I.

Introduction

Fossil, remains or traces of prehistoric plants and animals, buried and preserved in sedimentary rock, or trapped in organic matter. Fossils representing most living groups have been discovered, as well as many fossils representing groups that are now extinct. Fossils range in age from 3.5-billion-year-old traces of microscopic cyanobacteria (blue-green algae) to 10,000-year-old remains of animals preserved during the last ice age.

Fossils are most commonly found in limestone, sandstone, and shale (sedimentary rock). Remains of organisms can also be found trapped in natural asphalt, amber, and ice. The hard, indigestible skeletons and shells of animals and the woody material of plants are usually preserved best. Fossils of organisms made of soft tissue that decays readily are more rare. Paleontologists (scientists who study prehistoric life) use fossils to learn how life has changed and evolved throughout earth’s history (see Paleontology).

II.

Processes of Fossilization

Many factors can influence how fossils are preserved. Remains of an organism may be replaced by minerals, dissolved by an acidic solution to leave only their impression, or simply reduced to a more stable form. The fossilization of an organism depends on the chemistry of the environment and on the biochemical makeup of the organism. As a result, not all organisms in a community will be preserved.

A.

Carbonization

Plants are most commonly fossilized through carbonization. In this process, the mobile oils in the plant’s organic matter are leached out and the remaining matter is reduced to a carbon film. Plants have an inner structure of rigid organic walls that may be preserved in this manner, revealing the framework of the original cells. Animal soft tissue has a less rigid cellular structure and is rarely preserved through carbonization. Although paleontologists have found the carbonized skin of some ichthyosaurs, marine reptiles from the Mesozoic Era (240 to 65 million years before present), the microscopic structure of the skin was not preserved.

B.

Petrifaction

Another common mode of preservation of plants is petrifaction, which is the crystallization of minerals inside cells. One of the best-known forms of petrifaction is silicification, a process in which silica-rich fluids enter the plant’s cells and crystallize, making the cells appear to have turned to stone (petrified). Famous examples of silicification may be found in the petrified forests of the western United States (see Petrified Forest National Park). Petrifaction may also occur in animals when minerals such as calcite, silica, or iron fill the pores and cavities of fossil shells or bones.

C.

Replacement

Replacement occurs when an organism is buried in mud and its remains are replaced by sulfide (pyrite) or phosphate (apatite) minerals. This process may replace soft tissue, preserving rarely seen details of the organism’s anatomy. X-ray scanning of some German shales from the Devonian Period (410 million to 360 million years before present) have revealed limbs and antennae of trilobites (extinct ocean-dwelling arthropods) and tentacle arms of cephalopods (highly developed mollusks) that have been pyritised (replaced by pyrite). Paleontologists have used mild acids to etch the phosphatized fossil remains of ancient fish found in Brazil to reveal structures such as gills and muscles. Although mineral replacement is rare, fossils created in this way are important in helping paleontologists compare the anatomical details of prehistoric organisms with those of living organisms.

D.

Recrystallization

Many animal shells are composed of the mineral aragonite, a form of calcium carbonate that breaks down over millions of years to form the more stable mineral calcite. This method of preservation, called recrystallization, destroys the microscopic details of the shell but does not change the overall shape. Snail shells and bivalve shells from the Jurassic Period (205 million to 138 million years before present) and later are still composed principally of aragonite. Most older shells that have been preserved have recrystallized to calcite.

E.

Soft-Tissue Preservation

The soft tissues of animals are preserved only under extremely unusual conditions, and the preserved tissue usually lasts for only a short period of geological time. In the Siberian permafrost (earth that remains frozen year-round), for example, entire mammoths have been preserved in ice for thousands of years. The remains of the mammoths’ last meals have sometimes been preserved in the stomachs, allowing paleontologists to study the animals’ diet.

Mummification may occur in hot, arid climates, which can dehydrate organisms before their soft tissue has decayed fully. The skin itself is preserved for only a short time, but the impressions of the skin in the surrounding sediment can be preserved much longer if the sediment turns to rock. Paleontologists have found skin impressions of dinosaurs preserved by this method.

F.

Organic Traps

Whole organisms may become trapped and preserved in amber, natural asphalt, or peat (decaying organic matter). Amber is the fossilized remaining part of tree sap. When sap first flows from the tree, it is very thick and sticky, so as it runs down the trunk, it may trap insects, spiders, and occasionally larger animals such as lizards. These organisms can be preserved for millions of years with details of their soft tissue, such as muscles and hair-like bristles, still intact.

