Geological Time Scale, Practice Problems and FAQs

You all know that at some embryo stage inside the mother’s womb we all possess a tail. It is said that the human tail is relatively long in the beginning, but the length of the tail is reduced during embryonic development. But why are we all born without a tail?. Then where did that tail go? Our tail normally disappears at the end of the embryonic phase. 

Fig: Embryonic stages in different organisms

This occurs by a process called apoptosis (a programmed cell death). This is one reason that gives a proper shape to our body. Now you will be wondering why other mammals possess a tail?. We lost our tail during evolution. You all know about our present walking style, we walk with two legs and do not need a tail for counterbalance. This happened during human evolution. 

Fig: Human evolution

Now you will have lots of questions related to evolution in your mind. These questions are definitely related to the processes and changes that took place during evolution. So let’s understand when these changes occurred by taking a deep dive into the details of geological time scale in this article. 

Table of contents

• Evolution
• Geological time
• Geological time scale
• Significance of geological time scale
• Practice Problems
•  FAQs

Evolution

The word evolution means ‘the unfold or unroll’. It is considered as the changes in the characteristics of the population of organisms or groups of such populations over the course of generations. 

Geological time

The planet Earth and its living organisms have a long history which has been partly recorded in certain rocks of the Earth crust in the form of fossils. Geologists tried to estimate the age of Earth in various ways and determined the age of Earth to be around 4.5 billion years. Life appeared on Earth around 500 million years back. The entire lifespan of Earth is called the geological time. 

Plants and animals have changed gradually during the passage of geological time and present a chronological sequence of events, which led to the evolution of more and more complex forms from the simple ones. These evolutionary changes are read in the form of fossils that are found in the succeeding rock beds and strata. 

Geological time scale

The first geological time scale was developed by Giovanni Arduino in 1760. By reading the fossils in the rocks, the geologists divide the geologic time into intervals. These are characterised by significant events occurring in the organisation of organisms. The total span of geologic time is divided into six large divisions called eras. The eras are then subdivided into periods. Periods are then divided into epochs. These eras, periods and epochs are arranged on the time scale in an order of their age and this arrangement is called ‘geological time scale’. 

Reasons for formation of eras, periods and epochs

The two main reasons for the formation of ears, periods and epochs in the geological time scale include revolutions and disturbances. 

Revolutions

There is some evidence that says that each era was ended by widespread geological disturbances called revolutions. Each revolution includes heavy changes. It may be an upheaval of the Earth surface in certain regions which resulted in the formation of mountains. In certain regions lowering of the land (may be due to fall of meteorite) resulted in the formation or elimination of inland seas. These revolutions changed the distribution of not only sea but also the land organisms and wiped out many of the life forms too. 

GIF: Fall of meteorite

Disturbances

There were lesser events (a massive forest fire) in each of the eras, which were less widespread and created far less effects than the revolutions. These events are called disturbances in the Earth crust. These disturbances also marked in the geological time scale and divided the eras into periods and epochs. 

GIF: Massive forest fire

Eras

The geological time scale is divided into six eras. The various eras are as follows: 

• Azoic
• Archeozoic
• Proterozoic
• Palaeozoic
• Mesozoic
• Cenozoic

Azoic era

This is considered as the earliest time in the history of Earth. It is characterised by the complete absence of living organisms. During this particular era the Earth was formed, cooled and underwent many changes which created conditions favourable for the appearance of living organisms. 

GIF: Primitive Earth

Archeozoic era

During this era the conditions on Earth became suitable for life and the first living forms appeared. Catastrophic and widespread volcanic activities and upheavals were common in this era. As per scientists the organisms present in this era were simple unicellular organisms with soft bodies. Examples include bacteria like and alga like organisms. 

Fig: Cooling down of Earth in the Archeozoic era

Proterozoic era

It is commonly called ‘the era of ancient life’. This era showed the deposition of large amounts of sediments and glaciation. It is believed that life forms showed complex nature in this era. Examples include bacteria, cyanobacteria, sponges, jelly fishes, fungi, annelid worms etc.

This era ended by the Killarney revolution. This revolution resulted in the world wide continental uplift and erosion. 

Fig: Bacteria

Cryptozoic era

The Archeozoic and Proterozoic eras are collectively called the Cryptozoic era or the Precambrian time. This era is characterised by the availability of less fossil records.

Paleozoic era

It is also called ‘the cradle of ancient life’. Different types of plants and animals lived in both water and on land in this era. The fossil records of this era reflected the progression of different plants and animals. The fossils of first vertebrates appeared late in this era. Availability of fossils were more in this era. This era is divided into seven periods. They are as follows:

• Cambrian Period
• Ordovician Period
• Silurian Period
• Devonian Period
• Mississippian Period
• Pennsylvanian Period
• Permian Period 

Cambrian Period

In this period the melting of glaciers and slow rise of sea level happened. Climate became warm here. This period showed the presence of marine plants and animals. Examples include protozoans, sponges, coelenterates, annelids, molluscs, echinoderms and trilobites. 

