Biologically defined fluctuations in Cretaceous tropical reef boundaries of the Caribbean region record a dynamic rather than stable environmental history. These fluctuations may be related to major thermal changes resulting from ocean heat transport. With simultaneous poleward movement of surface and subsurface waters on sea-level highstands, the superheated middle Cretaceous tropics cooled, the reef line contracted, diversity decreased, and reef ecosystems collapsed, leading to mass extinction. Geologic data qualitatively test and support the hypothesis of enhanced Cretaceous ocean heat transport formulated from general circulation models. In these models, four times the present-day atmospheric concentration of CO2 and twice the present-day model value of ocean heat transport cooled the superheated tropics and provided the best match to the distribution of inferred middle Cretaceous temperature data. These dynamic changes suggest an important role for large-scale disturbance in the evolution of tropical ecosystems.