A Brief Introduction to


© Gastaldo, Savrda, & Lewis. 1996. Deciphering Earth History: A Laboratory Manual with Internet Exercises. Contemporary Publishing Company of Raleigh, Inc. ISBN 0-89892-139-2

Not every organism that ever lived could become part of the fossil record. If you eat an average of three meals a day, you test and prove this hypothesis daily. A large percentage of all biological entities end up as food for other organisms higher on the food chain. This fact alone may prevents these organisms from being preserved. Even those organisms that avoid being eaten have a low probability of becoming fossilized because most of them undergo decay and recycling of their chemical components. For example, you can examine any forest-floor litter and find that beneath the top layer of leaves, the organic matter has been degraded to an unrecognizable form (humus -- not hummus, the garlic-laden spread served in health-food restaurants). This recycling keeps the carbon, nitrogen, and sulfur cycles operating. In fact, many taphonomic biases impact the odds of any organism being preserved.

The paleontological subdiscipline called Taphonomy, from the Greek taphos (death), is concerned with the processes responsible for any organism becoming part of the fossil record and how these processes influence information in the fossil record. Many taphonomic processes must be considered when trying to understand fossilization. These include events that affected the organism during life (changes in rainfall, availability of food, and behavior for maximum growth, etc.), the transferral of that organism (or a part of that organism) from the living world (biosphere) to the sedimentary record (lithosphere; compare the death of a herd of vertebrates with the autumnal leaf fall from a forest), and the physical and chemical interactions that affect the organism from the time it is buried until the time it is collected in the field.

Any organism must successfully pass through three distinct, and separate, stages in order to be seen in a museum display. These stages span the entire time from death of the organism to collection. Necrology is the first stage, and involves the death or loss of a part of the organism. The vast majority of animals must die before they can become introduced to the next phase. It's true that if a starfish is cut in half, each half will regenerate itself. The result will be two animals. Not many animals have this capability. We suggest that you don't test this hypothesis with your beloved pet. On the other hand, most plants do not have to die to contribute one or more of their parts to the potential fossil record. When autumn leaves fall in temperate climates, the trees don't die. The oldest living organism, bristlecone pines, are more than 5300 years old (as determined by counting tree rings). Their present leaves are not the same ones that grew 5300 years ago. When plants disperse their reproductive bodies (spores, pollen, or seeds), most do not die thereafter. Of course there are exceptions, but these are a small percentage of all extant (living) plants.

Once an organism has died or sheds a part, all the interactions involving its transferral from the living world to the inorganic world (including burial) constitute the second taphonomic stage. This is the Biostratinomy stage. Besides the conspicuous fossil characteristics that you will be able to observe during this laboratory (those external and internal features of the fossilized remain), less-obvious details often record what happened to the organism (or part) before it became a fossil. By studying these details paleontologists are able to understand, in a Sherlock Holmesian way, the mode of death or disarticulation (breakup of an organism), any biological processes that may have modified the remains before burial (such as scavenging), the response of the part to transport (by animals, water and/or wind), and the amount of time the organism sat around in the environment before it was finally entombed.

Ultimately the organic matter is buried. Burial plays an important role in potential preservation of the organic matter. Very specific chemical and physical conditions must exist in the burial environment to allow preservation in a form recognizable to us. It is here that biological (e.g., enzymatic and bacterial) and chemical (e.g., enzymatic and dissolution) processes must be slowed or eliminated. Once buried, the organic material is subjected to the third taphonomic phase, or Diagenesis. Diagenesis involves all of the processes responsible for lithification of the sediment and chemical interactions with waters residing between clasts. The processes of fossilization appear to be site specific with respect to depositional settings, resulting in a mosaic of preservational traits in the terrestrial and marine realm. Few fossil assemblages are exactly identical, especially with regard to the way in which they were formed, but general patterns do exist. An understanding of taphonomic assemblage features within an environmental context allows for a more accurate interpretation of the fossil record.

Most organic matter on Earth is used by some organism higher on the food chain and is, therefore, ultimately recycled. This is the fate of almost all biomass on Earth. Most organic matter is composed of easily degraded and digested compounds that are not likely to be preserved even under the most favorable conditions. Those parts of an organism that are already mineralized (such as your calcium-fortified skeleton) and, hence have made the first step in the transition to "stone", have a higher probability of preservation than any of the soft, fleshy tissues either around or within the skeleton. The early inhabitants of Paris, France, the bones of whom are now stacked neatly in catacombs beneath the city streets, attest to this fact.

Although the fossil record is incomplete, it still provides a useful survey of the history of life because of the vast amounts of time represented within the rock record. Even if the conditions for preserving organic matter existed only once every 10,000 years in each contemporaneous depositional environment around the globe, a lithology that was 100 meters thick (330 feet) and encompassing 1 million years of time would contain 100 fossil assemblages. Such conditions are not unrealistic, particularly within the ocean basins. If we then consider contemporaneous depositional settings around the globe, the number of fossil assemblages that would be preserved during this 1 million years of time increases dramatically. Of course, not all of these fossil sites are or would be accessible for collection and study. Mountain-building processes associated with plate tectonic activity (metamorphism of fossil-bearing sedimentary rock beyond recognition) and the erosion of these folded (metasedimentary) and faulted (sedimentary) rocks depletes the number of fossil localities available at the Earth's surface through time. The quantity of fossiliferous rocks beneath ground level far exceeds those available at the surface to be sampled and studied. Nevertheless, there are far more fossils than paleontologists, which will continue to be the case far into the future. Paleontologists are not wanting in their search for the history of life on Earth.

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