Coccolithaceae
The coccolithophore Gephyrocapsa oceanica.
Scientific classification
Domain:	Eukaryota
Kingdom:	Chromalveolata
Phylum:	Haptophyta
Class:	Prymnesiophyceae

Emiliana huxleyi, the most commom coccolith in the seas today

Types of cocoliths

Coccoliths are the microscopic algae which form chalk. They are made by one of the most important eukaryote phytoplankton. These are single celled protists whose full name is coccolithophorids or coccolithophores. The spherical skeleton of coccoliths is a coccosphere.

Chalk makes up much of the marine strata in the Upper Cretaceous period, and is 95% to 99% made up of coccoliths, the calcium carbonate (CaCO₃) plates which coccolithophorids produce.

Coccoliths were first examined by Cristian Gottfried Ehrenberg (1795–1875), who thought they were inorganic products. They were named, and identified as made by living organisms by Thomas Henry Huxley. He realised they were a major part of the rock we call chalk. The story of how the tiny cell produces its coccosphere is told by Westbroek:

"The tiny coccolith-generating alga... is geological dynamite, a formidable force helping to conduct vast fluxes of calcium and carbonate towards the ocean bottom".