The Ordovician period is the second of the six (seven in North America) periods of the Paleozoic era. The Ordovician 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 situation where followers of Adam Sedgwick and Roderick Murchison were placing the same rock beds in the Cambrian and Silurian periods respectively. Charles Lapworth simply took all the conflicting strata and placed them in the new Ordovician period.

Ordovician dating

The Ordovician period started at a minor extinction event, possibly caused by a gamma ray burst, some time 490 million years ago (mya) and lasted for about 50-80 million years. It ended with a major extinction event 443.5 mya 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 oil and gas reservoirs in some regions.

Ordovician subdivisions

The Ordovician is usually broken into Lower (Tremadoc and Arenig), Middle (Caradoc, Llanvirn, Llandeilo) and Upper (Ashgill) subdivisions. The Faunal stages from youngest to oldest are:

• Hirnantian/Gamach (Upper-Ashgill)
• Rawtheyan/Richmond (Upper-Ashgill)
• Cautleyan/Richmond (Upper-Ashgill)
• Pusgillian/Maysville/Richmond (Upper-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 (Lower-Arenig)
• Arenig/Jefferson/Castleman (Lower-Arenig)
• Tremadoc/Deming/Gaconadian (Lower-Tremadoc)

Ordovician paleogeography

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

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.

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 appear in the Ordovician as do the first coral reefs. Solitary corals date back to at least the Cambrian. Molluscs, which had also appeared during the Cambrian, become 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.