DISTIBUTION IN TIME & SPACE -pg 2.
Radiolarians are present in the fossil record from as early as the Lower Cambrian, but the earliest well-preserved specimens are from the Lower Ordivician limestones of Spitsbergen. By this period, the three basic skeletal types were established, the spicule (spiny), sphere, and cone, indicating that environments in which they lived had already been developed for each type. This shows a lack of knowledge about the first radiolarians 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 additonal factors in the reorganization.