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PALEOZOIC ERA

END-PERMIAN EXTINCTION

258-251 million years ago

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End Permian Extinctions: The Permian Period ended with the worst mass extinction in earth's history.

The mass extinction that occurred at the end of the Permian was the worst mass extinction in history. An estimated 96% of marine species became extinct. The dominant groups of the Paleozoic seafloor (crinoids, bryozoans, and brachiopods) were replaced by groups which continue to dominate today (bivalves, gastropods, and echinoderms). Bivalves and gastropods were less affected and lost perhaps 30% of their diversity. Some groups had been reduced by earlier mass extinctions and now were completely wiped out (trilobites, blastoids, tabulate corals, rugose corals, orthid brachiopods, and many mollusks). Fusulinid foraminiferans and productid brachiopods were thriving in the Permian but went extinct at its end. Only one group of crinoids and two groups of ammonoids survived the end of the Permian.

What could have caused this extinction which lasted 8 million years?

A variety of factors have been considered and multiple factors could have contributed. One of the largest eruptions of flood basalts (the Siberian traps) and a large volcanic eruption might have changed the composition of the atmosphere and blocked sunlight (Erwin, 1994).

One of the most significant events of the Permian Period was the joining of the continents. The various landmasses on earth came together in the middle Permian to form a single supercontinent known as Pangea. As the ocean which separated what would someday become the Americas from Europe and Africa disappeared, many shallow marine habitats were lost. The change in bodies of water would have affected climate. Terrestrial ecosystems seem to have collapsed after the events which devastated marine ecosystems, and caused both animal and plant extinctions. Climatic fluctuations may have been a major cause (Looy, 2001, Erwim 1996). Some have argued that the halogens which would have been released by the Siberian Traps at the end of the Permian may have destroyed enough ozone to contribute to an ecological crisis (Visscher, 2004).

The formation of a supercontinent would certainly affect local climates since rainfall patterns would have been altered. In addition, all of the organic material from former marine continental shelves which were exposed to air would have decomposed. This would have used tremendous amounts of oxygen, perhaps reducing the atmospheric oxygen content by half. It appears that the world’s single ocean (named Panthalassa) suffered a major anoxic event which lasted millions of years at the end of the Permian. As oxygen levels dropped, carbon dioxide levels in the ocean seem to have increased substantially. Not only would this have been toxic for marine life, it appears that the carbon dioxide entered the atmosphere causing a brief greenhouse effect which melted the polar ice caps which had formed earlier in the Permian (for the first time in tens of millions of years. Whatever initiated the end Permian extinction (probably either mass volcanic eruptions and/or a meteorite impact), the result seems to have been a greenhouse effect which raised the earth’s temperature by an estimated 6 degrees Celsius. Both terrestrial and marine ecosystems were decimated and required tens of millions of years to recover their previous biodiversity (Benton, 2003; Isozaki, 1997, Erwin, 1996; Erwin 1994; Waters, 1997).