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Geological time scale : Palaeozoic era, Mesozoic era, Cenozoic era

Geological time scale : Palaeozoic era, Mesozoic era, Cenozoic era
The oldest known rocks are about 3.8 billion (3800 million) years old. It corresponds to 38, 000, 000 centuries. According to Geologists and Geo-physicists our planet earth is 4.7 billion (4,700 million) years old.

Geological time scale : Palaeozoic era, Mesozoic era, Cenozoic era


 

Geological time scale

 

The oldest known rocks are about 3.8 billion (3800 million) years old. It corresponds to 38, 000, 000 centuries. According to Geologists and Geo-physicists our planet earth is 4.7 billion (4,700 million) years old.

 

In this ancient time scale evidenes showed that the first life originated 2.5 billion (2,500 million) years ago. Hence from the formative stage for nearly 2,200 billion years there was no life on earth . Such a period in earth's history is known as the Azoic era (2,200 to 5,000 billion years ago). Evidences show that during this era the earth was a hot sphere. Gradually the upper surface of the earth cooled down. It resulted in solidifica-tion and formation of rocks and rocky terrain. Further water molecules were formed resulting in accumulation of water and water reservoirs as water masses and land surface were established. These transformations provided a suitable condition for the origin of the first life.

 

Once the life originated and established, evolutionary changes took place. Changes happened in the structure, organisation and living methods of organisms, depending on natural surroundings and changes in natural surround-ings. Thus fauna and flora started flourishing on earth. The water masses were fully exploited. Later land surfaces were invaded by plants and animals.

 

The Geological time succeeding Azoic Era, was dramatic and rich in life. This period is divided into three eras. These were significant periods in earth's history. Of these, the oldest era was the Paleozoic era. It ranged from 600 to 210 million years ago. Thus its duration was nearly 390 million years. This era saw the origin and adaptive radiation of sponges, starfishes, snails, insects, crabs, and terrestialised amphibians and reptiles.

 

The Paleozoic era was followed by middle period named as Meso-zoic era. This era ranged from 65 to 210 million years ago. Its duration was 145 million years. During this era, among animals the reptiles came to promi-nence. Hence this era is known as the golden age of reptiles. Further this era saw the origin and development of birds and reptiles.

 

The period ranging from 65 million years till date is named as the Coenozoic era. This era is characterised by rapid evolutionary changes inmammals. This era is known as the Age of mammals.

 

Geological time scale

 

A geological calendar has been formulated by assessing the age of rocks and rock sediments. Based on age, and events, the ancient period from earth's history is formulated into eras-periods-epochs. Each division in the geological calender is clearly identified and demarcated. Incidents pertaining to earth surface, plant and animals life are neatly recorded. The influence of geological and climatic changes on the life and the evolution of the living or-ganism had been well analysed.

 

 

Era       Duration         Significane

Palaeozoic       600 - 210 m. years ago           'Cradle of anicent life'.

Mesozoic         210 - 65 m. years ago 'Golden age of reptiles'.

Cenozoic          65 - 1 m. years ago     'Age of mammals'.

 

Era       Period Epoch  Duration

                                    (million years ago)

                                   

            Quarternary    Pliestocene      2 - 1

                                   

CENOZOIC       Tertiary           Pliocene          7 - 2

                        Miocene          26 - 7

                        Oligocene        38 - 26

                        Eocene            54 - 38

                        Palaeocene     65 - 54

            Cretaceous                  130      - 65

MESOZOIC      Jurassic                        160 - 130

            Triassic                        210      - 160

                                   

            Permian                      235 - 210

            Pennsylvanian             255 - 235

            Mississipian                 275 - 255

PALAEOZOIC   Devonian                     315 - 275

            Silurian                        350 - 315

            Ordovician                  440 - 350

            Cambrian                    600 - 440

                                   

            Precambrian               440 and before

 

 

Periods                                   E v e n t s

Quarternary - Pleistocene      2 - 1 m.years ago        Human evolution

TERTIARY - Pliocene               7 - 2 m. years ago       Rodents were successful

                                    Mammals increased

TERTIARY - Miocene               26 - 7 m. years ago     Priaries were formed

                                    Horses evolved.

