Taphonomy and the quality of the fossil record

Key points

• Plants and animals with hard tissues are most frequently preserved in the fossil record.

• Soft tissues usually decay rapidly, but rapid burial or early mineralization may prevent decay in cases of exceptional preservation.

• Physical and chemical processes may damage hard tissues during transport and compaction.

• Plants may be preserved as permineralized tissues, coalified compressions, cemented casts or as hard parts.

• There has been a longstanding debate about the fidelity and quality of the fossil record.

• The fossil record is clearly affected by the rock record, and apparent rises and falls in biodiversity can mimic rises and falls in sea level, for example.

• Perhaps the parallel patterns of biodiversity and rock record through time are driven by a third factor, such as sea-level change, at least at local and regional scales.

• Quantitative studies suggest that knowledge of the fossil record is improving.

• Paleontologists can use phylogenetic trees and fossil records, both largely independent of each other, to establish congruence between the two data sets, and so gain some measure of confidence that the fossil record tells the true history of life.

To examine the causes of life, we must first have recourse to death . . . I must also observe the natural decay and corruption of the human body. Darkness had no effect upon my fancy; and a churchyard was to me merely the receptacle of bodies deprived of life, which, from being the seat of beauty and strength, had become food for the worm. Now I was led to examine the cause and progress of this decay, and forced to spend days and nights in vaults and charnel-houses.

Mary Shelley (1813) Frankenstein

The paleontological study of taphonomy, which includes all the processes that occur after the death of an organism and before its fossilization in the rock, may seem ghoulish. In fact, many of the analytic approaches used by taphonomists are also used by forensic scientists. A crime scene investigator who is called to inspect a corpse may be asked how long ago the body was buried. The forensic scientist looks at the state of decay - is there any flesh remaining, do the bones still contain fat, what do the remnants of hair and finger nails look like? But now there is a whole armory of analytic techniques. For example, measurement of the chemistry of the bone and, in particular the assessment of the rare earth elements (scandium, yttrium and the 15 lanthanides), can help pinpoint the time of death. These forensic science methods are used by archeologists and, stepping back farther in time, also by paleontologists.

A related issue is the quality of the fossil record. Following the decay and loss of fossils, what is actually left? Can paleontologists trust the rock record and use their patchy fossil finds to somehow understand large-scale patterns of evolution? Critics are right to point out that paleontologists should be careful when they attempt to reconstruct a whole plant or animal, and try to understand its biomechanics, when they have just a few bones or bits of twigs. Care is required also in seeking to understand patterns of diversity change and evolution when many fossil species are missing. There is a heated debate about this issue, with some scientists claiming that the fossil record is desperately bad and next to useless, while others claim that the fossils do, in fact, tell us the history of life. We will look at taphonomy first, and the changes that have occurred in typical fossils since they were living organisms, and then consider the wider implications for paleobiology.

FOSSIL PRESERVATION Fossilization

When a plant or an animal dies, it is likely that it will not end up as a fossil. For those that do, there are several stages that normally occur in the transition from a dead body to a fossil (Fig. 3.1):

1 Decay of the soft tissues of the plant or animal.

2 Transport and breakage of hard tissues.

3 Burial and modification of the hard tissues.

In rare cases, soft parts may be preserved, and these examples of exceptional preservation are crucially important in reconstructing past life.

There are two kinds of fossil, body fossils, the partial or complete remains of plants or animals, and trace fossils, the remains of the activity of ancient organisms, such as burrows and tracks. In most of the book, "fossil" is used to mean "body fossil", which is the usual practice. Trace fossils are treated separately in Chapter 19.

Hard parts and soft parts_

Fossils are typically the hard parts - shells, bones, woody tissues - of previously existing plants and animals. In many cases these skeletons, materials used in supporting the bodies of the animals and plants when they were alive, are all that is preserved. Skeletons may nonetheless give useful information about the appearance of an extinct animal because they can show the overall body outline and may show the location of muscles, and woody tissues of plants may allow whole tree trunks and leaves to be preserved in some detail. The fossil record is biased in favor of organisms that have hard parts. Soft-bodied organisms today can make up 60% of the animals in certain marine settings, and these would all be lost under normal conditions of fossilization.

There are a variety of hard materials in plants and animals that contribute to their preservation (Table 3.1). These include inorganic mineralized materials, such as forms of calcium carbonate, silica, phosphates and iron oxides. Calcium carbonate (CaCO3) makes up the shells of foraminifera, some sponges, corals, bryozoans, brachiopods, mollusks, many arthropods and echinoderms. Silica (SiO2) forms the skeletons of radiolarians and most sponges, while phosphate, usually in the form of apatite (CaPO4), is typical of vertebrate bone, conodonts and certain brachio-pods and worms. There are also organic hard tissues, such as lignin, cellulose, sporopollenin potential body fossil

potential body fossil immediate burial^/ decay and transport burial burial preserved unaltered preserved unaltered

Brachio Sponge
Figure 3.1 How a dead bivalve becomes a fossil. The sequence of stages between the death of the organism and its preservation in various ways.

and others in plants, and chitin, collagen and keratin in animals, which may exist in isolation or in association with mineralized tissues.

Decay_

Decay processes typically operate from the moment of death until either the organism disappears completely, or until it is mineralized, though mineralization does not always halt decay. If mineralization occurs early, then a great deal of detail of both hard and soft parts may be preserved, so-called exceptional preservation (see below). If mineralization occurs late, as is usually the case, decay processes will have removed or replaced all soft tissues and may also affect many of the hard tissues.

Decay processes exist because dead organisms are valuable sources of food for other organisms. When large animals feed on dead plant or animal tissues, the process is termed scavenging, and when microbes, such as fungi or bacteria, transform tissues of the dead organism, the process is termed decay. Well-known examples of scavengers are hyenas and vultures, both of which strip the flesh from large animal carcasses. After these large scavengers have had their fill, smaller animals, such as meat-eating beetles, may continue the process of defleshing. In many cases, all flesh is removed in a day or so. Decay is dependent on three factors.

The first factor controlling decay is the supply of oxygen. In aerobic (oxygen-rich) situations, microbes break down the organic carbon of a dead animal or plant by convert

Table 3.1 Mineralized materials in protists, plants, and animals. The commonest occurrences are indicated with XX, and lesser occurrences with X.

Aragonite

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