Fig: Trilobite of Cambrian Period

Ordovician Period

This period is characterised by a uniformly warm environment. But in certain regions of the Earth glacial activities happened. The first vertebrates in the form of armoured jawless fishes called Ostracoderms which were present in the freshwaters appeared during this period. The first corals appeared in this period. Snails, squids and clams were also present in this period. 

Fig: Ostracoderms of Ordovician Period

Silurian Period

There was a mild climate with arid areas present in this period. Rise of sea level was observed in this period. Volcanic activities were also common during this period. During this period the appearance of first land plants and air breathing animals happened. Ferns were the first land plants. Huge coral reefs were present. Eurypterids (ancestors of scorpions) appeared. The Ostracoderms were present during this period too. The ancestral form of cartilaginous and freshwater fishes also appeared. In the Silurian period, two major types of plants existed. The Rhynia-type and Psilophyton which are considered as the ancestors of modern pteridophytes.

Fig: Plants of Silurian Period

Devonian Period

It is called ‘the age of fishes’. This period had a mild climate with dry areas. Volcanic activities were common. Formation of coal, oil and gas occurred during this period. The famous Arcadian disturbance happened in this period. The land plants appeared during this period. Seed ferns, horse tails and lycopods also appeared. The ammonites (a special group of cephalopods) also appeared in this period and the ostracoderms evolved into fishes during this period. Spiny skinned sharks formed during this period. Lobefins also came into existence in this period which are considered as the ancestors of modern day frogs and salamanders (amphibians). 

Fig: Lobefin of Devonian Period

 Mississippian Period

The climate was warm and dry in this period. It is believed that the formation of coal, oil, lead, zinc, gold, silver, gypsum and rock salts occurred during this period. The Earth was covered with plants during this period. Fauna included ferns, club mosses, primitive seed plants, spore bearing trees etc. Salamanders like amphibians were also present. Sharks, molluscs and arthropods were also present during this period. 

Fig: The vegetation of Mississippian Period

Pennsylvanian Period

The climate was mild and cold due to glaciation and desert formation in this period. The Appalachian Revolution occurred during this period. The land is flourished by horse tails, club mosses and woody plants. Dragon flies and cockroaches were present. The first reptiles appeared in this period. 

Fig: First reptiles of Pennsylvanian Period

Permian Period 

Climate showed lots of changes during this period. It was colder with dry areas. Woody plants were common. Mayflies, beetles, dragonflies and fishes were present. Therapsids originated in this period. 

Fig: Therapsid of Permian Period

Carboniferous Period

The formation of extensive coal beds occurred in this period. The Missisipian period and the Pennsylvanian period are together called Carboniferous period. This period is known as the age of ferns and it saw the emergence of various genera of seed ferns which are extinct spore bearing plants i,e they propagated with the help of spores. There existed mammal-like creatures called synapsids. They were viviparous and protected their unborn young inside the mother’s body.

Mammals were more intelligent in sensing and avoiding danger at least.

Fig: Seed ferns of Carboniferous Period

Mesozoic era

The Mesozoic era is dominated by reptiles, hence it is also called the age of reptiles. This era also witnessed the extinction of reptiles. Almost all modern phyla of plants and animals were established in this era. It is commonly called ‘the era of intermediate life’. Triassic, Jurassic and Cretaceous periods belong to the Mesozoic era. 

Fig: Reptiles of Mesozoic era

Triassic Period

The climate was harsh and dry during this period. Gymnosperms like conifers and cycads were present. Seed ferns became extinct in this period. Lizards like dinosaurs appeared. Ichthyosaurs and Plesiosaurs were present. Egg laying mammals also appeared in this period. 

Fig: Ichthyosaurs in Triassic Period

Jurassic Period

The climate was comparatively milder during this period. It was warm and humid with mild rains. Conifers, cycads and ferns were present. Angiosperms appeared for the first time during this period. Gigantic dinosaurs ruled the Earth during this period. Archaeopteryx was present. Lizards and crocodiles appeared for the first time during this period.

