Ordovician Period

Ordovician Period, in geology, second division of the Palaeozoic Era of the geological timescale, spanning a period from 500 million years ago to 448 million years ago. It is preceded by the Cambrian Period and succeeded by the Silurian Period. Its name is derived from that of an ancient tribe inhabiting the Welsh borderland, where rocks of this age are well developed. The name Ordovician was introduced by the geologist Charles Lapworth as a compromise between competing claims for the geological compass of the Cambrian and Silurian. However, it is assuredly a natural division of geological time in that the Ordovician records the greatest ever enrichment in the diversity of marine life. This was the period when the first true coral reefs appeared, along with recognizable fish, and important groups like bryozoans. The molluscs radiated into diverse groups of gastropods, clams, and nautiloid cephalopods, the descendants of which are found in today's oceans. It was also the highpoint of graptolite and trilobite evolution—both extinct kinds of animals that swarmed in great variety in the Ordovician seas. The succession of various species of graptolites (along with the stem-group vertebrate microfossils known as conodonts) provides the stratigraphical time framework for the period.

The Ordovician was a time when continents were as widely dispersed as they are at the present day. The continent of Laurentia, equivalent mostly to present-day North America, embraced the ancient tropics. A large Gondwana continent included a south pole in what is now northern Africa, but stretched eastwards to China, and included Australia. The Baltica continent was also independent, and there was an ocean along what is now the Ural Mountains. Siberia was another separate continent. As the Ordovician oceans separating these continents closed, great mountain belts were thrown up (see Orogeny). The greatest of these was the Caledonian-Appalachian chain stretching from Arctic Norway to the southern United States. The remnants of this chain comprise Norway, the Highlands of Scotland, and the east coastal ranges of the United States. At times there was violent volcanic activity: one huge eruption deposited an ashfall that is as great as any in the geological record. The climate was related to the ancient latitude. In Laurentia tropical seas favoured deposition of vast spreads of limestones. These were correspondingly rare in the higher palaeolatitudes of Gondwana, where sandstones and shales predominated. Generally, this was a 'greenhouse Earth'. However, at the end of the Ordovician there was a great glaciation, known as the Ashgill or Hirnantian event, which lowered sea levels and changed climate drastically for a short period, probably less than 1 million years. The event is associated with a major mass extinction, which is generally recognized as one of the 'Big Five' mass extinctions along with those at or near the ends of the Devonian, Permian, Triassic, and Cretaceous periods. Many Ordovician trilobites, graptolites, molluscs, and brachiopods were exterminated. A few refugia (havens for fauna and flora away from regions of climatic change)—such as in the island of Anticosti, Canada—may have been the source of most of the Silurian and younger faunas.