Permian extinction

Permian extinction, a series of extinction pulses that contributed to the greatest mass extinction in Earth’s history. Many geologists and paleontologists contend that the Permian extinction occurred over the course of 15 million years during the latter part of the Permian Period (299 million to 252 million years ago). However, others claim that the extinction interval was much more rapid, lasting only about 200,000 years, with the bulk of the species loss occurring over a 20,000-year span near the end of the period. The Permian extinction was characterized by the elimination of about 90 percent of the species on Earth, which included more than 95 percent of the marine species and 70 percent of the terrestrial species. In addition, more than half of all taxonomic families present at the time disappeared. This event ranks first in severity of the five major extinction episodes that span geologic time.

(Read E.O. Wilson’s Britannica essay on mass extinction.)

Marine invertebrates

Shallow warm-water marine invertebrates, which included the trilobites, rugose and tabulate corals, and two large groups of echinoderms (blastoids and crinoids), show the most-protracted and greatest losses during the Permian extinction. Using the maximum number of different genera in the middle part of the Guadalupian Epoch (about 272.3 million to 259.8 million years ago) as a benchmark, extinction within marine invertebrate faunas significantly reduced the number of different genera by 12 to 70 percent by the beginning of the Capitanian Age some 266 million years ago. The diversity levels of many of these faunas plummeted to levels lower than at any prior time in the Permian Period. Extinctions at the boundary between the Guadalupian and Lopingian epochs (259.8 million to 252.2 million years ago) were even more severe—bordering on catastrophic—with a reduction of 70 to 80 percent from the Guadalupian generic maxima. A great many invertebrate families, which were highly successful prior to these extinctions, were affected.

By the early part of the Lopingian, specifically the Wuchiapingian Age (some 259.8 million to 254 million years ago), the now substantially reduced invertebrate fauna attempted to diversify again, but with limited success. Many were highly specialized groups, and more than half of these became extinct before the beginning of the Changhsingian Age (some 254 million years ago), the last age of the period. Marine invertebrate faunas during the Lopingian accounted for only about 10 percent or less of the Guadalupian faunal maxima; that is, about 90 percent of the Permian extinctions were accomplished before the start of the Changhsingian Age.

The series of extinction episodes that occurred during both the last stage of the Guadalupian Epoch and throughout the Lopingian Epoch, each apparently more severe than the previous one, extended over about 15 million years. Disruptive ecological changes eventually reduced marine invertebrates to crisis levels (about 5 percent of their Guadalupian maxima)—their lowest diversity since the end of the Ordovician Period. The final extinction episode, sometimes referred to as the terminal Permian crisis, while very real, took 15 million years to materialize and likely eliminated many ecologically struggling faunas that had already been greatly reduced by previous extinction episodes leading up to the terminal Permian crisis.

The Permian extinction was not restricted to marine invertebrates. Several groups of aquatic vertebrates, such as the acanthodians, thought to be the earliest jawed fishes, and the placoderms, a group of jawed fishes with significant armour, were also eliminated. Notable terrestrial groups, such as the pelycosaurs (fin-backed reptiles), Moschops (a massive mammal-like reptile), and numerous families of insects also met their demise. In addition, a number of groups (such as sharks, bony fishes, brachiopods, bryozoans, ammonoids, therapsids, reptiles, and amphibians) experienced significant declines by the end of the Permian Period.

Causes

Temperature crises

Although other single-event causes have been suggested, current explanations of Permian extinction events have focused on how biological and physical causes disrupted nutrient cycles. Hypotheses of temperature crises, especially of those occurring in shallow marine (surface) waters, are based in part on studies of oxygen isotopes and the ratios of calcium to magnesium in Permian fossil shell materials. The highest estimated temperatures of ocean surface waters (estimated to be 25–28 °C [about 77–82 °F]) until that time occurred during the end of the Guadalupian Epoch and the beginning of the Lopingian Epoch. Subsequently, by the end of the Lopingian Epoch, calcium-to-magnesium ratios suggest that water temperatures may have dropped to about 22–24 °C (about 72–75 °F), decreasing further during the very beginning of the Triassic Period. One hypothesis proposes that water temperatures greater than 24–28 °C (about 75–82 °F) may have been too warm for many invertebrates; only those specialized for high temperatures, such as those living in shallow lagoons, survived.

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Another temperature-related hypothesis posits that photosynthetic symbionts, which may have lived within the tissues of some marine invertebrates, were unable to survive the higher ocean temperatures and abandoned their hosts. Some of the data have been interpreted to show that an increase in seawater temperature of about 6 °C (10.8 °F) occurred—perhaps increasing the overall temperature of seawater to about 30–32 °C (about 86–90 °F)—near the Permian-Triassic boundary.