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Essay on Fossils
Essay Contents:
- Essay on the Meaning of Fossils
- Essay on the Types of Fossils
- Essay on the Nomenclature of Fossils
- Essay on the Techniques for the Study of Fossils
- Essay on the Modes of Fossil Preservation
- Essay on Fossilisation
Essay # 1. Meaning of Fossils:
The word fossil is derived from the Latin verb ‘fodere’ which means to dig up. Thus, a fossil refers to anything that is excavated from the earth and not fashioned by man. Actually, the fossil refers to organic remains taken from the earth. So in common sense, fossils are the traces of the past life forms in the womb of the earth.
According to Arnold, the fossils are “the relic of some former living things, plants or animals, embedded in or dug out of the superficial deposits in past geological periods.” Schopf (1975) defines fossil as any specimen that demonstrates physical evidence of occurrence of ancient life.
Essay # 2. Types of Fossils:
The fossils may be categorised in the following types:
1. Chemical Fossils:
These are the remnants of organic compounds p reserved in sediments or in parts of fossilised structures without undergoing any or minimal change. These include amino acids, hydrocarbons, fatty acids, lipids, carbohydrates and the derivatives of other organic compounds.
The chemical composition of Pre-Cambrian rocks is an important criterion to establish the biogenicity of putative unicellular or multicellular organisms present in Pre- Cambrian rocks. The existence of insoluble kerogen is used as proof of biogenicity. Similarly, the occurrence of pristane and phytane, degradable products of chlorophyll molecule, may be used as proof of photosynthesis.
2. Trace Fossils or Ichnofossils:
Sometimes, indications of prior existence of organisms in the sediments of earth may be regarded as trace fossils or Ichnofossils. These include animal tracks or foot print preserved in rocks, burrows of invertebrates, coprolites (fossil excretes), gastroliths (polished stones in the abdomen of dinosaurs), gnawed bones, etc.
3. Microfossils:
Microscopic organisms like bacteria, spores and pollen grains, fungal and algal spores, foraminifera, diatoms, epidermal and wood fragments of plants etc. preserved in the sedimentary deposits are referred to as microfossils. Microfossils are visible only after maceration of sediments.
4. Megafossils:
Large parts of plants like leaf, stem, root, flower, seed, etc. and animal remains as whole organism or in parts, preserved in the sedimentary deposits are called megafossils. These are visible to naked eyes and are the better source of morphological as well as anatomical studies.
The megafossils may be categorised into the following five types on the basis of the nature of fossilisation:
(a) Compressions:
These are plant parts, compressed by the vertical pressure of the sediments (Fig. 1.83). The plant fragments like leaves, stem, seeds get flattened and are retained as thin carbonaceous films with outline of external features. Generally, internal structure is not preserved, however, in rare instances cuticles, stomate, etc. are retained.
(b) Impressions:
Impression may be defined as the negative of a compression. These are just impression of plant parts which do not contain organic matters as in compression (Fig. 1.84). The sediments containing the flattened plant parts become hardened and when split open shows the negative imprint i.e. impression.
(c) Petrifactions:
These are the best, but rarest types of fossil which preserve the external form as well as the internal structures. The cellular details are preserved due to the infiltration of minerals like SiO2, CaCO3, MgCO3, FeS, etc. into the tissue. The petrified fossils can be cut into small pieces and series of section can be made for anatomical studies (Fig. 1.85).
(d) Casts or Incrustations and Molds:
In these types, the deposition of iron and carbonate minerals occurs in the form of a hard cast around the plant parts. The internal structure is degraded to form a cavity which is completely filled up by the surrounding sediments. Thus, the external preserved surface of the plant part is called a mold (Fig. 1.86), while the replaced internal structure of the plant part is called a cast.
Stigmarian root system is an example of mold, while the pith cast of a Catamites stem is a common example of incrustation (Fig. 1.87). In these types, only external forms are preserved, while internal cellular details are not preserved.
5. Sub-Fossils:
A certain time period is required for the formation of a fossil. If the plant or animal parts are excavated before they completed their fossilisation process, they are called sub-fossils. Coal is a compressed fossil, while peat, an early stage of coalification is referred to as sub-fossil.
6. Pseudofossils:
Sometimes inorganic rocks that appear to be fossils are actually mineral deposition. These fake structures are mistaken for plant or animal remains. These are known as pseudofossils.
7. Derived Fossils:
The fossilised organisms that held in a stratum younger or older than the fossil themselves are called derived fossils. These are results of tectonic movement of earth or other geological upheaval.
8. Coal Balls:
The petrified spherical balls containing plant parts are commonly termed coal balls (Fig. 1.88). These spherical balls are formed as a result of infiltration of plant debris in swamps by carbonates of calcium or magnesium, thus restricting the conversion of the debris into coal. Coal balls occur in localised regions and they range in few centimeters to several meters and weigh from a few to several pounds. Coal balls are specifically significant in palaeobotanical studies.
