Sedimentary rocks can be dated using radioactive carbon, but because carbon decays relatively quickly, this only works for rocks younger than about 50 thousand years. So in order to date most older fossils, scientists look for layers of igneous rock or volcanic ash above and below the fossil.

What rocks are used to date fossils?

The volcanic material in tuff is well-suited for radiometric dating, which uses known decay rates for specific unstable isotopes to determine the age of the rock that contains that isotope. Feldspar crystals found in the tuff layers contain an unstable isotope of potassium that can be used for this dating method.

Which fossil will be more useful for dating the rock?

Organisms that lived for relatively short time periods are particularly useful for dating rocks, especially if they were distributed over a wide geographic area and so can be used to compare rocks from different regions. These are known as index fossils.