Permian-Triassic Extinction: Earth's Worst Mass Extinction

The Permian–Triassic extinction, often called the Great Dying, was the most severe mass extinction in the history of animal life. About 251.9 million years ago it wiped out an estimated 80 to 90 percent or more of species in the oceans and a large share of animals and plants on land. It marks the end of the Permian Period and of the Paleozoic Era, and it set the stage for the age of reptiles and, eventually, the dinosaurs.

Background: Earth at the end of the Permian

At the end of the Permian, almost all of Earth’s land was joined in a single supercontinent, Pangaea, surrounded by a vast ocean called Panthalassa. A smaller ocean, the Paleo-Tethys and later Tethys, cut into its eastern side. The interior of Pangaea was dry and seasonal, while coastal areas and high latitudes were wetter and supported forests.

Life on land was dominated by synapsids, the group that later gave rise to mammals. Large plant-eaters such as dicynodonts and predators such as the sabre-toothed gorgonopsians roamed the landscape. Early reptiles, amphibians and many insects lived alongside them, and seed plants such as Glossopteris formed extensive forests in the southern continents.

The seas were rich in life that looks unfamiliar today. Rugose and tabulate corals built reefs, brachiopods covered the sea floor, and crinoids (sea lilies) formed underwater meadows. Trilobites, which had existed for some 270 million years, still survived in small numbers.

Life had already been shaken earlier in the Permian. An extinction in the middle of the period, usually called the end-Guadalupian or Capitanian extinction, struck about 260 to 259 million years ago. Many researchers link it to the Emeishan Traps, a smaller volcanic province in south-west China. That earlier crisis was far less severe, but it shows that the late Permian world was already under stress.

What happened

The extinction is recorded in rock layers around the world. The best-studied site is at Meishan in Zhejiang Province, China, which in 2001 was ratified as the global reference point (the Global Stratotype Section and Point) for the base of the Triassic. There, the boundary is defined by the first appearance of a conodont, a tiny tooth-like fossil, called Hindeodus parvus.

High-precision uranium-lead dating of volcanic ash beds at Meishan, published by Seth Burgess, Samuel Bowring and Shu-zhong Shen in 2014, placed the main extinction pulse at about 251.94 million years ago and estimated that it lasted around 60,000 years, with an uncertainty of roughly 48,000 years. In geological terms this is close to instantaneous.

In the oceans, losses were catastrophic. The last trilobites disappeared, as did rugose and tabulate corals and many groups of brachiopods, ammonoids, bryozoans and crinoids. Reef ecosystems vanished for millions of years. Animals with heavy calcium carbonate skeletons and limited ability to control their internal chemistry suffered especially badly.

On land, many synapsid groups, including the gorgonopsians, died out, and the Glossopteris forests collapsed. Fossil soils and plant records show widespread loss of vegetation and increased erosion. The event is also the only known mass extinction of insects, with several orders lost.

Causes

The Siberian Traps

The leading explanation is the eruption of the Siberian Traps, an enormous volcanic province in what is now northern Russia. Dating shows that a major phase of magmatism coincided with the extinction. Much of the magma intruded into thick layers of coal, carbonate rock and salt, baking them and releasing huge amounts of carbon dioxide, methane and halogen gases.

A sharp negative shift in carbon isotopes, found in boundary rocks worldwide, shows that a large mass of carbon depleted in carbon-13 entered the oceans and atmosphere. Spikes of mercury in sediments of the same age, from China to Canada and the Arctic, are another fingerprint of volcanic activity and heated coal.

Global warming

The released carbon drove rapid warming. Oxygen isotope measurements of conodonts from South China suggest that tropical sea-surface temperatures rose by roughly 8 to 10 °C, possibly approaching 40 °C at the surface in the Early Triassic. Such temperatures are lethal to many marine animals and plants.

Loss of oxygen and ocean acidification

Warm water holds less oxygen, while marine animals need more oxygen as they warm up. A 2018 modelling study by Justin Penn and colleagues argued that this combination, known as temperature-dependent hypoxia, could explain much of the pattern of marine extinction, including heavier losses at higher latitudes. Rocks from this time show that large parts of the oceans became anoxic, and some became euxinic, meaning the water contained toxic hydrogen sulphide.

