Climate - Extinctions

The end of the Ordovician was a period of mass extinctions: 85% of all Ordovician species are estimated to have disappeared in the Hirnantian, which lasted about two million years, and the end of the Ordovician also saw extinctions in the Silurian period.

The extinction appears to have occurred in several stages. A first phase occurred at the end of the Ordovician, affecting hookworms, brachiopods and trilobites before sea level dropped significantly. Later, glaciation in Gondwanan Africa and South America caused a second phase of extinction, accompanied by sea level fall.

The cause of the Late Ordovician extinction is generally accepted to be related to the rapid cooling of the first wave at the end of the Ordovician, while the second wave was caused by the lowering of sea levels associated with the glacial period.

A small or background extinction occurred during the Ordovician. It occurred in the early Late Ordovician in the eastern United States. This extinction affected a wide variety of organisms with different habitats, including brachiopods, corals, trilobites, echinoderms and molluscs. Species restricted to the eastern United States were particularly threatened. The geographic range of many of the surviving species narrowed and they became extinct from the eastern region into the western US and Canada.