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CORE COURSE II BIODIVERSITY II (Unit-V PALEOBOTANY )

Concepts of Paleobotany, A general account on Geological Time Scale. Techniques for paleobotanical study. types: Compressions, incrustation, casts, molds, petrifactions, coalballs and compactions. Age determination and methods of study of . Systematic and Nomenclature of fossil . Paloclimates and fossil plants. Role of fossil in oil exploration and excavation, Paleopalynology.

Palaeobotany is the study of fossil plants. These fossils are found in the layers of earth and certain layers of rocks. It is also spelled as Palaeobotany (Gr. Palaeon = old; = study of plants). It is the branch of Palaeology. It deals with the identification of the plant remains from geological contexts and use for the biological construction of the plant environments (paleography) and both the evolutionary of plants with a bearing upon the of life in general. Palaeobotany includes the study of terrestrial plant fossils as well as the study of prehistoric marine autotrophs such as photo synthetic , weeds or . Its synonym is Palaeophytology. A closely related field is which is the study of fossilized and extinct and . Stenbery (1761-1838) is known as the father of Palaeobotany. The most ancient plant fossils were microscopic algae that lived more than one billion years ago during Precambrian times.

Concept of Paloebotany: Palaeobotany is the study of fossil plants. , i.e., the plants existed in the and now are entirely extinct. These fossils are found in the layers of earth and certain layers of rocks. It is also spelled as Palaeobotany (Gr. Palaeon = old; botany = study of plants)..It is the branch of Palaeology. It deals with the identification of the plant remains from geological contexts and use for the biological construction of the plant environments (paleography) and both the evolutionary history of plants with a bearing upon the evolution of life in general.

This is the difficult branch of Botany in respect that the fossil plants are difficult to obtain and they are rather scarce. Whenever the fossil plants are found, they are in parts which are to be coordinated. This is a tough process of the study. The fossils are cut in sections with a great difficulty and thereafter the preparations are made which require great labour, time and technique. The study of fossils is useful academically as well as economically. The academic interest lies in that their study clears up to a great extent the inter-relationships and evolution of the ancient groups of the plants. The economic interest lies in that some fossils are confined to definite strata of earth crust and they are associated with petroleum, coal and similar other things of economic value. Actually some coal fields were discovered only on account of the presence of certain fossils just above the coal mines. Another point of view of academic interest is that the fossils help in the determination of the of ancient time in different regions.

During period the earth was covered by very luxuriant forests and it is assumed that climate in those days was very uniform because the plants flourished in those times (i.e., Carboniferous period) were greatly the same on the whole surface of the earth. They also presume that the interior crust of the earth consists of many heavy metals like iron and lead. Whatsoever might have been the position but it is very clear that the first crust which formed the first surface much have been very uniform. Later on the water and air appeared and with the appearance of these two destructive factors the surface of the earth began to change. Rain and air affected the surface of the earth to great extent and at certain places the surface rose up in high mountains by bursting of interior of matter and at other places it sank down and formed seas and . The rocks withered off, the volcanic eruptions and other changes took place and the general topography of the earth was completely changed. Rivers coming down from mountains bring down pieces of rocks and large quantity of sand when they flow in the plains. This sand settles down at the bottom of the water. Along with this sand the parts of the plants and animals which get into that sand would have a chance of being preserved. This preservation takes place in different forms. The fossils are mainly found in the sand which has been brought down by the rivers from the mountains. This sand has become compressed and sedimentary rocks have been formed in the bottom of water. They contain the pieces of plants or entire bodies or bodies of animals which have been surrounded by mud and salt and they got the chance of being preserved.

This preservation takes in two forms: 1. Impressions: These impressions may be of the surface markings of the plant material or be the markings of the internal cavities in the plant material. The surrounding mud gets deposited on the surface of the plant material or gets into the cavity and their impressions are left. In some fine fossils even details of the venation, structure of stomata and epidermal have been studied. In such impressions the plant material itself has decayed or turned into coal. However, the surface impressions cannot be relied upon as we know that the plants, e.g., (a ). Ephedra (a ) and Casuarina (an angiosperm) gave same type of impressions, but all of them are widely differentiable and have no affinities and relationships with each other. 2. Petrifaction:

Another method of preservation of great importance is that the plant material when lying in water has become infiltrated with mineral matter such as lime, silica, magnesium salts and other similar substances. These salts penetrate the minute cells and the organic matter is being replaced by mineral salts. The original organic matter may be completely or partially changed but its outline and general structure is left in the form of these infiltrated substances. In coal mines generally the coal balls are found which are formed by the infiltration of calcium bicarbonate. In another method of preservation in which rare and small parts of plant material such as , grains, spores, etc., have been preserved. In this method the plant material happens to fall in resin or extruded by some plants, such as pine and others. Importance of Palaeobotany 1. Palaeobotany research is helpful in solving the problems connected with the formation of earth and evolutionary (gradual development) relationship among plants. 2. It helps to discover the earliest occurrence of different kinds of plants in the geological record. This knowledge of sequential occurrence of taxa is then used to develop an understanding of environmental relationship among groups of plants. 3. Palaeobotany research may be helpful in determining what fossil plants were like, and the kinds of animals that utilised then as food and habitat. This information may be helpful to infer the characteristics of the ancient environment including the type of climate in which plants grow (reconstruction of ancient ecological system and climate is known as palaeoecology and respectively.) 4. It is said that long ago continents moved. Palaeobotany helps in the study of problem. 5. The knowledge of Palaeobotany is also helpful in solving certain problems connected with the search of petroleum and coal. 6. Palaeobotany has also become important to the field of (the analysis and interpretation of plant tissues found at archaeological sites) primarily for the (a minute particle formed of mineral matter by a living plant and fossilized in rock) in and relative Palaeoethnobotany. (It is also called earth science, the study of fossil seeds and grains to farther archaeological knowledge, especially of the domestication of cereals. The proper chronological placement of a feature, object or happening in the geological time scale without reference to its absolute age is called relative dating).

