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Of all the plant parts, leaves give a very good impression.
After the plant part is entombed in the sediment if there is no compression the surrounding sedimentary material hardens to form a three dimensional structure called the mold. Incrustations may be defined may be defined as external molds of a plant usually in some incompressible materials like sandstone, ironstone or tufa. After the formation of mold, sometimes the plant part may decay.
This space later is filled by crystalline substances such as iron pyrites, sphalarites, chalerite, opal, agate etc. Such hardened structures are called casts. Casts exhibit the same external configurations as the original plant fragment.
No organic material is seen in a cast. Hence there is no question of cellular details.
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Of all the types of fossils, petrifactions are the best, but rarest. In these fossils external form, internal structure and sometimes the substances of original plants are preserved. Cellular details are preserved due to infilteration of some minerals into the plant tissues. During petrifaction, the entombed plant is impregnated with about twenty minerals such as carbonates, sulphates, silicates, phosphates, iron pyrites etc.
Solidi-faction of plant tissues takes place ultimately. Petrifactions are of great importance because they can be cut into sections for anatomical studies. A paper coal or leaf coal consists of thin dead leaves dispersed in an organic matrix. Sometimes decomposed stems and their cuticles have also been retained.
The matrix consists of carboniferous limestone. The best paper coal has been found at Tovarkovo, in Toula in Russia. Petrifactions of spherical specimens are generally termed coal balls. Coal ball plants are of great value in palaeobotanical studies. Plant fragments compressed by vertical pressure are called compactations. Aggregations of plant material found in peat, lignite etc. Any organic material that is related with ancient plant life, also comes under the category of fossils.
Fossil resin amber obtained from coniferous plants is one such example, Resinous substances exuded from plants due to injuries, insect bites etc.
Remnants of minute amounts of organic compounds in parts of fossilized structures without undergoing any or minimal chemical change are called chemical fossils. These are fake structures mistaken for plant remains. Because of the similarities with plant organs these are often mistaken for fossils. These may be products of minerals in rock crevices or creation of ancient human beings.
In the investigation of fossils, finding out their age is very important. It gives us the knowledge of the existence of a particular plant life in the remote past. Usually age of fossils is approximated by finding out the age of sedimentary rock in which the fossil is encased. When observing strata of sedimentary layers, we can easily assume that the lowest is the oldest and the top most is the recent.
The successive layers from below will be younger and younger. This of course is subject to certain reservations. As a result, an ocean bottom, due to upheaval may become land so that sedimentation stops.
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Once again an ocean may be formed due to geological changes. In such cases the layers of sediment will not be sequential. So when one is finding out the age and sequence of plant life all these have to be borne in mind. Generally there are two methods in finding out the age of fossils.
Uranium is a radioactive element. It gradually decays and gets converted into an isotope of lead Pb If in a given rock, both the U and Pb are present, by measuring their relative volumes since the rate of decay is known. One can measure the lime since the solidification of the rock. This gives an approximate Idea of the age of the fossil embedded in the rock. The percentage in any living body is always the same, because the body absorbs C 14 from the air at the same rate at which the isotope decays.
As soon as an animal or plant dies, C 14 is no more absorbed; and its amount begins to drop slowly. Since the original percentage of C 14 in an old piece of wood or bone is known a nuclear chemist can figure out how long ago the object started to decay.
If there is exactly half as much C 14 in the specimen as there should have been originally, the chemist can assumes that the object is 5, years old the half-life of C 14 is 5, years. The rate at which a radioactive material decays to reach one half of its original volume is called half-life. By careful uses of C 14 method the age of any fossil can be measured upto 50, years.
There are several methods to study the fossil specimens. The petrified specimens are cut into thin slices by different methods.
7. 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 ( U, U), Thorium ( U) and Potassium (40 K) are used. Fossils can include anything that gives an indication of the existence of prehistoric organisms. The Latin word Fossilium means 'dug out', which in earlier times meant to include any traces of body of animals and plants buried and preserved by natural causes. George Curvier is considered 'Father of palaeontology', who studied fossils scientifically to develop phylogenies. Fossils & Dating Methods Chapter Exam Instructions. Choose your answers to the questions and click 'Next' to see the next set of questions. You can skip questions if you would like and come back.
In one of the methods each slice is attached to glass plate and ground into sufficient thinness so that it can be viewed under the microscope. Another method of study of petrifactions is to prepare thin films by adopting special techniques. The first step is to smoothen the surface of the slice of the petrified specimen.
Due to this treatment the surface becomes rough and any organic matter remaining on the surface can be treated with hot gelatin.
This method is useful where a petrified specimen still retains some organic matter. The study of fossils provides an insight into plant life of the past; their structure, distribution and reproductive features. This helps us to understand and appreciate the present day flora better.
Dating of fossils biology discussion
Paleobotany introduces us to many extinct plants which may be looked upon as documents of nature recording the progressive changes and modifications undergone by various groups of plants through millions of years.
This helps us in understanding the history and evolution of modem vegetation. From the academic view point perhaps the best irrefutable evidence to evolutionary theory comes from the fossils. Fossils arranged in a chronological order clearly reveal the relationship between one group and other which are very diverse in the present day.
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.
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.
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.
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. Absolute Dating Absolute dating is used to determine a precise age of a rock or fossil through radiometric dating methods. This uses radioactive minerals that occur in rocks and fossils almost like a geological clock. It's often much easier to date volcanic rocks than the fossils themselves or the sedimentary rocks they are found in. Generally there are two methods in finding out the age of fossils. In the first method (uranium/lead method), the age of the rock is measured which gives an approximate idea about the age of the fossil and in the second method (C 14 dating method), the age of the fossil is measured directly. 1. Uranium/Lead Method: Uranium is a radioactive element.
Insects and flowers are often found preserved in amber. Amber has high economic value and used in jewelllery. Monograptus is an index fossil of Lower Devonian, while Myrepollenites is a marker of Eocene.
Foraminifera, pollen grains, spores etc.
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Essay 3. Nomenclature of Fossils : The whole plant is not preserved, but only detached plant parts like stem, root, cone, leaf, etc. 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 Each detached organs or fragments is given a different name.
Each of these names acquires the status of a genus.
One of the most frequent questions a Palaeobotanist or Palaeontologist hears concerns the method for dating sediments containing fossil plants and animals. Present knowledge is based on a long series of efforts to date the ages of various rocks. The half-life for 14 C is approximately years, therefore the 14 C isotope is only useful for dating fossils up to about 50, years old. Fossils older than 50, years may have an undetectable amount of 14 C. For older fossils, an isotope with a longer half-life should be used. Jun 12, Scientists have dated fossils found in South African caves to between , to , years old using several different geochronometers, including optically stimulated luminescence, a tool .
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.
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.
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 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. 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. 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.
Finally, the slice is mounted with a cover glass using suitable mounting media.
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A thin section of petrified wood can be made that preserves the cellular structure in an unchanged condition. 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. 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. 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. 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. 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.
This technique is most suitable for study of peat, lignite and coal. Essay 5. Modes of Fossil Preservation : J. Schopf has recognised four distinctly different modes of fosssil preservation.
Thus, the soluble minerals like carbonates, silicates etc. 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. 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 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.