Ordovician

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Ordovician

The Ordovician period is the second of the six (seven in North America) periods of the Paleozoic era. It follows the Cambrian period and is followed by the Silurian period. The Ordovician, named after the Welsh tribe of the Ordovices, was defined by Charles Lapworth in 1879, to resolve a dispute between followers of Adam Sedgwick and Roderick Murchison, who were placing the same rock beds in northern Wales into the Cambrian and Silurian periods respectively. Lapworth, recognizing that the fossil fauna in the disputed strata were different from those of either the Cambrian or the Silurian periods, realized that they should be placed in a period of their own.

While recognition of the distinct Ordovician period was slow in the United Kingdom, other areas of the world accepted it quickly. It received international sanction in 1906, when it was adopted as an official period of the Paleozoic era by the International Geological Congress.

Ordovician dating

The Ordovician period started at a minor extinction event, possibly caused by a gamma ray burst, some time about 488.3 million years ago (mya) and lasted for about 50-80 million years. It ended with a major extinction event about 443.7 mya (ICS, 2004) that wiped out 60% of marine genera. The dates given are recent radiometric dates and vary slightly from those used in other sources.This is the second period of the Paleozoic era.

Ordovician rocks contain abundant fossils and contain major petroleum and gas reservoirs in some regions.

Ordovician subdivisions

The Ordovician Period is usually broken into Early (Tremadoc and Arenig), Middle (Llanvirn into Abereiddian and Llandeilian) and Late (Caradoc and Ashgill) epochs. The corresponding rocks of the Ordovician System are referred to as coming from the Lower, Middle, or Upper part of the column. The Faunal stages (subdivisions of epochs) from youngest to oldest are:

Hirnantian/Gamach (Late-Ashgill)
Rawtheyan/Richmond (Late-Ashgill)
Cautleyan/Richmond (Late-Ashgill)
Pusgillian/Maysville/Richmond (Late-Ashgill)

Trenton (Middle-Caradoc)
Onnian/Maysville/Eden (Middle-Caradoc)
Actonian/Eden (Middle-Caradoc)
Marshbrookian/Sherman (Middle-Caradoc)
Longvillian/Sherman (Middle-Caradoc)
Soundleyan/Kirkfield (Middle-Caradoc)
Harnagian/Rockland (Middle-Caradoc)
Costonian/Black River (Middle-Caradoc)
Chazy (Middle-Llandeilo)
Llandeilo (Middle-Llandeilo)
Whiterock (Middle-Llanvirn)
Llanvirn (Middle-Llanvirn)

Cassinian (Early-Arenig)
Arenig/Jefferson/Castleman (Early-Arenig)
Tremadoc/Deming/Gaconadian (Early-Tremadoc)

Ordovician paleogeography

Sea levels were high during the Ordovician; in fact during the Tremadocian, marine transgressionss worldwide were the greatest for which evidence is preserved in the rocks.

During the Ordovician, the southern continents were collected into a single continent called Gondwana. Gondwana started the period in equatorial latitudes and, as the period progressed, drifted toward the South Pole. The Early Ordovician was thought to be quite warm, at least in the tropics. As with North America and Europe, Gondwana was largely covered with shallow seas during the Ordovician. Shallow clear waters over continental shelves encouraged the growth of organisms that deposit calcium carbonates in their shells and hard parts.

Ordovician rocks are chiefly sedimentary. Because of the restricted area and low elevation of solid land, which set limits to erosion, marine sediments that make up a large part of the Ordovician system consist chiefly of limestone. Shale and sandstone are less conspicuous.

A major mountain-building episode was the Taconic orogeny that was well under way in Cambrian times.

By the end of the period, Gondwana had neared or approached the pole and was largely glaciated.

Ordovician fauna

In North America and Europe, the Ordovician was a time of shallow continental seas rich in life. Trilobites and brachiopods in particular were rich and diverse. The first bryozoa appeared in the Ordovician as did the first coral reefs. Solitary corals date back to at least the Cambrian. Molluscs, which had also appeared during the Cambrian, became common and varied, especially bivalves, gastropods, and nautiloid cephalopods. It was long thought that the first true vertebrates (fish - Ostracoderms) appeared in the Ordovician, but recent discoveries in China reveal that they probably originated in the early Cambrian. Now-extinct marine animals called graptolites thrived in the oceans. Some cystoids and crinoids appeared. The first terrestrial plants appeared in the form of tiny plants resembling liverworts.

End of the Ordovician

Main article: Ordovician-Silurian extinction events.

The Ordovician came to a close in a series of extinction events that, taken together, comprise the second largest of the five major extinction events in Earth's history in terms of percentage of genera that went extinct. The only larger one was the Permian-Triassic extinction event.

The extinctions occurred approximately 444-447 million years ago and mark the boundary between the Ordovician and the following Silurian Period. At that time all complex multicellular organisms lived in the sea, and about 49% of genera of fauna disappeared forever; brachiopods and bryozoans were decimated, along with many of the trilobite, conodont and graptolite families.

The most commonly accepted theory is that these events were triggered by the onset of an ice age, in the Hirnantian faunal stage that ended the long, stable greenhouse conditions typical of the Ordovician. The ice age was probably not as long-lasting as once thought; study of oxygen isotopes in fossil brachiopods shows that it was probably no longer than 0.5 to 1.5 million years (Stanley, 358). The event was preceded by a fall in atmospheric carbon dioxide which selectively affected the shallow seas where most organisms lived. As the southern supercontinent Gondwana drifted over the South Pole, ice caps formed on it, which have been detected in Upper Ordovician rock strata of North Africa and then-adjacent northeastern South America, which were south-polar locations at the time.

Glaciation locks up water from the world-ocean, and the interglacials free it, causing sea levels repeatedly to drop and rise; the vast shallow intra-continental Ordovician seas withdrew, which eliminated many ecological niches, then returned carrying diminished founder populations lacking many whole families of organisms, then withdrew again with the next pulse of glaciation, eliminating biological diversity at each change (Emiliani, 1992 p. 491). Species limited to a single epicontinental sea on a given landmass were severely affected (Stanley, 360). Tropical lifeforms were hit particularly hard in the first wave of extinction, while cool-water species were hit worst in the second pulse (Stanley, 360).

Surviving species were those that coped with the changed conditions and filled the ecological niches left by the extinctions.

At the end of the second event, melting glaciers caused the sea level to rise and stabilise once more. The rebound of life's diversity with the permanent re-flooding of continental shelves at the onset of the Silurian saw increased biodiversity within the surviving Orders.

References

Ogg, Jim, June, 2004, Overview of Global Boundary Stratotype Sections and Points (GSSP's) http://www.stratigraphy.org/gssp.htm Accessed April 30, 2006.

Stanley, Steven M., Earth System History. New York: W.H. Freeman and Company, 1999. ISBN 0716728826