The goal of biostratigraphy is to use fossil occurrences within the rock record to establish correlations between time-equivalent rock strata as determined by the presence of a particular fossil species.
Although the concept is generally straightforward, i.e. the presence of a specific fossil species in two geographic localities indicates the rocks containing the fossil specimens were deposited at about the same time, in practice biostratigraphic studies tend to be complex.
The complexities of biostratigraphy result from aspects of the biology of the organisms including their environmental range, their evolutionary rates, as well as their tendency for preservation and probability of observation by the biostratigrapher.
Ultimately, the most rapidly evolving or short-lived, yet wide-ranging fossil taxa make the best biostratigraphic markers for correlation.
If a given taxa is both wide-ranging and evolutionarily short-lived, and if it is robust enough to be preserved in the fossil record, then the taxa is often referred to as an index fossil.
An index fossil identified in the rock record would constrain the age of the rock within which it is contained to a very specific interval of time when the organism lived.
Fossil taxa used in biochronologic investigations rarely satisfy all aspects of the ideal index fossil. That is, they often violate one or more of the following rules:
- Must have a widespread distribution (fossils tend to be limited to a small region or are found only in a particular depositional environment as opposed to globally)
- Must show rapid evolution (fossils change rapidly in preservable morphology so that distinctive identifiable species are easily recognized)
- Must be present in substantial numbers (so that fossils can be observed by the biostratigrapher)
- Fossils should be robust mineralogically (so that depositional and diagenetic processes do not remove the fossils from the rock record).
High resolution biostratigrapy
- Calcareous nannoplankton - (Qualitative and Quantitative)
- Micropaleontology (Foraminifera) - (Qualitative and Quantitative)
- Palynology (Pollen and spores) - (Qualitative and Quantitative)
- Palynofacies - (Qualitative and Quantitative)