                                    Carnivorous mammals

                                    were dominant

TERTIARY - Oligocene 38 - 26 m. years ago   Monkeys and apes

                                    originated

TERTIARY - Eocene                 54 - 38 m. years ago   Horses originated

                       

TERTIARY - Palaeocene          65 - 54 m. years ago   First flowering plants

                                    Mammals originated

                                    Dinosaurs disappeared.

CRETACEOUS  130 - 65 m.years ago  Dinosours became extinct

                       

JURASSIC         160 - 130 m. years ago           Birds originated

                                    Modern bony fishes.

                       

TRIASSIC          210 - 160 m. years ago           Dinosaurs evolved.

                                    Mammals originated.

PERMIAN         235 - 210 m. years ago           Origin of reptiles.

PENNSYLVANIAN         255 - 235 m. years ago           Land living insects.

MISSISSIPPIAN 275 - 255 m. years ago           Origin of Amphibia.

                                    Land living insects,

                                    Forests.

DEVONIAN      315 - 275 m. years ago           Age of fishes. Fernsand

                                    cycas.

SILURIAN         350 - 315 m. years ago           Jawed fishes originated

                       

ORDOVICIAN   440 - 350 m. years ago           First vertebrates,

                                    Jawless fishes,Bryophytes.

CAMBRIAN      600 - 440 m. years ago           Thallophytes,

                                    Arthopods,Molluscs,

                                    Echinoderms.

                       

PRECAMBRIAN            before 440 m. years ago        Protozoans,

                                    Poriferans and Annelids

                                    lived.

 

I. Paleozoic era :- This era produced revolutionary changes in the biosphere. Further this era saw the origin and the radiation of several groups of animals and plants that remained as the forefathers for the modern groups. Thus this era is known as the Cradle of ancient life.

 

1. Cambrian period :- (600 to 440 million years ago)


The period preceeding cambrian is known as Pre-cambrian period. During precambiran time simple algae, protozoans, poriferans, annelids, were well established. Thus the cambrian started with the plants and animals that were suceessful during the precambrian period. During cambrian among plants thallophytes were well establised. They diversified into various groups. (Chlorophyceae, Rhodophyceae etc.,). Among animals the aquatic arthropods and echinoderms came to prominence. The fossils of such organism were obtained from several places.

 

2. Ordovician period :- (440 to 350 million years ago)

This period was marked by formation of coral rocks and molluscs and echinoderms. Among plants the semi terrestrial bryophytes were getting es-tablished. Interestingly this period saw the origin of first vertebrates. These were the now extinct agnatha. (Jawless, armoured fishes). The origin of early vertebrates was the major event that happened in the evolution of animals. Among arthopods, the trilobites were more prominent during this period.

 

This period was marked by formation of coral rocks and molluscs and echinoderms. Among plants the semi terrestrial bryophytes were getting es-tablished. Interestingly this period saw the origin of first vertebrates. These were the now extinct agnatha. (Jawless, armoured fishes). The origin of early vertebrates was the major event that happened in the evolution of animals. Among arthopods, the trilobites were more prominent during this period.

 

3. Silurian period :- (350 to 315 million years ago)

 

The oldest land plant originated in this period. These plants possesed conducting tissues. They colonised the land. Among invertebrates except for insects all others flourised. The corals diversified. Several coral islands were formed. Jawed fishes originated. The fishes developed scales and paried fins, for the first time jaws originated in fishes. Origin of paired fins and jaws is considered as major events in chordate evolution.

 

4. Devonian Period :- (315 to 275 million years ago)

 

It is a significant period in the paleozoic era. During this period land living plants were more successful. The forests were filled with varieties of ferns and cycas. (non-flowering plants). Among aquatic animals fishes be-came dominant. They diversified by adapting themselves to live in various aquatic ecosystems. The forefathers of almost all modern fishes lived during this period. Due to these reasons this period is called as the Age of fishes.

