With their glassy skeletons of often perfect geometric form and symmetry, radiolarians are among the most beautiful of all protists. They are also an ancient group, going back all the way to the early Cambrian Period. Their abundance in many rocks, their long geologic history, and their diversity through time make them important sources of information on the geologic age and structure of many deposits.
Radiolaria can range anywhere from 30 microns to 2 mm in diameter. Their skeletons tend to have arm-like extensions that resemble spikes, which are used both to increase surface area for buoyancy and to capture prey. Most radiolarians are planktonic, and get around by coasting along ocean currents. Most are somewhat spherical, but there exist a wide variety of shapes, including cone-like and tetrahedral forms (see the image above). Besides their diversity of form, radiolarians also exhibit a wide variety of behaviors. They can reproduce sexually or asexually; they may be filter feders or predators; and may even participate in symbiotic relations with unicellular algae.
Though their silica skeletons have allowed us to find numerous fossils, scientists still have not been able to successfully develop a complete classification scheme for them. The evolution of the Radiolaria can be easily traced on the broad scale, with major transitions in the global fauna, but a concise taxomony reflecting the evolutionary relationships of major groups is still elusive. Until comparatively recently, radiolarians were primarily studied by micropaleontologists, and only at the end of the 20th century have scientists from other fields begun to study these fascinating protists as well.