Earth Science Activity 13
Name: _________________________________________ Period: __________ Date: __________
Index Fossils
Part 1: Investigate
Get the “Rock Layer Cutouts”. Cut out each lettered box. Each box represents a layer of rock and the fossils
they contain from a different Period in the Geologic Time Scale.
1. Use the ‘Key to Fossils’ to identify the fossils in each layer. Write the names of the fossils on the drawings.
2. The oldest rock layer is from the Cambrian Period. Some organisms in this layer will not be found in any other
layer. No organism in this layer still exists.
Which layer represents the Cambrian Period? ________
Check in with the Teacher ______
Now, order of all the layers from oldest to youngest.
3. Use the letters on the cards to record the correct sequence of layers in the data table. Record the names of
the fossils found in each layer. Use the Geologic Time Scale to identify the geologic period that corresponds to
each rock layer.
Earth Science Activity 13
4. How did you identify the layer that belonged next to the Cambrian layer?
5. Describe how some fossils were more helpful than others to figure out the relative ages of the layers. Provide
at least one example with your description.
Part 2: Background
An important tool for finding relative age is using index fossils. Index fossils provide evidence for the relative
age of rock layers. Not all fossils are index fossils. An index fossil must:
1. have lived for a relatively short period of time- The fossil of an organism that lived for only a million years or
so would be a good index fossil. You would know that any rocks containing that fossil are no more than a million
years different in age.
2. be found in many places- An index fossil should be widely distributed to be useful. For example, some of the
same index fossils are found in rocks of the same age in both the Grand Canyon and the Midwest.
If geologists identify an index fossil in a rock layer, they can be very sure of the age of the rock layer in which it
was found. They know it is about the same age as any other rock layers that contain the same index fossil.
STOP & THINK
1. Describe two important characteristics that make a good index fossil.
2. Identify one organism from the investigation that does not make a good index fossil and explain why it does
not help geologists determine the relative age of rock layers.
Earth Science Activity 13
Part 3: Practice
1. Based on the information from layers of rock at different locations pictured below, identify the index fossil
and describe why it makes a good index fossil.
Use the cross sections of rock below from three different locations separated by great distances to answer
question 2.
2. Using the index fossils in the diagram, identify the oldest layer of rock. Explain your reasoning.