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The Paleozoic Era of the Phanerozoic Eon:
542.0 to 251.0 million years ago
Early in the 300 million year history of the Paleozoic, atmospheric oxygen reached its present levels, generating the ozone shield that screens out ultraviolet radiation and allows complex life to live in the shallows and finally on land. This era witnessed the age of invertebrates, of fish, of tetrapods, and (during the Permian) reptiles. From the Silurian on, life emerged from the sea to colonize the land, and in the later Paleozoic and later gymnospermous plants flourished. The generally mild to tropical conditions with their warm shallow seas were interspersed with Ordovician and Permo-Carboniferous ice ages. Towards the end of the Paleozoic the continents clustered into the supercontinent of Pangea, and increasingly aridity meant the end of the great Carboniferous swamps and their unique flora and fauna. The Paleozoic was brought to an end by the end Permian mass-extinction, perhaps the most severe extinction the planet has seen.
The Paleozoic (also spelt "Palaeozoic") lasted from about 540 to 250 million years ago, and is divided into six The 290-odd million years of the Paleozoic era saw many important events, including the development of most invertebrate groups, life's , the evolution of , , , and , the formation of the supercontinent of , and no less than two distinct . The rotated faster than it does today so days were shorter, and the being nearer meant stronger tides.
The six periods, followed by the time they began are:
Since the continental all move with respect to each other, we need to pick an East-West point of reference to keep things straight. Paleozoic paleocartographers have somehow fallen into the habit of placing this reference longitude slightly east of . For most of the Paleozoic, Greenland remained close to the equator and, after Baltica sutured to Laurentia (North America plus Greenland) during the Silurian, this longitude came to correspond quite closely to the longitude of the future Greenwich, England, which defines the present conventional 0° longitude line. We will adopt this convention, although it is important to understand that it's just a convention. We have no absolute measures of East-West continental drift, and must be content with noting movements relative to some arbitrary geographical point.
The early Paleozoic saw many of the continents clustered around the equator, with (representing the bulk of old ) slowly drifting south across the South poles, and Siberia, Laurentia (North America plus Greenland) and Baltica converging in the tropics. There was a large ocean between Laurentia and Eastern Gondwanaland.
It seems that Gondwanaland underwent a large clockwise rotation around an axis close to Australia during the Early Paleozoic. Laurentia underwent a large eastward movement, as well as a northward drift.
joined with during the Silurian, drifting from a moderate southern hemisphere position in Cambro-Ordovician time to an equatorial position in Silurian-Devonian time. The combined continent is sometimes referred to as Euramerica, Laurasia, or Laurussia. Siberia, and possibly the Kazakhstan terranes, drifted across the equator to the northeast. All the East and Southeast Asian terranes, as well as the microcontinents which later formed Mexico, the east coast of North America, and southern Europe, were still part of the north coast (India-Australia margin) of Gondwana during the Early Palaeozoic.
During the middle and late Paleozoic (Devonian to Permian), about a third of the Gondwanan mass was torn into small pieces and moved rapidly to equatorial regions. Most of these blocks were assembled by a series of plate collisions into the supercontinent of Euramerica by the Devonian, which by addition of further landmasses became Laurasia by the late Carboniferous. Most of western Gondwana (South America and Africa), then rotated clockwise and moved northward to collide with Laurasia. By Permian time, Siberia and the Kazakhstan terranes were sutured to Euramerica (Laurussia) and the Chinese blocks started accreting to them. The result was the supercontinent Pangaea.
The climate was probably moderate at first, becoming warmer over the course of the , as the second-greatest sustained sea level rise in the got under way. However, as if to offset this trend, moved south with considerable speed, so that by the west Gondwana ( and ) was centered near 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, (Northern Europe and Russia) and (eastern North America and Greenland) remained in the tropical zone, while and lay in waters which were at least temperate. The Early Paleozoic ended, rather abruptly, with the short, but apparently severe, Ice Age. This cold spell caused the second-greatest mass extinction of Phanerozoic time.
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 and . 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 life simply became hardier, or both. At any event, the far southern continental margins of 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 Epoch 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 . These were far more severe than the brief Late Ordovician Ice; but, this time, the effects on world biota were inconsequential. By the , both oxygen and carbon dioxide had recovered to more normal levels. On the other hand, the assembly of created huge arid inland areas subject to temperature extremes. The is associated with falling sea levels, increased carbon dioxide and general climatic deterioration, culminating in the devastation of the end- extinction.
When the levitra sales era began () the seas were dominated by . That dominance continued in the period too, with the . In the , the bony-fishes were larger than early (), but were still jawless.
With the end of the Silurian period the first plants had evolved () and the first jawed fishes had also evolved. The fishes dominated the seas in the period and the plants evolved to trees and the first tetrapods evolved from fishes. The Devonian period ends with the extinction of many groups of fishes ( etc.).
Later, in the period the trees evolved into swamp forests and the first appeared (). As result of that, the oxygen which reached into air cause the evolution of large predators (, , etc.), but apart from this the full-oxygen air also caused strong storms. After the vast swamp forests the climate was drying and the arthropods had extinct. However, the reptiles could survive and the first medium to large-sized reptiles evolved. The period world was dominated by reptiles.
The continents were fusing together () and the world was covered by dry land - deserts. Later a group of reptiles appeared called 'mammal-like reptiles' and that group survived the great Permo-Triassic extinction and evolved later to . The Paleozoic era ends with the greatest mass extinction of many groups of animals.
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Credits (Paleozoic pages) - Kheper Site MAK980528, Palaeos com MAK020409, checked ATW020714, last modified ATW041219, transferred to Palaeos org MM060921, divided into several pages MAK060923, then combined them again :-) MAK091117.