Various stratigraphic sections, from New Mexico to Saskatchewan, reveal in their fossil record the disappearance of the sauropod dinosaurs, the extinction of dinosaurs generally, and the end of the brontotheres. In each case there is no evidence of drastic changes in the depositional environment at the time of the extinction. Rather, the sequence indicates a long period of gradually changing environment, at one moment in which the extinction occurred. Such extinctions appear to be a kind of breaking-point phenomenon, in which the adaptive powers of the animals to the slow changes abruptly reached their limits.