Using relative dating the fossil is compared to something for which an age is already known.For example if you have a fossil trilobite and it was found in the Wheeler Formation.
Fluoride ions are present in trace amounts in most soils and groundwaters.
Fluoride (or fluorine) dating is a relative dating method that can be used to date archaeological bone.
As a relative dating method, it can determine the relative age of specimens, but cannot provide a calendrical date unless the fluoride chronology is calibrated with an absolute dating method.
When it comes to determining the age of stuff scientists dig out of the ground, whether fossil or artifact, “there are good dates and bad dates and ugly dates,” says paleoanthropologist John Shea of Stony Brook University.
The good dates are confirmed using at least two different methods, ideally involving multiple independent labs for each method to cross-check results.