Sometimes, scientists already know the age of the fossil because fossils of the same species have been found elsewhere and it has been possible to establish accurately from those when the dinosaur lived. In a hypothetical example, a rock formation contains fossils of a type of brachiopod known to occur between and million years. The Wheeler Formation has been previously dated to approximately million year old, so we know the trilobite is also about million years old.
After about 60, years, all of the carbon will be gone. Studying the layers of rock or strata can also be useful. If a radioactive isotope is said to have a half-life of 5, years that means after 5, years exactly half of it will have decayed from the parent isotope into the daughter isotopes. This volcanic matter eventually settles and over time is compacted to form a special type of sedimentary rock called tuff. So, often layers of volcanic rocks above and below the layers containing fossils can be dated to provide a date range for the fossil containing rocks.