Ordovician

The Ordovician is a period of geological time within the Paleozoic Era. It falls between the earlier Cambrian period and the later Silurian period. It lasted from approximately 485 million years ago to 444 million years ago.

The name derives from the Ordovices, a Celtic people who inhabited western Wales. The geologist Charles Lapworth proposed the name in the 19th century to resolve a dispute over rock formations in Wales that different scientists had variously assigned to the Cambrian or Silurian periods.

Position in the geological time scale

The Ordovician is divided into three major subdivisions: Lower, Middle, and Upper Ordovician. Boundaries are defined by changes in fossil types, particularly graptolites and conodonts, which are small organisms that changed rapidly. This allowed rock sequences from different continents to be correlated.

Geography and climate

Most continental land was concentrated in the supercontinent Gondwana, which occupied the southern regions at that time. A large ocean covered the northern portion. Sea levels were high, and shallow seas flooded parts of the continents. The climate was initially mild, but toward the end of the period there was glaciation, with extensive ice sheets on Gondwana.

Life

Life remained largely confined to the oceans. The fauna included brachiopods, trilobites, bryozoans, straight-shelled cephalopods, echinoderms, and graptolites. This period is marked by the major diversification of marine life, known as the Great Ordovician Biodiversification Event. Early land plants—bryophytes (mosses)—appear in rock deposits from this time, though land was not yet fully colonized by vegetation.

End-period extinction

The end of the Ordovician saw one of the major extinction events in Earth's history. Gondwanan glaciation, falling sea levels, and changes in ocean temperature and chemistry are believed to have played a role. Many marine species were lost, but most major groups survived and continued into the Silurian.