Name: ______________________________
Pd: _____
Ast #: _____
Dating the Fossil Record: How old are the fossils?
Introduction: Geologists play an important role in helping us understand the order in which things occurred. The fossil
record consists of all of the fossils ever discovered on Earth. By analyzing where these fossils are found inside the Earth,
scientists can understand more about when and where different species lived. Geologists use two methods to
determine the age of rocks and fossils found inside the Earth.
Relative age dating allows scientists to determine when various events
occurred in relation to one another. This method helps scientist put the
events into a sequential order, from first to last or oldest to youngest. It can
be applied to different layers of rock found beneath the surface of the Earth
or to fossils found within those layers. Geologists use the Law of
Superposition to help them find the relative age of rocks and fossils. This law
states that sedimentary layers are deposited sequentially, with younger
layers forming over top of older ones. This means that the deeper geologists
dig, the further back in time they are able to see. As shown in Figure 1,
scientists can use the law of superposition to determine which species are
older and which are younger based on the rock layers in which they are found.
Absolute dating is a process which allows scientists to determine a numeric Figure 1: The Law of Superposition can be used to
age of something based on its physical or chemical properties. One common date fossils depending on the layers in which they
are found.
method of absolute dating is called radiometric dating, which uses known
properties of radioactive elements to determine how old a layer of rock or a fossil might be. This method can give
scientists an approximate time-frame in which the rock layer might have formed or the animal might have lived.
When these methods are combined, scientists can gain a more accurate picture of how events of the past unfolded,
whether it be the climate or landscape in a certain area, geologic events such as earthquakes, volcanic eruptions, or
floods, or whether certain species lived together at the same time.
Your Task: You have received nine rock samples from a paleontologist in California. Your job is to arrange the samples
in order from oldest to youngest according to their fossil content and to determine their relative ages using the
process of relative dating. Results from absolute dating methods will not be available from a laboratory for several
weeks, and the paleontologist needs the information immediately. You know from previous work that the rocks of
Sample 2 are the oldest.
Guiding Question: What is the relative age of the rock samples and the fossils within them? What is the age
range of each fossil species?
Materials:
- Sample Cards
- Fossil Key
- Data Table
- Colored Pencils
Getting Started:
1. Arrange the fossil cards from oldest to youngest. Begin with Sample 2 because you know this sample is the oldest. You
may need to try several different arrangements to get the cards in order. Hint: After an organism becomes extinct, it
doesn’t reappear in younger rocks. Also, filling in each type of fossil with a different color may help you see patterns.
2. In the data table, record the samples in order from bottom to top (oldest to youngest) in the first column. Sample 2 is
done for you.
3. Write the fossil names in order by age from left to right in the top row of the table. Hint: Examine your fossil cards
carefully to determine where each fossil appears in the rock record. Write an X in the appropriate column to indicate
which fossil or fossils are present in each sample.
4. You are planning to prepare a timeline for the paleontologist in California. But when the results of the radiometric
dating come in from the geology lab, you discover that the dates have become separated from the appropriate rock
samples. Absolute dating is very expensive, and you can’t have it done again. But wait! You have already determined
the relative ages of the samples. All you have to do is arrange the dates from oldest to youngest. Add these dates to
your data table.
1
Age Range of Each Species
Species
Earliest Date
Latest Date
2
Fossil Ages
The Geology lab has finished the radiometric analysis of the samples to provide an
absolute age. Unfortunately, in transport, the dates have become disorganized and
you must figure out which samples they belong to. Use your relative age data to
match the samples to the dates provided below (mya = million years ago).
28.5 mya
30.2 mya
18.3 mya
17.6 mya
26.3 mya
14.2 mya
23.1 mya
15.5 mya
19.5 mya
3