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An index fossil is a fossil representing a plant or animal that existed for a relatively short duration of time.
These are the fossils that we want to use for relative dating.
When a scientist finds a section of rock that has lots of different strata, he assumes that the bottom-most layer is the oldest, and the top-most layer is the youngest.
But sometimes, a scientist finds a couple of rock outcrops that are separated by a wide distance.
Smith was the first person to understand the principle of fossil succession.Index fossils help us to distinguish between rock strata from different time periods, so it's important that they don't cover too much historical ground.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!As a member, you'll also get unlimited access to over 55,000 lessons in math, English, science, history, and more.Plus, get practice tests, quizzes, and personalized coaching to help you succeed.