Ordovician-Silurian extinction
Ordovician-Silurian extinction, global extinction event that occurred during the Ordovician Period's Hirnantian Age (445.2 million to 443.8 million years ago) and the Silurian Period's subsequent Rhuddanian Age (443.8 million to 440.8 million years ago), eradicating an estimated 85 percent of all Ordovician species. In terms of the proportion of marine groups impacted, this extinction event is second only to the one that occurred around 251 million years ago at the border between the Permian and Triassic eras. The Ordovician-Silurian extinction event was nearly twice as severe as the K-T extinction event that happened at the end of the Cretaceous Period, around 66 million years ago, and is notable for bringing the dinosaurs to an end.
The extinction appears to have happened in stages. Some paleontologists believe that an early phase affecting graptolites, brachiopods, and trilobites occurred prior to the end of the Ordovician Period, before the major drop in sea level, and that it was caused by falling carbon dioxide levels associated with silicate rock erosion, which may have triggered a global cooling phase. However, most paleontologists agree that glaciation over the African and South American regions of Gondwana, as well as the accompanying drop in sea levels and changes in ocean current patterns, caused enormous disturbances to temperatures and ecosystems. The sea level decline would have emptied the huge epicontinental waters, reducing the accessible habitat for creatures that thrived in such environments.
The spread of cool-water brachiopod fauna into tropical latitudes was followed by glaciation in several locations, indicating the start of considerable global cooling. The rise in sea level caused by global warming and retreating glaciers during the Rhuddanian Age of the Silurian Period caused a third period of extinction. There was no iridium concentration around the extinction that would imply a bolide (meteorite or comet) impact like the one discovered at the end of the Cretaceous Period.
