What Was the Permian Extinction? The Day Life Nearly Ended

Palaeontologists estimate that about 81 percent of marine species and around 70 percent of land vertebrate species died out, along with most insects, the only mass extinction known to have hit insects hard. The trilobites, which had lived in the seas for 270 million years, vanished completely. Reef systems collapsed and did not rebuild for millions of years, and there is a stretch of rock in many parts of the world containing almost no fossils at all, a coal gap where no coal formed because there were too few plants to make it.

The victims were not obscure. The Permian world had been full of large, strange animals: by its end, the earlier sail-backed synapsids such as Dimetrodon had given way to therapsids, the group on the way to becoming mammals. Most of those lineages ended. What came through was a thin sample of what had existed before.

The prime suspect lies in Siberia. The Siberian Traps are the remains of one of the largest volcanic events in Earth's history: not a mountain erupting but a region the size of western Europe pouring out lava through fissures for something like a million years, leaving basalt in places several kilometres thick. Precise dating of the lava and of the extinction layer in rocks elsewhere shows the two coincide.

Lava alone would not have done it. The magma rose through thick beds of coal, oil and limestone, cooking them and releasing carbon dioxide and methane on a scale no ordinary eruption produces, along with sulphur and halogens that thinned the ozone layer. Estimates of the carbon released run into the tens of thousands of billions of tonnes, arriving over a few thousand years at a time.

The carbon warmed the planet by something like 10 degrees Celsius, and the warming did most of the damage indirectly. Warm water holds less oxygen, so the oceans began to suffocate. Dissolved carbon dioxide made seawater more acidic, which dissolved the shells of the animals that built them. As the deep ocean lost oxygen, bacteria that breathe sulphate instead thrived and pumped out hydrogen sulphide, which is poisonous to almost everything and may have leaked into the air. Sea surface temperatures near the equator may have reached 40 degrees, hotter than most animals can survive.

The evidence for each step is written in the chemistry of the rocks: a sharp shift in carbon isotopes marking the release, a spike in mercury from the volcanism, the disappearance of oxygen-loving species before oxygen-tolerant ones, and shells that grew thinner before they stopped appearing altogether.

Life did not bounce back. The early Triassic that followed stayed hot and unstable for several million years, and it took perhaps ten million years for ecosystems to regain their former complexity, the longest recovery after any extinction. For a while a single pig-sized plant-eating animal, Lystrosaurus, made up the great majority of land vertebrate fossils across the entire planet, a sign of how empty the world had become.

The emptiness also created the opening for what came next. With the synapsids reduced to a remnant, a different group of reptiles, the archosaurs, spread into the vacant space, and within the Triassic they had produced crocodiles, pterosaurs and the first dinosaurs. The age of dinosaurs began in the aftermath of the Great Dying, and the surviving synapsids, small and mostly nocturnal, were the ancestors of mammals.

The Permian extinction is the best natural experiment on record for what a large, rapid injection of carbon does to a planet, which is why climate scientists read the literature as closely as palaeontologists do. The Siberian Traps released carbon over thousands of years; human activity is currently releasing it at a rate estimated to be at least ten times faster, although the total so far is a small fraction of what the eruptions eventually delivered. The sequence of consequences, warming, ocean deoxygenation and acidification, is the same one being measured today at a much smaller scale. The chain of events, in order:

• •Flood basalt eruptions burn through coal and carbonate rock
• •Carbon dioxide and methane warm the planet by around 10 degrees
• •Oceans lose oxygen and turn acidic, reefs and shelled animals die
• •Hydrogen sulphide and ozone loss poison land and sea
• •Around 80 percent of species gone, recovery takes millions of years

Saying the extinction happened in a geological instant is only meaningful if the rocks can be dated precisely, and much of that work has been done at Meishan in southern China. Its limestone layers record the end of the Permian with unusual clarity, and it was chosen as the official reference section for the boundary between the Permian and Triassic. Geologists define that boundary at the first appearance of a particular conodont, a tiny eel-like animal whose tooth-like fossils are abundant and change quickly through time, so its arrival marks a recognisable moment anywhere the rocks are found.

The numbers come from thin beds of volcanic ash interleaved with the limestone. Ash contains small crystals of zircon, which lock in uranium when they form but exclude lead. Uranium decays to lead at a known, steady rate, so measuring the ratio in a crystal gives the age of the eruption that produced it. Refinements of this method have narrowed the main extinction pulse to a span of tens of thousands of years at most. Applying the same technique to the Siberian lavas is what allowed researchers to show that the eruptions and the extinction overlap rather than merely falling in the same rough era.

Fossilisation is rare, so the youngest fossil of a species almost never comes from the last individual that lived. A common species may leave fossils in nearly every layer, while a rare one might appear only every few metres of rock, and its final appearance can fall well below the true moment of its extinction. Palaeontologists call this the Signor-Lipps effect, after the two researchers who described it in 1982. Its consequence is that a sudden mass extinction tends to look gradual in the rocks, with rarer species seeming to disappear first and the common ones lasting right up to the boundary.

Researchers counter it by collecting intensively and by using statistics. If a species turns up regularly through a stretch of rock, the gaps between its fossils give a measure of how far beyond its last find it probably survived, and confidence limits can be placed on its true range. Applied to dense collections from boundary sections, this kind of analysis supports a sharp extinction rather than a long decline. The same patchiness explains Lazarus taxa, groups that vanish from the record and reappear millions of years later. They are unusually common in the early Triassic, because survivors were scarce and few of them were fossilised.

The survivors of the Great Dying were not a random sample. In the sea, the animals hit hardest tended to be those that built heavy skeletons of calcium carbonate and had sluggish metabolisms with little control over their internal chemistry: reef corals, the lamp shells called brachiopods, sea lilies and other animals fixed to the sea floor. Groups with more active physiology, better able to cope with low oxygen, high carbon dioxide and acidic water, fared better. That is one reason the Palaeozoic sea floor, long dominated by brachiopods, gave way to one dominated by clams, snails and other molluscs, a shift still visible in the shells on any beach.

On land the survivors were mostly small. Palaeontologists call the pattern the Lilliput effect: after the extinction, animals in many groups were on average smaller than their ancestors, and the big plant-eaters and predators of the late Permian were gone. Small size meant needing less food and breeding quickly, so numbers could recover fast. Burrowing helped too. Fossils of the small cynodont Thrinaxodon, a relative of the ancestors of mammals, have been found curled up inside burrows in early Triassic rocks of South Africa, where a hole in the ground would have offered shelter from heat and drought at the surface.

The end-Permian extinction, 252 million years ago, killed most species on Earth after a million years of Siberian eruptions burned through carbon-rich rock and cooked the planet. It cleared the way for dinosaurs and mammals, and it remains the clearest warning in the geological record of what a fast carbon release can do.