The Role of Environmental Conditions In Recovery from the Permian-Triassic Mass Extinction: Why the Greatest Mass Extinction In Earth History Is Associated with a Long, Complicated Recovery

The Permian-Triassic mass extinction was the most devastating mass extinction in the history of Earth with an estimated 95% of all marine species eliminated. Recovery from the mass extinction was complex, with some regions exhibiting rapid recovery, while other regions suffered until the Middle Triassic. Evidence suggests that the driving force behind the recovery was the presence or absence of harsh environmental conditions (e.g., Twitchett et al., 2004), and those areas that appear to have been subjected to persistent environmental stress suffered the longest. The post-extinction Lower Triassic sedimentary record is also marked by the widespread occurrence of anachronistic facies, or facies that are indicative of periods earlier in Earth history (i.e., the Precambrian). Anachronistic facies documented from Lower Triassic rocks include seafloor precipitates in the form of calcite cement fans and crusts, widespread subtidal microbialites, wrinkle structures, flat pebble conglomerates, and unusually large ooids. While the formation of anachronistic facies within Lower Triassic rocks is partially related to a decrease in bioturbation depth and intensity, the unusual carbonate chemistry of the oceans during the period was also responsible for enhancing the precipitation of calcium carbonate and placing stress on post-extinction faunas via hypercapnia. The occurrence of anachronistic facies are therefore indicative of the highly unusual and oppressive environmental conditions that were present during the Lower Triassic, and can be used to infer biotic stress, even when other indicators (e.g., laminated sediments) are absent. Anachronistic facies appear to be primarily associated with the Permian-Triassic extinction and recovery interval, and are relatively rare elsewhere during the Phanerozoic, although a handful of examples of anachronistic facies from other post-extinction intervals have been documented. The broad geographic range and temporally persistent nature of Lower Triassic anachronistic facies are therefore indicative of the unusual environmental conditions that persisted during the Early Triassic.