Different Kinds of Microfossils | Geology Optional Notes for UPSC PDF Download

Microfossils and Micropalaeontology

Microfossil Groups

• Microfossils represent diverse organisms like animals, plants, and protists.
• The term "microfossil" denotes the small size of these organisms rather than a specific group.

Mineral-Walled Microfossils

• These microfossils have mineralized structures, aiding in their preservation.
• Example: Foraminifera, which have calcium carbonate shells.

Organic-Walled Microfossils

• Comprising organic material, these microfossils include spores, pollen, and chitinous remains.
• Example: Pollen grains that provide insights into ancient plant species.

Significance of Microfossils

• Microfossils offer valuable data for understanding past environments and climates.
• Example: Diatoms, microscopic algae, indicate past marine conditions.

Collecting Microfossils

• Microfossils are collected through geological fieldwork, involving sampling, processing, and separation techniques.

Geological Fieldwork

• Fieldwork involves on-site collection of rock samples for subsequent analysis.
• Example: Geologists extracting sedimentary rock layers for microfossil examination.

Studying Microfossils

• Analyzing microfossils aids in deciphering past ecosystems and evolutionary patterns.
• Example: Researchers using microscope techniques to study microfossil structures.

Activity

• Engaging in hands-on activities like microfossil identification enhances learning.

Terminal Questions

• Questions at the end of the unit assess understanding of microfossil concepts.

Introduction to Palaeontology

• Microfossils are incredibly abundant and easily accessible fossils, providing a wealth of information in palaeontology.
• This unit delves into the detailed study of microfossils, emphasizing their significance.

Expected Learning Outcomes

• Define microfossils and micropalaeontology.
• Classify microfossils into mineral- and organic-walled groups.
• Highlight the importance of microfossils.
• Describe the techniques for collecting microfossils.
• Explain the methods used to study microfossils.

Microfossils and Micropalaeontology

• Microfossils represent the remains of minuscule single-celled and multi-celled organisms, typically under 2 millimetres in size.
• These remnants come from tiny creatures living in various water bodies and are preserved in sedimentary rocks post-mortem.
• The term 'microfossil' encompasses a range of groups like protists, bacteria, fungi, animals, and plants.
• Microfossils, which require microscopic examination, can vary from 0.001 mm to 2 mm in size.
• Fragmentary microscopic parts of macro-organisms like shell pieces and teeth are also considered microfossils.
• While some micro-organism remains are complete and valuable for study, others are often disregarded due to incompleteness.
• Foraminifers, radiolarians, ostracods, diatoms, and other micro-organisms' remains are of particular interest to micropalaeontologists.
• The classification of microfossils is based on their small size, with the method of study (microscope usage) being crucial.

Unit 10: Microfossil Groups

Microfossils are abundant in various environments such as marine, brackish water, and freshwater. Micropalaeontology is a branch of palaeontology that focuses on the study of microfossils and nanofossils, the latter being between 5 and 60 µm in size. The history of micropalaeontology dates back more than two centuries, but it gained significant importance during the early twentieth century. Initially, micropalaeontology primarily involved the study of fossil foraminifers, with a shift towards exploring other microfossil groups like radiolarians, ostracods, diatoms, dinoflagellates, pollen, and spores after 1925. Microfossils became crucial in petroleum exploration post-1945, leading to the field's extensive use and recognition as a vital branch of palaeontology.

MICROFOSSIL GROUPS

• Main microfossil groups
• Mineral-walled microfossils
• 
  
  • Foraminifers
  • Radiolarians
  • Diatoms
• Non-mineral (organic)-walled microfossils
• 
  
  • Acritarchs
  • Dinoflagellates
  • Pollen and spores

Mineral-walled Microfossils

This category includes micro-organisms with mineralized shells or tests like foraminifers, radiolarians, diatoms, ostracods, and conodonts. The shells of mineral-walled microfossils are typically composed of mineral matter such as calcium, silica, or phosphate, making them hard and resistant to external factors. As a result, these microfossils have a higher potential for preservation as fossils.

Foraminifers

Foraminifers, also known as "forams," are single-celled heterotrophic protozoans with hard, preservable tests or shells. What sets foraminifers apart from other protozoans is their possession of a hard test and a complex network of branched fiber-like pseudopodia. These pseudopodia form a net around the test and are utilized to capture prey.

