Biology : The Time of Ancient Life : Part Fourteen
The Great Permian Extinction
The end of the Permian, also the end of the Paleozoic era, was marked by the greatest extinction of the Phanerozoic eon. Despite its magnitude, the terminal Permian extinction has not received the amount of publicity or research that the more famous, but lesser, end-of-the Cretaceous extinction has. During the Permian extinction event, whose causes remain controversial, over 95% of marine species went extinct, while 70% of terrestrial taxonomic families suffered the same fate. The fusulinids went completely extinct, as did the trilobites. Brachiopod genera declined from 60 to 10. The majority of extinctions seem to have occurred at low paleolatitudes, possibly suggesting some event involving the world’s oceans
Plants seem to have missed the great extinction. Any floral changes occurred earlier in the Permian; The drying out of the Pangaea led to the evolution and spread of better adapted "dry forms" such as gymnosperms and seed ferns to replace the swamp trees of the Carboniferous. One major gymnosperm group disappeared, the swamp Cordaites.
The cause (or causes) of the Permian extinction remain in dispute.
- The Siberian Traps are a massive lava flow in eastern Asia that may have contributed to (or even caused) the Permian event by triggering a massive, sudden glaciation as well as other environmental consequences of volcanic eruptions. The period of greatest eruption in the traps coincides with the mass extinction. Ages of the lava flows suggest the traps formed over a one million year interval. The traps are near the city of Tura and also occur in Yakutsk, Noril'sk and Irkutsk, covering an area of slightly less than 2 million square kilometers (an area larger than Europe). The original volume of the traps is estimated at between 1 million and 4 million cubic km.
- The Cretaceous-Tertiary extinction that marks the close of the Mesozoic era has been hypothesized as a result of a large meteor or comet strike. Evidence of a similar (and larger) impact at the close of the Permian is not strongly supported, although some indirect evidence suggests an impact did occur during the Permian, although possibly not at the time of the extinction crisis.
- Climate change, possibly caused by glaciation and/or volcanic activity, has been associated with many mass extinctions. It seems likely that climate change is a consequence of the cause of extinction rather than the root cause itself.
- Formation of Pangaea has been invoked as a cause for the extinction. However, the formation of Pangaea had occurred well before the mass extinction. The mountain building and associated environmental disruption from volcanic activity would also have been extremely long term in duration, possibly lasting 10 million years. Pangaea's presence may have led to extreme environments with hotter interior areas of the continent and colder polar areas, possibly producing glaciation.
- There is evidence of a sudden drop in sea level at the end of the Permian. This could be attributed to the sudden cooling of the climate that produced glaciers at the southern polar areas in Gondwana. Glacial tillites and other geological evidence of late Permian age occur in Australia, Siberia and in the North Sea are interpreted as proof of the existence of large continental glaciers at the close of the Permian.
- Poisoning of the ocean has been suggested due to an apparent drop in carbon isotope data obtained from marine sediments formed at the time of the extinction. The cause of this apparent drop off in the photosynthetic rate in the seas has not yet been determined.
Whatever cause (or combination of causes), the terminal Permian extinction was a massive and severe crisis for life. Many groups of organisms went extinct at that time. Surviving groups diversified during the Triassic period and gradually a more modern world developed.
Time to take the Section 3 Quiz. This is worth 10 points.
Now go on to the next section.