About three quarters of all species on Earth died out in the Late Devonian mass extinction, though as a series of extinctions over several million years, rather than one event. Life that existed in shallow waters and seas were the the most affected; reefs were hit so hard that it was not until corals evolved over 100 million years later that reefs returned to their former glory. Aside from reef-building organisms, other groups that were hit hard include the brachiopods and the trilobites. During this time period, much of the sea bed became devoid of oxygen, which made it only sustainable for bacteria. Changes in sea level, asteroid impacts, climate change and new species of plants messing with the soil have all been probable causes for these extinctions. A major extinction, the Kellwasser event marked the beginning of the last phase of the Devonian period, the Famennian faunal stage about 375-360 million years ago. A second mass extinction, the Hangenberg event, closed the Devonian period. It is still unclear as to the extent of time during these extinctions, as it is uncertain whether there were two sharp mass extinctions or a series of smaller extinctions.
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