541 - 252.17 million years ago
The Paleozoic (or Palaeozoic) Era (// or //; from the Greek palaios (παλαιός), "old" and zoe (ζωή), "life", meaning "ancient life") is the earliest of three geologic eras of the Phanerozoic Eon, spanning from roughly (ICS, 2004). It is the longest of the Phanerozoic eras, 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 Period 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. Fish, arthropods, amphibians and reptiles 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 reptiles were dominant and the first modern plants (conifers) appeared.
The Paleozoic Era ended with the largest mass extinction in Earth's history, 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 to recover. Recovery of life in the sea may have been much faster.
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In North America, the era began with deep sedimentary basins along the eastern, southeastern, and western sides of the continent, while the interior was dry land. As the era proceeded, the marginal seas periodically washed over the stable interior, leaving sedimentary deposits to mark their incursions. During the early part of the era, the area of exposed Precambrian, or shield, rocks in central Canada were eroding, supplying sediment to the basins from the interior. Beginning in the Ordovician Period, mountain building intermittently proceeded in the eastern part of the Appalachian region throughout the rest of the era, bringing in new sediments. Sediments washing from the Acadian Mountains filled the western part of the Appalachian basins to form the famous coal swamps of the Carboniferous Period. In North America, carboniferous is not generally used. Instead, the time is divided between Mississippian and Pennsylvanian periods because of differences in the sedimentary rock deposited during that time. The Mississippian is characterized by limey sediments deposited in shallow seas, typically with abundant crinoidal fossils as in the Burlington Formation. The Pennsylvanian typically is characterized by terrestrial sediments such as sands, shale and most importantly coal. Most of our oil and gas are obtained from Pennsylvanian sediments. Where this has been stripped, as in the Ozark domal region, oil is not typically available.
Paleoclimatic studies and evidence of glaciers indicate that central Africa was most likely in the polar regions during the early Paleozoic. During the early Paleozoic, the huge continent Gondwanaland had either formed or was forming. 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 Pangaea and resulted in some of the great mountain chains, including the Appalachians, Urals, and mountains of Tasmania.
Periods of the Paleozoic
The Cambrian spans from 540 million years to 485 million years ago and is the first period of the Paleozoic and of the Phanerozoic Eon. The Cambrian sparks a boom in evolution in an event known as the Cambrian Explosion in which the largest number of creatures evolve in the history of Earth during one period. Creatures like algae evolve, but most of the water is populated by armored arthropods, like trilobites. Almost all marine phyla evolved in this period. During this time, the super-continent Rodinia begins to break up, most of which becomes the super-continent Gondwana. 
The Ordovician spans from 485 million years to 440 million years ago. The Ordovician is 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 went ashore to colonize the empty continent of Gondwana. By the end of the period, 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 staked a claim along coastal Gondwana. Glaciation caused a snowball earth, and the Ordovician-Silurian extinction in which 60% of marine invertebrates and 25% of families went extinct, and is considered the first mass extinction and the second deadliest extinction. 
The Silurian spans from 440 million years to 415 million years ago. The Silurian saw the healing of the earth that recovered from the 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, were still apex predators. Fully terrestrial life evolved, which included early arachnids, fungi, and centipedes. Also, the evolution of vascular plants (Cooksonia) allowed plants to gain a foothold on land. These early plants are the forerunners of all plant life on land. During this time, there are 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 provided many new species to thrive in water. 
The Devonian spans from 415 million years to 360 million years ago. Also known as "The Age of the Fish", the Devonian features 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 where the first trees evolved, as well as seeds. This event also diversified arthropod life. 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 went extinct in an event known as the Late Devonian extinction and is the second mass extinction event the world has seen. 
The Carboniferous spans from 360 million to 300 million years ago. During this time, average global temperatures were exceedingly high; the early Carboniferous averaged at about 20 degrees Celsius (but cooled down to 10 degrees during the Middle Carboniferous).  Tropical swamps dominated the earth, and the large amounts of trees created much of the carbon for the coal that is used today (hence the name "Carbon-iferous"). Perhaps the most important evolutionary development of the time was the evolution of amniotic eggs, which allowed amphibians to head farther inland and remained the dominant vertebrae throughout the duration of this 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. 
The Permian spans from 300 million to 250 million years ago and was the last period of the Paleozoic. At the beginning, all continents formed together to form the super-continent Pangaea and had 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 evolve, and dominate the terrestrial landscape. Nearing the end of the Permian, however, Pangaea got drier and drier. The interior was nothing but dry deserts, and new species such as Scutosaurus and Gorgonopsids filled the empty desert. 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 in world history.
Geologically, the Paleozoic starts shortly after the breakup of a supercontinent called Pannotia. Throughout the early Paleozoic, the Earth's landmass was broken up into a substantial number of continents. Towards the end of the era, the continents gathered together into a supercontinent called Pangaea, which included most of the Earth's land area.
The Ordovician and Silurian periods 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 cool period, the Early Palaeozoic Icehouse, culminating in the Hirnantian glaciation.
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 with considerable speed, 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 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.
The late Paleozoic was a time which has left us a good many unanswered questions. The Mississippian began with a spike in atmospheric oxygen, while carbon dioxide plummeted to unheard-of 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; but, this time, the effects on world biota were inconsequential. By the Cisuralian, 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 is associated with falling sea levels, increased carbon dioxide and general climatic deterioration, culminating in the devastation of the Permian extinction.
While macroscopic plant life appeared early in the Paleozoic and possibly late in the Neoproterozoic, it mostly remained aquatic until sometime in the Silurian and Devonian, when it 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.
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. The first animals to venture onto dry land were the arthropods. Some fish had lungs, and powerful bony fins that also could crawl onto land. The bones in their fins eventually evolved into legs and they became the first tetrapods. 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.
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References and further reading
|Wikisource has original works on the topic: Paleozoic|
- "Paleozoic". Online Etymology Dictionary.
- Sahney, S. & Benton, M.J. (2008). "Recovery from the most profound mass extinction of all time" (PDF). Proceedings of the Royal Society: Biological 275 (1636): 759–65. doi:10.1098/rspb.2007.1370. PMC 2596898. PMID 18198148.
- http://www.economist.com/node/16524904 The Economist
- University of California. "Cambrian". University of California.
- University of California. "Ordovician". University of California.
- University of California. "Silurian". University of California.
- University of California. "Devonian". University of California.
- Monte Hieb. "Carboniferous Era". unknown.
- University of California. "Carboniferous". University of California.
- Natural History Museum. "The Great Dying". Natural History Museum.
- University of California. "Permian Era". University of California.
- Munnecke, A.; Calner, M.; Harper, D. A. T.; Servais, T. (2010). "Ordovician and Silurian sea-water chemistry, sea level, and climate: A synopsis". Palaeogeography, Palaeoclimatology, Palaeoecology 296 (3–4): 389–413. doi:10.1016/j.palaeo.2010.08.001.
- Sahney, S.; Benton, M.J. & Falcon-Lang, H.J. (2010). "Rainforest collapse triggered Pennsylvanian tetrapod diversification in Euramerica" (PDF). Geology 38 (12): 1079–1082. doi:10.1130/G31182.1.
- British Palaeozoic Fossils, 1975, The Natural History Museum, London.
- "International Commission on Stratigraphy (ICS)". Home Page. Retrieved September 19, 2005.
|Preceded by Proterozoic Eon||Phanerozoic Eon|
|Paleozoic Era||Mesozoic Era||Cenozoic Era|