Hands-On Lab
Biostratigraphy
Timing: one 90-minute class session
Objective(s):
Students will investigate how index fossils are used to construct the geologic time scale. Students will
investigate the evidence used to construct the geologic time scale and recognize that the evidence
used to construct the geologic time scale comes from observations from all over the world and includes
fossil evidence, radiometric age data and comparative studies of different rock sequences. Students will
learn how fossils are used to construct the geologic time scale.
Safety Precautions:
There should not be any safety concerns with this lab.
Materials:
Per small group:
• Samples or high-quality pictures of the following index fossils:
o Phacopida trilobite
o Proetida trilobite
o Ammonite
o Orthid brachiopod
o Spiriferid brachiopod
o Pentamerid brachiopod
o Strophomenid brachiopod
o Calamites
o Lycopod
o Fern
o Pine needles/cone
o Leaf from an angiosperm
• Age Ranges of Index Fossils Chart provided in this lab
• Rock Sequences charts provided in this lab
• Safety scissors
• Tape
Teacher Preparation:
Set up samples or high-quality pictures of the fossils listed above under “Materials.” These fossil
samples or pictures should be labeled with their scientific names, as well as their common names (if
applicable), and any other relevant information about the fossil. Ideally, there will be a complete set of
samples and/or photos for each pair or group of students. However, this investigation could also be
accomplished as a class with each sample or photo being passed around to the students if supplies are
limited.
Procedure:
The Hands-On Labs include both Directed and Guided Inquiry approaches. If your students are new
to the investigational methods being used in the Hands-On Lab, it is recommended that the Directed
Inquiry approach be used to provide scaffolding that will ensure student safety and support the success
of their investigations. Often, the Directed Inquiry approach involves modeling the basic laboratory
techniques and methods to be used in the activity. A discussion of each step in the investigative
process will also be included. In some cases, students may then be asked to create a procedure based
on the one modeled for them. This may involve changing specific variables or adjusting the procedure
to determine the effect on the outcome.
The students will work on the the Guided Inquiry path after completing the Directed Inquiry activity.
During Guided Inquiry, students are allowed to conduct the investigations more independently. They
will be given opportunities to formulate their own questions, develop their own procedures, and/or
manipulate variables of their own choosing. It may be necessary to provide additional materials and
supplies for students using Guided Inquiry. It will also be important to set clear limits on students’
activities to ensure their safety and the relevance of their inquiry experience to the content you are
teaching.
Directed Inquiry
Demonstrate for students how scientists use index fossils, drawing on their knowledge of relative age
relationships and the law of superposition. A good way of demonstrating this is to use relative ages on
a human scale before proceeding to relative age dating on a geologic scale. One possible procedure
follows:
1. Explain to students the law of superposition. State that in undisturbed, horizontal rock
layers, the oldest layer is on the bottom, and the layers get progressively younger
proceeding up the rock sequence.
2. Relate the law of superposition to throwing laundry into a laundry basket. On Monday, a
student gets home and throws her jacket into the basket in her room. Tuesday, the student
gets home from soccer practice and throws her jersey on top of the jacket. Wednesday, the
student gets ready for bed and throws her clothing on top of the pile, and so on. The jacket
was “deposited” first, so it is the oldest layer in the pile of laundry.
3. Next explain the concept of relative age dating. This can be done by asking the students if
they have any siblings. For example, if Megan is the middle child, she is older than her
younger brother, Tom, and younger than her older sister, Susan.
4. Next explain the concept of index fossils to the class. An index fossil is the fossil of an
organism that is known to have existed only for a relatively short period of geologic time.
Since the age range of a fossil is known, it can be used to narrow down the age of a rock
layer. Returning to the example of siblings, explain that the absolute age of each family
member is unknown but the age ranges of some of the family members are known. Megan
is a teenager, and her older sister, Susan, is in her twenties. The age range of her younger
brother, Tom, is unknown. Using this information, we know that the oldest Susan can be is
29, and the youngest she can be is 20. We know that the oldest Megan can possibly be is
19, and the youngest she can be is 13. We don’t know Tom’s age range, but we know that
he is younger than Megan. This means that Tom cannot possibly be older than the oldest
that Megan can possibly be. Therefore, the oldest Tom could possibly be is 19.
5. Then explain that the same principle can be applied to fossils. The age ranges of fossils can
be used to narrow down the ages of rock layers. If more than one type of index fossil is
found in the same layer, the age of the rock layer can be narrowed down even more.
Because of the law of superposition, each successive layer can possibly be only as old as
the oldest the layer below it can possibly be.
6. Explain to students that this process of determining the relative ages of rock layers and
sequences using index fossils is called biostratigraphy.
