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.