When the isthmian land bridge triggered the Great American Interchange, a large majority of
land-mammal families crossed reciprocally between North and South America at about 2.5
Ma (ie, Late Pliocene). Initially land-mammal dynamics proceeded as predicted by
equilibrium theory, with roughly equal reciprocal mingling on both continents. Also as
predicted, the impact of the interchange faded in North America after about 1 my In South
America, contrary to such predictions, the interchange became decidedly unbalanced …