Divide students into pairs or small groups and proceed to the Guided Inquiry.
Guided Inquiry
Students will be given a description of several rock sequences, each containing index fossils. They will
also be given a chart that provides the age ranges for these index fossils. Students will use this to
determine the age of each rock sequence and the relative age of the whole set of rock sequences. Ask
the students some guiding questions to help them focus their inquiry:
• What are the age ranges of the index fossils in each layer?
• Does the overlap of any of these age ranges help to narrow the age range of the whole layer?
Explain.
• How does the law of superposition help to further narrow the age ranges of the rock layers?
Students should apply what they have learned about index fossils and the law of superposition to
determine the age range of each layer of the three Marine Rock Sequences below:
• Each layer in each rock sequence contains at least one of the index fossils shown on the Age
Ranges of Index Fossils Chart. Students can examine fossil samples and photos to familiarize
themselves with the real fossils named on the chart.
• Students should use the age ranges shown in the Age Ranges of Index Fossils Chart to
determine the age range of each layer of each rock sequence. Students should write the age
range (based on the periods on the geologic time scale—i.e., “Late Devonian to Permian” or
“Silurian to Devonian”) of each layer in the space provided. Note that index fossils with age
ranges that reach the top of the Age Ranges of Index Fossils Chart continue through the
Neogene Period to the present day.
• Students should then use the law of superposition to further narrow the age range of each layer
in the rock sequence where applicable, and write the new age range (if applicable) in the
second space provided.
•
•
•
After determining the age ranges of the layers of all of the Marine Rock Sequences, students
should cut out the three sequence charts, arrange the three sequences so that they are in order
from oldest to youngest, and tape them together.
Once students have completed this process with the Marine Rock Sequences, they should
repeat the process with the three Terrestrial Rock Sequences. Note that index fossils with age
ranges that reach the top of the Age Ranges of Index Fossils Chart continue through the
Neogene Period to the present day.
An answer key for each rock sequence has been provided below for the teacher’s reference.
Age Ranges of Index Fossils Chart
Marine Rock
Sequence 1
Layer 3
Index Fossils
Phacopida trilobite
Spiriferid brachiopod
Layer 1
Proetida trilobite
Orthid brachiopod
Marine Rock
Sequence 2
Layer 3
Index Fossils
Layer 1
Marine Rock
Sequence 3
Layer 4
Layer 3
Layer 2
Layer 1
Terrestrial
Adjusted Age Range
Based on Superposition
Age Range Based on
Index Fossils
Adjusted Age Range
Based on Superposition
Age Range Based on
Index Fossils
Adjusted Age Range
Based on Superposition
Phacopida trilobite
Proetida trilobite
Pentamerid brachiopod
Layer 2
Layer 2
Age Range Based on
Index Fossils
Pentamerid brachiopod
Phacopida trilobite
Spiriferid brachiopod
Proetida trilobite
Phacopida trilobite
Pentamerid brachiopod
Orthid brachiopod
Orthid brachiopod
Index Fossils
Ammonite
Spiriferid brachiopod
Spiriferid brachiopod
Strophomenid brachiopod
Ammonite
Stophomenid brachiopod
Spiriferid brachiopod
Spiriferid brachiopod
Orthid brachiopod
Proetida trilobite
Index Fossils
Age Range Based on
Adjusted Age Range
Rock
Sequence 1
Layer 3
Index Fossils
Based on Superposition
Age Range Based on
Index Fossils
Adjusted Age Range
Based on Superposition
Age Range Based on
Index Fossils
Adjusted Age Range
Based on Superposition
Pteridophytes
Gymnosperm
Layer 2
Calamites
Pteridophytes
Layer 1
Calamites
Terrestrial
Rock
Sequence 2
Layer 3
Index Fossils
Layer 2
Angiosperm
Gymnosperm
Layer 1
Pteridophytes
Gymnosperm
Terrestrial
Rock
Sequence 3
Layer 2
Index Fossils
Layer 1
Lycopod
Angiosperm
Lycopod
Calamites
ANSWER KEY FOR TEACHER REFERENCE
Marine Rock
Index Fossils
Age Range Based on
Sequence 3
Index Fossils
Layer 4
Ammonite
Triassic
Spiriferid brachiopod
Layer 3
Spiriferid brachiopod
Triassic
Strophomenid brachiopod
Ammonite
Layer 2
Stophomenid brachiopod
Silurian to Triassic
Spiriferid brachiopod
Layer 1
Marine Rock
Sequence 1
Layer 3
Layer 2
Layer 1
Marine Rock
Sequence 2
Layer 3
Layer 2
Layer 1
Spiriferid brachiopod
Orthid brachiopod
Proetida trilobite
Index Fossils
Phacopida trilobite
Proetida trilobite
Pentamerid brachiopod
Phacopida trilobite
