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Living things are made up of chemical compounds, most of which are organic
compounds. Organic compounds consist mainly of carbon, oxygen, and hydrogen.
After a plant or animal dies, it decomposes. As organisms decompose, their
organic compounds change into simpler compounds like carbon dioxide and
water. Decomposition is fastest when the organisms are in water that contains
dissolved oxygen. Organisms can also decompose even without oxygen. Some
kinds of bacteria feed on plant and animal tissues even though there is
no oxygen. These are called anaerobic ("no air") bacteria. Sooner or later,
almost all organic matter from plants and animals decays. Decay slows
down only when the organic matter is buried in very fine mud. That seals
the organic matter off from water with oxygen.
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| © Abi
Howe, American Geological Institute
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Sedimentary rocks seen in Dinosaur Provincial
Park in Alberta Canada. Dinosaur Provincial Park is considered
to be among the richest fossil sites in the world because
of the abundance and diversity of dinosaur remains, the rich
environments for plants and animals, and the spectacular badlands.
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The soft parts of an organism decompose the fastest. You know how little
time it takes for food to spoil and rot in warm weather when it is not
in the refrigerator. Bones and shells decompose much more slowly. Over
long times, their mineral materials dissolve. That can happen rapidly
when the shells and bones lie on the ground surface or on the sea bottom.
If the shell or bone is buried in sediment, it dissolves more slowly.
Shells are preserved without being dissolved only when they are buried
in sediments that consist of calcium carbonate minerals, like limestones.
The woody parts of plants that consist mostly of cellulose and lignin
decompose much more slowly than the softer parts.
Most animals become fossilized by being buried in sediment. For them
to be fossilized, they have to be buried and leave an imprint before they
decompose. Animals without skeletons are seldom fossilized, because they
decompose so quickly. Animals with hard skeletons are much easier to fossilize.
The most common fossils are shells of marine animals like clams, snails,
or corals. Insects, with thin outside skeletons of chitin, are not as
easy to fossilize. Sometimes an insect is trapped in sticky material,
resin, which comes out of some kinds of trees. The resin then hardens
to a material called amber. The insect fossil is preserved in the amber,
often perfectly.
Sometimes the actual shell or bone is preserved. Usually, however, you
see only its imprint. If it resists being dissolved for a long enough
time, the sediment around it turns into rock. Then, even though the shell
or bone dissolves, the imprint is preserved. When a hammer splits the
rock open, the fracture might pass through the imprint, and you see a
fossil.
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