Making an Impression: In which soil environment does a fossil most easily form?
(Excerpted from Experiment Central, published by U·X·L and available in Science Resource Center, a Gale online database.)
Purpose/Hypothesis
Paleontologists have found fossils on every continent, yet some areas contain more fossils than others. One of the key factors leading to fossil formation is the type of sediment or material in which a dead organism settles. (Most organisms settle where they die; in some cases a river, wind, or animals can carry the organism to another location.) Scientist use fossils to study and determine the lifestyles and adaptations of plants and animals. The more details found in a fossil, the more information the scientists gain.
In this experiment, you will determine how the soil makeup of different geographical areas impacts the number of fossil casts formed. You will make three fossil casts in three soils of varying moisture content. One of the soils will be dry sand. Sand is made up of large particles and does not hold moisture. A second type of soil will be a mixture between sand and moist topsoil, which is made up of smaller soil particles that retain water. The third soil will be a wet topsoil.
These soils will be the foundation layer for a plaster of paris cast. Using one object, a shell, you will first press the organism into each soil to equal depths. The plaster of paris will form a cast from this mold. This cast will be the fossil.
To begin this experiment, make an educated guess about the outcome of the experiment based upon your knowledge of fossils and sediment. This educated guess, or prediction, is your hypothesis. A hypothesis should explain these things:
- the topic of the experiment
- the variable you will change
- the variable you will measure
- what you expect to happen
A hypothesis should be brief, specific, and measurable. It must be something you can test through further investigation. Your experiment will prove or disprove whether your hypothesis is correct. Here is one possible hypothesis for this experiment: "The moist soil will make the best fossil impression; the dry material will not be firm enough to cause a fossil to form."
Once you have gathered your soil bases you need to make your impressions. It is best to use a seashell with distinguishing qualities such as scallops, ridges, and possibly an erosion hole or chip.
The variable you will change will be the soil. The variable you will measure will be the general shape and amount of detail of the impression. The item you use to make the impression should stay the same.
What Are the Variables?
Variables are anything that might affect the results of an experiment. Here are the main variables in this experiment:
- the soil makeup
- the consistency of the plaster of paris
- the object/organism
- the depth the object is pressed
In other words, the variables in this experiment are everything that might affect the ability of the object to make an impression. If you change more than one variable at the same time, you will not be able to tell which variable had the most effect on the impression.
Level of Difficulty
- Easy to Moderate.
Materials Needed
- plaster of paris (available at craft stores)
- shell, preferably one with identifiable features such as a hole, chip, or alternate mark
- three small disposable containers, such as a butter dish, large enough to fit the shell
- water
- disposable spoons
- measuring spoon
- ruler
- straw
- tweezers
- marking pen
- bowl
- 3 cups (0.75 liters) of sand (available at garden store)
- 3 cups (0.75 liters) of moist, organic topsoil (available at garden store)
Approximate Budget
- $5 to $10.
Timetable
- 1 hour for the experiment; overnight for the plaster of paris to harden.
How to Experiment Safely
- Be careful when removing your fossil casts; plastic containers may break.
Step-by-Step Instructions
- Make a sketch of your shell, noting the width, depth, and any identifiable features.
- In a bowl, mix 1 cup (about 0.25 liters) of moist topsoil with 1 cup (about 0.25 liters) of sand. This is the moist soil.
- Mix 2 cups (about 0.5 liters) of topsoil with 8 tablespoons (about 120 milliliters) water. This is the wet soil.
- Label each container with the type of soil and place each soil type into the appropriate container. The soil should be at least 2 inches (5 centimeters) deep. Even out the surface of the soil.
- Use the ruler to mark a line on the straw at 0.8 inches (2 centimeters).
- Place your shell in the soil with the ridges facing down. Gently place the straw in the center of the shell. Push the straw down with your pointer finger until the mark on the straw is level with the soil.
- Using tweezers, carefully remove your shell.
- Wash and dry the shell. Repeat Step 6 for the other two soil samples.
- Mix enough plaster of paris to make a 1-inch-deep (2.5-centimeters) layer in each container. The plaster should be the consistency of thick pudding.
- Pour a layer of plaster of paris into each container.
- Allow plaster to harden overnight.
- Remove your fossil casts by slipping a butter knife or similar thin object in the side between the soil, fossil, and container. It may be necessary to break the plastic containers.
- Make sketches of each cast. (If a digital camera or Polaroid is available, you could take pictures.) Include any measurements of width and depth you are able to determine from your fossil cast.
Summary of Results
Review the sketches of the casts compared to that of the shell. Which soil type is best for making fossils? What qualities did you compare to determine the best soil? Note on the sketch or photograph where any information can be observed on the fossil. For example, a shell may have a hole in one point that can indicate erosion. What type(s) of environments do you feel are most suitable for fossils to form? From your conclusions, how would the environment impact the study of species through fossils?
Troubleshooter's Guide
Below is a problem that may arise during this experiment, a possible cause, and a way to remedy the problem.
Problem: You have no fossils, only a lump of plaster.
Possible cause: All of your soil samples may have been too loose. It may be necessary to choose a soil sample that has a more solid consistency.