Paleocene was the time right around when the dinosaurs died, so about 65 million years ago. you've heard of Jurassic, and maybe you've even heard of cretaceous, this is the one that comes right after those two. Right now we're in the Holocene. The reason I mentioned it though is because (as far as we can tell) it was the hottest period in earth's history, with average temperatures 8 degrees Celsius higher than today (which is a ton, the fact that it's an average makes it seem less insane than it actually is). we're nowhere close to getting as warm as it was then, but even if we got half that hot in a relatively fast amount of time (like we are) it could still cause mass extinction.