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 mill