The second largest mass extinction of the Phanerozoic Era occurred during the Late Ordovician, about 445 million years ago. During that time, 22 percent of all families and more than 60 percent of genera became extinct. The Upper Ordovician extinction occurred in two pulses during the Ashgill (the latest part of the Ordovician), separated by about 500,000 years. The first extinction event occurred at the boundary between the Rawtheyan and Hirnantian stages and is related to changes in climatic cooling and changes in oceanic circulation related to the start of glaciation in Gondwana (the supercontintent composed of Africa, South America, Australia, India, and the Middle East).
The second wave of extinction occurred in the middle of the Hirnantian stage and is related to melting of the Gondwanan ice cap and the concomitant spread of anoxic bottom waters over the continental shelves. Although taxonomic losses were high dur
Extinction Horizons, Sea Level, and Environmental and Biotic Changes in the Late Ordovician
ing the Late Ordovician, the ecological severity of this event was not as great as for other mass extinctions, such as the Late Devonian extinction, inasmuch as only community-level changes occurred across the extinction boundary.
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