Permian Period

I

Introduction

Permian Period, last division of the Paleozoic Era of the geologic time scale (see Geology), spanning an interval from about 299 million to 251 million years ago. The Permian was preceded by the Carboniferous Period and followed by the Triassic Period. Life on Earth suffered the greatest mass extinction in its history at the end of the Permian. Scientists are trying to determine what events caused this global disaster, which killed about 90 percent of species living in the oceans and more than 70 percent of species on land. English geologist Sir Roderick Impey Murchison named the Permian after a village in eastern Russia where sedimentary strata of this age were correlated, on the basis of fossil content, with strata farther west in Germany. Throughout the world the rocks of the Permian period are rich in deposits of coal, oil, and gas.

Plate tectonics toward the end of the Paleozoic began to bring together the landmasses on Earth. Continents were raised from beneath the shallow seas of the preceding Carboniferous Period, and deposits that had accumulated in geosynclinal troughs were squeezed together and thrust upward to form mountain ranges: the central and southern Appalachians in North America and the Urals in Russia. Europe and Asia became joined—Siberia with Russia, and China with Siberia—while to the west a collision of continental plates welded North America to the ancestral supercontinent Gondwanaland. In this way, all of Earth’s landmasses collected as one, given the name Pangaea by German geophysicist Alfred Wegener. The southern regions of South America and Africa were clustered near the South Pole together with Antarctica, Australia, and India. North America and westernmost Europe, which straddled the Permian equator, were hot, dry regions, as indicated by thick deposits of evaporite minerals—such as salt and gypsum—that must have precipitated from the waters of enclosed seas..

II

Life During the Permian

The marine life of the beginning of the period was exceptionally rich, flourishing in the warm, shallow inland seas, and included large groups of corals, bryozoans, echinoderms, and other invertebrates.

The dominant animals on land during the Permian were reptiles. The most successful group of reptiles during the period was the synapsids, often called mammal-like reptiles because they included the direct ancestors of mammals and shared some mammal-like features in their skulls and teeth. During the Lower Permian, large sail-backed synapsids such as the meat-eating Dimetrodon and the plant-eating Edaphosaurus were common. More advanced mammal-like reptiles called therapsids appeared later in the period and included meat-eaters with saber-like teeth and squat plant-eaters with powerful beaked jaws. Another group of reptiles called diapsids lived alongside the synapsids and included the earliest forerunners of dinosaurs.

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Large amphibians lived in swamps and rivers. Oxygen levels in the atmosphere may have been higher than today, and insects were abundant and diverse. Land plants included the primitive seed ferns, which were later joined by conifers, cycads, and ginkgoes. See Paleontology.

III

Permian Mass Extinction

At the end of the Permian, 251 million years ago, most of the species of animals in the ocean and on land went extinct. This mass extinction was more extensive than the great extinction at the end of the Cretaceous Period 65 million years ago that killed the dinosaurs. The level of oxygen in Earth’s atmosphere had been about 30 percent at the beginning of the Permian, but fell to about 16 percent at the end, compared to 21 percent today. Oxygen levels in parts of the ocean also dropped dramatically.

Among the animals most affected by the Permian extinctions were the shelled brachiopods (see Lampshell), which had been abundant in the Paleozoic seas but were decimated and never recovered. Although some types of synapsids survived into the following Triassic Period, a group of diapsid reptiles called archosaurs became dominant after the Permian, leading to the dinosaurs.

Scientists debate whether there was a single mass extinction event at the end of the Permian or a series of extinctions, with mass extinctions in the ocean occurring before the mass extinctions on land. The scenarios that led to the mass extinctions are also debated. Huge volcanic eruptions in the region of what is now Siberia are thought to have released large amounts of carbon dioxide into the atmosphere, raising global temperatures through the greenhouse effect.

According to some theories, the environmental consequences of this global warming may have included the release of large amounts of methane hydrates that had been frozen on the ocean floor, depleting oxygen in the oceans and in the air. Warm temperatures may also have altered ocean currents, changing the flow of cold, oxygen-rich waters from the poles. Low oxygen in the oceans may have killed off some of the marine life and led to the proliferation of types of bacteria that deplete oxygen and release poisonous hydrogen sulfide gas, killing more animals in the sea and on land.

Other researchers have proposed alternative causes for the mass extinctions. A vast inland sea that lay over Central Europe dried up, forming salt lakes. The salt lakes may have contained bacteria that released toxic halocarbon gases into the air, changing the composition of the atmosphere and depleting the ozone layer. A few scientists have argued that an asteroid or comet struck the Earth at the end of the Permian Period, similar to the extinction at the end of the Cretaceous Period. However, the evidence for such a Permian impact remains controversial and the impact extinction theory is not widely accepted.

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