Ordovician

The Ordovician is a geologic period and system that spans roughly 485.4 to 443.8 million years ago, the second period of the Paleozoic Era. It follows the Cambrian and precedes the Silurian. The name derives from the Ordovices, a Welsh tribe; the term was introduced by Adam Sedgwick in the 19th century after he studied rocks in Wales.

During much of the Ordovician, global seas were extensive and sea levels high, producing widespread shallow marine habitats. The period saw the Great Ordovician Biodiversification Event (GOBE), a major rise in biodiversity among marine invertebrates including trilobites, brachiopods, bryozoans, molluscs, and echinoderms, as well as abundant planktonic organisms such as graptolites. Early jawless vertebrates appeared late in the period.

Stratigraphically, the Ordovician is divided into Lower, Middle, and Upper parts, with stages such as Tremadocian, Floian, Dapingian, Darriwillian, Sandbian, Katian, and Hirnantian. Climate gradually cooled through the late Ordovician, contributing to glaciation in the Hirnantian and preparing the way for transition to the Silurian.

At the end of the Ordovician, about 443.8 million years ago, a mass extinction event occurred, one of the largest in Earth history. It coincided with a severe ice age and rising sea-level fluctuations, wiping out a large proportion of marine species. The Ordovician extinction profoundly affected reef-building organisms and other marine communities and was followed by the Silurian diversification.