Fossils are typically used in determining the relative age of rocks rather than their absolute age. The principle of biostratigraphy allows scientists to correlate rock layers based on the presence of specific fossils. This helps to establish a sequence of geological events but does not provide precise numerical ages.
Absolute dating, on the other hand, is usually achieved through methods such as radiometric dating, which relies on the decay of radioactive isotopes in minerals to determine the age of rocks in years. Common isotopes used in absolute dating include uranium-lead, potassium-argon, and rubidium-strontium.
Fossils can aid in correlating rock layers and might help place them in a broader context when using absolute dating methods, but they are not used directly in assigning absolute ages. If one finds a fossil in a layer that has been dated using radiometric methods, the age of that rock can give a minimum age for the fossil. However, the dating of the rock itself would still rely on radiometric techniques or other absolute dating methods.