Introduction to Palaeontology

• Foraminifers Overview

  • Foraminifers are single-celled organisms found in marine waters at various depths.
  • They exhibit two main modes of life: benthic (living on or in the seabed) and planktic (drifting with ocean currents).
  • Planktic foraminifers are primarily located in equatorial and tropical regions.

• Taxonomic Classification

  • Foraminifers belong to Kingdom Protista, Phylum Sarcodina, Class Rhizopoda, and Order Foraminferida.

• Structure and Composition

  • The skeleton of foraminifers is called a test or shell.
  • The test is typically composed of calcium carbonate, tectin, and agglutinated matter.
  • Calcium carbonate is an inorganic substance secreted by foraminifers.
  • Tectin tests are made of an organic material consisting of complex carbohydrates and proteins.
  • Agglutinated tests may contain small sand grains and other particles cemented together.

• Morphology

  • Foram tests are usually less than 1 mm in diameter.
  • Tests can be single-chambered (unilocular) or multiple-chambered (multilocular).

Forms of Foraminifera Tests

• Linear and coiled forms are two main types of foraminifera tests.
• Linear tests can be uniserial, biserial, or triserial based on the number of rows in which chambers are arranged.
• Coiled tests can exhibit planispiral or helical forms depending on their coiling pattern.

Linear Forms:

• Uniserial form: Chambers arranged in a single row.
• Biserial form: Chambers arranged in two rows.
• Triserial form: Chambers arranged in three rows.

Coiled Forms:

• Planispiral form: Chambers arranged in a single plane.
• Helical form: Chambers arranged around a vertical axis.

Microfossils

• Single-chambered tests are typically flask or tabular in shape.
• Multi-chambered tests can be spherical or club-shaped.
• Each test has an opening known as an aperture.
• Forams also feature small internal openings called foramina.
• The external surface of foraminifera tests can vary in ornamentation, ranging from completely smooth to pitted, ribbed, or bearing spines and ribs.

Geological Range

• Foraminifera are a diverse group of microfossils that have been extensively studied.
• They have been found in rocks dating from the Cambrian period to the present.
• Their diversity notably increased during the Cretaceous period.

Introduction to Radiolarians

• Radiolarians are single-celled planktic protozoans with delicate internal skeletons.
• They capture food particles using thread-like extensions called pseudopodia.
• Most radiolarians display radial symmetry characterized by radial skeleton spines.
• Some radiolarians lack radial symmetry despite their name being derived from it.

Ecology and Morphology

• Radiolarians are exclusively marine organisms, existing as both solitary and colonial forms.
• The average diameter of an individual radiolarian ranges from 50 to 200 µm, with some colonies reaching up to 5 mm in length.

Taxonomy Classification

• Kingdom: Protoza
• Phylum: Radiozoa
• Subphylum: Radiolaria

Skeleton Composition

• The intricate skeleton of radiolarians is primarily composed of silica secreted by the organism.
• While most skeletons are silica-based, some are made of strontium sulphate, and a few consist of a combination of silica and organic material.

Living Conditions and Skeleton Composition of Radiolarians

• Radiolarian skeletons are primarily composed of cytoplasm, protecting them from dissolution in seawater.
• These skeletons are typically spherical or helmet to space-ship shaped, consisting of spines, bars, and perforated plates.
• Spines are elongated external features attached at one end, while bars are elongated internal features attached at both ends.
• A perforated plate has evenly spaced pores without a distinct plate boundary.

Radiolarian Symmetry Types

• Spumellar Radiolarians: Recognized by radial symmetry.
• Nassellar Radiolarians: Characterized by bilateral symmetry and conical to bell-shaped skeletons.

Geological Range of Radiolarians

• Radiolarians first appeared in the Cambrian period and continue to exist in present-day ocean waters.
• At the sea bottom, their skeletons contribute to the formation of silica-rich sedimentary rock known as siliceous or radiolarian chert.

Diatoms: Unicellular Photosynthesizing Algae

• Diatoms are unicellular algae with golden brown photosynthetic pigment, secreting a siliceous skeleton.
• They typically range between 20 and 200 microns in size, with some forms reaching up to 2 mm in length.
• Diatoms are mostly non-motile and can exist in solitary or colonial forms.

Diatom Classification and Morphology

• Frustule: The silica-based skeleton of a diatom, comprising two unequal valves that fit like a box lid.
• Pennales: Diatoms with linear to elliptical or rectangular frustules.
• Centrales: Diatoms characterized by circular, triangular, or other shaped frustules.

