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 fossil species below the ash must be slightly older than 507 million years, and the species above the ash must be slightly younger.

How do you know which fossil is the oldest?

Relative dating is used to determine a fossils approximate age by comparing it to similar rocks and fossils of known ages. Absolute dating is used to determine a precise age of a fossil by using radiometric dating to measure the decay of isotopes, either within the fossil or more often the rocks associated with it.