Taphonomy and Fossils — Introduction to Paleontology (Part IV)
- 演化之聲
- Mar 9
- 8 min read
Updated: Mar 22
Taphonomy and Fossils
Not all organisms have the opportunity to become fossils after death. The parts most likely to be preserved as fossils are usually the hard tissues of organisms, such as skeletons and shells composed of calcium carbonate, silica, or calcium phosphate, as well as the woody tissues of plants. Soft tissues, by contrast, are usually decomposed by microorganisms soon after death and therefore have a much lower chance of fossilization. As a result, when fossils are used to reconstruct past biodiversity, organisms composed only of soft tissues may be subject to problems such as survivorship bias.
Fossils can currently be divided into body fossils, trace fossils, and chemical fossils.
Body fossils: fossils formed from the body or body parts of an organism, such as bones, shells, horns, soft tissues, or embryos preserved within eggs (Figure 1).
Trace fossils: traces or remains left behind by the activities of organisms, such as footprints, bite marks, feces, and eggshells (Figure 2).
Chemical fossils: fossils produced through long-term chemical changes after the death of organisms, such as petroleum, natural gas, and coal (Figure 3).
Which biological components are more likely to become fossils?
Chitin, cellulose, keratin, phosphates, calcium carbonate, and similar substances are more likely to remain after an organism dies, and therefore have a relatively high chance of fossilization. The following chart broadly lists biological materials with a relatively high likelihood of fossilization, divided into two major categories: organic and inorganic.
Before biological remains can become fossils, they must pass through several obstacles, including scavenging and decay, damage and transport to other locations, and burial. These are discussed in greater detail below.
Scavengers and decay
After an organism dies, scavengers may arrive to feed on the carcass. Whether large animals or small insects, they may consume most of the soft tissues of the body, and sometimes even the hard tissues as well. Even if some soft tissues remain, they may still undergo putrefaction and decomposition because of microbial growth. The environmental conditions surrounding the carcass also affect the rate of decay. In colder regions, microorganisms are generally less able to proliferate, which slows decomposition; in warmer conditions, decay proceeds more quickly. If an organism dies in an environment with unusual acidity or alkalinity, decomposition may also proceed more slowly, thereby increasing the chance that soft tissues are preserved. Peat bogs provide an example of an acidic environment. In the Geisel Valley peat bog of Germany, muscle fibers and skin tissues from Eocene animals have been discovered.
A Lagerstätte is a sedimentary deposit containing exceptionally well-preserved fossils, and in some cases even exquisitely preserved soft tissues. The preservation of soft tissues in fossils is extremely difficult, because it depends on a race between mineralization and decay: the faster tissues are mineralized, the more likely they are to be preserved. There are several ways in which soft tissues may become fossilized, depending on the composition of the tissues and the speed of burial. Rapid burial, low organic content, and the presence of sulfides favor pyritization of biological tissues (Figure 4); slow burial and high organic content favor phosphatization (Figure 5); rapid burial combined with high organic content favors infilling by carbonates to form fossils (Figure 6). In rare cases, the soft tissues of small animals may also be preserved in amber or asphalt (Figure 7).
Damage and transport to other places
If an organism is not buried where it dies, its body may experience a number of fates that prevent complete preservation. For example, it may be dismembered. Whether during life by predators or after death by scavengers, the body may be torn apart and consumed, and some parts may be carried elsewhere and further eaten. Natural processes such as waves and storms may wash away parts of the body, while long exposure on the ground may allow wind-blown sand to erode the remains. These processes can destroy the original shape and surface details of the body, leaving the remains rounded and smoothed. Fossils formed under such conditions make it much harder to infer the organism’s original appearance.
Some biological remains, such as bones or shells left behind by organisms like mollusks, may also undergo bioerosion in aquatic environments. Sponges, algae, bivalves, and other mollusks may bore into rocks or into the hard tissues of dead organisms, damaging their structure to create living space for themselves (Figure 8). This results in remains covered with pits and holes.
Burial
If biological remains survive scavenging, decay, damage, transport, and erosion, they will eventually come to rest somewhere, where they become covered by sand and sediment and are buried progressively deeper over time. Once remains are buried beneath sediment, the physical and chemical processes involved in fossil formation begin.
The most common physical change is compression by the heavy sediment above, which flattens the remains. This is why many fossils are extremely flat. Harder structures, however, are more resistant to compression, and some may retain a shape closer to their original form. Even structures as difficult to flatten as tree trunks are often deformed by the pressure of overlying sedimentary rocks. During diagenesis, the remains may also become harder in texture, which helps prevent them from being crushed by surrounding rock.
Chemical changes begin soon after burial. These include the previously mentioned carbonate processes, pyritization, phosphatization, and others, but they proceed very slowly. Waters containing minerals gradually seep through the sediments and then slowly penetrate the remains, and the reactions may take anywhere from thousands to millions of years. Later diagenetic processes, including metamorphism and broader geological changes, generally begin only after millions of years have passed.
