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 (ICS, 2004), and is subdivided into six
geologic periods; from oldest to youngest they are: the
Cambrian,
Ordovician,
Silurian,
Devonian,
Carboniferous, and
Permian.
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.
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 to recover. Recovery of life in the sea may have been much faster.
Geology
During the hiatus between the late Precambrian and the Paleozoic most of the evidence of the earth's early history was destroyed by erosion. From the beginning of the Paleozoic, shallow seas began to encroach on the continents.
In North America, the era began with submerged geosynclines, or downward thrusts of the earth's crust, 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 geosynclines from the interior. Beginning in the Ordovician period, mountain building intermittently proceeded in the eastern part of the Appalachian geosyncline throughout the rest of the era, bringing in new sediments. Sediments washing from the Acadian Mountains filled the western part of the Appalachian geosyncline to form the famous coal swamps of the Carboniferous period. Uplift of the Appalachians meant that the region was never again inundated by vast marginal seas.
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 Tasmans.
Animal Life
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. A few primitive fishlike invertebrates, and then
vertebrates, appeared in the Cambrian and Ordovician,
scorpions in the Silurian period, land invertebrates and
amphibians in the Devonian, land
reptiles in the Carboniferous, and marine reptiles in the Permian.
Amphibians were the dominant vertebrates until the mid Carboniferous, then climate change greatly reduced their diversity; meanwhile reptiles prospered and continued to increase in number and variety by the late Permian.
Plant Life
Plant life of the Paleozoic era 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.
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
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 ~30 Ma 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 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.
Category:Geological history of Earth
