- Micropaleontology is concerned with microfossils and nanofossils (the latter being smaller than 50 μm), the study of which must, of necessity, be carried out using the light or electron microscope. Such microfossils are:-
1- The remains of unicellular and multicellular micro-organisms.
2- The dissociated elements and skeletal fragments of macro-organisms.
- They consist of these types: Foraminifera, Ostracods, Coccolithophora, Diatoms, Radiolaria, and Dinoflagellates.
- Among this, the Foraminifera, ("Hole Bearers") or forams for short, are single-celled protists with shells that can have either one or multiple chambers, some becoming quite elaborate in structure.
- Depending on the species, the shell may be made of organic compounds, sand grains and other particles cemented together, or crystalline calcite.
- Foraminifera are found in all marine environments, they may be planktic or benthic in mode of life.
- Because of their diversity, abundance, and complex morphology, fossil foraminiferal assemblages are useful for biostratigraphy, and can accurately give relative dates to rocks, in petroleum exploration, paleoclimatology, etc.
- The name Foraminiferida is derived from the foramen, the connecting hole through the wall (septa) between each chamber.
- The study of Foraminifera has a long history, their first recorded "mention" is in Herodotus (fifth century BC) who noted that the limestone of the Egyptian pyramids contained the large benthic foraminifera Nummulites.
- In 1835 Dujardin recognized foraminifera as protozoa and shortly afterward D'Orbigny produced the first classification.
- The famous 1872 HMS Challenger cruise, the first scientific oceanographic research expedition to sample the ocean floor collected so many samples that several scientists, including foraminiferologists such as H.B. Brady, were still working on the material well into the 1880s.
- Work on Foraminifera continued throughout the 20th century, workers such as Cushman in the U.S.A and Subbotina in the Soviet Union developed the use of foraminifera as biostratigraphic tools.
- Later in the 20th century, Loeblich and Tappan and Bolli carried out much pioneering work.
- Foraminifera are single-celled animals protected by hard shells of different types of materials (chitinous, calcareous, agglutinated, and siliceous). They are microscopic in size, and generally range from 0.1 to 1 mm. (approximately the size of a grain of sand or smaller) and some are more than that.
Animal consists of:
- Cell (Soft parts of foraminifera) has:-
Protoplasm (surrounded by membrane)
Protoplasm within the shell (test) = endoplasm (dark and granulous) contains:
- 1 or more nuclei
- systems for cell-secretion (Golgi)
- systems for gas-exchange (Mitochondria)
- systems for protein-synthesis (Ribosomes)
- fluid or gas-filled ‘ droplets’ (Vacuoles)
Protoplasm outside the test = ectoplasm (transparent) forms pseudopodia
Long strings of ectoplasm (2-3 to 20 times test diameter) Can branch very often, shaping web-like appearance around the test. Surrounded by a rather fluid layer.
Diagram showing live benthic foraminiferal style (Brasier, 1980)
WALL STRUCTURE AND COMPOSITION:-
- The test of Foraminifera are composed of two different kinds of material:-
a) one kind is secreted by the organism
b) the other consist of foreign particles gathered from bottom sediments and cemented together to form agglutinated test
- Some of them are as follows:-
Chitinous wall:- Composed of chitin, thin and transparent, present in freshwater benthic foraminifera. e.g. Chitiodendron fronconianom.
Agglutinated wall:- Only feature for benthic----foraminifer builds its test wall by cementing together exogenous grains (e.g. sand grains, oolites, fine grains of calcite, or sponge spicule) by carbonate mineralization. Wall is a simple layer that grades from fine grains inside to coarse grains outside. e.g. Rhabdammina.
Calcareous (Ca CO3) wall: Present for both benthic and planktonic foraminifera: (hyaline or porcelaneous). e.g. Globugerina.
Hyaline/glassy wall:- Transparent, perforate, crystals are radial, with lamellae (laminations) that separated by organic layers. e.g. Rotalina.
Porcelaneous wall:- Shiny, smooth, crystals randomly arranged. e.g. Miliolina.
Fig:-Examples of wall structures in foraminifera (Brasier, 1980)
CHAMBERS DEVELOPMENT, ARCHITECTURE AND SHAPE:-
- The test of Foraminifera may consist of a single chamber called unilocular (monothalamus) which chamber growth proceeds gradually along with protoplasmic growth. e.g. Lagena or of two or more chambers called multilocular (polithelamous) e.g. Nodosaria.
