How do you use fossils to tell time

Explanation:

The fossils at the bottom are the oldest and the fossils at the top are the newest. (Couldn't think of another way to say it >.<)

"To get an age in years, we use radiometric dating of the rocks. Not every rock can be dated this way, but volcanic ash deposits are among those that can be dated. The position of the fossils above or below a dated ash layer allows us to work out their ages. The volcanic ash layer is dated as 507 million years old."

the answer is a hope this