Although during the first half of the Cenozoic era South America did have a small population of primitive placental mammals, it was, like Australia, a bastion of the metatheres. It had been isolated since the Mesozoic era. However, during the Pliocene epoch of the Neogene period, the Central American land bridge was established between South America and North America which led to an exchange of terrestrial and freshwater faunas between the two areas, this event being called the Great American Interchange. The result was that the placental mammals from the north, being more versatile, almost entirely replaced the metatheres and the primitive placentals of the south. The northern fauna were more versatile because they had been subjected to greater selective pressures in the preceding 50 million years; they had been compelled to adapt radically in the face of changing environmental conditions brought about by such factors as ice ages and faunal exchanges with Asia. The result at the time of the collision with South America was a very hardy and adaptable stock of animals. The mammals of South America, on the other hand, had experienced a stable unchanging environment during the same period and therefore lacked this essential adaptability. A similar fate did not befall the marsupials of Australia, since that continent, in drifting northwards, presented its fauna with constantly changing conditions, resulting in a population of hardy species that were able to survive the faunal exchanges that occurred during the time shortly after Australia impacted with Asia.
20 million AD, the land connection with North America was again broken and South America became an island continent once more. Since the split, climatic conditions on the South American continent have remained unchanged and the fauna has therefore changed very little. This conservatism is well seen among the mammalian predators (a niche that has continued to be occupied by members of the order Carnivora despite the fact that this group has declined elsewhere).
Throughout its history, the movement of the crustal plate carrying the South American continent has been predominantly westwards, and hence the landmass has tended to remain within the same latitudes. This accounts for the constancy of the climatic regions and the conservatism of its fauna.
During the continent's early history the grasslands, or pampas, supported their own fauna of running hoofed animals called the meridiungulates, similar to, but totally isolated from, those in other parts of the world. These animals existed but diminished when the continent became joined by the land bridge to North America, when they and the native marsupial population were almost swept away completely by the influx of animals from the north. Strangely enough the northern ungulates (even-toed ungulates and odd-toed ungulates) did not find a truly permanent foothold on the pampas, but rather caviomorph rodents such as the maras (Dolichotis) and the capybaras (Hydrochoerus), present at the time of the Holocene, were the more successful. In this respect the South American continent anticipated the rise of the advanced running glires in the rest of the world.
Once the continent separated from the supercontinent of the north the rodent fauna (mostly the caviomorphs) developed along its own lines. The running animals of the pampas are dominated by strange bipedal grazers, which are descended from the jumping rodents that evolved in the rainshadow deserts along the western mountains of the Andes. Although long hind legs evolved independently among desert rodents in other continents only those of South America changed from a jumping to a running mode in the course of their evolutionary history. Along with this change of gait went an increase in size and a change of dentition that effected the final transition from the jumping, gnawing rodent of the desert to the striding grazer of the plains.
The continent's environments consist of temperate woodlands and grasslands, high and cold mountains, a southern desert at the narrow tail of the continent, tropical grasslands expanding south of the equatorial rainforests, and large tropical forests (still maintaining its great Amazon River that carries away copious rainwater).