South America broke from Antartica and Australia about 60 million years ago and became an island continent where its fauna evolved in isolation. The most common mammals were marsupials, edentates, and primitive hooved animals unfamiliar to most non-paleontologists. Circa 30 million years ago, caviomorph rodents and monkeys arrived from Africa via clumps of floating vegetation originating from that continent. During this time Africa and South America were closer together than they presently are. Floods in African rivers must have carried these little islands of vegetation with their clinging inhabitants far into the sea where favorable currents brought the lucky animals to South America. These new colonizers diversified into many species. South America remained an isolated continent until 9 million years ago when volcanic islands began emerging below North America. Although there was not yet a landbridge between the 2 continents, the fossil record shows there was an early exchange of species. Ground sloths went north, while tapirs, peccaries, and primitive mastodons known as gompotheres went south. All of these animals are excellent swimmers. Apparently, they island-hopped, swimming to reach new islands and eventually coming to a continent where they became successful newcomers.
Map showing the islands that collided with South America eventually forming the Isthmus of Panama. This joined North America with South America. Some species of mammals island-hopped and colonized the other continent before the landbridge completely emerged above sea level.
Elephants can swim for miles without getting tired. They can breath through their trunks while swimming. This explains how gompotheres (an elephant-like beast) colonized South America 9 million years ago, long before a land connection existed between the 2 continents.
Tapir swimming. They can breath through their snouts while swimming submerged. All of the early colonizers in the Great American Biotic Interchange were excellent swimmers. They island-hopped before the existence of a complete land connection.
A now extinct relative of the raccoon (Cyonasaura) was next to colonize South America 7.3 million years ago. An extinct species of raccoon (Chapmalania) that was as large as a bear lived in South America during the Pliocene. Sygmodontine rodents (rats, mice, gophers) from North America arrived in South America 6 million years ago. Pampatheres (a giant armadillo) and the terror bird spread to North America 5 million years ago. They were followed by glyptodonts and capybaras 4 million years ago. The Central American landbridge emerged above sea level ~3 million years ago bringing a cavalcade of animals south–llamas, horses, canids, and weasels; while porcupines headed north. A new climatic phase ~2 million years ago allowed even more mammals to head south–deer, rabbits, skunks, cats, and bears.
Other vertebrates took advantage of newly available habitat. South American forms that moved north included cichlid fish, bufo toads, tree frogs, parrots, tanagers, hummingbirds, and flycatchers. North American vertebrates that went south included lungless salamanders, ranid frogs, snapping turtles, wood turtles, trachemys turtles, rattlesnakes and other pit vipers, coral snakes, sparrows, and condors.
These faunal invasions occurred in pulses influenced by changes in climate. During Ice Ages sea level fell and this greatly expanded the land area of Central America. Average temperatures fell by as much as 14 degrees F here, though because the region is so close to the equator they were not subfreezing. Nevertheless, pollen records indicate tropical forests were replaced with grassy savannahs and temperate species of trees such as oak, sweetgum, myrtle, elm, walnut, and even spruce and fir at higher elevations. The change to a more open environment allowed horses and llamas to traverse Central America to colonize land further south. Warm interglacial conditions favored the expanded ranges of tropical species such as very large ground sloths and glyptodonts. These species eventually made it to southeastern North America. Some lineages of mammals colonized 1 continent, evolved to a new species, then returned to the original continent of their ancestor. Examples of these are the collared peccary and jaguarundi.
More species of South American-originating fauna survive in North America than vice-versa, but most are restricted to the Central American tropics. There are many species of edentates, marsupials, monkeys, and caviomorph rodents here. But, excepting Central America’s faunal composition, North America’s invasion to South America could be considered more successful. There are just 4 successful South American species living today in North America (not counting Central America)–opossum, armadillo, porcupine, and nutria whereas South America is home to tapirs, peccaries, llamas, foxes, cats, sygmodontine rodents, squirrels, rabbits, and shrews. However, a modern review of this disparity is misleading. Ground sloths, glyptodonts, and pampatheres were quite successful during the Pleistocene in North America, but humans caused their extinction. Toxodons and litopterns (primitive South American hooved animals that made it as far north as Mexico) were also vulnerable to human hunting and would likely still live today if not for man. North America also has a more temperate climate. It is difficult for species that evolved in tropical climates to adapt to freezing temperatures. The only real losers resulting from this faunal interchange were South America’s marsupial carnivores and the terror birds. North America’s carnivores eventually outcompeted South America’s meat-eaters.
“The GABI: Dispersals, Tectonics, Climate, Sea Level, and Holding Pens”
Journal of Mammalian Evolution 17 2010