Rock Cycle - Past Life (5)

  • Illustrating how fossils are preserved.
  • Making fossils.
  • cast
  • fossil
  • impression
  • mold

Students make a mold and cast.

Fossils often look "worn" or damaged, 
due to the effects of fossilization


The chance that an organism will be preserved as a fossil is low. Geological processes such as erosion, weathering, sedimentation, and leaching constantly "attack" the fossil, which may destroy it before anyone sees it.

There are two main types of fossil preservation, with alteration and without alteration.  Most common is fossil preservation with alteration; the original organic material is partially to fully changed into new material. 

There are several types of preservation with alteration:

  1. carbonization, a chemical reaction where water transforms the organic material of plant or animal to a thin film of carbon. Nitrogen, hydrogen, and oxygen are driven off as gases, leaving an outline of the organism. Organisms often preserved by carbonization include fish, leaves and the woody tissues of plants.
  2. permineralization or petrifaction takes place in porous materials such as bones, plants and shells. The material is buried; later, groundwater percolates through its pore spaces. A solution, commonly supersaturated in either calcium carbonate or silica, precipitates minerals in the spaces. The original wood or shell like material preserved.
  3. recrystallization changes the internal physical structure of a fossil. Recrystallization changes the microstructure of the original minerals; they often reform as larger crystals. The composition of the mineral does not change, only the crystal structure. For example, many shells originally composed of calcium carbonate in the form of the mineral aragonite recrystallize into the more stable form of calcium carbonate called calcite.
  4. replacement involves the complete removal of original hard parts by solution and deposition of a new mineral in its place. The Petrified Forest in Arizona is an excellent example of this type of preservation. Here the original organic material (wood) has been wholly replaced by silica.  

The second type of fossil creation is without alteration or direct preservation. The most common directly preserved fossils are unaltered hard parts of a living organism, like shells, teeth, and bones. This material is unchanged, except for the removal of less stable organic matter. Other examples of this type of preservation include fossil corals, shells, sponges, microscopic fossils and a host of other organisms with hard parts. In rare circumstances, preservation of the soft parts of an organism may occur.

Paleontologists can also study past life using indirect evidence about how the organisms lived. Types of indirect evidence include molds and casts, tracks and trails, burrows and borings, and coprolites.

The formation of a mold and cast is a very common type of indirect preservation. After the remains of an organism have been buried and cemented within sediment, water percolating through the sediment leaches out the fossil. This leaves a cavity in the rock, called a mold. A cast then forms when the mold is filled up with another substance. In some cases minerals such as calcite or quartz precipitate in the mold; elsewhere loose sediment may fill it up. The formation of a cast is similar to putting jello in a mold; when you remove the mold, you are seeing the cast of the mold.

The other types of indirect evidence are collectively called trace fossils. A trace fossil gives a paleontologist some evidence of the organism’s behavior. There are three main types of trace fossils. Tracks and trails are produced by an organism walking, crawling, foraging, or resting. For example, dinosaur tracks provide information about how large the dinosaur was, how fast it walked, and whether it walked alone or in a group. Burrows and borings are the tunnels or burrows left by organism digging into the ground, either on land or underwater. This may indicate whether the animal was feeding, dwelling, or just foraging. Finally, coprolites are fossilized animal excrements. They give some indication of the structure of the animal’s gut, and sometimes provide clues to its diet.

  1. In this lab the students prove that the "present is the key to the past" by using the remains of present day living organisms to make their own fossils. They will simulate the formation of mold and cast fossils.  They can make 5 different fossils (depending on time) including gastropod, scallop, clam, mushroom coral, and stone coral. 

    This lab can be extended by having each student group construct a "mini" analysis of their fossils. Have the students point out features such as the quality of fossil preservation, features that may have fossilized well or not at all, or any other observations or conclusions that you assign or that they discover.

    A short version of this lab is to prepare the molds and casts in advance, and have the students try and determine which impression belongs to which shell.

    Before lab, prepare the materials for each student group. If you have not made mold and cast "fossils" before, you may wish to practice before working with the plaster of Paris.

  2. Explain the principles of fossilization to the students. Be sure to describe the formation of mold and cast fossils. Tell the students they will make examples of molds and casts in the lab. Emphasize to the students that not all the information about an organism can be recorded in a fossil. Not all objects fossilize well. For example, objects with soft parts (like leaves) do not make good molds. Also, objects like gastropods (which are detailed inside the shell) only represent the outside, which can be misleading to whomever finds the fossil. Paleontologists sometimes have to act like detectives to trace what the organisms originally looked like. Sometimes they never find out!
  3. Each student group should make a mold and cast of a different type of fossil. If you have enough plaster of Paris, have the students make individual fossils. Use the instructions below to guide students in making fossil impressions.
    1. Flatten a piece of plasticine or modeling clay. This will be the base for making the mold and cast, so make sure the flattened clay is wider than the object to be "fossilized." To help guide your students (this is optional, but recommended), make a ring from a strip of thick paper. The diameter of the ring should be greater than the width of the object. Tape the ends of the strip together
    2. Push the ring into the clay. Cover the object with a little petroleum jelly. Press it into the clay, just enough so that it is firmly in place.
    3. Pour a thin layer of plaster of Paris into the ring, just enough to cover the object. Let it dry for a few hours. Drying time will depend on how wet you made the mixture and the thickness of the layer.
    4. Remove the ring and carefully separate the clay from the plaster of Paris mold. Gently remove the object. Now you have a model "mold" of the object.
  4. If desired, have the students paint their fossil with water-based paint. Have them try to make it look realistic.

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