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How severe was the Ordovician-Silurian mass extinction?

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Ordovician paleogeography — Ordovician Period The distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the Ordovician Period. Included in the paleogeographic reconstruction are the locations of the interval's subduction zones.

The Ordovician-Silurian extinction was a severe global mass extinction event that eliminated an estimated 85 percent of all Ordovician species. In terms of the percentage of marine families affected, this event ranks as the second most severe in Earth's history, surpassed only by the Permian-Triassic extinction. It was nearly twice as intense as the K–T extinction that ended the age of the dinosaurs.12

At the taxonomic level, the event claimed 26 percent of all marine invertebrate families and 60 percent of all marine invertebrate genera. Groups such as brachiopods, trilobites, bryozoans, and conodonts were particularly hard hit, especially those that were endemic to specific regions. The extinction occurred in multiple phases driven by glaciation, significant global cooling, and dramatic fluctuations in sea level, which destroyed critical habitats in shallow epicontinental seas.312