TRACE FOSSILS

Carol Mankiewicz* and Carl V. Mendelson^
Departments of Biology* and Geology*^
Beloit College, 700 College Street
, Beloit, WI 53511-5595


Level: Grades 4-6

Estimated time required: 3 to 4 hours, over three class periods

Anticipated Learning Outcomes

Background

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 across sand.

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.

Materials

 

Procedures:

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.

  1. 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.
  2. 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.
  3. 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.
  4. Allow the animal to run, walk, or crawl across the surface of each sediment type. All the groups will enjoy watching this part.
  5. 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.
  6. 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

  1. 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.
  2. 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.
  3. 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.
  4. 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.

Additional Activities

Selected References

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.

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