Carbon dioxide dissolving in seawater also made the oceans more acidic. Boron isotope evidence published in 2015 points to a pulse of acidification during the extinction, which would have made it harder for corals, shellfish and other calcifying organisms to build skeletons.

Other proposed factors

Other mechanisms have been suggested, including ozone damage from volcanic halogens, acid rain and the release of methane from sea-floor deposits. Researchers continue to debate their relative importance. An asteroid impact has also been proposed, but no convincing impact crater or global impact layer of the right age has been confirmed, and the impact idea is not widely accepted.

Key people

The extinction was first recognised as a turning point in the history of life in the nineteenth century. In 1841 the Scottish geologist Roderick Murchison named the Permian Period after the region of Perm in Russia, where he had studied its rocks. Around the same time the English geologist John Phillips divided the history of life into the Paleozoic, Mesozoic and Cenozoic eras, partly on the basis of the sharp change in fossils at the end of the Permian.

From the late twentieth century, palaeontologists including Douglas Erwin of the Smithsonian Institution and Michael Benton of the University of Bristol wrote influential studies and popular books on the event. Geochronologists such as Samuel Bowring and Seth Burgess, with Chinese colleagues led by Shu-zhong Shen, produced the precise dates that tied the extinction to the Siberian Traps.

Recovery

The earliest Triassic world was strikingly impoverished. On land, the stocky, tusked plant-eater Lystrosaurus became so widespread that its fossils make up a large share of finds from some Early Triassic rocks in South Africa, Antarctica, India, China and Russia. Such “disaster taxa” thrive when competitors have vanished.

There is a gap in coal deposits in the Early Triassic, reflecting the loss of peat-forming forests, and a “reef gap” in the oceans. Instability continued, with further warming pulses and smaller extinctions in the Early Triassic. Most researchers estimate that complex ecosystems took roughly 5 to 10 million years to rebuild, reaching a fuller recovery in the Middle Triassic, from about 247 million years ago.

The recovery created new opportunities. Archosaurs, the group that includes crocodiles, pterosaurs and dinosaurs, diversified rapidly, and the first dinosaurs appeared in the Middle to Late Triassic. In the seas, ichthyosaurs and other marine reptiles evolved, and modern-style corals (scleractinians) appeared. Another extinction about 50 million years later, the Triassic–Jurassic extinction, would open the way for dinosaurs to dominate.

By the numbers

Myths and facts

A common misconception is that the Great Dying killed off the dinosaurs. In fact dinosaurs did not yet exist; the extinction that ended their reign was the Chicxulub asteroid impact about 66 million years ago, some 186 million years later.

Another popular claim is that exactly 96 percent of species died. That figure came from a 1979 statistical estimate by David Raup and is still often repeated, but modern studies, which use larger fossil databases, usually give somewhat lower numbers for marine species, around 80 to 90 percent. Either way, it remains the largest known extinction.

Timeline

• c. 252.2 million years ago: Large flood basalt eruptions are under way in Siberia.
• c. 251.9 million years ago: The main extinction pulse begins, coinciding with widespread intrusion of Siberian sills into carbon-rich sediments.
• c. 251.9 million years ago: The Permian Period ends and the Triassic begins, marking the end of the Paleozoic Era.
• c. 251.9–247 million years ago: The Early Triassic, a period of extreme heat, unstable carbon cycles and slow recovery.
• c. 247 million years ago: The Middle Triassic begins, with ecosystems increasingly recovered.
• 1841: Roderick Murchison names the Permian Period after Perm, Russia.
• 2001: The Meishan section in China is ratified as the global reference point for the base of the Triassic.
• 2014: Burgess, Bowring and Shen publish a high-precision timeline of the extinction.

Why it matters

The Great Dying is the closest life on Earth has come to being wiped out since complex animals evolved. It reshaped the course of evolution, ending the dominance of Paleozoic marine life and early synapsids and making room for the reptiles that ruled the Mesozoic. Because it was driven largely by a rapid release of carbon, warming and ocean oxygen loss, it is also one of the most closely studied past events for understanding how the Earth system responds to large disturbances of the carbon cycle.