A GENERAL ACCOUNT ON GEOLOGICAL TIME SCALE

The (GTS) is a system of chronological dating that relates geological strata (stratigraphy) to time. It is used by geologists, paleontologists, and other Earth to describe the timing and relationships of events that have occurred during Earth's history. Geological time scale is a record of earth’s history based on the organisms that lived at different times. The geological time scale is a system of chronological measurement that related stratigraphy (the study of rock strata, especially the distribution, deposition and age of sedimentary rocks) to time, and is used by the geologists, palentologists and other earth scientists to describe the time and relationship between the events that have occurred throughout earth’s history. The first geological time scale was proposed in 1913 by the British geologist Arthur Holmes (1890-1965). This was soon after the discovery of the radioactivity and using it Holmes estimated that the earth was about 4 billion years old (evidence from radioactive dating indicates that earth is about 4.5 million years old). This was much greater than previously believed. The geological time scale is divided into five main eras: Coenozoic, , , Proterozoic and Archezoic. Each era is divided into periods and each period is divided into epochs. It is as follows:

There is another kind of time division used – the eon. The entire interval of the existence of visible life is called the Phanerozoic eon. The great Precambrian expanse of time is divided into the Proterozoic, Archean and Hadean eons in order of increasing age. The names of the eras in the Phanerozoic eon (the eon of visible life) are the Cenozoic (recent life), Mesozoic (middle life) and Paleozoic (ancient life). The further subdivision of the eras into 12 periods is based on in-identifiable but less profound changes in life-forms.

Technique of Palaobotany: The different methods of the study of fossil plants are as follows: It is a laborious process and requires sufficiently great time. Usually the petrified specimens are cut in serial sections which give an idea of the actual structure of the fossil plant. These petrified pieces are cut into very fine slices by different methods. In one method each such piece is attached to glass plate and grounded to sufficient thinness and thereafter studied under the microscope. Another improved method of the study of these petrified specimens is to prepare the films of the material by special techniques. The method of preparing thin films is as follows: First of all the surface of the section of the petrified material is made smooth. If the material consists of calcium carbonate, then on the smooth surface of the slice a film of 5 per cent hydrochloric acid is allowed to act for five minutes. If the slice of the petrified material is silica then the film of 10 per cent hydrofluoric acid (HF) is allowed to act on the smooth surface for ten minutes so that the silica is dissolved. The surface of the petrified section by the action of these acids becomes rough on account of the dissolution of the mineral matter. If any organic matter remains on this surface, now put hot gelatin on the surface. As soon as the things dry up, they are removed and studied under the microscope. This process may be successful only in the case when organic matter is left in petrified specimens. In cases where organic matters are already decayed, such preparations are never good. The fossils naturally would be pieces of plants. It is very rare that entire plant could have preserved. This way, only pieces can be studied. In such type of study the individual pieces are given botanical names, just as in living plants. The botanical names of the fossil plants are not so significant as those of living ones. As they are represented by the pieces of the plants and, therefore, their generic names would be according to stem, and or any reproductive structure. The stems are usually given the generic names which end with ‘dendron’ () or ‘xylon’ such as, Lygenodendron or Cladoxylon. The end with ‘pteris’ or ‘phyllum’ and reproductive parts end with strobilus. This way, the paleobotany is the study of the parts of fossil plants and in certain cases marvellous results have been obtained. In the case of Lygenopteris, one of the Cycadofilicales of Carboniferous period which was found in pieces and later on the palaebotanists supported that all the pieces belonged to one particular plant. Later on after few years the complete intact plant was found.

1. Ground Thin Section Technique: The specimen to be studied is cut into small-sized sections. Its surfaces are smoothed with 400-carborundum. The smooth surface of the section of the specimen is mounted on a glass slide. It is warmed and coated with melted resin. The latter hardens upon cooling. The fastened specimens are cut to form very thin slices which are ground on revolving 100-carborundum lap. Liquid resin-mounting medium is used for mounting the sections. 2. Peel Technique: The first step of this technique involves the etching of the fossil surface with the help of some mineral acids (e.g., hydrofluoric acid) and the final step involves transfer of the exact fossil structure. Another mixture usually used for etching is prepared by mixing butyl acetate (1000ml), nitrocellulose (115gm), toluol (10ml), amyl alcohol (200ml) and dehydrated castor oil (5ml). Before using for etching purposes, this mixture is aged for at least two weeks. After etching the specimen surface is washed with water, dried and covered with nitrocellulose. Wait for a few hours. The so formed film will dry during this period. It is peeled off from the specimen and studied. 3. Transfer Technique: Delicate fossil materials are studied by this technique. Several methods are used in the form of transfer technique In the Ash by film transfer method, peel solution is coated on the delicate fossil material adjoining the rock surface. When the solution dries, the portion of the rock having fossil material is removed. 25% hydrofluoric acid is then used for dissolving the rock material. 4. Maceration Technique: In the usual method of maceration technique, the fossil material is immersed in a mixture of 5% KOH and Cone. HNO3 for one week. The material is then washed thoroughly with water so that the acid is completely removed. It is then incubated with the solution of NaOH. Hydrofluoric acid is used for cleaning the thus separated cuticularized parts of the fossil material. 5. X-ray Technique: Highly sensitive celluloid films are used to obtain X-ray photographs of the fossil specimens. 6. Microtomy Technique: Fossil specimens, specially their macerated tissues, are embedded in celloidin or wax before microtomy. Sectioning of the embedded material is done by usual process of microtomy. The sectioned materials are stained and studied. Types of Fossils: (i) Sedimentary Rocks (e.g. Coal): Majority of plant materials are preserved as fossils in sedimentary rocks. Coal is the best known example of sedimentary rock. Sediments of plant origin are crushed by overlying pressure and form coal. Present coal belt in the world, therefore, represents dense forests of the world of earlier times. Least metamorphosed coal shows maximum details of fossilized or preserved plant material. Therefore, lignite’s (early stage of coal formation) carry less crushed plant parts and their details can be studied easily. Plant parts get excessively crushed in bituminous coal and anthracite coal because they show more degree of metamorphosis than lignite coal. Bituminous coal is of great importance in the study of palynology because pollen grains are best preserved in this type of coal.

