The American interchange of land mammals reached its acme during the late Blancan and early Irvingtonian in North America and during the Chapadmalalan and Uquian in South America. It lasted about two million years and included taxa adapted to diverse habitats. It was preceded in the early Hemphillian in North America and the Huayquerian in South America by the interchange of a few heralding genera. The MacArthur-Wilson faunal equilibrium hypothesis correctly predicts a marked increase in originations, number of genera, and turnover rate for the South American fauna during the peak of the interchange. Subsequent further increases were not so predicted but closely resemble patterns also observed in late Pleistocene land mammals of Europe and North America. The continued increase in South American land mammal genera after the interchange had largely ceased resulted principally from autochthonous evolution of northern immigrant stocks. A marked decrease in South American ungulate genera (from thirteen to three) coincided with the appearance of fourteen northern ungulate genera and therefore appears to be a replacement phenomenon. The area/diversity relationship predicts no important change in generic diversity if a maximum of only nine percent of North America is occupied by the interamerican mingled fauna. At the family level, however, diversity is seriously overestimated due to the nomenclatural artifact of increased relative diversity by filtering.
