Microfossils | Geology Optional for UPSC PDF Download

Introduction to Microfossils and Micropalaeontology

• Palaeontology is the study of fossils, which are the remains of ancient organisms preserved in rocks. Fossils come in two main types: macrofossils, which are large and usually found in rocks from the Phanerozoic era (about 541 million years ago to the present), and microfossils, which are tiny and can be found in sedimentary sequences from the Precambrian era to the present.
• Microfossils are found in large numbers in many sedimentary rocks and belong to various groups of animals, plants, and protists. The study of microfossils is called micropalaeontology, and it is an important part of palaeontology because microfossils are the most abundant and easily accessible fossils. A lot of information about the history of life on Earth is gathered from the study of microfossils. In this unit, we will focus on microfossils, their significance, major groups, and the methods used to collect and study them.

Expected Learning Outcomes

After studying this unit, you should be able to:

• define microfossils and micropaleontology;
• classify the microfossils into mineral- and organic-walled groups;
• outline the importance of microfossils;
• describe the techniques involved in collecting microfossils; and
• explain the methods used for studying them.

Microfossils and Micropaleontology

• Microfossils are the remnants of tiny organisms, both single-celled and multi-celled, typically measuring less than 2 millimeters (mm) in size. These remains consist of small animals and microorganisms that inhabit the surface and deep waters of oceans, as well as the ocean floor and other aquatic environments such as lakes and ponds. After their demise, these organisms are preserved in sedimentary rocks. The term "microfossil" is not limited to a specific kingdom of life; it encompasses the minuscule remains of various groups, including protists, bacteria, fungi, animals, and plants.
• While there isn't a universally accepted definition of microfossils, the term commonly applies to organic remains that require examination with a light or electron microscope. Microfossils are extremely small, with sizes ranging from 0.001 mm (1 micron) to 2 mm. Fossils larger than 2 mm are typically classified as macrofossils.
• Fragmentary microscopic parts such as shell pieces, bones, jaws, and teeth of larger organisms are also considered microfossils. However, many micropaleontologists tend to overlook these fragmentary remains, as they often provide incomplete or limited information. In contrast, the remains of certain microorganisms, such as foraminifers, radiolarians, ostracods, diatoms, dinoflagellates, spores, and pollen, are usually preserved as complete fossils and are of significant interest to micropaleontologists.
• While small size is the primary criterion for classifying microfossils, some foraminifer remains may exceed 2 mm or even reach 15 cm in diameter, yet they are still categorized as microfossils. The classification of microfossils in such cases is based on the method of study, with microscopy being the determining factor.
• Microfossils are found in abundance in various environments, including marine, brackish, and freshwater habitats. Micropaleontology, a branch of paleontology, focuses on the study of microfossils and nanofossils, the latter being organisms ranging from 5 to 60 micrometers (μm) in size. Although the history of micropaleontology spans over two centuries, it emerged as a prominent discipline in the early twentieth century. Initially, micropaleontology primarily focused on fossil foraminifers until 1925. The study of other microfossil groups, such as radiolarians, ostracods, diatoms, dinoflagellates, pollen, and spores, gained traction after 1925. Following 1945, micropaleontology found extensive applications in petroleum exploration, leading to a surge in the study of microfossils and establishing it as a crucial branch of paleontology.

Microfossil Groups

• Microfossils are the remains of the hard parts of tiny microorganisms. Most of these organisms do not have hard shells or tests, which is why they do not get preserved as fossils. However, there are some microorganisms whose shells or tests are made of mineral matter and tough organic material, and as a result, they often get preserved as fossils.
• Microfossils are divided into two groups based on the composition and structure of their shell or test:

Mineral-walled Microfossils

• Foraminifers: These are single-celled organisms with intricate mineral shells. They are found in marine environments and are important indicators of past climate and ocean conditions.
• Radiolarians: These are microscopic protozoa with intricate mineral skeletons, usually made of silica. They are found in oceanic waters and contribute to the sedimentary record.
• Diatoms: These are algae with unique silica cell walls. They are found in both marine and freshwater environments and play a significant role in the global carbon cycle.
• 2. Non-mineral (Organic)-walled Microfossils
• Acritarchs: These are organic-walled microfossils of uncertain origin, often found in marine sediments. They are useful for biostratigraphy and paleoenvironmental studies.
• Dinoflagellates: These are single-celled organisms with unique organic cell walls. They are found in marine and freshwater environments and are important for understanding past environmental conditions.
• Pollen and Spores: These are plant microfossils with organic walls. They are found in various sedimentary environments and are important for reconstructing past vegetation and climate.