(ii) Amber: The fossilized plant resin secreted by coniferous trees that grew in very early times is called amber. This “very early time” in the geological past ranged from Carboniferous (i.e. about 345 million years ago) to Pleistocene (i.e. about 2.5 million years ago). Fungal spores, pollen grains, etc. were trapped in this resin before fossilization. The resin fossilized into amber and inside this were left spores, pollen grains, etc. Amber is, therefore, an example of fossils within fossil. (iii) Diatomite: are unicellular algae belonging to class Bacillariophyceae. Their walls have silicon deposits. The sedimentary rock formed by the remains of diatoms is called diatomite. In due course of time, diatoms keep on depositing at the base of sea, oceans or lakes and form sedimentary rock. (iv) Pseudo-Fossils or Dendrites: Pseudo-fossils or dendrites are completely inorganic structures of various types. They often resemble plant organs. Their formation takes place by the deposition of minerals due to seepage or percolation of water in rock crevices. They superficially resemble leaves of . (v) Mummification: The process of the formation of fossils in ice-frozen environments in the polar regions is termed as mummification. The moisture of the of the organism gets converted to very small or microcrystals of ice. It is almost a process similar to deep freezing. (vi) Biochemical Fossils: These are the fossils which consists of chemical substances like , amino acids, aromatic acids, flavonoids, branched hydrocarbons and steroids. These have been reported to be present either in the fossilized remains of organisms or in the rocks. Niklas (1981) reported biochemical fossils of the substances related to sporopollenin, lignin, cutin, cellulose, etc. Type of fossilization (i) Impressions: This is a type of fossil in which impression or negative replica of the plant part is clearly visible on the rock upon splitting. The surface of the plant part involved is visible in this type of fossil. The entire shape of the organ is clearly visible in an impression but cellular details are not visible. During the course of the formation of impression type of fossil there is no involvement of the organic matter. A leaf or any organic part falling on semi-stiff clay easily leaves an impression on its surface. In course of time this impression becomes permanent when the clay turns into stone. Such impressions are often very clear showing full details of venation, etc.

The impression is often of a darker colour than the surface of the rock below because it very often retains some of the organic material. Some specimens are extremely beautiful looking like paintings or base-relief of the actual twigs. In some well-preserved material at least the skin or the remains intact so that structures like stomata are clearly seen in good preparations.

(ii) Molds: As the name indicates, molds are formed by three-dimensional structures, dissolved by seepage of ground water, leaving a hollow cavity in the rock. Such hollow cavities resemble with the original organ in size and shape. External features of the plant in three-dimensional view are best seen and studied in molds, e.g. ornamentation of seeds and . The process of mold formation is similar to modern day sculpture making. A sculptor makes the “original” with wax or and prepares the “mold”. At the time of preparation of mold, the original statue of wax or wood is removed from inside. The molten material is now poured into the mold to make statue of this material. Exactly in the similar fashion, molds are formed in the conditions when, prior to the crushing of plant part, the sediment surrounding it hardens. During course of time, the internal plant material dissolves, and the hollow of the sediment is the “mold”.

(iii) Casts: Decay of tissue in an organism results in the formation of a hollow. When this hollow gets filled with mineral matter i.e. sediments, it results in the formation of a cast. An exact replica of the original plant material is thus resulted. Like molds, there is also no involvement of the actual part of the plant in casts. Fruits, hard seeds and tree trunks are commonly fossilized as casts. In casts also, a three-dimensional view of the organ is seen similar to molds. Cast or Incrustation: One of the commonest types of fossils is the cast or incrustation. In the formation of this, the plant part gets covered up by sand or mud. In course of time the plant material inside rots away leaving a hollow. This cavity, a-gain, gets filled up by some rock forming material. In course of time the inside as well as the outside solidifies into stone from which the external part may be peeled off leaving an exact cast of the plant material showing all its surface features. The casts are as correct as one may obtain from clay or plaster or paris moulds today. Figure 506 shows a cast fossil of an ancient Lycopod stump. Internal casts of pith cavities may also be preserved in the same way (Fig. 507). A cast fossil does not actually contain any part of the original plant but

(iv) Compressions: When bulk of the plant material gets compressed in layers of sediment, the fossils are called compressions. By the laying of the additional sediments from above, water squeezes out of the parts of organisms, and this makes them more compact and flattened. Ultimately, a thin carbonaceous film remains in the compression fossil, and this corresponds to the original outline of the parts of the organisms. Differing from impression type of fossils (in which no cellular details can be seen) some cellular details can be seen in compression type of fossils. These cellular details include epidermal hairs, cuticularized epidermal cells, spores, etc. If compression type of fossils are formed in low pressure and low heat, some more cellular details (e.g. plasmodesmata, with grana, nuclei with chromatin, micro-fibrillar organization of , etc., may be observed, although rarely. Much of the organic matter of the plant is preserved in compression type of fossils.

