Temperature during incubation determines sex in turtles, crocodiles, and alligators and may have determined the sex of hatchlings in dinosaurs as well. Geologic evidence indicates that dinosaurs nested in upland sites in western Montana where eggs were exposed to fluctuating and/or lower temperatures as climate deteriorated at the end of the Cretaceous. Production of hatchlings of predominantly one sex, due to environmental shifts in nest incubation temperatures resulting from changing climates, would have altered drastically the population breeding structure and driven these dinosaurs toward extinction.