Therefore, we can use the succession of fossil assemblages to establish the relative ages of rocks.
Now, when we use fossils to date rocks, we have to be careful. Remember that some species of animals and plants lived for a very long time, while others existed only for a short period of time.
When a scientist finds a section of rock that has lots of different strata, he assumes that the bottom-most layer is the oldest, and the top-most layer is the youngest.William Smith collected fossils from his work sites and, over time, he learned to recognize which fossils tended to show up in which rock strata.He began to identify rock layers by the fossils they contained, and he even noticed that the general order of strata was identical over many different parts of the country.For example, human beings and modern elephants are part of the same assemblage because we live in the same time period.
Stegosaurus and Triceratops were not part of the same assemblage because they lived at different times.
The site also provides fact sheets on the age of the Earth and isochron dating.