In radiometric dating, scientists use the rate of decay of radioactive materials in rocks to estimate the absolute age of a rock. That tells them the age of fossils found in that rock.
Radiometric dating allows scientists to find fossils in only the lowest and oldest layers of sediment.
In radiometric dating, scientists place samples of a fossil in certain liquids until the samples dissolve. The rate at which they dissolve indicates the age of the fossil.
In radiometric dating, scientists mix the carbon in a fossil with carbon from similar fossils whose age they know. By comparing, the carbon they can tell the exact age of the fossil.