Fossils

Recognition of Fossils

Fossils are now recognized as the remains of once-living organisms; however, in the past the term was not so restricted and included anything dug out of the ground (Latin fossilis, “to dig up”). Thus minerals, gems, and archaeological remains were all included as fossils. The record of fossils goes back 3.4 billion years to preserved single-celled organisms such as bacteria and blue-green algae. Complex organisms or metazoans did not appear until about 700 million years ago, and hard parts were not preserved until about 544 million years ago, at the base of the Cambrian when there was a tremendous development of organisms often called the Cambrian Explosion. Since then, a diverse assemblage of fossil organisms is available for study by paleontologists.

Early ideas about the source of fossils were that they were the remains of mythical animals or that they had grown in the rocks like crystals. Aristotle suggested that the fossils of fish were the remains of sea animals that had swum into cracks in the rocks and become stranded, while others thought that they developed from seeds or grew from fish spawn washed into cracks during Noah’s Flood. These ideas remained an important influence until the Renaissance, when Leonardo da Vinci recognized that fossil shells in the Apennines represented ancient marine life, and Nicolaus Steno in the mid 1600’s showed that “tongue stones,” thought to be the petrified tongues of dragons, were actually ancient shark teeth. At about the same time that Steno was writing, the British scientist Robert Hooke studied and described fossils and used early microscopes to study their cellular structure. However, their ideas were slow to become accepted, and it was not until the mid 1700’s that naturalistic concepts of fossils began to prevail; when Linnaeus published his Classification of all organisms, the Systema Naturae (1735; A General System of Nature, 1800-1801), fossils were treated as living organisms. By 1800, Baron Georges Cuvier was able to apply comparative anatomy to fossil organisms, both showing their relation to modern organisms and arguing that they must represent extinct animals, as they had not been found living on even the most remote continents. Prior to this, the fact of extinction was not accepted, as it went against the view that God would not allow any of his creations to become extinct. Cuvier also recognized that fossils occur in a regular succession and can thus be used to date the sediments in which they are found. At about the same time, this fact was also recognized by William Smith in England, who developed it to the point that he was able to produce geological maps of England and Wales in 1815. This was the basis of dating and mapping rocks, and by the time of the publication of Charles Darwin’s On the Origin of Species in 1859, the understanding of the fossil record had reached the point where few scholars took biblical ideas about the history and evolution of life literally.

Fossilization

Fossilization is a rare occurrence, and it has been estimated that of the more than one million living species, only 10 percent are likely to be preserved as fossils. A single square meter of seafloor could, during a few million years, produce enough sea shells to swamp the museums of the world if they were all preserved. It must be appreciated, therefore, that the fossil record is selective and thus may be incomplete and biased. This is because some organisms with hard parts, such as shells or skeletons, tend to fossilize readily, and therefore much is known about their past. Others are soft-bodied and rarely if ever fossilize, so their fossil record is minimal. The study of how organisms become fossils, the study of death and decay, is called taphonomy. This is now an important area of paleontology, because to understand and interpret the fossil record it is necessary to understand the processes that have resulted in fossilization. Once an organism dies, its remains are subject to decay and perhaps transport and mechanical breakdown before they are buried. The more of that lost information that can be reconstructed, the more reliable the final hypothesis about the original community will be.

Preservation is dependent on a number of factors, of which the most important are the composition and structure of the organism, its abundance, the sedimentary environment in which it lives, and the postdepositional changes that occur. All organisms are composed of delicate tissues, known as soft parts, but many also have more resistant tissues, referred to as hard parts. The hard parts may be mineralized, as in the shell of bivalves, or composed of organic material, such as the chitin that makes up the exoskeleton of arthropods. Although in general the possession of mineralized hard parts is a prerequisite for fossilization, soft-part preservation can occur, and some fossil groups have only organic soft parts.