Natural asphalt (also called tar) is a residue from oil that has seeped to the earth’s surface from deposits in the rock below. When an asphalt pit is covered by water, thirsty animals that come to the pit to drink may become trapped in the sticky substance and be preserved. One well-known example of such an area is the La Brea Tar Pits of the Pleistocene Epoch (1.6 million to 10,000 years before present) in Los Angeles, California.

Animals may also be preserved in peat, although the acidic environment of this decaying organic matter may cause bones to lose their rigidity. Some human remains have been found in peat bogs in Denmark (2000 years old) and England (2200 years old).

G.

Molds and Casts

Acidic conditions may slowly dissolve away the skeleton of fossil animals preserved in rock, leaving a space where the organism used to be. The impression that is left in the rock becomes a mold. This process commonly occurs in fossil shells where the calcite shell dissolves easily. The impression of the outside of the shell is the external mold. Sometimes the inside of the shell is filled with sediment before the shell is dissolved, leaving an internal impression of the shell called an internal mold. If the space where the shell used to be is then filled with a new mineral, the replica of the shell forms a cast.

H.

Tracks and Trails

When animals walk through soft sediment such as mud, their feet, tails, and other body parts leave impressions that may harden and become preserved. When such an impression is filled with a different sediment, the impression forms a mold and the sediment that fills the mold forms a cast. Molds and casts of dinosaur tracks are relatively common and help paleontologists understand how these creatures moved.

I.

False Fossils

Minerals can sometimes grow within rocks into shapes that resemble fossils. Dendrite crystals are often mistaken for fernlike fossils. Flint nodules in chalk can look like a variety of different life forms. Mineral concretions in sediments are sometimes mistaken for fossilized eggs. It is only with close study that the true nature of false fossils can be discovered.

Modern animals and plants sometimes become mummified or coated in travertine (calcium carbonate salts from springwater), or they may die while trapped in cracks in older rock strata. These remains are not true fossils, but trapped animals and plants may eventually fossilize with time.

III.

Where Fossils Form

Fossils are found in all parts of the world, from Greenland to Antarctica. They can be found in cores drilled in and retrieved from the ocean floor, and on top of the highest mountains. Their wide geographical distribution is a result of the way the earth’s surface has changed throughout its history.

The earth’s crust is made up of several large tectonic plates that float on the earth’s liquid mantle (see Plate Tectonics). These tectonic plates have moved throughout geological time, forming large land areas and mountain ranges, and forming and closing off seas. Some land that is now in the polar regions was once closer to the equator, and many modern mountain ranges were once under water.

The global climate has also changed over geological time, alternating between periods of warmth and ice ages. These climatic conditions affected the distribution of life on the earth and are reflected in the fossil record. Fossils are abundant in rocks that were formed in tropical and equatorial regions for the same reason that life is most abundant at these latitudes today—a warm, tropical climate supports a wider variety of life forms than does a cold climate.

The types of fossils found in a particular region depend on the age of the rocks that are currently eroding at the surface. Some areas have become famous for the types of fossils found there, such as China and the badlands (rugged, rocky areas with little vegetation) of the United States and Canada, where an abundance of dinosaur fossils from the Cretaceous Period (138 million to 65 million years before present) have been found. Some fossils are restricted to small areas and some are distributed globally. The most widespread fossils are the remains of organisms that lived in oceans and could move with the currents, such as foraminifera, and those that lived on land and were spread by wind, such as spores. Fossils of graptolites (marine invertebrates that lived in colonies) in rocks of marine origin and of ferns on land are now found on all continents. Certain species of shallow-water trilobites, and dinosaurs that were restricted to land, are found only at particular localities.

Different types of fossils are found in different geological formations, depending on the prehistoric environment represented and the age of the rock. Older rocks are found on low, eroded continents near the edges of large oceans. Younger rocks are found more commonly where there is active mountain building and volcanic activity. Old fossils are most commonly found where an old mountain range has eroded, such as in eastern North America and northern Europe, or where two old continents have collided, such as in Russia. Younger fossils are found at the ocean side of young mountains where an ocean plate is colliding with a continental plate, such as in western North and South America and in New Zealand.

IV.

Learning From Fossils

Paleontologists use fossils to reconstruct how prehistoric organisms might have looked. Fossils that are found grouped together can suggest how an organism interacted as part of a community. Sometimes the microscopic structure of an organism is preserved, as well as different growth stages from embryo to adult. Such remains allow paleontologists to determine how closely related fossil organisms are to one another and to living organisms. When studying extinct organisms with no obvious living relatives, such as graptolites, paleontologists look at the microscopic structure and chemical composition of the remains to determine if there is a living relative.

