What is a fossil? The word is Latin, and it means, “dug up.” One of the earliest known research publications to use this term was Agricola’s work on systematic mineralogy, De natura fossilium. It wasn’t until Lamarck’s Hydrogeologie was published in 1802 that the term was restricted to “the still recognizable remains of organized bodies.” Even as late as the mid-1800s, the concept of fossilization was, except in the minds of a very few, considered the result of a recent worldwide deluge. For over 1,000 years, just the discussion of events beyond the scope of the biblical account of creation could cause one to end up in prison or worse. Sir Walter Raleigh is one of the better-known of these victims. His conviction for treason and subsequent beheading was precipitated in part by his suggestion, in his volume entitled History of the World, that civilization was older than the biblical account allowed. The current definition of the term fossil has thus only very recently evolved.
Currently, the term “fossil” refers to any remains or traces of once-living organisms. Although this definition is much narrower than the original term’s Latin origin, it still covers a broad range of preserved structures. A fossil may be as simple as a single preserved mark in a sediment or as complex as a mummified dinosaur. It may be as old as 3.7 billion years or as recent as a few hundred. Fossils are as varied and diverse as the life forms that produced them and the climatic and structural conditions that preserved them. However, this vast array of preserved evidence of past life has survived to the present through only a very few natural processes. These processes include permineralization, tracks and traces, molds and casts, carbonization, and original preservation. The following is a brief examination of each of these processes and a description of the types of fossils preserved through each process.
It should be noted that one process is common to all fossil preservation: the process of protection. Regardless of the method of preservation, if the specimen had not been suitably protected, it would not have survived. Most often, this protection involves being buried and remaining undisturbed until discovery. Rapid encasement and removal of oxygen from the environment are the key protection criteria for virtually all fossils. These conditions are necessary to keep the organic tissue from either deteriorating or becoming damaged while the other processes take place.
Permineralization and Replacement
The permineralization process is the one most often thought of when one describes fossilization. It is defined as the molecule-by-molecule replacement of the organic remains with inorganic minerals. Through this process, the original structure of the specimen is preserved even at the microscopic level. Although this process has been performed in the lab in a matter of hours, it is a process that usually takes millions of years in nature. In fact, the process is almost never carried to completion; an average of 3% to 8% of the original organic molecules remain in permineralized specimens. The preservation of hominid remains range from virtually no permineralization to almost complete permineralization. The oldest recovered DNA from hominid remains is about 70,000 years. Mineral replacement in the older specimens has progressed too far to allow DNA recovery.
The permineralization process generally occurs in two stages. The first stage happens rapidly, and the second much more slowly. The first stage in this process consists of filling the pore spaces and cavities, and the second stage is the replacement of the tissue itself. The speed at which these processes take place is determined by heat, pressure, mineral type, and quantity available, and groundwater flow. If an animal carcass is preserved in a hot-springs environment, it will fossilize much more rapidly than if it were in a sand dune in a temperate desert. However, the fossil produced in the desert environment would likely be much stronger and more durable, since the primary mineral available would be silica rather than the carbonate usually found in hot-springs environments.
Molds and Casts
If an object such as a tree trunk or animal carcass falls into mud that subsequently hardens, the mud produces a mold. This mold preserves the exact shape of the object. If the object then deteriorates and is removed, the cavity can be refilled. This infill is a natural casting. Natural molds and casts are some of the most common fossils. Silica infilling of the mold often produces beautiful agate and jasper. Fossilized wood limb casts preserved in this manner are often displayed with a polished cross section, and such specimens are sought by jewelers and rock collectors.
Soft body parts are rarely preserved. When they are, it is usually through the process of carbonization. When preservation conditions are just right, the organic material present in the original specimen is converted to various gases through chemical reactions, leaving only the carbon behind. The result is a thin film of carbon that often perfectly preserves the original body shape. This form of fossilization is responsible for such diverse remains as the Cambrian sea fauna of the Burgess Shale and the Eocene plants of the Green River Formation. This process, on a larger scale, is responsible for coal.
Tracks and Traces
This form of preservation is unlike any other, in that it preserves life events rather than death events. It is also unlike any other listed here in that it is not a single process that preserves, but rather a multitude of processes through which the life event was recorded.
These remains include trackways as well as event traces, such as wing brushings, tail drag marks, nest structures, and coprolites (fossilized droppings). It is from these fossils that we gain our best estimates of speed, movement, and diet. This evidence is generally observed as impressions contained within a single sedimentary layer. Once the impression was made, the area remained undisturbed until another layer of sediment was deposited over the top, protecting the impression. Such impressions are among the most numerous of fossils, and they were the first recognized evidence of dinosaurs in the western hemisphere. Fossil hominid tracks have been found in Tanzania, and these tracks preserve some of the earliest life data for Australopithecus. The tracks were made over 3.5 million years ago and seem to represent a family unit, of father, mother, and child, walking together, with the child following the adults.
Organic remains may be preserved unaltered in several ways. The common element for original preservation is extremely unusual circumstances. The cycling of organics from previously living plants and animals as part of their diet, then in turn their organics repeating the process, is responsible for supporting the variety and diversity of life on Earth today. Many mechanisms exist to perform the breakdown, and it is extremely unusual for any remains to escape all these breakdown processes. Encasement and desiccation are two processes through which unaltered remains may survive.
Encasement is a term reserved for preservation in an airtight medium such as tar, ice, amber, or copal. Organic remains preserved in this manner generally exhibit a certain amount of decay often leading to carbonization. However, freeze-dried mummies of Wooly Mammoths and a 5,000 year-old male human, a woman thrown into an asphalt pool at what is now Rancho La Brea, and mosquitoes from Dominican Republic amber (made popular in a major motion picture) are all exceptions. Each of these specimens has provided scientists with valuable clues from these exceptionally well-preserved remains.
Desiccation was one of the earliest observed preservation phenomena. Perhaps due to observations of animal and human carcasses that had perished in the desert, this same process has long been used by humans for the preservation of bodies. Egyptian mummies were, in earliest times, created by being buried in the hot desert sand. However, as time went by, more and more procedures were added, and so in more recent times, desiccation was a minor factor in the total process.
Regardless of the means by which the fossil is preserved, original preservation retains the greatest amount of usable data for any specimen. However, much information has been gleaned from permineralized specimens, and even from natural molds and castings. Fossils provide a valuable resource for understanding life in the past.
- MacFall, R. P., & Wollin, J. C. (1983). Fossils for amateurs. New York: Van Nostrand Reinhold.
- Mayr, H. (1992). A guide to fossils. Princeton, NJ: Princeton University Press. (Original work published 1985, as Fossilien: Uber 500 Verlagsgesellshaft in Farbe. Munchen: BLV Verlagsgesellshaft mbH)
- Parker, S. (1990). The practical paleontologist. London: Quarto.
- Raup, D. M., & Stanley, S. M. (1978). Principles of paleontology. San Francisco: Freeman.