In general, it is thought that fossils found in the lower strata of rock are older than those found in the upper layers.
There are two methods that scientists employ to date rocks and fossils, according to their relative position younger rocks are placed on top of older rocks.
Relative age dating is used to identify whether a rock layer or the fossils contained within it is older or younger than another.
Therefore, because they reveal the relative ages of the rock layers in which they are found, index fossils are helpful.
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