Mineral-walled Microfossils

• Mineral-walled microfossils are the remains of tiny organisms that had hard, mineralized shells or tests. These include foraminifers, radiolarians, diatoms, ostracods, and conodonts. The shells of these microfossils are typically made of minerals like calcium, silica, or phosphate, which makes them tough and resistant to physical and chemical breakdown. This is why they have a higher potential for preservation as fossils. 
• Let's discuss some important fossils that belong to the category of mineral-walled microfossils. 

(i) Foraminifers

• Foraminifers, commonly known as "forams," are single-celled protozoans that feed on organic material and are characterized by their hard, preservable shells or tests. What sets foraminifers apart from other protozoans is their intricate shell and a complex network of branched, fiber-like pseudopodia. These pseudopodia, which form a net around the shell, are used to capture food particles from the water. Foraminifers inhabit marine waters at various depths and have two primary modes of life: benthic and planktic. Most foraminifers are benthic, living on or within the seabed, while planktic foraminifers drift with ocean currents, predominantly residing in equatorial and tropical regions.
• Foraminifers are classified under the Kingdom Protista, Phylum Sarcodina, Class Rhizopoda, and Order Foraminferida. Their skeleton, known as the test or shell, is a key feature of their morphology.
• The test of foraminifers is typically composed of at least three types of hard materials: calcium carbonate, tectin, and agglutinated matter. Calcium carbonate is an inorganic substance secreted by the foraminifers, while tectin tests are made of an organic material consisting of complex carbohydrates and proteins. The agglutinated test is formed by gluing together small sand grains and other particles. Foraminifer tests are usually less than 1 mm in diameter and can be unilocular (single-chambered) or multilocular (multiple-chambered).
• The arrangement of chambers within the test gives rise to various forms, including linear and coiled tests. Linear tests can be uniserial, biserial, or triserial, depending on the number of rows of chambers, while coiled tests exhibit planispiral or helical forms, based on how they are coiled around an axis.
• The shape of the chambers in foraminifer tests varies, with single-chambered tests typically being flask or tabular in shape, and multi-chambered tests displaying spherical or club shapes. Each test features an aperture, which is an opening, and multiple small internal openings called foramina. The external surface of the test can be smooth, pitted, ribbed, or adorned with spines and ribs.
• Foraminifers have a geological range spanning from the Cambrian period to the present, with significant diversification occurring during the Cretaceous period. They are among the most diverse and extensively studied groups of microfossils.

(ii) Radiolarians

Radiolarians are single-celled, planktonic protozoans known for their intricate internal skeletons. Similar to foraminifers, these organisms use thread-like extensions called pseudopodia to capture food particles. Pseudopodia extend outward from the center of their skeletons.

