sentences:  The seaway had isolated South America for much of the Cenozoic, allowing the evolution of a wholly unique diverse mammalian fauna there; when it closed, a faunal exchange with North America ensued, leading to the extinction of many of the native South American forms.The evidence for when the Central American landmass emerged and the closing of the Central American Seaway can be divided into three categories.The first is the direct geologic observation of crustal thickening and submarine deposits in Central America.