 

5. Mississippian Period :- (275 to 255 million years ago)

Several changes happened to the land structure. There were massive upraising of land in several places. This resulted in the formation of several mountain ranges. Huge water bodies were broken into smaller lakes. These major changes on earth's surface were known as revolutions(eg. Caledonian revolution). Such changes were the cause for the origin of lungs in fishes. Lungs evolved for the purpose of living temporaily on land. It helped such fishes to find new water bodies. Such practises encouraged the origin of the amphibians. The origin of land living amphibians were further increased by the proliferation of several land living insects.

 

6. Pennsylvanian :- (255 to 235 million years ago)

 

The land living forms became more successful during this period. There were huge forests of ferns and cycas. Due to geotectonic changes several forests got burried under the soil. Today's coal and petroleum are obtained from such resources only. Hence the Pennsylvanian and the earlier Mississip-pian were collectively krown as Carboniferous (carbon bearing) period.

 

7. Permian Period :- (235 to 210 million years ago)

 

It was the last period in the Paleozoic era. This period was marked by extinctions of several older groups of animals and plants. Nearly 60% of the organism that survived at that time became extinct . Some of the amphibians dramatically laid land eggs (cleidoic eggs). Specifically the group of organisms that laid such eggs are identified as Seymouria. These are considered as inter-connecting links between amphibians and reptiles.


II. Mesozoic Era :-

 

This middle period in the history of life was marked by the prominence of land living forms. Among animals the reptiles became more dominant. They increased in size and in number. Hence this era is named as the Golden age of reptiles.

 

1. Triassic Period :- (210 to 160 million years ago)

 

For the first time fossils of turtles, crocodiles, and dinosaurs have been obtained from this period. Fossil evidences show that aquatic and flying reptiles thrived during this time. The mammals orginated from reptiles during this period.

 

2. Jurassic Period :- (160 to 130 million years ago)

 

There was a marked adaptive radiation among dinosaurs. They diver-sified into carnivorus and herbivorus forms. The first birds originated from the reptiles. The earlist bird thus originated is known as the Archeopteryx. The origin of birds was a major physiological change among animals. From a more common poikilothermic condition through feathers the birds became homeo-thermic.

 

The modern bony fishes were diversified into several groups.

Cretaceous Period :- (130 to 65 million years ago)

 

The larger marine molluscs became extinct. The fossils of such or-ganisms are available in places like Ariyaloor, of Tamil Nadu, today.

 

The Dinosaurs of the Mesozoic era abruptly became extinct during this period. Several reasons are given for the extinction of the dinosaurs. Fos-sils of dinosaurs were not obtained from later periods.


III. Cenozoic Era :- (65 million years ago till date)

 

Plenty of fossil of organisms belonging to this era had been obtained. All modern animals and plants were represented in these fossils. This era is subdivided into Tertiary and Quarternary periods. Further this era contains seven epochs. Through fossils we can trace the origin and evolution of inde-pendent groups of animals, camels and man.

 

 

1. Paleocene epoch :-

 

Modern placental mammals originated during this time.

 

2. Eocene Epoch :-

 

Ungulates originated. The ancestral form of modern horses lived during this epoch.

 

3. Oligocene epoch :-

 

Several animals with ancient characteristics became extinct. Modern mammalian families were established. The apes originated during this epoch.

 

4. Miocene epoch :-

 

Several varieties of grasses evolved in Europe and N. America. Thus large Priaries were formed. These changes encouraged the evolution of fast running herbivorus mammals and their predators. Thus the carnivorous mammals came to prominence.


5. Pliocene Epoch :-

 

The priaries enlarged still further in several regions. The rodents became more successful. The mammals increased in number.

 

6. Pleistocene epoch :-

 

Several glaciations happened during this time. This epoch is popularly called the 'Ice age'. The evolution of horses and man reached the final stages during this period. The melting of ice that happened 1,500 years ago is considered as the last stage of this epoch. Today we are living in an inter-glacial Period.


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