It may never be as well known as the Cretaceous extinction, the one that killed off the dinosaurs. Yet the much earlier Permian extinction – 252 million years ago – was by far the most catastrophic of the planet’s five known paroxysms of species loss.

No wonder it is called the Great Dying: Scientists calculate that about 95 percent of marine species, and an uncountable but probably comparable percentage of land species, went extinct in a geological heartbeat.

The cause or causes of the Permian extinction remain a mystery. Among the hypotheses are a devastating asteroid strike, as in the Cretaceous extinction; a catastrophic volcanic eruption; and a welling-up of oxygen-depleted water from the depths of the oceans.

Now, painstaking analyses of fossils from the period point to a different way to think about the problem. And at the same time, they are providing startling new clues to the behavior of modern marine life and its future.

In two recent papers, scientists from Stanford and the University of California, Santa Cruz, adopted a cellular approach to what they called the “killing mechanism”: not what might have happened to the entire planet, but what happened within the cells of the animals to finish them off.

Their study of nearly 50,000 marine invertebrate fossils in 8,900 collections from the Permian period has allowed them to peer into the inner workings of the ancient creatures, giving them the ability to describe precisely how some died while others lived.

”Before, scientists were all over the map,” said one of the authors, Matthew E. Clapham, an earth scientist at Santa Cruz. “We thought maybe lots of things were going on.”

Clapham and his co-author, Jonathan L. Payne, a Stanford geochemist, concluded that animals with skeletons or shells made of calcium carbonate, or limestone, were more likely to die than those with skeletons of other substances. And animals that had few ways of protecting their internal chemistry were more apt to disappear.

Being widely dispersed across the planet was little protection against extinction, and neither was being numerous. The deaths happened throughout the ocean. Nor was there any correlation between extinction and how a creature moved or what it ate.

Instead, the authors concluded, the animals died from a lack of dissolved oxygen in the water, an excess of carbon dioxide, a reduced ability to make shells from calcium carbonate, altered ocean acidity and higher water temperatures. They also concluded that all these stresses happened rapidly and that each one amplified the effects of the others.

That led to a wholesale change in the ocean’s dominant animals within just 200,000 years, or perhaps much less, Clapham said.

Among the hardest hit were corals; many types, including the horn-shaped bottom-dwellers known as rugose corals, disappeared altogether. Sea sponges were also devastated, along with the shelled creatures that commanded the Permian reefs and sea. Every single species of the once common trilobites, with their helmetlike front shells, vanished for good.

No major group of marine invertebrates or protists, a group of mainly one-celled microorganisms, went unscathed. Instead, gastropods like snails and bivalves like clams and scallops became the dominant creatures after the Permian. And that shift led directly to the assemblage of life in today’s oceans. “Modern marine ecology is shaped by the extinction spasms of the past,” Clapham said.

So what happened 252 million years ago to cause those physiological stresses in marine animals? Additional clues from carbon, calcium and nitrogen isotopes of the period, as well as from organic geochemistry, suggest a “perturbation of the global carbon cycle,” the scientists’ second paper concluded – a huge infusion of carbon into the atmosphere and the ocean.

But neither an asteroid strike nor an upwelling of oxygen-deprived deep-ocean water would explain the selective pattern of death.

Instead, the scientists suspect that the answer lies in the biggest volcanic event of the past 500 million years – the eruptions that formed the Siberian Traps, the stairlike hilly region in northern Russia. The eruptions sent catastrophic amounts of carbon gas into the atmosphere and, ultimately, the oceans; that led to long-term ocean acidification, ocean warming and vast areas of oxygen-poor ocean water.

The surprise to Clapham was how closely the findings from the Great Dying matched today’s trends in ocean chemistry. High concentrations of carbon-based gases in the atmosphere are leading to warming, rapid acidification and low-oxygen dead zones in the oceans.

The idea that changes in ocean chemistry, particularly acidification, could be a factor in a mass extinction is a relatively new idea, said Andrew H. Knoll, a Harvard geologist who wrote a seminal paper in 1996 exploring the consequences of a rapid increase in carbon dioxide in the atmosphere on the physiology of organisms.

”In terms of the overall pattern of change, what we’re seeing now and what is predicted in the next two centuries is riding a parallel track to what we think happened in the past,” he said.

Clapham noted that Permian and modern similarities are not exact. The Permian ocean was easier to acidify than today’s ocean because it had less deep-water calcium carbonate, which offsets the acid. But he said that corals are the most vulnerable creatures in the modern ocean for the same reason they were during the Permian extinction. They have little ability to govern their internal chemistry and they rely on calcium carbonate to build their reefs.

Chris Langdon, a University of Miami biologist who is a pioneer in ocean acidification research, said corals are undoubtedly in danger across the globe.

”Corals, I think, are going to take it on the chin,” he said.

In a recent study, Langdon examined the effects of naturally high acidification on coral reefs in Papua New Guinea. They showed drastic declines in coral cover at acidity levels likely to be present in the ocean by the end of this century, especially among branching corals that shelter fish.

Hans Portner, an animal ecophysiologist at the Alfred Wegener Institute in Bremerhaven, Germany, said his work showed that a warmer ocean with less dissolved oxygen and greater acidity had an array of negative physiological effects on modern marine animals.

The Permian extinction provides an archive of effects suggesting how modern marine creatures will fare as the carbon load in the atmosphere incr eases, he said.

Like Clapham, he cautioned that the trends between the two periods were not exactly comparable. Back in the Permian, the planet had a single supercontinent, Pangea, and ocean currents were different.

And he and Langdon noted that carbon was being injected into the atmosphere today far faster than during the Permian extinction. As Knoll put it, “Today, humans turn out to be every bit as good as volcanoes at putting carbon dioxide into the atmosphere.”

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Alanna Mitchell, The New York Times, 1 May 2012. Full article.