Micropalaeontology

 Introduction:-

Paleontology is a branch of geology which deals with the study of the remains of plants or animals (fossils), preserved in the sediments which are the evidence of life during the geological past. Palaeobotany studies plant fossils such as stems, leaves, roots, flowers, fruits and seeds while the palaeozoology deals with the ancient fossil fauna of vertebrates or invertebrates. 

One sixth of the earth's surface is covered by a thin blanket of soft ooze which is white to buff-colored. It contains millions of tiny shells resembling miniature shuttlecocks, water wheels, footballs, garden sieves or space ships. 

                             Fig:- Different types of microfossils

Definition : The study of the microfossils is called micropalaeontology. It is a special branch of palaeontology which studies microscopic plant or animal fossils under microscopes.

 

Size of microfossils : These fossils range in size from less than 1 µm in diameter to about 100 µm. Some of the spores may have a size range of 200-400 µm. 

Composition of microfossils: These minute organisms have varied chemistry. Foraminifera, ostracods and calpionellids are calcareous. The diatoms and Radiolaria are composed of silica while conodonts are phosphatic. 

Branches of micropalaeontology :-

Micropaleontological studies are meant for collection, separation, preservation and identification of microscopic plant and animal remains. For the sake of simplification, it is studied under three disciplines namely, micropalaeontology, nannopalaeontology and palynology. 

Micropalaeontology includes study of mineral walled foraminifera,ostracods, calpionellids, diatoms, Radiolaria and conodonts. 

 

Nannopalaeontology covers the study of Calcareous nannofossils, which are the smallest of the microfossil groups. This group includes coccoliths and nannoliths and also calpionellids.

palynology studies organic-walled microfossils such as spores, pollens and acritarch. They are collectively termed as palynomorphs include dinoflagellate cysts (dinocysts), acritarchs, marine / fresh water algae and chitinozoa. In addition, this branch also covers other fossil remains such as foraminiferal test-linings, arthropod fragments, fungal material and organic debris containing amorphous organic matter. 

Types of microfossils : -

There are several types of microfossils possessing characteristic features which help in geological interpretation. These include : 

Foraminifers : Unicellular organisms having a pore bearing shell which is less than 1 µm in diameter to about 100 µm.

 

Calcareous nannoplanktons / coccoliths : These include Very small, flat, button-shaped or elliptical disc shaped calcareous organisms, known an calcareous nanoplanktons . 

They require very high magnifying microscope or scanning electron microscope (SEM) as their size range is 0.25 to 200 µm. They are common in Late Jurassic limestones and marls; and range from Jurassic to the Present. Today, they are moderately abundant and their remains contribute to deposits of calcareous ooze on the ocean floors.

Tasmanitids : These are microfossils with a size range of 100-600 µm and consisting of spheroidal, disc-like or ovoidal unicellular organisms. These range in colours from reddish brown, black to those with yellow tinge.

Radiolarians : These are marine zooplanktons having an endoskeleton comprising of silica. Their endoskeletons are ornamented with spines and ridges. 

                                                                                                                                                  Fig  – Radiolarian shell    

Ostracods : These are bivalve crustaceans, showing some external similarities with lamellibranches or pelecypods.

                                                             Fig.  Different types of Ostracod shells

 

          Their carapaces are bean-shaped, oval or kidney shaped. 

Conodonts : These are tooth-like microfossils found in marine sediments. They range in size from 0.1 to nearly 5 mm in length and have a greyish to translucent brown colour. 

Diatoms : These are unicellular algae characterized with a golden brownish pigment. They range in size from 20 to 200 µm and are confined to both, fresh water and marine environments. Most commonly they occur as circular, elongated or hemispherical forms. They are found in all latitudes in oceans as well as in fresh-water. They play an important role in the composition of recent sediments. Fossil diatoms are common in Tertiary marine as well as fresh water deposits particularly in siliceous shales. Diatomaceous earth is used in industry as a filtering medium.

Pollens and spores : Fossilised pollens and spores are characteristic of both, marine and non-marine sediments. Pollens range in size from 5-200 pm while spores range in. size from 200-400 pm. Well-preserved pollens and spores are useful in coal and petroleum exploration. Their study is, therefore, treated as a special branch of palaeontology, called as palynology.  

                          

Sponge spicules : These are marine, siliceous or calcareous skeletal elements found scattered in mesogloea of sponges. After the decomposition, these spicules get scattered and are preserved in marine sediments. Their shapes include straight, b t needles or rods. Sometimes, they also occur as triradiate forms 

 Cynobacteria

Cynobacteria are rounded to cylindrical cells upto about 25 microns in diameter and arranged in series to form single or clustered filaments. Cynobacteria are among the most ancient known organisms dating back some 2800 to 3500 million of years. They are involved in the formation of Stromatolites, which are of particular geological interest since they also occur in sedimentary rocks of early Pre-Cambrian times.

Uses of microfossils 

Despite their minute, microscopic size, the microfossils are found to be extremely useful for the following 

• Stratigraphic correlation : Microfossils are very sensitive to temperature and other climatic changes. Hence, microfossils are important in correlating distant strata based on fossil similarity. Microfossils, especially pollens and spores of foraminifers, are used for biostratigraphy correlation of surface and subsurface sediments. 

• Geological age of rocks : These microfossils also serve as marker horizons of certain geological time limit and therefore, geological age determination of rocks is made possible. 

• Geological exploration : Pollen and spores, along with foraminifers, are useful in identifying hydrocarbon reserves and the source rocks in petroleum exploration. 

• Palaeo-environment : Some species of microfossils serve as basic evidences for organic evolution because of their worldwide geographical distribution and limited geological time range. They also help in understanding conjectures about ancient and present life as they occupy a place at or near the base of the food web.  Microfossils such as ostracods, foraminifers or coccoliths are used for the determination of palaeo-environment with regard to their habitat. These also suggest about the fresh, marine or terrestrial water conditions. Depth of shallow to deep aquatic environment, salinity, temperature or geographical location (e.g., sub-glacial, temperate, subtropical or tropical) are also inferred by study of the microfossils.

• Ecological studies : Micropalaeontological studies also provide guidelines in understanding the development and stability of the present global ecosystem.
• Organic evolution : The importance of microfossils is further augmented by recent discoveries in Precambrian rocks which have documented the organic evolution through more than three-quarters Of the history of life on the earth. 
•  Physico-chemical studies : The study of microfossils also reveals the degree of light penetration, current direction, turbulence, turbidity, type of substrate and the concentration of dissolved gases in water. 
•  Mineral studies : Some ore bodies owe their formation to microbial leaching process. Hence, microfossils will gain Importance in understanding palaeo-geology of these deposits in