Thermal barriers provide an explanation for the geologically sudden extinction of benthic faunas of epeiric seas when these seas disappeared by contracting to the open ocean. Biotic interactions could not have caused the sudden extinctions, and neither could reduction of the regional area of seafloor because substantial areas of shallow seafloor remained along neighboring continental shelves when epeiric seas drained. Instead, temperature contrasts must have been responsible. Epeiric seas had strongly seasonal climates, and when some receded to continental margins, many of their species would have encountered waters that failed to provide the maximum or minimum temperature required for reproduction. When epeiric seas receded poleward, equatorward, or into Coriolis-driven currents, many species faced lethal temperatures. The history of the Jurassic Sundance Sea provides a striking example of the fate of a warm-adapted fauna driven westward into an area dominated by a cool, Coriolis-driven current.