The earth is made up of different type of rocks namely igneous, sedimentary and metamorphic rocks. On observation, the sedimentary rocks often contain certain remains and markings, which resemble the present day organisms which live upon the surface of the earth. On further examination they are found to be either parts or impressions of ancient plants or animals which are preserved by nature in the sedimentary rock.
These naturally preserved parts or impressions of ancient animals and plants are known as fossils. The ‘Salagrams’ described in various Hindu mythology are nothing but sedimentary rock pebbles rolled down from The Himalayas by the rivers, which are associated with fossils.
The term fossil, which has been derived from an ancient European word (fodder = to dig up ) has been defined as, ‘remains of geologically ancient plants and animals that resemble the parts of the present day animals and plants which have been preserved in the rocks of the earth’s crust by natural processes and agencies’.
1. Development of modern living world and fossils: We must take the help of fossils for the study of ancient living world. But before that one must acquire detailed knowledge about the present day animal and plant kingdom.
The fossils are nonliving bodies and are certain hard parts like bones, outer shells of ancient animals, leaves, spores and trunks of ancient plants or their casts and impressions. Hence, before studying the ancient living world with the help of fossils one should be well versed with the present day animals and plants.
While going through the available fossil records, we wonder how the modern animals and plants have slowly evolved from their ancient ancestors. This is clearly evident when we compare the modern elephants and horses with their available fossils.
A brief classification of animals which are presently living under various conditions of land and water, on the surface of the earth is given below and illustrated in.
2. Mollusks –
3. Chordates –
4. Protozoan’s –
Certain ancient plants and animals are known to us only through their fossils because, they have been extinct since long from the surface of the earth, while a few other animals and plants still continue to be in the
5. Worms –
6. Other invertebrates-21,000
1. Flowering plants – 2, 59,000
2. Ferns and conifers – 10,000
3. Mosses etc.-
Primitive stage since their origin. A short classification of the modem plants sustaining under different conditions of land and water on the surface of the earth is given below and illustrated in.
4. Algae and fungi –
Changes are the unchanged truth of nature. The environment, on the surface of the earth, changes from time to time. Those animals and plants which adjust themselves with the changed environment progress and survive for a long period while those which are unsuccessful in adjusting themselves to the changed environment become extinct. Further, each distinct environment such as a desert, a river or a pond, a snow caped mountain each support more or less a unique type of animal and plant community.
The fleshy stems of a cactus which adapts and survives in desert conditions or the streamlined shape of a fish along with its fins and tail are adaptations to live in water are a few examples only.
Sedimentary rocks are often rich in fossils. On examination they indicate that along with geomorphologic evolution of the earth, the contemporaneous living world has also gradually evolved. Long after the creation of the earth and with the formation of the atmosphere rain fall must have started.
The rain water coming down the high lands and hill tops through small nasal and gullies gradually increase their width as they reach the valley area forming small and big rivers and ultimately discharge the
water into shallow regions which gave rise to modern seas and oceans. The running water on its way denuded the surface of the earth by breaking down the rock formation into smaller and smaller size and transported and deposited in the lakes and oceans.
Simultaneously birth and death continued in the living world. After the death, some of the dead bodies of animals and plants must have been carried away by the running water and it is natural that they might have been buried under sand and mud deposited in the lakes and oceans.
In due course of time the rock debris deposited under the sea water converted into sedimentary rocks as various geological processes acted upon them. Along with the consolidation of the sediments into sedimentary rocks, the dead bodies of animals and plants or their impressions were also preserved within these sedimentary rocks in selective areas. These animals and plants or their impressions, which are preserved within the sedimentary rocks formed millions of years back, are known as fossils.
The process of formation of sedimentary rocks continued unabated, one above the other and buried the dead bodies of animals and plants which were gradually evolving from simpler to complicated ones.
The processes of formation of the sedimentary rocks indicate that different sedimentary beds have been formed during a particular geological period. Hence, each sedimentary layer has its own age.
Thus, if they are not disturbed by different geological forces, the oldest or the first formed rock strata of a geological formation remain at the bottom while the newer layers are deposited above the older ones. Sedimentation and formation of sedimentary rocks is continuing in the present day lakes and oceans burying the dead animals and plants and preserving them for consideration as fossils in the future.
Quite a good number of animals and plants which were flourishing on the surface of the earth at one time or other, starting from the origin of life, do not exist today as they are dead and gone. Very often, we take the help of their available fossils to understand them. Unfortunately, the fossil record is very often found to be incomplete. Further, the early formed organisms were soft bodied having neither internal nor external hard parts.
As most of the sedimentary rocks are formed inside water, the land plants and animals do not get adequate chance to be fossilised. However, the geological disturbances are mainly responsible for the hindrances in the process of fossilisation.
It is said that the earth itself is the book of history of the earth, the sedimentary rock layers being its pages, while the fossils and such other.
Imprints and impressions are the letters, which can give us a complete picture of the geological past of our mother earth. As has been mentioned earlier, quite good numbers of pages along with the descriptive writings have been completely destroyed or not available for our perusal.
Whatever lost is lost, but if we can systematically arrange and study the available fossils, we will be able to get a picture of the past history of the ancient living world, animals and plants. By reconstructing the ancient living world with the help of their traces and broken parts preserved as fossils in the light of the present day representatives, it is possible to have a better understanding of the extinct animals and plants.