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 PermianTriassic (PT) boundary extinction event, sometimes popularly called the Great Dying.
It was more devastating than the much more famous KT 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 KT 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, acanthodians, placoderms, and pelycosaurs, which did not survive beyond the PT boundary.
Other groups that were substantially reduced include the bryozoans, brachiopods, nautiloids, ammonites, sharks, bony fish, crinoids, eurypterid arthropods, ostracodes, and echinoderms. Terrestrial fauna affected included insects, amphibians, reptiles, as well as the dominant terrestrial group, the therapsids (mammallike 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 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 evaporites, 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 PermianTriassic 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 PT mass extinction. Later, the carbon dioxide emitted by the Siberian Traps eruptions caused the possibly cyclical climatic warming (greenhouse effect).
The main challenge to this hypothesis is the need to prove that the emissions of dust and aerosols were enough to explain the extinction event. Estimations suggest that the Siberian Traps eruptions occurred within a period of no less than 200,000 years, doubling the atmosphere’s carbon dioxide content and raising global temperatures by 1.5o C to 4.5o C.
This seems to be insufficient to explain the PT extinction, but the greenhouse effect might have exponentially increased due to the release of methane from oceanic methane hydrates trapped in deepsea sediments.
Carbon isotopic analyses suggest giant methane hydrate gasification across the PT boundary, and oxygen isotopic analyses indicate that global temperatures increased by about 6 oC near the equator and therefore by more at higher latitudes.
Methane is a greenhouse gas about 62 times as powerful as carbon dioxide. Because the stability of the methane hydrates depends on temperature, it is possible that a light deepsea warming caused a giant transfer of methane from oceanic hydrates to the atmosphere, increasing the greenhouse effect and triggering the PT extinction event.
There are questions as to the velocity of the extinctions during the Late Permian event, because some specialists consider that those extinctions were compatible with a more gradual mass extinction model and with the gradual volcanic theories.
However, other studies indicate that the extinctions occurred very abruptly, consistent with a catastrophic, possibly extraterrestrial, cause. These data suggest that an impact event (asteroidal or cometary) accompanied the PT extinction event, as was the case for the KT boundary event.
PT boundary impact evidence has been reported, including chondritic fragments in Antarctica, shocked quartz, fullerenes trapping extraterrestrial noble gases, and several potential impact craters: the 120kilometerdiameter Woodleigh crater (western Australia), the 250kilometerdiameter Bedout crater (northeastern coast of Australia), and even the giant 500kilometerdiameter Wilkes Land crater (Wilkes Land, Antarctica).
It has been suggested that the largescale volcanism in the Siberian Traps was triggered by some of these impacts. Nevertheless, for all proposed craters, the size, impact origin, and precise age of the impacting object have not been yet completely demonstrated.
Therefore, the real initial cause for the Permian mass extinction event is under debate. At present, the most broadly accepted hypothesis considers a multicausal scenario including climatic cycles between glacial and greenhouse conditions, volcanic eruptions, dissociations of methane hydrate, and probable meteoritic impacts.