On the other hand, soft-bodied animals are probably represented very poorly. Animals with hard shells and skeletons are represented well in the fossil record. Other kinds of organisms are much better represented by fossils. That means that some kinds of organisms are much scarcer as fossils than they were when they were alive. Paleontologists are sure that the fossil record is biased. Under just the right conditions, a delicate impression of the animal might be preserved. Some animal bodies then escape decomposition. Mud often contains a lot of other organic matter as well, and that uses up oxygen faster. Water seeps through mud much more slowly than through sand, so the body does not decompose as fast. Sometimes, however, the body is buried rapidly by fine mud. Even then, it is likely to decompose, because water that seeps through the sediment around it usually is rich in oxygen. 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!). For a soft-bodied animal to be fossilized, its body must be protected from decomposition. When paleontologists find a well-preserved fossil of a soft-bodied animal, it's an occasion for celebration. Often the shells end up just as rounded grains of sand or gravel, which no longer look like fossils.įor animals without skeletons, like worms or jellyfish, fossilization is a very rare event. After a shelled animal dies, the strong water motions cause the hard body parts to be broken and worn. Some ocean environments that support animal life are exposed to very strong currents and waves. Fossils are fairly common in sediments deposited on river floodplains. Deposition on land is common only in river valleys. Unless there is deposition, fossils cannot be preserved. Fossilization on land is very uncommon, however, because most areas of the land are being eroded. Most environments on the land surface are populated with animals. 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.Ĭourtesy USGS Cascade Volcano Observatory Animals cannot live without oxygen, so no animals live there. The bottom water is never exchanged with surface waters, so the water contains no dissolved oxygen. The water above many lake bottoms and many areas of the deep ocean bottom are stagnant.

First of all, the animal had to live in the given area! Animals live in many environments on Earth, but not everywhere. For a fossil to form, several conditions have to be met.

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