Great American Interchange
The borhyaenids and the sabertooth Thylacosmilus were once considered to be marsupials.  They are sparassodont metatherians, the sister group of the marsupials.  Sparassodonts were the only South American mammals to specialize as carnivores.  Their relative inefficiency left opportunites for nonmammalian predators to be more prominent than usual (similar to the situation in Australia).