With the acceptance of the hypothesis of continental drift, paleontologists have been able to employ two concepts that have had important implications for paleobiogeographic distributions of organisms. One concept is that continents previously combined into one have separated, and vice versa, and the other is that a single continent may have been at different latitudinal positions during its history. Paleontologists have used this latter concept to explain, for example, present-day polar positions of ancient tropical forests or to reconstruct pole-to-equator diversity gradients in benthic marine communities. The model underlying these reconstructions and explanations is that climatic belts are wholly temperature-controlled and parallel with latitude. An important feature f climatic zonation that has been largely ignored is that along ocean margins, climatic zonation is asymmetric east to west. This is because the major ocean currents, which affect the climate, are asymmetric, with colder currents dominating the regime on the eastern sides of the oceans and warmer currents dominating the western sides.

Atmospheric circulation maps have been constructed for several Paleozoic periods, from which ocean-current maps have been derived. In these reconstructions, the asymmetry of climate along the ocean margins, caused by the asymmetry of the currents, is reflected in the distribution of ancient terrestrial and marine communities.

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