9. Paper Coal:
It consists of thin dead leaves, dispersed in organic matrix. The inner tissues of leaves are destroyed, thus the paper coal consists of layer after layer of cuticles, often with decomposed stems. The carbonaceous lime stone horizon at Tovarkovo, in Toula in Russia, is an example of paper coal.
10. Amber:
The fossilised resin of extinct coniferous trees, Pinus succinifera in particular, is called Amber. The resinous exude flowed due to injury caused by boring insects which eventually accumulated on the forest floor and got hardened forming amber. Insects and flowers are often found preserved in amber. Amber has high economic value and used in jewelllery.
11. Index Fossils:
The organisms that help in dating other fossils found in the same sedimentary layer are called index fossils. Such fossils are found widely distributed geographically, and limited in time span having very distinct characteristic features. Monograptus is an index fossil of Lower Devonian, while Myrepollenites is a marker of Eocene. Foraminifera, pollen grains, spores etc. are also used as index fossils.
Essay # 3. Nomenclature of Fossils:
The whole plant is not preserved, but only detached plant parts like stem, root, cone, leaf, etc. are preserved as fossils. These detached plant parts are being discovered in different times by different authors. Thus, these detached plant parts or organs are given a bionomial (generic and specific name) by the same set of rules under the International Rules of Botanical Nomenclature which have been framed for living plants.
The first valid description of Lepidodendron came into existence from the publication of Sternberg in 1820. Thus, this date has been considered as the starting point of palaeobotanical nomenclature like that of Linnaeus’s ‘Species Plantarum’ in 1753 for the nomenclature of modern vascular plants.
Each detached organs or fragments is given a different name. Each of these names acquires the status of a genus. The generic name in fossils is applicable for only a plant part like root, stem, leaf, cone or other organ, without indicating to what plant it belongs. Thus, the genus is termed form genus or artificial genus in contrast to natural genus for living plants.
A form genus cannot reliably be assigned to a single family, however, it may be assigned to an order or other higher taxonomic rank. For example, Stigmaria is a form genus of the order Lepidodendrales which cannot be assigned to any one of the three families: Lepidodendraceae, Sigillariaceae or Bothrodendraceae.
When the relationships among different organs like stem, root, leaf and reproductive structures are established and can be assigned to the same family, then the genera can be called organ genera.
For example, stem genus Bucklandia, leaf genus Ptilophyllum, male fructification Weltrichia and female fructification Williamsonia are genetically related and assigned to the same family Williamsoniaceae. Thus, all are considered to be organ genera. However, there is no provision in the International Rules of Botanical Nomenclature for the use of organ genera.
During reconstruction, the palaeobotanists should select the earliest (after 1820) validly published generic name applied to any one of its parts as per Rule of Priority. He or she will use any one of the form genera as the generic name for the whole organism. Say, for example, the validly published female fructification, Williamsonia has been used for naming the whole plant.
Rules for naming form genera:
A particular suffix is used for naming a form genus which signifies the organ it belongs.
The suffixes applied to different plant parts are:
Essay # 4. Techniques for the
Study of Fossils:
The following points highlight the top four techniques used for the study of fossils. The techniques are: 1. Ground Thin Section 2. Film or Peel Technique 3. Transfer Technique 4. Maceration Technique.
1. Ground Thin Section:
This technique is suitable for the study of petrified fossils which preserve cellular details.
To begin with, the specimen is cut to a convenient size with a lapidiary’s saw or similar instrument and is smoothened on one surface with abrasive (300 or 400 carborundum). The smooth surface is attached to a glass slide with melted resin. Then, the specimen is cut as close to the glass as possible, thus a moderately thin section is obtained.
The thin section fixed to the glass slide is further ground on a revolving lap with 100 carborundum till sufficiently translucent, so that the section can be viewed under the microscope. Finally, the slice is mounted with a cover glass using suitable mounting media.
Advantage:
A thin section of petrified wood can be made that preserves the cellular structure in an unchanged condition.
Disadvantages:
(a) The technique needs extensive labour and is time-consuming,
(b) A small number of sections can be made from a given specimen, thus a lot of material is wasted.
2. Film or Peel Technique:
This is a conventional and most suitable technique for study of well-preserved petrifactions with considerable organic materials.
The specimen to be studied is ground and smoothened and is etched with 5% hydrochloric acid (if the material has calcium carbonate) and 10% hydrofluoric acid (if the material has silica) for 5 and 10 mns respectively. The etched surface is gently washed in running water for the removal of acid and is air-dried and then covered with a solution or a thin film of nitrocellulose.
The specimen is air-dried for at least 6 hours. Then the film is carefully peeled by loosening one edge and is permanently mounted on a slide with a cover- glass using suitable mounting media. An improvement in the peel technique has been made that makes possible rapid preparation of serial sections.
