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Cephalaspis, a
jawless fish. |
Eras of the
Phanerozoic
The Phanerozoic is divided into three eras: the
Paleozoic, Mesozoic, and Cenozoic, which are further
subdivided into 12 periods. The Paleozoic features the
rise of fish, amphibians and reptiles. The Mesozoic is
ruled by the reptiles, and features the evolution of
mammals, and more famously, dinosaurs, including birds.
The Cenozoic is the time of the mammals, and more
recently, humans.
Paleozoic Era
The Paleozoic is a time in Earth's history when complex
life forms evolved, took their first breath of oxygen on
dry land, and when the forerunners of all life on Earth
began to diversify. There are six periods in the
Paleozoic era: Cambrian, Ordovician, Silurian, Devonian,
Carboniferous and Permian.
Cambrian Period
The Cambrian is the first period of the Paleozoic Era
and ran from 541 million 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
the fauna was dominated by armored arthropods, such as
trilobites. Almost all marine phyla evolved in this
period. During this time, the super-continent Pannotia
began to break up, most of which later recombined into
the super-continent Gondwana.
Ordovician Period
The Ordovician spans from 485 million to 444 million
years ago. The Ordovician was a time in Earth's history
in which many species still prevalent today evolved,
such as primitive fish, cephalopods, and coral. The most
common forms of life, however, were trilobites, snails
and shellfish.[clarification needed] More importantly,
the first arthropods crept ashore to colonize Gondwana,
a continent empty of animal life. By the end of the
Ordovician, Gondwana had moved from the equator to the
South Pole, and Laurentia had collided with Baltica,
closing the Iapetus Ocean. The glaciation of Gondwana
resulted in a major drop in sea level, killing off all
life that had established along its coast. Glaciation
caused an icehouse Earth, leading to the
Ordovician–Silurian extinction, during which 60% of
marine invertebrates and 25% of families became extinct.
This is considered the first mass extinction and the
second deadliest in the history of Earth.
The Silurian spans from 444 million to 419 million years
ago, which saw a warming from an icehouse Earth. This
period saw the mass evolution of fish, as jawless fish
became more numerous, jawed fish evolved, and the first
freshwater fish evolved, though arthropods, such as sea
scorpions, remained the apex predators. Fully
terrestrial life evolved, which included early
arachnids, fungi, and centipedes. The evolution of
vascular plants (Cooksonia) allowed plants to gain a
foothold on land. These early terrestrial plants are the
forerunners of all plant life 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.
Devonian Period
The Devonian spans from 419 million to 359 million years
ago. Also informally known as the "Age of the Fish", the
Devonian features a huge diversification in fish,
including armored fish like Dunkleosteus and lobe-finned
fish which eventually evolved into the first tetrapods.
On land, plant groups diversified; the first trees and
seeds evolved. By the Middle Devonian, shrub-like
forests of primitive plants existed: lycophytes,
horsetails, ferns, and progymnosperm. This event also
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. |
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Eogyrinus (an
amphibian) of the Carboniferous. |
Carboniferous Period
The Carboniferous spans from 359 million to 299 million
years ago. During this period, average global
temperatures were exceedingly high: the early
Carboniferous averaged at about 20 degrees Celsius (but
cooled to 10 degrees during the Middle Carboniferous).
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 and remain the dominant vertebrates
throughout the period. Also, the first reptiles and
synapsids 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, in an event known
as the Permo-Carboniferous glaciation or the
Carboniferous rainforest collapse. |
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Dimetrodon. |
Permian Period
The Permian spans from 299 million to 252 million years
ago and was the last period of the Paleozoic era. 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 the climate of the interior of
Pangaea wasn't regulated by large bodies of water.
Reptiles and synapsids flourished in the new dry
climate. Creatures such as Dimetrodon and Edaphosaurus
ruled the new continent. The first conifers evolved,
then dominated the terrestrial landscape. Nearing the
end of the period, Scutosaurus and gorgonopsids filled
the arid landmass. 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. |
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Plateosaurus (a
prosauropod). |
Mesozoic Era
The Mesozoic ranges from 252 million to 66 million years
ago. Also known as "the age of the dinosaurs", the
Mesozoic features the rise of reptiles on their
150-million-year conquest of the Earth on the land, in
the seas, and in the air. There are three periods in the
Mesozoic: Triassic, Jurassic, and Cretaceous.
Triassic Period
The Triassic ranges from 252 million to 201 million
years ago. The Triassic is a transitional time in
Earth's history between the Permian Extinction and the
lush Jurassic Period. It has three major epochs: Early
Triassic, Middle Triassic and Late Triassic.
