Fossil Occurrence and the Age of Rocks

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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.