The Ancestors of South American Mammals May Have Been Overtaken and Replaced By Their Counterparts from North America

A long time ago, in a galaxy, well, I guess right here on Earth, North American mammals poured into South America, when the two continents joined. But for some reason, South American mammals didn’t follow suit, and did not migrate to the North, and now scientists and researchers have a clue as to why.

Over one million decades ago, the Pacific tectonic plate crashed into and slid under the South American and Carribean plates, causing the Panama Isthmus to form. The Isthmus acted as a land bridge between North and South America, which allowed animals to move between the two. Now known as the Great American Biotic Interchange, the mass migration had a large influence on today’s distribution of animals. 

“The interchange was relatively balanced at first”, says Juan Carrillo, a paleobiologist for the National Museum of Natural History in Paris. However, the exchange soon became unbalanced, with many more North American animals moving southwards, while most South American animals never even managed to move to the north. 

The migration pattern affected today’s mammals, as almost half of South American mammal origins can be traced to North America, while only 10 percent of North American mammals are from the south. 

As for a reason why, Carrillo and other experts aren’t sure. For one, northern migrants might have split and evolved into more species than their southern counterparts, leading to a greater biological diversity. Alternatively, northern mammals may have been better or had an easier time migrating, due to geographical or climate-based factors. Or maybe the southern animals just died out and went extinct more often than their northern counterparts. 

Jaguars and llamas are actually from North America, while porcupines, armadillos, and opossums have ancestors from South America.

To test the three different possibilities, Carrillo and his team in Paris used 20,000 fossils and a computer simulation to estimate how the mammals evolved, migrated, and went extinct. Carrillo’s team found that while the two continents’ animals evolved and moved at similar rates, the South American animals started to go extinct much more during the Pliocene Epoch, about 2-5 million years ago. 

It isn’t clear what the reason for the extinctions was. The Pliocene was characterized by colder climate across the globe, while South America was becoming drier than ever, which may have destroyed environments and led to extinction. 

Additionally, the main predator in South America at the time – the Sparassodonta – was in decline, and went extinct around the same time the isthmus formed. Carrillo says that the Sparassodonta dying off may have left a spot for predators from North America. It’s also possible that direct competition between the Sparassodonts and northern carnivores accelerated the extinction of the South American predators, says Jens-Christian Svenning, who works as an ecologist at Aarhus University in Denmark. 

While the study done by Carrillo’s team paired with previous knowledge of mammal ancestors in the Americas reveals a lot, there is still so much more to be discovered. Very little is known about animals in the bodies of water and in tropical areas. 

Uncovering more information about the Great American Biotic Interchange is vital to understanding how animals evolved, Carrillo says. “It shows this very close link between geology and biology, and how historical events, like a higher extinction of mammals, end up having important repercussions on the patterns of diversity that we see today.” 

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