Here is your short essay on Fossils

The word ‘fossil’ has been derived from the Latin word ^‘Fossilium’, which literally means anything dug out of the earth’. These are recognisable remains of once-living plants or animals, most of which have been extinct for many thousands of years. They were preserved in sediments, rocks and other materials such as ice, tar, amber etc. prior to historic times. Thus, the remnants of plants or animals of the past geologic ages preserved in the rocks of the earth’s crust by natural processes are known as ‘fossils’.

Nature and mode of preservation:

The varieties in the fossils correspond to what is preserved and how it is preserved. Thus they are of the following nature:

(i) Fossils may comprise the remains of the complete animal but that is very rare. Such fossils chiefly include insects preserved in amber, animals in asphalt, and mammoths and other mammals frozen in ice. But such fossils are of rare occurrence and are of recent origin.

(ii) There may be petrified remains of hard parts of body in rocks. These are often incomplete and are found in broken fragments.

(iii) There may be just the impressions of footprints or leaf- prints and not the original part of the organism.

(iv) Fossils might be also in the form of casts or mould.

Conditions favourable for preservation:

We know that millions of animals and plants had lived, died and were destroyed without leaving a trace. But it has been observed that two factors are favourable for the preservation of organisms as fossils

(a) The possession of hard parts such as shells and bones, and

(b)Quick burials of the remains by different processes to prevent destruction by scavengers and decay.

Any animal or plant satisfying the above two conditions can be preserved as fossils under normal conditions. The condition hi which fossils occur depends on their original composition and on the material in which they are embedded.

Fossilisation may occur in several ways. Sometimes the soft parts remain unaltered in fossilisation; sometimes only hard parts remain unaltered and sometimes the hard parts are also altered.

1. Unaltered-soft-parts (Actual remains):

In such cases the whole of the organism, including its soft parts is preserved as such. It may be possible due to entombment of the animals under a thick cover of ice. Sometimes insects become entangled in soft and sticky secretions (resin) of trees On exposure, this hardens and changes to amber and with it the entangled are perfectly preserved.

2. Unaltred hard parts:

Shells and internal skeletons are frequently preserved for long periods of time. Many of the best of these are fossils of marine animals that fell into the soft sediment on the sea-floor when they died. Of the land dwellers, those that live near swamps, lakes or sea are most likely to be preserved. Corals, mollusca and protozoans are examples of this category.

3. Altered hard parts:

In this case, the actual remains of an organism are likely to undergo changes through time. These changes are fostered by the slowly circulating ground waters that carry elements in solution. It includes the following processes:

(a) Petrifaction:

It is a slow process which involves removal in solution, of each individual molecule; of the material constituting the hard parts and simultaneous precipitation of an equivalent quantity of the replacing mineral. This molecule by molecule replacement of one substance by another helps in preserving even the most delicate organic structures, as such. In this manner the bones, shells or plant tissues are transformed into calcite, silica or pyrite and the processes are known as calcification, solidification and pyritization respectively.

(b) Carbonisation:

In this process, the organism is decomposed and it loses nitrogen, oxygen and other volatile constituents. As a result, it is enriched in carbon and is said to have been carbonised. Coal seams are the carbonised remains of plants.

(c) Moulds and casts:

Sometimes, the hard parts preserved within the accumulating sediments, may be totally removed in solution. As a result, hollows are left within the rock beds which are called moulds. Porous and permeable rock-beds are able to have seepage of sub-surface water which can remove in solution the shells of organisms, thus facilitate the formation of moulds.

When the moulds are filled up subsequently with mineral matter; it is known as ‘cast’, Thus casts only retain the external form of the hard parts.

(d) Impression:

Plants and animals devoid of hard parts, do sometimes leave a record of their existence, in the form of imprints within the rock beds. Impression of leaves, feathers of extinct birds are the examples.

(4) Tracks and trails:

While moving on soft and damp ground, the foot prints or trail of the animals are entombed in the mud and when this ground hardens into a rock the foot prints present are pre­served. Even though they do not form any part of the animal, yet they are regarded as fossils.

