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Phanerozoic Eon: Paleozoic Era, Mesozoic Era & Cenozoic Era

  • The boundary between the Proterozoic and the Phanerozoic eons was set when the first fossils of animals such as trilobites appeared.
  • Life remained mostly small and microscopic until about 580 million years ago, when complex multicellular life arose, developed over time, and culminated in the Cambrian Explosion about 541 million years ago.
  • This sudden diversification of life forms produced most of the major life forms known today.
  • Plant life on land appeared in the early Phanerozoic eon.
  • Complex life, including vertebrates, begin to dominate the Earth’s ocean.
  • Pangaea forms and later dissolves into Laurasia and Gondwana.
  • Gradually, life expands to land and all familiar forms of plants, insects, animals and fungi begin appearing.
  • Birds, the descendants of dinosaurs, and more recently mammals emerge.
  • Modern animals—including humans—evolve at the most recent phases of this eon (2 million years ago).

The Phanerozoic eon is divided into three eras:

  1. the Palaeozoic, an era of arthropods, amphibians, fishes, and the first life on land;
  2. the Mesozoic, which spanned the rise, reign of reptiles, climactic extinction of the non-avian dinosaurs, the evolution of mammals and birds; and
  3. the Cenozoic, which saw the rise of mammals.
  • The Phanerozoic is divided into three eras: the Paleozoic, Mesozoic, and Cenozoic, which are further subdivided into 12 periods.

Phanerozoic Eon Geological Time Scale

Paleozoic Era

  • There are six periods in the Paleozoic era: Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian.

Cambrian Period

  • Spans from 541 to 485 million years ago.
  • The Cambrian sparked a rapid expansion in evolution in an event known as the Cambrian Explosion during which the greatest number of creatures evolved in a single period in the history of Earth.
  • Plants like algae evolved, and arthropods dominated the fauna.
  • Almost all marine phyla evolved in this period.

Lifeforms during the Cambrian Period

Lifeforms during the Cambrian Period (OpenStax, via Wikimedia Commons)

Ordovician Period

  • Spans from 485 million years to 440 million years ago.
  • Many species still prevalent today evolved, such as primitive fish, corals, etc.
  • The most common forms of life, however, were trilobites, snails and shellfish.
  • More importantly, the first arthropods crept ashore (the beginning of terrestrial lifeforms).
  • By the end of the Ordovician, Gondwana had moved from the equator to the South Pole.
  • The glaciation of Gondwana resulted in a major drop in sea level, killing off all life along its coast.
  • Glaciation caused a snowball Earth, leading to the Ordovician-Silurian extinction (First Mass Extinction).

Ordovician–Silurian extinction (First Mass Extinction)

  • This is considered as the second deadliest in the history of Earth.
  • This event greatly affected marine communities.
  • As the southern supercontinent, Gondwana drifted over the South Pole, ice caps formed on it.
  • A combination of lowering of sea level and glacially driven cooling were likely driving agents.
  • A fall in atmospheric carbon dioxide preceded the late Ordovician glaciation event.
  • The dip is correlated with a burst of volcanic activity that deposited new silicate rocks, which draw CO2 out of the air as they erode.

Silurian Period

  • The Silurian spans from 440 million years to 415 million years ago.
  • It saw warming from Snowball Earth.
  • This period saw the mass evolution of fish.
  • The first freshwater fish evolved, though arthropods, such as sea scorpions, remained the apex predators.
  • Fully terrestrial life evolved, which included fungi, and centipedes.
  • The evolution of vascular plants allowed plants to gain a foothold on land.
  • During this time, there were four continents: Gondwana (Africa, South America, Australia, Antarctica, India), Laurentia (North America with parts of Europe), Baltica (the rest of Europe), and Siberia (Northern Asia).
  • The recent rise in sea levels provided new habitats for many new species.

Lifeforms during the Silurian Period

Lifeforms during the Silurian Period (

Devonian Period

  • Spans from 415 million years to 360 million years ago.
  • Also known as the Age of the Fish, the Devonian features a huge diversification in fish.
  • On land, plant groups diversified; the first trees and seeds evolved.
  • By the Middle Devonian, shrub-like forests of primitive plants existed.
  • This event allowed the diversification of arthropod life as they took advantage of the new habitat.
  • The first amphibians also evolved, and the fish were now at the top of the food chain.
  • Near the end of the Devonian, 70% of all species became extinct in an event known as the Late Devonian extinction, which is the second mass extinction known to have happened.

