Biostratigraphy

Biostratigraphy is that branch of the study of layered rocks—stratigraphy—that focuses on fossils. Its goals are the identification and organization of strata based on their fossil content. Biostratigraphy thus investigates one of the principal bases of the geologic time scale of Earth history.

Biostratigraphic Zones

Biostratigraphy is the method of identifying and differentiating layers of sedimentary rock (strata) by their fossil content. Strata with distinctive fossil content are termed biostratigraphic units or zones. Zones vary greatly in thickness and in lateral extent. A zone may be a single layer that is a few centimeters thick and of a very local extent, or it may encompass thousands of meters of rocks extending worldwide. The defining feature of a zone is its fossil content. The fossils of a given zone must differ in some specific way from the fossils of other zones.

Zones are usually recognized after fossils have been collected extensively over the lateral and vertical extent of a rock sequence or at many sequences over a broad region. The positions of the fossils in the strata are carefully recorded in the field. Fossils that co-occur in a single layer are noted, as are fossils found isolated in the strata. In the laboratory, the biostratigrapher, usually a paleontologist, then tabulates the vertical and lateral ranges of the fossils collected. It is from these ranges that the paleontologist recognizes zones. Different types of zones are recognized depending on the way in which the fossils in the strata prove to be distinctive. Assemblage zones are strata distinguished by an association (assemblage) of fossils. Thus, not one type but many types of fossils are used to define an assemblage zone. All dinosaur fossils, for example, can be thought of as defining an assemblage zone that encompasses Earth's history from about 220 to 66 million years ago.

Range zones are strata that encompass the vertical distribution, or range, of a particular type of fossil. Thus, one fossil type—not many—is used to define a range zone. In contrast to the example just given, one type of dinosaur, Tyrannosaurus rex, lived only between 68 and 66 million years ago. Its fossils thus define a range zone that corresponds temporally to this two-million-year interval.

Acme zones are rock layers recognized by the abundance, or acme, of a type (or types) of fossil (or fossils) regardless of association or range. Horned dinosaurs (Triceratops and its allies) reached an acme between seventy and sixty-six million years ago. That is, during this period, they were most diverse and numerous. This acme zone thus overlaps the Tyrannosaurus rex range zone and represents a small portion of the dinosaur assemblage zone.

Finally, interval zones are recognized as strata between layers where a significant change in fossil content takes place. For example, the mass extinctions that took place 250 and 66 million years ago bound a 184-million-year-long interval zone that is popularly referred to as the “age of reptiles.”

Development of Biostratigraphy

Biostratigraphy developed independently in England and France just after 1800. British civil engineer William Smith worked in land surveying throughout the country. From his vast field experience, he recognized that a given stratum usually contains distinctive fossils and that the fossils (and the stratum) could often be recognized across a large area. Smith's work culminated in his geological map of England (1815), based on his tracing of rock-fossil layers across much of the country.

Meanwhile, in France, Georges Cuvier and Alexandre Brongniart studied the succession of rocks and fossils around Paris. They, too, discovered a definite relationship between strata and fossils and used it to interpret the geological history of the rocks exposed near Paris. In this history, Cuvier saw successive extinctions of many organisms coinciding with remarkable changes in the strata. To him, these represented vast “revolutions” in geological history, which Cuvier argued were of worldwide significance. It is now known that Cuvier was mistaken, but the discovery that a particular fossil type (or types) was confined to a particular stratum became the basis for biostratigraphy. This allowed geologists to identify strata from their fossil content and to trace these strata across broad regions of the Earth's crust.

Almost simultaneous with the development of biostratigraphy was the development of biochronology. Biochronology is the recognition of intervals of geologic time by fossils. It stemmed from the realization that during Earth's history, different f organisms lived during different intervals of time. Thus, the fossils of any organism represent a particular interval of geologic time. (Such fossils are called index fossils because they act as an “index” to a geologic time interval.) Biochronology thus identifies intervals of geologic time based on fossils. These time-distinctive fossils are the fossils by which zones are defined, which is to say that each zone represents, or is equivalent to, some interval of geologic time.

The time value of zones made them more useful in tracing strata and deciphering local geological histories. Biostratigraphy became one of the central methods of stratigraphic correlation. With the aid of fossils, it became possible to determine the ages of strata and thus demonstrate the synchrony or diachrony of these strata in different areas.Through the use of stratigraphic correlation, biostratigraphy became one of the bases for constructing the relative geological time scale of Earth’s history composed of eons, eras, periods, epochs, and ages. This time scale is the “calendar” by which all geologists temporally order their understanding of the history of the Earth.

Application of Biostratigraphy

Biostratigraphy is generally used as a method of stratigraphic correlation—the process of determining the equivalence of age or stratigraphic position of layered rocks in different areas. Stratigraphic correlation by biostratigraphy is extremely important in deciphering geological history. It reveals the sequence of geological events in one or more regions. Understanding geological history interests scientists and laypersons alike for its own sake. It is crucial to discovering mineral deposits and energy resources within the Earth's crust. In addition, it provides insight into the biological events that have taken place on this planet for the last 3.9 billion years.