In a few exceptional cases, organisms are preserved entire, and this results from unusual preservational circumstances. Woolly mammoths have been preserved by freeze-drying, so that their soft tissues and even their last meals are preserved, and humans have been found completely preserved in peat bogs. Preservation in amber occurs when small organisms are trapped by tree resin that subsequently hardens around them, and in some cases the preservation is so good that original biomolecules can be extracted and deoxyribonucleic acid (DNA) sequencing carried out. Although this was the premise of the film Jurassic Park (1993), it is clear that the genetic material is so incomplete that it will never be possible to reconstruct a complete organism in this way.

In most cases, the soft tissue has been lost and some change has taken place to the hard tissues during the process of fossilization. Permineralization is the process by which the natural pores in wood, bone, or shell are filled with minerals by groundwater that percolates through them in the sediment. In this case, none of the original material is lost, and even in petrified wood there is still some of the original material, although the logs have been completely permeated by silica. Only part of the original material is preserved in carbonization, a process in which most of the volatile organic materials disperse, leaving a black carbon film. This may result in exceptional preservation in insects, for instance, or ichthyosaurs (marine reptiles) in which the outline of the body is preserved as a carbon film. More frequently, the hard tissues are recrystallized or are replaced by some other mineral. Recrystallization results in the loss of internal detail, and the same is true of replacement, as in both cases none of the detailed structure is left. However, replacement by more resistant minerals, such as the replacement of a calcite shell by silica, can improve the survival potential of a fossil and preserve delicate structures, such as spines, that might otherwise be lost.

Preservation Potential

The preservation potential of an organism is affected by a number of factors that operate at different stages in the process of fossilization. Necrolysis, or the breakup and decay of an organism after death, reduces the potential for preservation of all but the most resistant organisms, and it has been estimated that only 20 to 30 percent of shallow marine organisms are likely to be preserved. The numerical abundance of organisms is generally considered to be important as, all other things being equal, abundant organisms would seem to be more likely to be preserved than rare ones. However, in a number of cases this has been shown not to be true, and this differential preservation is often related to the preservability of the hard tissues. Mobile organisms tend to have lighter and less durable skeletons that would be more easily destroyed than those of static animals such as corals, in which the skeleton is formed of a solid mass of calcite.

A number of agents may operate to destroy even resistant skeletons. Biological agents include scavengers, which may break up shells and bones to extract nutrition, and burrowing organisms, which may use the hard tissues as a substrate and thus weaken them. Mechanical agents such as wind, waves, and currents can also be very effective, particularly in shallow environments where energy is highest. Experiments in which shells were tumbled with pebbles have shown that thin-shelled organisms break up quickly, while thicker shells last longer. Similar studies for terrestrial vertebrates have shown that the least durable parts of a skeleton are the ribs and vertebrae, while the skull, jaws, and particularly the teeth are the most resistant to breakdown. In general, it appears that the shape, density, and thickness of the bone or shell are the most important factors in determining its survival during mechanical transport.

Many apparently unfossiliferous rocks may have had fossils at one time but lost them during the changes that take place in sediments after deposition. Many snails and bivalves have shells composed of the unstable form of calcite called aragonite, and this may dissolve early, leaving no trace of the fossil, while other organisms composed of more stable calcite will not be dissolved. In these cases, therefore, the representation of the original community will be biased more by the chemical composition of shells than by their mechanical durability or by their relative abundance.

Trace Fossils

Trace fossils (or ichnofossils, from the Greek ichnos, “track”) are sedimentary structures created by organisms and, therefore, reflect their behavior. They include tracks, trails, burrows, and borings and may occur in sediments that contain no body fossils, thus providing the only record of life and certainly the only record of soft-bodied organisms that are not normally preserved. One important aspect of trace fossils is that they are almost always preserved in place, as reworking would result in their destruction, and thus they can always be directly related to the sediments in which they are found. Because of this, trace fossils are particularly important in understanding and reconstructing paleoenvironments.

Trace fossils are difficult to classify, as they represent behavior rather than the remains of an animal. They are also difficult to relate to the animal that formed them, as traces are rarely found with the organism that made them, different organisms may form similar traces, and the same organism may form a variety of different traces in different circumstances. Because of these problems, traces are organized within behavioral categories, including dwelling structures made by organisms that feed outside their burrows; mining structures that are formed by animals feeding in the sediment; grazing traces made by animals feeding across a surface; locomotion traces formed by animals during travel; and resting traces formed by an animal during a temporary stop. Thus, dinosaur tracks would come within the locomotion category and the burrow of a filter-feeding worm would be recognized as a dwelling structure.

