Radiolaria are found as zooplankton throughout the ocean, and their skeletal remains cover large portions of the ocean floor as radiolarian ooze. Radiolaria have existed since the beginning of the Palaeozoic era. They take their name from the radial symmetry of most species. They are valuable markers or diagnostic fossils, because they have a rapid turn-over of species. This helps to identify strata, and to correlate strata at different sites.