Millions of years before humans set foot in the Americas, a rush of alien animals began arriving in South America.
As the Isthmus of Panama came up from the waves, bridging the North and South American continents, llamas, raccoons, wolves, bears and many other species headed south. At the same time, the ancestors of armadillos, possums and porcupines headed north.
Paleontologists call the event the Great American Interchange. But they’ve long been puzzled by one aspect of it: Why did the majority of mammal 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 think they have an answer: a nasty extinction event struck South American mammals during the interchange, leaving fewer of them available to head north. Their research was published Monday in the Proceedings of the National Academy of