Fossil and Fossilization
Fossil and Fossilization
Fossils are a window into Earth’s history and the evolution of life. The term fossil literally means something that has been “dug up,” but its modern meaning is restricted to preserved evidence of past life. Such evidence may take the form of body fossils (both plant and animal), trace fossils or ichnofossils (physical features formed in rock due to animal-sediment interaction), and chemical trace fossils (chemical evidence of life processes preserved in minerals within the rocks).
Fossilization refers to the series of postmortem (after-death) processes that lead to development of a body, trace, or chemical fossil. For original hard parts (e.g., shell, skeleton, and teeth), which are composed of various kinds of organic minerals, fossilization may include replacement by new minerals,
permineralization (filling open spaces with minerals). Fossil shells may be represented by external or internal (steinkern) sediment molds. Soft parts of plants or animals may also be mineralized and preserved as fossils in the process of carbonization. Soft tissue can be preserved as fossil material under special conditions where bacteria and moisture are excluded (e.g., fossils buried in glacial ice, anoxic peat bogs, and amber).
Fossils and their enclosing sediment (or sedimentary rock) are carefully studied in order to reconstruct ancient sedimentary environments and ancient ecosystems. Such analysis is called paleoecology, or the study of ancient ecologic systems. Fossils occur in nearly all sediments and sedimentary rock, and some volcanic rocks (e.g., ash deposits) as well. The bulk of these fossils are invertebrates with hard parts (e.g., clam shells). Vertebrates, the class that includes
reptiles (e.g., dinosaurs) and mammals (e.g., mastodons), are a relatively late development, and the finding of a large, complete vertebrate fossil, with all its parts close together, is rather rare. Microfossils, on the other hand, are extremely common. Microfossils include very early bacteria and algae; the unicellular organisms called foraminiferans, which were common in the Tertiary periods, and fossil pollen. The study of microfossils is a specialized field called micropaleontology.
Fossils of single-celled organisms have been recovered from rocks as old as 3.5 billion years. Animal fossils first appear in Upper Precambrian rocks dating back about a billion years. The occurrence of fossils in unusual places, such as dinosaur fossils in Antarctica and fish fossils on the Siberian steppes, reflects both shifting of continental position by plate tectonics and environmental changes over time. The breakup of the supercontinent Pangaea during and since Triassic pulled apart areas that were once contiguous and thus shared the same floras and faunas. In particular, Earth’s tectonic plates carrying the southern hemisphere continents-South America, southern Africa, the Indian subcontinent, Australia, and Antarctica-moved in different directions, isolating these areas. Terrestrial vertebrates were effectively marooned on large continental “islands.” Thus, the best explanation for dinosaurs on Antarctica is not that they evolved there, but that Antarctica was once part of a much larger land mass with which it shared many life forms.
An important environmental factor influencing the kinds of fossils deposited has been radical and episodic alteration in sea levels. During episodes of high sea level, the interiors of continents such as North America and Asia are flooded with seawater. These periods are known as marine transgressions. The converse, periods of low sea level when the waters drain from the continents, are known as marine regressions. During transgressions, fossils of marine animals may be laid down over older beds of terrestrial animal fossils. When sea level fall occurs, thus exposing more land at the edges of continents, sediments with fossils of terrestrial animals may accumulate over older marine animals. In this way, plate tectonics and the occasional marine flooding of inland areas could result in unusual collections of fossil floras and faunas in sediments and sedimentary rocks where the living plants or animals could not exist today—such as fishes on the Siberian steppes.
Changes in sea level over the past million years or so have been related to episodes of glaciation. During glaciation, proportionately more water is bound up in the polar ice caps and less is available in the seas, making the sea levels lower. It is speculated, but not certain, that the link between glaciation and lower sea levels holds true for much of Earth’s history. The periods of glaciation in turn are related to broad climatic changes that affect the entire Earth, with cooler weather increasing glaciation and with warmer temperatures causing glacial melting and a rise in sea levels. Global climatic change has had a profound effect on Earth’s fauna and flora over time. This is strongly reflected in the fossil record and the record of paleoecology of Earth found in sedimentary strata.
