Morphotaxonmy, classification of Radiolaria

Morphology, classification and geological distribution of radiolaria

1. Radiolaria Radiolaria are holoplanktonic protozoa and form part of the zooplankton, they are non-motile (except when flagella-bearing reproductive swarmers are produced) but contain buoyancy enhancing structures; they may be solitary or colonial. Formally they belong to the Phyllum Protista , Subphylum Sarcodina , Class Actinopoda , Subclass Radiolaria .

2. The sister Subclass Acantharia have skeletons composed of strontium sulphate which is easily dissolved in seawater and are not preserved in the fossil record. Within the Subclass Radiolaria there are two important super-orders. 1)The Tripylea which includes the Phaedaria which have skeletons composed of hollow silica bars joined by organic material, which are not commonly preserved 2) The Polycystina which form skeletons of pure opal and are therefore more resistant to dissolution in seawater and hence more commonly preserved in the fossil record. The Polycystina may be divided into two suborders the Spumellaria and the Nassellaria . They are wholly marine, the most relatively commonly preserved and therefore studied members of the formal Subclass Radiolaria . It must be remembered, however, that seawater is under saturated with respect to silica and the degree of preservation of Radiolaria depends on the robustness of the skeleton, depositional and burial conditions and diagenesis .

3. Crystallized from  opaline silica , this unusual and often strikingly beautiful characteristic of these organisms is their primary morphological characteristic, providing both a basis for their classification and an insight into their ecology. Silica is an important material, found extensively in the Earth's crust as a raw material. It is also vitally important to the protection and survival of radolarians , as well as for preserving the record of their existence in the world's oceans throughout prehistoric and modern times. Silica (SiO 2 ), being a highly ceramic compound with a complex crystal structure, affords a very durable skeleton. It is because of this sturdy crystal structure of silica that the fossil record for Radiolaria is well preserved.

4. Varying slightly from one subclass to another, the skeletons of radiolarians are generally organized around  spicules , or spines, which are sharp, dense outcroppings from the main skeletal mass. Formed from the fusion of many of these spines is the outermost skeleton, 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 assembly. Within and extending into the many chambers created by this complex structure is the single cell of the organism.

5. When viewed on a larger scale, Radiolaria are incredibly diverse in the form their skeletons may take, ranging from spherical to rod-shaped, and radial to bilaterally symmetrical. To this day, the process by which a single cell is able to produce such amazing complexity remains under dispute, and continues to be one of the most active areas of research involving Radiolaria . Proposed methods of production include direct chemical metamorphosis of the cytoplasm, or perhaps scretion of a fluid form of silica from the cell that later  polymerizes  to form the skeleton. Additional research is being focused toward determining the benefits and selective advantage of producing a skeleton at all.

6. Although originally thought to be quite simple internally, Radiolaria are actually some of the most complex extant protists . Indicative of the high degree of specialization they exhibit, their  cytoplasmic mass , which constitutes the majority of the space within the cell, is divided into two regions separated by a perforated membrane. The first of these regions is the central mass, also known as the  central capsule , and the second is the  extracapsulum , a peripheral layer of cytoplasm surrounding the central capsule. The central capsule contains the organelles common to all  eukaryotic cells , such as the mitochondria and vacuoles, while the extracapsulum is characterized by its thread-like extensions of cytoplasm, the  rhizopodia . Aiding in the capture of prey, the rhizopodia are crucial in obtaining the energy necessary for the successful completion of the Radiolarian life cycle. Additionally, the rhizopodia act to increase the surface area of the cell, improving the rates of release of metabolic wastes and the uptake of oxygen. The separation of the cytoplasm is thought to allow for increased control of the diffusion of large molecules within the cell, such as fat globules, and organelles.

7. Range First recorded occurrences of Radiolaria are from the latest Pre-Cambrian, they are generally thought to have been restricted to shallow water habitats. By the Silurian deep water forms are believed to have evolved. All early Radiolaria are spumellarians , the first possible nassellarians appear in the Carboniferous and definite true nassellarians do not appear until the Triassic. During the late Palaeozoic Radiolaria show a gradual decline until the end of the Jurassic when there is a rapid diversification, this coincides with the diversification of the dinoflagellates which may have represented an increased source of food for the Radiolaria . It is thought that the evolution of diatoms in the Cretaceous may have had a significant effect on radiolarian evolution due to competition for silica (diatoms also use silica to build their skeleton); it is commonly accepted that radiolarian skeletons have become finer and less robust from this time.

8. Classification Extant radiolaria are classified using features of both the preservable skeleton and the soft parts, which makes the classificaiton of fossil forms extremely difficult. Most workers in this field today use classification schemes based on Nigrini and Moore's and Nigrini and Lombari's works on modern and Miocene radiolarians. A major problem with radiolarian classification is that separate classifications have been established for the Palaeozoic , Mesozoic and Cenozoic, and little has been done to integrate them. The two suborders, the spumellarians and the nassellarians are subdivided into informal groups which equate to family level.

9. Applications Radiolarian assemblages often contain 200-400 species so they can potentially be very useful biostratigraphic and palaeo environmental tools. They have an unusually long geological range, from latest Pre-Cambrian to Recent. Because Radiolaria have a skeleton composed of silica and have an extremely long geological range they have become useful in the study of sediments which lack calcareous fossils, either because of deposition below the CCD (Carbonate Compensation Depth) or because the strata being examined are too old. Cherts and particularly nodules within chert bands are often good sources for Radiolaria . Ophiolites and accretionary terrains often include chert bands and Radiolaria may be the only palaeontological aid available in these situations and as such have proved invaluable in the study of these geological settings.

10. Preparation Techniques Radiolaria are often found in standard micropalaeontological preparations (i.e. those aimed at recovering foraminifera). However for the best results samples are washed using a weak (10%) concentration of hydroflouric acid. It is also possible to differentially etch Radiolaria from cherts using hydrofluoric acid. This is extremely dangerous and must only be carried out in a fume cupboard with full protective clothing and as such should be left to trained personel only.

11. Observation Techniques Radiolaria are often smaller than foraminifera but may be veiwed using the same techniques as those described for foraminifera, and they can be picked and mounted in the same way. They can also be prepared in strew mounts on glass slides.