The geologic time scale is based principally on the relative ages of sequences of sedimentary strata. Establishing the ages of strata within a region, as well as the ages of strata in other regions and on different continents, involves stratigraphic correlation from place to place. Although correlation of strata over modest distances often can be accomplished by tracing particular beds from place to place, correlation over long distances and over the oceans almost invariably involves comparison of fossils. With rare exceptions, fossils occur only in sedimentary strata. Paleontology, which is the science of ancient life and deals with fossils, is mutually interdependent with stratigraphy and with historical geology. Paleontology also may be considered to be a branch of biology.
Organic evolution is the essential principle involved in the use of fossils for stratigraphic correlation. It incorporates progressive irreversible changes in the succession of organisms through time. A small proportion of types of organisms has undergone little or no apparent change over long intervals of geologic time, but most organisms have progressively changed, and earlier forms have become extinct and, in turn, have been succeeded by more modern forms. Organisms preserved as fossils that lived over a relatively short span of geologic time and that were geographically widespread are particularly useful for stratigraphic correlation. These fossils are indexes of relative geologic age and may be termed index fossils.
Fossils play another major role in geology because they serve as indicators of ancient environments. Specialists called paleoecologists seek to determine the environmental conditions under which a fossil organism lived and the physical and biological constraints on those conditions. Did the organism live in the seas, lakes, or bogs? In what type of biological community did it live? What was its food chain? In short, what ecological niche did the organism occupy? Because oil and natural gas only accumulate in restricted environments, paleoecology can offer useful information for fossil fuel exploration.
Invertebrate paleontology
One of the major branches of paleontology is invertebrate paleontology, which is principally concerned with fossil marine invertebrate animals large enough to be seen with little or no magnification. The number of invertebrate fossil forms is large and includes brachiopods, pelecypods, cephalopods, gastropods, corals and other coelenterates (e.g., jellyfish), bryozoans, sponges, various arthropods (invertebrates with limbs—e.g., insects), including trilobites, echinoderms, and many other forms, some of which have no living counterparts. The invertebrates that are used as index fossils generally possess hard parts, a characteristic that has fostered their preservation as fossils. The hard parts preserved include the calcareous or chitinous shells of the brachiopods, cephalopods, pelecypods, and gastropods, the jointed exoskeletons of such arthropods as trilobites, and the calcareous skeletons of frame-building corals and bryozoans. The vast variety of organisms lacking hard parts are poorly represented in the geologic record; however, they sometimes occur as impressions or carbonized films in finely laminated sediments.
Vertebrate paleontology
Vertebrate paleontology is concerned with fossils of the vertebrates: fish, amphibians, reptiles, birds, and mammals. Although vertebrate paleontology has close ties with stratigraphy, vertebrate fossils usually have not been extensively used as index fossils for stratigraphic correlation, vertebrates generally being much larger than invertebrate fossils and consequently rarer. Fossil mammals, however, have been widely used as index fossils for correlating certain nonmarine strata deposited during the Paleogene Period (about 65.5 to 23 million years ago). Much interest in dinosaurs has arisen because of the evidence that they became extinct approximately 65.5 million years ago (at the Cretaceous-Tertiary, or Cretaceous-Paleogene, boundary) during the aftermath of a large meteorite or comet impact.
Micropaleontology
Micropaleontology involves the study of organisms so small that they can be observed only with the aid of a microscope. The size range of microscopic fossils, however, is immense. In most cases, the term micropaleontology connotes that aspect of paleontology devoted to the Ostracoda, a subclass of crustaceans that are generally less than one millimetre in length; Radiolaria, marine (typically planktonic) protozoans whose remains are common in deep ocean-floor sediments; and Foraminifera, marine protozoans that range in size from about 10 centimetres to a fraction of a millimetre.
Generally speaking, micropaleontology involves successive ranges of sizes of microscopic fossils down to organisms that must be magnified hundreds of times or more for viewing. The study of ultrasmall fossils is perhaps the fastest growing segment of contemporary paleontology and is dependent on modern laboratory instruments, including electron microscopes. It is an important aspect of oil and natural gas exploration. Microfossils, which are flushed up boreholes in the drilling mud, can be analyzed to determine the depositional environment of the underlying sedimentary rocks and their age. This information enables geologists to evaluate the reservoir potential of the rock (i.e., its capacity for holding gas or oil) and its depth. Ostracods and foraminifera occur in such abundance and in so many varieties and shapes that they provide the basis for a detailed classification and time division of Mesozoic and Cenozoic sediments in which oil may occur.
Filamentous and spheroidal microfossils are important in many Precambrian sediments such as chert. They occur in rocks as old as 3,500,000,000 years and are thus an important testimony of early life on Earth.
Paleobotany
Paleobotany is the study of fossil plants. The oldest widely occurring fossils are various forms of calcareous algae that apparently lived in shallow seas, although some may have lived in freshwater. Their variety is so profuse that their study forms an important branch of paleobotany. Other forms of fossil plants consist of land plants or of plants that lived in swamp forests, standing in water that was fresh or may have been brackish, such as the coal-forming swamps of the Late Carboniferous Period (from 320,000,000 to 286,000,000 years ago).
Palynology
Palynology deals with plant spores and pollen that are both ancient and modern and is a branch of paleobotany. It plays an important role in the investigation of ancient climates, particularly through studies of deposits formed during glacial and interglacial stages. Study of a sequence of spore- or pollen-bearing beds may reveal successive climatic changes, as indicated by changes in types of spores and pollen derived from different vegetative complexes. Spores and pollen are borne by the wind and spread over large areas. Furthermore, they tend to be resistant to decay and thus may be preserved in sediments under adverse conditions.