Radiolaria are a subclass of single celled organisms which are distinguished by their intricate mineral exoskeletons, usually made of silica and often with many spines extending outward. The outer shells are generally spherical, symmetrical and sometimes several millimeters wide. Radiolarian skeletons are extremely diverse, and they are the key feature on the basis of which the subclasses are classified into smaller orders, families, genera and species. Most species vary slightly from one subclass to another, and the skeletons of radiolarians are generally organized around spines, which are sharp, dense outcroppings from the main skeletal mass.
The outermost skeleton is formed from the fusion of many spines to create the cortical shell. Connecting this shell to the many concentrically organized inner shells are bars or beams, which also serve to strengthen and support the entire structure. Within and extending into the many chambers created by this complex structure is the single cell of the organism. The siliceous bars or rods merge with each other leaving empty spaces between them. The perforated holes allow the pseudopods to extend for capturing food. When radiolarians die, their shells sink, forming the radiolarian ooze of deep ocean floors that has generated many formations of sedimentary rock over the course of geological time.
Because of their rapid evolution, their skeletons are important diagnostic fossils found from the Cambrian period over 500 million years ago. Since radiolarians are well preserved in sediment after death, they have been instrumental in their application to paleontological studies. Radiolarian fossils in sediments allow a variety of studies, including determinations of the age of the sediments that contain them, analyses of the spatial relationships between sedimentary layers (of particular importance for the oil industry), and studies of the geological evolution of the continental land masses and ocean basins. Furthermore, studying the changes in the morphology of radiolarian skeletons through time yields first hand information on the evolution of the group, and on the rates and modes of the species formation and extinction. This information can yield important facts pertaining to climatology during a certain time period including the present.
A radiolarite is a sedimentary rock with fine grains presenting an alternation of dark and light layers. It is primarily made up of siliceous hulls of radiolairia known as Actinopode in warm seas. The red color is due to the presence of iron. Sometimes the iron is in ferrous form, which gives the rock a green color.