11 Geological Time Scale

Dr. Jyoti Ratan Ghosh

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Contents of this unit

 

1. Introduction

 

2. Precambrian Earth and Life

 

3.  Phanerozoic Earth and Life

 

A.  The Paleozoic Era

 

i) The Cambrian Period

 

ii) The Ordovician Period

 

iii) The Silurian Period

 

 iv) The Devonian Period

 

 v) The Carboniferous period

 

 vi) The Permian Period

 

 B. The Mesozoic Era

 

 i) The Triassic Period

 

 ii) The Jurassic Period

 

 iii) The Cretaceous Period

 

C. The Cainozoic Era

 

i) The Paleogene Period

 

a. The Palaeocene Epoch

 

b. The Eocene Epoch

 

c. The Oligocene Epoch

 

ii)  The Neogene Period

 

a. The Miocene Epoch

 

b. The Pliocene Epoch

 

iii) The Quaternary Period

 

a. The Pleistocene Epoch

 

b. The Holocene Epoch

 

 

Learning objectives

  • What is geologic time scale?
  • What are the different subdivisions of geologic time scale?
  • How the environment changed with geological time?
  • How Earth changed from its origin with geological time?
  • How life forms changed since its origin?
  1. Introduction

    Geological time scale is a system of organizing the earth’s history into natural eras, periods and epochs (Mai et al., 2005). According to the geological time scale the estimated age of the earth is about 4.6 billion years. The geologic time was measured from the information collected by geologists from rock samples and fossil evidences and then sequencing them in a chronological order through time. Later on, with the discovery of absolute dating techniques, geologists have been able to assign absolute ages in years to the subdivisions of the geologic time scale.

 

  According to the geological time scale the time periods since the origin of Earth can be divided into four major subdivisions or Eon. These are Hadean, Archean, Proterozoic and Phanerozoic. The first three Eons are collectively known as Precambrian Eon (Figure 1).

 

2. Precambrian Earth and Life

The earliest episode of geologic time in earth’s history is termed as Precambrian eon. It was started with the formation of Earth since 4600 million years ago (mya) and continued up to the Cambrian i.e. about 545 mya. Thus, the Precambrian occupies about 88% of the geological time period, with a span of 4055 mya.

 

  There are two subdivisions of the Precambrian eon, the older Archaean (4000 mya to 2500 mya) and younger Proterozoic eon (2500 mya to 545 mya). The Archean eon is subdivided into four eras: the Neoarchean (2800 mya to 2500 mya), Mesoarchean (3200 mya to 2800 mya), Paleoarchean (3600 mya to 3200 mya), and Eoarchean (4000 to 3600 mya). The Proterozoic eon is further subdivided into the Neoproteroxoic (1000mya to 545mya), Mesoproterozoic (1600 mya to 1000 mya) and Palaeoproterozoic (2500mya to 1600mya). The Neoproterozoic is again subdivided into the Riphean (1000 mya– 600mya) and Vendian (600 mya – 545 mya).

 

  The Archaean and Proterozoic subdivisions of the Precambrian are dated and identified on the basis of absolute dating of rock samples belonging to these eons. However, no rocks on Earth are known that are older than the Archaean eon i.e. 4000 mya. The time period between the formation of Earth and the Archaean eon was designated by the term Hadean (4600 mya-4000 mya).

 

 With the beginning of Cambrian period Rodinia, the supercontinent formed during the a part of Precambrian eon, began to fragment into smaller continents. Invertebrates with hard parts first appeared in the earlier part of Cambrian period. During this period many new forms of algae also appeared and diversified. Earliest vertebrate jawless fish called ostracoderms first appeared at the end of Cambrian period.

 

ii. The Ordovician Period

Ordovician is the second period of the Paleozoic era, and lasted from 505 to 438mya. The climate of the early and middle Ordovician was mildly warm with high humidity in the atmosphere. However, the later part of the Ordovician was characterized by glaciations. One of the most important features of the Ordovician period was the colonization of the land. As for the first time in the Earth’s history land plants appeared in Ordovician.