The principal use of fossils by geologists has been to date rock strata (layers) that have been deposited over millions of years. As different episodes in Earth’s history are marked by different temperature, aridity, and other climatic factors, as well as different sea levels, different life forms were able to survive in one locale or period but not in another. Distinctive fossilized life forms that are typically associated with given intervals of geologic time are known as index fossils, or indicator species. Over the past 200 years, paleontologists have determined an order of successive index fossils that not only allows geologists to date strata, but also is the foundation for understanding organic evolution.
The temporal relationship of the strata is relative: it is more important to know whether one event occurred before, during, or after another event than to know exactly when it occurred. Recently geologists have been able to subdivide time periods into progressively smaller intervals called epochs, ages, and zones, based on the occurrence of characteristic indicator (index fossil) species, with the smallest time slices
about one-half million years. Radiometric dating measures that measure the decay of radioactive isotopes have also been used to derive the actual rather than relative dates of geological periods; the dates shown on the time scale were determined by radiometry. The relative dating of the fossil clock and the quantitative dating of the radiometric clock are used in combination to date strata and geological events with accuracy.
The fossil clock is divided into units by index fossils. Certain characteristics favor the use of one species over another as an index fossil. For example, the ammonoids (ammonites), an extinct mollusk, function as index fossils from Lower Devonian through Upper Cretaceous—an interval of about 350 million years. The ammonoids, marine animals with coiled, partitioned shells, in the same class (Cephalopoda) as the present-day Nautilus, were particularly long-lived and plentiful. They evolved quickly and colonized most of the seas on the planet. Different species preferred warmer or colder water, evolved characteristically sculpted shells, and exhibited more or less coiling. With thousands of variations on a few basic, easily visible features—variations unique to each species in its own time and place—the ammonoids were obvious candidates to become index fossils. For unknown reasons, this group of immense longevity became extinct during the Cretaceous-Triassic mass extinction. The fossils are still quite plentiful; some are polished and sold as jewelry or paperweights.
Index fossils are used for relative dating, and the geologic time scale is not fixed to any one system of fossils. Multiple systems may coexist side by side and be used for different purposes. For example, because macrofossils such as the ammonoids may break during the extraction of a core sample or may not be frequent enough to lie within the exact area sampled, a geologist may choose to use the extremely common microfossils as the indicator species. Workers in the oil industry may use conodonts, fossils commonly found in oil-bearing rocks. Regardless of which system of index fossils is used, the idea of relative dating by means of a fossil clock remains the same.
The likelihood that any living organism will become a fossil is quite low. The path from biosphere to lithosphere—from the organic, living world to the world of rock and mineral—is long and indirect. Individuals and even entire species may be “snatched” from the record at any point. If an individual is successfully fossilized and enters the lithosphere, ongoing tectonic activity may stretch, abrade, or pulverize the fossil, or the sedimentary layer housing the fossil may eventually be subjected to high temperatures in Earth’s interior and melt, or be weathered away at the Earth’s surface. A fossil that has survived or avoided these events may succumb to improper collection techniques at the hands of a human.
Successful fossilization begins with the conditions of death in the biosphere. Fossils occur in sedimentary rock and are incorporated as an integral part of the rock during rock formation. Unconsolidated sediments such as sand or mud, which will later become the fossiliferous (fossil-bearing) sandstone or limestone, or shale, are an ideal matrix for burial. The organism should also remain undisturbed in the initial phase of burial. Organisms exposed in upland habitats are scavenged and weathered before they have an opportunity for preservation, so a low-lying habitat is the best. Often this means a watery habitat. The fossil record is highly skewed in favor of organisms that died and were preserved in calm seas, estuaries, tidal flats, or the deep ocean floor (where there are few scavengers and little disruption of layers). Organisms that died at altitude, such as on a plateau or mountainside, and are swept by rivers into a delta or estuary may be added to this death assemblage, but are usually fragmented.
