What are Radiolarians? Radiolarians are unicellular protists that build external, glassy skeletons (tests) of silica. Their intricate tests come in various shapes and often have delicate spikes and projections that help increase buoyancy. Most radiolarians live as single cells, but some form long colonies up to 1 meter long. Radiolaria are members of the marine plankton, floating around the ocean at the whim of currents.
First known fossil occurrence: Cambrian.
Last known fossil occurrence: Quaternary. This group has living relatives.