Millions of years ago, North American mammals flooded South America after the two continents joined. But South American mammals failed to return the favor, and now scientists have an idea why.
A new analysis of fossils suggests that many native South American mammal groups were declining early in the continental coupling, leaving fewer species available to head north, researchers report October 5 in the Proceedings of the National Academy of Sciences.
More than 10 million years ago, as the Pacific tectonic plate slid under the South American and Caribbean plates, the Isthmus of Panama began to rise out of the ocean, bridging North and South America. Animals began to move between the continents, in a trickle at first and then in a massive wave after the isthmus had fully formed around 3 million years ago. This exchange, known as the Great American Biotic Interchange, had a major influence on the distribution of mammals in the Americas today.
South American mammals at the time of the event — having evolved for tens of millions of years on an island continent — were stupendously strange. Club-tailed armadillo relatives the size of small cars shuffled about (SN: 2/22/16). Vaguely camel-like and rhinolike herbivores grazed the landscape. Immense ground sloths shambled on land and even swam offshore.
“This interchange was relatively balanced at first,” says Juan Carrillo, a paleobiologist at the National Museum of Natural History in Paris. But eventually, the switcheroo became asymmetric, with many more mammals with North American origins showing up in the south than vice versa. In fact, most of South America’s extraordinary creatures never managed to move north and persist into the modern era.
That pattern persists to this day. Nearly half of modern South American mammal genera can trace their origins to North America, but only 10 percent of northern mammals (excluding those in Central America) are descended from South American migrants. We now think of jaguars and llamas as quintessential examples of South American wildlife, for example, but their ancestors came across the isthmus from North America. Today, the remaining transplants from South America include animals like porcupines, armadillos and opossums.
The reasons for this pattern are unclear, Carrillo says. Northern mammals could have become more prevalent because, for some reason, they were better at spreading into South America than southern mammals were at moving north. Alternatively, perhaps northern newcomers to South America evolved into many more species once they infiltrated the new landmass. Or perhaps South America’s native mammals went extinct more frequently than their northern counterparts, forming an imbalance as they disappeared. Or some combination of these scenarios could have occurred.
To test these possibilities, Carrillo and his team analyzed roughly 20,000 fossils of mammals from the Americas, using a computer simulation to estimate how fast the creatures were diversifying into new species, migrating or going extinct. The team found that overall, mammals from both continents evolved and spread at roughly the same rates. However, the simulation showed that South American mammals started disproportionately dying out during the Pliocene, about 5 million to 2.5 million years ago.
