Permian Extinction

After more than 100 million years of relative stability during the Carboniferous and the Permian eras, this last period ended 251 million years ago with the largest extinction event in the Earth's history. This biotic crisis is known in paleontology as the Permian-Triassic (P-T) boundary extinction event, sometimes popularly called the Great Dying. It was more devastating than the much more famous K-T boundary extinction event, when the dinosaurs were extinguished. It has been estimated that as many as 52% of families and 90% to 95% of species were lost, far more than were lost in the K-T extinction, in which 11% of families and 75% to 80% of species were extinguished. Some authors have considered that perhaps 99.5% of individuals died as a result of the event. The primary marine and terrestrial victims included the fusulinid foraminifera, trilobites, rugose and tabulate corals, blastoid echinoderms, acan-thodians, placoderms, and pelycosaurs, which did not survive beyond the P-T boundary. Other groups that were substantially reduced include the bryo-zoans, brachiopods, nautiloids, ammonites, sharks, bony fish, crinoids, eurypterid arthropods, ostra-codes, and echinoderms. Terrestrial fauna affected included insects, amphibians, reptiles, as well as the dominant terrestrial group, the therapsids (mammal-like reptiles).

During the Carboniferous and Permian, life flourished with crinoids, nautiloids, and ammonites. Corals and fishes dominated the oceans, and amphibians and reptiles progressively invaded the terrestrial environment. This scenario changed in the end of the Permian due to causes that still are under debate. Many causes have been proposed, including meteorite impacts, volcanic activity, glaciations, and fluctuations in sea level. It is known that the formation of the supercontinent Pangea occurred in the Permian, collecting all the earth's major landmasses; that it extended from the North to South poles, and that it caused an effect on ocean currents. These large continental landmasses created climates with extreme variations of heat and cold (continental climates), and the deserts were widespread on Pangea. One of the first scenarios proposed for explaining the Late Permian extinctions was the reduction of shallow continental shelves as a result of the formation of this [Page 982]supercontinent. Such reduction would cause an ecological competition for space, acting as an agent for extinction. In fact, it is suggested that the marine environment was more affected than the terrestrial one, estimating that more than 95% of marine species and only 70% of land species became extinguished. Although this is a viable hypothesis, it is known as the formation of Pangea and the putative destruction of the continental shelves occurred in the Early and Middle Permian—that is, unrelated to the Late Permian mass extinction.

A second possible mechanism for the Permian extinction was severe climatic fluctuations produced by concurrent glaciation events on the North and South poles, and subsequent sea level changes. There is sedimentological evidence of significant cooling and drying in temperate latitudes, such as thick sequences of dune sands and evapor-ites, and prominent glaciation in the polar latitudes, such as glacial tillites.

The hypothesis for the Permian extinction most broadly accepted by paleontologists posits an increase in volcanic activity. There is abundant evidence that massive flood basalts from magma output contributed to rapid climatic turnovers and environmental stress. A massive eruptive event spanning the Permian-Triassic transition, about 252 to 250 million years ago, formed the famous Siberian Traps, a large igneous province in Siberia that covered over 200,000 square kilometers. Extensive pyroclastic deposits suggest that numerous large explosive eruptions occurred during or before the eruptions of basaltic lavas. The combination of a worldwide ash cloud and sulphates in the atmosphere might have initiated sudden climatic changes. Dust clouds and acid rain might have disrupted photosynthesis and caused the collapse of the food chains, triggering the P-T mass extinction. Later, the carbon dioxide emitted by the Siberian Traps eruptions caused the possibly cyclical climatic warming (greenhouse effect).

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