Using shells as an example, shells contain abundant calcium carbonate, which exists in the form of calcite or aragonite. One common diagenetic process involving calcium carbonate is the transformation of aragonite into calcite. When a shell is buried in sediment, its aragonite dissolves into the water occupying the pore spaces of the sediment, and this water already contains some dissolved calcium carbonate. The result is a supersaturated solution of calcium carbonate, while empty spaces form where the original aragonite once existed in the shell. Calcium carbonate from the pore water then crystallizes in the form of calcite and fills these spaces. Put simply, the aragonite in the shell is replaced by calcite.
Carbonate concretions can often be observed in black shale (Figure 9). When biological remains are buried, bacteria in the sediment decompose them, consuming oxygen and creating a low-oxygen environment while producing carbon dioxide. This carbon dioxide dissolves in nearby water to form carbonic acid, which then reacts with calcium ions or iron ions from the remains to form compounds such as iron carbonate and calcium carbonate. Pyritization likewise requires microbial activity, in which sulfate is reduced and combined with iron ions to form iron sulfide, replacing the original material of the remains during diagenesis.
The incompleteness of the fossil record
The burial process of biological remains has a major influence on how scientists understand and reconstruct ancient organisms. The more incomplete a fossil is, the more difficult it becomes to infer the organism’s appearance and habits. For this reason, scientists always hope that a species will be represented by multiple fossil specimens that can be compared and verified against one another. In reality, however, most species are not fortunate enough to leave behind numerous fossil records. Studies of the Burgess Shale Lagerstätte suggest that only about 5 to 10 percent of species are represented in the fossil record, and these are mainly trilobites and brachiopods. The remaining 90 to 95 percent of species have vanished into the long flow of Earth’s history.
The condition of remains during the earliest stages of decay may also influence how fossils are later reconstructed and where they are placed in evolutionary trees. Rob Sansom once conducted an experiment in which lampreys and several other modern basal vertebrates were allowed to decay for 200 days. In the end, most tissues had decomposed, leaving only the rough outline of the remains. These decayed experimental remains looked just as primitive as some early vertebrate fossils that have been excavated. Sansom therefore argued that one should not casually assume that early vertebrate fossils that appear primitive necessarily represent a simple-to-complex sequence of evolution, because the loss of crucial organs and tissues may already have blocked our ability to reconstruct them correctly.
David Raup proposed several reasons why the fossil record can never be complete:
Anatomical factors: organisms with hard tissues have a much greater chance of becoming fossils. Organisms made almost entirely of soft tissues, such as jellyfish, worms, and many mollusks, have an extremely low chance of fossilization.
Biological factors: the behavior and abundance of organisms also affect the likelihood of fossilization. For example, mice, which reproduce rapidly, are numerous, and have short lifespans, are more likely to leave fossils than pandas, which reproduce less frequently, are rare, and live longer.
Environmental factors: organisms living in shallow marine environments, lakeshores, riverbanks, and other shallow-water settings are more likely to become fossils.
Sedimentary factors: some areas are favorable for the accumulation of biological remains, such as lagoons and lakes, whereas places that are frequently washed or eroded, such as slopes and coastlines, offer much less opportunity for remains to be deposited.
Preservational factors: after burial, remains require a suitable environment for diagenesis. Acidic water can destroy bones or shells. If remains lie in turbulent water, they may be re-exposed after burial and repeatedly washed out, eventually being completely eroded away.
Diagenetic factors: even after remains have already become fossils, the movement of mineral-rich water through sedimentary rocks may deform or alter the rock and may also damage the fossil’s appearance.
Metamorphic factors: after millions of years of burial, if sedimentary rocks are subjected to high heat and pressure, metamorphism may occur, transforming mudstone into slate or limestone into marble, and fossils are very likely to be destroyed in the process.
Vertical movement factors: sedimentary rocks accumulate layer by layer upward. If crustal movements do not bring deeper layers back toward the surface, fossils at the bottom will never be excavated, leaving gaps in our understanding of ancient life.
Human factors: even when fossils reappear at the surface, many enter private trade. Most fossils available for scientific study are excavated by researchers themselves or come from museums, but privately owned fossils are not always made available to scientists, which also hinders the fossil record.
Fossils are one of the most important sources of information for the study of ancient life. Yet before an organism can become fossilized, it must pass through many conditions and obstacles, and even if a fossil is preserved, it may not be complete. For these reasons, our interpretations of some ancient organisms may at times be biased. By understanding taphonomy and related knowledge, we can bring these potential problems into discussion and make our explanations more reasonable and closer to the true life appearance of extinct species in their own time.
Author: Shui-Ye You
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