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Mystery Undersea Extinction Cycle Discovered

John Roach
for National Geographic News
March 9, 2005
 
Robert Rohde and Richard Muller are vexed. For the past 542 million years the number of animal species living in the world's oceans has risen and fallen in a repeating pattern, and the scientists haven't the foggiest idea why.

"I wish I knew what it all meant," said Muller, who is a professor of physics at the University of California, Berkeley.

The pattern includes a rise and fall of marine animal diversity every 62 million years and a weaker cycle of rising and falling marine diversity, which repeats every 140 million years. The researchers think that expanding and retreating glaciers may explain the 140-million-year cycle, but they are stumped over what drives the 62-million-year cycle.

The declines in the 62-million-year cycle correspond with some of the best known mass extinctions on Earth.

Among them are the die-off caused by the asteroid or comet widely believed to have doomed the dinosaurs 65 million years ago and the "Great Dying" of 250 million years ago. During the Great Dying, some unknown cause wiped out most life on Earth.

The patterns found in the new study identify five additional declines in marine-animal diversity. Muller and his graduate student Rohde say the pattern is too regular to occur by chance. But they have failed to find a plausible explanation for its existence.

The pair looked for a pattern of asteroid and comet impacts, global climate shifts, volcanic eruptions, fluctuating sea levels, changes in the total amount of plant and animal life, and the reshuffling of the continents. None fit.

Muller went so far as to purchase a lava lamp, plug it in, and studiously gaze at how often the blobs rose to the surface. He thought perhaps magma bubbles from the Earth's core rise in a cyclical pattern. "It's not regular enough," he said.

The researchers' report will appear in tomorrow's issue of the science journal Nature.

Computer Analysis

Rohde and Muller started with Sepkoski's compendium, a database of every marine-animal fossil ever found. Then they correlated this database with the dates of when the species appeared and disappeared from the fossil record. The dates were gleaned from the International Commission on Stratigraphy's 2004 timescale.

"We took all the best data and put it with the best timescale. It's not what we were looking for, but [Rohde] plotted it up and there it was. We've been wrestling with it ever since," Muller said.

Wolfgang Kiessling is a paleontologist with the Museum of Natural History at Humboldt University in Berlin, Germany. He said Rohde and Muller's statistical analysis is robust and that the 62-million-year cycle is distinct in the data—but its finding comes as a surprise.

"Looking at my own analysis of Sepkoski's compendium, I do see a similar pattern, although I have resolved the data more coarsely," he said. "Perhaps the periodicity did not strike my eye because so many previous analyses on regularities in extinctions and diversity have proved erroneous in the past."

John Alroy coordinates the Paleobiology Database at the National Center for Ecological Analysis and Synthesis in Santa Barbara, California. He is skeptical about the accuracy of Rohde and Muller's statistical analysis. He does, however, applaud their exhaustive search for the cycle's cause.

"Nice of them to argue in detail that there is no known mechanism, out of many considered that could possibly explain this pattern," he said.

Muller invites the scientific community at large to help find an answer and is working to make publicly available their computerized database of Sepkoski's compendium correlated with the International Commission on Stratigraphy's 2004 timescale.

Head Scratching

Kiessling said the pattern most likely stems from fluctuations in the fossil record caused by fluctuations in sea level. The number and abundance of fossils preserved on the seafloor is less when sea levels are low, he explained.

Rohde and Muller looked at sea level as a possible explanation. They said they found no 62-million-year cycle in the best obtainable records of sea level change.

Kiessling said their sampling is insufficient to rule out sea-level fluctuations as a cause of the cycle.

"However, what remains to be analyzed are the reasons for these large-scale cycles in the quality of the fossil record. So the results of Rohde and Muller remain interesting and provoking," Kiessling said.

Alroy, the paleobiology-database coordinator, said two unrelated things drive the observed pattern: mass extinctions, such as the Great Dying, and low points in the fossil record, such as the early Cretaceous (144 to 127 million years ago).

He believes that this factor rules out the possibility that a single mechanism alone is responsible for the pattern. Alroy added that the observed cyclical patterns could simply be the result of an error in the statistical analysis.

Rohde and Muller stand by the statistical analysis, but remain vexed over the driving force. "We have a bet," Muller said. "I'm betting it will be astronomy, and he's betting it will be something inside of the Earth."

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