The Late Ordovician mass extinction

Near the end of the Late Ordovician, in the first of five mass extinctions in the Phanerozoic, about 85% of marine species died. The cause was a brief glacial interval that produced two pulses of extinction. The first pulse w as at the beginning of the glaciation, when sea-level decline drained epico ntinental seaways, produced a harsh climate in low and mid-latitudes, and i nitiated active, deep-oceanic currents that aerated the deep oceans and bro ught nutrients and possibly toxic material up from oceanic depths. Followin g that initial pulse of extinction, surviving faunas adapted to the new eco logic setting. The glaciation ended suddenly, and as sea level rose, the cl imate moderated, and oceanic circulation stagnated, another pulse of extinc tion occurred. The second extinction marked the end of a long interval of e cologic stasis tan Ecologic-Evolutionary Unit). Recovery from the event too k several million years, but the resulting fauna had ecologic patterns simi lar to the fauna that had become extinct. Other extinction events that elim inated similar or even smaller percentages of species had greater long-term ecologic effects.