Name:
Date:
Period:
Fossil Occurrence and the Age of Rocks
Groups of fossil organisms occur throughout the geologic record for specific intervals of
time. This time interval is called the fossil’s range. Knowing the range of the fossils of
specific organisms or groups of organisms can be used to relatively date rocks and
sequences of rocks. In this laboratory exercise, you will use such information to assign a
date to a hypothetical unit of rock.
Problem How can the occurrence of fossils and their known age ranges be used to date
rocks?
Skills Interpreting Diagrams, Graphing, Hypothesizing, Inferring
Procedure
1. 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. Make a bar graph using the information shown in the Data Table. Begin by
listing the numbers of the individual fossils on the x-axis. Use the Geologic Time Scale to
list the time units of the Geologic Time Scale on the y-axis.
2. Transfer the range data of each fossil onto the graph. Draw an X in each box,
beginning at the oldest occurrence of the organism up to the youngest occurrence.
Shade in the marked boxes. You will end up with bars depicting the geologic ranges of
each of the fossils listed.
3. Examine the graph. Are there any time units that contain all of the fossils listed?
Write this time period at the bottom of the graph.
Analyze and Conclude
1. Reading Graphs What is the age of the hypothetical rock layer that these fossils were
collected from?
2. Inferring Based on the age determined, do you think that this group of fossils could be
considered index fossils? Why or why not?
3. Inferring A species of the trilobite listed in line 7 of the Data Table (Paciphacops logani) is
limited to rocks of lower Devonian age. Trilobite fossils are widespread throughout North
America. Can this fossil be considered an index fossil? Why or why not?
4. Connecting Concepts These fossils were collected from limestone and shale rocks.
Based on what you have learned about the formation of these rock types, what type of
environment did these organisms live in?
5. Understanding Concepts Shale often contains fossils of leaves. If the gastropods
listed in line 3 and line 6 were collected from shale containing leaf fossils, could you use
radiocarbon dating to assign a numerical date to this rock unit? Explain.