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Fossils - Moldy Oldies

The great fossil mystery
Some plants and animals which lived on the Earth long ago left clues of past life that science detectives have discovered. These clues are called fossils. So grab your notepad and magnifying glass and get started on the most exciting case ever. It is time to investigate the "Great Fossil Mystery."

If rocks could talk
Fossils are the evidence of plants or animals which lived on Earth long ago. Scientists examine this evidence to make inferences about what happened on Earth millions of years ago. Fossils are usually found in rocks and are the preserved remains of a once living thing. To become a fossil, most organisms must have hard parts such as bones or shells since the soft parts decay before they are fossilized. The plant or animal must be buried quickly to prevent decay. It must also remain undisturbed for a long period of time. A fossil might be the thigh bone of a triceratops or the petrified trunk of a tree.

How do you explain it?
Sometimes a fossil is just a mark left behind by an organism when it was alive such as a trail, a burrow, or a track. These are called trace fossils. Dinosaur footprints are often found in the roofs of coal mines in Carbon County. Were the dinosaurs walking upside-down? Think about it...does this make sense? Perhaps it would be more accurate to say that the coal formed under the material of which the dinosaur stepped.

Leave no trace...
Some rare organisms are preserved unaltered (unchanged). This might happen when an animal falls through ice or into a tar pit. Some insects such as mosquitos have been preserved in amber (the fossilized sap from ancient trees). Most plants and animals that lived in the past died without leaving a trace.

What changes must happen to allow a fossil to form?

Replacement fossils are made when water dissolves part of the dead plant or animal and washes it away. This leaves an imprint of an organism - which is kind of like a sponge filled with holes. Minerals fill in the holes and then harden to form a fossil. The fossil is the same shape and size of the original organism. Sometimes the minerals fill in each cell so perfectly, that you can see very detailed parts of the once living organism like the rings in a tree trunk. Replacement fossils can be very colorful because the minerals which fill in the holes may be different colors.

Choose one of the following activities to make your own model fossil.

Replacement Fossil
Option		 Film Trace
Option		 Mold / Cast
Option
Animal option Plant option  
Materials:
  • Sponge you can cut up
  • Salt
  • Very warm water
  • Three or four paper cups
  • Food coloring or liquid tempera paint
  • Pie pan or plate

Procedure:

  1. Cut the sponge into the shape of a plant or animal part. Be creative. How about a raptor claw or a stegosaurus thigh bone?
  2. Place the sponge "organism" on the plate. 
  3. Make a "Mineral" mixture by doing the following: 
    • Put three tablespoons of warm water into each paper cup. 
    • Pour salt into each cup and stir until the mixture is very cloudy.
    • Add different food colorings or paints to each cup and mix to represent various minerals.
  4. Pour a little of each mineral mixture over different areas of the sponge organism. 
  5. Wait a few days for the fossil to harden.
  6. Observe your "replacement fossil" and record your observations. Did the minerals fill in the holes in the organism? How did the organism stay the same? How did it change?
 Materials:
  • Tree or plant leaves (Ferns and maples are great!)
  • Black tempera paint
  • Paintbrush
  • Paper plate
  • Art paper cut like a tombstone
  • Paper towels

Procedure:

  1. Place the leaf on the paper plate. Paint the back side with black paint. 
  2. Shake off the extra paint.
  3. CAREFULLY place the leaf painted side down on the art paper. 
  4. Place a paper towel over the leaf and gently press down all parts of the leaf. (Be sure the leaf doesn't move.)
  5. Carefully lift the leaf.
  6. You have now made a stamp of your leaf. How is this procedure different from nature's method of "stamping?"
 Materials:
  • Clay homemade or from the store (two colors)
  • Shells, bones, etc.
  • Cooking oil spray

Procedure:

  1. Flatten a small ball of clay and spray the top lightly with cooking spray.
  2. Press shells or bones into the clay and spray lightly again.
  3. Flatten another small ball of clay and press over the top of the shell.
  4. Carefully separate the two balls of clay and remove the shell.
  5. Allow the clay to harden for a few days. You have two "mold fossils."

Cast Fossil:
In order for a cast fossil to form, the molds must be filled in with minerals, mud or other sediments and allowed to harden.

  1. Roll a new color of clay into a small ball. Push it gently into one side of the mold fossil.
  2. Put the other side of the mold on top and press gently together.
  3. Open the molds and gently remove the "cast fossil." Allow a day or two for it to harden.
Utah State Office of Education This Sci-ber Text was developed by the Utah State Office of Education and Glen Westbroek.