The Great American Biotic Interchange

With its many excellent fossil sites, Argentina has lots of evidence for how life evolved through time. One of the largest effects on how organisms evolved was the position of the continents. For example, when the continents were united into the “super continent” called Pangea at the end of the Paleozoic, land animals could migrate for huge distances. In contrast, once the continents began to separate in the Mesozoic, animals could not move so far and so they evolved independently on the separated continents. 

One relatively recent change was between the continents of North and South America, which were separated by ocean until about 3 million years ago, when the Isthmus of Panama closed, and animals could start migrating between the continents. This resulted in what has been called “The Great American Interchange”. 

Many groups moved from North America to South America; for example, the great cats, including the Saber-toothed tiger (Smilodon genus—shown above). If you’ve been to the La Brea Tar Pits museum, you know that these tigers roamed the area that is now Los Angeles until about 10,000 years ago (they also went extinct in South America).  Other groups that moved south were elephants, bears, horses, deer, and camels. It’s interesting that camels subsequently went extinct in North America, but persist in South America as the camellid groups of llamas, vicuñas, guanacos, and alpacas.

Many groups also moved from South America to North America. One of the most interesting was the glyptondon genus (shown above) whose members were strange-looking herbivores that got as large as cars and that are now recognized as members of the armadillo family. Other groups that moved north included sloths, bats, porcupines, and various types of birds (including the “terror birds”).

Another interesting example is the giant ground sloth (megatherium genus—shown above) that was endemic to South America. When you go to Costa Rica to search for the shy little sloths that hang out in trees, it’s hard to imagine the elephant-sized animal that was the megatherium. You can see fossil remains of large sloths in North America (but not as large as megatherium) in the La Brea museum in Los Angeles. 

This biotic interchange caused extinctions in both North and South America, as new predators arrived in each continent to compete with the endemic fauna. But most of these so-called “megafauna” went extinct a mere 10,000 years ago, perhaps because humans had arrived and developed sophisticated enough weaponry to kill off the animals for food, sport, and/or safety. Paleontologists continue to debate the reasons.

All of the photos in this post were taken in the Museum of Natural Science in Córdoba.