A good example of using biostratigraphy in this last regard comes from the study of dinosaur extinction. When dinosaurs were first discovered in England in 1824, and when the term “dinosaur” was coined by the British anatomist Sir Richard Owen in 1841, no one realized that dinosaurs had lived on Earth for only 150 million years and that their extinction had taken place rather rapidly about 66 million years ago. By 1862, enough dinosaur fossils had been collected around the globe that a biostratigraphic pattern was beginning to emerge. In that year, the American geologist James Dwight Dana, in his classic Manual of Geology, noted that all dinosaurs disappeared before the end of the Mesozoic era, which is now considered as the interval of Earth history between 250 and 66 million years ago. This biostratigraphic generalization was possible because geologists noticed that many Mesozoic rocks (but no older or younger rocks) were full of dinosaur fossils, and thus, the Mesozoic came to be termed “the age of reptiles.” It might just as well be referred to as the “dinosaur zone,” except for the first thirty or so million years of the Mesozoic, during which dinosaurs did not exist.

More than a century of research has confirmed Dana's biostratigraphic generalization and considerably refined it. Scientists generally agree that the last dinosaurs disappeared worldwide approximately sixty-six million years ago. It is also known that dinosaurs first appeared about 220 million years ago. Thus, scientists can recognize a dinosaur zone and erect many types of zones based on the ranges and acmes of specific types of dinosaurs. This biostratigraphy of dinosaurs is the basis for informed discussion of the sequence and timing of events during the evolution of the dinosaurs. For example, scientists are now confident that Stegosaurus lived long before Tyrannosaurus and that stegosaurs as a group of dinosaurs became extinct long before the end of the Mesozoic.

Although the discussion here has relied heavily on dinosaurs for examples of biostratigraphy at work, the fossils of these giant reptiles are not ideal for use in biostratigraphy because it is not easy to identify most dinosaur fossils precisely and because most dinosaurs were not animals with broad geographic ranges. Indeed, the fossils of most use in biostratigraphy, index fossils, are those that are easy to identify precisely and that represent organisms that had wide geographic ranges, enjoyed broad environmental tolerances, and lived only for a brief period of geologic time.

Usually, an entire skull or skeleton is needed to identify a dinosaur fossil precisely. The isolated bones most often found are not enough, although they indicate the fossil is that of a dinosaur. Most dinosaurs (with some notable exceptions) seem to have lived in one portion of one continent; indeed, fossils of the horned dinosaur Pentaceratops (a cousin of Triceratops) have been found only in New Mexico. There is strong evidence that some dinosaurs preferred coastlines, whereas others preferred dry areas. Thus, many, if not most, dinosaurs did not live in a wide range of environments. Finally, although many dinosaurs apparently lived for only brief intervals of geologic time, the fossil record of most of these giant reptiles is not extensive enough to pin down their exact interval of existence.

The factors that argue against the use of most dinosaur fossils in biostratigraphy are quite different for microscopic fossils of pollen grains and the shelled protozoans known as foraminiferans. These microscopic fossils fit well the four criteria that identify fossils most useful in biostratigraphy. Indeed, such “microfossils” (studied by micropaleontologists) are some of the mainstays of biostratigraphy.

Significance

Biostratigraphy—the recognition of strata by their fossil content—is a cornerstone of stratigraphic correlation. To identify bodies of rock, the presence of fossils can be traced over broad areas, and their sequence in distant areas can often be determined. Stratigraphic correlation by biostratigraphy is critical to deciphering geological history. Without it, the search for mineral deposits and energy resources would be considerably more difficult. Furthermore, understanding the history of geological disasters—earthquakes, volcanic eruptions, meteorite impacts, and the like—and thereby being able to predict future disasters relies on knowledge of the sequence and timing of geological events, knowledge often derived from biostratigraphy. Deciphering the history of life on this planet, including the myriad appearances, changes, and extinctions of Earth's biota during the last 3.9 billion years, largely depends on the sequence and timing established by biostratigraphy.

Biostratigraphy has also given rise to biochronology, the recognition of intervals of geologic time based on fossils. As a result, scientists have been able to construct a relative global geologic time scale, and it is within the context of this time scale that all geological and biological events in Earth's history have been placed.

Principal Terms

correlation: the determination of the equivalence of age or stratigraphic position of two strata in separate areas or, more broadly, the determination of the geological contemporaneity of events in the geologic histories of two areas

fossils: remains or traces of animals and plants preserved by natural causes in the Earth's crust

index fossil: a fossil that can be used to identify and determine the age of the stratum in which it is found

sedimentary rocks: rocks formed by the accumulation of particles of other rocks or of organic skeletons or of chemical precipitates or some combination of these

stratigraphy: the study of layered rocks, especially of their sequence and correlation

stratum (plural, strata): a single bed or layer of sedimentary rock

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