The Early Triassic lasted between 252 million to 247
million years ago, and was dominated by deserts as
Pangaea had not yet broken up, thus the interior was
arid. The Earth had just witnessed a massive die-off in
which 95% of all life became extinct. The most common
life on Earth were Lystrosaurus, labyrinthodonts, and
Euparkeria along with many other creatures that managed
to survive the Great Dying. Temnospondyli flourished
during this time and were dominant predators for much of
the Triassic.
The 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 and nothosaurs, 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 that had long dominated the freshwater
environment.
The Late Triassic spans from 237 million to 201 million
years ago. Following the bloom of the Middle Triassic,
the Late Triassic featured frequent rises of
temperature, as well as moderate precipitation (10 to 20
inches per year). The recent warming led to a boom of
reptilian evolution on land as the first true dinosaurs
evolved, as well as pterosaurs. 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, in which all
archosaurs (excluding ancient crocodiles and dinosaurs),
most synapsids, and almost all large amphibians became
extinct, as well as 34% of marine life in the fourth
mass extinction event. The extinction's cause is
debated. |
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Sericipterus. |
Jurassic Period
The Jurassic ranges from 201 million to 145 million
years ago, and features three major epochs: Early
Jurassic, Middle Jurassic, and Late Jurassic.
The Early Jurassic Epoch spans from 201 million to 174
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, with species such as
Dilophosaurus at the apex. 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.
The Middle Jurassic Epoch spans from 174 million to 163
million years ago. During this epoch, dinosaurs
flourished as huge herds of sauropods, such as
Brachiosaurus and Diplodocus, filled the fern prairies
of the Middle Jurassic. 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 were quite common, and ichthyosaurs were
flourishing. This epoch was the peak of the reptiles.
The Late Jurassic Epoch 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.
Sea levels rose, destroying fern prairies and creating
shallows. Ichthyosaurs became extinct whereas sauropods,
as a whole, did not; in fact, some species, like
Titanosaurus, lived until the K–T extinction. The
increase in sea-levels opened up the Atlantic sea way
which would continue to get larger over time. The
divided world would give opportunity for the
diversification of new dinosaurs. |
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Artist's 1901
depiction of a Stegosaurus (inaccurately
portrayed with a dragging tail). |
Cretaceous Period
The Cretaceous is the Phanerozoic's longest period, and
the last period of the Mesozoic. It spans from 145
million to 66 million years ago, and is divided into two
epochs: Early Cretaceous, and Late Cretaceous.
The Early Cretaceous Epoch spans from 145 million to 100
million years ago. The Early Cretaceous saw the
expansion of seaways, and as a result, the decline and
extinction of sauropods (except in South America). Many
coastal shallows were created, and that caused
ichthyosaurs to die out. Mosasaurs evolved to replace
them as apex species of the seas. Some island-hopping
dinosaurs, like Eustreptospondylus, evolved to cope with
the coastal shallows and small islands of ancient
Europe. Other dinosaurs, such as Carcharodontosaurus and
Spinosaurus, rose to fill the empty space that the
Jurassic-Cretaceous extinction had created. Of the most
successful would be the Iguanodon which spread to every
continent. Seasons came back into effect and the poles
grew seasonally colder. Dinosaurs such as the
Leaellynasaura inhabited the polar forests year-round,
while many dinosaurs, such as the Muttaburrasaurus,
migrated there during summer . Since it was too cold for
crocodiles, it was the last stronghold for large
amphibians, such as the Koolasuchus. In this epoch
Pterosaurs reached their maximum diversity and grew
larger, as species like Tapejara and Ornithocheirus took
to the skies. The first true birds evolved, possibly
sparking competition between them and the pterosaurs. |
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Tylosaurus (a
mosasaur) hunting Xiphactinus. |
The Late Cretaceous Epoch spans from 100
million to 66 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 and areas beyond the tropic
lines featured extreme seasonal changes of weather.
