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Paleontology

Paleontology

Paleontology (from Greek: palaeo, “old, ancient”; on, “being”; and logos, “speech, thought”) is the study of ancient life. Life appeared on Earth about 3,550 million years ago in the oceans, subse­quently evolved from simple bacteria-like cells to complex multicellular forms, and colonized the land. Countless adaptations resulted in a great diversity of biological forms and in addition changed the planet itself. We have learned about extinct organisms through the examination of fos­sils, the visible evidences left behind by them and preserved in rocks and sediments. Fossils include mineralized, carbonized, mummified, and frozen remains of bodies after death, or of cast-off parts, normally of the skeleton or portions, such as teeth, that became partially mineralized during life; the preservation of soft tissues, however, is extremely rare. Many other fossils consist of casts or impressions, tracks, burrows, fossilized feces (coprolites), as well as chemical residues.

People have collected fossils ever since recorded history began, and probably before that, but the nature of fossils and their relationship to life in the past became better understood during the modern era as a part of the changes in natural philosophy that occurred during the 17th and 18th centuries. The emergence of paleontology, in association with comparative anatomy, as a scientific disci­pline occurred at the end of the 18th century, when Georges Cuvier clearly demonstrated that fossils were left behind by species that had become extinct. Paleontology therefore is the study of fossils throughout geological time. The totality of fossils, both discovered and undiscovered, and their placement in sedimentary layers or strata is known as the fossil record. The fossil record ranges in age from the Holocene, the most recent geologi­cal epoch that began 12,000 years ago and contin­ues until present, to the Archean eon, which extends from about 3.8 to 2.5 billion years ago.

Fossils vary in size from the microscopic (micro­fossils), such as fossilized shells of unicellular organisms, to those of gigantic proportions, such as the fossil bones of dinosaurs. Micropaleontology studies microscopic fossils, including organic­walled microfossils, the study of which is called palynology. The study of microfossils requires a variety of physical and chemical laboratory tech­niques to extract them from rocks and the use of light or electron microscopy to observe them. Macrofossils are usually studied with the naked eye or under low-power magnification, but the observation of fine skeletal details often needs high-powered magnification.

The study of macrofossils is undertaken by several specialties. Invertebrate paleontology deals with fossils of animals with no vertebral column, while vertebrate paleontology deals with those of animals with a vertebral column, including fossil hominids (paleoanthropology). Paleobotany under­takes the study of macrofossils of plants. There are many developing specialties, such as paleoichnol­ogy (the study of trace fossils), molecular paleon­tology (the study of chemical fossils or biomarkers), and isotope paleontology (the study of the isotopic composition of fossils).

Two of the most important portions of knowl­edge that paleontologists obtain from fossils include first how they were formed (taphonomy), that is, the process of fossilization through which some material or information was incorporated from the biosphere to the lithosphere; and second, what the organisms were that produced them (paleobiology), as well as how and where they lived and what their evolutionary history was. The

source information for this purpose is the biology and ecology of present-day organisms, applying the uniformitarian principle. Fossils usually con­tain morphological information that allows us to recognize most of them as living organisms and then to identify and classify them according to the Linnaean taxonomy, as well as to study their rela­tionships to other taxa.

Fossils are generally found in sedimentary rock with differentiated strata representing a succession of deposited material. To place the fossils in con­text in terms of the time, setting, and surroundings in which the organisms lived, paleontologists require knowledge of the precise geological loca­tion where the fossils were found and details of their source rock strata. Paleontology has provided important tools for both geologists and biologists.

In geology, fossils are important in the analysis of the order and relative position of strata and their relationship to the geologic timescale, and also in correlating successions of rock strata from the same time interval across the globe. The deep time of Earth’s past has been organized into a timescale composed of various units that are usually delim­ited by major geological or paleontological events, such as mass extinctions. In a pioneering applica­tion of stratigraphy, at the end of the 18th century, William Smith in England and Georges Cuvier and Alexandre Brongniart in France made extensive use of fossils to help correlate rock strata in different locations. They observed that sedimentary rock strata contain particular assemblages of fossilized flora and fauna and that these assemblages succeed each other vertically in a specific, reliable order that can be recognized even in widely separated geologic formations (principle of faunal succes­sion). Later, Darwin’s theory of evolution closely described the causal mechanism of the observed faunal and floral succession preserved in rocks.

This principle is of great importance in deter­mining the relative age of rocks and strata by using the fossils contained within them (biostratigra­phy). As the distribution and diversity of living organisms are limited by environmental factors, and as vestiges of biochemistry of the original organism and isotopic signatures of ancient envi­ronments are preserved on fossilized skeletal remains, fossils provide an insight into the envi­ronment once inhabited by living organisms (paleo­ecology) and help in the interpretation of the nature of ancient sedimentary environments and the diagenetic processes undergone by the rocks that contained them. The primary economic impor­tance of paleontology lies in both applications to geology. The study of the fossils, especially micro­fossils, contained in a rock remains one of the fast­est, cheapest, and most accurate means to determine the age and nature of the rocks that contain them or the layers above or below. This information is vital to the mining industry and especially the petroleum industry.

