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How are Jurassic fossils used to help calculate the age of rock formations?

Fossils are remnants of plants and animals that once lived. When they died, some of them had their bodies buried in the soil or water. Over time, deposition caused the formation of rocks, and these dead plants and animals were fossilized and preserved in these rocks. When we dig through rock layers, we sometimes come across fossils and scientists can use them to determine the age of the rocks they were buried in.


The age of a fossil is determined by radiometric dating. The method uses the knowledge of half lives of radioactive isotopes of elements and the presence of these isotopes in animal/plant bodies. One such isotope is carbon-14, which has a half life of 5,730 years. This obviously cannot be used with Jurassic fossils or rocks, since the Jurassic period existed between 150-200 million years ago. For such samples, we can use Uranium-235, which has a half-life of about 700 million years and it decays to lead-207. Thus, knowing the ratio of U-235 and Pb-207, we can determine the age of the samples and hence know the age of the rocks.


Hope this helps. 

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