Fossils found in deeper rock layers are generally older than those found in shallower layers. This is because new layers of rock are constantly being deposited on top of older layers, so the deepest layers are the oldest. This principle, known as the principle of superposition, is the foundation of relative dating in geology.

Determining the exact age of a fossil requires more sophisticated techniques, such as radiometric dating. Radioactive elements decay at a constant rate, and the amount of decay products accumulated in a fossil can be used to calculate its age.