Paleontologists must sometimes compare the fossils of extinct organisms with living organisms to draw conclusions about the nature, behavior, or habits of prehistoric life forms. For example, the inner chambers of the extinct ammonites (squidlike mollusks with a spiral shell) can be compared with the inner chambers of the living nautilus. The sharp, serrated teeth of Tyrannosaurus are similar to those of living carnivores, indicating that this dinosaur was also a meat eater. Similarly, the flattened teeth of Hadrosaurus, which resemble the teeth of living herbivores, suggest that this duck-billed dinosaur was a plant eater.

Some fossils reveal information about how a species grew. Paleontologists have found fossils of the empty shells of trilobites, for example, that reveal that the animals shed their shell-like skeletons as they grew into adult forms, much as shrimps and crabs do today. Vertebrates have internal skeletons that cannot be shed at different growth stages. In order for paleontologists to gather information about the growth stages of vertebrates, therefore, they must study the fossilized bones of animals that died during certain stages. For example, paleontologists have discovered a dinosaur nesting site in Montana that contains skeletal fossils of the duck-billed Maiasaura that represent various stages from embryo to adult.

Prehistoric organisms interacted with one another in much the same way as living organisms do today. Paleontologists have identified predators and their victims using evidence such as the teeth marks of mosasaurs (large, carnivorous marine lizards) on ammonites. Evidence of fighting between rivals can be seen in the fossils of some crocodiles, in which the jaws or ribs have been broken and have healed. Prehistoric animals also suffered from disease and deformities, as evidenced by such fossils as arthritic hip joints of plesiosaurs or split segments of trilobites. Fossil plants show evidence of parasitism and disease, as well as evidence of having been fed on by insects and larger animals.

A.

Evolution

The fossil record contains evidence of how life has changed and evolved throughout the earth’s history. The earliest fossils are more than 3.5 billion years old. They are simple, microscopic, single-celled bacteria called blue-green algae. There is little evidence of change in the life forms on earth over the next 3 billion years, except that cyanobacteria (formerly known as blue-green algae) began growing in layered colonies called stromatolites. The first complex life—jellyfish and worms—appears in the fossil record about 680 million years ago. The first vertebrates evolved about 570 million years before present, at the border between Precambrian time and the Paleozoic Era. At this point, the seas also became abundant with a variety of life forms.

About 400 million years before present, some living organisms migrated onto land, and pioneering plants and arthropods became common. Vertebrates soon took advantage of this new habitat, and reptiles appeared about 330 million years before present. Early mammals appeared about 100 million years later, during the Mesozoic Era, when dinosaurs roamed the earth. After the extinction of the dinosaurs 65 million years before present, mammals moved into habitats left vacant by the dinosaurs and developed, with other survivors, into the creatures that exist today. Flowering plants appeared about 120 million years before present, becoming abundant after the extinction of the dinosaurs.

The fossil record also reveals how individual species evolved over time. It is possible to study such changes by comparing older fossils found lower in a sedimentary formation with the younger fossils found higher in the formation. The study of the succession of geological time represented by these sediments and fossils is called stratigraphy.

The fossil record suggests that evolution may have progressed at different rates—sometimes gradually, and at other times in short bursts. This is difficult to prove, however, because sedimentation is rarely continuous over long periods of time. Paleontologists theorize that rapid evolutionary events commonly occur after a major extinction, such as that of the dinosaurs. This may be so because populations of different species move into the newly unoccupied position, or niche, within a community left vacant by the extinct species.

Convergent evolution occurs when an animal with a shape that was well suited to its function becomes extinct, and a new animal that replaces the extinct one evolves a similar shape to perform a similar function. This type of evolution has occurred in dolphins and porpoises that moved into the environmental niche left vacant by the extinction of ichthyosaurs. Although there are substantial differences between the extinct ichthyosaurs and today’s dolphins and porpoises, and although their ancestry is very different, the basic form that dolphins and porpoises adapted for living in the ocean is similar to that of ichthyosaurs.

B.

Climate and Landscape

Paleontologists can also gather information about the climates of prehistoric times by studying fossils and sediments. This field of study is called paleoclimatology. In general, animal and plant life is more abundant in warm, humid equatorial climates and less abundant in both hot and cold dry climates. In the sea, corals may provide evidence of changes in climate as well as in water depth because they generally grow best in warm, shallow seas. Studies of isotopes in fossilized calcite skeletons can help determine the water temperature in which animals, such as belemnites (an extinct group of marine organisms resembling small squid), lived.