  During Ordovician period the Rodinia supercontinent was separated by oceans into subcontinents. The largest of which was Gondwana continent and it moved towards the South Pole during Ordovician.

 

iii. The Silurian Period

  The Silurian extended from 438 mya to 408 mya. Silurian period is subdivided into four epochs, namely the Llandovery, Wenlock, Ludlow and Pridoli. The climate of the Silurian period was cold at the beginning, but towards the end it was warm to hot. During the Silurian, Baltica continent moved towards north and collided with Laurentia continent and resulted in the formation of larger Laurasia continent.

   Vascular land plants and coral reefs were made their first appearance during the middle Silurian. Silurian was also significant for the evolution of fish. Fishes became progressively more abundant during the period and the first jawed fish evolved during late Silurian.

 

iv. The Devonian Period

The Devonian period dated between 408 and 360 mya and was marked by diversification of vertebrate. There were three major continental masses during Devonian, North America and Europe sat together near the equator and a super continent consisting of South America, Africa, Antarctica, India, and Australia at the southern hemisphere. The first flowerless seed plants (gymnosperms) appeared at the late Devonian period. The evolution of the largest reef ecosystems in Earth’s history also occurred during the Devonian period.

  Amphibians first appeared in the fossil record during the Devonian period. The Devonian period is commonly known as the “Age of Fish”, because all major fish groups were present during this time period. Both cartilaginous fish (Chrondrichthyes) and bony fish (Osteichthyes) first evolved during the Devonian period.

 

v. The Carboniferous period

The Carboniferous period lasted from about 360 to 286 mya during the late Paleozoic Era. The majority of the earth’s known coal deposits belong to this period. The Carboniferous period is subdivided into the Mississippian, dated 360 to 320 mya (early Carboniferous and characterized by mostly limestone layers) and the Pennsylvanian, dated 320 to 286 mya (late Carboniferous and characterized by coal-bearing layers). The temperature during the Carboniferous was mild and uniform. The major geological change of the Carboniferous period was the formation of the supercontinent Pangaea due to the collision of Laurasia and Gondwana.

  The first land snails and insects with wings appeared, flourished and radiated during this period. One of the most important evolutionary features of this period was the evolution of amniote egg, which gave the ancestors of birds and reptiles the ability to lay their eggs on land. Reptiles made their appearance for the first time during the Mississippian. During Pennsylvanian period amphibians were abundant and diversified widely.

 

vi. The Permian Period

The Permian period was the last period of the Paleozoic era, lasting from approximately 286 to 245 mya. During Permian period formation of Pangaea was completed and the Appalachian Mountains formed. The beginning of the Permian was marked by glaciations, but by the end of the middle Permian there was no evidence of glaciations. Life in the Permian seas and oceans were similar to the Carboniferous period.

 

B. The Mesozoic Era

The Mesozoic era was the second geologic era of the Phanerozoic eon. The Mesozoic era continued about 179 mya, from 245 mya to 66 mya. Mesozoic rocks contain a variety of valuable mineral resources, including coal, petroleum, uranium, gold, and copper. The Mesozoic era is divided into three time periods, namely, the Triassic (245-208 mya), the Jurassic (208-144 mya), and the Cretaceous (144-66 mya).

i. The Triassic Period

The Triassic period (245 mya to 208 mya) was the oldest period of Mesozoic era. The major geological event of the Triassic period was the beginning of fragmentation of Pangaea supercontinent to form Gondwana (South America, Africa, India, Antarctica, and Australia) in the south and Laurasia (North America and Eurasia) in the north. Triassic climate was warm-temperate to tropical. For the first time palm like gymnosperms (cycads) plants made their appearance during the Triassic. Mammals for the first time made their appearance during the Triassic period. Dinosaurs, both giant quadruped herbivores (Apatosaurus, Brachiosaurus) and bipedal carnivores (Allosaurus, Coelophysis) evolved during the late Triassic. Late Triassic was also marked by the appearance of flying reptiles (pterosaurs).

