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.
1. Petrifaction
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.
4. Coprolites
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.
5. Impressions
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.