Coccoliths – tiny fossils with immense paleontological importance

Microfossils, such as foraminifera, diatoms, ostracods and conodonts, are usually studied using a magnifying glass or under a stereo-microscope. However, nannofossils, such as coccoliths – with sizes measured in micrometres – are way beyond the resolving power of these optical tools.

Figure1
Biology of a coccolithophorid alga. Abbreviations: cl — coccolith, cp — chloroplast, f — flagellum, Ga — Golgi- apparatus, hn — haptonema, m — mitochondria, n — nucleus, nl — nucleolus, v — vesicle (modified from Bown, 1998).

In general, coccoliths are very regularly shaped, fine calcite platelets that are produced by unicellular, autotrophic (that is, capable of synthesizing their own food from inorganic substances), marine algae – the so-called Coccolithophorida (phylum: Haptophyta; class: Prymnesiophyceae). They are arranged in spheres (coccospheres) that completely enclose the organisms. After the death of the algae, the coccospheres are either preserved entirely or fall to pieces. Then, either way, they settle on the seafloor and can do so continuously over periods of millions of years.

Coccoliths are of immense value to the palaeontologist, because of their highly specific shapes. For this reason, they are frequently used for biostratigraphic investigations, where the ages of lithological strata are estimated by using the fossils found in them.

Palaeontology distinguishes between two types of coccoliths – the holococcoliths and the heterococcoliths. The first type is composed of calcite crystals of identical size, whereas the second consists of variably-sized calcite crystals. Extant Coccolithophorida preferentially produce heterococcoliths during the phase of their life cycle when they are immobile.

In this article, I will provide a brief overview of the biology of the Coccolithophorida, their biostratigraphic significance and the main steps that have to be undertaken for a successful preparation of sediment samples to look at coccoliths.

Figure2
Examples of some recent and fossil coccospheres: a — Arkhangelskiella specillata, b — Gephyrocapsa oceanica, c — Micula concava, d — Watznaueria barnesae, e — Emiliana huxleyi.


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