| Phanerozoic eon|
| Paleozoic era|
| Mesozoic era|
| Cenozoic era|
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 pteridophyte 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") era lasted from about 540 to 250 million years ago, and is divided into six periods The 290-odd million years of the Paleozoic era saw many important events, including the development of most invertebrate groups, life's conquest of land, the evolution of fish, reptiles, insects, and vascular plants, the formation of the supercontinent of Pangea, and no less than two distinct ice ages. The Earth rotated faster than it does today so days were shorter, and the Moon being nearer meant stronger tides.
The six periods, followed by the time they began are:
Permian 299 Mya
Carboniferous 359 Mya
Devonian 416 Mya
Silurian 444 Mya
Ordovician 488 Mya
Cambrian 542 Mya
Since the continental cratons 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 Greenland. 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 Gondwana (representing the bulk of old Rodinia) 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.
Baltica joined with Laurentia 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 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 under way. However, as if to offset this trend, Gondwana moved south with considerable speed, so that by the Ordovician west Gondwana (Africa and South America) 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, 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.
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 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 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 Pangea 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 end-Permian extinction.
When the Paleozoic era began (Cambrian explosion) the seas were dominated by invertebrates. That dominance continued in the Ordovician period too, with the mollusks. In the Silurian, the bony-fishes were larger than early vertebrates (Haikouichthys), but were still jawless.
With the end of the Silurian period the first plants had evolved (cooksonia) and the first jawed fishes had also evolved. The fishes dominated the seas in the Devonian period and the plants evolved to trees and the first tetrapods evolved from sarcopterygian fishes. The Devonian period ends with the extinction of many groups of fishes (placoderms etc.).
Later, in the Carboniferous period the trees evolved into swamp forests and the first amniotes appeared (reptiles). As result of that, the oxygen which reached into air cause the evolution of large arthropod predators (Meganeura, Megarachne, 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 Permian period world was dominated by reptiles.
The continents were fusing together (Pangaea) 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 mammals. The Paleozoic era ends with the greatest mass extinction of many groups of animals.
Andrews, SM & TS Westoll (1970), The postcranial skeleton of Eusthenopteron foordi Whiteaves. Trans. Roy. Soc. Edin. 68: 207–329.
Andrews, SM & TS Westoll (1970a), The postcranial skeleton of rhipidistian fishes excluding Eusthenopteron. Trans. Roy. Soc. Edin. 68: 391–489 (1970).
Clack, JA (2002), Gaining Ground: the Origin and Evolution of Tetrapods. Indiana Univ. Press, 369 pp.
Draganits, E, B Grasemannt & SJ Braddy (1998), Discovery of abundant arthropod trackways in the ?Lower Devonian Muth Quartzite (Spiti, India): implications for the depositional environment. J. Asian Earth Sci. 16:109-118.
Dudley, R (1998), Atmospheric Oxygen, giant Paleozoic insects and the evolution of aerial locomotor performance. J. Exper. Biol. 201: 1043-1050.
Klok, CJ, RD Mercer & SL Chown (2002), Discontinuous gas-exchange in centipedes and its convergent evolution in tracheated arthropods. J. Exper. Biol. 205: 1019-1029.
Orr, PJ, DEG Briggs, DJ Siveter & DJ Sivter (2000), Three-dimensional preservation of a non-biomineralized arthropod in concretions in Silurian volcanoclastic rocks from Herefordshire, England. J. Geol. Soc. Lond. 157: 173-186.
Stanley, SM (1998), Earth System History. WH Freeman & Co., 615 pp.
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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.