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Fossils Are Generally Defined As Any Direct Or Indirect Evidence of Pre-Historic Life (Excluding Those Fossil-Like Objects That

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Fossils Are Generally Defined As Any Direct Or Indirect Evidence of Pre-Historic Life (Excluding Those Fossil-Like Objects That 1 Diversity of Seed Plants and their Systematics Gymnosperms I Fossils Dr. NUPUR BHOWMIK Department of Botany University of Allahabad Senate Hall Allahabad – 211002 [email protected] Date of submission: 27/06/2006 2 Fossils Fossils are generally defined as any direct or indirect evidence of pre-historic life (excluding those fossil-like objects that have collected on earth since the beginning of recorded history, approximately 6000 years) found in rocks extending from Arctic to the Antarctic. The exposed strata of rock containing fossils may be found along rivers, streams, along sea-shores, in excavations etc. Some fossils have been used to identify stratigraphic layers of earth’s surface and have been called as “index fossils”. A good "index fossil" is abundantly represented in the rock strata and can be easily identified. Pollen grains and spores have been used as "index" fossils in biostratigraphy and in correlation of rock units. Some megafossils such as leaves and seeds have aided in correlating rock units that are widely separated geographically. A vast majority of plant fossils are preserved in sedimentary rocks. Formation of sedimentary rock occurs when deposition of rock particles of various sizes collects in a body of water. The particles are derived from erosion of igneous, metamorphic or other sedimentary rocks by agents of denudation like wind, water, freezing, erupting volcanoes and movement of glaciers. Moving water carries sand grains, silt particles, pebbles, seeds, leaves, twigs etc. and slows down when it meets a body of standing water such as pond, lake or sea, where the load begins to be dropped or to sediment out. The heavier particles are dropped first, closer to the shore and lighter particles are dropped farther from shore. In this way a delta is built up. This deltaic sediment may become converted into sedimentary rock – the sand becoming sandstone and the mud becoming shale. Often plant parts carried by the stream, sinks along with mud and sand and if not decayed, become incorporated into the sediment and finally included in the rock. As sediments accumulate, water is squeezed out of them, so they become much more compact and plant fragments contained within them become flattened. But plant fossils are preserved in a variety of ways and different kinds of physical and chemical processes are involved at the time of preservation. Moreover, plant fossils are formed under very special environmental conditions, usually a swampy environment where deposited plant parts remains undisturbed and intact in a deep body of acidic water. Acidity being formed by decomposition of metabolic wastes. In such an acidic environment rate of activity of decomposers becomes restricted and anaerobic organisms contribute little to the decay process. The swamps also have sedimentary material like soluble silicates, carbonates, iron compounds or other minerals. A lower pH (increase in acidity) enhances deposition of insoluble compounds into the plant parts – permineralizig them. Plant parts are preserved as compressions in sedimentary rocks if there is abundance of sedimentary material like silt, clay or fine grained sand in the environment (see figs. 1C, 2A, 3I). As the plant parts accumulate in the body of water, they become covered with the sediment and entombed in the subsequently formed rocks. Sometimes parts like twigs, seeds and fruits are also found preserved in the environments other than swamps. They may have been retrieved from sediments in the floors of desert caves, where high temperature and low humidity of desert environment and protection of the cave eliminated decomposition. Another excellent means of preservation was by-refrigeration when organisms become trapped in snow and ice- fields of Siberia like the mammoth which has been preserved for thousands of years. Even tillites deposited at the time of glacial retreat during the Pleistocene glaciation contained unaltered branches and trunks of trees. Besides these, preservation has also occurred in oil saturated environment where large tree trunks about 100 million years old have been found preserved in black, gooey oil at the base of Cretaceous oil sands of northern Alberta. Ancient environments that allowed excellent preservation of plant parts resulted in different categories of fossilization. Schopf (1975) recognized four distinct modes of preservation (see figs 1,2 & 3) : A. Petrifaction : Where cellular permineralization of plant parts occurred due to infiltration of soluble silicates, carbonates, iron compounds etc. The dissolved minerals permeated all the cells and tissue