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
(SiO2), 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.

8. • 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.

9. • 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.
•

10. • 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.

11. • 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.

12. • 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.

13. 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.

14. 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.