Fossils can be preserved in a number of ways
A fossil is a remnant, or trace of an organism, from a past geologic age. Geologists, or scientists who study the history, origin and structure of the earth, use fossils to further their studies. Often the preserved remnants or traces are then thrust to the surface of the earth through natural geological processes. It takes much luck and sheer happenstance for a fossil to form, as the vast majority of living things are quickly recycled upon death, generally leaving only bones and shells behind. Where fossils do form though, it can happen in a number of ways.
Permineralization and Sedimentation
Permineralization is by far the most common type of fossil. This occurs when minerals fill the cellular spaces and crystallize. Eventually, the shape of the original plant or animal is preserved as rock. In some cases, none of the organic material remains.
When an animal dies and lays undisturbed, sedimentation can occur. Over time, sediments bury the exoskeleton, sealing it off from scavengers, or from bacteria that would normally decompose it. The quality of the fossil depends upon a number of factors. Fine-grained sediments, for instance, will produce more detailed fossils. Time is of the essence when creating a fossil by sedimentation, and such events as landslides and volcanic eruptions help to bury creatures quickly and create fossils in this way.
Carbonization or Distillation
Carbonization has occurred when plant leaves, or the soft body parts of fish or other animals decompose leaving only the carbon behind. This carbon resolves itself into an outline of the animal on the surrounding rock. These impressions can be very detailed. Carbonization also is called distillation.
Asphalt and Amber
As crude oil seeps through fissures in the earth, it occasionally forms pools. In these pools, the lighter parts of the oil evaporate, leaving behind an asphaltlike substance. One famous example of this is the La Brea tar pits. When animals became stuck in these, they sank and were preserved in the asphalt, which acts as an excellent preservative. Bones, teeth, shells have been pulled from tar pits. Even more fragile remains like the exoskeletons of insects and plant seeds have been pulled from tar pits.
Amber is the hardened fossil resin formed from the sap of trees. Sometimes insects or spiders that were caught in the sticky sap of trees millions of years ago become covered in it, and are preserved inside.
Refrigeration and Desiccation
The best fossils are formed by refrigeration. In places like Alaska and Siberia, fossilized mammoths and wooly rhinoceroses have been found in remarkable condition. In some cases, flesh, skin and even hair has been found intact. For these fossils to occur, the entire specimen has to have remained continually frozen from death until it is discovered. These fossils are very rare.
Desiccation, or drying, occurs in arid parts of the world. Occasionally, mummified bodies of animals, including humans, are discovered. These fossils are valuable in that the drying process preserves the soft tissues, skin and organs. Like the products of refrigeration, these are among the rarer forms of fossils.
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