Process of relative dating
We want fossils of plants and animals that lived for a relatively short amount of time, like a few hundred thousand years or so.
I know that doesn't seem like a very short time span, but it is when we're talking about geologic time.
We talked about relative dating of rocks and how scientists use stratigraphic succession to compare the ages of different rock layers.
We wouldn't want to use a horseshoe crab fossil, because horseshoe crabs have existed for over 400 million years and are still alive today!Now, when we use fossils to date rocks, we have to be careful. Remember that some species of animals and plants lived for a very long time, while others existed only for a short period of time.We don't want to use fossils belonging to species that lived for too long; these fossils would show up in more than one rock layer.Let's say in the first outcrop, he found an upper rock layer containing ammonite fossils and a lower layer containing scallops.In the second outcrop, miles and miles away, he also found two layers; but these layers were different.
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An index fossil is a fossil representing a plant or animal that existed for a relatively short duration of time.