Evolution seems to be in one direction. Once a species becomes
extinct, the likelihood of that organism coming back to life is
remote. It is this belief that allows us to date rocks. For instance, we
will not have dinosaurs again. If you find dinosaur bones in rocks,
those rocks are in the Mesozoic. Rocks without dinosaurs are younger
(Cenozoic) or older (Paleozoic).
Relative time is very important and based on fossil occurrence. The
ordering or the Principle of Faunal Succession will remain the same.
Look at your fossils and try to familiarize yourself with the organisms
and when they lived in geologic time.