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| Trilobites. |
Paleozoic
The Paleozoic (or Palaeozoic) Era ( /ˌpæl.i.əˈzoʊ.ɪk, -i.oʊ-,
ˌpeɪ.li.ə-, -li.oʊ-/ pal-ee-ə-ZOH-ik, -ee-oh-,
pay-lee-, -lee-oh-; from the Greek palaiós (παλαιός),
"old" and zōḗ (ζωή), "life", meaning "ancient life") is
the earliest of three geologic eras of the Phanerozoic
Eon. It is the longest of the Phanerozoic eras, lasting
from 541 to 251.902 million years ago, and is subdivided
into six geologic periods (from oldest to youngest): the
Cambrian, Ordovician, Silurian, Devonian, Carboniferous,
and Permian. The Paleozoic comes after the
Neoproterozoic Era of the Proterozoic Eon and is
followed by the Mesozoic Era.
The Paleozoic was a time of dramatic geological,
climatic, and evolutionary change. The Cambrian
witnessed the most rapid and widespread diversification
of life in Earth's history, known as the Cambrian
explosion, in which most modern phyla first appeared.
Arthropods, molluscs, fish, amphibians, synapsids and
diapsids all evolved during the Paleozoic. Life began in
the ocean but eventually transitioned onto land, and by
the late Paleozoic, it was dominated by various forms of
organisms. Great forests of primitive plants covered the
continents, many of which formed the coal beds of Europe
and eastern North America. Towards the end of the era,
large, sophisticated diapsids and synapsids were
dominant and the first modern plants (conifers)
appeared.
The Paleozoic Era ended with the largest extinction
event in the history of Earth, the Permian–Triassic
extinction event. The effects of this catastrophe were
so devastating that it took life on land 30 million
years into the Mesozoic Era to recover. Recovery of life
in the sea may have been much faster. |
|
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| Cephalaspis (a
jawless fish). |
Geology
The Paleozoic era began with the breakup of the
supercontinent of Pannotia and ended with the assembly
of the supercontinent of Pangaea. The breakup of
Pannotia began with the opening of the Iapetus Ocean and
other Cambrian seas and coincided with a dramatic rise
in sea level. Paleoclimatic studies and evidence of
glaciers indicate that Central Africa was most likely in
the polar regions during the early Paleozoic. The
breakup of Pannotia was followed by the assembly of the
huge continent Gondwana (510 million years ago). By
mid-Paleozoic, the collision of North America and Europe
produced the Acadian-Caledonian uplifts, and a
subduction plate uplifted eastern Australia. By the late
Paleozoic, continental collisions formed the
supercontinent of Pangaea and created great mountain
chains, including the Appalachians, Ural Mountains, and
mountains of Tasmania.
Periods of the Paleozoic Era
There are six periods in the Paleozoic Era: Cambrian,
Ordovician, Silurian, Devonian, Carboniferous
(alternatively subdivided into the Mississippian Period
and the Pennsylvanian Period), and the Permian.
Cambrian Period
The Cambrian spanned from 541 to 485 million years ago
and is the first period of the Paleozoic era of the
Phanerozoic. The Cambrian marked a boom in evolution in
an event known as the Cambrian explosion in which the
largest number of creatures evolved in any single period
of the history of the Earth. Creatures like algae
evolved, but the most ubiquitous of that period were the
armored arthropods, like trilobites. Almost all marine
phyla evolved in this period. During this time, the
supercontinent Pannotia begins to break up, most of
which later became the supercontinent Gondwana.
Ordovician Period
The Ordovician spanned from 485 to 444 million years
ago. The Ordovician was a time in Earth's history in
which many of the biological classes still prevalent
today evolved, such as primitive fish, cephalopods, and
coral. The most common forms of life, however, were
trilobites, snails and shellfish. The first arthropods
went ashore to colonize the empty continent of Gondwana.
By the end of the Ordovician, Gondwana was at the south
pole, early North America had collided with Europe,
closing the Atlantic Ocean. Glaciation of Africa
resulted in a major drop in sea level, killing off all
life that had established along coastal Gondwana.
Glaciation may have caused the Ordovician–Silurian
extinction events, in which 60% of marine invertebrates
and 25% of families became extinct, and is considered
the first mass extinction event and the second
deadliest.
Silurian Period
The Silurian spanned from 444 to 419 million years ago.
The Silurian saw the rejuvenation of life as the Earth
recovered from the previous glaciation. 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,
were still apex predators. Fully terrestrial life
evolved, including early arachnids, fungi, and
centipedes. The evolution of vascular plants (Cooksonia)
allowed plants to gain a foothold on land. These early
plants were the forerunners of all plant life on land.
During this time, there were four continents: Gondwana
(Africa, South America, Australia, Antarctica, Siberia),
Laurentia (North America), Baltica (Northern Europe),
and Avalonia (Western Europe). The recent rise in sea
levels allowed many new species to thrive in water.
Devonian Period
The Devonian spanned from 419 to 359 million years ago.
Also known as "The Age of the Fish", the Devonian
featured a huge diversification of fish, including
armored fish like Dunkleosteus and lobe-finned fish
which eventually evolved into the first tetrapods. On
land, plant groups diversified incredibly in an event
known as the Devonian Explosion when plants made lignin
allowing taller growth and vascular tissue: the first
trees evolved, as well as seeds. This event also
diversified arthropod life, by providing them new
habitats. 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 was
the Earth's second mass extinction event. |
 |
| Eogyrinus (an
amphibian) of the Carboniferous. |
Carboniferous Period
The Carboniferous spanned from 359 to 299 million years ago.
