Calcareous phytoplankton fossil. Coloured scanning electron micrograph (SEM) of a fossil of the skeleton (coccosphere) of a coccolithophore, a small marine algal organism. The coccosphere consists of plates (coccoliths) of calcium carbonate. These skeletons can make up the major component of a limestone rock. This fossil was found in chalk that was laid down in the Cretaceous period, between 142 and 65 million years ago. Magnification: x5500 when printed at 10 centimetres wide.