What is a fossil?
Fossils represent the preserved remains of life of the past. The word "fossil" comes from the Latin word "fossilis" which means "dug up"; remains must be buried to become preserved. The study of fossils is paleontology.
Fossils come in two major types: body fossils and trace fossils. Body fossils represent an actual body part (rarely the entire body). Usually an organism dies, but a body part may have been discarded (a molt of an arthropod, a shed tooth of a shark, for example). Trace fossils are evidence of the activity of an organism, for example, burrows left by worms or an organism's footprints.
How are fossils preserved?
The fossil record is biased, especially in the types of organisms preserved. When an organism dies, bacteria act to decompose the soft parts. The only parts that usually survive are skeletal structures, if present, especially if they are mineralized skeletons ("hard parts") impregnated with crystals of calcium carbonate (calcite or aragonite), calcium phosphate (apatite), or silica. Animals with mineralized skeletons include corals, brachiopods, bryozoans, mollusks, echinoderms, and many chordates. It is no wonder that the fossil record is biased toward these groups, while those lacking a mineralized skeleton, such as worms, jellyfish, and insects, are poorly represented.
Once they are successfully buried, remains usually are altered during or after the fossilization process. Soft parts may be preserved under unusual circumstances, such as burial in an anoxic environment, surrounding them with an "air-tight" material such as amber or tar, or even freezing. Organic compounds may volatize to leave thin films of carbon, a process called carbonization.
Hard parts may be preserved in their original unaltered state, but this becomes rarer the further back in time one goes. Minerals such as calcite and (especially) aragonite are prone to recrystallization, so even though shell material may be preserved, the original crystal arrangement is often destroyed. Hard parts may be replaced by quartz (silica) or other minerals. Porous materials (commonly bone or wood) may be preserved by a process called permineralization, with individual void spaces filled by minerals such as silica to preserve minute detail. Hard parts may also be buried in sediment that then becomes firm, only to have them dissolve away to leave a cavity or mold that may later be filled by mineral matter to form a cast. If the inside of a shell is filled with sediment or minerals and the shell is then dissolved away, an internal mold, or steinkern, is formed that has on its outer surface the details of the inside surface of the shell.
The scientific value of fossils
Once fossils are found, they are useful in several types of investigation. Biostratigraphy uses fossils to assign relative ages to rocks, based on the idea that species evolved through time so that a given species lived only during a relatively brief interval of geologic time. Oil companies need people who can date rocks using fossils (usually using microfossils since they are abundant and easily recovered from drill holes). Paleoecology is the study of the interactions between fossil species and each other and the environment. It is valuable in interpreting what conditions were like in the past, where land and sea was, and even where to look for fossil fuels. Ancient environment interpretation is also aided by taphonomy, the study of the preservation of fossils. The state of preservation reflects environmental conditions and processes that acted on the remains in the environment both before and after burial.