Abstract

Mass biotal extinctions have been attributed to numerous causes ranging from cosmic radiation to epidemic diseases. One cause commonly mentioned to explain the mass extinctions of Permian and Triassic times is a eustatic sea-level change during which epicontinental seas — the breeding grounds of thousands of marine taxa — were virtually eliminated from the continents. As a result of late Paleozoic orogenies and the sea withdrawals, large land areas were exposed and ecologic niches were eliminated and/or reduced drastically in size. It is almost certain that large-scale eustatic sea-level lowering and exposure of large land areas were related closely to the mass extinctions of Permian and Triassic times. However, these are not likely to have been the only causes, because the extinctions of numerous marine taxa took place during both Late Permian and Late Triassic times, whereas major extinctions of tetrapod faunas and land floras are mainly Late Triassic phenomena.

The great sea withdrawals during Permian time were accompanied by a worldwide climate change from an Early Permian glacial-maximum condition (frigid, temperate, and torrid zones were well developed) to a latest Permian-Triassic evaporite-maximum condition (a nearly uniform worldwide climate in which only temperate and torrid zones were developed). Evaporite-maximum conditions persisted through Triassic time, and epicontinental seas continued to be scarce. At the end of Triassic time and during Early and Middle Jurassic time, an abrupt change took place to a cool, wet climate similar to that of the middle part of the Permian. This climatic change caused a marked alteration to take place in the vegetation, and many herbivorous tetrapods, accustomed to certain plant foods, no longer had these accessible to them. Almost simultaneously, epicontinental seas began to transgress large areas of the continents through Jurassic time.

Therefore, the great faunal and floral “crisis” of Permian and Triassic times appears to have been caused by several factors acting together: eustatic sea-level lowering, orogeny, continental emergence, major climate change, and important changes in vegetation. Finally, Late Permian through Triassic time seems to have been a period when the solar system was passing through the perigalactic part of its orbit, with the result that the effects of climatic change were intensified.