• Skeleton Structure: Radiolarians have a unique skeleton consisting of a perforated membranous central capsule. Most radiolarians exhibit radial symmetry, characterized by radial skeleton spines, although some lack this symmetry. The name "radiolarians" comes from the radial symmetry of their skeletons.
• Habitat: These organisms are exclusively marine, primarily living as solitary individuals, although a few species are colonial. The average size of an individual radiolarian ranges from 50 to 200 micrometers, with some colonies reaching up to 5 millimeters in length. After their death, radiolarians are deposited on the ocean floor, where they contribute to the formation of radiolarian ooze by mixing with deep-sea sediments.
• Classification: Radiolarians belong to the Kingdom Protozoa, Phylum Radiozoa, and Subphylum Radiolaria.
• Skeleton Composition: The intricate skeletons of radiolarians are primarily made of silica secreted by the organisms. However, some skeletons are composed of strontium sulfate, and a few are made from a combination of silica and organic material. In living conditions, the skeleton is entirely composed of cytoplasm, which protects it from dissolution in seawater.
• Skeleton Shape:Radiolarian skeletons are typically spherical or shaped like helmets or spaceships, formed by spines, bars, and perforated plates.
  • Spines: Elongated external features attached at one end.
  • Bars: Elongated internal features attached at both ends.
  • Perforated Plates: Plates with evenly spaced pores and no specific boundary.
• Types:Radiolarians are categorized into two types based on symmetry:
  • Spumellar Radiolarians: Characterized by radial symmetry.
  • Nassellar Radiolarians: Identified by bilateral symmetry and conical to bell-shaped skeletons.
• Geological History: Radiolarians have a long geological history, first appearing in the Cambrian period and still existing in present-day oceans. At the sea bottom, their skeletons contribute to the formation of silica-rich sedimentary rock known as siliceous or radiolarian chert.

(iii) Diatoms

Diatoms are single-celled algae that can make their own food using sunlight. They have a golden-brown pigment that helps them in this process. These tiny organisms are usually between 20 and 200 microns in size, but some can grow as long as 2 mm. Diatoms are mostly non-moving and can live alone or in groups.

Classification: Diatoms are classified under the Kingdom Plantae, Sub-kingdom Chromista, Infra-kingdom Diatomea, and Class Bacillariophyceae.

Morphology: The skeleton of a diatom, known as a frustule, is made of silica. It consists of two uneven valves, with the larger valve fitting over the smaller one, similar to a box lid. Diatoms are categorized into two types based on their frustule morphology: (a) Pennate Diatoms: Their frustules are linear, elliptical, or rectangular. These diatoms are typically found in freshwater environments. (b) Centrate Diatoms: Their frustules are circular, triangular, or quadrate, exhibiting radial symmetry. These diatoms are usually found in marine conditions. The frustules of diatoms are often dotted with tiny holes called punctae, which are sealed by porous plates.

Geological Range: Diatoms are believed to have first appeared during the Mesozoic era, likely in the Jurassic period, and they continue to exist today. Their frustules are commonly preserved in deep-sea sediments, forming a diatom-rich rock known as diatomite.

Non-mineral (Organic) Walled Microfossils

Non-mineral (organic) walled microfossils consist of the remains of microorganisms whose shell walls are made of tough, non-mineralized, proteinaceous material. These shells are resistant to microbial and chemical attacks, as well as to extreme temperature and pressure conditions after burial in sediments. As a result, these microfossils are easily fossilized and are commonly found in sedimentary rocks. There are three main types of organic-walled microfossils:

• Spores
• Pollen

Other less common forms of organic-walled microfossils include Chitinozoa and some fungal remains. Broadly, organic-walled microfossils are referred to as palynomorphs. The study of palynomorphs is a specialized branch of paleontology known as palynology. Palynomorphs can include microscopic remains of plants and animals ranging in size from about 5 to 500 µm. These remains are composed of organic compounds like chitin, which are highly resistant to biological, chemical, and other forms of destruction.

(i) Dinoflagellates

• Dinoflagellates are tiny, water-dwelling, single-celled organisms with a complex cell structure, often classified as a type of algae. They are equipped with two whip-like tails, known as flagella, which they use for movement. While they usually drift from place to place with the help of water currents, they can also move independently. Most dinoflagellates live alone, but some form colonies. What makes dinoflagellates unique is that they have characteristics of both plants and animals. Some of them make their own food (autotrophic), while others rely on consuming other organisms (heterotrophic). They typically inhabit the photic zone, the upper layer of oceans and other water bodies where sunlight penetrates.
• The life cycle of dinoflagellates alternates between a swimming stage and a resting stage known as a cyst. The swimming stage is rarely preserved in the fossil record, while cysts, made from durable organic material, are commonly found. These cysts range in size from 40 to 150 micrometers and can be found in various aquatic environments, from marine to freshwater. Dinoflagellates are classified under Kingdom Protozoa, Phylum Dinozoa, and Subphylum Dinoflagellata. The name "dinoflagellates" comes from Greek words meaning "whirling whip," referring to their flagella's movement.

