sentences:  The second is the Great American Interchange of vertebrates between North and South America which required a continuous land bridge across the two areas for the organisms to travel along with a climate that was very different from the climate today.Lastly is the development of differences in marine assemblages and their isotopic signatures in the Caribbean from those in the Pacific.The Central American Seaway was closed by the elevation of the Central American Isthmus which is proposed to have occurred three and a half to five million years ago.