A coccolithophore (or coccolithophorid, from the adjective ) is a unicellular, eukaryotic phytoplankton (alga). They belong either to the kingdom Protista, according to Robert Whittaker's Five kingdom classification, or Chromalveolata, according to the newer Thomas Cavalier-Smith Biological Classification system. Within the Chromalveolata, the coccolithophorids are in the phylum or division Haptophyta, class Prymnesiophyceae (or Coccolithophyceae). Coccolithophorids are distinguished by special calcium carbonate plates (or scales) of uncertain function called coccoliths, which are also important microfossils. However, there are Prymnesiophyceae species lacking coccoliths (e.g. in genus Prymnesium), so not every member of Prymnesiophyceae is coccolithophorid. Coccolithophores are almost exc