The Permian Triassic (P-T, P-Tr) extinction event, also known as the End Permian Extinction and very commonly known as the Great Dying, formed the boundary between the Permian and Triassic geologic periods. Not only within the periods but between the Paleozoic and Mesozoic eras, around approximately 251.9 million years ago. It is one of Earth's most severe known extinction events to have taken place, with the extinction of 57% of biological families, 83% of genera, 81% of marine species and 70% of terrestrial vertebrate species. It was the largest known mass extinction, especially of insects.
Brief Account of End Permian Extinction
There is evidence for one to three distinct pulses, or phases, of the end Permian mass extinction. The scientific consensus is that the Permian extinction causes included the increase in the temperature and the widespread oceanic anoxia which was due to the large amounts of carbon dioxide emitted by the eruption of the Siberian Traps. It has also been put forward that the burning of hydrocarbon deposits, including oil and coal, by the eruption of the Siberian Traps and the emissions of methane by methanogenic microorganisms contributed largely to the extinction.
The speed of recovery from the Permian extinction is controversial. Some scientists estimate that it took almost 10 million years, until the Middle Triassic time, because of both the severity of the extinction and the naturally worst conditions that returned periodically for another 5 million years. But, studies in Bear Lake County, near Paris, Idaho, showed a comparatively quick rebound in a localized Early Triassic marine ecosystem, taking almost 2 million years to recover, suggesting that the impact of the extinction may have been felt less in some areas than others.
Patterns of Extinction
Marine invertebrates suffered the greatest losses during the P–Tr extinction. Evidence of this has been found in samples that come from south China sections at the P–Tr boundary. Here, 286 out of 329 marine invertebrate genera disappear within the final two sedimentary zones containing conodonts from the Permian. The decrease in diversity was probably caused by a sharp increase in extinctions, rather than a decrease in speciation.
The extinction primarily affected organisms with calcium carbonate skeletons, especially those reliant on stable CO2 levels to produce their skeletons. These organisms were susceptible to the effects of the ocean acidification that resulted from increased atmospheric CO2.
Among benthic organisms, the extinction event multiplied background extinction rates, and thus caused the largest species loss to taxa that had a high background extinction rate (by implication, taxa with a high turnover). The extinction rate of marine organisms was catastrophic. Surviving marine invertebrate groups included articulate brachiopods (those with a hinge), which had undergone a slow decline in numbers since the P–Tr extinction; the Ceratitida order of ammonites; and crinoids ("sea lilies"), which very nearly became extinct but later became abundant and diverse. The groups with the highest survival rates generally had active control of circulation, elaborate gas exchange mechanisms, and light calcification; more calcified organisms with simpler breathing apparatuses suffered the greatest loss of species diversity. In the case of the brachiopods, at least, surviving taxa were generally small, rare members of a previously diverse community.
The ammonoids, which had been in a long-term decline for the 30 million years since the Roadian (middle Permian), suffered a selective extinction pulse 10 million years before the main event, at the end of the Capitanian stage. In this preliminary extinction, which had greatly reduced the disparity, or the range of different ecological guilds, environmental factors have been found to be apparently responsible. Diversity and disparity fell further until the P–Tr boundary; the extinction here (P–Tr) was non-selective, consistent with a catastrophic initiator. During the Triassic, diversity rose very fast, but disparity remained low.
The range of morphospace occupied by the ammonoids, that is, their range of possible forms, shapes or structures, became more restricted as the Permian progressed. A few million years into the Triassic, the original range of ammonoid structures was once again reoccupied, but the parameters were now shared distinctly among clades.
The Permian had great diversity in insect and other invertebrate species, including the largest of the insects ever to have existed. The end-Permian is the largest known mass extinction of insects. According to some sources, it is the only insect mass extinction. Eight or nine insect orders became extinct and ten more were greatly reduced in diversity. Palaeodictyopteroidea which are insects with piercing and sucking mouthparts began to decline during the mid-Permian. These extinctions have been linked to a change in the animal kingdom. The greatest decline occurred in the Late Permian. It was in all probability not directly caused by the weather-related floral transitions.
Most fossil insect groups found after the Permian–Triassic boundary differs in a significant manner from those before. Amongst the Paleozoic insect groups, only the Glosselytrodea, Miomoptera, and Protorthoptera have been discovered in deposits that have survived from after the time of the extinction. The caloneurodeans, monurans, palaeodictyopteroidea, protelytropterans, and protodonates became extinct by the end of the Permian. In the very well documented Late Triassic deposits, the fossils majorly consist of modern fossil insect groups.
In the wake of the extinction event, the ecological structure of the present-day biosphere evolved from the stock of surviving taxa. In the sea, the Modern Evolutionary Fauna became the dominant force over elements of the Palaeozoic Evolutionary Fauna. Typical taxa of shelly benthic faunas were now bivalves, snails, sea urchins and Malacostraca, whereas bony fishes and marine reptiles diversified in the pelagic zone. On land, dinosaurs and mammals arose in the course of the Triassic. The profound change in the taxonomic composition was partly a result of the selectivity of the extinction event, which affected some of the taxa, for example, brachiopods, some more severely than others for example the bivalves. But, recovery was also differential between taxa. Some survivors became extinct some million years after the extinction event without having re-diversified (dead clade walking e.g. the snail family Bellerophontidae), whereas others rose to prominence over geologic times such as the bivalves.
Permian Mass Extinction Cause
Pinpointing the exact causes of the Permian–Triassic extinction event is difficult. This is because it occurred over 250 million years ago, and since then most of the evidence that would have pointed to the cause has been destroyed. Another difficulty is that it might be hidden deep within the Earth under many layers of rock. The seafloor is completely recycled every 200 million years. And because of the ongoing process of plate tectonics and seafloor spreading, no useful proof that must be beneath the ocean would be found. Yet, scientists have gathered significant evidence for causes, and several mechanisms have been put forward. The proposals include - the catastrophic and the gradual processes (like the ones theorized for the Cretaceous–Paleogene extinction event).
The catastrophic group includes one or more large bolide impact events, increased volcanism, and sudden release of methane from the seafloor, either due to dissociation of methane hydrate deposits or metabolism of organic carbon deposits by methanogenic microbes. The gradual group includes sea-level change, increasing anoxia, and increasing aridity. Any hypothesis about the cause must explain the selectivity of the event, which affected organisms with calcium carbonate skeletons most severely; the long period (4 to 6 million years) before recovery started, and the minimal extent of biological mineralization, despite inorganic carbonates that might be deposited, once the recovery began.
Possible causes supported by strong evidence appear to describe a sequence of catastrophes, each worse than the last. At first, the Siberian Traps eruptions, that were bad enough alone. But because they occurred near coal beds and the continental shelf, they also triggered very large releases of carbon dioxide and methane. The resultant global warming may have caused possibly the most severe anoxic event in the oceans' history. According to this theory, the oceans became so anoxic that anaerobic sulfur-reducing organisms dominated the chemistry of the oceans and caused the massive emissions of toxic hydrogen sulfide.
But, there may be some weak links in this chain of events. The changes in the 13C/12C ratio expected to result from a massive release of methane do not match the patterns that have been seen throughout the Early Triassic. Also, the types of oceanic thermohaline circulation that may have existed at the end of the Permian are not likely to have supported deep-sea anoxia.