Foraminifera
Foraminifera are unicellular organisms belonging to rhizopod protozoa.
The body (protoplasm) is emitted in the form of pseudopodia, this pseudopodia is used for catching prey, locomotion and in the creation of the hard skeleton (test).
Life cycle:
-There are two forms of foraminiferal species:
* Form A (
Megalospheric form): small in size but has largeinitial chamber.
* Form B (
Microspheric form): large in size but has smallinitial chamber.
The life cycle starts with the microspheric form (B) whose protoplasm is divided into a large number of small masses each with one nucleus giving rise to megalospheric form (A) [Asexual reproduction].
The megalospheric form (A) emits male and female gametes which unite together to form zygote that grow into microspheric form (B) [Sexual reproduction].
This alternation of sexual and asexual generations is called “Alternation of generations”
|
Microspheric form (B) |
Megalospheric form (A) |
|
1-small initial chamber 2-large test 3-large number of chambers 4-less common in nature 5- reproduce asexually |
1-large initial chamber 2-small test 3-few number of chambers 4-more common in nature 5- reproduce sexually |
Feeding:
foraminifera feed on particles and on microscopic organisms (diatoms).
Ecology:
- Most foraminifera are benthonic; some move freely over the seabed, others are sessile and fix themselves to rocks or shells by pseudopodia.
- They are mostly marine and stenohaline (can tolerate only very small variations in the salinity of water).
-
Benthonic foraminifera occur at all depths but those with a porcellaneous test live in shallow water whereas those with a hyaline test occur everywhere but in the deepest areas. Species with agglutinated tests are similar but they can live at depths below 4000-5000m.-
Planktonic foraminifera on the other hand reach about 40 species, they live at depth between 6-30m. Below 200 they are absent.
Foraminiferal Test
The test of foraminifera is either one chamber (monothalamous) or more than one chamber (polythalamous).
Shape of monothalamous test:
Globular cylindrical flask-shape
Stellate coiled undivided tube
Arrangement of chambers: (polythalamous test):
either straight or curved.
2-
Biserial: two rows of alternating chambers.
3-
Triserial: three rows of alternating chambers.
4-
Planispiral: addition of chambers along one plane, this may be:
a-
Evolute: all whorls are obvious.
b-
Involute: the last whorl covers all earlier whorls.
5-
Trochospiral: arrangement of chambers in more than one plane.
6-
Chevron-shape: the chambers form an inverted v-shape.
7-
Annular: semi-circular chambers overlapping each other.
Aperture:
- It is an opening in foraminiferal test, through which pseudopodia protrude and reproductive bodies escape.
Shape:
Circular elliptical fissurine radial
Phyaline virguline dendritic
(With lip &neck) (Comma-shape)
Wall
The wall of foraminifera is very important since it may indicate the environment of deposition. There are different types of wall:
chance of preservation.
2- Calcareous: composed of (CaCO3) secreted by the protoplasm.
This may be:
* Porcellaneous: like porcelain.
* Hyaline: like glass.
3- Agglutinated: composed of materials not secreted by the protoplasm
but collected from the environment. This material may
be sand grins (arenaceous), mica flask or sponge
spicules.
Geologic significance:
Foraminifera are very important in oil exploration, for age determination and biostratigraphic zonation of oil-bearing horizons. Besides; there are some kinds of foraminifera which can be seen by naked eye, these are known as large foraminifera. Although they can be seen by naked eye, yet their study requires thin sections for microscopic inspection. Most of these large forams are good index fossils and so useful for age determination as well as biostratigraphic zonation.
Ex. Nummulites Eocene
Fussulina Carboniferous
Orbitulina
Cretaceous
