However, by itself a fossil has little meaning unless it is placed within some context.
Index fossils are fossils from common, easily identifiable species that lived in wide ranges during restricted time periods.
Generally, deeper rocks and fossils are older than those found above them.
* Observations of the fluctuations of the Earth's magnetic field, which leaves different magnetic fields in rocks from different geological eras.
Here is some information from a good web site: Paleontologists use many ways of dating individual fossils in geologic time.
Paleontologists can estimate the amount of time that has passed since the stratum containing the fossil was formed.