The following points highlight the top five types of fossils. The types are: 1. Petrification 2. Cast or Incrustation 3. Impression 4. Compression 5. Rocks, Minerals, etc. of Organic Origin.
Type # 1. Petrification:
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, phosphates, 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 microscope. 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.
Type # 2. 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
Type # 3. Impression:
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. (Fig. 508).
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 epidermis remains intact so that structures like stomata are clearly seen in good preparations.
Type # 4. Compression:
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.
Type # 5. Rocks, Minerals, etc. of Organic Origin:
Any object which might have connection with ancient organisms is considered as a fossil. A stone showing a footprint of an animal or a trail of a worm is a fossil. Gums of ancient resinous, coniferous trees are found in fossilised forms as amber which is of great commercial value. Amber sometimes encloses beautiful fossils of flowers or insects. Coal is nothing but a highly compressed fossil derived from primeval forests.
Diatomaceous Earth is formed of skeletons of billions of diatoms depositing on the sea bed. Even petroleum may be considered as a fossil as organisms are responsible for its formation. Graphite used in lead pencils is a fossil in the same sense as this type of carbon is supposed to be of organic origin.
There are different types of algal limestone’s (Fig. 510) in the formation of which specific algae took part. All such minerals are to be considered as fossils.
On the other hand, it should also be noted that sometimes certain minerals, etc., during their formation or crystallisation resembles some plants (e.g., algae) or animals. There are instances of these being considered wrongly as fossils. Such structures have been termed as pseudofossils.