Fossil record

The fossil record is the record of life on Earth as it is preserved in rock as fossils . The fossil record provides evidence of when and how life began on the planet, what types of organisms existed and how long they persisted, how they lived, died, and evolved, and what the climate was and how it changed. The fossil record also has allowed scientists to correlate rocks on a worldwide basis and to determine the relative ages of rock formations.

Fossils record life by preserving remains of organisms. A fossil is a rare thing. Most organisms decay and disappear quickly after dying. Of the tiny minority of organisms that do become preserved as fossils, an even smaller fraction survives the geologic cycle to become exposed and visible. As a result the fossil record is incomplete; there is no record of most organisms that probably lived and died.

The interpretation of the fossil record requires describing fossils, classifying them to place them in a biological context, and determining their age to give them chronological context. Fossil classification follows the same system of taxonomy as modern biology. Fossil organisms are placed in a genus, species, etc. Owing to the incompleteness of the fossil record, the classification of fossil organisms includes only about 250,000 species, a small number when compared to the over 2 million species of modern organisms that have been identified.

The most direct information the fossil record provides is of an organism's physical structure and what it may have looked like, thereby enabling it to be classified. Other information such as its environment, its diet, and its life cycle is deduced from its physical attributes, from other fossils found in association, and from the types of rocks containing the fossils. Trace fossils, or fossilized marks left as a result of the activities of creatures such as trails, footprints, and burrows also provide important information.

Of critical importance to the fossil record is the age of fossils. Many theories about how Earth and life on it evolved would not be possible without knowing the time sequence of the fossil record. The age of fossils is determined by two methods: relative dating and absolute dating. Relative dating involves comparing one rock formation with another and deciding the relative ages of the two formations. For example, when one formation is found above another, the lower formation and the fossils it contains must have been deposited prior to the overlying formation and so must be older. This rule, known as the principle of superposition , holds as long as the rocks have not been overturned by faulting or folding. In determining an absolute age, radiometric age dating is used. This method measures the abundance of a radioactive element in a fossil or an associated rock. An absolute age is then reverse-calculated based on the rate of decay, or half-life of the element.

Often, certain fossils are found in a limited vertical sequence of rock and are assumed to represent a limited time period. These fossils, known as index fossils, are useful for determining relative ages and for correlating rock formations on a worldwide basis. Early workers used index fossils and rock correlation to develop the geologic scale. Originally, the geologic scale was relative, based largely on the fossil record. Subsequently, absolute ages have been applied to the geologic scale.

By synthesizing the fossil record, classifying fossils, aging them, and placing them in the context of the geologic scale, scientists have revealed the sequence of life on Earth. In many cases, the scale shows how some organisms evolved systematically over time, each subsequent version of an organism displaying modifications over the earlier. In other cases, there are large gaps in the fossil record and the developmental process for some organisms is not as clear. Often, the evolution of organisms leads to a dead end. The fossil record shows that throughout geologic time , life often evolved slowly, punctuated by explosions of life when a large number new organisms appeared. For example, the beginning of the Cambrian Period of the geologic scale contains a phenomenal number of new organisms. It also shows that, periodically, mass extinctions occurred, such as at the end of the Cretaceous Period , when a majority of species came to an end over a relatively short amount of time.

The fossil record begins with 3.5 to 3.0 billion-year-old rocks from Australia and South Africa , which preserve the remains of blue-green algae. These fossils resemble the modern stromatolites that grow in oceanic tidal areas. The fossil record shows a steady increase in the complexity of marine organisms over the next three billion years. Eventually, about 435 million years ago, terrestrial organisms appeared. The subsequent rise and fall of different creatures, from insects to fish, dinosaurs to mammals, has all been deduced from the fossil record.

In addition to outlining the history of life on Earth, the fossil record provides clues to climatic and tectonic evolution of the planet. Plant fossils and microscopic fossils such as pollen are particularly useful for the evidence they provide about the climate of the earth. For example, the Carboniferous period must have been very warm and moist because of the presence of abundant fossils of ferns and other tropical plants from that time. Also, the lack of fossilized remains of oxygen breathing organisms and the dominance of photosynthetic algae fossils from the very early Earth suggest that the primordial atmosphere was devoid of oxygen. The concept of plate tectonics was greatly aided by the observation that fossils now found widely spaced across the globe must have actually lived on the same original landmass that subsequently split apart.

See also Evolution, evidence of; Fossils and fossilization; Uniformitarianism

Fossil

Fossils are the preserved remains of a once-living organism. They form in many different ways and they can provide us with information on the climate, geology, and geography of ancient Earth. Fossils also provide strong evidence for evolution (the process by which living things change over generations).

