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 true marsupials.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.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.Additionally, a find of seven Ma-old apparent cebid teeth in Panama suggests that South American monkeys had dispersed across the seaway separating Central and South America by that early date.
The large Neotropic metatherian predators fared no better.
The pyrotheres and astrapotheres were also strange but were less diverse and disappeared earlier, well before the interchange.
The North American fauna was a typical boreoeutherian one, supplemented with Afrotherian proboscids.
Native South American ungulates also did poorly, with only a handful of genera withstanding the northern onslaught.
It has long been recognized that several of the largest forms, macraucheniids and toxodontids , survived to the end of the Pleistocene.