Calcareous nannoplankton represent a major component of oceanic phytoplankton, ranging in size from 0.25 to 30 μm. The first records are from the Late Triassic. Their calcareous skeletons can be found in fine-grained pelagic sediments in high concentrations and the biomineralization of coccoliths is a globally significant rock-forming process. This heterogeneous group includes coccoliths, discoasters and nannoconids.

Coccolithophores are unicellular marine golden-brown algae differing from other Chrysophyta in having two flagella and a third flagella-like appendage called a haptonema. They also posses calcified scales, called coccoliths, at some stage in their life as a protective armour that eventually falls to the ocean floor to build deep-sea ooze and fossil chalks.

The discoasters are an extinct group of stellate calcareous nannofossils and the nannoconids are cone-shaped microfossils very useful in Cretaceous biostratigraphy in the absence of other groups.

Typically coccolithophores are autotrophic but they can be heterotrophs under certain environmental conditions. They are restricted to the photic zone of the water column (0–200 m depth). The algal cell is generally spherical and includes two golden-brown pigment, a nucleus, two flagella of equal length and a haptonema, mitochondria, vacuoles and the Golgi body which is the site of coccolith secretion in many species.

In some living genera there is also an alternation between a motile and a non-motile stage. The first one has a flexible skeleton with coccoliths embedded in a pliable cell membrane and in the non-motile stage, the calcification of the membrane forms a rigid shell called a coccosphere.

Coccoliths are composed of calcium carbonate in the form of calcite with a low amount of magnesium, although it has been some of vaterite or aragonite. It is thought they are formed for protection from intense sunlight, to concentrate light, buoyancy control, or for the biochemical efficiency of the cell.

The coccolith morphology is the basis for classification of both living and fossil members of the group. They can be divided in two basic morphological types: heterococcoliths and holococcoliths. While the holococcoliths are usually formed by rhombohedral calcite and always disintegrate after they are shed, the heterococcoliths provide the bulk of the microfossil record. They are built of different submicroscopic elements such as plates, rods and grains imbricated into a relatively rigid structure.

In some cases, some living coccolithophores, like Scyphosphaera produce two layers of morphologically distinct coccoliths (dithecism).

Ehrenberg in 1836, was the first to use the term “coccoliths” while he was studying the chalk from the island of Rugen in the Baltic Sea, but he thought they had an inorganic origin. G. C. Wallich in 1860, was the first to suggest the organic origin of coccoliths. Later, in 1872, the HMS Challenger expedition recovered coccospheres from the upper water layers and correctly concluded that they were the skeletons of calcareous algae.

Coccolithophores has a great radiation in the Early Jurassic, an event that parallels the radiation of the peridinialean dinoflagellate cysts and it’s related to the opening of the Atlantic Ocean. During the Late Cretaceous, there was a second radiation that led to the deposition of chalk in several areas of continental plataform, but were very affected by the extinction event at the end of the Cretaceous. Since then, Coccolithophores have regained their dominance in tropical and temperate waters but are significantly less diverse than in the Mesozoic.

Coccoliths and discoasters has an extraordinary value as biostratigraphic markers for the Mesozoic and Cenozoic, and are good indicators of surface water chemistry and reflect surface productivity.

References:

Armstrong, H. A., Brasier, M. D., 2005. Microfossils (2nd Ed). Blackwell, Oxford.

Jörg Mutterlose, André Bornemann, Jens O. Herrle, Mesozoic calcareous nannofossils — state of the art, Paläontologische Zeitschrift, March 2005, Volume 79, Issue 1, pp 113-133.