FORCE OF CHANGE. “Extinction itself promotes biotic interchange. Asymmetries in ancient biotic interchange generally appear to reflect geographic differences in extinction intensity. The end-Cretaceous extinction shows, however, that although biotic interchanges pervade the postextinction world, simple linear relationships can break down to produce unexpected source-sink patterns.“
http://www.pnas.org/content/98/10/5393.full/Donnie)