 

ii. The Jurassic Period

The Jurassic period continued from 208 mya to 144 mya in the Mesozoic Era. The major geological event of the Jurassic period was the breakup of Gondwana continent, which resulted in the separation of Antarctica, India, and Australia from the South America and Africa. Later on India broke away from the landmass containing Antarctica and Australia and began to move northward. This was followed by the separation of South America and Africa at the late Jurassic period. With regard to the climate, the mild Triassic climate continued during the Jurassic period.

  During the Jurassic period land fauna was dominated by dinosaurs. Giant quadruped herbivores Dinosaurs (Brachiosaurus, Apatosaurus) were abundant. The first birds, including the Archaeopteryx also evolved during this period. Coccolithophores phytoplankton also first evolved during the Jurassic period. Fauna in the seas included giant marine crocodiles, and modern-looking sharks and rays during the Jurassic period.

 

iii. The Cretaceous Period

The last period of the Mesozoic era was the Cretaceous period and continued from 144 mya to 66 mya. The important geological events of the Cretaceous period was the separation of Australia and Antarctica continental mass at the later part of Cretaceous and by that time Indian continental mass nearly reached the equator. South America, Africa and Greenland also became independent landmass by the end of Cretaceous period. The climate of the Cretaceous period remained equable, but there was marked difference between north and south.

 

  One of the most important incidents in terrestrial life of floral evolution was the appearance of the first flowering plants during Cretaceous period. Coccolithophores was the dominant phytoplankton during Cretaceous period. There was another mass extinction at the end of the Cretaceous period which resulted in the extinction of dinosaurs along with majority of other species of that time.

 

C. The Cenozoic Era

The Cenozoic era is the most recent of the Phanerozoic eon and we are at present living in this era. The Cenozoic (66 mya to the present) is divided into three periods, the Paleogene (66 to 23.7 mya), Neogene (23.7 to 1.6 mya) and the Quaternary (1.9 mya to present). The Paleogene and Neogene periods were previously called as the Tertiary period. The Paleogene is again subdivided into three epochs namely, the Paleocene (66 mya to 57.8 mya), the Eocene (57.8 to 36.6 mya), and the Oligocene (36.6 to 23.7 mya). The Neogene is subdivided into two epochs namely, the Miocene (23.07 to 5.3 mya) and Pliocene (5.3 to 1.6 mya). The Quaternary is also subdivided into two epochs namely, Pleistocene (1.6mya to 0.01mya) and Holocene or Recent (0.01 mya to Present).

i. THE PALEOGENE PERIOD

a. The Paleocene Epoch

 

The Paleocene epoch is the first epoch of the Paleogene period, which was lasted from 66 to 57.8mya. The early Paleocene was cooler and dryer than the preceding Cretaceous; however, the climate changed and became warm and humid towards the end of Paleogene. Paleocene was marked by the development of modern plant species and diversification of the flowering plants. During the Paleocene epoch, small carnivorous mammals (miacids) with short heavy limbs made their first appearance. The primates (prosimians) also evolved during the Paleocene.

 

b. The Eocene Epoch

The Eocene is the second epoch of the Paleogene period. The Eocene continued from 57.8 to 36.6mya. During the early Eocene Antarctica and Australia as well as Asia, North America and Europe were connected through land mass. But later on during middle Eocene, Antarctica and Australia started to separate. The Eocene was also marked by the formation of Himalayas and Alps. The average temperature of the Eocene epoch was about 300C, however, by the end of the Eocene temperature gradually reduced and a global cooling event was observed at the end of the Eocene.

   

Eocene was the time when most living mammal orders were present and widely diversified. The largest terrestrial mammal, like Elephant, was evolved for the first time during the Eocene. The fossil evidence indicated that whales also first appeared during the early Eocene. During the end of Eocene epoch, the anthropoids (higher primates) evolved from a prosimian lineage.