Block 3: Introduction to Palaeontology

• Overview of Diatoms:
  • Diatoms are single-celled algae with intricate frustules.
  • They exhibit radial symmetry and can be pennate or centrale in shape.
  • Pennate diatoms are found in freshwater, while centrales thrive in marine environments.
• Structure of Diatom Frustules:
  • The frustules of diatoms are dotted with tiny holes called punctae.
  • Punctae are covered by porous plates on the frustules.
  • Fig. 10.8 illustrates the schematic views of diatom frustules.
• Geological Range of Diatoms:
  • Diatoms emerged during the Mesozoic era, likely in the Jurassic period.
  • They are still present today and are predominantly found in deep sea sediments, forming diatom-rich rocks known as diatomite.

Short Answer Question 1

• a) Organic-walled vs. Mineral-walled Microfossils:
  • Organic-walled microfossils have shells made of tough organic material, while mineral-walled microfossils have mineralized shells.
  • Organic-walled microfossils are more resistant to environmental factors and are commonly found in sedimentary rocks.
• b) Siliceous or Radiolarian Chert:
  • Radiolarians are the fossil group associated with making siliceous or radiolarian chert.
• c) Key Morphological Features of Radiolarians:
  • Radiolarians typically exhibit intricate skeletal structures with spines and intricate patterns.

Non-mineral Walled Microfossils

• Organic-walled microfossils:
  • These micro-organisms have shells made of tough organic or non-mineralised material.
  • They are highly resistant to decay and common in sedimentary rocks.
• Types of Organic-walled Microfossils:
  • Dinoflagellates
  • Acritarchs
  • Spores and pollen
• Microscopic Study of Organic Remains:
  • Organic remains requiring a microscope for study are categorized as microfossils.

Organic-Walled Microfossils

• Organic-walled microfossils less common than other types like Chitinozoa and fungal remains.
• Collectively known as palynomorphs, forming a branch of palaeontology called palynology.
• Palynomorphs can be microscopic remnants of plants or animals, ranging from 5 to 500 µm.
• Composed of resilient organic compounds like chitin, making them highly resistant to various forms of degradation.

Dinoflagellates

• Dinoflagellates are small, aquatic, single-celled eukaryotic organisms often considered a type of algae.
• They possess two whip-like flagella for movement and can be found both individually and in colonies.
• Some dinoflagellates exhibit characteristics of both plants and animals, with variations in their feeding mechanisms.
• Most dinoflagellates inhabit the well-lit regions of oceans and other water bodies.

Dinoflagellate Life Cycle

• The life cycle of dinoflagellates involves motile (swimming) and cyst (sedentary) stages.
• Motile dinoflagellates are less commonly preserved compared to cyst stage dinoflagellates.
• Cysts, ranging from 40 to 150 µm, are formed from durable organic material that fossilizes easily.
• Dinoflagellates are found in various aquatic environments, from marine to freshwater.

Morphology of Dinoflagellates

• About 10% of dinoflagellates develop cysts, tough structures that aid in fossilization.
• Dinoflagellates have two flagella: a transverse flagellum in the cingulum and a longitudinal flagellum in the sulcus.
• The cingulum is a transverse furrow around the central equatorial position, while the sulcus is a longitudinal furrow at the top.

Block 3: Introduction to Palaeontology

• Dinoflagellate Morphology
• 
  
  • The body of a dinoflagellate is divided into two parts: the epitheca and the hypotheca.
  • The top part of the dinoflagellate cyst is called the apex, while the bottom part is known as the antapex.
  • A cyst can consist of multiple small plates referred to as theca or thecal plates.
• Types of Cysts
• 
  
  • Proximate Cysts: These cysts have theca plates that are similar in size and shape, closely attached to the thecal wall.
  • Chorate Cysts: They lack a distinct sulcus or cingulum.
  • Cavate Cysts: These cysts feature inner and outer walls, with various surface characteristics like granules, ridges, or spines.
• Geological Range of Dinoflagellates
• 
  
  • Dinoflagellates likely emerged during the Palaeozoic era, possibly in the Silurian period, and continue to exist today.
• Acritarchs
• 
  
  • Acritarchs are tiny, organic-walled vesicular microfossils with uncertain biological connections.
  • They form a diverse group possibly originating from various organisms like bacteria, protists, fungi, algae, or animal eggs.
  • Acritarchs are thought to represent cyst stages or benthic phases in the life cycles of planktic algae.