Compression is only a degree of impression when the organic remain of the plant part actually remains in the fossil but is highly compressed. The great pressure under which fossilisation takes place flattens out all round or solid organs so that what remains in the fossil is usually a carbonaceous film. But, in good compressions it has been possible to swell out the organ by some chemical treatments so that some details become visible. A good type of compression fossil is the clay nodule. In this the plant material gets encased in a ball of clay, gets compressed and the clay ball turns into stone. On splitting open this nodule the organic remains is found very much intact (Fig. 509) although not as perfectly as in a petrified fossil.

(v) Petrifactions: Petrifactions are formed when parts of the plant are completely submerged in water reservoirs containing dissolved minerals. In the process of their formation, several types of soluble minerals infiltrate the cells and intercellular spaces replacing the water and organic molecules. The soluble minerals include carbonates, silicates and iron compounds. The minerals surrounding the cellular remains precipitate and form the rock matrix. This precipitation is resulted due to gradual evaporation of moisture. A state of super-saturation is resulted. In this way a petrified fossil is a mass of plant tissue infiltrated with the hardened mineral substances so that a large part of the internal structure is preserved. It is like that of a process of embedding the plant material in paraffin wax in the laboratory. The air and water in tissues and cells are replaced by the impregnating material in a petrifaction. The cellular details are much better preserved in petrifaction than in a compression.

Petrification is the best but perhaps the rarest type of fossilisation. This literally means transformation of the organic tissues into stone. Although the actual process of petrification is not very well understood, it is clear that no ‘molecule by molecule’ replacement of the organic, molecules by mineral molecules takes place. The buried plant material absorbs mineral solutions like silicates, carbonates, sulphates, phos- phates, etc., and infiltration followed by precipitation takes place so that silica, calcium carbonate, magnesium carbonate, iron sulphide, etc., get impregnated within the tissues. Most of the organic material may get destroyed but at least some original cell wall compounds often remain. The whole structure becomes stone like so that fine sections may be obtained by stone sectioning methods and exact tissue structures (Fig. 504) may be observed under the micro- scope. Anatomical structures of ancient plants are beautifully obtained from such petrifications. Petrifications are usually bits of stems, twigs, seeds, sporangia, etc. Silicified bits of wood are often found. Calcified fossils are also known. The best examples are, however, the coal balls. Goal balls (Figs. 504 & 505) are irregularly rounded masses ranging in diameter from a few millimetres to a metre. These occur often in great numbers within chunks of coal. Each ball is a mass of calcium and magnesium carbonate with, sometimes, iron sulphide. These show petrified remains of a great number of plant fragments representing the debris of those days. Even delicate parts remain intact in the coal balls so that the anatomy as well as the morphology is clear.

