Fossil record

The fossil record provides evidence that addresses fundamental questions about the origin and history of life on the earth. When did life evolve? How do new groups of organisms originate? How are major groups of organisms related? This record is neither complete nor without biases, but as scientists’ understanding of the limits and potential of the fossil record grows, the interpretations drawn from it are strengthened.

Overview

The term “fossil” originally referred to any object dug up from the earth and included minerals as well as the petrified remains of once-living organisms. The term is now used in a restricted sense to describe the preserved remains of organic life, both plant and animal. Probably the most familiar kinds of fossils are body fossils, which are the fossilized remains of the actual organisms, but there is also indirect fossil evidence of organic activity, such as footprints as evidence of walking. Collectively, these tracks, trails, and burrows are termed “trace fossils.”

The term “fossil record” refers to the sum total of fossils preserved in geological strata on the earth. The fossil record extends back in time to rocks 3.5 billion years old. The first entries in the fossil record are single-celled, plantlike organisms. There are about 250,000 known fossil species of plants and animals. That seems to be a large number until one compares it with the approximately 4.5 million species of plants and animals alive today. The entire fossil record of ancient life amounts to 5 percent of the total number of modern species. What is the reason for the paucity of fossils compared with the abundance of modern species? Does this difference in number of fossils and recent species reflect true differences in diversity, or does it reflect limitations in the compilation of the fossil record? In other words, were there fewer species in the geological past than in the recent past, or is the fossil record incomplete?

English naturalist Charles Darwin, famous for his contributions to evolutionary theory, came to the conclusion that the fossil record is incomplete. Darwin was one of the earliest naturalists to publish on what he termed the “imperfection” of the fossil record. In his book On the Origin of Species by Means of Natural Selection (1859), Darwin compared the preserved record of life on the earth to a set of books in which several volumes are missing, and from the remaining volumes of which chapters are missing, and from the remaining chapters of which pages are missing, and from the surviving pages of which words are missing. Darwin and others concluded that not every kind of organism that once lived is fossilized and that the fossil record is biased toward the preservation of some forms over others.

Rapid Burial and Anoxia

The biases inherent in the fossil record stem from the fact that fossilization of organic material is the exception, not the rule, and very specific and relatively rare conditions must be met for an organism to become fossilized. Fossilization favors organisms with hard parts—for example, an exterior shell (exoskeleton) or internal skeleton (endoskeleton). Fossilization also favors organisms living in certain environments. Two particular environmental conditions favor fossilization: rapid burial and anoxia (lack of oxygen). Rapid burial protects organic remains from predators or scavengers and physical reworking by tides and waves. Oxygen supports bacteria and decomposition of organic material. Burial in an oxygen-free (reducing) environment insulates organic material from decay and thus favors fossilization.

The most exceptional fossils known are from environments in which one or both of these two environmental conditions were met. German paleontologists call exceptionally preserved fossil assemblages fossil “lagerstatten,” or mother lodes. Famous fossil lagerstatten include the Mazon Creek fauna, from Pennsylvanian-period strata (300 million years old) in Illinois, in which insects, crustaceans, and previously unknown soft-bodied organisms are preserved in ironstone concretions; the Burgess Shale fauna, from Middle Cambrian (500-million-year-old) strata in British Columbia, famous for the discovery of a great variety of unusual arthropod and annelid-like animals; the Solnhofen Limestone, from Jurassic strata (200 million years old) in Bavaria, in which Archaeopteryx (a reptile with feathers) was discovered; insects preserved in amber (fossilized tree sap), of the Oligocene epoch (40 million years old) in Germany; and the La Brea Tar Pits, from the Pleistocene epoch (10,000 years old) in California, in which various animals, including saber-toothed tigers and mastodons, were ensnared and preserved in natural asphalt springs.

From this brief survey, it is clear that fossil lagerstatten occur in various geological and geographical settings and through a wide range of geologic time. These lagerstatten share the characteristic of rapid burial or burial in a biologically inert environment. They provide information on the morphology of previously unknown groups or a better record of known groups but do such assemblages reveal anything about how the organisms lived? Some of these lagerstatten represent environments in which the animals died rather than the environments in which they lived. The La Brea animals, for example, certainly did not live in the tar pits. Lagerstatten, however, can be used to reconstruct paleocommunities, to create a picture of organisms that lived contemporaneously, even though that picture of paleocommunity structure or paleoecological relationships might not be complete. Each different fossil lagerstatten provides an exceptional view of a geologically unique situation.

