A group of marine protists, regarded as a subclass of Actinopodea in older classifications, but not recognized as a natural group in some modern systems owing to its heterogeneity. Radiolarians occur almost exclusively in the open ocean as part of the plankton community, and are widely recognized for their ornate siliceous skeletons produced by most of the groups (illus. a–c). Their skeletons occur abundantly in ocean sediments and are used in analyzing the layers of the sedimentary record (biostratigraphy). Although most radiolarians occur only in open ocean locations, a unique skeleton-bearing nasselarian radiolarian (Lophophaena rioplatensis) has been identified in a coastal location in the Rio de la Plata estuary in South America.