FIRST there was the Permian era; then came the Triassic period. Now palaeontologists have discovered an intriguing interlude, the porcine age, when pig-like creatures ruled the earth.

The animals, known as lystrosaurs, were among the only survivors of the greatest mass extinction event the world has seen, when, around 251m years ago, 95% of all living species were wiped out by a series of volcanic eruptions.

The eruptions eliminated every large predator, so for a million years or more the lystrosaurs had the planet — and all its succulent plant life — almost entirely to themselves.

“They fed and spread. We think there were billions of them,” said Paul Wignall, professor of palaeo-environment at Leeds University. “Their fossils are everywhere.”

The lystrosaurs evolved towards the end of the Permian period, around 260m years ago, long before the time of modern mammals and reptiles and even of the dinosaurs. They belonged to a group called the cynodonts. Reconstructions from fossils suggest they were similar in size and stature to modern pigs, complete with snouts and small tusks for rooting around in vegetation.

This was a period in Earth’s history when complex life had established itself on land and sea, with new species evolving rapidly into every available niche. Early lystrosaurs, with their sprawling gait, ungainly bodies and bulbous heads, would have been easy meat for the abundant predators and so seem to have evolved as underground beasts. Fossils show they dug burrows, which in turn implies they may have been nocturnal and have had the ability to hibernate.

In that period all the continents were joined together in one huge land mass now called Pangaea. Geological records suggest the world was then much warmer, with forests covering Australia, then lying over the south pole, and no icecaps.

About 251m years ago everything changed. “A massive volcanic eruption began in the northern part of Pangaea in what is now Siberia,” said Wignall, speaking at a conference last week. “Over thousands of years about 5,000,000km3 of basalt erupted onto the Earth’s surface and billions of tons of CO2 poured into the air.”

The initial result of each eruption was rapid cooling as the volcanic haze blotted out the sun. Then, over the following decades, the world warmed sharply because of the CO2. The eruptions came in pulses over thousands of years, so this pattern of warming and cooling was repeated several times.

“We can only speculate on how lystrosaurus survived while the rest died, but perhaps its ability to burrow and hibernate protected it from the worst periods,” Wignall said.

A number of species did survive, including other cynodonts. These would later give rise to the mammals and eventually humans. Back then, however, they were just tiny shrew-like creatures. A second group of survivors were diapsids, the group that would later give rise to the dinosaurs, reptiles and birds. They, too, were tiny.

Wignall said: “The remarkable thing about the lystrosaurs was their size. Nothing else that big seems to have got through the destruction, and that is why they were able to dominate the earth for so long afterwards.”

One mystery is why the lystrosaurs did eventually disappear. But Wignall believes their fate and that of other Permian species holds clear lessons for modern humans: “The amounts of CO2 we are emitting are roughly equivalent to those poured into the atmosphere during the Permian eruptions. Our climate is changing like theirs did.”

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