How is Fossils Studied? (i) Impressions: These are studied generally by preparing artificial casts which are also called positives. (ii) Molds: These are studied exactly like that of impressions. (iii) Compressions: These can be studied either by transfer technique or by maceration in Schulze’s solution, or by first softening and then cutting thin sections by microtomy. Microtomy is done by embedding the material in plastic. (iv) Petrifactions: Petrified materials are studied by peel section method. In the earlier days, palaeobotanists used to study fossils by very hard and time-consuming techniques. They used to cut thin sections of rock containing fossils by a circular toothless saw. Periphery of this saw had diamond particles. Etching technique or peel method was then discovered In this method polished surface of the rock is etched by sulphuric acid or nitric acid. This surface is then flooded with acetone and a film of cellulose acetate is rolled down. After some time, when the film is dry, it is removed from the surface of specimen. A clear impression of specimen develops on the film. By definite methods, it is made permanent and studied under microscope. (v) Some Modern Methods of Studying Fossils: Fossils are now studied in modern laboratories by using transmission and scanning electron microscope, interference microscope, phase contrast microscope and methods of X-ray analysis. How are Fossils Reconstructed and Named? Fossils of the entire plants have only rarely been reported. Generally an organ or a part of the plant is seen preserved as fossils. Reconstruction of the fossils of various vegetative and reproductive parts in the form of a single plant is, therefore, a very big problem of the palaeobotanists. A detailed scientific knowledge is required to reconstruct a plant. Besides this, some direct or indirect evidences are also applied to conclude that the organ belongs to a particular plant. Some Factors Which Help in Reconstruction of Plant Fossils: Some of the factors, on the basis of which the fossils are reconstructed, are under mentioned: 1. Discovery of an organic connection between the fossils of two parts. It acts as a direct evidence to conclude that these two parts belong to the same plant. 2 Structural similarity between the fossils. 3. Regular occurrence of the same type of fossils in the same area. 4. of different fossils containing similar type of pollen grains. 5. Relationship between different fossils is also assigned on the basis of . Concept of Form Genus or Organ Genus: Separate generic names are given by the palaeobotanists to the fossils of the detached organs or fragments. Each of these organs or fragments is called a form genus or organ genus. Similar to the present day living plants, binomial system of nomenclature is also applied to name these form genera. Considering the organ genera of Pentoxylales as an example, various organs of the Type genus Pentoxylon have been named as under: (i) Stem has been named as Pentoxylon sahnii. (ii) Leaf has been named under organ genus Nipaniophyllum raoi. (iii) Male fructification has been named under organ genus Sahnia nipaniensis. (iv) Female fructification has been named under organ genus Carnoconites compactum. Using all these organ genera, Sahni (1948) named the complete plant as the Type genus Pentoxylon. Determination of Age of Fossils: By finding the age of rock, the age of fossils is calculated. The age of rock is calculated by using radiometric dating techniques. In these techniques, various radioactive isotopes of Uranium (236U, 238U), Thorium (232U) and Potassium (40K) are used. All these radioactive isotopes are also called “geological clocks”. The radioactive isotopes decay and lead to stable isotopes. In this process, the energy is released. The rate of decaying of any radioactive isotope and its giving rise to stable isotope, is always constant. The age of rock and plant can thus be calculated by measuring relative quantities of radioactive isotope and the stable isotope. The use of radioactive carbon is called carbon dating. Carbon dating technique is used in ascertaining the age of specimens of plants and animals, back to about 60,000 years. It is because of the fact that half-life of carbon is 5568 + 30 years. Important Strata of Paleobotany: The Palaeozoic and Mesozoic strata are very important from the study point of view of the fossil plants. It is in the upper middle Palaeozoic, i.e., the strata, we come across with the first land plants such as lycopods belonging to Lycopodiales, Equisetales, the seeded ferns, the primitive , the , etc. In the late Palaeozoic, i.e., both in the upper and lower. Carboniferous strata the earth was covered up by the very luxuriant forests. These forests were formed by lycopods, horsetails, seeded ferns and later on with primitive gymnosperms. The Carboniferous strata is most important. The coal mines are situated in this strata. The coal mines are the result of dense forests having got submerged in those times. The Mesozoic is also very important from the point of view that the first angiosperms made their appearance; otherwise the higher gymnosperms formed luxuriant forests in those times. In the later Mesozoic some of the gymnosperms disappeared. Majority of the Cycads disappeared and only a few forms have been left up to the present day. In India the most important strata is described technically the ‘Gondwana system’ named after Gondwana Kingdom. For the first time the rocks of this period were discovered near Narmada river. Coal ball

A coal ball is a type of concretion, varying in shape from an imperfect sphere to a flat-lying, irregular slab. Coal balls were formed in Carboniferous Period swamps and mires, when peat was prevented from being turned into coal by the high amount of calcite surrounding the peat; the calcite caused it to be turned into stone instead. As such, despite not actually being made of coal, the coal ball owes its name to its similar origins as well as its similar shape with actual coal. Coal balls often preserve a remarkable record of the microscopic tissue structure of Carboniferous swamp and mire plants, which would otherwise have been completely destroyed. Their unique preservation of Carboniferous plants makes them valuable to scientists, who cut and peel the coal balls to research the geological past.

Coal balls may be found in coal seams across North America and Eurasia. North American coal balls are more widespread, both stratigraphically and geologically, than those in Europe. The oldest known coal balls date from the Namurian stage of the Carboniferous; they were found in Germany and on the territory of former Czechoslovakia.

Coal balls are not made of coal; they are non-flammable and useless for fuel. Coal balls are calcium-rich permineralised life forms, mostly containing calcium and magnesium carbonates, pyrite, and quartz. Other minerals, including gypsum, illite, kaolinite, and lepidocrocite also appear in coal balls, albeit in lesser quantities Although coal balls are usually about the size of a man's fist, their sizes vary greatly, ranging from that of a walnut up to 3 feet (1 m) in diameter. Coal balls have been found that were smaller than a thimble. Coal balls commonly contain dolomites, aragonite, and masses of organic matter at various stages of decomposition. Hooker and Binney analysed a coal ball and found "a lack of coniferous wood ... and of ferns" and noted that the discovered plant matter "appear[ed] to [have been arranged] just as they fell from the plants that produced them". Coal balls usually do not preserve the leaves of plants. In 1962, Sergius Mamay and Ellis Yochelson analysed North American coal balls. Their discovery of marine organisms led to classification of coal balls were sorted into three types: normal (sometimes known as floral), containing only plant matter; faunal, containing animal fossils only; and mixed, containing both plant and animal material. Mixed coal balls are further divided into heterogeneous, where the plant and animal material was distinctly separated; and homogeneous, lacking that separation. The quality of preservation in coal balls varies from no preservation to the point of being able to analyse the cellular structures. Some coal balls contain preserved root hairs,pollen, and spores, and are described as being "more or less perfectly preserved", containing "not what used to be the plant", but rather, the plant itself. Others have been found to be "botanically worthless" with the organic matter having deteriorated before becoming a coal ball Coal balls with well-preserved contents are useful to paleobotanists. They have been used to analyse the geographical distribution of vegetation: for example, providing evidence that Ukrainian and Oklahoman plants of the tropical belt were once the same. Research on coal balls has also led to the discovery of more than 130 genera and 350 species. Three main factors determine the quality of preserved material in a coal ball: the mineral constituents, the speed of the burial process, and the degree of compression before undergoing permineralisation. Generally, coal balls resulting from remains that have a quick burial with little decay and pressure are better preserved, although plant remains in most coal balls almost always show differing signs of decay and collapse. Coal balls containing quantities of iron sulfide have far lower preservation than coal balls permineralised by magnesium or calcium carbonate, which has earned iron sulfide the title "chief curse of the coal ball hunter". Coal balls were first found in England, and later in other parts of the world, including Australia, Belgium, the Netherlands, the former Czechoslovakia, Germany, Ukraine, China, and Spain. They were also encountered in North America, where they are geographically widespread compared to Europe; in the United States, coal balls have been found from Kansas to the Illinois Basin to the Appalachian region. The oldest coal balls were from the early end of the Namurian stage (326 to 313 mya) and discovered in Germany and former Czechoslovakia, but their ages generally range from the Permian (299 to 251 mya) to the Upper Carboniferous. Some coal balls from the US vary in age from the later end of the Westphalian (roughly 313 to 304 mya) to the later Stephanian (roughly 304 to 299 mya). European coal balls are generally from the early end of the Westphalian Stage. In coal seams, coal balls are completely surrounded by coal.] They are often found randomly scattered throughout the seam in isolated groups, usually in the upper half of the seam. Their occurrence in coal seams can be either extremely sporadic or regular; many coal seams have been found to contain no coal balls, while others have been found to contain so many coal balls that miners avoid the area entirely.

