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 for a Welsh tribe -- was defined by Charles Lapworth in 1879 to resolve a situation where followers of Adam Sedgwick and 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.
The Ordovician period started at a (apparently minor?) extinction event sometime around 490 million years ago and lasted for about 50-80 million years. It ended with a major extinction event that wiped out 60% of marine genera. It is unusual to find any two sources that agree exactly on the absolute time of the start and end of the period.
Ordovician rocks contained abundant life and contain major oil and gas reservoirs in some regions.
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)
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 the Cambrian at least. It was long thought that the first true vertebrates (Fish) appeared in the Ordovician, but recent discoveries in China reveal that they probably orginated in the Early Cambrian. Now extinct marine animals called graptolites thrived in the oceans. The Southern continents were collected into a single continent called Gondwana during the Ordovician. Gondwana started the period in equatorial latitudes and drifted toward the South Pole during the period. 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.