Fossilization and taphonomy

Taphonomy is the subfield of paleontology that addresses the processes by which the evidence of once-living organisms passes from the biosphere into the lithosphere, where they are fossilized and eventually discovered. In all but the rarest situations, the resulting fossil is much different from the original organism from which it was derived. The paleontologist unravels the taphonomic history of a fossil in order to gain an accurate impression of the organism when it was living.

Evidence of Past Life

Of the many millions of kinds of living things that have inhabited the Earth over the past several billion years, only a small fraction has left any evidence of their existence. Paleontologists call these evidence of past life fossils and use them to try to reconstruct the appearance and lifestyle (for example, ecology) of the original organism. From the time an organism dies until its remains are discovered by the paleontologist, however, many things may occur that cause a loss of information and make the task of reconstruction very difficult. Therefore, understanding how fossils are formed becomes essential to their interpretation. A separate subfield of paleontology called taphonomy has arisen, which is devoted to the study of how organisms become fossils. Taphonomy is traditionally divided into two parts. The first concerns the many destructive events that take place between the death of an organism and its final burial. The second concerns the many changes that can take place following burial and prior to discovery. It is this second part that traditionally has been called fossilization.

To illustrate the role of events that take place between death and burial, it is convenient to imagine a scenario involving a typical vertebrate organism—for example, a deer. When the organism dies—by predation, disease, or accident—the body of the animal begins to change, and the information that was contained in the living animal begins to decrease. Even the mode of death can be destructive since predators can crush and tear parts of the deer, often leaving telltale marks on the skeleton. Should the death itself be quiet, however, the body will quickly be attacked by scavengers that will tear at the flesh and will tend to dismember the animal, sometimes taking parts (for example, a leg) far from the site for consumption. Thus, almost immediately, the remains of the vertebrate would no longer be complete at the death site. With the majority of the flesh stripped off the skeleton, smaller scavengers will remove the last of the tendons and ligaments holding bones together, and the entire skeleton will then be separate bones. Once again, many of the smaller scavengers will chew on bones and antlers, weakening them or consuming them completely. Because most natural waters are acidic (a result of dissolved atmospheric carbon dioxide), the rain that falls on the isolated bones will have a tendency to dissolve them. Eventually, the last part of the skeleton remaining will be the teeth, as they are the hardest and most compact parts of the animal. Thus, a moderately large animal, such as a deer, may be reduced to only a few teeth prior to burial. This explains why most vertebrate fossils, particularly of smaller vertebrates, tend to be teeth.

Preservation of Organic Material

A scenario similar in broad outline to what happens to land animals could be written for marine invertebrates. Many processes that are common to the environment of an organism can reduce a complex living individual to a few fragments. Therefore, one of the major factors that will determine the survivability of a fossil is the durability of its skeletal material. Second, the environment in which the organism lives will be of great significance. For the deer in the example, the wooded terrestrial environment is not a place where sediments that aid in the fossilization process, such as sand, lava, or sticky tar, accumulate rapidly to bury animal remains quickly, and preservation is far less probable. For many organisms living on or in the floor of the sea, the chances of preservation are much greater, as sediment accumulates in these environments at much greater rates than on land. Organisms living in the sea floor (infauna) are especially prone to burial because they are already within soft sediment; those living on the sea floor (epifauna) are also well positioned for such preservation. Thus, marine invertebrates are far better represented in the fossil record than terrestrial vertebrates. A clam's living environment naturally facilitates its preservation and fossilization much better than an animal that lives far from sediment sources or near predators, like squirrels.

Once buried, the remains are still not guaranteed preservation. What is recovered will depend upon the mode of fossilization. In the best of situations, permitting the greatest amount of information retrieval, no alteration of the original material will have occurred so that what is recovered is compositionally and morphologically identical to what was buried. Typically, unaltered fossils are relatively recent, as the probability of change increases with geological age. Good examples are the insect and plant remains that become entombed in tree sap that hardens to produce amber. Enclosing the organisms in the thick, sticky sap effectively excludes oxygen and protects against putrefaction and the scavenging of other organisms. The chitinous exoskeleton of the insect is likely to be preserved without any change. Spectacular preservation of ants, wasps, and other insects has been reported from the famous Baltic amber of the Eocene epoch (about 65 to 55 million years ago). Similarly, many of the bison bones that have been found in prehistoric sites in the western United States are unaltered. Plant material is often slower to be modified during fossilization, and some wood from the Cretaceous period (about 140 to 65 million years ago) appears unchanged, even under the microscope.

