from the American Geological Institute: Learning about Fossils
http://www.k5geosource.org/content/dd/fossil/pg2.htmlHow do fossils form?
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.
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.
Under what conditions do fossils form?
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.
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.
How can we tell how old rocks are?
Knowing the fossil record lets a geoscientist place a particular fossiliferous rock layer into the scale of geologic time. But the time scale given by fossils is only a relative scale, because it does not give the age of the rock in years, only its age relative to other layers. Long after the relative time scale was worked out from fossils, geologists developed methods for finding the absolute ages of rocks, in years before the present. These methods involve radioactivity. Here's how one of the important ones works.
Some minerals contain atoms of the radioactive chemical element uranium. Now and then, an atom of uranium self-destructs to form an atom of lead. Scientists know the rate of self-destruction. They grind up a rock to collect tiny grains of minerals that started out containing some uranium but no lead. Then they use a very sensitive instrument, called a mass spectrometer, to measure how much of the uranium has been changed to lead. Using some simple mathematics, they can figure out how long ago the mineral first formed. It is possible to date rocks as old as four billion years this way.
Absolute dating of rocks has provided many "tie points" for the relative time scale developed from fossils. The result is an absolute time scale. When you collect a fossil from a rock, you can place it in the relative time scale. Then you also know about how old it is in years (or usually millions, or tens of millions, or hundreds of millions of years).
