Earth & Space Science: Session 2
A Closer Look: Fossils
What is a fossil?
Fossils are either the preserved remains of ancient animals and plants, their imprints, or the traces of their activities. Fossils are most often preserved in sedimentary rock. However, organisms preserved in ice, or organic matter such as amber or tar, are also considered fossils if the organisms are from the prehistoric past.
Fossilization is a rare event. The chances of a given individual plant or animal becoming a fossil are very small. The hard skeletons and shells of animals and hard woody plant structures are more likely to become fossilized than the soft tissues of organisms. Fossilization is also more likely to occur in marine environments. Even if fossils are able to form, they may not be preserved intact. For example, in some cases, sedimentary rock that is buried and compacted experiences temperature and pressure increases that can melt and deform it, changing it to metamorphic rock. Fossils preserved in sedimentary rock that becomes metamorphic rock are usually distorted or destroyed.
How are fossils formed?
When organisms become trapped in substances such as amber, tar, or ice, their remains can be preserved unchanged. In some cases, the structure of the organism's soft tissue is even preserved.
When petrifaction occurs, mineral rich fluids slowly seep into the cells of an organism and crystallize, displacing the original organic matter. Petrifaction is also referred to as permineralization. Most bone and wood fossils are permineralized.
When an organism is buried in mud, the minerals that make up its hard parts can dissolve and be replaced by different minerals such as calcite or silica. This process can preserve the soft tissue of organisms, providing important information on ancient organismal structures.
This process is also referred to as coalification. Plants are usually preserved through this process when bacteria decay the plant material and in doing so release elements like hydrogen, oxygen, and nitrogen. This reduces most of the remaining matter to carbon.
Many aquatic animals have shells composed of a form of calcium carbonate that will change into calcite, a more stable mineral, over millions of years. This process also rearranges small calcite crystals into larger crystals, preserving the original material but destroying its original structure.
Fossilized organisms preserved in rock can be destroyed or dissolved leaving an empty space in the rock where there once was an organism. The impression that is left behind in the rock is called an external mold. Sometimes the inside of a shell is filled with sediment before the shell dissolves or is destroyed. This is an internal mold. If the shell is filled with a new mineral, the shell replica is called a cast.
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