BACKGROUND:
Evolution is the documented change in organisms, leading to the creation
of new species, through time. Evolution is a non-reversible process. For
example, dinosaurs will never exist again. Present day evolutionary theories
are based not only from biochemical data from living organisms, but also
from the remains of organisms preserved as fossils in rocks. Discoveries
about the details of the evolutionary process continue, but the basic
accuracy of the theory is not in doubt.
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 the
fossil before anyone sees it.
There are two main types of fossil preservation. 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:
- 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.
- 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.
- 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.
- 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 direct preservation, the
preservation of fossils without alteration. 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
withing 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 actually eating 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.
PROCEDURE:
- Discuss with students what fossils tell us about that organism. The
remains of organisms tell us about what it was and how it lived.
However, think about how organisms live today; they do not live in
isolation, but interact with other organisms in their environment. The
biological subdiscipline of ecology studies the relationships between
communities of organisms, as well as interactions between organisms in
their environment. Ecologists must answer a broad range of questions to
document ecological relationships. These include:
- How does the physical and chemical environment influence the
organisms?
- How do individuals gather food?
- How do individuals occupy living space, and what position of the
food chain do they occupy?
- How do organisms interact with each other?
- How does the community evolve through time?
- The physical, chemical and biological properties of communities can
limit the distribution and abundance of species. Limiting factors can
include parameters like temperature, salinity of water, oxygen, pH,
sunlight, water turbulence, nutrients, substrate, and predator/prey
interactions, to name a few.
Note that this list of factors includes very few items that are
recorded in the fossil record. You can illustrate this point by using a
fish tank. You might go home and find a few dead fish in your tank. You
can then check the temperature of the water, the pH, or other factors
and probably determine what caused the fish to die. However, in the
fossil record you cannot check many of these parameters. You must rely
on the remains of the organisms and from them determine the overall
ecology of the community. This is difficult. It would be as if you went
on vacation and came home to an empty fish tank. Maybe your fish died
while you were gone, and your maid disposed of the fish and drained and
cleaned the tank. How would you find out what killed those fish? Was it
the temperature, pH, the cat, or was the maid hungry?
The only source for ecological information on fossil organisms is the
rock surrounding the fossil. Sometimes the rock records the environment
where the organism lived. This is not always the case, as dead organisms
may be transported before they are fossilized. However, if the fossil
formed in rocks reflecting its "living" environment, the rocks
can be used to make ecological inferences. These include the type of
environment, i.e., land vs. ocean, the energy of the environment, and
sometimes basic environmental chemistry.
- Use the worksheet to have students reinact what the fossil might have
looked like as a living organisms. Both of them lived in the ocean,
because the legs are modified as fins. Color should be similar to
organisms that presently live in the oceans so they would be gray to
black
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