Radiolaria are planktonic marine protozoa showing unicellular organization and heterotrophic mode of nutrition. They build tests varying in shape from simple scattered spicules to highly ornated geometric-shaped shells (Adl et al., 2005). Sizes usually range from hundredths to tenths of millimeters. The tests are made from opal (hydrated silica), some from silica and organic material, and some from strontium sulphate. The name “Radiolaria” derives from the marked radial skeletal spines that characterize many species. Radiolarians are known from the very beginning of the Paleozoic (early Cambrian of the Yangtze Platform; Braun et al., 2007). They are important index fossils and may significantly contribute to marine silica-rich sediments. Radiolarian cherts (radiolarites), a variety of chert (see entry Cherts) composed of radiolarian remains, indicate deep water deposition at depths below which siliceous sediments are stable, but carbonates are dissolved. See entry “ Protozoa...
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