Further Reading

Prothero, Donald R. Bringing Fossils to Life: An Introduction to Paleobiology. New York: McGraw-Hill, 2004.

Permian Permian refers to the last period in the Paleozoic era, lasting from 290-248 million years ago, and to the corresponding system of rocks. Sir Roderick Murchison named it in 1841 after the Perm region of northern Russia, where rocks of this age were first studied in detail. The supercontinent of Pangaea included most of the planet's landmasses during the Permian. This continental landmass extended from the South Pole, across the equator to high northern latitudes, with a wide Tethys sea forming an open wedge of water near the equator. The Siberian continental block collided with Lau-rasia in the Permian, forming the Ural Mountains. Most of Pangaea was influenced by hot and dry climate conditions and saw the formation of continental red-bed deposits and large-scale, cross-bedded sandstones, such as the Coconino sandstone of the southwestern United States and the New Red sandstone of the United Kingdom. Ice sheets covered the south-polar region, amplifying already low sea levels so they fell below the continental shelves, causing widespread mass extinctions. The glaciations continued to grow in intensity through the Permian, and together with weathering of continental cal-silicates, were able to draw enough C02 out of the atmosphere to drastically lower global temperatures. This dramatic climate change enhanced the already widespread extinctions, killing off many species of corals, brachiopods, ammonoids, and forams in one

Fossil reptile Orobates pabsti Diadectomorpha Diadectidea from early Permian period. Found in the Bromacker, Germany, area (Phil Degginger/Carnegie Museum/Alamy)

of history's greatest mass extinctions, in which about 70-90 percent of all marine invertebrate species perished, as did large numbers of land mammals. This greatest catastrophe of Earth history did not have a single cause, but reflects the combination of various elements.

The complex multi-factor cause of the end-Permian mass extinction is explained as follows. Plate tectonics was bringing many of the planet's landmasses together in a supercontinent (Pangaea), causing greater competition for fewer environmental niches by Permian life-forms. Drastically reduced were the rich continental shelf areas. As the continents collided mountains were pushed up, reducing the effective volume of the continents available to displace the sea, so sea levels fell, putting additional stress on life by further limiting the availability of favorable environmental niches. The global climate became dry and dusty, and the supercontinent formation led to widespread glaciation. This lowered sea level even more, lowered global temperatures, and put many life-forms on the planet in a very uncomfortable position, and many perished.

In the final million years of the Permian, the northern Siberian plains became the site of one of the largest volcanic effusions in Earth history. The Siberian flood basalts began erupting at 250 million years ago, becoming the largest known outpouring of continental flood basalts ever. Carbon dioxide was released in hitherto unknown abundance, warming the atmosphere and melting the glaciers. Other gases were also released, perhaps including methane, as the basalts probably melted permafrost and vaporized thick deposits of organic matter that accumulated in high latitudes like that at which Siberia was located 250 million years ago.

The global biosphere collapsed, and evidence suggests that the final collapse happened in less than 200,000 years, and perhaps in less than 30,000 years. According to this scenario, entirely internal processes may have caused the end-Permian extinction, although some scientists now argue that an impact may have dealt the final death blow. After it was over, new life-forms populated the seas and land, and these Mesozoic organisms tended to be more mobile and adept than their Paleozoic counterparts. The great Permian extinction created opportunities for new life-forms to occupy now empty niches, and the most adaptable and efficient organisms took control. The toughest of the marine organisms survived, and a new class of land animals grew to new proportions and occupied the land and skies. The Mesozoic, time of the great dinosaurs, had begun.

See also evolution; fossil; mass extinctions; Pangaea; stratigraphy, stratification, cyclothem; supercontinent cycles.

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