Paleozoic

Of, belonging to, or designating the era of geologic time that includes the Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, and Permian periods and is characterized by the appearance of marine invertebrates, primitive fishes, land plants, and primitive reptiles.

The Paleozoic Era.

A major division of time in geologic history, extending from about 540 to 250 million years ago (Ma). It is the earliest era in which significant numbers of shelly fossils are found, and Paleozoic strata were among the first to be studied in detail for their biostratigraphic significance.

The Paleozoic Era is divided into six systems; from oldest to youngest they are Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. The Carboniferous is subdivided into two subsystems, the Mississippian and the Pennsylvanian which, in North America, are considered systems by many geologists. The Silurian and Devonian systems are closer to international standardization than others; all the series and stage names and lower boundaries have been agreed upon, and most have been accepted. See also Cambrian; Carboniferous; Devonian; Ordovician; Permian; Silurian.

Because Alpine and Appalachian mountain chains were among the first studied in detail, orogenies were first named there. In eastern North America, mountain-building effects during the early Paleozoic were ascribed to the Taconic orogeny (Middle and Late Ordovician); middle Paleozoic events were assigned to the Acadian orogeny (Middle and Late Devonian); and late Paleozoic movements were called Appalachian (more accurately Alleghenian) for Permian and, perhaps, Triassic events. See also Dating methods; Isotope; Orogeny; Plate tectonics; Unconformity.

The major changes in lithofacies during the Paleozoic were also effected by biotic evolution through the era. Limestone facies became more abundant and more diversified in the shallow warm seas as calcium-fixing organisms became more diverse and more widespread. Sediment input from the land was modified as plants moved from the seas to the low coastal plains and, eventually, to the higher ground during the Devonian. Primitive vertebrates evolved during the Cambro-Ordovician, but true fishes and sharks did not flourish until the Devonian. Amphibians invaded the land during the Late Devonian and early Carboniferous at about the same time that major forests began to populate the terrestrial realm. These changes produced an entirely new suite of nonmarine facies related to coal formation, and the Carboniferous was a time of formation of major coal basins on all continental plates.

Major cycles of cold and warm climates were overlaid on depositional and evolutionary patterns, producing periods of continental glaciation when large amounts of the Earth's water were tied up in ice during the Late Ordovician, the Late Devonian, and the late Permian. During the earliest and latest of these periods, icesheets were concentrated in the Southern Hemisphere on a single large Paleozoic continental mass—Gondwana. See also Depositional systems and environments; Facies (geology); Paleoclimatology.

The Paleozoic featured a single southern landmass (Gondwana) for most of the era. This megaplate moved relatively sedately northward during this entire time interval (540–250 Ma) and always contained the magnetic and geographic south poles. Consequently, many of the facies and biologic provinces in the Gondwanan region were influenced by the cooler marine realms and continental and mountain glaciers in nearly every Paleozoic period. Most of the tectonic action that produced major periods of collision, mountain building, carbonate platform building, back-arc fringing troughs with their distinctive faunas and lithofaces, and formation of coal basins and evaporites took place in the Northern Hemisphere. These pulsations produced combinations of Laurentian (North American), Euro-Baltic, Uralian, Siberian, and Chinese plates a various times during the Paleozoic; and these combined units, in turn, moved slowly across the latitudes, producing climatic change; lithofacies changed in response to both the climate and the plate tectonics. See also Paleogeography.

There were fewer and simpler life forms in the Cambrian—often termed the Age of Trilobites. All groups of invertebrates and plants became more numerous through geologic time. For example, 7 major invertebrate animal groups at the beginning of the Cambrian doubled to 14 by the end of the period, 20 by the end of the Ordovician, 23 at the end of the Devonian, and 25 at the end of the Paleozoic. The pattern for plant diversification, although starting later, is similar. Three simple plant groups became 5 by the end of the Silurian, 7 at the end of the Devonian, and 13 at the end of the Paleozoic. The vertebrates also diversified very slowly. From one or two groups in the Cambro-Ordovician (conodonts are now considered primitive vertebrates), the number of major kinds rose to 6 at the end of the Devonian and 8 at the end of the Paleozoic. See also Biogeography; Geologic time scale; Index fossil; Paleoecology; Stratigraphy; Trilobita.

