The age of the fossil must be determined so it can be compared to other fossil species from the same time period.
Understanding the ages of related fossil species helps scientists piece together the evolutionary history of a group of organisms.
To deal with many of these problems geologists utilize two types of geologic time: relative time and absolute time.
Relative time places events or formations in order based on their position within the rock record relative to one another using six principles of relative dating.
An absolute age is one determined usually by mass-spectrometry where an isotope is measured and then an age can be calculated (a very very basic explanation).
So in the end you can say this fossil is 50 thousand years old (always with an associated uncertainty).
Although there might be some mineral differences due to the difference in source rock, most sedimentary rock deposited year after year look very similar to one another.