Unit 10: Acritarchs and Microfossils

Acritarchs

• Acritarchs are small marine micro-organisms.
• Morphology:
  • An acritarch consists of a central cavity called a vesicle, typically ranging from 50 to 100 µm in size.
  • The vesicle wall, made of a tough material called sporopollenin, may have one to three layers.
  • The external surface of the vesicle can be smooth or ornamented with various features like spines, ridges, or pores, which help classify acritarchs.
• Geological Range:
  • Acritarchs are among the oldest known fossils, dating back to around 1.8 billion years ago in the Precambrian period.
  • They continue to exist in modern oceans and are recognized as complex Precambrian fossils.

Spores and Pollen

• Spores and pollen are reproductive structures of plants:
  • Spores are produced by lower plants like bryophytes and ferns, while pollen grains are produced by higher plants including gymnosperms and angiosperms.
  • Both spores and pollen have durable walls made of sporopollenin and can fossilize easily.
  • They are small grains, typically ranging from 10 to 200 µm in size, and are dispersed by air and water, settling in various environments.

Morphology of Spores and Pollen

• Spore Morphology:
  • Tetrad Formation: The morphology of spores is primarily determined by divisions of the spore mother cell. This cell divides to form a tetrad consisting of four small spores. There are two types of arrangements observed in tetrads: tetrahedral and tetragonal.
    • Tetrahedral Arrangement: In this arrangement, the cell divides into four spores simultaneously, with all four spores in contact at three areas, forming a Y-mark. Spores with a Y-mark exhibit radial symmetry and are known as trilete spores.
    • Tetragonal Arrangement: In this arrangement, the cell undergoes successive divisions to form spores. Initially, the cell divides into two spores, which further subdivide. Spores in this arrangement have bilateral symmetry and are termed as monolete spores.
• Pollen Characteristics:
  • Pollen Size and Shape: Pollen grains typically range from 20 to 150 µm in size. They can be oblate, spheroidal, or prolate in shape. Some pollen grains lack pores and are known as inaperturate, while others have one or more pores.
    • Types of Pores: Pollen grains can be classified based on the number of pores they possess. Those with one pore are monoporate, two pores are diporate, and three pores are triporate.
  • Wall Structure: Both spores and pollen generally have a double-layered wall structure comprising inner and outer layers. The outer layer is highly durable and easily fossilizes, exhibiting variable ornamentation.

Unit 10: Microfossils and Their Significance

Forms and Classification of Spores and Pollen

• Spores and pollen exhibit various forms such as monoporate, diporate, and triporate.
• The shape and arrangement of pores play a crucial role in their classification.

Geological Range

• The earliest spore occurrences date back to the Silurian rocks, while the oldest pollen traces are from the Devonian rocks.
• Spores and pollen have continuously existed from ancient times to the present day.
• The evolution of spores and pollen is closely linked to the evolution of land plants.

Significance of Microfossils

• Microfossils are tiny organic remains found in sedimentary and occasionally in low-grade metamorphic rocks.
• Characteristics like global distribution, distinctive morphology, and rapid evolutionary rate make microfossils ideal for stratigraphic studies.
• Microfossils are valuable for stratigraphic correlation, dating rocks, and understanding past environments and climates.
• They play a crucial role in industries like oil well logging and the identification of oil and gas source rocks.

Collecting Microfossils

• Microfossils are more abundant and widely distributed in sedimentary rocks compared to larger fossils.
• Microfossil-bearing rocks offer insights into various aspects of the geological past.
• Macrofossils are visible to the naked eye in the field, while microfossils are microscopic and require specialized methods for collection and preparation.

Main Methods for Collecting Microfossils

• Geological fieldwork
• Sampling
• Processing
• Separation

Introduction to Palaeontology

Geological Fieldwork

• Fieldwork is crucial for both palaeontological and geological studies, tailored to the study's specific objectives.
• During fieldwork, samples are systematically collected at the intended geological site, with detailed recording of the bed attitudes, lithology, and field sketches.
• Geographic coordinates are noted using GPS or topographic maps, alongside photographic documentation.

