Great American Interchange - South America's Endemic Fauna

South America's Endemic Fauna

After the late Mesozoic breakup of Gondwana, South America spent most of the Cenozoic era as an island continent whose "splendid isolation" allowed its fauna to evolve into many forms found nowhere else on earth, most of which are now extinct. Its endemic mammals initially consisted of metatherians (marsupials and sparassodonts), xenarthrans, and a diverse group of native ungulates: notoungulates (the "southern ungulates"), litopterns, astrapotheres (e.g. Trigonostylops, Astrapotherium), and pyrotheres (e.g. Pyrotherium). Monotremes, gondwanatheres and possibly multituberculates were also present in the Paleocene, but did not survive very long.

Marsupials appear to have traveled via Gondwanan land connections from South America through Antarctica to Australia in the late Cretaceous or early Tertiary. One living South American marsupial, the monito del monte, has been shown to be more closely related to Australian marsupials than to other South American marsupials; however, it is the most basal australidelphian, meaning that this superorder arose in South America and then colonized Australia after the monito del monte split off. A 61-Ma-old platypus-like monotreme fossil from Patagonia may represent an Australian immigrant. It appears that ratites (relatives of South American tinamous) migrated by this route around the same time, more likely in the direction from South America towards Australia/New Zealand. Other taxa that may have dispersed by the same route (if not by flying or floating across the ocean) are parrots, chelid turtles and (extinct) meiolaniid turtles.

Marsupials present in South America included didelphimorphs (opossums) and several other small groups; larger predatory relatives of these also existed, like the borhyaenids and the sabertooth Thylacosmilus (sparassodont metatherians which are no longer considered to be marsupials).

Metatherians were the only South American mammals to specialize as carnivores; their relative inefficiency created openings for nonmammalian predators to play more prominent roles than usual (similar to the situation in Australia). Sparassodonts shared the ecological niches for large predators with fearsome flightless "terror birds" (phorusrhacids), whose closest extant relatives are the seriemas. Through the skies over late Miocene South America (6 Ma ago) soared the largest flying bird known, the teratorn Argentavis, with a wing span of 6 m or more, which may have subsisted in part on the leftovers of Thylacosmilus kills. Terrestrial ziphodont sebecid crocodilians were also present at least through the middle Miocene. Some of South America's aquatic crocodilians, such as Gryposuchus, Mourasuchus and Purussaurus, reached monstrous sizes, with lengths up to 12 m (comparable to the largest Mesozoic crocodyliforms). They shared their habitat with one of the largest turtles of all time, the 3.3 m (11 ft) Stupendemys.

Xenarthrans are a curious group of mammals that developed morphological adaptations for specialized diets very early in their history. In addition to those extant today (armadillos, anteaters and tree sloths), a great diversity of larger types were present, including pampatheres, the ankylosaur-like glyptodontids, various ground sloths, some of which reached the size of elephants (e.g. Megatherium), and even semiaquatic marine sloths.

The notoungulates and litopterns had many strange forms, like Macrauchenia, a camel-like litoptern with a small proboscis. They also produced a number of familiar-looking body types that represent examples of parallel or convergent evolution: one-toed Thoatherium had legs like those of a horse, Pachyrukhos resembled a rabbit, Homalodotherium was a semi-bipedal clawed browser like a chalicothere, and horned Trigodon looked like a rhino. Both groups started evolving in the Lower Paleocene, possibly from condylarth stock, diversified, dwindled before the great interchange, and went extinct at the end of the Pleistocene. The pyrotheres and astrapotheres were also strange but were less diverse and disappeared earlier, well before the interchange.

The North American fauna was a pretty typical boreoeutherian one (supplemented with Afrotherian proboscids).