Morphology: About 10% of dinoflagellates form tough and resistant structures called cysts, which are responsible for their fossilization and preservation in the geological record. The two flagella of dinoflagellates are described as follows:

• Transverse Flagellum: This flagellum encircles the body in the cingulum, a central equatorial position marked by a transverse furrow.
• Longitudinal Flagellum: This flagellum lies in the central area of the cell and forms a sulcus, a longitudinal furrow, towards the top or apex of the organism.

The upper half of a dinoflagellate cell, from the center to the top (apex), is called the epitheca, while the lower half, from the center to the antapex, is known as the hypotheca. The top of a cyst is referred to as the apex, and the bottom is called the antapex.

Cysts may be composed of small plates known as theca or thecal plates. There are three basic types of cysts:

• Proximate Cysts: In these cysts, the theca is similar in size and shape and is in close contact with the thecal wall.
• Chorate Cysts: These cysts show no evidence of a reflected sulcus or cingulum.
• Cavate Cysts: These cysts have both an inner and outer wall.

Cysts may have smooth surfaces or feature granules, ridges, indentations, crests, short spines, or horns.

Geological Range: Dinoflagellates likely first appeared in the Palaeozoic era, possibly during the Silurian period, and they continue to exist today.

Acritarchs

Acritarchs are tiny microfossils with organic walls, and their exact biological origins are unclear. They likely come from various ancestors and could include a range of organisms from bacteria to single-celled protists or even multicellular eukaryotes like fungi, algae, or animal eggs. It's thought that acritarchs represent specific stages in the life cycles of planktonic algae, similar to the cyst stage of dinoflagellates. These micro-organisms vary in size, typically ranging from less than 10 microns to over 1 millimeter, but most acritarchs fall between 15 and 80 microns. They are found exclusively in marine environments.

Morphology

• The acritarch test features a central cavity that can be spherical, oval, or triangular, known as the vesicle. Vesicles usually measure between 50 to 100 micrometers in size.
• The vesicle wall, made of durable organic material called sporopollenin, can consist of one to three layers.
• The vesicle's central cavity may be open to the exterior through a pore called the pylome.
• The outer surface of the vesicle can be smooth or decorated with spines, ridges, processes, indentations, or pores, which are all outgrowths of the vesicle wall.
• These characteristics, including the vesicle's shape and the presence or absence of ornamentations and processes, are used to classify acritarchs.

Geological Range

• Acritarchs are among the oldest known fossils, appearing in the Precambrian approximately 1.8 billion years ago. They are still found in modern oceans and are considered the most complex fossils from the Precambrian era.

Spores and Pollen

Spores are reproductive units produced by lower plants like bryophytes and ferns. Pollen grains are produced by higher plants, including gymnosperms and angiosperms, and carry sperm. Both spores and pollen have tough outer walls made of a substance called sporopollenin, which makes them resistant and capable of fossilization.

• Size and Structure: Spores and pollen grains are very small, typically ranging from 10 to 200 micrometers in size. They are produced in large quantities and can travel easily through air and water.
• Fossilization and Sedimentation: Due to their resistant walls, spores and pollen can fossilize easily. They settle at the bottoms of lakes, ponds, rivers, and oceans as part of sediment.
• Morphology of Spores: The shape and arrangement of spores are determined by how the spore mother cell divides. There are two main types of spore arrangements:
• Tetrahedral Arrangement: In this arrangement, the spore mother cell divides into four spores at once, which are in contact with each other at three points, creating a Y-shaped mark. These spores are known as trilete spores.
• Tetragonal Arrangement: Here, the spore mother cell divides into two spores, which further divide. These spores have a single contact mark and are called monolete spores.
• Morphology of Pollen Grains: Pollen grains vary in size from 20 to 150 micrometers and can be oblate, spheroidal, or prolate in shape. Some pollen grains have no pores (inaperturate), while others have one (monoporate), two (diporate), or three (triporate) pores.
• Wall Structure: Most spores and pollen have a double-layered wall, with an inner and outer layer. The outer layer is resistant and fossilizes easily, often featuring granulations, pits, and rod-like extensions. The shape and arrangement of pores are important for classifying spores and pollen.

