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For a fossil to form, several conditions have to be met. First of all,
the animal had to live in the given area! Animals live in many environments
on Earth, but not everywhere. The water above many lake bottoms and many
areas of the deep ocean bottom are stagnant. The bottom water is never
exchanged with surface waters, so the water contains no dissolved oxygen.
Animals cannot live without oxygen, so no animals live there. In these
situations, the only possibility of fossilization is if a fish or other
swimming animal dies in oxygen-rich waters above, sinks down into the
stagnant muddy bottom, and is buried by sediments.
Most environments on the land surface are populated with animals. Fossilization
on land is very uncommon, however, because most areas of the land are
being eroded. Unless there is deposition, fossils cannot be preserved.
Deposition on land is common only in river valleys. Fossils are fairly
common in sediments deposited on river floodplains. Some ocean environments
that support animal life are exposed to very strong currents and waves.
After a shelled animal dies, the strong water motions cause the hard body
parts to be broken and worn. Often the shells end up just as rounded grains
of sand or gravel, which no longer look like fossils.
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© Michael
Collier
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The Green River meanders across its floodplain
after descending from Wyoming's Wind River Mountains.
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For animals without skeletons, like worms or jellyfish, fossilization
is a very rare event. When paleontologists find a well-preserved fossil
of a soft-bodied animal, it's an occasion for celebration. For a soft-bodied
animal to be fossilized, its body must be protected from decomposition.
The body is usually exposed to air and water with a lot of oxygen, so
it decomposes rapidly.The animal is likely to be fossilized only if it
is buried soon after it dies (or when it is buried alive!). Even then,
it is likely to decompose, because water that seeps through the sediment
around it usually is rich in oxygen. Sometimes, however, the body is buried
rapidly by fine mud. Water seeps through mud much more slowly than through
sand, so the body does not decompose as fast. Mud often contains a lot
of other organic matter as well, and that uses up oxygen faster. Some
animal bodies then escape decomposition. Under just the right conditions,
a delicate impression of the animal might be preserved.
Paleontologists are sure that the fossil record is biased. That means
that some kinds of organisms are much scarcer as fossils than they were
when they were alive. Other kinds of organisms are much better represented
by fossils. Animals with hard shells and skeletons are represented well
in the fossil record. On the other hand, soft-bodied animals are probably
represented very poorly. It's likely that most soft-bodied species that
ever existed are gone forever without a trace. Land animals are probably
very poorly represented as well. For example, most animals that are now
alive, or ever have lived, are insects, but the fossil record of insects
is poor.
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