Because of the movement of the earth’s tectonic plates, most continents have drifted through various climatic zones over geological time. As a result, a particular region may have passed more than once through equatorial regions with rain forests, through tropical latitudes with deserts, and through temperate zones. The fossil record suggests that climatic variation is greater now than it was during the Jurassic Period. In Antarctica, Australia, and New Zealand, which were all close to the South Pole during the Jurassic Period, fossils of plants and animals that are normally associated with warm climates have been found.

V.

Fossil Discovery and Collection

Before paleontologists begin new fieldwork, they first study the geology of the region to determine if it is likely that fossils are present. Sometimes they visit a site that has already been documented. The typical tools of a paleontologist include a hammer, chisels, eye protection, gloves, a hard hat, a notebook and pen, collecting bags, maps, and a compass. Paleontologists take field notes as fossils are collected: For each fossil, they record the precise locality, stratigraphic level, and any associated fossils. Each fossil is given a unique identifier (such as a number) that is attached to it so that data recorded from the site can be related to individual fossils. After returning from a trip, paleontologists examine any unidentified fossils more closely.

Paleontologists usually donate fossils of a new species or of some other importance to museums, where the fossils are preserved and displayed. Although fossils may have survived for many millions of years, it may take only a very short time for them to disintegrate once they are exposed. Scientists have a variety of tools at their disposal to slow or halt this disintegration. The method of preservation they select depends on the kinds of minerals in the fossil. If a fossil has been pyritized, it can be very difficult to prevent so-called pyrite-rot, or oxidation of iron sulfides, which destroys the fossil. In general, stable humidity and temperature and an acid-free environment help protect fossils from decay.

VI.

Dating and Classifying Fossils

Paleontologists have established a basic history of life on earth based on the known fossil record. They can determine the relative age of a fossil of a new species by examining the fossils in its surroundings. Some organisms lived for only a short period of geological time, and paleontologists use the fossils of these organisms as indicators to establish the age of fossils found in association with them. If similar fossils have been found over a wide geographic range, the fossils may be used to correlate the dates of formations in different localities. A stratigraphy (a map of rock layers) can be drawn up based on the occurrence of fossils. Many ammonites from the Jurassic and Cretaceous periods are used in this way, as are graptolites in older rocks.

Paleontologists use radiometric dating to determine more precisely the age of fossils (see Dating Methods: Radiometric Dating). In this process, they study the isotopes of minerals in the rock surrounding the fossil. Knowing the rates at which the isotopes decay, and having determined how much of the isotope has decayed in the rock sample, paleontologists can determine the age of the rock—and thus the age of the fossil preserved in the rock.

Fossils are classified using several techniques. The three most popular techniques are evolutionary taxonomy, numerical taxonomy, and cladistics. Evolutionary taxonomy is the method that was most commonly used in the past. It is based on comparing the shape, structure, and relationships of organisms within a stratigraphic framework. Many paleontologists believed this method was too subjective and developed numerical taxonomy as an alternative. Numerical taxonomy uses a mathematical comparison of organisms in which measured features of the organisms are related. In an effort to achieve still greater objectivity, some paleontologists developed a third method, cladistics, based on classifying organisms according to certain features that are either primitive or derived. Primitive features are those that are common to all organisms within a group, whereas derived features are evolutionary novelties. Paleontologists have had problems with subjectivity in cladistics as well, and the method also does not easily take into account the time dimension of the geological record. A combination of the methods used in cladistics and the geological record may provide a clearer picture of the evolution of life on earth.

VII.

History of Paleontology

The collection and study of fossils began in the late 17th century when English naturalist Robert Hooke examined fossils of marine creatures from England. He realized that these animals must have lived in different climatic conditions and were now extinct. The field of paleontology grew as more fossils of different ages were discovered around the globe. English scientist Charles Darwin used the fossil record to form his theory of evolution in the 1830s. Modern paleontologists have used the fossil record to further develop the theory of evolution and to divide earth’s history into periods based on the kinds of life that were present. These periods begin with Precambrian time (about 4 billion to 570 million years before present), when earth was populated by soft-bodied organisms whose remains were not well preserved, and extend through the current time period, the Recent, or Holocene, Epoch (10,000 years before present to the present time).