Bipolar bivalve genera probably existed through the greater part of late Mesozoic (i.e. late Jurassic-late Cretaceous) time. Of the various theories put forward to account for their presence, those based on some form of global climatic zonation seem most appropriate. Although equatorial-polar temperature gradients were substantially less in late Mesozoic time than at the present day, high latitude regions were subjected to temperate (or even cool-temperate) climatic regimes. Macro-benthonic marine faunas may have been more susceptible to differentiation in response to these climatic patterns than nektonic ones. This is particularly so in the late Mesozoic strata of the Southern Hemisphere.