Calcareous nannofossils are a class of tiny (less than 30 microns in diameter) microfossils that are similar to coccoliths deposited by the modern-day coccolithophores. The nannofossils are a convenient source of geochronological data due to the abundance and rapid evolution of the single-cell organisms forming them (nannoplankton) and ease of handling of the sediment samples. The practical applications of calcareous nannofossils in the areas of biostratigraphy and paleoecology became clear once the deepwater drilling took off in 1968 with the Deep Sea Drilling Project, and they have been extensively studied ever since. Nannofossils provide one of the most important paleontological records with the contiguous length of 220 million years.
History of research
Christian Gottfried Ehrenberg, while examining the chalk from Ruegen, recorded in 1836 an observation of what was later termed "coccolith" and had pictured the coccoliths and Discoasters in his Mikrogeologie (1854), erroneously classifying these discs as a kind of complex spheric concretion. T. H. Huxley coined the term coccoliths in 1858 (due to their shape resembling the Protococcus), while agreeing with their inorganic nature. In 1861 George Charles Wallich and, independently, Henry Clifton Sorby, figured out the organic nature of coccoliths after observing their aggregations, coccospheres. Huxley then changed his views and declared that coccoliths are skeletal elements of an unknown organism, Bathybius haeckelii, a primordial form of organic life. One of the goals of the Challenger expedition was to understand the nature of the Bathybius, but the scientists aboard the ship reached the conclusion that the gel-like substance apparently holding the disks in a coccosphere together was a result of processing the samples and later declared the coccoliths to constitute the defensive armor of tiny nannoplankton algae (the term was coined in 1909 by to identify the tiniest plankton, less than 60 microns in size, that passed through the regular phytoplankton nets).
Research of the nannoplankton systematics in the early 20th century (Erwin Kamptner, , and Trygve Braarud) enabled M. N. Bramlette and W. R. Riedel to successfully use the nannofossils for biostratigraphy (1954). The Deep Sea Drilling Project (DSDP, 1968) revealed the power of the technique: stratigraphic positions were found within minutes after the drilling core was hauled aboard the ship. At the same time, the continuous DSDP cores provided a solid foundation for setting up the nannofossil biozones. It took decades to establish comprehensive chronological schemes (e.g. Martini 1971; Sissingh 1977; Roth 1978; Okada & Bukry 1980).
The researchers started to use the transmission electron microscopes in the mid-1950s, switching to scanning electron microscopes in the 1960s and 1970s. Optical microscopes with cross-polarization and phase-contrast illumination, techniques introduced in 1952 by Kamptner and Braarud & Nordli respectively, are still used for routine field work.
Terminology
The terminology in the field evolved over time and nannofossils are also sometimes called "nannoplankton" and "coccoliths" as well as some other names, especially in the literature published in 1950s and 1960s. The term "calcareous nannofossil" was chosen in the DSDP publications (although it was rarely used prior to that) and gained popularity afterwards, in the early 1970s. "Calcareous" is derived from , "lime", and means "containing lime".
Siesser & Haq describe the general use as follows: coccolith is restricted by some authors to designate round-shaped elements similar to the ones produced by the living coccolithophores. For differently-shaped objects (e. g., stars and horseshoes), nanolith is being used. Some other authors, however, use coccolith in a broader sense for all calcareous nanofossils; nannoplankton is sometimes used to identify the living organisms, with nannofossils referring to the now-extinct species. Other researchers use the nannoplankton for all forms, both living and extinct arguing that even though the true taxonomy of the extinct ones might never be known, their planktic way of living is subject to little doubt. Siesser & Haq themselves use nannoplankton as a generic way to refer to all organisms, whether living or extinct and nannofossils when describing specifically the fossil forms.