Devonian Period ― Age of Fish

Devonian Period ― Age of Fish (Joseph Smit (1836-1929), via Wikimedia Commons)

Late Devonian extinction (Second Mass Extinction)

  • The Late Devonian extinction occurred about 376–360 million years ago.
  • The extinction seems to have only affected marine life.
  • The causes of these extinctions are unclear.
  • Leading hypotheses include changes in sea level and ocean anoxia (lack of oxygen), possibly triggered by global cooling or oceanic volcanism.

Carboniferous Period

  • Spans from 360 million to 300 million years ago.
  • Tropical swamps dominated the Earth, and the large amounts of trees created much of the carbon that became coal deposits (hence the name Carboniferous).
  • The high oxygen levels caused by these swamps allowed massive arthropods, normally limited in size by their respiratory systems, to proliferate.
  • Perhaps the most important evolutionary development of the time was the evolution of amniotic eggs, which allowed amphibians to move farther inland. (Amniotic fluid is a clear, slightly yellowish liquid that surrounds the foetus).
  • Also, the first reptiles evolved in the swamps.
  • Throughout the Carboniferous, there was a cooling pattern, which eventually led to the glaciation of Gondwana as much of it was situated around the south pole.

Permian Period

  • Spans from 300 million to 250 million years ago.
  • At its beginning, all continents came together to form the super-continent Pangaea, surrounded by one ocean called Panthalassa.
  • The Earth was very dry during this time, with harsh seasons, as large bodies of water didn’t regulate the climate of the interior of Pangaea.
  • Reptiles flourished in the new dry climate.
  • Creatures such as Dimetrodon and Edaphosaurus ruled the new continent.

Dimetrodon and Edaphosaurus

(English Wikipedia)

  • The first conifers evolved, then dominated the terrestrial landscape.
  • Nearing the end of the period, Scutosaurus and gorgonopsids filled the arid landmass.

Scutosaurus and gorgonopsids


  • Eventually, they disappeared, along with 95% of all life on Earth in an event simply known as “the Great Dying“, the world’s third mass extinction event and the largest in its history.

Permian–Triassic extinction event (Third Mass Extinction)

  • The Permian–Triassic (P-T) extinction event is also known as the Great Dying.
  • It occurred about 252 Ma (million years) ago, forming the boundary between the Paleozoic and the Mesozoic eras.
  • It is the Earth’s most severe known extinction event, with up to 96% of all marine species and 70% of terrestrial vertebrate species becoming extinct.
  • It is the only known mass extinction of insects.
  • Suggested causes include large meteor impact events, massive volcanism such as that of the Siberian Traps, runaway greenhouse effect triggered by the sudden release of methane from the sea floor due to methane-producing microbes known as methanogens.
  • Possible contributing gradual changes include sea-level change, increasing anoxia, increasing aridity, and a shift in ocean circulation driven by climate change.

Mesozoic Era

  • Spans from 250 million to 66 million years ago.
  • Also known as “the Age of the dinosaurs“, the Mesozoic features the rise of reptiles.
  • There are three periods in the Mesozoic: Triassic, Jurassic, and Cretaceous.

Triassic Period

  • Spans from 250 million to 200 million years ago.
  • It is a transitional time between the Permian Extinction and the lush Jurassic Period.
  • It has three major epochs: Early Triassic, Middle Triassic and Late Triassic.

Early Triassic

  • Spans from 250 million to 247 million years ago.
  • Deserts dominated Pangaea (not yet broken up; thus the interior was arid).

Supercontinent Pangea

Supercontinent Pangea (User: Kieff, via Wikimedia Commons)

  • The Earth had just witnessed a massive die-off in which 95% of all life became extinct.
  • The most common life on Earth was Lystrosaurus, labyrinthodonts, along with many other creatures that managed to survive the Great Dying.

Nobu Tamura

(Nobu Tamura, from Wikimedia Commons)

Middle Triassic

  • Spans from 247 million to 237 million years ago.
  • The Middle Triassic featured the beginnings of the breakup of Pangaea and the beginning of the Tethys Sea.
  • The ecosystem had recovered from the devastation of the Great Dying.
  • Phytoplankton, coral, and crustaceans all had recovered, and the reptiles began increasing in size.
  • New aquatic reptiles, such as ichthyosaurs, proliferated in the seas.
  • Meanwhile, on land, pine forests flourished, as well as mosquitoes and fruit flies.
  • The first ancient crocodilians evolved, which sparked competition with the large amphibians.

Late Triassic

  • Spans from 237 million to 200 million years ago.
  • It featured frequent rises of temperature, as well as moderate precipitation.
  • The recent warming led to a boom of reptilian evolution on land as the first true dinosaurs evolved, as well as pterosaurs.