Preservational Biases

Because of the preservational biases inherent in the fossil record, most fossilized species represent only a few major groups—the numerically abundant and well-skeletonized organisms that lived in or near an anoxic environment or an environment that was subjected to rapid, episodic influxes of sediment. The environment with the best fossil preservation potential on the earth is the shallow marine shelf: Most marine life lives in the shallow shelf; the shelf is subject to rapid influxes of sediment (via storms and rivers, for example), and many marine invertebrates have exoskeletons. Thus, hard-shelled invertebrates from shallow marine environments constitute the bulk of the fossil record. The major marine invertebrate groups that dominate the fossil record include corals, bryozoans, brachiopods, mollusks (clams, cephalopods, and snails), arthropods (especially trilobites), and echinoderms (starfish and their relations).

The fossil record of other groups, including marine and terrestrial vertebrates and plants, is neither as abundant nor as complete as the marine invertebrate record. Leaves and flowers are rare as fossils, often preserved as imprints in sediments deposited in ancient inland lakes. Woody tissue of trees is often preserved by a process called replacement, in which mineral-rich water percolates through the porous wood and minerals (especially quartz) precipitate from the water, filling the voids in the plant structure. The logs of the Petrified Forest in Arizona are examples of replacement by silica. Pollen spores and seed pods are important constituents of the paleobotanical record because these structures are abundant and often have tough outer coverings. Paleobotanists face special problems in the identification and classification of plant fossils because entire plants are rarely found as fossils. Stems, roots, leaves, flowers, and seeds are described separately as they are found. Consequently, a single plant species is likely to have separate species names given to each of these structures.

Quality and quantity of fossil material are restricted for the vertebrate paleontologist, compared to the wealth of fossil material available to the invertebrate paleontologist, because vertebrates have an endoskeleton that is more easily broken apart (disarticulated) after death of the organism than the well-calcified exoskeletons of marine invertebrates. Terrestrial vertebrates also live and die in environments in which their remains are subject to destruction by predators, scavengers, and bacterial decomposition. The vertebrate fossil record is dominated by teeth, which contain the mineral apatite, which is hard and resistant to weathering. Often, entire fossil vertebrate species are defined on the basis of solitary bits of jaws, teeth, and bone.

Origin and

The biases associated with fossilization do not diminish the importance of the fossil record for scientists’ understanding of the origin and evolution of life. The fossil record not only enables scientists to reconstruct the morphology of long-extinct individuals but also provides evidence from which ecological relationships—for example, interactions between different organisms and the structure of ancient communities of organisms, or paleocommunities—can be inferred. Sometimes, the evidence for ancient ecological relationships is striking, as in the find of an ammonite (related to the modern genus Nautilus) with circular holes in the shell associated with the circular teeth of a mosasaur (marine reptile)—evidence of a predator-prey relationship.

Trace fossils contribute their own unique information to scientists’ understanding of ancient life. They are often found in rocks devoid of body fossils and provide the only evidence of biological activity in these ancient sediments. Unlike body fossils, trace fossils cannot be transported by wind or currents. Traces were made where they are found, and there is little fear of erroneous interpretations based on mixing trace fossils from different environments. Trace fossils also provide important evolutionary information: The earliest evidence of multicellular organisms is small vertical burrows in rocks 2 billion years old. There are no body fossils in these rocks; the trace-makers were undoubtedly soft-bodied organisms that were poor candidates for fossilization. The presence of the burrows in these ancient rocks suggests that the evolutionary transformation from unicellular to multicellular life took place in a shallow marine, nearshore environment. The diversity of trace-fossil forms reveals various animal behaviors: burrowing, crawling, walking, feeding, and grazing. For example, the gait of some extinct arthropods and details of their appendages can be determined from the pattern of their fossilized footprints, even though body fossils of these animals are rarely found with intact appendages. The spacing of dinosaur footprints has led to the idea that many dinosaurs were able to move rapidly and were not the slow-gaited, lumbering giants formerly imagined.

The fossil record documents evolution and provides data from which scientists can infer the processes by which evolution works. Darwin believed that evolution was a slow and gradual process of small changes, accumulating over time to transform one fossil species into another. This gradualistic view of evolution, however, is not strongly supported by the fossil record. Gradualism predicts that numerous intermediate forms (transitional in appearance between the old species and the new species) should exist. Many fossil groups have left no record of these intermediate forms. Darwin attributed the absence of transitional forms to the incompleteness of the fossil record. This view of the fossil record as incomplete provided a convenient explanation for gradualists, and gradualism was widely accepted as an explanation of how evolution works for many years.

