In: Anatomy and Physiology
There are two main methods determining a fossils age, relative dating and absolute dating-
1.Relative dating is used to determine a fossils approximate age by comparing it to similar rocks and fossils of known ages.
2.Absolute dating is used to determine a precise age of a fossil by using radiometric dating to measure the decay of isotopes, either within the fossil or more often the rocks associated with it.
Relative dating-
Using relative dating the fossil is compared to something for which an age is already known.Scientists can use certain types of fossils referred to as index fossils to assist in relative dating via correlation. Index fossils are fossils that are known to only occur within a very specific age range. Typically commonly occurring fossils that had a widespread geographic distribution such as brachiopods, trilobites, and ammonites work best as index fossils. If the fossil you are trying to date occurs alongside one of these index fossils, then the fossil you are dating must fall into the age range of the index fossil.
Absolute Dating-
Absolute dating is used to determine a precise age of a rock or
fossil through radiometric dating methods. This uses radioactive
minerals that occur in rocks and fossils almost like a geological
clock.The atoms in some chemical elements have different forms,
called isotopes. These isotopes break down at a constant rate over
time through radioactive decay. By measuring the ratio of the
amount of the original (parent) isotope to the amount of the
(daughter) isotopes that it breaks down into an age can be
determined.
We define the rate of this radioactive decay in half-lives. If a
radioactive isotope is said to have a half-life of 5,000 years that
means after 5,000 years exactly half of it will have decayed from
the parent isotope into the daughter isotopes. Then after another
5,000 years half of the remaining parent isotope will have
decayed.