Fossils are usually thought of as the remains of once-living things that have been mineralized or turned into rock. While this describes a fossil, the notion of fossilization includes several other methods of preserving the evidence of ancient life. The use of fossils to study ancient life and its development, or evolution, is called paleontology. The word fossil comes from the Latin word fossilis, meaning "something dug up." This is actually how most fossils are discovered. Most are found below the surface of Earth in a preserved form, since they had been covered up at one time or another. A fossil can be the partial or complete bodily remains of a plant or animal, or it can be the more common "trace fossil" which is more like some evidence of the organism's life or activities. Trace fossils are things like trails, coprolites (fossilized animal dung), or footprints.

THE FORMATION OF FOSSILS

Fossils can be formed in many ways, but they all have one process in common. They all replace the relatively fragile organic structure of a living thing with something that is harder and which lasts longer. This can only happen when certain conditions apply. Under normal conditions, if an animal falls dead on the ground, it will eventually begin to rot, or decompose and will be reduced to its basic organic compounds, which are then recycled. In the end, it completely and totally disappears. However, if the animal falls dead and slides into a tar pool, asphalt lake, or peat bog, it may be completely preserved because of the lack of oxygen or the absence of bacteria, which are both needed in order for decomposition to occur. In an extremely hot and dry climate, an animal may undergo mummification, or be rapidly dehydrated, under the right conditions; therefore, it will be preserved. In other extreme climates in the north, completely preserved specimens of woolly mammoths and even humans have been recovered after being frozen under glacial conditions for thousands of years. A final way that nature can capture and preserve a complete specimen is in amber (the sticky resin of a pine tree). Insects were usually fossilized this way when they became trapped inside the amber. When the resin itself became fossilized, the insect was forever frozen in a drop of what looks like yellow glass.

Although these are examples are more like preservation than true fossilization, they are all considered to be fossils. True fossils are the remains of once-living organisms that are preserved in stone. When geology (the study of the Earth) became more of a real science in the late eighteenth and early nineteenth centuries, geologists began exploring below Earth's surface. These scientists went deeper into successively older and older layers of what is called sedimentary rock (formed by the gradual settling of sediments). It was in these buried layers that they found entire communities of fossils in each layer. Often, the deeper layers of rock contained animals that were very different from any known animals, since the deeper the geologists dug, the older the fossil. These unknown animal bodies that were preserved were those that had been covered by sediments very quickly before they could start to decay. However, they needed to be more than just covered to become fossils. They also needed to be deprived of oxygen. This often happened when a dead animal sunk to the bottom of a body of water and was deeply covered by mud. As more sediments piled up and pressure increased, the organic material that made up the animal was slowly replaced by minerals and the body eventually became a fossil made of stone. These type of fossils might be described as a replica of the real living thing.

In the rare process known as petrifaction, both the external shape and the internal structure of a plant or animal are perfectly reproduced. The Petrified Forest of Arizona contains trees that were buried by volcanic eruptions and underwent petrifaction. In this rare process, a molecule-by-molecule replacement occurred, with the end result being the replacement of natural wood fibers by silica. This replacement is usually so accurate that even the cell structure of the tree can be determined.

Paleontology, or the study of fossils, informs scientists about the organisms that lived on Earth long before human life evolved. It also tells us a great deal about Earth's climate in those ancient times. The oldest known fossils are 3,500,000-year-old bacteria. They represent not only the oldest known form of life on Earth but help scientists to learn more about the origins of life itself. Fossils also tell scientists about extinction that has taken place. Finally, most of what is known about evolution is based on the fossil record. It is fossils that provide the most reliable evidence for evolution since for some organisms, paleontologists are able to compare their fossils from different layers of rock (which are from different ages) and actually trace how they evolved physically.

[See alsoGeologic Record ]

Fossil Record

Geologists and other scientists use fossils to correlate the ages of different rock strata (thin layers or beds of rock that differ in some way from adjacent layers) in different places on Earth. If two different rock strata contain the same set of fossil species, then the two different rock strata were probably deposited about the same time. Fossils also can give clues about the environment of Earth in the past. For example, certain fossils are only found in the ocean. When these fossils are found in a rock strata, it is a sure sign that the rock strata was deposited in an ocean even if it is now on top of a high mountain.

Fossils can also help to establish the relative ages of rocks (which rocks are older and which are younger). If a fossil species can be assigned an absolute date by radioactive dating, then that same fossil species can be used to help determine the absolute dates of other rocks that contain it. The fossil record also gives clues as to how life has evolved.

A fossil is any preserved remains of ancient life. There are several different categories of fossils. Trace fossils include such things as tracks, burrows, and coprolites (fossilized excrement). Body parts or whole bodies of organisms can be preserved by a process known as mineralization, in which minerals gradually replace the organic remains and the fossil is turned to stone.

