R adiolaria

Abstract Radiolaria are marine unicellular zooplankton characterised by the presence of two types of pseudopodia called axopodia (pseudopodia having axial portion) and reticulopodia (pseudopodia make reticulated network). Class Phaeodarea, traditionally included in Radiolaria, were revised their taxonomic position (out of Radiolaria but in Cercozoa) on the basis of molecular studies. Also, a molecular perspective shows us that Radiolaria have five orders (Spumellaria, Nassellaria, Collodaria, Acantharia and Taxopodida) in two classes (former three in Polycystina, latter two in Spasmaria). Most of Radiolaria (excepting from some species of Collodaria) possess siliceous or celestite shells or spines, and both the fine structure and architectural diversity of the skeletons are used as taxonomic criteria. Good preservation of siliceous shells allows Radiolaria to be used as stratigraphic markers in Paleontology. Key Concepts Radiolaria are marine heterotrophic protists, and some harbour symbionts. Most radiolarians, in particular class Polycystina, have good fossil record from the Cambrian era due to possessing siliceous shells. Phylogenetic relationship of Radiolaria was revised through molecular phylogeny. The closest lineage of Radiolaria is the other important microfossil group, Foraminifera. Radiolaria are a member of eukaryotic supergroup Rhizaria, together with Cercozoa, Endomyxa and Foraminifera.