Five natural methods which explain how living organisms are preserved to become fossils.
Being a paleontologist is like being a coroner except that all the witnesses are dead and all the evidence has been left out in the rain for 65 million years.
- Mike Brett-Surman, 1994
On Earth, there are over one and a half million species which have been identified, and estimates arrive at three and a half million which remain. Fossils preserve only a select portion, such as organisms with hard body parts; shells, bone, wood, for instance, fossilize more easily, making them easier to study and learn about their past. Soft-bodied organisms on the contrary, seldom fossilize, making it far more difficult for paleontologists to study. The study which focuses on how organisms become fossilized is taphonomy, Greek for "laws of burial".
Taphonomy has increased in popularity over the last decades. It's importance is gaining insight into the preserved fossil record. From the time an organism dies, many things can happen before it becomes covered, such as decay, trampling, broken, fed upon, and the more information scientists can reconstruct about the creature, the more accurate the hypothesis.
First in the process of reconstruction, is determining what type of fossilization took place. The majority of fossils have been altered from their original shape and texture which presents the paleontologist with a challenging task in making a determination on the appearance of the original specimen. In rare occurences, an organism became fossilized with original tissue still intact. For instance, in the Siberian Tundra, some Wooly Mammoths were discovered thawing with freeze-dried soft tissue and food remains in their digestive tract. Some of them so well preserved, the thirty thousand yr. old meat could be eaten without adverse effect. Another example is of a Wooly Rhinoceros found fossilized in an oil seep in Poland, the specimen was pickled which prevented decay. Though such cases are rare, when they do occur they preserve physical traits such as texture, color and diet.
Forty thousand year old bones pickled in tar, retain the original composition, but are black and scent of petroleum. These were found fossilized in the Rancho La Brea tar pits. Enough of the original material remains for scientists to extract DNA for comparative studies with living relatives.Photo by Natural History Museum of Los Angeles County, California.
2000 years ago the Celts and their kin believed the bogs of Northern Europe were entrance to the realm of the gods. Bogs are filled with a natural enbalming fluid, acidic water, low in oxygen and rich with tannins (the same chemicals used to cure leather). Over time dead vegetation turns into peat, harvested for heating fuel.
More than 1000 bog mummies have been found. Most around 2000 years old. Often their bones are dissolved, while the skin has been transformed into leather, with tremendous lifelike detail. Many bear signs of a violent death, slit throats, strangulation or hanging. Many scholars believe they were sacrificed to (sun-agricultural related) fertility gods.
Images (some alterations required for Web) and Notes, Adapted from National Geographic Video, Mummies: Voices of the Dead
Many shells from the Pleistocene age contain unaltered composition, which includes the mother of pearl coating, (the shiny coat lining the interior surface of the shell.) Some Cretaceous ammonite fossils contain this layer, but most fossils that date older, lack the original aragonite.
Insects trapped in Amber are preserved with the finest details though most of the organic material will decompose over time.
Photo adapted from Briggs and Crowther, 1990.
PRESERVATION IN AMBER
Amber comes from resin, a thick sticky liquid which often traps insects and other organisms when it seeps from tree bark. The resin dries, hardens, with the insect preserved inside, often with incredible detail. The majority of such specimen are mere carbonized films while sometimes the organism has been discovered with original bio-chemicals intact. Such molecules can be extracted and sequenced for study. However, the degraded composition of the material makes it impossible to ever reconstruct an entire organism.
NATURAL METHODS OF FOSSILIZATION
- Permineralization
Bone, especially marrow and wood both contains pores and cavities. After soft material decays, the harder material is left. If it becomes buried, it may become permeated in calcium deposits from groundwater. Calcium carbonate or silica settles into the bone or wood, cementing it and turning to rock. Unlike Replacement (below), new material settles into hard remains, but no original material is removed. Fossil logs from the Petrified Forest in Arizona were permineralized by silica, while other petrified wood and bone specimens are fossilized by carbonate. This process can be so thorough, even details of cell structure remain intact. - Recrystallization
Some shells are made of aragonite and other unstable materials which sometimes reverts to a form of calcium carbonate, or calcite. In some cases, the calcite may recrystallize into larger crystals, preserving the original shape, but under a microscope the alteration in original texture becomes apparent. - Dissolution and Replacement
Sediments of bone or shell may be exposed to water, which will cause a decay of the original material. When a fossil dissolves, its original shape will be preserved in a void by the surrounding sediments. This internal filling is known as steinkern, German for "stone cast". The void sometimes is replaced with sediments, which mimmicks the shape of the original fossil. This method of fossilization is identified when a fossil is comprised of minerals that are clearly not the original. - Carbonization
Fossils may also be preserved as thin films of carbon in such environment as sandstone or shale. When an organism dies, its soft tissue decays leaving a residue of carbon, a dark film which preserves the outline of the organism. This method is common with plant fossils, though there are examples of carbonized animal fossils.
The outline of an Ichthyosaur preserved in a carbonized film around the skeleton.
Original Photo by R. Wild, from Bringing Fossils to Life, An Introduction to Paleobiology, by Donald R. Prothero
REFERENCES 
Bringing Fossils to Life, An Introduction to Paleobiology, McGraw Hill Publishers, Donald R. Prothero
FURTHER SUGGESTED READING
Adrienne Mayor's books
1) The First Fossil Hunters (Princeton 2000) explains how ancient Greek and Roman discoveries of mysterious petrifed bones of extinct dinosaurs and mastodons led to myths about griffins, giants, and monsters. Watch for "Ancient Monster Hunters" on the History Channel.
2) Fossil Legends of the First Americans (Princeton 2005) gathers exciting Native American discoveries and myths about fossils, from tiny shells to enormous dinosaur bones, with stories from more than 45 different tribes, beginning with the Aztecs & Incas.
Stephen Meyer's article, "Are Dinosaurs Mentioned in the Bible?"
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