End–Ordovician extinction event
The End–Ordovician extinction event is the third-largest extinction event of the Phanerozoic eon. The Ordovician period followed the Cambrian and was followed by the Silurian. There were no living things on the land except for bacteria and perhaps some single-celled algae. The biota was almost entirely marine.
The extinction came in two steps, at the start and the finish of the Hirnantian stage, which was the last stage of the Ordovician.
- 1. Pre-event: warm climate, deep ocean anoxic event. ocean bottoms were anoxic (little or no oxygen). Black shales were laid down in deep ocean strata; carbonates laid down on oxygenated continental shelves.
- 2. First step: climate turns cold; turnover of water in seas. Rising anoxic water kills most of the plankton, and shrinking seas remove habitats. Cold stage with clear evidence of widespread glaciation.
- 4. Second step: warming ocean re-established; glaciers melt, anoxic conditions reach continental shelves and kills fauna again.
Basic mechanism: climate changes from very warm to very cold and back to very warm. Changes in ocean circulation were the results of the climate changes. Both benthic (ocean bottom) and pelagic fauna were faced with conditions they were unable to cope with.
More than 100 invertebrate families becam extinct in the End–Ordovician extinction event, and a total of almost half the genera. The brachiopods and bryozoans were decimated, along with many of the trilobite, conodont and graptolite families.
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