The PHANEROZOIC Eon (
British English PHANæROZOIC) is the current
geologic eon in the geologic time scale , and the one during which
abundant animal and plant life has existed. It covers 541 million
years to the present, and began with the
Cambrian Period when diverse
hard-shelled animals first appeared. Its name was derived from the
Ancient Greek words φανερός (phanerós) and ζωή (zōḗ),
meaning visible life, since it was once believed that life began in
Cambrian , the first period of this eon. The time before the
Phanerozoic, called the
Precambrian supereon, is now divided into the
Hadean , Archaean and
The time span of the
Phanerozoic starts with the rapid emergence of a
number of animal phyla ; the evolution of those phyla into diverse
forms; the emergence and development of complex plants ; the evolution
of fish ; the emergence of insects and tetrapods ; and the development
of modern fauna . Land plant life appeared in the early Phanerozoic
eon. During this time span, tectonic forces caused the continents to
move and eventually collect into a single landmass known as
which then separated into the current continental landmasses.
* 1 Proterozoic-
* 2 Eras of the
* 4 See also
* 5 Notes
* 6 References
* 7 External links
Life timeline view • discuss • edit -4500 — – -4000 —
– -3500 — – -3000 — – -2500 — – -2000 — – -1500 —
– -1000 — – -500 — – 0 — WATER Single-celled
life PHOTOSYNTHESIS EUKARYOTES Multicellular
life LAND LIFE DINOSAURS MAMMALS FLOWERS ←
Earliest Earth (−4540 ) ← Earliest water ← Earliest
life ← LHB meteorites ← Earliest oxygen ←
Atmospheric oxygen ← Oxygen crisis ← Earliest sexual
reproduction ← Ediacara biota ←
← Earliest humans P
n Pongola Huronian
Cryogenian Andean Karoo
Axis scale : millions of years .
Orange labels: known ICE AGES.
Human timeline and Nature timeline
Phanerozoic boundary is at 541 million years ago. In
the 19th century, the boundary was set at time of appearance of the
first abundant animal (metazoan ) fossils but several hundred groups
(taxa ) of metazoa of the earlier
Proterozoic era have been identified
since the systematic study of those forms started in the 1950s. Most
geologists and paleontologists would probably set the
Phanerozoic boundary either at the classic point where the
first trilobites and reef-building animals (archaeocyatha ) such as
corals and others appear; at the first appearance of a complex feeding
Treptichnus pedum ; or at the first appearance of a
group of small, generally disarticulated, armored forms termed 'the
small shelly fauna '. The three different dividing points are within a
few million years of each other.
In the older literature, the term
Phanerozoic is generally used as a
label for the time period of interest to paleontologists, but that use
of the term seems to be falling into disuse in more modern literature.
ERAS OF THE PHANEROZOIC
Phanerozoic is divided into three eras : the
Paleozoic , Mesozoic
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, birds and more famously, dinosaurs. The
Cenozoic is the time
of the mammals, and more recently, humans.
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
Cambrian is the first period of the
Paleozoic Era and starts 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 the fauna was
dominated by armored arthropods, such as trilobites . Almost all
marine phyla evolved in this period. During this time, the
Pannotia began to break up, most of which later
recombined into the super-continent
Cephalaspis , a jawless fish
Ordovician spans from 485 million years to 440 million years. 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. More importantly, the first arthropods crept
ashore to colonize
Gondwana , a continent empty of animal life. By the
end of the Ordovican,
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 a snowball Earth , leading to the
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
Silurian spans from 440 million years to 415 million years, which
saw a warming from
Snowball Earth . This period saw the mass evolution
of fish, as jaw-less 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
Gondwana (Africa, South America, Australia, Antarctica,
Laurentia (North America with parts of Europe),
rest of Europe), and
Siberia (Northern Asia). The recent rise in sea
levels provided new habitats for many new species.
Devonian Eogyrinus (an amphibian) of the
Devonian spans from 415 million years to 360 million years. Also
known as the "Age of the Fish", the
Devonian features a huge
diversification in fish, including armored fish like
lobe-finned fish which eventually evolved into the first tetrapods .
On land, plant groups diversified incredibly in an event known as the
Devonian Explosion during which the first trees evolved, as well as
seeds. 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.
Carboniferous spans from 360 million to 300 million years. During
this period, average global temperatures were exceedingly high: the
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
Carboniferous glaciation or the
Permian spans from 300 million to 250 million years 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
Panthalassa . The Earth was very dry during this time,
with harsh seasons, as the climate of the interior of
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 empty desert. 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.
Mesozoic ranges from 252 million to 66 million years. 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 ranges from 250 million to 200 million years. The
Triassic is a desolate transitional time in Earth's history between
Extinction and the lush
Jurassic Period. It has three
major epochs: Early Triassic, Middle
Triassic and Late Triassic.
