There are three distinct transitions in the evolution of Radiolaria, corresponding to three transitions in the geologic time scale, the Permo-Triassic, Cretaceous-Tertiary, and Paleogene-Neogene. Each phase includes extinction of families at the transition between periods, and a subsequent increase in the number of families as the new period progressed.
Radiolaria are present in the fossil record from as early as the Lower Cambrian, but the earliest well-preserved specimens are from the Lower Ordovician limestones of Spitsbergen (Fortey and Holdworth, 1971). By this period, the three basic skeletal types of the Radiolaria were established, the spicule (spiny), sphere, and cone, indicating that niches had already been developed for each type, and emphasizes the lack of knowledge about the first Radiolara due to an incomplete early fossil record. The early Radiolaria are suspected to have lived in shallow water because they were found in the same layer of limestone as shallow fauna. True deep-water Radiolaria appear in the Silurian, possibly as a result of the initiation of the mid-Paleozoic cold-water sphere, which might have encouraged the evolution of deep, cold-water radiolarians. The initiation of a cold-water sphere in the late Paleozoic is a possible factor in the major Radiolaria reorganization during the late Paleozoic-Mesozoic transition. The reduction in the number of water masses due to the movement of the continents and the formation of glaciers may be addiitonal factors in the reorganization.
A huge proliferation of Radiolaria occurred in the Mesozoic, with more than ten major speciation events. About half of the groups existing today are descendants of Radiolaria that evolved during Mesozoic. The first indisputable Nassalleria appear in the Triassic (Steineck and Casey, 1990). Common polycystine Radiolaria found in the Mesozoic include the Amphipyndacids, Dictyomitrids, and Hagiastrids, all in the order Spumellaria, distinguished by having a spherical skeleton.
The third era of the Radiolaria fossil record begins at the Cretaceous-Tertiary transition. The major reorganization of Radiolaria involved some extinction in the late Mesozoic, but not the drastic reduction characteristic of most other planktonic fossils. The transition is characterized instead by major radiations of the Cenozoic groups. This rediversification in Radiolaria was probably due in part to environmental pressures from the rediversification of Foraminifera and other plankton groups after the Cretaceous extinctions.
The sharing of niches within the newly formed groups of the Cenozoic, and the rise of dominant diatoms during this period probably had a major impact on radiolarian diveristy. Competition between diatoms and radiolarians for dissolved silica is suspected to be the cause of an observed decrease in the average weight of radiolarian skeletons during the Cenozoic. Only warm-water sphere Radiolaria showed this loss in weight; cold-water sphere groups did not, lending support to the idea that only those groups, in ecological contact with diatoms were affected.
The Paleogene-Neogene transition is the last major reorganization of the Radiolaria. Several groups evolved and/or diversified at this transition, including the Tholonids, Antarctisins, Collosphaerids, Artiscins, Spongasterins, and Pyloni. The new intermediate and circumpolar water masses formed in the Neogene are thought to have been a factor in the evolution of the Tholonoids and the Antarctisins, since both are warm-water sphere forms. The development of nutrient-poor subtropical water masses provided a niche for the Collosphaerids, Artiscins, Spongasterins, and Pylonids, which are all warm-water sphere forms.
