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The Phanerozoic has seen five major extinctions, as indicated in Figure 8.10.

The most significant of these was at the end of the Permian, which saw the extinction of over 80% of all species and over 90% of all marine species.

Throughout the history of life, different organisms have appeared, flourished and become extinct.

Many of these organisms have left their remains as fossils in sedimentary rocks.

Relative dating is used to arrange geological events, and the rocks they leave behind, in a sequence.

The method of reading the order is called stratigraphy (layers of rock are called strata).

The oldest well-understood fossils are from rocks dating back to around 600 Ma, and the sedimentary record from that time forward is rich in fossil remains that provide a detailed record of the history of life.

However, as anyone who has gone hunting for fossils knows, that does not mean that all sedimentary rocks have visible fossils or that they are easy to find.

Some of the most useful fossils for dating purposes are very small ones.

For example, microscopic dinoflagellates have been studied and dated in great detail around the world.

Suppose you find a fossil at one place that cannot be dated using absolute methods.

That fossil species may have been dated somewhere else, so you can match them and say that your fossil has a similar age.

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