Some scientists study what happened in the past. This includes those that study the dinosaurs of the Jurassic Period.
Scientists like to break up time into different periods just as historians do.
Historians break down time according to what they know of what humans were doing. Scientists break down time according to what animals and other life on the planet where doing. By breaking down our past into chronological order scientists help to add a structure and a framework for making sense of the past. They use this approach so they can take all of the complicated things that have happened over a billion years and chunk it up into smaller manageable pieces that have some things in common. So using chronological order as a organizing device they make their job of discovering and uncovering facts more manageable.