First, scientists like to date the rock in which the fossil is on. Radiometric dating is a technique used to date materials such as rocks or carbon.Scientists combine several well-tested techniques to find out the ages of fossils. The most important are Relative Dating.
In which fossils and layers of rock are placed in order from older to younger, and Radiometric Dating, which allows the actual ages of certain types of rock to be calculated.
Until the middle of the last century, “older” or “younger” was the best scientists could do when assigning ages to fossils. There was no way to calculate an “absolute” age (in years) for any fossil or rock layer.
But after scientists learned that the nuclear decay of radioactive elements takes place at a predictable rate, they realized that the traces of radioactive elements present in certain types of rock, such as hardened lava and tuff (formed from compacted volcanic ash), could be analyzed chemically to determine the ages, in years, of those rocks.