The gradualist view was challenged by paleontologists who saw a different pattern in the geological record of some groups of organisms. Many fossil groups appear suddenly in the fossil record and persist largely unchanged in appearance through most of their geologic history. This pattern suggests that evolution (as measured by the change in the appearance of an organism) happens very quickly and involves a major change in appearance, as the old form “jumps” to the new form. According to this alternative view, the sudden appearance of new species in the fossil record and the absence of intermediate fossils is real and not an artifact of an imperfect fossil record. This alternative model of evolution, termed “punctuated equilibrium,” holds that evolution is not the gradual, constant process envisioned by Darwin but rather a rapid process involving major changes in morphology. Speciation in this punctuated model of evolution does not require the hypothetical (and problematic) transitional intermediates proposed by Darwin. Both gradualistic and punctuated modes of evolution have been documented for different groups of organisms. The evidence suggests that groups of organisms evolved through different evolutionary pathways—some gradual, some punctuated.

Study of the Fossil Record

Study of the fossil record begins in the field, where paleontologists collect fossils and record observations on the orientation of the fossils and the character of the sediment in which the fossils are found. The specific technique used in collecting fossils depends on the kinds of fossils sought, the nature of the research, and time constraints. A major problem faced by paleontologists in collecting fossils in the field is separating the fossils from the surrounding sediment. Certain microfossils commonly found in limestone are collected by dissolving many kilograms of the rock in hydrochloric acid. The microfossils themselves are insoluble in weak acid and are easily recovered from the acid bath. Other microfossils may be picked from deep-sea cores that consist of unconsolidated mud. Macrofossils can be removed from the outcrop with the aid of hammers and chisels, although vertebrate paleontologists sometimes use brushes in excavating fragile vertebrate material. Unless the fossils have weathered naturally from the outcrop, they probably need to be cleaned of adhering sediment. Macrofossils can be cleaned by boiling in solvents, by sandblasting with a miniature air-abrasion unit, or by meticulous, time-consuming hand cleaning with small probes (old dental instruments make good fossil-cleaning tools).

Strategies for collecting fossils from outcrop exposures include collecting fossils exposed at the surface from weathering, and “mining,” digging down to or along targeted fossiliferous bedding surfaces. First-time collectors usually begin by collecting everything in sight. Once the novelty has worn off, they select only the best, or most complete and well-preserved, specimens and toss aside broken or deformed fossils. Perfect specimens make good museum exhibits, but experienced paleontologists realize that important information is often revealed by broken or imperfect specimens. The pattern of shell wear or breakage may reveal important information about the transport and burial history of the fossil after its death. The study of these postmortem processes is called taphonomy. Taphonomic studies require detailed observation about the preservation of the fossils and the original orientation of the fossils in the outcrop.

In the laboratory, fossils are examined with a binocular microscope, or they may be ground down to translucent slices and examined under a petrographic microscope. This thin-section technique permits microscopic examination of shell structure. The scanning electron microscope is used to examine surficial features of the shell, at magnifications on the order of 30 to 5,000 times. The composition of fossil shells might be determined by microprobe or isotopic analysis.

Much paleontological study concerns the description of fossil species. Paleontologists seek to differentiate between species and discover the relationship between species by describing and comparing the key morphologic characteristics of different species. Computers and digital imaging equipment enable paleontologists to quickly measure many morphologic features and rapidly process large sets of numerical data. Relationships between species can be represented numerically through various mathematical techniques, and the results are presented in graphical form in computer printouts.

Not all paleontological discoveries are made in the field. A wealth of fossil material resides in museum collections, awaiting future study. For example, a new dinosaur species was discovered when crates of fossils collected one century earlier were unpacked and examined in detail for the first time.

As research continues, technology advances, and scientists better understand fossil records, the Earth's complexities allow better predictions, evaluations, and data management of the planet's future, present, and history. For example, in 2024, scientists at the University of Toronto documented the oldest fossilized human skin dated 286 to 289 million years back, predating the dinosaurs. The same year, scientists identified the first individual using ancient DNA to have Turner Syndrome.

Principal Terms

body fossil: the petrified remains of a plant or animal

fossil: evidence of organic activity, usually preserved in sedimentary rock strata

gradualism: an explanation of how evolution works involving slow, constant change through time

lagerstatte: an assemblage of exceptionally well-preserved fossils

macrofossil: a fossil that is large enough to study with the unaided eye, as opposed to a microfossil, which requires a microscope for examination

morphology: the appearance (shape and form) of an organism

paleontologist: a scientist who studies ancient life; invertebrate paleontologists study fossil invertebrate animals, vertebrate paleontologists study fossil vertebrates, and micropaleontologists study microfossils

punctuated equilibria: an alternative model of how evolution works, by rapid speciation events that involve major changes in morphology

speciation: the evolutionary process of species formation; the process through which new species arise

species: a group of similar, closely related organisms

taphonomy: the study of the sequence of events that led to the burial and preservation of fossils

trace fossil: indirect evidence of an organism's presence through tracks, trails, and burrows

Bibliography

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