Coccolithophorida

(botany) A group of unicellular, biflagellate, golden-brown algae characterized by a covering of coccoliths.

A group of unicellular, biflagellate, golden-brown algae characterized by a covering of extremely small interlocking calcite plates called coccoliths. The Coccolithophorida are mainly classified by the shape of their coccoliths into two groups: the holococcoliths, with simple rhombic or hexagonal crystals arranged like a mosaic, and the heterococcoliths, with complex crystals arranged into boat, trumpet, basket, or collar-button shapes (see illustration). The Coccolithophorida are usually considered plants but possess also some animal characteristics. Botanists assign them to the class Haptophyceae of the phylum Chrysophyta, and zoologists to the class Phytamastigophorea of the phylum Protozoa. See also Phytamastigophorea; Protozoa.

coccosphere; (c) a heterococcolith. (A. McIntyre, Lamont-Doherty Geological Observatory of Columbia University)">
Coccolithophorida: (a) Discoaster; (b) a coccosphere; (c) a heterococcolith. (A. McIntyre, Lamont-Doherty Geological Observatory of Columbia University)

Chiefly photosynthetic, although epiphytism, phago-trophism, and saprophytism have also been shown, the Coccolithophorida form a significant percentage of the nanno-plankton of the tropic through subarctic-subantarctic waters of all oceans; a few brackish and fresh-water species also exist. Together with the diatoms they constitute the primary producers of the open ocean food chain. See also Phytoplankton.

Coccoliths preserve well and have a fossil record dating back into the Jurassic, 180,000,000 years ago. Their long and involved evolutionary record makes them useful to geologists for dating ancient sediments. Because they live in surface waters, coccoliths are under direct climatic control. Thus many modern species with relatively long fossil records are excellent temperature indicators. See also Micropaleontology.