Fossilization | Research & Encyclopedia Articles

Fossilization

Fossils are preserved evidence of once-living organisms—typically in sediment or rock. Body fossils are preserved remains of an organism or indications of remains; trace fossils are preserved indications of an organism's life activities, such as trails or burrows. Fossilization is the preservation of an organism as a fossil. Paleontology is the study of fossils, and the use of fossils for the study of Earth history.

As of 1998, the oldest body fossils discovered are 3.8 billion-year-old cyanobacteria, or blue-green algae. The oldest known trace fossils are 3.5 billion-year-old stromatolites--sediment layers formed by colonies of blue-green algae, known as algal mats, as a means of acquiring food from seawater.

The large and diverse collection of fossils that can be seen at a good science museum might indicate that fossils are common--and they are--but the likelihood that any single organism will be preserved, or fossilized, in the rock record is almost infinitely small. Likewise, the likelihood that any particular species will be preserved is also very small. For this reason, according to most paleontologists, the rock record is not a good representation of the variety of organisms that have inhabited Earth during its long history.

So why is one organism fossilized while another is not? The answer to that question is complex, but can be summarized fairly easily. To be fossilized, an organism must be preservable, that is, its remains must be capable of being preserved, and it must be preserved--it must avoid biological, chemical and mechanical destruction. At that point, it is a fossil, but to be discovered and studied it must also avoid being eroded away until someone comes along and unearths it.

Random chance obviously is an important factor in preservation, but certain characteristics make an organism more likely to be preserved. Specifically, organisms with hard parts--bones or shells, for example--are much more likely to be preserved than those with only soft tissues. However, as paleontologists have learned, almost anything can be fossilized under the right conditions.

The critical step in fossilization is how the organism becomes fossilized—the mode of preservation--and this is controlled in part by the life habitat and in part by the site of preservation, which in some cases, are one in the same. If the habitat of an organism is a beach and the organism has no hard parts, or its hard parts are rather delicate, its remains are unlikely to avoid disintegration in this habitat. However, if this same organism is swept offshore and deposited in a low energy, oxygen-poor environment, the remains might escape mechanical destruction if they arrive intact. Biological consumption and biochemical decay may also be avoided through rapid burial and the anaerobic conditions.

In general, preservation is favored in low-energy aquatic environments with rapid, relatively fine-grained sedimentation. Organisms from terrestrial environments are less likely to be fossilized than those from aquatic environments because rapid decay and scattering are more likely on dry land. Rapid burial with fine-grained sediment isolates the remains and inhibits decay. However, organisms that graze and burrow in sediment for food--worms, snails, etc--are still likely to disturb the remains. This is why an oxygen-poor environment is preferable, since it tends to discourage the presence of such organisms.

The degree of preservation is contingent on the mode of fossilization. Generally, preservational mode can be divided into two broad categories: unaltered preservation and altered preservation. Unaltered preservation usually involves hard parts, such as bones or shells, which are relatively young--100s to 100,000s of years old. Typically, these have not been buried deeply nor subjected to intense weathering. Given sufficient time, they would probably be altered or destroyed.

Fossilization by alteration involves a variety of processes. Some provide fairly thorough preservation of fossil morphology, others only preserve superficial details. The best preserved are those that experience freezing. This may even preserve cell structure, body fluids, etc., although certain morphological characteristics may be distorted. The most common examples are probably mammals recovered from glaciers during partial melting. Perhaps the most spectacular of these was the 5,300 year-old Stone Age hunter recovered from the Italian Alps in 1991, nicknamed "The Ice Man."

The next best type of altered fossilization in terms of preservation is mummification. In this case the body fossil has been preserved through minor chemical additions or losses. Saturation with oil in a tar pit, burial in a peat bog, entombment in amber, and desiccation are the most common examples. Each alters the fossil in its own unique way but all produce only relatively minor changes in cell structure and chemistry. All except amber are confined to relatively young fossils. Amber can preserve fossils almost indefinitely--at least for 100s of millions of years.

The remaining types of fossilization by alteration involve some variety of mineralization and shallow to deep burial. All but one, carbonization, offer little chance of preserving any trace of soft tissue. The least destructive and the most common of all types of altered fossilization is permineralization. This involves filling the pores of original material with mineral crystals. By far the best-known example is petrified wood, in which open pores in the cellulose are filled with silicon dioxide, the mineral known as quartz.

Replacement, on the other hand, involves the chemical removal of all of the original material from the organism's hard parts and replacement by another mineral. For example, the shells of many types of marine organisms are composed of aragonite, one of the crystal forms of calcium carbonate. This is routinely replaced by another crystal form of calcium carbonate, calcite, after burial. In many instances, this completely obliterates fine details, but not always. The degree of detail preserved usually depends upon whether the replacement occurs all at once, or whether the original material is replaced molecule by molecule, making the preservation of detail more likely. Quartz and calcite are most commonly involved in replacement.

Carbonization involves not a replacement of material but distillation. Volatile organic compounds are driven off during burial, leaving behind a thin carbon film that may preserve finely detailed, soft-tissue structure--a rarity in burial fossilization. The formation of coal is actually a type of carbonization involving thick deposits of plant tissue. However, the starting materials are very carbon rich so large volumes of carbon remain behind, creating coal.

Impressions are perhaps the simplest type of fossil. They consist of an organism's features outlined in soft, fine-grained sediment, which is preserved after the organism's body has decayed away. The Burgess Shale, a famous fossil-bearing rock from a locality in British Columbia, Canada, contains impressions of primitive, soft-bodied organisms with exquisite detail preserved. This, however, is extremely unusual. In a sense, impressions are very similar to trace fossils, but instead of footprints the form of an entire side of the body may be preserved. Two preservation modes closely related to impressions are molds and casts. Molds are formed when a shell, for example, is buried in mud and then dissolved away. This leaves an unfilled space in the sediment that faithfully represents the size, shape, and perhaps some morphologic details of the shell. Casts form if the mold is later filled in with material.

Once a fossil is discovered, it can be used in a variety of ways. The most common use is probably for systematic paleontology, the study of fossils for the purposes of understanding evolution. Each fossil's form is studied and compared to other fossils and living organisms in hopes of understanding their evolutionary relationships. In addition, its temporal range is also determined. The temporal range of an organism is the time from its first appearance (evolution) to when it became extinct. Another common use is biostratigraphy, using fossils to understand the spatial and temporal relationships between different sedimentary rocks. Fossils are used in biostratigraphy to temporally equate, or correlate, sedimentary rocks in different geographic areas by comparing the ranges of the various fossils in the rock. By doing so, relative dating--determining whether a rock layer is older or younger than other rocks--becomes an easy task. Some organisms, known as index fossils, are ideal for this activity due to their short ranges.

Paleontologists also study fossils to learn about their paleoecology. Ecology is the study of the interactions between a living organism and its environment, including other organisms living with it. Paleoecology is ecology applied to extinct organisms. A closely related area of research is paleogeography. Based upon paleoecology studies, paleontologists attempt to understand the distribution of various types of geographic environments through time. Similarly, paleontologists may attempt to reconstruct the distribution of fossil organisms through time, a research area known as paleobiogeography. Whatever the goal, fossils have proven to be extremely useful resources in the quest to better understand Earth history, and without preservation through fossilization, this undertaking would be much less fruitful.