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What happened to the missing megamammals in South America?

Millions of years before humans set foot in the Americas, a wave of alien animals began arriving in South America.

As the Isthmus of Panama rose from the waves, connecting the North and South American continents, llamas, raccoons, wolves, bears and many other species headed south. At the same time, the ancestors of armadillos, opossums, and porcupines headed north.

Paleontologists call the event the Great American Interchange. But they have long been baffled by one aspect: why did most mammalian immigrants go south, rather than the other way around? What happened to the southern mammals?

After a detailed analysis of fossil data from both continents, a group of researchers thinks they have an answer: A nasty extinction event hit South American mammals during the exchange, leaving fewer available to head north. Their research was published Monday in the Proceedings of the National Academy of Sciences.

After the extinction of the dinosaurs and the gradual recovery of the Earth’s biosphere, there have been millions of years of animal trade between North America and Asia. During this time, the ancestors of modern horses, camels and cats crossed land bridges back and forth. But South America spent most of this time period, the Cenozoic, as an island continent, complete with its strange bestiary: giant sloths, bizarre native ungulates, tank-like armadillo relatives, and saber-toothed marsupial predators.

Then, 10 million years ago, a series of tectonic events gave rise to the Isthmus of Panama, connecting two very different faunas.

“This exchange was relatively balanced at first,” said Juan Carrillo, a colleague at the Smithsonian Tropical Research Institute in Panama and author of the study. “There was the same amount of mammals that migrated in both directions. But what we found is that five million years ago in the Pliocene there was a disproportionate decline in diversity. “

The source of that extinction event remains unclear, Dr. Carrillo said. The climate became drier in the Pliocene, repelling South American forests and initiating a global cooling cycle that eventually led to the ice ages. As habitats changed, South American ecosystems were likely subjected to considerable stress.

Competition from northern species likely also played a role, with northern predators such as dogs, bears and saber-toothed cats benefiting from relatively larger brains and more efficient teeth. Some of those southward migrants may have also brought parasites and diseases with them.

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