How Fossils are Formed

Excerpt from Introduction to Fossil Collecting

(C) 1994-2005, Glen Kuban, E-mail:
Part of Kuban's K-Paleo Place

When an animal or plant dies, it usually is soon eaten by scavengers or decomposed by bacteria. However, in some cases a flood, mudslide, sandstorm, or other event quickly buries a creature, or it may become entombed in ice, tar, or tree resin. When such an event happens, an organism is largely protected from decay, and may remain buried for millions of years. Through geologic time, and interactions with mineral seepage, pressure, and other factors, the organism or material around it may change in various ways. The changes may involve distortions, infillings, color changes, and the partial or complete conversion to rock (discussed below). Eventually, the specimen may be exposed again (this time as a fossil) through erosion or other factors, including human excavators.

In general, the hard parts of an organism such as teeth, bones, shells, and wood, are more likely to be preserved than soft parts, since hard parts are more resistant to scavenging and decay. Fortunately, well-preserved specimens including soft parts are sometimes found, and missing parts often can be deduced with fair confidence by studying the structures of the existing parts, and by comparisons with similar species living today.

The process by which dead organisms or their parts are transformed into fossils is called fossilization. The study of the factors and conditions that affect the fossilization process is called taphonomy. One of the most common changes fossils undergo through time is the partial or complete conversion to rock. This process (which can happen in various ways), is called petrifaction or petrification. You have probably heard the term "petrified," meaning "turned to stone." Certain types of petrification are given special names. If only the open spaces or soft parts of an organism are filled with minerals (such as silica or calcite), leaving the solid parts intact, the process is called permineralization. If an organism's bones, shell, or other hard parts are dissolved and replaced with other minerals, the process is called replacement. Sometimes the original shell or skeleton will remain, but undergo a change in crystal structure called recrystallization. If the entire organism dissolves away, leaving a hollow cavity, the cavity is referred to as a natural mold. If a natural mold is filled with minerals, the infilling is called a natural cast, or if you like fancy words, a pseudomorph. Often molds and casts occur together. Sometimes the area inside the shell of a mollusk (such as a clam) will fill with sediment, after which the shell dissolves away. This internal mold is sometimes called a steinkern, which is German for "stone kernel."

In some cases an organism's remains may be preserved through freezing (also called refrigeration), or through drying (desiccation), as sometimes happens with droppings of cave animals. Some fossil plants and insects are compressed into thin carbon films, sometimes called carbonizations, or distillations. Other fossils comprise only the outward impression of an organism or its parts, such as an impression of tree bark. If the impression or trace that records the living movements or functions of an ancient organism (as in the case of animal burrows, trails and trackways), the fossil is called a trace fossil or ichnite. Trace fossils (as distinguished from "body fossils") also include eggs, tooth marks, stomach contents, and coprolites (fossil excrement), and any other product or trace made while an ancient organism was still alive.

For further reading see: Fossilization - How Fossils Form