A group of unicellular, marine, planktonic algae which are, at least at some stage in their life cycle, covered in calcareous plates (coccoliths) embedded in a gelatinous sheath. They are spherical or oval, and less than 20 μ‎m in diameter. They range in age from the Upper Triassic to Holocene, although dubious examples have been described from the Upper Precambrian and Palaeozoic. Because of their abundance, with large seasonal blooms, and the preservation of coccoliths in sediments, coccolithophorids are believed to play a major role in the global carbon cycle.

Coccolithophorid