Processes of Fossilization

Fossils are often said to be “petrified.” This is a nonscientific term referring to three processespermineralization, replacement, and distillation. Permineralization is the infilling of pores and cells by the precipitation of mineral matter. The pore walls and cell walls remain unchanged. The mineral involved is typically opaline silica; many other minerals have been found, although less commonly. Replacement refers to the substitution of some other mineral for the substance of the fossil. These substitutions may take place on a molecule-by-molecule basis, leaving a nearly perfect copy of the original, with much of the detail intact. While opaline silica is again very common, many other minerals are also common, including pyrite, hematite, and calcite.

Distillation, unlike the other two processes of fossilization, involves the soft tissues. These tissues are composed of compounds of carbon, hydrogen, and oxygen. Under burial pressures and temperatures and in the absence of oxygen, destructive distillation takes place liberating water and carbon dioxide until only free carbon remains. The carbon forms a thin, black film in the rocks. The outline of the organism is often preserved, sometimes around its skeleton, giving clues to the fleshy shape of the organism, which could not be determined from the skeleton alone. Plants are often preserved this way. The three-dimensional form of the plant may be completely distorted. In some cases, however, the surface textures are molded into the surrounding sediment before distillation, leaving very thin carbon films lining the molds, which preserve the details of veins and stems. Many fern and fernlike fossils are preserved in this manner.

Perhaps the most common form of preservation is molds and casts. A mold is a fossil that preserves the surface features, which may be either external or internal to the organism, in negative relief—that is, with raised areas on the original appearing as depressions on the fossil. To obtain an accurate impression of the original, paleontologists often pour rubber into the mold; when hardened, it will have the surface features of the original in true relief, that is, with projections now appearing as projections of rubber. The rubber reproduction is termed a cast. Nature may also produce casts by filling a mold in rock with minerals or sediment. Because shelled organisms such as clams have large internal cavities that may become sediment-filled, it is easy to imagine this material as a cast, yet in fact, it is a mold of the interior of the shell, known as a steinkern. An indentation on the interior of the clamshell will appear as a raised area (that is, in negative relief) on the steinkern. Internal molds are common in large bivalved organisms, and spiral cones of sediment may be all that remains of a coiled gastropod, having filled the interior of the shell. When the shell was subsequently dissolved away, only the solidified filling was left.

A subtler change may take place if the skeletal material was originally aragonitea variety of calcium carbonate often used by marine organisms. Aragonite is chemically unstable at or near the Earth's surface and will recrystallize as calcite. Calcite has a slightly different crystal shape and size. As a result, when the aragonite crystallizes, the fine details of the microstructure of the skeleton are often obliterated, even though the gross external form appears unchanged.

Traces of Organic Activity

Tracks, trails, and traces of organisms are another category of fossils that record the activity of the animals that made them. These trace fossils are extremely useful in revealing behavioral habits that cannot be determined from skeletal remains. The best known of these are the dinosaur footprints found in numerous localities around the world. Less impressive but more commonly found are the burrows and feeding marks of various marine worms. While a particular trace can seldom be assigned to a specific animal, comparison with the habits and markings of modern organisms has allowed paleontologists to make general animal identifications and understand the historical composition of the Earth. For example, scientists have discovered fossilized ammonitescreatures that inhabit the seain the Himalayas in Nepal. This implies the highest mountains in the world were once an ocean floor.

Because none of the original substance of the organism is involved, the process of preservation of trace fossils depends entirely upon preserving the texture of the sediments in which they were formed, both before and after burial. Where there are abundant organisms living on and in the sea floor, their activities continually churn and disturb the sediment, usually wiping out most traces before they can be fossilized. Rapid sedimentation, however, can often bury the traces below the reach of most infaunal organisms, which tend to live near the surface, thus preventing the subsequent disruption of the traces.

Finally, regardless of the type of fossil preservation, many fossils are destroyed when the sediments containing them are compacted, thus crushing and distorting any fossils within. Moreover, if the pressures and temperatures of burial become great enough, the rock will become metamorphosed, and all the original features, including the fossils, may be lost, especially if the rock undergoes recrystallization.

Principal Terms

cast: a fossil that displays the form of the original organism in true relief

distillation: the driving of carbon dioxide and water out of tissues, leaving only free carbon

epifauna: organisms that live on the sea floor

fossil: any remains or evidence of a once-living organism

fossilization: the processes by which the remains of an organism become preserved in the rock record

infauna: organisms that live in the sea floor

mold: a fossil that displays the form of the original organism in negative relief

permineralization: the filling of pores and cells with minerals without changing the material surrounding the pores or cells

replacement: molecule-by-molecule substitution of the original material of the organism by a different mineral material

steinkern: an internal mold that preserves the interior form of an organism in negative relief

taphonomy: the study of all the processes that take place between the death of an organism and its discovery as a fossil

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