Triassic lasted between 250 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
with many other creatures that managed to survive the Great Dying .
Temnospondyli evolved during this time and would be the dominant
predator for much of the Triassic.
Plateosaurus (a prosauropod )
Triassic spans from 247 million to 237 million years. The
Triassic featured the beginnings of the breakup of
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 ,
evolved. 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 since
rule the freshwater world.
Triassic spans from 237 million to 200 million years.
Following the bloom of the Middle Triassic, the Late
frequent rises of temperature, as well as moderate precipitation
(10-20 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 . The climactic change, however, resulted in a large
die-out known as the
Triassic-Jurassic extinction event , in which all
archosaurs (excluding ancient crocodiles), 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.
Jurassic ranges from 200 million years to 145 million years, and
features three major epochs: Early Jurassic, Middle Jurassic, and Late
Jurassic Epoch spans from 200 million years to 175 million
years. 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 dominanted 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.
Jurassic Epoch spans from 175 million to 163 million
years. During this epoch, reptiles flourished as huge herds of
sauropods, such as
Diplodicus , filled the fern
prairies of the Middle Jurassic. Many other predators rose as well,
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. Artist’s 1901 depiction of a Stegosaurus
(inaccurately portrayed with a dragging tail).
Jurassic Epoch spans from 163 million to 145 million years.
Jurassic featured a massive extinction of sauropods and
ichthyosaurs due to the separation of
Gondwana in an extinction known as the Jurassic-
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.
Cretaceous is the longest period in the Mesozoic, spans from 145
million to 66 million years, and is divided into two epochs: Early
Cretaceous, and Late Cretaceous.
Tylosaurus (a mosasaur )
Cretaceous Epoch spans from 145 million to 100 million
years. 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
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
migrated there during summer . Since it was too cold for crocodiles,
it was the last stronghold for large amphibians, such as the
Koolasuchus . Pterosaurs grew larger as species like Tapejara and
Ornithocheirus evolved. More importantly, the first true birds evolved
sparking competition between them and the pterosaurs.
Cretaceous Epoch spans from 100 million to 65 million years.
Cretaceous featured a cooling trend that would continue into
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
the food web. Pterosaurs , however, were going into a decline as birds
took to the skies. The last pterosaur to die off was
Marsupials evolved within the large conifer forests as scavengers. In
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
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-T 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.
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:
Neogene and Quaternary.
Paleogene spans from the extinction of the dinosaurs, some 66
million years ago, to the dawn of the
Neogene 23 million years ago. It
features three epochs :
Basilosaurus (a whale, despite the name)
Paleocene Epoch began with the
K-T extinction event caused by the
impact of a metorite 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
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.
During this time there were no land creatures over 10 kilograms.
Mammals were still quite small.
Eocene Epoch ranged from 56 million years to 34 million years. In
the early Eocene, land animals were small and living in cramped
jungles, much like the Paleocene. None had a mass over 10 kilograms.
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 . It is the only time in recorded history
that birds ruled the world (excluding their ancestors, the dinosaurs).
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.
Oligocene Epoch spans from 33 million to 23 million years. The
Oligocene featured the 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 in an event known as
Oligocene extinction event (Grand Coupure).
Neogene Animals of the
Hyenadon , entelodont )
Neogene spans from 23.03 million to 2.58 million years ago. It
features 2 epochs: the Miocene, and the Pliocene.
Miocene spans from 23.03 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 a 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
Pliocene lasted from 5.333 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
million years. 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
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.
Pleistocene (mammoths , cave lions , woolly rhino ,
American horses )
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 . 100,000 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. All the continents were
affected, but Africa to a lesser extent. That continent retains many
large animals, such as hippos.
Holocene began 11,700 years ago and lasts until to present day.
All recorded history and "the history of the world " lies within the
boundaries of the
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.
During the Phanerozoic, biodiversity shows a steady but not
monotonic increase from near zero to several thousands of genera.
It has been demonstrated that changes in biodiversity through the
Phanerozoic correlate much better with the hyperbolic model (widely
used in demography and macrosociology ) than with exponential and
logistic models (traditionally used in population biology and
extensively applied to fossil biodiversity as well). The latter models
imply that changes in diversity are guided by a first-order positive
feedback (more ancestors, more descendants) or a negative feedback
that arises from resource limitation, or both. The hyperbolic model
implies a second-order positive feedback. The hyperbolic pattern of
the human population growth arises from a second-order positive
feedback, caused by the interaction of the population size and the
rate of technological growth. The character of biodiversity growth in
Phanerozoic Eon can be similarly accounted for by a feedback
between the diversity and community structure complexity. It is
suggested that the similarity between the curves of biodiversity and
human population probably comes from the fact that both are derived
from the superposition on the hyperbolic trend of cyclical and random
* ^ "Phanerozoic".
Oxford Dictionaries .
Oxford University Press
Oxford University Press .
* ^ "Phanerozoic".