252 Million B.C.T. - The Great Dying Occurred

Around 252 Million B.C.T., the Permian-Triassic extinctinon event -- the most severe extinction event in the history of the Earth -- occurred. 

The Permian–Triassic (P–Tr) extinction event, informally known as the Great Dying, was an extinction event that occurred 252.28 million years ago, forming the boundary between the Permian and Triassic geologic periods, as well as the Paleozoic and Mesozoic eras. It is the Earth's most severe known extinction event, with up to 96% of all marine species and 70% of terrestrial vertebrate species becoming extinct. It is the only known mass extinction of insects.  Some 57% of all families and 83% of all genera became extinct. Because so much biodiversity was lost, the recovery of life on Earth took significantly longer than after any other extinction event, possibly up to 10 million years.

Researchers have variously suggested that there were from one to three distinct pulses, or phases, of extinction. There are several proposed mechanisms for the extinctions; the earlier phase was likely due to gradual environmental change, while the latter phase has been argued to be due to a catastrophic event. Suggested mechanisms for the latter include large or multiple bolide impact events, increased volcanism, coal/gas fires and explosions from the Siberian Traps, and sudden release of methane clathrate from the sea floor; gradual changes include sea-level change, anoxia, increasing aridity, and a shift in ocean circulation driven by climate change.

Pin-pointing the exact cause (or causes) of the Permian–Triassic extinction event is a difficult undertaking, mostly because the catastrophe occurred over 250 million years ago, and much of the evidence that would have pointed to the cause has either been destroyed by now or is concealed deep within the Earth under many layers of rock. The sea floor is also completely recycled every 200 million years by the ongoing process of plate tectonics and seafloor spreading, thereby leaving no useful indications beneath the ocean. With the fairly significant evidence that scientists have managed to accumulate, several mechanisms have been proposed for the extinction event, including both catastrophic and gradualistic processes (similar to those theorized for the Cretaceous–Paleogene extinction event). The former include large or multiple bolide impact events, increased volcanism, or sudden release of methane hydrates from the sea floor. The latter include sealevel change, anoxia, and increasing aridity. Any hypothesis about the cause must explain the selectivity of the event, which primarily affected organisms with calcium carbonate skeletons, the long (4– to 6-million-year) period before recovery started, and the minimal extent of biological mineralization (despite inorganic carbonates being deposited) once the recovery began.

Evidence that an impact event may have caused the Cretaceous–Paleogene extinction event has led to speculation that similar impacts may have been the cause of other extinction events, including the P–Tr extinction, and therefore to a search for evidence of impacts at the times of other extinctions and for large impact craters of the appropriate age.

Reported evidence for an impact event from the P–Tr boundary level includes rare grains of shocked quartz in Australia and Antarctica; fullerenes trapping extraterrestrial noble gases; meteorite fragments in Antarctica; and grains rich in iron, nickel and silicon, which may have been created by an impact.  However, the accuracy of most of these claims has been challenged. Quartz from Graphite Peak in Antarctica, for example, once considered "shocked", has been re-examined by optical and transmission electron microscopy. The observed features were concluded to be not due to shock, but rather to plastic deformation, consistent with formation in a tectonic environment such as volcanism.

Several possible impact craters have been proposed as possible causes of the P–Tr extinction, including the Bedout structure off the northwest coast of Australia and the hypothesized Wilkes Land crater of East Antarctica. In each of these cases, the idea that an impact was responsible has not been proven, and has been widely criticized. In the case of Wilkes Land, the age of this sub-ice geophysical feature is very uncertain – it may be later than the Permian–Triassic extinction.
If impact is a major cause of the P–Tr extinction, the crater likely would no longer exist. As 70% of the Earth's surface is sea, an asteroid or comet fragment is more than twice as likely to hit ocean as it is to hit land. However, Earth has no ocean-floor crust more than 200 million years old, because the "conveyor belt" process of seafloor spreading and subduction destroys it within that time. Craters produced by very large impacts may be masked by extensive flood basalting from below after the crust is punctured or weakened. Subduction should not, however, be entirely accepted as an explanation of why no firm evidence can be found: as with the K-T event, an ejecta blanket stratum rich in siderophilic elements (e.g., iridium) would be expected to be seen in formations from the time.
One attraction of large impact theories is that theoretically they could trigger other cause-considered extinction-paralleling phenomena, such as the Siberian Traps eruptions (see below) as being either an impact site or the antipode of an impact site. The abruptness of an impact also explains why species did not rapidly evolve in adaptation to more slowly manifesting and/or less than global-in-scope phenomena.