Paleontological Method

the method of determining the relative age of sedimentary strata of the earth’s crust according to fossil remains of organisms preserved in them. The paleontological method is used for solving problems of stratigraphy. The method was devised by the English geologist W. Smith (1769–1839), who, realizing that various strata of rocks have their characteristic remains of organisms, showed that the remains may be used to compare the strata of different regions and to compile geological maps. The development of the paleontological method was furthered by C. Darwin, who established the evolutionary approach to paleontology.

The paleontological method is based on the successive replacement of groups of animals and plants during the historical development of the organic world, the nonrecurrence of plant and animal forms (see IRREVERSIBILITY OF EVOLUTION), and the optional simultaneous change of certain groups throughout the world.

The geological age of a deposit is often determined by an index fossil. However, index genera and species may under certain conditions appear at a level not characteristic of them. A more reliable means of determining the age of a deposit involves the study of an entire community of fossil organisms. Comparisons are reliable if a vertical section establishes the same succession of several animal species. Great difficulties arise in comparing marine and continental deposits that contain, as a rule, organisms of different groups. The groups of organisms most useful in the paleontological method are those that changed rapidly over time, are widely distributed and well preserved in fossil form, and are found in significant numbers. Of increasing significance in the paleontological method are microorganisms and the mi-croremains of organisms (foraminiferans, ostracodans spores, pollen, conodonts). Important marine macroorganisms include graptolites, cephalopods, and brachiopods; useful continental macroorganisms include vertebrates and plants.