How do paleontologists know how old their fossils are?
Paleontologists deal with two types of dating, absolute and relative. Absolute dating, which estimates the age of a rock or fossil in years, is most usually done by measuring the amounts of a radioactive isotope and its decay product; since isotope decay rates are known to be constant, the age can be calculated from the relative amounts of parent isotope to daughter product. Fossils up to about 40,000 years old can be dated using carbon-14 if there is enough organic matter present. Older rocks can be dated using potassium-40, which decays to argon-40, or uranium-235, which decays to lead-207. However, many sedimentary rocks cannot be dated directly by these methods; dates usually are obtained from igneous rocks within a sedimentary sequence, such as lava flows or ash beds. Such dates are maximum age estimates for fossils above the dated beds, or minimum estimates for fossils below the beds. Relative dating has been practiced for nearly 200 years, arising from the observation that diffe
