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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.

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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.

Using Fossils to Correlate Strata

So, how exactly is an index fossil used for relative dating of rocks? Well, let’s go back to our surveyor, William Smith. He was often presented with the problem of finding two different rock outcrops from two different periods.

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. The upper layer had scallop fossils, and the lower layer had trilobites. Smith would have brought these two arrangements together, overlapping the common scallop layer, to produce a larger succession of three rock strata! Now we have a more complete piece of geologic history: a piece that says the trilobite layer is the oldest, the ammonite layer is the youngest and the scallop rock layer is somewhere in between.

Index fossils can be used to correlate the relative ages of rocks that are separated by vast distances. The cool thing is that we can even correlate rocks from different continents! For example, scientists found Barosaurus fossils inside a layer of Tendaguru rocks in East Africa. They also found Hypsilophodon fossils inside a layer of Wealden rocks in Europe. Scientists didn’t know how old either of the rocks were, or even which dinosaur was older than the other.

But in North America, they found a big chunk of rock which contained both fossils. The Barosaurus was in the upper Morrison Formation, while the Hypsilophodon was in the lower Lakota Sandstone. Therefore, the Hypsilophodon had to be older than the Barosaurus. And, even though the rock types were different, scientists could assign relative ages to the other rocks based on their fossils. They could safely assume that the Tendaguru rocks in East Africa were older than the Wealden rocks in Europe.

Lesson Summary

When rocks are made up of distinct strata, we use stratigraphic succession to determine the relative ages of each of the layers in the rock. However, another form of relative dating is the use of fossil succession: the principle that certain assemblages of fossils can be tracked in a stepwise fashion through geologic time.

In order to use fossils for relative dating, scientists focus their efforts on index fossils. These fossils represent plants and animals that lived for a relatively short period of time. We use index fossils to identify periods of geologic history and to match up pieces of rock strata that have been separated by large distances. When one outcrop contains two index fossils from two different time periods, it acts as a ‘missing link’ between other outcrops that have only one of the two fossils.

Learning Outcomes

When this lesson is completed, you should be able to:

  • Define and explain relative dating of rock and fossils
  • Determine what index fossils are
  • Discuss the missing link of two different time period fossils found in one strata

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