Fossils can't form in the igneous rock that usually does contain the isotopes.
The extreme temperatures of the magma would just destroy the bones.
To determine the ages of these specimens, scientists need an isotope with a very long half-life.
Some of the isotopes used for this purpose are uranium-238, uranium-235 and potassium-40, each of which has a half-life of more than a million years.
A dating method used by Richard Bailey at Oxford University measures the amounts of radiation absorbed by sand grains in and around a bone specimen by measuring radioactive isotopes in the sand and comparing that to a sophisticated radiation transport model using data from a CT scan of the specimens.
You can learn more about fossils, dinosaurs, radiometric dating and related topics by reading through the links on the next page.
Scientists have also made improvements to the standard radiometric measurements.
For example, by using a laser, researchers can measure parent and daughter atoms in extremely small amounts of matter, making it possible to determine the age of very small samples [source: New Scientist].
By using radiometric dating to determine the age of igneous brackets, researchers can accurately determine the age of the sedimentary layers between them.
Using the basic ideas of bracketing and radiometric dating, researchers have determined the age of rock layers all over the world.