Spiriferid brachiopod
Proetida trilobite
Orthid brachiopod
Index Fossils
Pentamerid brachiopod
Phacopida trilobite
Spiriferid brachiopod
Proetida trilobite
Phacopida trilobite
Pentamerid brachiopod
Orthid brachiopod
Orthid brachiopod
Adjusted Age Range
Based on Superposition
Silurian to Permian
Age Range Based on
Index Fossils
Ordovician to Devonian
Adjusted Age Range
Based on Superposition
Silurian to Devonian
Silurian to Devonian
Ordovician to Permian
Ordovician to Devonian
Age Range Based on
Index Fossils
Silurian to Devonian
Adjusted Age Range
Based on Superposition
Ordovician to Devonian
Early Cambrian to Permian
Early Cambrian to Devonian
ANSWER KEY FOR TEACHER REFERENCE
Terrestrial
Index Fossils
Age Range Based on
Rock
Index Fossils
Sequence 2
Layer 3
Angiosperm
Jurassic to Present
Layer 2
Angiosperm
Jurassic to Present
Gymnosperm
Layer 1
Pteridophytes
Gymnosperm
Late Devonian to Present
Terrestrial
Rock
Sequence 1
Layer 3
Index Fossils
Age Range Based on
Index Fossils
Pteridophytes
Gymnosperm
Late Devonian to Present
Layer 2
Calamites
Pteridophytes
Late Devonian to Permian
Layer 1
Calamites
Late Devonian to Permian
Terrestrial
Rock
Sequence 3
Layer 2
Index Fossils
Age Range Based on
Index Fossils
Lycopod
Calamites
Late Devonian to
Carboniferous
Layer 1
Lycopod
Early Silurian to
Carboniferous
Adjusted Age Range
Based on Superposition
Adjusted Age Range
Based on Superposition
Adjusted Age Range
Based on Superposition
Analysis and Conclusions:
In order to help students analyze and interpret their results, consider discussing some or all of the
following questions, or assigning them as homework:
1. Observe that the age ranges of many index fossils in the Age Ranges of Index Fossils Chart end
at the boundaries of geologic time scale divisions. Explain and analyze why index fossils end at the
boundary of a division in the geologic time scale. (Note that index fossils with age ranges that reach the
top of the Age Ranges of Index Fossils Chart continue through the Neogene Period to the present
day.)
Sample Response: Many divisions on the geologic time scale are marked by mass extinctions, such
as the Permo-Triassic extinction event and the Late Devonian extinction event. These index fossils
were likely wiped out by these mass extinctions.
2. Once scientists narrow the age ranges of a rock layer in the field using index fossils and the law of
superposition, what technique could scientists then use in the lab to determine the absolute ages of the
rock layers?
Sample Response: radiometric dating
3. Observe that certain species of the same type of animal, such as brachiopods or trilobites, were
around for a long period of time, and then new species of the same type of animal appear in the fossil
record. Analyze how this can be explained?
Sample Response: New species of the same animal evolve from other species.
Inquiry and Nature of Science Skills in this Lab:
• Design Investigations
o Make or use models that:
 Have as many details as possible replicated from the real thing.
 Function exactly like or similarly to the real thing.
 Have been revised as new knowledge and information has been obtained.
 Are based on logic or evidence.
o Explain the investigative processes by:
 Describing the logical sequence that was used to conduct the investigation.
• Gather Data
o Use senses to observe:
 Seeing (color, shape, size, texture)
o Use the appropriate format to record data:
 Table
 Chart
 Diagram
 Photograph/Image
• Interpret Data
o Sort and classify using scientific reasoning by:
 Sorting objects, substances, or organisms by characteristic
 Applying a classification scheme to objects, substances, or organisms
o Identify and interpret patterns using:
 Tables and graphs
 Analysis of data collected during an investigation
• Communication in Science
o Report results using:
 Table/graph showing data
• Scientific Investigation
o Scientific Investigation:
 Science takes place in many locations including labs, offices, fields, and under
the ocean.
o Scientific Data and Outcomes:
 Scientific claims are based on data and reliable scientific sources.
 Collecting and analyzing data is the best way to understand a changing pattern.
• Scientific Endeavor
o Characteristics of Science:
 One way to make sense of something is to think of how it relates to something
more familiar.
 Scientific claims can be substantiated using data and observation.
 Scientific theories are based on accumulated evidence.
 A law is a description of a specific relationship under given conditions in the
natural world.