- In multilocular forms protoplasmic growth is gradual but test growth is periodic and the addition of new and larger chambers will take place in different ways to produce different growth planes.
- Planispiral - When chambers arranged spirally around an axis of coiling and spiral lies in a single plane. e.g. Nodosaria.
- Trocospiral - When the spiral does not lie in one plane, but progress up the axis of coiling the chamber arrangement becomes helicoids. e.g. Ammonia.
- Uniserial - Arranged in a single raw or arcuate if curve and rectilinear if a straight series. e.g. Dentaline
- Biserial - Chambers are arranged in two rows. e.g. Textularia.
- Non-laminar - When there is no overlap of previous chamber walls by the new wall. e.g. Fusulinina.
- Multilaminar – If overlap occurs, a thin section will reveal the layers of successive walls. e.g. Rotalina.
- Whorl or Coil – When a series of chambers is arranged spirally or coiled about an axis.
- Involute – The majority of the previous coil is hidden and shows the ventral side. e.g. Cilicidis.
- Evolute(convolute) – The majority of the previous coil is visible, show the dorsal side.
- Umbo – When central depression (umbilicus) is filled with secondary material.
- Proloculus – The initial chamber of foraminifera.
- Dimorphism – A significant feature which exhibits two distinct morphological character i.e. megalosphiric and microsphiric. e.g. Nummulites.
- Dextral and Sinistral – Clockwise and anticlockwise coiling.
- Suture – The joint of each whorl with the other.
- Aperture – Opening found in the wall of the final chamber.
PALEOECOLOGY AND ITS SIGNIFICANCE:-
- It has been estimated that more than 80,000 species of Foraminifera are described in the literature.
- Foraminifera are adapted to all aquatic habitats, most in marine waters, but some can exist in salt or brackish water and the member of one family (Allogromidae) live in freshwater.
- Most are typically slow-moving bottom-dwellers and some are pelagic.
- Foraminifera are usually extremely sensitive to an environmental condition such as temperature, salinity, bathymetry, etc. and therefore their study is significant to understand the palaeoecological condition of the time of their living.
- The benthonic forms are indicative of the temperature, salinity, depth, and bottom conditions while the temperature, bathymetry, and paleocurrent patterns are indicated by planktonic forms.
- Foraminifera are thought to have been present in the Precambrian seas, but as yet no undepted fossils of this order have been described from rocks older than the Cambrian.
- The Foraminifera found in the sediments are of all the ages ranging from Cambrian to Recent.
- They made their first appearance in Cambrian and comparatively rare till Carboniferous, but they became prominent and of great geological importance during upper Carboniferous and Permian.
- Again during Triassic, they become less abundant, but during Jurassic and Cretaceous they are represented by large numbers, they reached the maximum developed during Tertiary and Recent.
USES OF FORAMINIFERA:-
- Because of their diversity, abundance, and complex morphology, fossil Foraminiferal assemblages are useful for biostratigraphy, and can accurately give relative dates to rocks. The oil industry relies heavily on microfossils such as forams to find potential oil deposits.
- Calcareous fossil Foraminifera are formed from elements found in the ancient seas they lived in. Thus they are very useful in Palaeoclimatology and Palaeoceanography.
- They can be used to reconstruct past climate by examining the stable isotope ratios of oxygen, and the history of the carbon cycle and oceanic productivity by examining the stable isotope ratios of carbon.
- Foraminifera can also be utilized in Archaeology in the provenancing of some stone raw material types. Some stone types, such as limestone, are commonly found to contain fossilized Foraminifera.
- Foraminifera are abundant in most marine environments so only small sediment samples (a few grams) are needed to obtain statistically significant numbers of microfossils to perform environmental analysis.
- They are sensitive to environmental change and their mineralized shells normally get preserved in the sediment after the death of the organism.
- The quantitative importance of Foraminifera is large and they may constitute a major portion of the Biomass in many marine ecosystems. They must, therefore, be considered as one of the most significant organism groups living today.
- Some recent studies indicate that Foraminifera are not only a group of great ecological importance as food organisms for fish and invertebrates, but also play an important role in the turnover of nutrients and energy in the sea.
- They are extremely useful in Applied Geology, such as in the history of Climate, Stratigraphy, and in Oil Prospecting.
- Foraminiferal analyses of dated sediment cores represent a quick and cost-efficient way to evaluate possible environmental differences between present and pre-impact (reference) conditions without previous knowledge of the area being examined.