Fossil and Fossilization

The principal use of fossils by geologists has been to date rock strata (layers) that have been deposited over One of the most complete early hominid fossils is this Australopithecus afarensis specimen commonly known as "Lucy," which was found by Donald Johanson in the Afar region of Ethiopia. © John Reader/Science Photo Library/Photo Researchers, Inc. Reproduced by permission. millions of years. As different episodes in Earth's history are marked by different temperature, aridity, and other climatic factors, as well as different sea levels, different life forms were able to survive in one locale or period but not in another. Distinctive fossilized life forms that are typically associated with given intervals of geologic time are known as index fossils, or indicator species. Over the past 200 years, paleontologists have determined an order of successive index fossils that not only allows geologists to date strata, but also is the foundation for understanding organic evolution.

The temporal relationship of the strata is relative: it is more important to know whether one event occurred before, during, or after another event than to know exactly when it occurred. Recently geologists have been able to subdivide time periods into progressively smaller intervals called epochs, ages, and zones, based on the occurrence of characteristic indicator (index fossil) species, with the smallest time slices about one-half million years. Radio-metric dating measures that measure the decay of radioactive isotopes have also been used to derive the actual rather than relative dates of geological periods; the dates shown on the time scale were determined by radiometry. The relative dating of the fossil clock and the quantitative dating of the radiometric clock are used in combination to date strata and geological events with accuracy.

The fossil clock is divided into units by index fossils. Certain characteristics favor the use of one species over another as an index fossil. For example, the ammonoids (ammonites), an extinct mollusk, function as index fossils from Lower Devonian through Upper Cretaceous—an interval of about 350 million years. The ammonoids, marine animals with coiled, partitioned shells, in the same class (Cephalopoda) as the present-day Nautilus, were particularly long-lived and plentiful. They evolved quickly and colonized most of the seas on the planet. Different species preferred warmer or colder water, evolved characteristically sculpted shells, and exhibited more or less coiling. With thousands of variations on a few basic, easily visible features—variations unique to each species in its own time and place—the ammonoids were obvious candidates to become index fossils. For unknown reasons, this group of immense longevity became extinct during the Cretaceous-Triassic mass extinction. The fossils are still quite plentiful; some are polished and sold as jewelry or paperweights.

Index fossils are used for relative dating, and the geologic time scale is not fixed to any one system of fossils. Multiple systems may coexist side by side and be used for different purposes. For example, because macrofossils such as the ammonoids may break during the extraction of a core sample or may not be frequent enough to lie within the exact area sampled, a geologist may choose to use the extremely common microfossils as the indicator species. Workers in the oil industry may use conodonts, fossils commonly found in oil-bearing rocks. Regardless of which system of index fossils is used, the idea of relative dating by means of a fossil clock remains the same.