NASA now tells us that ozone is still being depleted around boreal sunrise, despite the reduction in CFCs, because the same changes in the greenhouse effect that cause global warming also happen to induce stratospheric cooling, which hastens ozone depletion. This totally plausible theory will ultimately serve to ho-hum away yet another environmental disaster. Be aware that correlation here does not imply causation. All of this means that when the planet was teeming with dinosaurs, cycads and whatever else fossilized itself into General Motors, the earth's natural greenhouse effect was several times greater than it is today. Which means that the stratosphere was several degrees colder.
Nature really does bat last.
She survived through all those innings when ultraviolet radiation rained down through the naturally low stratospheric ozone. In fact, she prospered, and, as a result, we evolved.