If our stratigraphic methods show that fossil A was always deposited below fossil B whenever we are in a position to compare their dates of deposition, then we can conclude that species A is older than species B.
There are three main advantages of using fossils for dating in this manner.
First of all, we may want to date a stratum which is a long way up or down from any rocks we can date using radiometric methods.
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.
Sedimentary rocks are rarely useful for dating because they are made up of bits of older rocks.
A common problem with any dating method is that a sample may be contaminated with older or younger material and give a false age.