A second factor contributing to successful fossilization is the presence of hard parts. Soft-bodied organisms rarely make it into the fossil record, which is highly biased in favor of organisms with hard parts—skeletons, shells, woody parts, and the like. An exception is the Precambian Burgess Shale, in British Columbia, where a number of soft-bodied creatures were fossilized under highly favorable conditions. These creatures have few relatives that have been recorded in the fossil record; this is due to the unlikelihood of the soft animals being fossilized.
From the time of burial on, an organism is technically a fossil. Anything that happens to the organism after burial, or anything that happens to the sediments that contain it, is encompassed by the term diagenesis. What is commonly called fossilization is simply a postmortem alteration in the mineralogy and chemistry of the original living organism.
Fossilization involves replacement of minerals and chemicals by predictable chemical means. For example, the shells of molluscs are made of calcium carbonate, which typically remineralizes to calcite or aragonite. The bones of most vertebrates are made of calcium phosphate, which undergoes subtle changes that increase the phosphate content, while cement fills in the pores in the bones. These bones may also be replaced by silica.
The replacement of original minerals and chemicals takes place according to one of three basic schemes. In one scheme; the skeleton is replaced one to one with new minerals. This scheme is known as replacement. In a second scheme, the hard parts have additional mineral material deposited in their pores. This is known as permineralization. In a third scheme, both hard and soft parts dissolve completely and a void is left in the host rock (which may later be filled with minerals). If in the third scenario, the sediments hardened around the hard part and took its shape before it dissolved, and the dissolved hard part was then not replaced (i.e., there is a void), a thin space remains between two rock sections. The rock section bearing the imprint of the interior face of the shell, let us say, is called the part, or internal mold, and the rock section bearing the imprint of the exterior of the shell is called the counterpart, or external mold. External molds are commonly but mistakenly discarded by amateur fossil collectors.
Because of the nature of fossilization, fossils are often said to exist in communities. A fossil community is defined by space, not time. Previously fossilized specimens of great age may be swept by river action or carried by scavengers into young sediments that are just forming, there to join the fossil mix. For this reason, it may be difficult to date a fossil with precision on the basis of a presumed association with nearby fossils. Nevertheless, geologists hope to confirm relationships among once living communities by comparing the makeup of fossil communities.
One of the larger goals of paleontologists is to reconstruct the prehistoric world, using the fossil record. Inferring an accurate life assemblage from a death assemblage is insufficient and usually wrong. The fossil record is known for its extreme biases. For example, in certain sea environments over 95% of species in life may be organisms that lack hard parts. Because such animals rarely fossilize, they may never show up in the fossil record for that locale. The species diversity that existed in life will therefore be much reduced in the fossil record, and the proportional representation of life forms greatly altered.
To gain some idea of the likelihood of fossilization of an individual or a species, scientists have sampled the death assemblages—decaying plants and animals that have gained the security of undisturbed sediments—in modern-day harbor floors and offshore sediments, and compared those death assemblages with actual life assemblages in the overlying waters. It seems that no more than 30% of species and 10% of individuals are preservable after death. The death assemblage is still millions of years away from becoming a fossil community, however, and once such factors as consumption of the decaying organisms by scavengers, transport of the organisms out of the area, disturbance of sediments, reworking of the rock after it has formed, and erosion are added to the picture, the fossilization rate falls well below the preservation rate.
In some cases, however, a greater than usual proportion of preservable individuals in a community has fossilized in place. The result is a bed of fossils, named after the predominant fossil component, “bone bed” or “mussel bed,” for example. Geologists are divided over whether high-density fossil deposits are due to reworking and condensation of fossiliferous sediments or to mass mortality events. Mass mortality—the contemporaneous death of few to millions of individuals in a given area—usually is attributed to a natural catastrophe. In North America, natural catastrophe is thought to have caused the sudden death of the dinosaurs in the bone beds at Dinosaur National Park, Colorado, and of the fossil fishes in the Green River Formation, Wyoming. These are examples of local mass mortality. When mass mortality occurs on a global scale and terminates numerous species, it is known as a mass extinction. The greatest mass extinctions have been used to separate strata formed during different geological eras: the Permian-Triassic extinction separates the Paleozoic era from Mesozoic; the Cretaceous-Tertiary extinction, which saw the demise of the dinosaurs and the rise of large mammalian species to fill newly available biological niches, separates Mesozoic from Tertiary. Thus, mass extinctions are recorded not only in the high-density fossil beds, but in the complete disappearance of many species from the fossil record.
