Diatoms

Photosynthetic, unicellular algae inhabiting aquatic and subaquatic environments. Most are phytoplankton, although some are sessile benthic dwellers. They secrete an external wall (frustule) composed of silica. Of biostratigraphic importance in the Cenozoic, although some species found in Cretaceous rocks. Diatom assemblages are also indicative of distinctive paleoclimatic realms.