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 databut 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.
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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