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“Exploration Lab: Fossil Occurrence & Age of Rocks”
Introduction:
Groups of fossilized organisms occur throughout the geologic record for specific “intervals” (periods) of time.
This time interval is called the fossil’s age range. A Fossil’s Age Range is the period of time between its oldest
occurrence and its youngest occurrence. Recall that Relative Dating is the process by which scientists organize
rocks in a general sequence or timeline without dates. Scientists can use the age Range of the fossils of specific
organisms to relatively date sequences of rocks. If a group of fossils are widespread geographically, occur in
large numbers, and are limited to a short span of geologic time, they can be considered Index Fossils, that are
helpful in the process of relative dating of rocks.
Purpose:
We will use fossil age ranges to assign a date to a hypothetical (made-up) unit of rock.
Problem: How can the occurrence of fossils and their known age ranges be used to date
rocks?
Pre-Lab: (complete sentences!)
1. How many different types of fossils were found in the rock sample? _______________
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2. Using the Geologic Time Scale (Figure A), decide which geologic period is the oldest
period represented by fossils found in the rock sample: _________________________
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3. Which fossilized organism could have been alive during this oldest period?___________
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4. Which geologic period is the most recent period represented by fossils found in the rock
sample? ____________________________________________________________
5. Which fossilized organism could have been alive during this most recent period? ______
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Materials:
 Geologic Time Scale
 Graph Paper
 Pencil
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Procedure:
A section of rock made up of layers of limestone and shale has been studied and
samples have been taken. A large variety of fossils were collected from the rock
samples.
1. Use a sheet of graph paper to make a bar graph using the information shown in the
Fossil Data Table 1. Begin by listing categories of individual fossils on the horizontal
axis. Use the Geologic Time Scale (Figure A) to list the units of the geologic time scale
on the vertical axis.
2. Transfer the age range data of each fossil onto your graph. Draw an “X” in each box,
beginning at the oldest occurrence of the organism up to the youngest occurrence.
3. Shade in the marked boxes. You will end up with bars showing the geologic ranges of
each of the fossils listed.
Observations:
1. Follow the given procedure to create a bar graph showing the age ranges for each fossil
listed in the Fossil Data Table 1.
2. Examine your graph. Are there any time units that contain all of the fossils listed?
(HINT: Look at the vertical axis for this information!). Write this period at the bottom
of your graph!
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Y-axis: _______________________________
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X-axis: ____________________________________________
Analysis and Conclusion
1. Which geologic period contains samples of each fossil? _________________________
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2. What is the age of the hypothetical rock layer that these fossils were collected from? _
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3. Based on the age determined, do you think that this group of fossils could be considered
index fossils? WHY? (Hint: Look at your introduction ) _______________________
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4. Which fossil was found in the most geologic periods? ___________________________
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5. Which fossilized organism had the shortest age range? _________________________
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