Dinosaurs and Pterosaurs

Dinosaurs and Pterosaurs (Durbed, via Wikimedia Commons)

  • By the end of the period the first gigantic dinosaurs had evolved and advanced pterosaurs colonised Pangaea’s deserts.
  • The climactic change, however, resulted in a large die-out known as the Triassic-Jurassic extinction event (Fourth Mass Extinction), in which all large amphibians became extinct.

Triassic–Jurassic extinction event (Fourth Mass Extinction)

  • It marks the boundary between the Triassic and Jurassic periods, 201 million years ago.
  • This event happened in less than 10,000 years and occurred just before Pangaea started to break apart.
  • On land, all archosaurs except a few and many of the large amphibians became extinct.
  • This event vacated terrestrial ecological niches, allowing the dinosaurs to assume the dominant role.
  • Gradual climate change, sea-level fluctuations, oceanic acidification reached a tipping point.
  • Massive volcanic eruptions might have caused intense global warming (release of carbon dioxide or sulphur dioxide) or intense global warming (release of aerosols).

Jurassic Period

  • Spans from 200 million to 145 million years ago, and features three major epochs: Early Jurassic, Middle Jurassic, and Late Jurassic.

Early Jurassic

  • Spans from 200 million to 175 million years ago.
  • The climate was much more humid than the Triassic, and as a result, the world was very tropical.
  • In the oceans, plesiosaurs, ichthyosaurs and ammonites dominated the seas.
  • On land, dinosaurs and other reptiles dominated the land.
  • The first true crocodiles evolved, pushing the large amphibians to near extinction.
  • The reptiles rose to rule the world.
  • Meanwhile, the first true mammals evolved, but never exceeded the height of a shrew.

Middle Jurassic

  • Spans from 175 million to 163 million years ago.
  • During this epoch, dinosaurs flourished.
  • Many other predators rose as well, such as Allosaurus.
  • Conifer forests made up a large portion of the world’s forests.
  • In the oceans, plesiosaurs and ichthyosaurs were flourishing.
  • This epoch was the peak of the reptiles.

Late Jurassic

  • Spans from 163 million to 145 million years ago.
  • The Late Jurassic featured a massive extinction of sauropods and ichthyosaurs due to the separation of Pangaea into Laurasia and Gondwana in an extinction known as the Jurassic-Cretaceous extinction.
  • The increase in sea-levels opened up the Atlantic seaway which would continue to get larger over time.
  • The divided world would give an opportunity for the diversification of new dinosaurs.

Cretaceous Period

  • Spans from 145 million to 66 million years ago, and is divided into two epochs: Early Cretaceous, and Late Cretaceous.

Early Cretaceous

  • Spans from 145 million to 100 million years ago.
  • The Early Cretaceous saw the expansion of seaways.
  • Seasons came back into effect, and the poles grew seasonally colder.
  • Since it was too cold for crocodiles, it was the last stronghold for large amphibians.
  • In this epoch, Pterosaurs reached their maximum diversity and grew larger.
  • The first true birds evolved, possibly sparking competition between them and the pterosaurs.

Late Cretaceous

  • Spans from 100 million to 65 million years ago.
  • The Late Cretaceous featured a cooling trend that would continue into the Cenozoic Era.
  • Eventually, tropical ecology was restricted to the equator.
  • Dinosaurs still thrived as new species.
  • Pterosaurs went into a decline as birds radiated.
  • Marsupials evolved within the large conifer forests as scavengers.
  • Also, the first flowering plants evolved.

Cretaceous–Paleogene extinction event (Fifth Extinction)

  • The Cretaceous-Paleogene (K-Pg) or Cretaceous-Tertiary (K-T) extinction, was a sudden mass extinction on Earth approximately 66 million years ago.
  • At the end of the Cretaceous, the Deccan Traps and other volcanic eruptions were poisoning the atmosphere.
  • As this was continued, it is thought that a large meteor smashed into Earth, creating the Chicxulub Crater (Yucatan Peninsula Mexico) creating the event known as the K-T Extinction.

Chicxulub crater

  • Every living thing with a body mass over 10 kilograms became extinct, and the age of the dinosaurs came to an end.
  • In its wake, many groups underwent remarkable adaptive radiation—sudden and prolific divergence into new forms and species.
  • Mammals in particular diversified in the Paleogene, evolving new forms such as horses, whales, bats, and primates. Birds, fish, and perhaps lizards also radiated.

Cenozoic Era

  • The Cenozoic featured the rise of mammals as the dominant class of animals.
  • There are three divisions of the Cenozoic: Paleogene, Neogene and Quaternary.

