The great Permian extinction

The story

The “Great Dying”

In the history of life on Earth there have been five great mass extinctions, moments when, in a time that is very short for geology, a large share of the planet's species disappears. The worst of them all happened at the end of the Permian period, about 252 million years ago. Scientists call it the “Great Dying”. It was so devastating that life took millions of years to recover.

According to estimates, between 80 and 90 per cent of the species in the seas vanished. If an aquarium held a hundred species of animals, only ten or twenty would have been left. On land too the losses were very severe: about two thirds or more of the known species of land animals disappeared. It is the most severe mass extinction for insects, with whole orders wiped out. Gone for ever were the trilobites, which had lived in the seas for almost 270 million years, many ancient corals and a great many other forms.

The world before the disaster

The Permian was a different world. The continents were gathered into a single supercontinent, Pangaea, surrounded by a great ocean. The climate was dry and there were large deserts in the interior. The seas held reefs with corals, sponges, molluscs and fish. On land lived large mammal-like reptiles, the synapsids, such as Dimetrodon, with its sail on its back, and the gorgonopsids, predators with sabre-like teeth. There were no dinosaurs yet.

What happened?

The main cause, according to most researchers, was volcanic eruptions of unbelievable size in a region that is now Siberia. For hundreds of thousands of years lava poured out of the ground in enormous flows, covering millions of square kilometres. The rocks it formed are called the “Siberian Traps” (from a Swedish word meaning “steps”). But lava was not the only problem: enormous quantities of gas came out of the volcanoes, above all carbon dioxide, but other substances too. As the magma rose, it also burned or heated deposits of coal and other materials, releasing more gas.

The effect was a chain of disasters. The carbon dioxide warmed the climate very quickly: the temperatures of tropical seas rose so much that they became unbearable for many species, perhaps above 35 degrees. Warmer water holds less oxygen, and in many places the oceans became poor in oxygen or had none. Some of the gas dissolved in water, making the seas more acidic, which makes it hard to build shells and skeletons of limestone. On land, the heat and the acid rain killed many plants, and without plants the food chains collapsed.

How we know

Scientists have rebuilt this story from the rocks. In a quarry near Meishan, in China, there is a layer of rock that marks exactly the boundary between the Permian and the Triassic, and it is used as a reference all over the world. With precise measurements of the age of zircon crystals in volcanic ash, studied in 2011 and 2014, researchers have established that the extinction in the seas was concentrated into a short time, perhaps a few tens of thousands of years. Other evidence comes from changes in the amounts of types of carbon and oxygen in the rocks, which point to warming and to an upheaval in the carbon cycle.

Some scientists propose additional hypotheses, such as the action of methane-producing microbes, and the debate goes on. Almost all of them, however, agree on the role of Siberian volcanism.

Who survived

Not everything died. Among the survivors was Lystrosaurus, an animal the size of a pig, with a beak and two tusks, which after the catastrophe became the most common land vertebrate on the planet. Also surviving were reptiles and small ancestors of mammals, some fish, molluscs such as the nautiloids, the first archosaurs, the group from which crocodiles, pterosaurs and dinosaurs would come. It took 5 to 10 million years in the seas, and up to about 30 million for land animals, for ecosystems to become rich again, and many environments stayed poor and unstable.