Rodents and monkeys rafting from Africa reshaped South America's mammals
Long before Panama joined the Americas, a few rodents and monkeys drifted from Africa to South America on mats of flotsam. Their descendants now number 372 species, more than twice the 136 marsupial and xenarthran species descended from the continent's older residents. Then the land bridge rose and everything changed again.
After Gondwana broke apart, South America spent most of the Cenozoic as an island continent, and in that isolation its animals evolved into forms found nowhere else. There were marsupial relatives, armadillos, anteaters and sloths, and a menagerie of native hoofed mammals. Macrauchenia resembled a camel with a small trunk, Thoatherium had horse-like one-toed legs and Trigodon carried a horn like a rhinoceros. Ground sloths such as Megatherium grew to the size of elephants. Because mammal carnivores were scarce, flightless terror birds hunted alongside saber-toothed Thylacosmilus, while the teratorn Argentavis, with a wingspan of 6 m or more, soared overhead. Crocodilians like Purussaurus reached 12 m, sharing rivers with the 3.3 m turtle Stupendemys.
Newcomers arrived across open ocean. Caviomorph rodents landed about 40 million years ago, probably from Africa when the Atlantic crossing was far narrower, and gave rise to capybaras, chinchillas and New World porcupines. Primates followed by 36 million years ago. Fossils described in 2020 show a second group of African monkeys, Ucayalipithecus, briefly reached Peru as well. Bats made similar trips, flying rather than floating.
The great exchange itself sped up dramatically about 2.7 million years ago, when the volcanic Isthmus of Panama rose from the seabed and united the two continents. Mammals were most affected, but reptiles, amphibians, insects, flightless birds and even freshwater fish crossed too. For ocean life the effect was reversed: the isthmus split the Caribbean from the Pacific, a separation dubbed the Great American Schism, driving both diversification and extinction.
Alfred Russel Wallace, who had spent five years collecting in the Amazon, first described the interchange in 1876, and Henry Fairfield Osborn discussed its Pliocene timing in 1910. Similar mixings happened when India and Africa met Eurasia about 56 and 30 million years ago.
Source: Great American Interchange
