There are two major types of fossils - body fossils and trace fossils. Both are the remains of living organisms. Body fossils reveal the body structure of the organism while trace fossils reveal the activities of these organisms.
The process of fossilization is called taphonomy. There are three main components. First, there is the death of the organism. Then, there are certain processes that can happen to the organism before it is buried. These processes can include body decay due to natural elements such as wind, water or attack from predators. Finally, there are certain processes that occur after the organism's body is buried. These processes result in the different categories of fossils.
What are some factors that can affect fossilization?
Body construction: Does the organism have hard or soft body parts?
Environment: Do the surrounding conditions allow for rapid burial and preservation? Will the body be exposed to many elements of erosion?
Predators: Are other organisms going to destroy the body before it can be preserved?
How do fossils form?
There are many ways fossils can be formed including permineralization, freezing, compression, and entrapment by amber. (See informational links.) Methods of fossilization often involve rapid burial in such a way that predators and erosional effects are eliminated. This allows for preservation of the body parts or trace evidence. Below are some examples.
Permineralization is a major method that involves the hardening of minerals that have entered the small pores and cavities of dead organisms. As hard water (water containing minerals) enter these pores, minerals are deposited and, under high pressures, becomes solid.
Natural molds and casts form when the hard parts (such as shells) are buried in sediment leaving behind an impression of its shape. If the interior of the shell were to be filled with sediment that hardened, an internal mold would be created. However, if the shell should dissolve away over the years, it will leave behind a mold of the shell (external mold).
How is fossilization dependent upon the environment?
The environment plays a crucial role in an organisms ability to fossilize. The best scenario would be in which an organism is buried at the bottom of a lake where it is then covered by a lot of sediment. In this type of environment, the organism is protected from other animals and natural elements that would cause the body's breakdown. It is crucial that the body be in an environment that allows for rapid burial. Areas in which there is a high rate of sediment deposition is ideal because of the presence of minerals and the increase of pressure.
The environment can also affect where the fossil is found. For example, river currents can carry a body away from the site of death before it is buried.
Drier environments, such as land, are more susceptible to the effects of erosion and so it is more difficult to preserve the organism before it decays.
Where are fossils found?
Both body and trace fossils are commonly found buried in sedimentary rocks. There are major sediment deposits in such places as the mouths of rivers or at the bottom of sea and lake beds. Because rock layers are laid down oldest at the bottom and youngest at the top, fossils can provide an important clue to the Earth's history. (See info on the Fossil Record.)
Fossils can be found in more unusual places as well, such as inside amber or tar pits. Fossils can be found in many places around the world provided the conditions are right.
What does fossilization tell us about the fossil record?
The fossil record does not represent all of the living things once found on Earth. The reason for this is because not all organisms have correct properties that allow for fossilization. Some organisms may have decayed before fossilization could begin. Organisms that live on land or have soft body parts are less likely to fossilize than those that live in water or have hard body parts. Therefore, the fossil record can only provide a piece of Earth's history.
Activities and Lessons
Trace Fossils (Warning: This one can get messy.)
Casts and Molds
Creative Writing Ideas (Just for Fun)
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