Fossil

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.

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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).