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Fossilization

by
Chris Nedin

Two claims commonly made by literalist fundamentalist creationists about the fossil record are (1) that if there have been animals on Earth for hundreds of millions of years, then the fossil record should contain millions and millions of fossils, many of which should be transitional forms, so that there should be numerous examples of transitional fossils. (2) Uniformitarian sedimentation rates are too slow to preserve organisms.

Ignoring the contradiction inherent in these two statements, are they correct? To answer this we must have an understanding of the fossilization process (taphonomy) from

life -> death -> preservation -> survival -> discovery.

While it would appear that the fossilization process must start with the death of the organism, it actually starts in life (as the upper echelons of academia occasionally shows).

Life

By life I mean that the life strategy of the organism, which will have a profound impact into its fossilization potential (FP - the likelihood of being found as a fossil). For an organism even to begin considering a career as a fossil it must be buried, any life strategy which enhances the likelihood of burial will increase its FP. Consequently, organisms which live on land have a much lower FP than organisms which live in shallow, near-shore marine environments. For example, the FP of your average marine Lesser Spotted Leaping Clam has a higher FP than your average Lesser Spotted Leaping Mountain Goat, due to the periodic, rapid sedimentation events experienced in near-shore environments. Also, organisms which do not possess hard parts or whose hard parts are fragile will have a low FP since they are unlikely to survive the burial process intact.

The position of the organism in the food chain is important since those lower down tend (for excellent ecological reasons) to be more abundant than those towards the top, so that organisms which occur in large populations, due to their sheer numbers, have a higher FP than those that occur in smaller populations. Having a lifestyle which incorporates living within the sediment provides added bonus points to the organisms FP since it is already buried. So:

High chance of burial + low in the food chain = high FP (with bonuses for already buried).

Low chance of burial + high in the food chain = low FP (it can be argued that the Lesser Spotted Leaping Mountain Goat has a good chance of being buried by the occasional landslide, living as it does in the mountains, but see Survival below.)

Death

How an organism dies also affects its FP. Shuffling off this mortal coil out in the open is not good since there tends to be a large number of organisms around to take advantage of such events to reduce your remains and FP quite rapidly. Even if you are already buried, something higher in the food chain digging you out and crushing you into a thousand small bits has something of a disastrous effect on your FP. Similarly your FP can be badly affected if you are rolled around by storms. Rapid smothering is still the best way of increasing your FP. This cuts down the possibility of further contributing to the food chain and in certain instances provides an environment suitable for preservation. So we have:

smothering by storm or flood sedimentation = high FP
being eaten or rolled around = low FP

Preservation

This covers the survival of the organism's remains after death and its transition into a fossil. Burial is the premier method of ensuring passage into the preservation stage. Rapid burial is a common process which occurs on a regular basis. Floods, storms, landslides, and volcanic ash eruptions lay down sediments over a period of hours to days. In some cases several meters of sediment can be dumped in near-shore environments by severe storms. Sedimentation does not have to occur on a slow, steady, incremental basis. Rapid sedimentation is part and parcel of current geological theories and is an important causal constituent of the rock record. However, not all environments are preserved equally in the rock record, leading to a heavy bias towards shallow, near-shore marine environments. Also, all periods of geological history are not preserved equally. Far more is preserved of recent Earth history than of more ancient times.

Burial protects the organism from higher food chain organisms with bad intentions and from the general environment - against mechanical (abrasion and break-up) and chemical processes (decay and disintegration). In the vast majority of cases decay is inevitable. Therefore organisms which possess some robust hard parts (shell, bone) have a much higher FP than soft-bodied organisms which have no hard parts, or possess fragile hard parts. Only under exceptional circumstances are soft-bodied organisms, or non-mineralized tissues, preserved. This usually occurs when the environment surrounding the organism either is low or is lacking in oxygen. Such environments are not needed to produce all fossils. The vast, vast, vast majority of organisms were fossilized in environments with normal oxygen levels, although these organisms are only represented by their hard parts with no soft parts remaining. Fossils containing preserved soft parts are therefore exceedingly rare compared with fossils composed of hard parts.

There are a number of possible preservation pathways once burial is complete. Some result in chemical alteration of the remains. Most result in the continued destruction of soft tissues. So:

Rapid burial = High FP
Hard parts = High FP
Slow burial = Low FP
No hard parts = Low FP

Survival

Once an organism has been successfully buried and preserved the fossil must survive. If the sediments containing the fossil erode away the fossil will be lost. If the sediments are buried too deeply the resultant increase in temperature can destroy the fossil.

Loss by erosion is especially likely if the organism was buried above sea level. Sediments above sea level are far more likely to be eroded than sediments buried in basins which are sinking. Sinking basins can accommodate more sediments (e.g. in the near-shore marine environment) allowing more sediment to be deposited, protecting the sediments beneath. Our Lesser Spotted Leaping Mountain Goat (or any other non-marine animal) may well have become a fossil but its fossil is much more likely to be eroded and destroyed than that of a fossilized clam which is buried below sea level. So:

High FP burial below sea level
burial in sinking basin
shallow burial
Low FP burial above sea level
deep burial

Discovery

This encompasses the chance of the fossil being discovered. A fossil cannot be discovered unless the sedimentary rock containing it is exposed at the Earth's surface. At any one time only a tiny fraction of the fossil bearing sedimentary rocks are exposed and available for being searched. Of all the time that current fossil-bearing rocks have been exposed at the Earth's surface only for the last few hundred years have they been searched. Of all the fossil-bearing rocks that have ever been exposed at the Earth's surface only the ones exposed now have been searched. Rocks in past times have been eroded away leaving little, if any, trace of their fossil content. Thus the fossil-bearing rocks we have searched are only a tiny fraction of the total fossil-bearing rocks.

The fossil record we have reflects these influences on fossilization. The vast majority of fossils are of organisms with hard parts which lived in environments conducive to rapid burial, i.e., in basins that were below sea level and were sinking. In other words the vast (up to 90%) majority of all fossils are of near-shore marine organisms with hard parts. Organisms which lived on land represent a tiny fraction of the fossils record. This is understandable given what we know about the fossilization process but is hard to explain if all the fossils were laid down during one event.

Conclusion

In reply to the claims mentioned at the start:

(1) We cannot expect that all or even most of the organisms that have lived on Earth will be represented in the fossil record. On the contrary, only a few, living in favoured environments and possessing hard parts, will have any likelihood of being preserved (a large number of known phyla have no members which possess hard parts, and most phyla have members which possess no hard parts). On top of this, the vast majority of fossils have not been discovered since they have either eroded away previously or remain buried and out of reach. So the fossil record is not a comprehensive record of all life that has existed on Earth. Nor is the rock record a comprehensive record of all environments that have existed. Further, with regard to transitional fossils, is has become apparent in recent years that evolution does not necessarily proceed in a constant, gradual manner over geological time, but can operate rapidly, in isolated populations. Such evolution results in small populations of intermediates separated by very short periods of geological time. such scenarios are unlikely to make it into the fossil record to begin with, let alone survive to be found. Thus preservation of any fossil from a transitional series will be rare.

(2) The rate of sedimentation is not too slow to preserve organisms. The vast majority are preserved by rapid sedimentation entirely in accordance with modern geological theories.

This page was last updated February 24, 1998.