planktonic foraminifera

Marine zooplankton that passively float or weakly swim, and have calcium carbonate skeletons that are present in large numbers on the surface of the ocean. The skeletons of planktonic foraminifers are preserved in large numbers in deep-sea sediments, providing a rich fossil record of the environmental conditions of the upper ocean. The size of the shells is typically from 50 to 100 microns.