Typically commonly occurring fossils that had a widespread geographic distribution such as brachiopods, trilobites, and ammonites work best as index fossils.
The method compares the abundance of a naturally occurring radioactive isotope within the material to the abundance of its decay products, which form at a known constant rate of decay.
and is now the principal source of information about the absolute age of rocks and other geological features, including the age of fossilized life forms or the age of the Earth itself, and can also be used to date a wide range of natural and man-made materials.
This uses radioactive minerals that occur in rocks and fossils almost like a geological clock.
It’s often much easier to date volcanic rocks than the fossils themselves or the sedimentary rocks they are found in.
Different methods of radiometric dating vary in the timescale over which they are accurate and the materials to which they can be applied.