Coccolithophorids are unicellular planktonic algae: each living cell (coccosphere) is surrounded by a test made of usually oval calcite (or aragonitic) plates (coccoliths), and measures between 5 and 35 µm. On the death of the organism, the membrane holding the coccoliths plates breaks up and releases the individual coccoliths which then fall to the seafloor to make a major contribution to the calcareous oozes. The stratigraphical range of coccolithoporids is long, from the Triassic to Recent. Detailed biostratigraphic scales based on the appearance, evolution, and disappearance of the different species were devised by the micropaleontologists and are helpful to date marine sediments through microscopic examination of smear slides.

Changes in composition of specific assemblages contribute to a better understanding of ancient oceanic environments: temperature, fertility, and currents…
The picture (obtained with a Stereo Electronic Microscope) features a 10µ large coccosphere of a common living form Emiliana huxleyi.