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.The atoms in some chemical elements have different forms, called isotopes.Sedimentary rocks are rarely useful for dating because they are made up of bits of older rocks.Uranium is present in many different rocks and minerals, usually in the form of uranium-238.
Accurate dates also allow us to create sequences of evolutionary change and work out when species appeared or became extinct. These are: Where possible, several different methods are used and each method is repeated to confirm the results obtained and improve accuracy.
There are two main methods determining a fossils age, relative dating and absolute dating.
Relative dating is used to determine a fossils approximate age by comparing it to similar rocks and fossils of known ages.
Instead, other methods are used to work out a fossil’s age.
These include radiometric dating of volcanic layers above or below the fossils or by comparisons to similar rocks and fossils of known ages.
This problem is now reduced by the careful collection of samples, rigorous crosschecking and the use of newer techniques that can date minute samples.