals once hailed from South America. Now, researchers who reviewed some 20,000 fossils may have an answer, according to the Times.

According to the paper, published this week in the journal the Proceedings of the National Academy of Sciences, the asymmetry of immigrant mammal diversity we see today was the result of droves of South American mammals going extinct, leaving gaping ecological holes waiting to be filled by northern species and reducing the pool of potential immigrant species to make the trek north, reports Christine Janis, an ecologist at