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.
They include amino acid racemization, useful in measuring shells and other bicarbonates between 500 and 300,000 years old, and obsidian hydration, used to measure natural glass between 500 to 200,000 years old.
These methods are not so useful in measuring the age of fossils and bones but they are useful in measuring the age of objects such as beads and shellfish eaten as food found with fossils and bones.
Fossils, however, form in sedimentary rock -- sediment quickly covers a dinosaur's body, and the sediment and the bones gradually turn into rock.