Uses of fossils:

1. The study of fossils provides evidences in favour of organic evolution and migration of plants and animals through ages.

2. They help in establishing the geological age of rock beds and their correct order of succession in any area.

3. In the reconstruction of palaeogeography of earth, fossils are paramount significance. For example, presence of fossil trees or stumps indicates the terrestrial environment and in a similar way the fossils of corals and echinoderms, etc. are suggestive of marine environment.

4. Fossils also help in ascertaining the palaeoclimate.

5. In correlating rock-beds of one area with those of another.

6. The relics of ancient life can be used to establish the time sequence of sedimentary rocks.

7. Fossils provide evidences of “Ontogeny recapitulates phyllogeny’. Ontogeny means the development of the individual, Recapitulation means brings and phyllogeny means the race history. This is obvious as the fossils include the ancestors of modern forms.

8. The study of fossils constitutes an integral part of any investigation leading to the discovery of new deposits of coal and petroleum.

The important features of a brachiopod shell are as follows:

(i) The shell is equilateral and unequal.

(ii) Both the valves are produced, into what is called beak or umbo, each. They mark the posterior end of the shell. The umbo of the ventral valve is more prominent than that of the dorsal valve.

(iii) The surface of the shell is nearly smooth, but at irregular intervals it is marked by concentric growth lines, which are parallel to shell margin.

(iv) Through the foramen of the ventral valve, a muscular stalk which extrudes gels attached to the substratum. This is known as pedicle.

(v) Directly beneath the beak, the inter-area of either valve may be interrupted by a triangular open space termed as dehhyrium in the ventral valve, and not otherium in the dorsal valve.

(v) Covering of the delthyrium is called a deltidium and that of nototherium is called a chilidium.

(vii) The two valves of a brachiopod are joined together by means of a hinge and in some cases they are held together by muscles. On the basis of the mode of union of the valves, there are two major classes known as (a) Inarticulata and (b) Articulata. In case of inarticulata brachiopods both the valves are held together by means of muscles and those of articulata class are united by hinge, at their posterior end.

(viii) In the inarticulate brachiopods, the muscles which open the valves are called divaricators and those which close them are known as aducters.

(ix) In case of articulate brachiopods, a pair of teeth is developed on the ventral valve near the umbo and these fit into two sockets that occur near the umbo of the dorsal valve. The teeth and sockets together form the hinge and the portion of the posterior margin where they occur is known as hinge-line or cardinal-margin. In certain cases, a triangular area exists between the umbo and the hinge-line. This area may be flat or slightly concave and is described as the hinge-area (or) cardinal area or inter-area or palintrope.

(x) Sometimes the teeth are supported by plates known as the dental plate.

(xi) Inside the shell of the brachiopods, the following organs are found to be present; besides those which have been described earlier, viz. crura, median septum, septilial plates, spondylium, brachidium which consists of a pair of simply curved on doubly bent arms known as loops and the skeletal connections joining both the arms are known as jugum.

(xii) On the external surface of the valves sometimes ridges are found which extend radically from beak. Sometimes there occurs radial ribs, spines, knobs or tubercles, which ornament the external surface of the shell.

The ridges on the shell of the brachiopods are known as plica­
tions or costae and shells possessing them are described as plicate or costate.

(xiii) The shell form of the brachiopods may be elongated, ellipsoidal, sub-circular, alate (winged), concavo-convex, biconvex, plano-convex, resupinate etc.

(xiv) When the shells are found to possess minute holes, they- are known as punctate shell.

(xv) The shells are mainly calcareous in composition, and are formed of three layers:

(a) Inner calcareous layer.

(b) Middle layer of flattened prisms of calcite.

(c) Outer part consists of chitinous material.

(xvi) The junction between the valves is known as commissure.

(xvii) The initially formed shells are called ‘protegulum’. In old age, the shells become thick and their margins truncated. The ornamentation tends to disappear.