(By Dr. Girish Chandra)
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 (1769-1832) is considered ‘Father of palaeontology’, who studied fossils scientifically to develop phylogenies.
Types of fossils
Based on the mode of formation of fossils, they can be categorised in several types. Fossilisation is a rare phenomenon, which takes place under specialised conditions. The study of natural process of death, burial, decomposition, preservation and transformation into fossil is called taphonomy. Fossils are the only direct evidence of the biological events in the history of earth and hence important in the understanding and construction of the evolutionary history of different groups of animals and plants.
Petrifaction is molecule-by-molecule replacement of organic matter by inorganic compounds, viz. silica, calcium carbonate or iron pyrites. It literally means “turned into stone” and takes place in buried situations, particularly at the bottom of lakes, ponds or sea, where there are sediments rich in calcium carbonate and silica. Over millions of years, inorganic matter replaces the entire bony material, making an exact replica of the original. By this time sediments transform into sedimentary rocks, in which fossils can remain preserved for a long time. Most of the old fossils are petrified, e.g. shells of molluscs, arthropods and fish skeletons.
2. Preservation of footprints
When animals walk on wet soil and sand, they leave trail of footprints or limbless animals and worms may leave tracks and trails in mud. If these footprints are covered by volcanic ash, they can be preserved for a long time as the clay containing footprints and the volcanic ash covering it will harden to form different types of rocks. Mary Leakey discovered footprints of prehistoric man along with those of giraffes, elephants, guinea fowls etc. in Kenya.
3. Burrows and borings
Annelid worms, arthropods and molluscs make burrows in soil or bore into hard wood, rocks or corals. These are preserved as the soil hardens to form rock.
Sometimes excreta of animals is buried and fossilised, which gives an indication of the diet of animals. For example, carboniferous nodular excreta of salamanders have been found in Illinois in USA. Huge deposits of guano of sea birds have been found in South American seacoast.
Impressions of body parts, skin, feathers, leaves etc. are formed when they are pressed hard against the soft clay, which subsequently hardens to form rock. Fossil of Archaeopteryx is such an impression. More bird fossils in the form of impressions have been discovered in China recently, e.g. fossils of Sinosauropteryx, Caudipteryx and Confusiusornis.
6. Moulds and casts
Moulds are formed by hardening of the material surrounding the body of an animal, such as volcanic ash or lava that flows into the sea and traps molluscs and arthropods. Animal body disintegrates leaving the hollow mould, which gives an indication of the contour of the animal. If this mould is filled with inorganic material such as calcium carbonate or silica that will harden to form a cast, it will be an exact replica of the animal body.
7. In petroleum and asphalt
In some coastal areas where there is abundance of petroleum pools and tar-pits, small animals, birds and insects get entrapped in them. In asphalt the process of decay is very slow and if these are buried in soil, they are preserved for a long time.
8. In resin and amber
Resin is a sticky substance that exudes from coniferous trees and traps small insects such as flies and mosquitoes. Resin hardens to form amber and preserves the insects with all fine details. Most of the insect fossils are fossilized in this way.
9. In ice
Ice is a good preservative even of softer body parts. If animals are buried deep in snow that never melts, then they are preserved entirely for a long time. Fossil of woolly mammoth from the permafrost in Siberia is an excellent example of preservation in ice, in which even flesh, skin and hair are preserved.
Index fossils: Fossils that are found in undisturbed sedimentary rocks and in short geological time period generally lie in recognisable strata of older rocks below and subsequently formed layers above. Based on the presence of such fossils in rocks, age of other fossils in the same rock can be determined without dating, because such fossils are an index to a particular geological period. Ammonites are considered good index fossils as different species represent specific geological periods in rocks.
Determination of the age of fossils
Age of fossils is determined by Radioactive Clock Method, or Radiometric dating, which was devised by Boltwood in 1907 and later by Rutherford in 1955. The rate of disintegration of radioactive material is always constant and is not affected by the environmental factors. The time taken by 50% of the radioactive material to disintegrate into stable element is known as its half-life. If we know the half-life of an element, then the age of the fossil can be calculated by finding out the ratio of radioactive element and its stable daughter element using scintillation counter or Geiger-Muller counter.
For example, half-life of Uranium238 is 4.5 billion years. With this rate of decay, one million grams of U238 would yield about 1.7400 grams of Pb206 in one year. OR, in one year one gram of U238 would yield 1/7,400,000,000 grams of Pb206. The radioactive isotope U238 disintegrates into its stable isotope, Pb206 by emitting 8 alpha particles and 6 beta emissions through a chain of daughter radioactive elements. This method is also explained in the following table.
Different radioactive materials have different half-life and, therefore, age of recent as well as very old fossils can be determined by selecting the appropriate radioactive element, provided that the element is present in that particular rock.
This method was devised by W.F. Libby in 1956 by recognising the fact that all living organisms have C14, the radioactive isotope of carbon in their bodies, which starts disintegrating after their death into C12 or N14 by the release of an electron, as it is no longer replenished after the death of animal or plant. This radioactive carbon is taken up by plants in the form of carbon dioxide, and animals get it into their bodies from plants. Since after death, animals and plants do not replenish C14 through food, it only decays at a constant rate. The rate of decay is found out by counting the number of beta particles emitted by C14 per unit time, using Scintillation counter, Wilson-cloud chamber or Geiger-Muller counter (that use CO2 gas) and then feeding the data into a computer.
When there is water seepage through the bones, fluorine in water combines with calcium in bones to form fluorapatite, whose proportion in the bones can be detected to find out the age of the fossil.
This method works on collagen degradation in the bones and accumulation of other materials such as fluorine. How much nitrogen is retained in the bone gives an idea of the age of the fossil.
Bones buried in soil absorb uranium at a constant rate and longer they lay in those strata more uranium they will absorb. Since uranium is radioactive, its presence in the bones can be detected using scintillation counter.
Trace amount of Uranium238 impurities are commonly found in nature and spontaneous fission of this radioactive element produces small, permanent, microscopic damage trails in the insulating solids through long periods. Simple count of numbers of such trails gives the age of the fossils.
Potassium is abundant in nature and its isotope K39 comprises over 93% of the total, while K40 is only 0.0118% and K41 6.8%. K40 decays into its daughter isotopes, viz. Ca40 by beta decay and A40 by K capture, in a half life of 1.3 billion years. The ratio of the daughter isotopes can be detected in a mass spectrophotometer. This method is used to detect the age of very old fossils but theoretically potassium rich samples as young as 5,000 year old can also be dated.
Amino acid racemization dating
The method is based on the fact that all animals have L-amino acids in their proteins which after death transform by racemization into D-amino acids, whose proportion increases with time. The ratio of the two types of amino acids will give the age of the skeletal material in the fossil.
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