Fossils are any part or trace of an organism that lived long ago. Time, since the beginning of planet Earth, is measured in geologic time units. The largest measure in geologic time is the eon. Proterozoic, 2.5 to .59 BYA 3. Phanerozoic, .59 BYA to the present.
Eons have eras.
Cenozoic Eras have periods. Earth was dominated by bug-like creatures called trilobites 2. Earth was dominated by coral 4. Earth was dominated by fish with jaws 5. Earth was dominated by small reptiles 2.