Molds, casts, and imprints make up another category of fossils. They are formed when the sediment has solidified about an organic object and the object is subsequently dissolved, leaving a hole in the rock—a mold. Deposition of mineral matter from underground solutions may fill the hole, producing a cast. Molds of thin objects (such as leaves of ferns often found in coal) are called imprints.

Organisms with hard tissues are more likely to be preserved. Organisms that are more abundant are more likely to be preserved. Organisms that live in swamps or near water are less likely to decay when they die and are more likely to be preserved. All these factors make the fossil record somewhat incomplete. Nevertheless, the fossil record extends back at least 3.5 billion years. During this immense span of time, tens of millions of different species have lived on Earth.

The fossil record can also be used to determine the ages of rocks. The geological principle of superposition states that if rock layers are undisturbed, older rock layers are found below younger layers. If the rock layers contain fossils, then the relative ages of those fossils can be determined from the relative ages of the rock in which they were found. Then those same fossils can be used to help determine the relative ages of rocks found elsewhere. A bed of sedimentary rock can be identified by its fossils.

Using these ideas, geologists working in the first part of the nineteenth century at many different places gradually developed a theory of the history of life on Earth. This life history is now known as the geological time scale. Although the early researchers dramatically underestimated the age of Earth, they did establish the principle of determining the age of rocks by looking at the fossils found in those rocks.

Even a superficial examination of the fossil record shows that many species existed in the distant past that no longer exist today. Likewise, even an incomplete fossil record reveals that species living today did not exist in the distant past. Thus the fossil record and the geological time scale provided the background for Charles Darwin and other scientists to develop their theories of evolution.

see also Biological Evolution; Geological Time Scale; Morphological Evolution in Whales; Tetrapods—From Water to Land.

Elliot Richmond

Bibliography

Dott, Robert H., Jr., and Roger L. Batten. Evolution of the Earth, 3rd ed. New York: McGraw-Hill, 1994.

Faul, Henry, and Carol Faul. It Began with a Stone: A History of Geology from the StoneAge to the Age of Plate Tectonics. New York: John Wiley & Sons, Inc., 1983.

Foster, Robert. Geology, 3rd ed. Columbus, OH: Charles E. Merrill, 1976.

Schneer, Cecil J. "The Rise of Historical Geology in the 17th Century." Isis. 45, no. 141 (1954):256-268.

Stanley, Stephen. Earth and Life through Time. New York: W. H. Freeman & Co., 1989.

Toulmin, Stephen, and June Goodfield. The Discovery of Time. New York: Harper & Row, 1965.

fossil The remains or traces of any organism that lived in the geological past. In general only the hard parts of organisms become fossilized (e.g. bones, teeth, shells, and wood) but under certain circumstances the entire organism is preserved. For example, virtually unaltered fossils of extinct mammals, such as the woolly mammoth and woolly rhinoceros, have been found preserved in ice in the Arctic. Small organisms or parts of organisms (e.g. insects, leaves, flowers) have been preserved in amber.

In the majority of fossils the organism has been turned to stone – a process known as petrification. This may take one of three forms. In permineralization, solutions originating underground fill the microscopic cavities in the organism. Minerals in these solutions (e.g. silica or calcite) may actually replace the original material of the organism so that even microscopic structures may be preserved; this process is known as replacement (or mineralization). A third form of petrification – carbonization (or distillation) – occurs in certain soft tissues that are composed chiefly of compounds of carbon, hydrogen, and oxygen (e.g. cellulose). After the organism has been buried, and in the absence of oxygen, carbon dioxide and water are liberated until only free carbon remains. This forms a black carbon film in the rock outlining the original organism. Moulds are formed when the original fossil is dissolved away leaving a mould of its outline in the solid rock. The deposition of mineral matter from underground solutions in a mould forms a cast. Palaeontologists often produce casts from moulds using such substances as dental wax. Moulds of thin organisms (e.g. leaves) are commonly known as imprints. Trace fossils are the fossilized remnants of the evidence of animal life, such as tracks, trails, footprints, burrows, and coprolites (fossilized faeces).

The ideal conditions for the formation of fossils occur in areas of rapid sedimentation, especially those parts of the seabed that lie below the zone of wave disturbance. See also chemical fossil; index fossil; microfossil; taphonomy.

fos·sil / ˈfäsəl/ • n. the remains or impression of a prehistoric organism preserved in petrified form or as a mold or cast in rock. ∎ derog. or humorous an antiquated or stubbornly unchanging person or thing: he can be a cantankerous old fossil at times. ∎  a word or phrase that has become obsolete except in set phrases or forms, e.g., hue in hue and cry.ORIGIN: mid 16th cent. (denoting a fossilized fish found, and believed to have lived, underground): from French fossile, from Latin fossilis ‘dug up,’ from fodere ‘dig.’