Simply put, fossils are the remains of prehistoric life, or direct evidence that such life existed. These can be the whole creature or plant preserved with all its tissues and structure intact, or simply traces left behind it like we might leave fingerprints on a doorknob. But the most common fossils are the bony or woody portions of an organism that have, to one degree or another, been turned to stone.
The oldest known fossils -- the remains of the first primitive cells -- are 3.4 billion years old.
For organic* material to survive the eons in any recognizable form, it must be preserved from decay. Most often this preservation is the result of a quick burial in sand, silt, or other materials which with time turn to sedimentary rock. Embedded this way, shells and teeth may remain relatively unchanged. Bone and wood, however, usually become permineralized -- a process in which the pores are filled with minerals -- or petrified as groundwater carries away their original material, replacing it with minerals. A dramatic example of petrifaction is the replacement of the shell of an ammonite with pyrite.
Limestone is a kind of sedimentary rock that is actually formed of shells that have been largely reduced to calcium carbonate, although many whole fossil shells and fossil molds can be found in it.
Coal -- another sedimentary rock -- along with petroleum and natural gas, are known as fossil fuels because they are the remains of plant and animal matter which were transformed by bacteria, heat, and pressure. Fossil plants (and occasionally dinosaurs!) are found embedded in seams of coal. Tar, which forms when petroleum is exposed to the air long enough for the lighter compounds to evaporate, has played a crucial role in preserving unchanged the bones and teeth of prehistoric animals which became trapped in sticky tar pits.
The soft parts of an organism are much more vulnerable to decay than bone or shell and are very rarely petrified. If conditions are just right, however, they may be preserved essentially unchanged. Ice does an excellent job of this, and prehistoric mammals have been found still frozen in permafrost** in the Artic. This process is known, as you might imagine, as refrigeration. Carcasses buried in hot sand or salt beds can desiccate (lose all their moisture), which is a natural form of mummification. Bacterial spores have been recovered and revived from salt beds laid down 250 million years ago! Amber -- which is itself the fossilized remains of ancient tree sap -- preserves pollen, insects, and other small creatures through the complete exclusion of oxygen.
Soft tissues may also be preserved by chemical means. The acidity of peat bogs can mummify the soft tissues of plants and animals (including humans). Whether these mummified remains count as fossils depends on the paleontologist you consult, and whether the species so preserved is extinct. Peat bog mummies are very young in geologic terms -- 15 thousand years at most, as the bogs in question have developed since the last period of glaciation.
A process known as distillation can preserve evidence of an organism's soft tissue as a carbon film on rock, when all the other elements have been dissolved away. Distilled fossils are frequently paired with an impression of the organism in the rock itself, which is lucky as the carbon film is extremely fragile and may be destroyed either before or during collection.
Sometimes when all direct remains of an organism have been decayed or otherwise destroyed, evidence of it can still be found. This evidence generally falls into one of two categories: molds or casts of the organism itself, or traces of its passage.
Molds form when the organism has completely decayed or dissolved away, leaving an imprint in the surrounding matrix. The imprints left by distilled leaves are an example of this. Molds of ancient marine life that was buried in mud are also very common. Casts are formed when a mineral fills in this mold, creating what is called a pseudomorph, from the Greek words for "false" and "form."
Traces of an organism's passage can be anything from a dinosaur's tracks to the tunnels left by a feeding insect in what was to become petrified wood, to coprolites, or fossilized dung.
Past, Present, and Future
Each fossil, whether a petrified log of ancient cherry wood, a frozen auroch, or a tiny frog caught in amber, is a fragment of the record of life here on Earth -- of our own history, however distant. And the processes of fossilization go on today. Peat bogs (if we don't strip-mine them into extinction) will become the coal of future ages. Both a bark beetle caught in pine sap and a badger entombed in a mudslide are on their way to becoming fossils, though few organisms ever complete that journey.
Our own footprints left in a muddy field -- given exactly the right conditions -- might one day become fossils, too!
Information for this article came, in part, from:
The Audubon Society Field Guide to North American Fossils;
An Illustrated Guide to Fossil Collecting, by Casanova & Ratkevich; and
Fossils: A Golden Guide, by Rhodes, Zim, & Shaffer
* "Organic" in this sense means that the material was part of an organism.
**Permafrost is permanently frozen subsoil.