Editing Paleontology (section)

===Biostratigraphy===
{{main|Biostratigraphy}}
[[File:Western Europe latest Oligocene ruminant biostratigraphy evolution.png|thumb|left|Biostratigraphy of [[ruminant]]s from the [[Oligocene]] of western Europe]]
Fossils have been used for stratigraphic correlation since at least the 18th century. Observed changes in the fossils found through geologic time led to the principle of [[ecological succession]], however this study was not elaborated on until the 1960s. The first and last appearance of a taxon in the fossil record can be used to compare the relative ages of different [[lithography|lithographic]] sections of sediment. This principle allows for relative ages of different sediments to be determined more precisely. These "[[index fossil]]s" are combined with measurements of [[volcanic ash]], paleomagnetic reversals, or pre-dated sediments to make precise measurements of geologic time.<ref name="kelley2013"/> For example, the [[Jurassic Period]] was named and defined based on ten main subdivisions identified through the English and French assemblages of [[ammonites]], some of which are still in use today.<ref name="ogg2012"/> Biostratigraphy is also applied to the analysis of [[stratotype]] sections and boundaries of geologic time units. It can also use the first or last appearance date of a taxon to establish time periods that are independent of their constituent strata.<ref name="kelley2013"/>

The geologic time scale is based primarily on the biostratigraphy (correlating strata) and equivalent biochronology (correlating times) of the appearance and disappearance of various fossil taxa. Some factors can introduce uncertainty into this process including the quality or quantity of sampled fossils. Different graphical and numerical methods are used in the construction of the geologic time scale. Even the [[Ediacaran]], which is poorly represented through fossils, can be assessed using biostratigraphy in combination with [[chemostratigraphy]] and absolute dating.<ref name="narbonne2012"/> The biostratigraphy of the [[Ordovician]] and [[Silurian]] is based primarily on fossils of [[graptolites]] and [[conodonts]].<ref name="cooper2012"/> Other common groups used in zonation include ammonites, [[foraminifera]], and [[palynology|plant pollen]], where it is preserved.<ref name="davydov2012"/>