Geological Range. Spores are believed to have originated in the Silurian period, while pollen dates back to the Devonian period. Both spores and pollen continue to exist today, and their evolutionary history is linked to the evolution of land plants.

Significance of Microfossils

Microfossils are tiny, microscopic organic remains found in most sedimentary rocks and occasionally in low-grade metamorphic rocks. Their characteristics, such as global distribution, unique morphology, sensitivity to climate, and rapid evolutionary rate, make them valuable for stratigraphic studies. Microfossils are used for:

• Stratigraphic correlation
• Dating of rocks
• Reconstructing past environments, climates, and oceanic circulation patterns
• Industrial applications, such as oil well logging and locating source rocks for oil and gas

(a) Geological fieldwork:  (b) Sampling:  (c) Processing:  (d) Separation:  <

Geological Fieldwork

Fieldwork is a crucial aspect of both palaeontological and geological studies, tailored to the specific aims and objectives of the research. During fieldwork, researchers visit geological sites to systematically collect samples, record the orientation and composition of geological layers, and create sketches in a field diary. Additionally, geographic coordinates are documented using GPS or topographic maps, and photographs of the site are taken.

A field kit is essential for effective fieldwork and may include the following equipment:

• Hammer
• Chisels and shovels
• Compass clinometers or Brunton compass
• Measuring tape and hand lens
• Topographic and geologic maps
• Field notebook or diary
• Plastic and cloth sample bags
• Plastic acid bottle for limestone identification
• Knife, field camera, and GPS
• First aid box

Sampling for Micropalaeontological Studies

• Micropalaeontological studies involve the examination of tiny fossils, which are often not visible in the field due to their small size. However, some larger forms of foraminifers and ostracods can be seen with the naked eye, helping researchers identify specific layers for sample collection. In most cases, samples are collected randomly, and their potential for containing microfossils is only determined during laboratory processing. Depending on the study's nature, samples can be collected vertically at regular or irregular intervals from a geological section.
• For micropalaeontological studies, it is preferable to collect samples from within the rock rather than from the weathered surface to avoid contamination. Samples can be taken from any geological site with well-exposed rock succession, both horizontally and vertically. Subsurface samples can be obtained from core samples during drilling by oil companies or from open mines.

When collecting samples, it is important to follow certain precautions:

• Avoid collecting samples from weathered rock exposures.
• Clean the top or weathered surface of exposure before sampling.
• Always collect samples from fresh rock surfaces.
• Clean tools like hammers, chisels, and shovels before use to prevent contamination.
• Place each sample in a new bag, label it with locality and bed information, and record the details in a field notebook.
• For micropalaeontological studies, sample size should be between 200 grams to 1 kilogram.
• Microfossils are more prevalent in fine-grained sediments like clay, mudstone, or silt compared to coarse-grained rocks like sandstone.
• Collect samples from soft fine-grained rocks such as mudstone, siltstone, or clay.
• Samples may be collected from lower to higher elevation levels within a section.
• Take care to avoid contamination at every step of the sampling process and ensure safety during fieldwork.

Processing

After collecting samples in the field, they are taken into the laboratory for processing or preparation to separate or recover microfossils from them.

Processing of Soft Sediments

• Initial Steps: Soft sediments like shales, clay, or mudstones are broken into smaller pieces (usually less than 3 cm) and dried.
• Water Soaking: Dried samples are placed in a container covered with water for 10-12 hours, allowing them to disintegrate into a mud slurry.
• Screen Washing: The slurry is then washed through sieves of varying mesh sizes (e.g., 80, 100, 200, or 280) to remove unwanted fine particles of clay or sand. This method is efficient for processing soft sediments.

Processing of Hard Samples

• Chemical Disintegration: Hard samples like limestone and sandstone may require chemicals such as sodium bicarbonate, sodium hydroxide, or sodium sulfate for disintegration.
• Freezing and Thawing: Alternating freezing and thawing methods can also be employed for hard samples.
• Thin Section Preparation: Very hard rocks like chert can be studied by preparing thin sections.