Advantages:
(a) A series of sections can be made from a single specimen.
(b) It is less expensive and quicker to prepare.
(c) The sections (peels) obtained are translucent, thinner and durable.
3. Transfer Technique:
This technique is most suitable for study of coalified compression which reveals additional details of venation, epidermal pattern and hairs.
The face of the specimen adjoining the rock surface is cleaned either mechanically or by washing in an acid for removal of rock particles. The prepared surface of the rock is coated with a solution of nitrocellulose or with a cellulose acetate film. When the film is dried, it is loosened from the rock surface.
Sometimes coalified materials are adhered to the film. Occasionally the film is treated with strong oxidising agent to make the film more transparent. Finally, the film is dried and permanently mounted on the slide with a coverglass using a suitable mounting medium.
Advantage:
This technique is very useful for study of coalified compression. It helps to learn about leaf form, venation pattern, stomatal and epidermal characteristics which are important features used in establishing systematics and phylogeny of extinct plants.
4. Maceration Technique:
This technique is most suitable for study of peat, lignite and coal. It is very useful for pollen and spore analysis.
The methods of peat, lignite and coal analysis are discussed schematically:
(a) Peat Analysis (Acetolysis Technique):
(b) Lignite and Coal Analysis:
Advantages:
1. A small portion of sediment is enough to get considerable amount of pollen and spores.
2. Due to the presence of resistant chemical sporopollenin in exine, pollen and spores are considerably preserved in deposits like peat/ coal/lignite, etc.
3. The maceration process is very simple and reliable.
Essay # 5. Modes of Fossil Preservation:
J. M. Schopf (1975) has recognised four distinctly different modes of fosssil preservation.
These are:
(a) Cellular permineralisation,
(b) Coalified compression,
(c) Authigenic preservation, and
(d) Duripartic preservation.
(a) Cellular Permineralisation:
This includes ail of the specimens that have previously been called petrifaction fossils. This involves infiltration followed by intracellular and interstitial precipitation of soluble minerals like silicates, carbonates, iron compounds through cell walls. The buried plant part undergoes partial disintegration to release free carbons which interact with the sulphides present in water and lead to the formation of carbonates of Ca, Mg, Fe.
Thus, the soluble minerals like carbonates, silicates etc. are deposited within cell walls through infiltration and precipitation. As mineral deposition continues within plant tissues, water is expelled as a result of compaction of the sediments. This causes the sediments and the buried plant part to solidify, thus completing cellular permineralisation.
This is analogous to the embedding of plant parts in paraffin blocks for microtome sectioning. Some of the well-known deposits that contain permineralised fossils are Rhynie chert, Gunflint chert, Bitter Spring Formation, petrified forest of Arizona, Deccan Intertrappean bed in India, etc.
Since a cellular permineralisation reveals the cellular details of the plant, such as nature of cortical cells, vascular bundle, pit connections, secondary wall thickenings, nature of ray cells etc. — it has served as a tool to unravel the taxonomic dispute.
(b) Coalified Compression:
Unmineralised parts are deposited in sediment, followed by softening of cell walls and collapse of internal cell spaces. This leads to loss of gas moisture and soluble materials. As a result of pressure exerted by accumulated sediments and water, the residues are altered and consolidated to form a black coaly deposit.
The distortion or compression is directional and only in the vertical plane. The splitting of rocks commonly yields the coalified compression on one face and its counterpart i.e. impression on the opposite face. On weathering, coaly part is lost or exfoliated, thus an impression may be revealed on the rock.
The extent of compression varies with the degree of hard nature of plant parts. Leaves are commonly retained in their natural form, but cylindrical or rounded organs such as stem, root and seed become dorsiventrally flattened. Most of the fossils recovered as coalified compression are remains of leaves, occasionally stems, roots, flowers, cones and seeds.
Since a coalified compression reveals leaf form, venation pattern, epidermal characteristics, etc., it has served as a tool in establishing systematic position and affinity of extinct plants.
(c) Authigenic Preservation or Cementation:
It involves very early cementation in soft sediments by iron and carbonate compounds. The plant material develops an electric charge as soon as it starts to decay. Thus, it attracts oppositely charged ionised particles of sediments. As a result sediments comprising of iron pyrite sphalerite, chalerite, agate, opal, carbonate along with mud and sand accumulate around the plant part.
Later, the sediments become cemented and the plant part is entombed in the sediment. The internal structure is degraded to form a cavity which is completely filled up by the surrounding sediments- After lithification, the external surface of the plant part is preserved as mold and the replaced internal structure of the plant part is called a cast.
In this process, the internal cellular details are not preserved. Stigmaria stump of Lepidodendron stem is a mold, while the pith cast of Calamites stem is an example of cast.