In biology, fossils are the most direct evidence of the evolution of life on Earth; they have helped to establish evolutionary relationships and to date the divergences between taxa (phylogeny). An expand­ing knowledge of the fossil record encouraged the formulation of early evolutionary theories. In fact, Darwin himself collected and studied South American fossils during his trip on the H.M.S. Beagle. After Darwin’s evolutionary theory was published in 1859, much of the focus of paleontol­ogy shifted to understanding lineage evolution, including human evolution. George Gaylord Simpson and, later, Stephen J. Gould played a cru­cial role in incorporating ideas from paleontology to evolutionary theory.

Fossils indicate long-term patterns of biodiver­sity in the geological past. The story of the devel­opment of life on Earth, of the biosphere, forms the subject of paleontology. Modern paleontology sets ancient life in its context by studying how, over this vast time span, life has adapted to a changing world; this change is barely discernible during a single human lifetime. Long-term physical changes of global geography (continents and oceans pushed by plate tectonics, mountains formed and eroded) and long-term fluctuations between hot and cold climates (ice ages driven by orbital factors, warm periods in response to rapid increases in atmospheric carbon dioxide) triggered changes in living things: Populations, species, and whole lineages disappeared, and new ones emerged. Ecosystems have responded to these changes and have adapted to the planetary environment in turn. These processes continue, and today’s biodiversity is affected by these mutual responses.

Very few species, known as living fossils, sur­vive virtually unchanged for tens or hundreds of millions of years. Most species today appeared very recently in geological terms. It has been estimated that more than 95% of species that ever lived have become extinct. Paleontology has shown that extinction is a natural process that generally happens at a continuously low rate. Throughout geologic history, very few mass-extinction events have occurred in which many species have disap­peared in a relatively short period of geological time. Thus, paleontology evidences the fragility of the world. Humans appeared only about 2.5 mil­lion years ago, and several human species have become extinct. Modern man appeared very recently in geological terms, no more than 200,000 years ago. At the end of the last glacial period, around 12,000 years ago, many mammals weigh­ing more than 40 kilograms (megafauna) disap­peared. There is a debate as to the extent to which this extinction event can be attributed to environ­mental and ecological factors, to the onset of warmer climates, or to human activities, directly by overkilling megafauna or indirectly. Megafaunal extinctions continue to the present day, and this deteriorating situation is being referred to as “the sixth mass-extinction.”

The present biodiversity of an area is the conse­quence of the natural evolution of the species in its dating back to more than 3 million years ago. This evolution has been conditioned, in many ways, by geological history and certain other natural phe­nomena. But for the first time, a single species— ours—appears to be almost wholly responsible for an extinction crisis. Natural environments are now so degraded that we must go back in time to true known natural environments to understand natu­ral processes. Paleontology furnishes an extensive database that, when integrated with neontological data, allows us to define models that explain better the past and present biodiversity and that would be useful in prospective studies.

As a consequence, the strategies aimed at the protection of biodiversity should also take into account the preservation of paleobiodiversity (pale­ontological heritage) and of geological materials (geological heritage and geodiversity), which consti­tute proof of past natural processes. Although fos­sils are also preserved in museums and private collections, the paleontological heritage exists in the natural environment as fossil sites. These sites com­pose an irreplaceable and finite resource for science, education, and recreation. Paleotourism, as part of adventure tourism, is poised for dramatic growth in the decades ahead, a fact directly related to the demography of wealthy nations. As this industry grows in years to come, it will be important for scientists and government officials to work together with the local inhabitants of the fossiliferous regions to create effective partnerships to educate the public and to protect and develop our paleontological heritage. This element of natural and cultural heri­tage is vulnerable to abuse and damage and there­fore needs safeguarding and management to ensure its survival for future generations.

Beatriz Azanza

See also Archaeopteryx; Dating Techniques; Dinosaurs; Fossil Record; Fossils, Interpretations of; Geology; Stromatolites; Trilobites

Further Readings

Briggs, D., & Crowther, P. R. (2001). Palaeobiology II. Osney Meads, Oxford, UK: Blackwell Science.

Foote, M., & Miller, A. I. (2007). Principles of paleontology (3rd ed.). New York: Freeman.

Gould, S. J. (Ed.). (1993). The book of life: An illustrated history of the evolution of life on earth. New York: Norton.

Prothero, D. R. (2004). Bringing fossils to life: An introduction to paleobiology. Boston: McGraw-Hill.

Prothero, D. R. (2007). Evolution: What the fossils say and why it matters. New York: Columbia University Press.

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