What is the fossil record?

For a fossil to size, several conditions have sầu lớn be met. First of all, the animal had to live sầu in the given area! Animals live in many environments on Earth, but not everywhere. The water above sầu many lake bottoms & many areas of the deep ocean bottom are stagnant. The bottom water is never exchanged with surface waters, so the water contains no dissolved oxyren. Animals cannot live sầu without oxygene, so no animals live sầu there. In these situations, the only possibility of fossilization is if a fish or other swimming animal dies in oxygen-rich waters above sầu, sinks down into lớn the stagnant muddy bottom, and is buried by sediments.

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Courtesy USGS Cascade Volcano Observatory

Most environments on the l& surface are populated with animals. Fossilization on l& is very uncomtháng, however, because most areas of the l& are being eroded. Unless there is deposition, fossils cannot be preserved. Deposition on lvà is common only in river valleys. Fossils are fairly comtháng in sediments deposited on river floodplains. Some ocean environments that tư vấn animal life are exposed to very svào currents & waves. After a shelled animal dies, the strong water motions cause the hard toàn thân parts khổng lồ be broken & worn. Often the shells kết thúc up just as rounded grains of svà or gravel, which no longer look like fossils.

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For animals without skeletons, lượt thích 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 lớn be fossilized, its toàn thân must be protected from decomposition. The body is usually exposed to air và water with a lot of oxyren, 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 lớn decompose, because water that seeps through the sediment around it usually is rich in oxyren. Sometimes, however, the body is buried rapidly by fine mud. Water seeps through mud much more slowly than through s&, so the body toàn thân does not decompose as fast. Mud often contains a lot of other organic matter as well, và that uses up oxygene 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 sầu. Other kinds of organisms are much better represented by fossils. Animals with hard shells và 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. Lvà 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.