By LEE BOWMAN
Scripps Howard News Service
April 14, 2005
Not only did plummeting oxygen levels over a 20 million-year span directly contribute to an event called the "Great Dying," but the changes also made most dry land above sea level uninhabitable for many animals and plants for millions of years.
"Oxygen dropped from its highest level to its lowest level ever (during the time Earth has supported life) in only 20 million years, which is quite rapid, and animals that once were able to cross mountain passes easily suddenly had their movements severely restricted," said Raymond Huey, a University of Washington biology professor and co-author of the report. It is being published Friday in the journal Science.
Scientists calculate that 90 percent of all marine life and three-quarters of all land plants and animals became extinct during the episode of low oxygen, greenhouse conditions and lowered sea levels at the boundary of the Permian and Triassic geological periods.
Huey and co-author Peter Ward, a paleontologist at the university who specializes in extinction events, say that global warming - triggered by massive volcanic activity and a lower sea level - was the biggest contributor to the Great Dying.
Ward and other researchers, in another report published by Science in January, reported that fossil evidence collected in South Africa showed that the die-off was gradual over about 10 million years, then accelerated for another 5 million years. Oxygen levels - and ecological diversity - didn't fully recover for another 100 million years.
Ward doesn't believe an asteroid impact played a part at the time. He says that evidence of the gradual decline is very different from the evidence of the sudden demise of most dinosaurs and many other life forms following an asteroid impact 66 million years ago. Other scientists still believe that an asteroid impact contributed to climate change 250 million years ago.
What Huey and Ward did in the new study was calculate just how unpleasant a world the low oxygen conditions created. During most of the Permian period, all continents were clumped into a single landmass, called Pangea.
Sediment analysis from early in the period shows oxygen saturation of the air at sea level was about 30 percent, lung candy even compared to today's 21 percent. By the end of the Permian period, the air was only about 16 percent oxygen, falling to a low of 12 percent -the ratio now found atop some of the world's highest mountains - about 10 million years into the Triassic.
Ward said researchers have previously assumed that Pangea was not only a supercontinent but also a great highway of migration for most species during the early Permian. Fossils of the same animals from the period had turned up in today's South Africa, China and Russia.
But now it seems that thinner air not only starved many animals and plants into extinction, it forced survivors into low-lying niches where they could breathe more easily. And those survivors not only could live with less oxygen but also adapted to live in new terrain.
"Declining oxygen levels and warming temperatures would have been doubly stressful for late Permian animals," Huey said. "As the body warms, body temperatures and metabolic rates go up. That means oxygen demand is going up, so animals would have faced an increased oxygen demand and a reduced supply."
The researchers calculate that about half of all land on the planet - any spot more than about 1,500 feet above sea level - would have had air too thin to sustain life.
"I think we have to go back and look at oxygen and its role in evolution and how different species developed," Ward said. "You can go without food for a couple of weeks. You can go without water for a few days. But how long can you go without oxygen? A couple of minutes? There's nothing with a greater evolutionary effect."
A few mammal-like animals that lived in burrows and had lungs adapted to low-oxygen conditions survived in the thin air, as did the ancestors of dinosaurs and birds.
"We know that birds can live at much lower oxygen concentrations than we do, and we think there were similar lung adaptations in dinosaurs," Ward said.
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