Dinosaurs still thrived as new species such as
Tyrannosaurus, Ankylosaurus, Triceratops and Hadrosaurs
dominated the food web. Whether or not Pterosaurs went
into a decline as birds radiated is debated; however,
many families survived until the end of the Cretaceous,
alongside new species such as the gigantic
Quetzalcoatlus. Marsupials evolved within the large
conifer forests as scavengers. In the oceans, Mosasaurs
ruled the seas to fill the role of the ichthyosaurs, and
huge plesiosaurs, such as Elasmosaurus, evolved. Also,
the first flowering plants evolved. 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 creating the
event known as the K–Pg extinction, the fifth and most
recent mass extinction event, during which 75% of life
on Earth became extinct, including all non-avian
dinosaurs. Every living thing with a body mass over 10
kilograms became extinct, and the age of the dinosaurs
came to an end. |
|
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Basilosaurus was an
ancient cetacean ancestor. |
Cenozoic Era
The Cenozoic featured the rise of mammals as the
dominant class of animals, as the end of the age of the
dinosaurs left significant evolutionary vacuums. There
are three divisions of the Cenozoic: Paleogene, Neogene
and Quaternary.
Paleogene Period
The Paleogene 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 Paleocene Epoch began with the K–Pg extinction event
caused by the impact of a meteorite in the area of
present-day Yucatan Peninsula and caused the destruction
of 75% of all species on Earth. The Early Paleocene saw
the recovery of the Earth from that event. The
continents began to take their modern shape, but all
continents (and India) were separated from each other.
Afro-Eurasia was separated by the Tethys Sea, 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. The oceans were dominated by sharks
as the large reptiles that had once ruled became
extinct. Archaic mammals, such as creodonts and early
primates that evolved during the Mesozoic filled the
world. Mammals were still quite small, meanwhile
enormous crocodiles and snakes like Titanoboa radiated
to fill the niche of top predator.
The Eocene Epoch ranged 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. At
the top of the food chains were huge birds, such as
Gastornis. Carnivorous flightless birds continued to be
top predators for much of the rest of the Cenozoic,
until their extinction in the Quaternary period. The
temperature was 30 degrees Celsius with little
temperature gradient from pole to pole. In the Middle
Eocene Epoch, 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, which
were, by now, almost fully aquatic. Mammals like
Andrewsarchus were now at the top of the food-chain and
sharks were replaced by Basilosaurus, whales, as rulers
of the seas. The late Eocene Epoch saw the rebirth of
seasons, which caused the expansion of savanna-like
areas, along with the evolution of grass. At the
transition between the Eocene and Oligocene epochs there
was a significant extinction event, the cause of which
is debated.
The Oligocene Epoch spans from 34 million to 23 million
years ago. The Oligocene was an important transitional
period between the tropical world of the Eocene and more
modern ecosystems. 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, along with many other
perissodactyls.
Neogene Period
The Neogene spans from 23.03 million to 2.58 million
years ago. It features two epochs: the Miocene, and the
Pliocene.
The Miocene spans from 23.03 million to 5.333 million
years ago and is a period in which grass spread further
across, effectively dominating a large portion of the
world, diminishing forests in the process. Kelp forests
evolved, leading to the evolution of new species, such
as sea otters. During this time, perissodactyla thrived,
and evolved into many different varieties. Alongside
them were the apes, which evolved into 30 species.
Overall, arid and mountainous land dominated most of the
world, as did grazers. 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.
The Pliocene lasted from 5.333 million to 2.58 million
years ago. The Pliocene featured dramatic climactic
changes, which ultimately led to modern species and
plants. The Mediterranean Sea dried up for several
thousand years in the Messinian salinity crisis. Along
with these major geological events, Australopithecus
evolved in Africa, beginning the human branch. The
isthmus of Panama formed, and animals migrated between
North and South America, wreaking havoc on the local
ecology. Climatic changes brought savannas that are
still 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, save for changes brought about by the
glaciations of the Quaternary, such as the Great Lakes. |
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Megafauna of the
Pleistocene (mammoths, cave lions, woolly rhino,
reindeer, horses). |
Quaternary Period
The Quaternary spans from 2.58 million years ago to
present day, and is the shortest geological period in
the Phanerozoic Eon. It features modern animals, and
dramatic changes in the climate. It is divided into two
epochs: the Pleistocene and the Holocene.
The Pleistocene lasted from 2.58 million to 11,700 years
ago. This epoch was marked by ice ages 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. Meanwhile, Africa
experienced a trend of desiccation which resulted in the
creation of the Sahara, Namib, and Kalahari deserts.
Many animals evolved including mammoths, giant ground
sloths, dire wolves, saber-toothed cats, and most
famously Homo sapiens. One hundred thousand years ago
marked the end of one of the worst droughts of Africa,
and led to the expansion of primitive human. 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. All the
continents were affected, but Africa to a lesser extent.
That continent retains many large animals, such as
hippos. The extent to which Homo Sapiens were involved
in this extinction is debated.
The Holocene began 11,700 years ago and lasts until the
present day. All recorded history and "the Human
history" 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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