 

c. The Oligocene Epoch

The Oligocene was the last Epoch of the Paleogene period and lasted from about 36.6 to 23.7mya. The cold climate of the later part of the Eocene was prevalent throughout the Oligocene epoch. During this epoch both the toothed whales and the baleen whales evolved. On land, mammals including horses, deer, camel, elephants, cats, dogs, and earlier primates began to dominate during Oligocene. Diurnal raptors and rodents also first appeared during the Oligocene epoch. Oligocene epoch was also marked by the evolution of both Old World and New World monkeys.

ii. THE NEOGENE PERIOD

a. The Miocene Epoch

 

The Miocene is the first epoch of Neogene period, lasted 23.7 to 5.3 mya and the climate was comparatively warmer than the Oligocene epoch. Artiodactyls including cattle, goats, bison, and many species of antelope made their first appearance during the Miocene. The hominoids, including apes and humans probably diverged from Old World monkeys at the beginning of Miocene epoch. By the end of the Miocene 95% of modern seed plant families existed.

b. The Pliocene Epoch

The Pliocene epoch continued from 5.3 mya to 1.6 mya and was characterized by cold climate, after the warmer Miocene epoch. The change in climate from warmer to cold and dry during Pliocene resulted in the replacement of many forests by grasslands. The major geological event of the Miocene epoch was the joining of the tectonic plates of North and South America. Moreover, during this epoch the tectonic plates of India and Asia also collided, which resulted in the formation of the Himalayas. Pliocene epoch also played an important role in human evolution, because during this time some apes came down from trees and started to exist on the plains in Africa. Australopithecus was believed to probably first evolved in the late Pliocene.

 

iii. THE QUATERNARY PERIOD

a. The Pleistocene Epoch

The Pleistocene epoch (also called the Ice Age), started 1.6mya and ended about 10,000 (0.01mya) years ago. During Pleistocene epoch, several intervals of widespread continental glaciations took place, which covered about 30% of earth’s land surface and separated by several warmer inter glacial periods. Most of the present-day mammals evolved by Pleistocene epoch. However, many large mammals become extinct at the end of the Pleistocene. The Pleistocene was marked by the evolution and expansion of Homo sapiens, and by the end of the Pleistocene, humans dispersed all over the world.

 

b.  The Holocene Epoch

The Holocene is the name given to the last 10,000 years of the earth’s history, which was started since the end of the last major glacial epoch or the Pleistocene epoch. Holocene climate has been fairly stable with a relatively warm period. Animal and plant life have not evolved much during the Holocene epoch, but there have been major shifts in the distributions of plants and animals. Holocene is marked by the development of human knowledge, technology and civilizations.

 

 

you can view video on Geological Time Scale

 

 

References:

  • Mai LL, Owl MY, Kersting MP. 2005. The Cambridge dictionary of human biology and evolution. Cambridge University Press, New York.
  • Bhattacharya DK. 1989. An outline of Indian prehistory.PalakaPrakashan, Delhi
  • Monroe JS, Wicander. 2006. The Changing Earth: exploring geology and evolution. Brooks/Cole Cengage Learning. USA.
  • Selley RC, Cocks LRM, Plimer IR. 2005. Encyclopedia of Geology, Volume – 3. Elsevier/Academic Press, Amsterdam.
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  • Eldredge N. 2002. Life on earth: an encyclopaedia of biodiversity, ecology and evolution. ABC-CLIO, Inc, USA.
  • Miall AD. 2010. The geology of stratigraphic sequences. Springer, Heidelberg.

Web references:

  • http://australianmuseum.net.au/the-geological-time-scale; accessed on 5.2.15
  • http://earthscience.brookscole.com/changingearth4e; accessed on 5.2.15
  • http://www.esse.ou.edu/fund_concepts/Fundamental_Concepts4/Geologic_Time_Scale.html; accessed on 4.2.15
  • http://www.paleoportal.org/index.php?globalnav=time_space; accessed on 6.2.15
  • http://www.ucmp.berkeley.edu/help/timeform.php; accessed on 4.2.15