A "Great Dying\ took place 250 million years ago at the boundary between the Permian and Triassic geologic periods. A new paper in Science by Yadong Sun and an international team of scientists documents extraordinarily warm temperatures in the marine environment of that time, and shows that both marine and terrestrial animals were displaced toward the poles.
The Permian/Triassic extinction event resulted in a loss of about 70% of terrestrial vertebrate species and possibly over 90% of marine species. Even insects were not immune, with about 83% of genera lost. There was even a dearth of coal formation during this period, possibly related to a loss of vegetation.
Sun et al used oxygen isotope ratios in skeletal remains of conodonts (an extinct eel-like fish) as an indicator of water temperature in the ancient ocean. A sudden rise seen at the end of the Permian period, from 21°C to 36°C over a period of 800,000 years, was followed later by an additional rise that may have taken sea surface temperatures above 40°C in the late Smithian period. Temperatures in the equatorial latitudes may have exceeded that at which most marine life can survive. Metabolic oxygen demand is greater for sea animals at elevated temperatures while the amount of soluble oxygen decreases. The high sea temperatures were reflected by even higher ambient temperatures on land. Most plant life cannot survive sustained temperatures above 40C. The authors document the relative paucity of both marine and terrestrial vertebrate fossils at equatorial latitudes.
The cause of the Permian/Triassic extinction is not known with certainty, but evidence points to a massive release of greenhouse gasses by a series of volcanic eruptions in the Siberian Traps, which ultimately covered an area the size of Europe with volcanic rock. Carbon dioxide may also have released as the result of continuous coal and peat fires started by the volcanoes. Release of methane from marine clathrates may have occurred as the ocean warmed.
The Earth of the Triassic area was a very different place than today, with almost all of the land collected in one huge continent called Pangaea. From Sun's paper, one gets an impression of a huge, hot dry desert with animal and plant life clinging precariously to the polar edges. Likewise, the fish would have been largely segregated into northern and southern populations separated by a warm anoxic and acidic microbial swamp that comprised most of the single ocean.
James Lovelock's Gaia hypothesis maintains that the biosphere of the Earth is a homeostatic mechanism that functions to keep the planet close to a physiological optimum for its plants and creatures. That may well be, but deviations from equilibrium can last for a very long time. In the Triassic area, it took 5 million years for the Earth to return to more normal temperatures.
The story of the Permian/ Triassic extinction is a cautionary tale for our own time. While volcanic activity is now relatively minimal, greenhouse gasses are being released by power plants and smokestacks and millions of vehicles, resulting in the warming of the planet.
More great images of paleogeographic maps are available from Ron Blakey and NAU Geology on their website.