How do we know the ages of fossils and fossil-bearing rocks? Scientists combine several well-tested techniques to find out the ages of fossils. The most important are Relative Dating, in which fossils and layers of rock are placed in order from older to younger, and Radiometric Dating, which allows the actual ages of certain types of rock to be calculated. Relative Dating: Fossils are found in sedimentary rocks that formed when eroded sediments piled up in low-lying places such as river flood plains, lake bottoms or floors. Sedimentary rock typically is layered, with the layers derived from different periods of sediment accumulation. Almost any place where the forces of erosion - or road crews - have carved through sedimentary rock is a good place to look for rock layers stacked up in the exposed rock face. When you look at a layer cake, you know that the layer at the bottom was the first one the baker put on the plate, and the upper ones were added later. In the same way, geologists figure out the relative ages of fossils and sedimentary rock layers; rock layers, and the fossils they contain, toward the bottom of a stack of sediments are older than those found higher in the stack. Radiometric Dating: Until the middle of the last century, "older" or "younger" was the best scientists could do when assigning ages to fossils. There was no way to calculate an "absolute" age (in years) for any fossil or rock layer. But after scientists learned that the nuclear decay of radioactive elements takes place at a predictable rate, they realized that the traces of radioactive elements present in certain types of rock, such as hardened lava and tuff (formed from compacted volcanic ash), could be analyzed chemically to determine the ages, in years, of those rocks. Putting Relative and Radiometric Dating Together:Once it was possible to measure the ages of volcanic layers in a stack of sedimentary rock, the entire sequence could be pinned to the absolute time scale. In the Wyoming landscape shown below left, for example, the gray ash layer was found to be 73 million years old. This means that fossils in rock layers below the tuff are older than 73 million years, and those above the tuff are younger. Fossils found embedded within the ash, including the fossil leaves shown below right, are the same age as the ash: 73 million years old. Despite seeming like a relatively stable place, the Earth's surface has changed dramatically over the past 4.6 billion years. Mountains have been built and eroded, continents and oceans have moved great distances, and the Earth has fluctuated from being extremely cold and almost completely covered with ice to being very warm and ice-free. These changes typically occur so slowly that they are barely detectable over the span of a human life, yet even at this instant, the Earth's surface is moving and changing. As these changes have occurred, organisms have evolved, and remnants of some have been preserved as fossils. A fossil can be studied to determine what kind of organism it represents, how the organism lived, and how it was preserved. However, by itself a fossil has little meaning unless it is placed within some context. The age of the fossil must be determined so it can be compared to other fossil species from the same time period. Understanding the ages of related fossil species helps scientists piece together the evolutionary history of a group of organisms. For example, based on the primate fossil record, scientists know that living primates evolved from fossil primates and that this evolutionary history took tens of millions of years. By comparing fossils of different primate species, scientists can examine how features changed and how primates evolved through time. However, the age of each fossil primate needs to be determined so that fossils of the same age found in different parts of the world and fossils of different ages can be compared. There are three general approaches that allow scientists to date geological materials and answer the question: "How old is this fossil?" First, the relative age of a fossil can be determined. Relative dating puts geologic events in chronological order without requiring that a specific numerical age be assigned to each event. Second, it is possible to determine the numerical age for fossils or earth materials. Numerical ages estimate the date of a geological event and can sometimes reveal quite precisely when a fossil species existed in time. Third, magnetism in rocks can be used to estimate the age of a fossil site. This method uses the orientation of the Earth's magnetic field, which has changed through time, to determine ages for fossils and rocks. Relative dating to determine the age of rocks and fossils Geologists have established a set of principles that can be applied to sedimentary and volcanic rocks that are exposed at the Earth's surface to determine the relative ages of geological events preserved in the rock record. For example, in the rocks exposed in the walls of the Grand Canyon (Figure 1) there are many horizontal layers, which are called strata. The study of strata is called stratigraphy, and using a few basic principles, it is possible to work out the relative ages of rocks. The principles of original horizontality, superposition, and cross-cutting relationships allow events to be ordered at a single location. However, they do not reveal the relative ages of rocks preserved in two different areas. In this case, fossils can be useful tools for understanding the relative ages of rocks. Each fossil species reflects a unique period of time in Earth's history. The principle of faunal succession states that different fossil species always appear and disappear in the same order, and that once a fossil species goes extinct, it disappears and cannot reappear in younger rocks (Figure 4).

