Biostratigraphy

A stage is a major subdivision of strata, each systematically following the other each bearing a unique assemblage of fossils. Therefore, stages can be defined as a group of strata containing the same major fossil assemblages. French palaeontologist Alcide d'Orbigny is credited for the invention of this concept. He named stages after geographic localities with particularly good sections of rock strata that bear the characteristic fossils on which the stages are based.

In 1856 German palaeontologist Albert Oppel introduced the concept of zone (also known as biozones or Oppel zone). A zone includes strata characterized by the overlapping range of fossils. They represent the time between the appearance of species chosen at the base of the zone and the appearance of other species chosen at the base of the next succeeding zone. Oppel's zones are named after a particular distinctive fossil species, called an index fossil. Index fossils are one of the species from the assemblage of species that characterize the zone.

Biostratigraphy uses zones for the most fundamental unit of measurement. The thickness and range of these zones can be a few meters, up to hundreds of meters. They can also range from local to worldwide, as the extent of which they can reach in the horizontal plane relies on tectonic plates and tectonic activity. Two of the tectonic processes that run the risk of changing these zones' ranges are metamorphic folding and subduction. Furthermore, biostratigraphic units are divided into six principal kinds of biozones: Taxon range biozone,[6] Concurrent range biozone,[6] Interval biozone, Lineage biozone, Assemblage biozone, and Abundance biozone.

The Taxon range biozone represents the known stratigraphic and geographic range of occurrence of a single taxon. Concurrent range biozone includes the concurrent, coincident, or overlapping part of the range of two specified taxa. Interval biozones include the strata between two specific biostratigraphic surfaces and can be based on lowest or highest occurrences. Lineage biozones are strata containing species representing a specific segment of an evolutionary lineage. Assemblage biozones are strata that contain a unique association of three or more taxa within it. Abundance biozones are strata in which the abundance of a particular taxon or group of taxa is significantly greater than in the adjacent part of the section.

Amplexograptus, a graptolite index fossil, from the Ordovician near Caney Springs, Tennessee.

Index fossils (also known as guide fossils, indicator fossils, or dating fossils) are the fossilized remains or traces of particular plants or animals that are characteristic of a particular span of geologic time or environment, and can be used to identify and date the containing rocks. To be practical, index fossils must have a limited vertical time range, wide geographic distribution, and rapid evolutionary trends. Rock formations separated by great distances but containing the same index fossil species are thereby known to have both formed during the limited time that the species lived.

Index fossils were originally used to define and identify geologic units, then became a basis for defining geologic periods, and then for faunal stages and zones.

Ammonites, graptolites, archeocyathids, inoceramids, and trilobites are groups of animals from which many species have been identified as index fossils that are widely used in biostratigraphy. Species of microfossils such as acritarchs, chitinozoans, conodonts, dinoflagellate cysts, ostracods, pollen, spores and foraminiferans are also frequently used. Different fossils work well for sediments of different ages; trilobites, for example, are particularly useful for sediments of Cambrian age. A long series of ammonite and inoceramid species are particularly useful for correlating environmental events around the world during the super-greenhouse of the Late Cretaceous.[7][8]

To work well, the fossils used must be widespread geographically, so that they can be found in many different places. They must also be short-lived as a species, so that the period of time during which they could be incorporated in the sediment is relatively narrow. The longer lived the species, the poorer the stratigraphic precision, so fossils that evolve rapidly, such as ammonites, are favored over forms that evolve much more slowly, like nautiloids.

Often biostratigraphic correlations are based on a faunal assemblage, rather than an individual species — this allows greater precision as the time span in which all of the species in the assemblage existed together is narrower than the time spans of any of the members. Furthermore, if only one species is present in a sample, it can mean either that (1) the strata were formed in the known fossil range of that organism; or (2) that the fossil range of the organism was incompletely known, and the strata extend the known fossil range. For instance, the presence of the trace fossil Treptichnus pedum was used to define the base of the Cambrian period, but it has since been found in older strata.[9] If the fossil is easy to preserve and easy to identify, more precise time estimating of the stratigraphic layers is possible.

Image displaying newly discovered fossil being introduced into the succession sequence.

The concept of faunal succession was theorized at the beginning of the 19th century by William Smith. When William was studying rock strata he began to recognize that rock outcrops contained a unique collection of fossils.[10] The idea that these distant rock outcrops contained similar fossils allowed for Smith to order rock formations throughout England. With Smith's work on these rock outcrops and mapping around England, he began to notice some beds of rock may contain mostly similar species, however there were also subtle differences within or between these fossil groups. This difference in assemblages that appeared identical at first, lead to the principle of faunal succession, where fossil organisms succeed one another in a definite and determinable order, and therefore any time period can be categorized by its fossil extent.[11]