Carol Mankiewicz* and Carl
Departments of Biology* and Geology*^
Beloit College, 700 College Street, Beloit,
Level: Grades 4-6
Estimated time required: 3 to 4 hours,
over three class periods
Anticipated Learning Outcomes
- Students will learn about different types of fossils,
particularly trace fossils.
- Students will learn how the sediment type affects fossil
- Students will learn how to design an experiment.
Paleontologists study fossils. When most people think of
fossils, they think of shells, bones, or teeth; these are called body fossils.
But by definition, a fossil is any evidence of pre-existent life. If a footprint
were preserved in a rock, that would be evidence that an animal had once
traveled over the surface that was later preserved in the rock; therefore,
the footprint would be a fossil. Footprints represent one type of fossil
known as trace fossils. Trace fossils record the past behavior of organisms.
They include tracks, trails, burrows, and any other mark made by an animal
walking, running, crawling, resting, or feeding on or within soft sediment,
such as sand or mud. Plants also can leave traces--think of a leaf blowing
Trace fossils commonly occur in rocks that do not contain body fossils;
in fact, the trace fossils may be the only evidence that organisms had once
lived in a certain area. Thus, it is important to understand as much as
we can about how traces are made and what they mean. This is easier said
than done because the animal or plant is gone, leaving no hard parts to
study. In order to better understand fossil traces, many paleontologists
study living organisms and their traces. They ask questions such as "How
does the weight of the animal affect the appearance of the trace?"
"How does the type of sediment (mud versus sand) affect the trace?"
"How does the dampness (moisture content) of the sediment affect the
trace?" "Can an animal make more than one kind of trace?"
"Can two animals make similar traces?" You don't have to be a
paleontologist to study traces; anyone can do it.
- Plaster of Paris
- Container for mixing the plaster
- Sticks for stirring the plaster
- A flat piece of plastic or wood (e.g., ruler) for smoothing
- Shallow aluminum pans or one-gallon plastic milk jugs
with the upper third cut off
- Mud and sand, preferably a few grain sizes
- Small pets (e.g., hamsters, mice, lizards), large slow
insects (e.g., some beetles) or other small animals (e.g., snails or slugs)
- Optional: a large cage or terrarium
Note: Procedures 1 and 2 can be completed the first day,
3 through 5 on the second day, and 6 on the final day of the experiment.
- Have the students work in groups of three to four. Each
group needs to decide what to test in its experiment. This is an important
step; the students will probably need guidance. One group might use dry
sand and then do the same experiment with damp sand. Another might use
damp mud and damp sand. These two combinations will probably work the best.
It's okay if two groups do the same experiment; they can compare their
results at the end. Or, one group can use the same conditions, but use
a different animal. The main design to avoid is testing more than one variable
at a time. For example, advise them against using damp mud and dry sand.
In this case, they wouldn't be able to decide whether the fidelity of preservation
of the traces was due to dampness or to grain size.
- After they have chosen which variable they will test,
the groups must make sure they have appropriate materials. The main decision
here is what kind of container to use. The choice will probably be a function
of which kind of animal will be studied. For example, a shallow pan will
accommodate the slow-moving snail, but not the fast lizard. A modified
milk jug might be more appropriate for the lizard; a shallow pan also could
be used if placed inside of a large cage or terrarium. Two containers will
be needed for each group.
- Put the chosen sediment types in the appropriate containers.
Add water to the sediment if this is part of the design; drain off excess
water (you don't want a sediment soup!). Smooth the surface of the sediment
using a flat piece of plastic or wood; a cheap ruler should work well.
- Allow the animal to run, walk, or crawl across the surface
of each sediment type. All the groups will enjoy watching this part.
- Mix the plaster of Paris with water. Use a ratio of 2
to 1 (e.g., 200 ml plaster to 100 ml water). The amount needed will depend
on the depth of the trace and the total area occupied by the trace. Pour
enough of the mixture to fill the trace and to cover the adjacent sediment
to a depth of a couple of millimeters. Allow to dry overnight.
- Check to make sure the plaster casts are dry, and then
carefully remove them. If necessary, carefully rinse off excess sediment
with water. Study the plaster casts. Let each group report on the results
of their experiment.
Results and Discussion
- As the animal moves across or through the sediment, it
leaves a depression. This depression can be thought of as a record of the
animal's movement. The plaster fills in the depression, imitating the shape
of the trace. Most trace fossils are preserved in a similar fashion--they
are preserved best by the sediment that fills in the tracks or trails.
Thus, the experiment mimics preservation in the fossil record.
- For heavier animals, the most distinct, recognizable
traces will probably be produced in damp sand because the dampness increases
the cohesion of the sediment. If the sand is too dry or too wet, the sides
of the trace will cave in, thus modifying the shape. Damp mud tends to
stick to the foot or body as the animal moves across or through the sediment;
again, the shape of the trace might be altered.
- For very light animals, the best trace might be produced
in dry, finer sand or possibly damp mud; however, this trace may not be
very distinct. Traces might not be preserved in damp sand if the animal
is not heavy enough.
- A consideration of the points made in 2 and 3 above should
provoke a discussion of the value of studying modern traces. The shape
of the trace might tell the paleontologist something about the environment:
a distinct trace might suggest damp sediment. The depth of the trace might
tell the paleontologist something about the animal: the deeper the trace,
the heavier the animal.
- As an alternative to using live animals, clam shells
could be pressed into the sediment, thus leaving an impression. Although
this impression of the clam shell is not a trace fossil (it does not record
behavior), the results would yield information regarding fidelity of preservation
of shape in different grain sizes or moisture contents; such results would
not yield information on weight.
- If you have access to sieves, you may wish to perform
these experiments using different sand sizes. Sand, as defined by a geologist,
ranges from 1/16 mm to 2 mm in diameter. You will find that the fidelity
of preservation of traces will depend a great deal on the sand size you
- If your school is located near a sandy lake or ocean
shore, you could extend the experiment using the students as trace makers.
On the basis of the original experiment, you might conclude that the most
distinct human footprints would be left in damp sand. Have each student
walk naturally on damp sand near the shore. Have each determine her/his
average stride and average depth of penetration of the heel. Each student
should also record her/his height and weight. The students could pool all
of this information and summarize it on two graphs. In graph one, they
could plot length of stride against height; in graph two, they could plot
depth of heel print against weight. Both of these graphs should show a
positive correlation between the variables. Repeat the experiment, but
this time invite the students to run across the sediment. Plot the data
on the same graphs and compare running to walking. Discuss how paleontologists
could use this information to determine behavior of ancient humans or even
of animals like dinosaurs.
ALEXANDER, R.M., 1991, How dinosaurs ran: Scientific American,
v. 264, no. 4, p. 130-136.
EVANS, R.H., 1990, The physics of dinosaurs: The Physics Teacher, September
1990, p. 364-371.
FREY, R.W. and WHEATCROFT, R.A., 1989, Organism-substrate relations and
their impact on sedimentary petrology: Journal of Geological Education,
v. 37, p. 261-279.
LOCKLEY, M., 1984, Dinosaur tracking: Science Teacher, v. 51, p. 18-24.
WHITE, T.D., 1980, Evolutionary implications of Pliocene hominid footprints:
Science, v. 208, p. 175-176.