(Similar large terrestrial predatory birds, the bathornithids, were found in North America during the early Cenozoic, but they died out in the Early Miocene, about 20 million years ago.) 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.
Its most dramatic effect is on the zoogeography of mammals but it also gave an opportunity for reptiles, amphibians, arthropods, weak-flying or flightless birds, and even freshwater fish to migrate.
(Island-hopping caviomorphs would subsequently colonize the West Indies as far as the Bahamas,) Over time, some caviomorph rodents evolved into larger forms that competed with some of the native South American ungulates, which may have contributed to the gradual loss of diversity suffered by the latter after the early Oligocene. Like caviomorph rodents, South American monkeys are believed to be a clade (i.e., monophyletic).
However, although they would have had little effective competition, all extant New World monkeys appear to derive from a radiation that occurred long afterwards, in the Early Miocene about 18 Ma ago.
Metatherians (and a few xenarthran armadillos like Macroeuphractus) 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 and giant opossums shared the ecological niches for large predators with fearsome flightless "terror birds" (phorusrhacids), whose closest extant relatives are the seriemas.