A fossil identified in the field is not immediately chiseled out of its matrix. First, photographs are taken to show the relationship of the fossil fragments, and the investigator notes the rock type and age, and the fossil’s orientation. Then a block of rock matrix that contains the entire fossil is cut out with a rock saw, wrapped in muslin, and wrapped again in wet plaster, a process known as jacketing. The jacketed fossils may additionally be stored in protective crates for air transport.
In the laboratory, the external wrappings are removed, exposing the fossil in its matrix. The technique used to remove the excess rock varies with the type of rock and type of fossil, but three methods are common. Needle-sharp pointed tools, such as dental drills and engraving tools, may be used under a binocular microscope; or pinpoint blasting may be done with a fine abrasive powder; or acid may be used to dissolve the rock. Because some fossils also dissolve in some acids, the fossil’s composition must be determined before a chemical solvent is used. If the investigator wishes to see the complete anatomy of the fossil, the entire rocky matrix may be removed. Thin slices of the fossil may be obtained for microscopic study. If replicas are desired, the fossil may be coated with a fixative and a rubber cast made. For security purposes, most prehistoric skeletons on display in museums and public institutions are models cast from a mold, and not the original fossil itself.
The study of fossils is not limited to freeing the fossil from its matrix, looking at it microscopically, or making articulated reproductions to display in museum halls. Since about 1980, a variety of techniques developed in other fields have been used to make discoveries about the original life forms that were transformed into fossils. Immunological techniques have been used to identify proteins in fossilized dinosaur bones. The ability to recover DNA, not only from insects preserved in amber but also from fossilized fish and dinosaurs, may soon be realized. Studies of temperature-dependent oxygen isotopes formed during fossilization have been used to support the theory that dinosaurs were warm-blooded. And even as laboratory research is moving toward the molecular biology of fossilized organisms, aerial reconnaissance techniques for identifying likely locales of fossil beds are being refined. The true value of a fossil, however, is realized only when its relationships to other organisms, living and extinct, and to its environment are known.
The fossil record—the sum of all known fossils— has been important in tracing the phylogeny, or evolutionary relations, of ancient and living organisms. The contemporary understanding of a systematic, phylogenetic hierarchy descending through each of the five kingdoms of living organisms has replaced earlier concepts that grouped organisms by such features as similar appearance. It is now known that
Bone bed— High-density accumulation of fossilized bones.
Diagenesis— The processes to which a dead organism is exposed after burial as it becomes a fossil; for example, compaction and replacement.
External mold— Fossilized imprint of the exterior portion of the hard part of an organism, left after the fossilized organism itself has been dissolved; related to internal mold, bearing the imprint of the interior portion of the hard part, for example, of a clam shell.
Fossil record— The sum of fossils known to humans, and the information gleaned from them.
Fossiliferous— Fossil bearing; usually applied to sedimentary rock strata.
Ichnofossil— A trace fossil, or inorganic evidence of a fossil organism, such as a track or trail.
Index fossil— A distinctive fossil, common to a particular geological period, that is used to date rocks of that period; also called indicator species.
unrelated organisms can look alike and closely related organisms can look different; thus, terms like “similar” have little analytical power in biology. In recent years, interpretation of the fossil record has been aided by new knowledge of the genetics of living organisms. Modern genomes contain large amounts of information about evolutionary relationships. Debates over the ancestry of whales, for example, have been decided using a combination of fossil and DNA evidence.
In addition to providing important information about the history of Earth, fossils have industrial uses. Fossil fuels (oil, coal, petroleum, bitumen, natural gas) drive industrialized economies. Fossil aggregates such as limestone provide building material. Fossils are also used for decorative purposes.
Pough, Harvey. Vertebrate Life, 6th ed. Upper Saddle River, NJ: Prentice Hall, 2001.