Radiometric dating and index fossils are two methods we can use to figure out how old fossil-containing chunks and layers of rock are. Sedimentary rock forms layers as particles settle on sea beds over millions of years. If we see one of these fossils in a layer of rock, this tells us how old that rock must be.

Are index fossils used for absolute dating or relative dating?

Keyed to the relative time scale are examples of index fossils, the forms of life which existed during limited periods of geologic time and thus are used as guides to the age of the rocks in which they are preserved.