The pyrotheres and astrapotheres were also strange but were less diverse and disappeared earlier, well before the interchange.
Terror birds may have also island-hopped to North America as early as 5 Ma ago.
This was due to the prevailing direction of oceanic currents, rather than to a competition between North and South American forms.
Although terror birds were able to invade part of North America, their success was temporary; this lineage disappeared about two million years ago.
The large Neotropic metatherian predators fared no better.
One group has proposed that a number of large Neartic herbivores actually reached South America as early as 9—10 Ma ago, in the late Miocene, via the "Baudo pathway", an early land bridge that was probably incomplete and required some swimming and island-hopping to traverse.
The limited evidence for these early immigrants may reflect their presence primarily in the Amazon basin , an area where fewer fossils have been collected.Sparassodonts and giant opossums 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.This included the immigration into South America of North American ungulates including camelids , tapirs , deer and horses , proboscids gomphotheres , carnivorans including felids like cougars and saber-toothed cats , canids , mustelids , procyonids and bears and a number of types of rodents [n 10].The larger members of the reverse migration, besides ground sloths and terror birds, were glyptodonts , pampatheres , capybaras and the notoungulate Mixotoxodon the only South American ungulate known to have invaded Central America.It was long thought that they had come from North America, but a recent comparative genetic analysis concludes that the South American genus Chelonoidis formerly part of Geochelone is actually most closely related to African hingeback tortoises.