Paleogene Period

  • Spans from the extinction of the non-avian dinosaurs, some 66 million years ago, to the dawn of the Neogene 23 million years ago. It features three epochs: Paleocene, Eocene and Oligocene.


  • The Early Paleocene saw the recovery of the Earth from the K-T extinction event.
  • The continents began to take their modern shape, but all continents were separated from each other.
  • The Tethys Sea separated Afro-Eurasia, and the Americas were separated by the strait of Panama, as the Isthmus of Panama had not yet formed.
  • This epoch featured a general warming trend, and jungles eventually reached the poles.
  • Sharks dominated the oceans as the large reptiles that had once ruled became extinct.
  • Archaic mammals, such as early primates that evolved during the Mesozoic filled the world.
  • Mammals were still quite small; meanwhile enormous crocodiles and snakes were top predators.


  • Spans from 56 million to 34 million years ago.
  • In the early Eocene, most land mammals were small and living in cramped jungles, much like the Paleocene.
  • Among them were early primates, whales and horses along with many other early forms of mammals.
  • Carnivorous flightless birds continued to be top predators, until their extinction in the Quaternary period.
  • The temperature was 30 degrees Celsius with a little temperature gradient from pole to pole.
  • The circum-Antarctic current between Australia and Antarctica formed which disrupted ocean currents worldwide, resulting in global cooling, and caused the jungles to shrink.
  • This allowed mammals to grow; some such as whales to mammoth proportions.
  • The late Eocene Epoch saw the rebirth of seasons, which caused the expansion of savanna-like areas, along with the evolution of grass.


  • Spans from 33 million to 23 million years ago.
  • This period featured a global expansion of grass which had led to many new species to take advantage, including the first elephants, cats, dogs, marsupials and many other species still prevalent today.
  • Many other species of plants evolved during this epoch also, such as the evergreen trees.
  • The long-term cooling continued, and seasonal rains patterns established.
  • Mammals continued to grow larger.
  • Paraceratherium, the largest land mammal to ever live evolved during this epoch.


Paraceratherium (ABelov2014, via Wikimedia Commons)

Neogene Period

  • Spans from 23.03 million to 2.58 million years ago.
  • It features 2 epochs: the Miocene, and the Pliocene.


  • Spans from 23.03 to 5.333 million years ago.
  • Grass spread further across diminishing forests in the process.
  • The Tethys Sea finally closed with the creation of the Arabian Peninsula and in its wake left the Black, Red, Mediterranean and Caspian Seas. This only increased aridity.
  • Many new plants evolved, and 95% of modern seed plants evolved in the mid-Miocene


  • Spans from 5.333 to 2.58 million years ago.
  • The Pliocene featured dramatic climatic changes, which ultimately led to modern species and plants.
  • The Mediterranean Sea dried up for several thousand years.
  • Australopithecus evolved in Africa, beginning the human branch.


Australopithecus. Image Credits: Wikimedia

  • The isthmus of Panama formed, and animals migrated between North and South America.
  • Climatic changes brought savannas that are continuing to spread across the world, Indian monsoons, deserts in East Asia, and the beginnings of the Sahara Desert.
  • The Earth’s continents and seas moved into their present shapes.
  • The world map has not changed much since except for changes brought about by the glaciations of the Quaternary, such as the Great Lakes.

Quaternary Period

  • Spans from 2.58 million years ago to the present day.
  • It features modern animals and dramatic changes in the climate.
  • It is divided into two epochs: the Pleistocene and the Holocene.


  • Spans from 2.58 million to 11,700 years ago.
  • Ice ages marked this epoch as a result of the cooling trend that started in the Mid-Eocene.
  • There were at least four separate glaciation periods marked by the advance of ice caps as far south as 40 degrees N latitude in mountainous areas.
  • Africa experienced a trend of desiccation which resulted in the creation of the Sahara, Namib, and Kalahari deserts.
  • Many animals evolved including mammoths, dire wolves, and most famously Homo sapiens.
  • 1,00,000 years ago, marked the end of one of the worst droughts of Africa, and led to the expansion of primitive man.
  • As the Pleistocene drew to a close, a major extinction wiped out much of the world’s megafauna, including some of the hominid species, such as Neanderthals.


Neanderthals (Charles R. Knight, Wikimedia)


  • The Holocene began 11,700 years ago and lasts until to the present day.
  • All recorded history and “the history of the world” lies within the boundaries of the Holocene epoch.
  • Human activity is blamed for a mass extinction that began roughly 10,000 years ago, though the species becoming extinct have only been recorded since the Industrial Revolution.
  • This is sometimes referred to as the “Sixth Extinction“.
  • More than 322 species have become extinct due to human activity since the Industrial Revolution.
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