Fig: Dinosaurs of Jurassic period

Cretaceous Period

The climate was warm during this period. The flowering plants established everywhere. Deciduous trees dominated the Earth’s surface. Sharks and bony fishes were present. Snakes appeared for the first time. Mammals were small and less in number. Egg laying Prototherians were present. Eutherian or the placental mammals appeared for the first time in this period. Sphenopsids or horsetails were seen which were primitive vascular plants characterised by jointed ribbed stems, small leaves usually in whorls at distinct stem nodes. Ginkgo, also known as the maidenhair tree which had characteristic fan shaped leaves, was present. Gnetales are the ancestors of angiosperms. Unlike other gymnosperms, they possess vessel elements in the xylem. Dicotyledons, the plants which possess an embryo with two cotyledons, also evolved during this period. 

Fig: Sphenopsids of Cretaceous Period

Tyrannosaurus rex is the largest carnivorous dinosaur which was present during this period. 

GIF: Tyrannosaurus rex of Cretaceous Period

 Cenozoic era 

 The Cenozoic era is the latest era or the current era in which we are living. The cenozoic era has seen the emergence of mammals and has been mostly dominated by different kinds of mammalian species. It is also known as the age of mammals. This era is divided into two periods as follows:

• Tertiary Period
• Quaternary Period

Fig: Animals of Cenozoic era

 Tertiary Period

The climate was warm but gradually it became colder during this period. This period ended with the Cascadian revolution. The mountain ranges attained their present shape in this period. Grasslands appeared in this period. Hardwood trees and conifers were present. In the tertiary period, the bryophytes which exist even today came into existence.Monocotyledons evolved during this period. Modern mammals formed in this period. This period is divided into the following epochs:

• Eocene epoch
• Oligocene epoch
• Miocene epoch
• Pliocene epoch

Fig: Animals of Tertiary Period

Eocene epoch

Angiosperms become more abundant during this epoch. Monotremes were present. Diversification of placental mammals occurred in this epoch. Ancestors of camels, pigs, horses (Eohippus), rats and monkeys were present. Bats (aerial) and sea cows (water) were also present.

Fig: Eohippus of Eocene epoch

Oligocene epoch

Climate was pleasant in this epoch. Ancestors of camels, pigs, horses, rats and monkeys were present. Bats and sea cows were also present. Monocotyledons flourished on the land in this epoch. Hoofed mammals like rhinoceros were present. 

Miocene epoch

There was a general lowering of the temperature in this epoch. Deciduous trees were more abundant. The development of modern mammals continued in this epoch also. The ancestors of man (Dryopithecus) and apes also appeared. 

Fig: Dryopithecus of Miocene epoch

Pliocene epoch

It ended with the beginning of one of the two greatest ice ages. Climate was becoming colder in this epoc. Grasslands were present. Placental mammals were present in this epoc. Elephants, rhinoceros, hippopotamus, camels, giraffes, deers, antelope and wild horses were present. The ancestors of man, Australopithecus were present during this epoch. 

Fig: Australopithecus of Pliocene epoch

Quaternary Period

This period witnessed four epochs of ice ages. The intervals between glaciations in this period represent interglacial periods with warm climates. It is believed that we are still in an interglacial period and the ice age has not ended yet. All the modern forms of life were represented in this period. It include two epochs as follows:

• Pleistocene epoch
• Recent epoch

Pleistocene epoch

Repeated glaciations due to unusual solar activity was present in this epoch. The Saber-toothed tigers, mammoths, elephants, and ground sloths become extinct in this epoch. They were not able to adjust to the cold. Hairy elephants, woolly rhinoceros and cave bears became adapted to the climate and survived. Ancestral forms of man (Homo Habilis) and apes were also present in this epoch. 

Fig: Homo Habilis of Pleistocene epoch

Recent epoch

It includes the time after the last glaciation. Spreading of forest occurred in some parts of the Earth during this epoch. At the same time in some parts of the Earth deserts grew up. Modern man (Homo sapiens sapiens) evolved and underwent cultural evolution in this epoch. 

Fig: Animals of Recent epoch

An overview of geological time scale

The evolution took place in different eras and their respective periods which are represented in the image given below. 

Fig: Geological time scale

Significance of geological time scale

The geological time scale is a chronological measurement system that is related to the study of rocks, strata, its distribution as well as age of sedimentary rocks with respect to time. This information can be used by geologists, palaeontologists or other scientists to understand and describe the relationship between time and events that took place in Earth’s history.

Practice Problems

1. From the following options, identify the ascending order of eras in the geological time scale for the hypothetical vertebrate evolution?

1. Cenozoic, Mesozoic, Palaeozoic
2. Cenozoic, Palaeozoic, Mesozoic
3. Mesozoic, Palaeozoic, Cenozoic
4.  Palaeozoic, Mesozoic, Cenozoic

Solution: The geological time scale is the representation of the duration of the eras and periods with their dominant forms of life in the history of Earth. The geological time scale is divided into eras and each era is divided into periods. The correct chronological order of various eras in the geological time scale for the hypothetical vertebrate evolutionis Palaeozoic → Mesozoic → Cenozoic. Hence the correct option d. 

