The Great American Biotic Interchange

Once the isthmus is complete, animals cross in both directions. Ground sloths, glyptodonts, armadillos, opossums and porcupines move north; cats, dogs, bears, horses, camels, deer, tapirs and rodents move south. The exchange is strikingly asymmetrical: North American groups came to dominate South America, while relatively few southern lineages established themselves in the north. The likeliest explanation is that the northern fauna had been tested by repeated Eurasian invasions, while South America's had evolved in isolation. Most of South America's endemic ungulates and marsupial carnivores disappear.