Further Reading

McKinney, Michael L. Evolution of Life, Processes, Patterns, and Prospects. Englewood Cliffs, N.J.: Prentice Hall, 1993.

Prothero, Donald R. Bringing Fossils to Life: An Introduction to Paleobiology. New York: McGraw-Hill, 2004.

Raup, David M., and Steven M. Stanley. Principles of Paleontology. 3rd ed. San Francisco: W. H. Freeman, 2007.

Stanley, Steven M. Earth and Life through Time. New York: W. H. Freeman, 1986.

Paleozoic The era of geologic time that includes the interval between 544 and 250 million years ago and the erathem of rocks deposited in this interval, called the Paleozoic, includes seven geological periods and systems of rocks: the Cambrian, ordovician, Silurian, Devonian, Carboniferous (Mississippian and Pennsylvanian), and Permian. The name Paleozoic, meaning ancient life, was coined by British geologist Adam Sedgwick in 1838 for the deformed rocks underlying the old Red Sandstone in Wales. The base of the Paleozoic is defined as the base of the Cambrian period, conventionally taken as the lowest occurrence of trilobites. Recently however, with the recognition of the advanced Vendian and Ediacaran fauna, the base of the Cambrian was reexamined and has been defined using fossiliferous sections in eastern Newfoundland and Siberia to be the base of an ash bed dated at 544 million years ago.

At the beginning of the Paleozoic, the recently formed supercontinent of Gondwana was breaking apart, but later regrouped as Pangaea by the Carboniferous. This supercontinent included the southern continents in the Gondwanan landmass and the northern continents in Laurasia, separated by the Pleionic and Tethys oceans, and surrounded by the Panthalassa ocean. With the breakup of the late Precambrian supercontinent, climates changed from icehouse to hothouse conditions. The volume of carbon dioxide (CO2) emitted to the atmosphere by the mid-ocean ridge system caused this dramatic change. During supercontinent periods, the length of the ridge system is small, and relatively small amounts of CO2 are emitted to the atmosphere. During supercontinent breakup however, much more Co2 is released during enhanced volcanism associated with the formation of new ridge systems. Since Co2 is a greenhouse gas, supercontinent breakup is associated with increasing temperatures and the establishment of hothouse conditions. The Pangaean supercontinent then experienced continental climates ranging from hot and dry to icehouse conditions, with huge continental ice sheets covering large parts of the southern continents. Many collisional and rifting events occurred, especially along the active margins of Pangaea, necessitating the subduction of huge tracts of oceanic crust in order to accommodate these collisional events.

The dramatic changes in continental configurations, the arrangement of ecological niches, and the huge climatic fluctuations at the base of the Paleozoic are associated with the most dramatic explosion of life in the history of the planet. Hard-shelled organisms first appeared in the lower Cambrian and are abundant in the fossil record by the mid-Cambrian. Fish first appeared in the Ordovician. All of the modern animal phyla and most of the plant kingdom are represented in the Paleozoic record, with fauna and flora inhabiting land, shallow seas, and deep-sea environments. There are several mass extinction events in the Paleozoic in which large numbers of species suddenly died off and were replaced by new species in similar ecological niches.

In addition to the development of hard-shelled organisms and skeletons, the Paleozoic saw the dramatic habitation of the terrestrial environment. Bacteria and algae crept into different environments such as soils before the Paleozoic, with land plants appearing in the silurian. Dense terrestrial flora expanded by the Devonian and culminated in the dense forests of the Carboniferous. This profoundly changed the weathering, erosion, and sedimentation patterns from that of the Precambrian, and also significantly affected the atmosphere-ocean composition. Terrestrial fauna rapidly followed the plants onto land, with tetrapods roaming the continents by the Middle or Late Devonian. By the Devonian, invertebrates including spiders, scorpions, and cockroaches had invaded the land, and fish became abundant in the oceans.

In the Carboniferous much organic carbon got buried, reducing atmospheric Co2 levels and ending the hothouse conditions. With the formation of Pan-gaea in the Carboniferous and Permian, global ridge lengths were reduced, and less Co2 was released to the atmosphere. Together with the burial of organic carbon, new icehouse conditions were established, stressing the global fauna and flora. The largest mass extinction in geological history marks the end of the Paleozoic (end-Permian mass extinction) and the start of the Mesozoic. The causes of this dramatic event seem to be multifold. Conditions on the planet included the formation of a supercontinent (Pangaea), falling sea levels, evaporite formation, and rapidly fluctuating climatic conditions. At the boundary between the Paleozoic and Mesozoic Periods (245 million years ago), 96 percent of all species became extinct, including marine organisms such as the rugose corals, trilobites, many types of brachio-pods, and many foraminifera species.

The Siberian flood basalts were erupted over a period of less than 1 million years 250 million years ago, at the end of the Permian at the Permian-Tri-assic boundary. They are remarkably coincident in time with the major Permian-Triassic extinction, implying a causal link. They cover a large area of the Central Siberian Plateau northwest of Lake Baikal and are more than half a mile thick over an area of 210,000 square miles (544,000 km2), but they have been significantly eroded from an estimated volume of 1,240,000 cubic miles (5,168,500 km3). The rapid volcanism and degassing could have released enough sulfur dioxide to cause a rapid global cooling, inducing a short ice age with an associated rapid fall of sea level. Soon after the ice age took hold the effects of the carbon dioxide took over and the atmosphere heated, resulting in a global warming. The rapidly fluctuating climate postulated to have been caused by the volcanic gases is thought to have killed off many organisms, which were simply unable to cope with the wildly fluctuating climate extremes.

Another possibility is that the impact of a meteorite or asteroid with the Earth aided the end-Permian extinction, adding environmental stresses to an already extremely stressed ecosystem. If additional research proves this to be correct, it will be shown that a one-two-three punch, including changes in plate configurations and environmental niches, dramatic climate changes, and extraterrestrial impacts together caused history's greatest calamity.

See also Cambrian; Carboniferous; Devonian; evolution; fossil: Gondwana, Gondwana-land; Pangaea; Permian; Silurian.

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