Sample Preservation:

• During processing, half of the sample should be preserved in the laboratory for reference or re-investigation if needed.

Residue Drying and Packaging:

• Once disintegration is complete, the obtained residue must be dried completely.
• Dried residue is then placed in plastic sample bags, numbered according to their location and stratigraphic position.

Separation of Microfossils

• Tools Required: High-resolution binocular light microscope, picking tray, fine-hair brush, and micropalaeontological assemblage slide.
• Initial Examination: Dried residue containing microfossils is placed in the picking tray and examined under the microscope.
• Picking Microfossils: Microfossils are picked up using a fine-hair brush and stored in the micropalaeontological assemblage slide.

Viewing Thin Sections

• Thin sections can be directly viewed under the microscope, preferably with a digital camera and computer attachment.
• Microfossils found in thin sections can be photographed for further studies.

Study of Microfossils

• The study of microfossils involves their description and identification, which is a challenging task requiring knowledge of taxonomy and biological classification. It often necessitates consulting reference books such as fossil identification guides and textbooks of paleontology.
• To describe and identify microfossils, a high-resolution binocular microscope and, in some cases, a Scanning Electron Microscope (SEM) are needed. After isolating microfossils from their matrix, they are examined under the microscope, where all morphological details are noted and photographed. Line drawings are also helpful in illustrating important features.
• During examination, microfossils are compared with previously described specimens to determine if the material is new or a known species. If it represents a new form, a name may be assigned according to the code of biological nomenclature.

Activity

List of Important Microfossils

You are required to study the similar facts for other microfossil groups and accordingly, fill in the table as done in case of foraminifers.

Summary

In this unit, you have learnt about the following:

• Microfossils are the remains of very small single-celled and multi-celled organisms. These are usually less than 2 mm in size.
• The term microfossil is applicable to all those organic remains whose study requires the use of a light or electron microscope.
• The study of microfossils is known as micropalaeontology. The history of micropalaeontology is more than two centuries old.
• Foraminifers, radiolarians and diatoms are the main groups of mineral-walled microfossils, whereas acritarchs, dinoflagellates, spores and pollen are the main groups of organic-walled microfossils.
• Foraminifers are single-celled protozoans and may be planktic or benthic in mode of life. The majority of foraminiferal tests are multi-chambered bearing many small internal openings known as foramina. They range in age from Cambrian to present.
• Radiolarians are single-celled planktic protozoans characterised by radial symmetry. They are exclusively marine and range in age from Cambrian to present.
• Diatoms are unicellular photosynthesising non-motile algae that live in almost all kinds of environments from marine to freshwater. Their skeleton, called frustule, is made up of silica and consists of two unequal valves. They range in age from Mesozoic to present.
• Dinoflagellates are small, aquatic, single-celled eukaryotic micro-organisms commonly regarded as algae. They possess both plant-like and animal-like characters. About 10% of dinoflagellates develop tough and resistant organic structures called cyst, which fossilise freely. They range in age from Palaeozoic to present.
• Acritarchs are microscopic organic-walled vesicular microfossils with unknown biological affinities. They are exclusively marine and are the oldest known group of microfossils. They range in age from Precambrian to present.
• Spores and pollen both are part of the plant reproductive system. Spores are the reproductive bodies of lower vascular plants and their earliest occurrences are known from Silurian rocks. Pollen are the sperm-carrying reproductive bodies of seed plants. They occur first in Devonian rocks. The occurrences of spores and pollen reflect the evolutionary history of land plants.
• Microfossils are broadly used for stratigraphic correlation, dating the rocks, reconstruction of environment, climate and oceanic circulation patterns of the geological past and are also useful in oil industry.
• The collection of samples from the field and extraction of microfossils from them in laboratory involves fieldwork, sampling, processing or preparation of samples and sorting of microfossils under microscope.
• The study of microfossils involves their description and identification. For this purpose, a high-resolution biological microscope and sometimes Scanning Electron Microscope are used.