Understanding the behavioral genesis of trace fossils has enabled their use in reconstructing past environments. They are grouped in recurrent ichnofacies (traces reflecting a particular environment), and these are in turn linked to a set of environmental parameters, such as salinity, light level, and temperature, that are generally correlated with depth. This approach has been particularly valuable in recognizing shallow marine facies and has the advantage of being independent of time, as particular environments and, therefore, behavior persist through time, although the trace formers themselves may change.

The study of marine traces is relatively recent; however, terrestrial traces have been studied for much longer, as the first dinosaur tracks were reported in 1802, although they were misidentified as bird tracks at that time. Terrestrial trackways can give a considerable amount of information about the locomotion and behavior of the trace former, and can lead to quite precise information about speed, as well as insights into posture, activity levels, and metabolism. Relative speed is related to the length of the leg and the stride length (the distance between one foot impression and the next by the same foot). Hip height (calculated from foot length) is divided into the stride length (measured from the trackway) to give a proportional estimate of the speed. At 2.0 an animal moves from walking to trotting, and above 2.9 it is running, information that has been used in studies of dinosaur locomotion. In addition, the fact that trackways show dinosaurs moving together in herds has been used to suggest a level of social organization greater than that demonstrated by any modern reptiles.

Fossils and Time

In geology, both relative time and measured time are recognized. Measured time is obtained from the decay rates of certain radioactive minerals in rocks and gives a date in years. Relative time refers only to the sequence of strata, showing that some beds are older or younger than others. Although the relative age of rocks can be obtained from their sequence, as those lower in the sequence are older and those higher in the sequence are younger, the lithology of the rocks cannot be carried laterally, because sedimentary environments change over distance. This problem was resolved in the late 1700’s in England by William Smith, who recognized that there was a sequence to the fossils in the sedimentary record and that this could be used to characterize the rocks and correlate them to each other over a distance. This recognition of “faunal succession,” which represents the successive preservation of organisms as they evolved through time, has been extremely important in the development of the study of biostratigraphy, in which fossils are used to provide dates for rocks. In this system, the basic unit is the zone, which is represented by zone or index fossils that characterize a period of time. The potential of different organisms as zone fossils will vary, and to be most useful they should embody certain characteristics. They must have a high preservation potential, be relatively common, and also be distinctive, so that accurate identification is possible. They must also have a wide lateral distribution but a short vertical range, so that they characterize a short time period over a wide area. It is also helpful if they are independent of environment. Because of this and because most sediments are marine in origin, the organisms used are often planktonic, that is, they float near the surface of the ocean but will be preserved in various sedimentary environments when they die. Microfossils, such as spores and pollen, are very useful because they are wind-borne and thus may be found in both terrestrial and aquatic environments.

The oldest known fossil was discovered in Greenand in 2016 and is 3.7 billion years old.

Principal Terms

biostratigraphy: the dating of rocks using fossils

Cambrian: the period of time between 544 and 505 million years ago

ichnology: the study of trace fossils

paleontologists: scientists who study fossils

taphonomy: the study of the processes that lead to fossilisation

zone fossils: fossils that characterize a period of time and can be used to provide a relative date for a rock

Bibliography

Ausich, William I., and N. Gary Lane. Life of the Past. 4th ed. Upper Saddle River: Prentice, 1999. Print.

Briggs, Derek E. G., and Peter R. Crowther, eds. Palaeobiology: A Synthesis. Boston: Blackwell, 1990. Print.

Gillette, David D., and Martin G. Lockley, eds. Dinosaur Tracks and Traces. New York: Cambridge UP, 1989. Print.

Prothero, Donald R. Bringing Fossils to Life. Boston: WCB/McGraw-Hill, 1998. Print.

Rudwick, Martin J. S. The Meaning of Fossils. Chicago: U of Chicago P, 1985. Print.