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Low O2 Levels Behind Some Extinctions, Spurred Bird Breathing System
Oct. 31, 2003
Ultra-low oxygen could have triggered mass extinctions, spurred bird
Recent evidence suggests that oxygen levels were suppressed worldwide 175
million to 275 million years ago and fell to precipitously low levels
compared with today's atmosphere, low enough to make breathing the air at
sea level feel like respiration at high altitude.
Now, a University of Washington paleontologist theorizes that low oxygen
and repeated short but substantial temperature increases because of
greenhouse warming sparked two major mass-extinction events, one of which
eradicated 90 percent of all species on Earth.
In addition, Peter Ward, a UW professor of biology and Earth and space
sciences, believes the conditions spurred the development of an unusual
breathing system in some dinosaurs, a group called Saurischian dinosaurs
that includes the gigantic brontosaurus. Rather than having a diaphragm to
force air in and out of lungs, the Saurischians had lungs attached to a
series of thin-walled air sacs that appear to have functioned something
like bellows to move air through the body.
Ward, working with UW biologist Raymond Huey and UW radiologist Kevin
Conley, believes that breathing system, still found in today's birds, made
the Saurischian dinosaurs better equipped than mammals to survive the
harsh conditions in which oxygen content of air at the Earth's surface was
only about half of today's 21 percent.
"The literature always said that the reason birds had sacs was so they
could breathe when they fly. But I don't know of any brontosaurus that
could fly," Ward said. "However, when we considered that birds fly at
altitudes where oxygen is significantly lower, we finally put it all
together with the fact that the oxygen level at the surface was only 10
percent to 11 percent at the time the dinosaurs evolved.
The Permian-Triassic extinction is believed to have eradicated 90 percent
of all species, including most protomammals, a group of mammal-like
reptiles that were the immediate ancestors of true mammals. The
Triassic-Jurassic extinction killed more than half the species on Earth,
with mammal-like reptiles and true mammals, which evolved during the
Triassic Period, hit particularly hard. But dinosaurs, which also evolved
between the two extinctions, had little problem with conditions during the
"The seminal observation is that dinosaurs skated across the second of
these mass extinctions, actually increasing in number as they went along,
while everything else was dropping around them," Ward said.