In undisturbed sedimentary rock the bottom layers are the oldest and the top layers are the youngest. Fossils found in the lower layers are older. Those found in the upper layers are younger. Thus a fossil's position in sedimentary rock beds gives its age relative to other fossils. The absolute age of a fossil is determined by dating the fossil with radioactive isotopes.
Radioactive isotopes have unstable nuclei that break down, or decay, and form other elements.
These isotopes decay at a constant, known rate. Remember that isotopes of an element differ in number of neutrons Carbon-12, the most common and stable isotope of carbon, has 6 protons and 6 neutrons and therefore has an atomic mass of 12. Organisms take up the two isotopes of carbon in about the same ratio that the isotopes are found in the atmosphere. However, carbon-12 does not decay. Scientists use other radioactive isotopes, such as potassium-40, to date older fossils.