(d) Duripartic (Hard Part) Preservation:
Certain hard parts of both plants and animals are resistant to decay and oxidation and also resistant to physical distortion. Preservation of such hard parts without being changed by chemical or physical factors is referred to as duripartic preservation.
Such sites of preservation are abundant in the Phanerozoic epicontinental seas. Skeletal parts of lime-precipitating algae (Dasycladaceae, Characeae), coccoliths, diatom frustules (diatomite) and radiolarian skeletons (radiolarian ooze) are some examples of duripartic preservation.
Essay # 6. Fossilisation:
Fossilisation is a process of preservation of an organism or its parts in the form of fossils. In other way, fossilisation can be defined as what happens to an organism after its death and until its discovery as a fossil. It includes decomposition, postmortem transport, burial, compaction and other physical, chemical or biological activities which affect the remains of the organism.
The process that occurs between the death of an organism and its subsequent burial in the sediments are termed biostratinomy. This generally includes decomposition, abrasion, bioerosion, etc. On the other hand, diagenesis is the physical and/or chemical effects after burial. This includes dissolution, replacement or recrystallisation, etc.
Schematic representation of fossilisation process:
Conditions of Fossilisation:
It is a rare instance that an organism is preserved intact. Most of the known fossils are imperfect, where only external features are preserved. The perfect permineralised fossils showing cellular details are very rare.
The conditions for perfect fossilisation process can be categorised under the following heads:
1. Sites of fossilisation.
2. Nature of the tissue undergoing fossilisation.
3. Events that occur before, during and after fossilisation.
1. Sites of Fossilisation:
The preservation of plants depends on removing the organic materials from the zone of aerobic decomposition. This can most easily be accomplished by burying the plants in enclosed water reservoir in which sediments accumulate on the surface at a fast rate. Therefore, swamps, deltas, lakes, lowland flood plains are good sites for fossilisation.
2. Nature of the Tissue Undergoing Fossilisation:
The extent of preservation is directly proportional to the mechanical resistance of the cells, tissues and organs undergoing fossilisation. Thin-walled soft organs like flowers, juicy fruits, thin delicate leaves are unsuitable for preservation, while thick walled hard organs like stem, seeds etc. are preserved as compressions, casts or petrifactions for their resistant nature.
Plant cell walls primarily comprising of cellulose together with hemicellulose, lignin and suberin are far more resistant to decomposition. Cutinised epidermal layers of leaf, sporopolleninous exine of spores and pollen grains, siliceous wall of diatoms, hard skeletal parts of calcareous algae, suberised bark layers are better preserved for their high degree of resistance to destruction.
In a similar way, soft tissues like cortex, phloem and xylem parenchyma, sieve tubes, pith, etc., are rapidly decayed by microorganisms, while hard tissues such as tracheary elements, fibres, sclerenchyma, etc. are well preserved.
3. Events that Occur before, during and after Fossilisation:
(a) Events that Occur before Fossilisation:
Plant parts undergoing fossilisation must be deposited in an enclosed or protected water reservior in which fine-grained sediments accumulate on the surface with sufficient rapidity to cause quick burial.
In deep water body, the oxygen content is low, thus the intensity of microbial disintegration is also very low. Moreover, environment rich in humic acids and clay minerals can retard decay by blocking the chemical sites onto which decomposers fasten their degrading enzymes.
The plant parts must be preserved in situ (autochthonas deposition) which provides an environment with minimum destruction of plant parts. On the other hand, transportation of plant parts (allochthonas deposition) for long distance results in breakage and decay.
(b) Events that Occur during Fossilisation:
A plant part is subjected to many factors during the course of fossilisation which ultimately determines the characteristic of a fossil.
The depth of water in which the plant parts sink is important, because it can avoid two disintegrative forces such as (a) decay and hydrolysis, (b) mechanical action of water, wind, scouring (rubbing action) sand, and rolling stones.
The activities of bacteria and saprophytic fungi are greatly reduced, because in deep water the concentration of decomposers are very poor. Moreover, it affords good protection from wave action as intensive wave action or rolling boulders can reduce even the most resistant plant tissues to pulp.
The sediment that accumulates on the surface of plant part is usually of fine textures, such as clay, silt or fine sand favouring good preservation in greater details.
Most decomposers are aerobic and require oxygen for their metabolism. In deep water, oxygen content is low and plant parts are generally preserved in such anaerobic sediments. Thus, low oxygen content, and relatively high concentration of toxic substances also retard decay and hydrolysis.
The cells of the plant parts must be quickly filled with mineral deposits to avoid pilling up of the overlaying sediments.
(c) Events that Occur after Fossilisation:
Well-preserved fossils may suffer severe damages during destructive processes of earth, such as seismic activity, earthquake, volcanic eruption, etc., at the sites of their origin. Thus, undisturbed sediments provide a proper fossil record.