The Paleozoic Era, "old" and zoe, "life", meaning "ancient life") is the earliest of
three geologic eras of the Phanerozoic Eon, spanning from roughly 541 to 252.17 million
years ago. It is the longest of the Phanerozoic eras, and is subdivided into six geologic
periods: 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 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.
Geology 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.
Tectonic activity
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.
Climate The Ordovician and Silurian periods were warm
greenhouse periods, with the highest sea levels of the Paleozoic; the warm climate was interrupted
only by a 30 million years 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
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 and Laurentia 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. Flora
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.
Fauna 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.
See also Geologic timescale
Precambrian Cenozoic
Mesozoic References and further reading
British Palaeozoic Fossils, 1975, The Natural History Museum, London.
"International Commission on Stratigraphy". Home Page. Retrieved September 19, 2005.
External links 60+ images of Paleozoic Foraminifera