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. 


Some of the critters went extinct (fossils!)

This province (Provincia Chubut) is super rich in life forms. But even more diverse than the organisms living today are those that have gone extinct. From east (near the coast) to west (near the Andes), sedimentary deposits are progressively older and have fossils from the Cenozoic (age of mammals), Mesozoic (age of reptiles, including dinosaurs), and Paleozoic (old life, including trilobites). Paleontologists from around the world flock here to help with excavations and study the specimens.

We are staying near the town of Trelew, which is known for its Museo Paleontológico Egidio Feruglio (fossil museum – if you google the name you’ll find their excellent web site). We were fortunate to have two scientists from the museum guide us on a field trip to Parque Bryn Gwyn, where we could see fossils in sediments ranging in age from 40 – 10 million years ago. Here you can see Anita and our awesome guides: Pedro, an astronomer who leads the educational programs for the museum, and Amalia, a biologist/paleontologist who explained what we were seeing along the way.


For a sedimentologist (me!) there were lots of cool features to see – strata formed by moving ripples, ancient soils, beds of volcanic ash. But the features most interesting to nongeologists are usually the fossils – our record of how life has evolved through time. There were land animals – anteaters, several types of marsupials and other life forms not living today. Then the ocean flooded the land and the sediments contained sharks, whales, and smaller life forms such as oysters. The most impressive of these was a thick layer of shells that were preserved in their life position (see picture below). In Anita’s hand you can see their large size – imagine having these on the half shell!



Then it was off to the museum. You have probably read about some of the fossils from the area, because they include the largest dinosaur yet found. It is a vegetarian sauropod that is named Argentinosaurus! It was 80-100 tons and 10-12 meters long. The photo shows its legs with Anita for scale. Are we glad they don’t live now? But they eat plants; more frightening would have been the tyrannosaurus who was even bigger than the Rex. The museum has a large number of fossils from all of the time periods found in Chubut province and is well worth a visit should you find yourself in Trelew (meanwhile, check out their website).