DEFINITION OF PALEOZOIC

The era of geological time that includes the

interval between 544 million and 250 million years ago, and

the erathem of rocks deposited in this interval. It includes

seven geological periods and systems of rocks deposited in

those periods, including the Cambrian, Ordovician, Silurian,

Devonian, Carboniferous (Mississippian and Pennsylvanian),

and Permian. The Paleozoic was named by Adam Sedgwick

in 1838 for the deformed rocks underlying the Old Red Sand-

stone in Wales, and the name means ancient life. 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 by the

Carboniferous it regrouped as Pangea. 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 cause of this dramatic change was the volume of CO2

emitted to the atmosphere by the mid-ocean ridge system. 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 Pangean 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. There were many collisional and rifting

events, especially along the active margins of Pangea, and

huge tracts of oceanic crust must have been subducted 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 also 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 those

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 Middle or Late Devonian. By the Devonian, invertebrates

including spiders, scorpions, and cockroaches had invaded

the land. Fish became abundant in the oceans.

In the Carboniferous much organic carbon got buried,

ending the reduction of atmospheric CO2, and ending the hothouse

conditions. With the formation of Pangea 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 (Pangea), sea-level regression, 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

the rugose corals, trilobites, many types of brachiopods, and

marine organisms including many foraminifera species.

The Siberian flood basalts were erupted over a period of

less than one million years 250 million years ago, at the end of

the Permian at the Permian-Triassic 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 have been significantly eroded from

an estimated volume of 1,240,000 cubic miles (5,168,500

km3). It has been postulated that the rapid volcanism and

degassing released enough sulfur dioxide to cause a rapid global

cooling, inducing a short ice age with 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.

It has also been postulated that the end Permian extinction

was aided by the impact of a meteorite or asteroid with

the Earth, adding environmental stresses to an already

extremely stressed ecosystem. If additional research proves

this to be correct, it will be shown that a 1–2–3 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; GONDWANA;

MASS EXTINCTIONS; ORDOVICIAN; PANGEA; PERMIAN;

SILURIAN; VENDIAN.

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