Radiolarian

Radiolarians are unicellular eukaryotes about 0.1 to 0.2 millimeters across that produce intricate mineral skeletons, usually made of silica, with a central capsule dividing the cell into inner and outer portions. They are found as zooplankton throughout the global ocean and are primarily heterotrophic, though many carry photosynthetic endosymbionts and are considered mixotrophs. The skeletal remains of some radiolarians make up a large part of the ocean floor as siliceous ooze, and because they change rapidly as species and have intricate skeletons, radiolarians are an important diagnostic fossil found from the Cambrian period onward.