The noun has one meaning:

Meaning #1: from 230 million to about 544 million years ago
  Synonym: Paleozoic era

Paleozoic era 542 - 251 million years ago

Key events in the Paleozoic view • discuss • edit -550 — – -500 — – -450 — – -400 — – -350 — – -300 — – -250 — – Neoproterozoic Mesozoic Cambrian Ordovician Silurian Devonian Carboniferous Permian           P a l e o z o i c  Phanerozoic  Proterozoic An approximate timescale of key Paleozoic events. Axis scale: millions of years ago.

The Paleozoic or Palaeozoic Era (from the Greek palaios (παλαιός), "old" and zoe (ζωή), "life", meaning "ancient life") is the earliest of three geologic eras of the Phanerozoic eon. The Paleozoic spanned from roughly 542 to 251 million years ago (ICS, 2004), and is subdivided into six geologic periods; from oldest to youngest they are: the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian

Contents 1 Paleozoic life 2 Tectonics 3 Climate 4 See also 5 References and further reading

Paleozoic life

Trilobites flourished throughout the lower Paleozoic era until becoming extinct in the Permian period

The Paleozoic covers the time from the first appearance of abundant, soft-shelled fossils to the time when the continents were beginning to be dominated by large, relatively sophisticated reptiles and modern plants. The lower (oldest) boundary was classically set at the first appearance of creatures known as trilobites and archeocyathids. The upper (youngest) boundary is set at a major extinction event 300 million years later, known as the Permian extinction. Modern practice sets the older boundary at the first appearance of a distinctive trace fossil called Trichophycus pedum.

At the start of the era, all life was confined to bacteria, algae, sponges and a variety of somewhat enigmatic forms known collectively as the Ediacaran fauna. A large number of body plans appeared nearly simultaneously at the start of the era -- a phenomenon known as the Cambrian Explosion. There is some evidence that simple life may already have invaded the land at the start of the Paleozoic, but substantial plants and animals did not take to the land until the Silurian and did not thrive until the Devonian. Although primitive vertebrates are known near the start of the Paleozoic, animal forms were dominated by invertebrates until the mid-Paleozoic. Fish populations exploded in the Devonian. During the late Paleozoic, great forests of primitive plants thrived on land forming the great coal beds of Europe and eastern North America. By the end of the era, the first large, sophisticated reptiles and the first modern plants (conifers) had developed.

Tectonics

Geologically, the Paleozoic starts shortly after the breakup of a supercontinent called Pannotia and at the end of a global ice age. (See Varanger glaciation and Snowball Earth). Throughout the early Paleozoic, the Earth's landmass was broken up into a substantial number of relatively small continents. Toward the end of the era, the continents gathered together into a supercontinent called Pangaea, which included most of the Earth's land area.

Climate

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Land distribution in the Early Paleozoic, 514 Ma

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 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 Permian extinction.

See also

• Geologic timescale

References and further reading

• British Palaeozoic Fossils, 1975, The Natural History Museum, London.
• "International Commission on Stratigraphy (ICS)". Home Page. http://www.stratigraphy.org/. Retrieved on September 19 2005. 

Wikimedia Commons has media related to: Paleozoic

Preceded by Proterozoic eon	542 Ma - Phanerozoic eon - Present
	542 Ma - Paleozoic era - 251 Ma						251 Ma - Mesozoic era - 65 Ma			65 Ma - Cenozoic era - Present
	Cambrian	Ordovician	Silurian	Devonian	Carboniferous	Permian	Triassic	Jurassic	Cretaceous	Paleogene	Neogene	Quaternary

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