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Relative Dating with Fossils: Index Fossils as Indicators of Time

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  • 0:07 Review of Relative Dating
  • 1:04 Fossil Succession
  • 2:14 Index Fossils
  • 3:36 Using Fossils to…
  • 5:27 Lesson Summary
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Lesson Transcript
Instructor: April Koch

April teaches high school science and holds a master's degree in education.

You may already know how to date a fossil with a rock. But did you know that we can also date a rock with a fossil? Watch this video to find out how we use index fossils to establish the relative ages of rocks.

Review of Relative Dating

In previous lessons, we talked about the Geologic Time Scale and how scientists use it to piece together the history of the earth. We talked about relative dating of rocks and how scientists use stratigraphic succession to compare the ages of different rock layers. You should already understand that the lower rock strata are generally older than the strata found higher up in the rock. 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. One outcrop shows layers from one geologic time period, while the other outcrop represents a different time. What can a scientist do with these two outcrops? Can he put the pieces together to make the story more complete? Can he match one set of strata to the other? Let's find out how scientists deal with this common problem by using the fossils inside the rocks.

Fossil Succession

Back in 1793, there lived a land surveyor named William Smith. He worked in Southern England, and he got to see all kinds of different rock strata that were exposed in outcrops and canals. William Smith collected fossils from his work sites and, over time, he learned to recognize which fossils tended to show up in which rock strata. He began to identify rock layers by the fossils they contained, and he even noticed that the general order of strata was identical over many different parts of the country. Smith was the first person to understand the principle of fossil succession.

Fossil succession is based on the observation that certain assemblages, or groups, of animals and plants have lived during certain time periods over geologic history. For example, human beings and modern elephants are part of the same assemblage because we live in the same time period. Stegosaurus and Triceratops were not part of the same assemblage because they lived at different times. Obviously, the fossil assemblages change from period to period. They follow an ordered progression that is very clear and predictable. Therefore, we can use the succession of fossil assemblages to establish the relative ages of rocks.

Index Fossils

Now, when we use fossils to date rocks, we have to be careful. We can't just use any fossil that we find. 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. 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.

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. 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! We'd want to use a more short-lived fossil, like the dodo bird. We also want our index fossils to be common, widely-distributed species that are easy for scientists to identify. Some of the scientists' favorite index fossils are trilobites, ammonites and scallop shells.

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