systems. Later, precipitation of dissolved minerals formed a rock matrix supporting the plant tissues and hardening of plant fragment took place from within and outside. Cell walls consisting of organic matter becoming chemically altered and intercellular spaces and lumens became filled with mineral material. By this process sometimes various cellular contents got preserved which included structures like starch grains, nuclei, various types of membranes, tapetal deposits, megagametophytes of seed plants etc. When mineral was completely solidified, plant fragment became entombed within solid rock. (see figs. 1A,B,D; 2A-D) Permineralized specimens revealed not only external but also internal form of plants and were most useful when studying internal structure. 3 To study petrifactions, thin sections of the rock are cut by special saws to allow passage of transmitted light. The cut surface of the rock is opaque and to make it further transparent, cut surface is ground with a grinding machine, so that more light can pass through the rock and the rock is thin enough to be examined with a microscope. Usually, study of permineralizations are made by the peel technique which is a simple method for preparing sections of petrified materials. A few examples of silicified permineralizations (see figs. 1A,B) are the Devoinan Rhynie Cherts, Precambrian Gunflint Cherts, Triassic woods from petrified forests of Arizona, the giant tertiary Sequoia trunks in the Yellowstone Park and silicified specimens from Tertiary Deccan Intertrappean series of India. Coal deposits formed by fossilization of tropical forests are sometimes associated with calcareous specimens commonly called Coal balls. They have also been of immense help in Palaeobotanical studies. Coal balls are found only in the Carboniferous rocks associated with seams of bituminous coal. (see fig. 1D) They are variously shaped (often spherical or ovoid) limestone rocks containing within them, plant parts generally preserved with cellular details in calcium carbonate. Coal balls represent peat that was infiltrated by carbonate before there was extensive compaction of the plants within them. They provide detailed information that helps in investigating reproductive biology of plants that lived in peat swamps or they may reveal ontogeny and phylogeny of Carboniferous plants. B. Compression : Coalified compression fossils are formed when plants get deposited in sedimentary environment. Sedimentary rocks generally preserve fossil plants. (see figs. 1C, 2A, 3I) Formation of sedimentary rocks takes place by accumulation of small rock particles derived by weathering. Plant parts are fossilized in areas where sediment is accumulating usually in a delta, where course of the river constantly shifts. The delta remains undisturbed for sometime and plant fragments carried to abandoned water bodies where a high concentration of sediment buries the plant parts and fills in the water body. Compressions may also be formed in lagoons, along the meandering rivers, ponds, swamps or other situations. As sediments accumulate water is squeezed out of them and they become more compact and plant fragments contained within the sediments become flattened. Cell walls of plant parts soften and then collapse becoming squashed. In the meantime there is loss of gas, moisture and other soluble materials because of pressure exerted by accumulated sediments and water. The residues are altered and consolidated to form a carbonaceous film that conforms to the original outline of the plant part involved. This is one of the most common type of fossils and is called a compression. Most well preserved compressions are found in clay or shale. (see fig. 1C) Sometimes fossils represent compression of plants growing near an area of volcanic activity. These compressions are found in consolidated volcanic ash. The rain water rand ash make a fine grained mud that picks up and buries plant parts as it moves down the slope. When the mud hardens, it entombs pieces of plant material. Other unique forms of sedimentary rocks containing plant and animal fossils are diatomite and amber. Diatomite, a rock formed from cell walls of a group of unicellular algae called diatoms. They occur in abundance in fresh water and marine environments. On dying, the highly resistant silica cell walls of diatoms become deposited on bottoms of lakes, seas and oceans, where with the passage of time and consolidation they form a white, light weight sedimentary rock called, diatomite. Diatomite is fine-grained, may contain well preserved remains of fossilized plants and animals. It is an unusual situation where sedimentary matrix is composed of fossils in which fossils are embedded. Amber is a semiprecious stone of rare beauty. It is a sedimentary material formed from fossilized plant resins that have undergone chemical change during the process of fossilization. Because of its sticky nature when it was produced
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