Rock Cycle - Past Life (3)

  • Exploring the present is the key to the past.
  • Discovering trace fossils.
  • carbon
  • coprolite
  • trace fossil
  • tracks

Students examine how organisms become fossils.



The pre lab exercise introduced different types of fossil preservation. All of these are direct evidence of past life. 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 dissolves or leaches out the fossil. This leaves a cavity in the rock, called a mold. A cast 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. The formation of a cast is similar to putting jello in a mold. When you remove the mold, you are looking at 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 excrement. They give some indication of the structure of the animal’s gut, and sometimes provide clues to its diet.

  1. Before lab, prepare sample collections for each student group, using the specimens in the Rock Cycle - Past Life (3) or other materials that you may have available.
  2. Show the plastic lion and giraffe to the students (or other appropriate animals). Ask them to name differences between the two animals, for example, the lion is a carnivore, while the giraffe is a herbivore. Tell the students that both animals presently live on the African Savannah. Students should  imagine that both creatures are no longer living; all that remains are their trace fossils.
    Have the students predict what the fossil tracks of both animals would look like. Student should  consider both running and walking tracks. The giraffe would leave hoof like impressions, compared to the paw prints of the lion. In general, the giraffe's prints would be further apart than the lion’s tracks, because it is a bigger animal. Likewise, for both creatures, running tracks would have wider spacing than walking tracks.
    Next ask the students to consider the coprolites (fossilized poop)  of the two animals. How would they differ? You may wish to have them consider the "coprolites" of everyday animals, like cows and cats, as analogues. Note that the lion coprolite would probably have bone fragments, and the giraffe coprolite would not.
    Students should then consider the skeletons of the lion and giraffe. How would they differ? For example, the giraffe, compared to the lion, would have longer neck bones (although the same number of bones as a lion), and larger bones in general.
  3. In lab students will be looking at direct evidence of fossils. Students should  examine the fossils in the Rock Cycle - Past Life (3) kit. Introduce each of the specimens using the information below. Be sure to give them some information to guide their answers, but leave a great deal for them to determine.

    TRILOBITE - (from Morocco) These arthropods are now extinct, but the individual segments make it an obvious "bug." Trilobites are found in marine sedimentary rocks; they thus lived on the sea floor. The organisms were probably buried in mud after they died. The fossils are replacements of the original organism.

    AMMONITE (from Morocco). This is related to the present day nautilus (Mollusca). Ammonites were squid-like animals that appear to have been predators. The animal lived in the outermost chamber of its shell.

    CRINOID STEM (from Morocco) - Crinoids belong to the same family of marine animals as do seastars and sand dollars. Crinoids were animals, but they lived attached to the sea floor. The top part of the organism had feathery tentacles, which trapped food. The fossil in the kit is actually part of the stem which connected this feathery top to the sea floor. Like the trilobite, this crinoid fossil is an example of replacement.

    IRREGULAR ECHINODERM (from Morocco). This is a "fat" sand dollar which would be related to sea biscuits. They like warm, shallow water. They burrow in the sandy mud, and tend to be preserved where they die. This is an example of replacement.

    SHARK TEETH (from Morocco). These are from several species of sharks that lived in the ancient Mediterranean Sea. Compare this with the present day shark jaw that is included in the kit. Sharks’ teeth will fall out as soon as the shark eats something, so there are always rows and rows of teeth to take the place of the fallen ones. You can see this within the shark jaw.

    ORTHOCERAS (from Morocco) Is an extinct organism related to the Ammonites and the present day nautilus (Mollusca). Unlike these creatures, Orthoceras had straight or linear chambers instead of a spiral shape. This organism swam in the water, much like an octopus or squid.

    DINOSAUR BONES (from Alaska) These pieces represent a duck bill dinosaur that lived on the Arctic side of Alaska.  Notice the holes within the piece, which are diagnostic of dinosaur bones.  We know they are duck billed because of the larger pieces found. 

    STINGRAY TEETH  (from Morocco) These flattened-out fish are related to sharks. Stingray mouths are conveniently located on their bellies. The fossils in this kit are part of their mouth structure. 

    BRACHIOPOD (location unknown) Brachiopods had two shells and lived attached to the sea bottom or some object on the sea bottom.  Brachiopods are always bilaterally symmetric, although the top and bottom shells usually differ in shape

    PETRIFIED WOOD (from Arizona)  These pieces are remnants of giant trees from ancient forests of the Triassic Period over 200 million years old, these logs turned from wood to rock after the trees were buried under layers of sand and silt.  Quartz replaced the wood, that is why they are so hard.

    MOLLUSK FOSSIL  Clam and snail fossils can be seen in this specimen.

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