Geological time scale
Geological time scale:-
> Earth’s history is approximately 350 million years old.
> Geologists have used two major units for subdividing the geological history of the earth these are- time and strata.
> On the basis of time the geological history of earth has been divided into five Eras namely Archaeozoic, Proterozoic, Palaeozoic, Mesozoic and Coenozoic.
> Eras have been divided into periods and periods into epochs.
> Similarly on the basis of strata, the geological history of the Earth is divided into system,series, stage and zone.
> The various eras and their age along with plant groups evolved during that period have been described in the given figure.
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1. Precambrian Era:- Aarchaeozoic and Proterozoic eras are together known as Precambrian. It is said that life originated on the earth in Precambrian. The Rocks of this era have only a few fossils because either the environment was not suitable for fossilization or the organisms present during that time had so soft tissues that it was not fit for fossilization. Therefore, the absence of any fossil records makes it very difficult to tell something about the environment of Precambrian era.
2. Palaeozoic Era:-
> This era had vegetation including both aquatic and land plants. The aquatic plants included species such as Nematothallus and Prototaxites. These two species show similarities with thallophytes. The first fossils of land plants were found in the Silurian and Devonian periods of this era (225-350 million years ago). Manograptus, Sporogonites, Zosterophyllum, Cooksonia, Rhynia, Horneophyton and Psilophyton were important fossil genera discovered.
> In the Carboniferous and Permian periods, Lycopsids, Sphenopsids and Pteropsids formed dense forests. Seed ferns (Pteridospermatophytes) were also abundant in this period (age of seed ferns). Presence of annual rings in certain fossil plants of this period indicates seasonal changes.
> Significant changes in environment occurred in late carboniferous resulting in the development of swampy regions. This type of habitat was unstable for woody vegetation and in the late Devonian period the vegetation present on the earth was converted into coal due to geological events. The coal beds of Pennsylvania and West Virginia we are formed due to coalification of Lycopsids, Sphenopsids, seed ferns and gymnosperms of that period. Fortunately few Psilopsids, Lycopsids and Sphenopsids escaped destruction and eventually gave rise to the present forms.
3. Mesozoic Era:-
> The environment for the Triassic period of this era was very arid (dry) and thus unsuitable for the growth of plants requiring in humid climate. Fossil records however indicate the presence of many ferns, Cycadophytes and conifers in the Triassic period. This period is also known as the age of Cycads. Although flowering plants also originated in the period, their number was negligible.
> Significant changes occurred in the vegetation of the Cretaceous. Of the 300 cretaceous species collected from North America 200 species belong to angiosperms. It shows that angiosperm formed a significant part of the cretaceous vegetation.
4. Coenozoic Era:-
> This era is known as the “age of angiosperms”. High mountain ranges such as Himalayas in Asia and Alps in Europe were formed during this era. As a result, the effect of hot winds got reduced and ICE AGE started. Gradually the ice shifted towards the poles.
> Simultaneously the development of tropical and temperate regions occured. These changes had major effect on vegetation. The seed plants gradually migrated towards tropical regions and those which could not migrate became extinct. As most plants of the late coenozoic era were deciduous, it shows that there was seasonal variations too.
> The vegetation of the temperate and the Polar Regions mostly consisted of annual and biennial herbs which could withstand low temperature.