The technique can, however, provide the relative ages of bones from the same site.
Most fossils are found in sedimentary rocks deposited in layers.
This problem is now reduced by the careful collection of samples, rigorous crosschecking and the use of newer techniques that can date minute samples.
Volcanic rocks – such as tuff and basalt – can be used in dating because they are formed at a particular moment in time, during an eruption.
This technique is, however, useful for providing relative dates for objects found at the same site.
Another useful chemical analysis technique involves calculating the amount of nitrogen within a bone.
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.
This relatively new technique was developed in order to achieve more accurate dates than those obtained from the potassium-argon method.
The older method required two samples for dating and could produce imprecise dates if the argon was not fully extracted.
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.
The level of nitrogen gradually reduces as the bone decays.