During this time, average global temperatures were
exceedingly high; the early Carboniferous averaged at about
20 degrees Celsius (but cooled to 10 °C during the Middle
Carboniferous). Tropical swamps dominated the Earth, and the
lignin stiffened trees grew to greater heights and number.
As the bacteria and fungi capable of eating the lignin had
not yet evolved, their remains were left buried, which
created much of the carbon that became the coal deposits of
today (hence the name "Carboniferous"). 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 for the
duration of this period. Also, the first reptiles and
synapsids evolved in the swamps. Throughout the
Carboniferous, there was a cooling trend, which led to the
Permo-Carboniferous glaciation or the Carboniferous
Rainforest Collapse. Gondwana was glaciated as much of it
was situated around the south pole.
Permian Period
The Permian spanned from 299 to 252 million years ago and
was the last period of the Paleozoic Era. At the beginning
of this period, all continents joined together to form the
supercontinent Pangaea, which was encircled by one ocean
called Panthalassa. The land mass was very dry during this
time, with harsh seasons, as the climate of the interior of
Pangaea was not regulated by large bodies of water. Diapsids
and synapsids flourished in the new dry climate. Creatures
such as Dimetrodon and Edaphosaurus ruled the new continent.
The first conifers evolved, and dominated the terrestrial
landscape. Near the end of the Permian, however, Pangaea
grew drier. The interior was desert, and new taxa such as
Scutosaurus and Gorgonopsids filled it. Eventually they
disappeared, along with 95% of all life on Earth, in a
cataclysm known as "The Great Dying", the third and most
severe mass extinction. |
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| Synapsid: Dimetrodon. |
Climate
The early Cambrian climate was probably moderate at
first, becoming warmer over the course of the Cambrian,
as the second-greatest sustained sea level rise in the
Phanerozoic got underway. However, as if to offset this
trend, Gondwana moved south, so that, in Ordovician
time, most of West Gondwana (Africa and South America)
lay directly over the South Pole. The early Paleozoic
climate was also strongly zonal, with the result that
the "climate", in an abstract sense, became warmer, but
the living space of most organisms of the time—the
continental shelf marine environment—became steadily
colder. However, Baltica (Northern Europe and Russia)
and Laurentia (eastern North America and Greenland)
remained in the tropical zone, while China and Australia
lay in waters which were at least temperate. The early
Paleozoic ended, rather abruptly, with the short, but
apparently severe, late Ordovician ice age. This cold
spell caused the second-greatest mass extinction of
Phanerozoic time. Over time, the warmer weather moved
into the Paleozoic Era.
The Ordovician and Silurian were warm greenhouse
periods, with the highest sea levels of the Paleozoic
(200 m above today's); the warm climate was interrupted
only by a 30 million year cool period, the Early
Palaeozoic Icehouse, culminating in the Hirnantian
glaciation, 445 million years ago at the end of the
Ordovician.
The middle Paleozoic was a time of considerable
stability. Sea levels had dropped coincident with the
ice age, but slowly recovered over the course of the
Silurian and Devonian. The slow merger of Baltica and
Laurentia, and the northward movement of bits and pieces
of Gondwana created numerous new regions of relatively
warm, shallow sea floor. As plants took hold on the
continental margins, oxygen levels increased and carbon
dioxide dropped, although much less dramatically. The
north–south temperature gradient also seems to have
moderated, or metazoan life simply became hardier, or
both. At any event, the far southern continental margins
of Antarctica and West Gondwana became increasingly less
barren. The Devonian ended with a series of turnover
pulses which killed off much of middle Paleozoic
vertebrate life, without noticeably reducing species
diversity overall.
There are many unanswered questions about the late
Paleozoic. The Mississippian (early Carboniferous
Period) began with a spike in atmospheric oxygen, while
carbon dioxide plummeted to new lows. This destabilized
the climate and led to one, and perhaps two, ice ages
during the Carboniferous. These were far more severe
than the brief Late Ordovician ice age; but, this time,
the effects on world biota were inconsequential. By the
Cisuralian Epoch, both oxygen and carbon dioxide had
recovered to more normal levels. On the other hand, the
assembly of Pangaea created huge arid inland areas
subject to temperature extremes. The Lopingian Epoch is
associated with falling sea levels, increased carbon
dioxide and general climatic deterioration, culminating
in the devastation of the Permian extinction. |
|
 |
| An artist's
impression of early land plants. |
Flora
While macroscopic plant life appeared early in the
Paleozoic Era and possibly late in the Neoproterozoic
Era of the earlier eon, plants mostly remained aquatic
until the Silurian Period, about 420 million years ago,
when they began to transition onto dry land. Terrestrial
flora reached its climax in the Carboniferous, when
towering lycopsid rainforests dominated the tropical
belt of Euramerica. Climate change caused the
Carboniferous Rainforest Collapse which fragmented this
habitat, diminishing the diversity of plant life in the
late Carboniferous and Permian periods.
Fauna
A noteworthy feature of Paleozoic life is the sudden
appearance of nearly all of the invertebrate animal
phyla in great abundance at the beginning of the
Cambrian. The first vertebrates appeared in the form of
primitive fish, which greatly diversified in the
Silurian and Devonian Periods. The first animals to
venture onto dry land were the arthropods. Some fish had
lungs, and powerful bony fins that in the late Devonian,
367.5 million years ago, allowed them to crawl onto
land. The bones in their fins eventually evolved into
legs and they became the first tetrapods, 390 million
years ago, and began to develop lungs. Amphibians were
the dominant tetrapods until the mid-Carboniferous, when
climate change greatly reduced their diversity. Later,
reptiles prospered and continued to increase in number
and variety by the late Permian period. |
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 Kiddle: Paleozoic
Wikipedia: Paleozoic |
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