Field Kit Essentials

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

Field Equipments

• a) Geological hammer
• b) Chisel
• c) Brunton compass
• d) Measuring tape
• e) Hand lens

Sampling

• Microfossils are typically not visible in the field due to their small size.
• For larger microorganisms like foraminifers and ostracods, specific beds/layers can be identified for collection.
• In cases where microfossils are not visible, samples are collected randomly, and their microfossil content is only revealed during laboratory processing.
• Sampling methods include vertical collection at regular or irregular intervals from bottom to top based on the study's nature.

Unit 10: Geological Section

Collecting Samples

• When conducting micropalaeontogical studies, it is crucial to gather samples from within the rock rather than from the weathered surface to avoid contamination risks.
• Samples should be obtained from geological sites with well-exposed rock successions both horizontally and vertically.
• Subsurface samples can be sourced from core samples extracted during drilling operations by oil companies or from open mines.
• Precautions during sample collection:
  • Do not collect samples from weathered rock exposures.
  • Clean the top or weathered surface before sampling.
  • Always gather samples from fresh rock surfaces.
  • Ensure equipment like hammers, chisels, and shovels are clean to prevent contamination.
  • Place each sample in a new bag, label it with locality and bed information, and record details in a field notebook.
  • For micropalaeontological studies, sample sizes should range from 200 grams to 1 kilogram.
  • Microfossils are more abundant in fine-grained sediments like clay, mudstone, or silt compared to coarse-grained rocks such as sandstone.
  • Prefer collecting samples from soft, fine-grained rocks like mudstone, siltstone, or clay.
  • Collect samples from lower to higher elevation levels within the section.
  • Exercise caution to prevent contamination and accidents during fieldwork.

Processing Samples

• Upon field collection, samples are transported to the laboratory for processing to extract or recover microfossils.
• Due to varying softness or hardness of samples, different processing treatments are applied based on lithology.
• For softer sediments like shales, clay, or mudstones, the processing typically involves breaking them into smaller pieces (< 3 cm), followed by drying.
• Subsequently, samples are placed in a container with water for 10-12 hours to disintegrate and form a mud slurry, especially for very soft rocks.
• The mud slurry can then undergo screen washing using sieves of different mesh sizes to remove unwanted fine particles like clay or sand.
• Screen washing, employing sieves of varying mesh sizes such as 80, 100, 200, or 280, is an efficient technique for processing soft sediments.

Block 3: Separation Techniques in Palaeontology

• Introduction to Palaeontology:
  • Palaeontology is the study of ancient life forms, including fossils and traces of past life on Earth.
• Methods for Sample Disintegration:
  • Chemicals like sodium bicarbonate, sodium hydroxide, and sodium sulphate are used for disintegrating hard samples.
  • Alternating freezing and thawing is another method employed for sample disintegration.
• Preparation and Preservation of Samples:
  • Thin sections of very hard rocks like chert are prepared for study.
  • Half of the sample should be preserved in the laboratory for reference or re-investigation purposes.
• Processing Residues:
  • Residues obtained post-disintegration should be completely dried and stored in numbered plastic sample bags.
• Separation of Microfossils:
  • High-resolution binocular light microscopes are used to separate microfossils from dried residues.
  • Microfossils are picked using fine-hair brushes and stored in micropalaeontological assemblage slides.
  • Thin sections of samples can be directly viewed under microscopes, with the option of using digital cameras and computer attachments for detailed analysis.

Unit 10: Studying Microfossils

Important Groups of Fossils Belonging to Organic-Walled Microfossils:

• Acritarchs
• Chitinozoans
• Scolecodonts

Composition of Shells of Organic-Walled Microfossils:

• The shells of organic-walled microfossils are primarily composed of organic materials like chitin and sporopollenin.

Fossil Spores and Pollen:

• Fossil spores and pollen are the preserved remnants of the reproductive parts of plants.

Equipment of the Geological Field Kit:

• High-resolution binocular microscope
• Scanning Electron Microscope (SEM)

Studying Microfossils:

• The study of microfossils involves detailed description and identification processes.
• It requires knowledge of taxonomy, biological classification, and consultation of reference materials such as fossil identification guides and palaeontology textbooks.
• Microfossils are examined under a microscope after isolation from the matrix, with morphological details noted and photographs taken.
• Comparison with previously described fossils helps determine if the material is a new species, which may lead to assigning a new name following biological nomenclature guidelines.