Figure 4: The principle of faunal succession allows scientists to use the fossils to understand the relative age of rocks and fossils. Fossils occur for a distinct, limited interval of time. In the figure, that distinct age range for each fossil species is indicated by the grey arrows underlying the picture of each fossil. The position of the lower arrowhead indicates the first occurrence of the fossil and the upper arrowhead indicates its last occurrence – when it went extinct. Using the overlapping age ranges of multiple fossils, it is possible to determine the relative age of the fossil species (i.e., the relative interval of time during which that fossil species occurred). For example, there is a specific interval of time, indicated by the red box, during which both the blue ammonite and orange ammonite co-existed. If both the blue and orange ammonites are found together, the rock must have been deposited during the time interval indicated by the red box, which represents the time during which both fossil species co-existed. In this figure, the unknown fossil, a red sponge, occurs with five other fossils in fossil assemblage B. Fossil assemblage B includes the index fossils the orange ammonite and the blue ammonite, meaning that assemblage B must have been deposited during the interval of time indicated by the red box. Because, the unknown fossil, the red sponge, was found with the fossils in fossil assemblage B it also must have existed during the interval of time indicated by the red box. Fossil species that are used to distinguish one layer from another are called index fossils. Index fossils occur for a limited interval of time. Usually index fossils are fossil organisms that are common, easily identified, and found across a large area. Because they are often rare, primate fossils are not usually good index fossils. Organisms like pigs and rodents are more typically used because they are more common, widely distributed, and evolve relatively rapidly. Using the principle of faunal succession, if an unidentified fossil is found in the same rock layer as an index fossil, the two species must have existed during the same period of time (Figure 4). If the same index fossil is found in different areas, the strata in each area were likely deposited at the same time. Thus, the principle of faunal succession makes it possible to determine the relative age of unknown fossils and correlate fossil sites across large discontinuous areas. Determining the numerical age of rocks and fossils Unlike relative dating methods, absolute dating methods provide chronological estimates of the age of certain geological materials associated with fossils, and even direct age measurements of the fossil material itself. To establish the age of a rock or a fossil, researchers use some type of clock to determine the date it was formed. Geologists commonly use radiometric dating methods, based on the natural radioactive decay of certain elements such as potassium and carbon, as reliable clocks to date ancient events. Geologists also use other methods - such as electron spin resonance and thermoluminescence, which assess the effects of radioactivity on the accumulation of electrons in imperfections, or "traps," in the crystal structure of a mineral - to determine the age of the rocks or fossils. All elements contain protons and neutrons, located in the atomic nucleus, and electrons that orbit around the nucleus (Figure 5a). In each element, the number of protons is constant while the number of neutrons and electrons can vary. Atoms of the same element but with different number of neutrons are called isotopes of that element. Each isotope is identified by its atomic mass, which is the number of protons plus neutrons. For example, the element carbon has six protons, but can have six, seven, or eight neutrons. Thus, carbon has three isotopes: carbon 12 (12C), carbon 13 (13C), and carbon 14 (14C) (Figure 5a).

Figure 5: Radioactive isotopes and how they decay through time. (a) Carbon has three isotopes with different numbers of neutrons: carbon 12 (C12, 6 protons + 6 neutrons), carbon 13 (C13, 6 protons + 7 neutrons), and carbon 14 (C14, 6 protons + 8 neutrons). C12 and C13 are stable. The atomic nucleus in C14 is unstable making the isotope radioactive. Because it is unstable, occasionally C14 undergoes radioactive decay to become stable nitrogen (N14). (b) The radioactive atoms (parent isotopes) in any mineral decay over time into stable daughter isotopes. The amount of time it takes for half of the parent isotopes to decay into daughter isotopes is known as the half-life of the radioactive isotope.

Most isotopes found on Earth are generally stable and do not change. However some isotopes, like 14C, have an unstable nucleus and are radioactive. This means that occasionally the unstable isotope will change its number of protons, neutrons, or both. This change is called radioactive decay. For example, unstable 14C transforms to stable nitrogen (14N). The atomic nucleus that decays is called the parent isotope. The product of the decay is called the daughter isotope. In the example, 14C is the parent and 14N is the daughter.

Some minerals in rocks and organic matter (e.g., wood, bones, and shells) can contain radioactive isotopes. The abundances of parent and daughter isotopes in a sample can be measured and used to determine their age. This method is known as radiometric dating. Some commonly used dating methods are summarized in Table 1. The rate of decay for many radioactive isotopes has been measured and does not change over time. Thus, each radioactive isotope has been decaying at the same rate since it was formed, ticking along regularly like a clock. For example, when potassium is incorporated into a mineral that forms when lava cools, there is no argon from previous decay (argon, a gas, escapes into the atmosphere while the lava is still molten). When that mineral forms and the rock cools enough that argon can no longer escape, the "radiometric clock" starts. Over time, the radioactive isotope of potassium decays slowly into stable argon, which accumulates in the mineral. The amount of time that it takes for half of the parent isotope to decay into daughter isotopes is called the half-life of an isotope (Figure 5b). When the quantities of the parent and daughter isotopes are equal, one half-life has occurred. If the half life of an isotope is known, the abundance of the parent and daughter isotopes can be measured and the amount of time that has elapsed since the "radiometric clock" started can be calculated. For example, if the measured abundance of 14C and 14N in a bone are equal, one half-life has passed and the bone is 5,730 years old (an amount equal to the half-life of 14C). If there is three times less 14C than 14N in the bone, two half lives have passed and the sample is 11,460 years old. However, if the bone is 70,000 years or older the amount of 14C left in the bone will be too small to measure accurately. Thus, is only useful for measuring things that were formed in the relatively recent geologic past. Luckily, there are methods, such as the commonly used potassium-argon (K-Ar) method, that allows dating of materials that are beyond the limit of radiocarbon dating (Table 1).

