Toxic Deep-Ocean Water Triggered "Great Dying"

Helen Scales
for National Geographic News
November 26, 2007

The finger of blame for the greatest mass die-off in Earth history points to a slow, drawn-out demise that came from below, a new study shows.

Researchers discovered that bryozoans—a common type of colonial marine creature also known as "moss animals"—began slowly declining in oceans across the world many millions of years before the mass-extinction event at the end of the Permian period about 251 million years ago.

(Related news: "Mystery Undersea Extinction Cycle Discovered" [March 9, 2005].)

This so-called Great Dying event wiped out about 90 percent of marine species and 70 percent of land species.

"It was a very gradual process," said Catherine Powers, a paleobiologist from the University of Southern California who co-authored a study on the new finding.

The first to go were the deeper-dwelling animals, followed by the shallow-water inhabitants.

"It indicates that something from the deep ocean is coming up," Powers said.

"Whatever killed these organisms and probably led to this mass extinction is tied to ocean circulation processes."

Toxic Upwellings

Bryozoans form an excellent "environmental dipstick" for studying the effects of mass extinctions, Powers said.

"They live in all sorts of marine environments, from tropical reefs to muddy shallow waters [to] the deep sea," she said.

"And they respond to environmental factors, with different growth forms dominating under different conditions."

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