Radiometric dating uses what isotopes
For example if you have a fossil trilobite and it was found in the Wheeler Formation.
The Wheeler Formation has been previously dated to approximately 507 million year old, so we know the trilobite is also about 507 million years old.
It’s often much easier to date volcanic rocks than the fossils themselves or the sedimentary rocks they are found in.
So, often layers of volcanic rocks above and below the layers containing fossils can be dated to provide a date range for the fossil containing rocks.
All rocks and minerals contain long-lived radioactive elements that were incorporated into Earth when the Solar System formed.
Then after another 5,000 years half of the remaining parent isotope will have decayed.Radiometric dating (often called radioactive dating) is a technique used to date materials such as rocks or carbon, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates.The use of radiometric dating was first published in 1907 by Bertram Boltwood and is now the principal source of information about the absolute age of rocks and other geological features, including the age of the Earth itself, and can be used to date a wide range of natural and man-made materials.The number of parent atoms originally present is simply the number present now plus the number of daughter atoms formed by the decay, both of which are quantities that can be measured.Samples for dating are selected carefully to avoid those that are altered, contaminated, or disturbed by later heating or chemical events.
The majority of the time fossils are dated using relative dating techniques.