Figure : Evolution of vertebrates

The Paleozoic era includes Silurian, Devonian, Carboniferous and Permian periods in the chronological order. Invertebrates (non-chordates) evolved 500 mya. The first chordates were seen during the Paleozoic era. Fishes evolved during the Paleozoic era and the Devonian period is considered the age of fishes. From lobe-finned fishes, amphibians evolved during the Devonian Period and the Carboniferous Period is generally considered as the age of amphibians.The first reptiles appeared during the Carboniferous Period of the Paleozoic era and further diversified during the Mesozoic era. The Mesozoic era is considered the age of reptiles. The Mesozoic era has various periods like the Triassic, Jurassic and Cretaceous in chronological order. The evolution of the first mammals and birds occurred during the Triassic Period and Jurassic Period respectively. Mammals flourished during the Cenozoic era, which is called the age of mammals.

Figure : Geological time scale showing the evolution of vertebrates

2. The Cenozoic era is referred to as the age of mammals. It is divided into ____(i)______ periods namely _____(ii)______and____(iii)_____.

1. (i) - 2, (ii) -Tertiary, (iii) - Quaternary
2. (i) - 3, (ii) - Carboniferous, (iii) - Devonian
3. (i) - 2, (ii) - Triassic, (iii) - Permian 
4. (i) - 2, (ii) Createous, (iii) - Tertiary

Solution : The Cenozoic era has seen the emergence of mammals and has been mostly dominated by different kinds of mammalian species. It is also known as the age of mammals. This era is further divided into two periods namelyTertiary and Quaternary. Carboniferous, Devonian and Silurian periods belong to the Paleozoic era along with the Permian period. The Cretaceous Period belongs to the Mesozoic era along with Triassic and Jurassic periods.

The following geological time scale depicts the cenozoic era and the respective periods.

Figure : Cenozoic era

3. The Mesozoic era is also called the golden age of the ____________ .

1. amphibians
2. reptiles
3. mammals 
4. fishes

Solution : The total span of Earth’s evolution or geological time is divided into eras. 

The Mesozoic era is known as ‘the golden age of the reptiles’ as it was marked with the emergence and evolution of a variety of reptilian species. Dinosaurs (giant reptiles) were dominant on the Earth in this era along with the ancestral form of the birds like toothed birds. The Cenozoic era is the era in which we are currently living, and it is called the ‘age of mammals’ whereas the Devonian Period of the Paleozoic era is known as the age of fishes. 

Figure : Geological time scale with the dominant organisms

FAQs

Q1. Prove that there is an evolutionary connection between reptiles and birds?

Answer: Reptiles are ancestors of birds, their feathers developed to form an insulating coat on the body so as to prevent the loss of body heat. But later, these feathers became useful in flight. This supported the view that birds are very closely related to reptiles. As per studies, birds descended from a group of two-legged dinosaurs known as Theropods, whose members included the Tyrannosaurus rex.

Figure : Theropods

Fossil study of Archaeopteryx also supported the evolution of birds from reptiles. Archaeopteryx is a fossil bird which has both avian and reptilian features and hence it is considered as a missing link between reptiles and birds. It acts as an evolutionary connection between birds and reptiles. The avian characters include the jaws being modified into a beak and the exoskeleton of feathers.

The reptilian characters include jaws bearing teeth (modern birds lack teeth), spongy bones and many tail vertebrae (birds have short tails with fused vertebrae). This implies that birds have evolved from reptiles. Birds, for that matter, can fly. It means the feathers which were mostly functionless in the dinosaurs gained utmost importance in birds and gave them the ability to fly. 

Figure : Archaeopteryx

Q2. What are the 3 things needed for evolution?

Answer: The essence of Darwin's theory is that natural selection will occur if three conditions are met. These conditions are as follows: 

• A struggle for existence
• Variations
• Inheritance

Q3. Why is evolution important to life?

Answer: Evolution explains how living things are changing today and how modern living things have descended from ancient life forms that no longer exist on Earth. As living things evolve, they generally become better suited for their environment. This is because they evolve adaptations.

Q4. Why do organisms change over evolution?

Answer: Biological evolution refers to the cumulative changes that occur in a population over time. These changes are produced at the genetic level as the genes of the organisms mutate and/or recombine in different ways during reproduction and are passed onto future generations.

Q5. What causes evolution?

Answer: There are five key mechanisms that cause a population, a group of interacting organisms of a single species, to exhibit a change in allele frequency from one generation to the next. These are as follows:

• Mutation
• Genetic drift
• Gene flow
• Non-random mating
• Natural selection

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