Age Range of Name of Method Material Dated Methodology Application

Organic material such as Radioactive decay of 14C in Radiocarbon 1 - 70,000 years bones, wood, , organic matter after removal shells from biosphere

1,000 - billion of Potassium-bearing minerals Radioactive decay of 40K in K-Ar dating years and glasses rocks and minerals

Radioactive decay of 10,000 - billion of Uranium-Lead Uranium-bearing minerals uranium to lead via two years separate decay chains

1,000 - 500,000 Uranium-bearing minerals, Radioactive decay of 234U Uranium series 230Th years corals, shells, teeth, CaCO3 to

Measurement of damage 1,000 - billion of Uranium-bearing minerals tracks in glass and minerals Fission track years and glasses from the radioactive decay of 238U

Burial or heating age based on the accumulation of Luminescence (optically or 1,000 - 1,000,000 Quartz, feldspar, stone radiation-induced damage to thermally stimulated) years tools, pottery electron sitting in mineral lattices

Electron Spin Resonance 1,000 - 3,000,000 Uranium-bearing materials Burial age based on (ESR) years in which uranium has been abundance of radiation- absorbed from outside induced paramagnetic sources centers in mineral lattices

Radioactive decay of Typically quartz or olivine 1,000 - 5,000,000 cosmic-ray generated Cosmogenic Nuclides from volcanic or years nuclides in surficial sedimentary rocks environments

Measurement of ancient 20,000 - billion of Sedimentary and volcanic polarity of the earth's Magnetostratigraphy years rocks magnetic field recorded in a stratigraphic succession

Uses chemistry and age of 100 - billions of volcanic deposits to establish Tephrochronology Volcanic ejecta years links between distant stratigraphic successions

Table 1. Comparison of commonly used dating methods.

Radiation, which is a byproduct of radioactive decay, causes electrons to dislodge from their normal position in atoms and become trapped in imperfections in the crystal structure of the material. Dating methods like thermoluminescence, optical stimulating luminescence and electron spin resonance, measure the accumulation of electrons in these imperfections, or "traps," in the crystal structure of the material. If the amount of radiation to which an object is exposed remains constant, the amount of electrons trapped in the imperfections in the crystal structure of the material will be proportional to the age of the material. These methods are applicable to materials that are up to about 100,000 years old. However, once rocks or fossils become much older than that, all of the "traps" in the crystal structures become full and no more electrons can accumulate, even if they are dislodged.

Using paleomagnetism to date rocks and fossils:

The Earth is like a gigantic magnet. It has a magnetic north and south pole and its magnetic field is everywhere (Figure 6a). Just as the magnetic needle in a compass will point toward magnetic north, small magnetic minerals that occur naturally in rocks point toward magnetic north, approximately parallel to the Earth's magnetic field. Because of this, magnetic minerals in rocks are excellent recorders of the orientation, or polarity, of the Earth's magnetic field.

Figure 6: The earth’s magnetic field can be measured to determine the polarity of a rock sample. (a) The earth is surrounded by a magnetic field generated by the magnetism in the core of the earth. Small magnetic grains in rocks will orient themselves to be parallel to the direction of the magnetic field pointing towards the north pole. (b) The geomagnetic polarity time scale shows how the polarity of the earth’s magnetic field has changed through time. Black bands indicate times of normal polarity and white bands indicate times of reversed polarity. Through geologic time, the polarity of the Earth's magnetic field has switched, causing reversals in polarity. The Earth's magnetic field is generated by electrical currents that are produced by convection in the Earth's core. During magnetic reversals, there are probably changes in convection in the Earth's core leading to changes in the magnetic field. The Earth's magnetic field has reversed many times during its history. When the magnetic north pole is close to the geographic north pole (as it is today), it is called normal polarity. Reversed polarity is when the magnetic "north" is near the geographic south pole. Using radiometric dates and measurements of the ancient magnetic polarity in volcanic and sedimentary rocks (termed paleomagnetism), geologists have been able to determine precisely when magnetic reversals occurred in the past. Combined observations of this type have led to the development of the geomagnetic polarity time scale (GPTS) (Figure 6b). The GPTS is divided into periods of normal polarity and reversed polarity. Geologists can measure the paleomagnetism of rocks at a site to reveal its record of ancient magnetic reversals. Every reversal looks the same in the rock record, so other lines of evidence are needed to correlate the site to the GPTS. Information such as index fossils or radiometric dates can be used to correlate a particular paleomagnetic reversal to a known reversal in the GPTS. Once one reversal has been related to the GPTS, the numerical age of the entire sequence can be determined.

Acknowledgement www.google.com All the above study materials were downloaded from Google search engine and edited by me.