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Fossil records show biodiversity comes and goes

n taxonomy, because genera classifications are more manageable and less often revised than species classifications; and includes only marine fossils because the records are longer and better preserved than records of land fossils.

For their study, Muller and Rohde defined fossil diversity as the number of distinct genera alive at any given time. This yielded a total of 36,380 genera, whose history the Berkeley scientists tracked over time, using the International Commission on Stratigraphy's 2004 time scale.

"Putting the timescale in a chronologic format was critical to our findings, because there were no specific years assigned to the geologic timescale used by Sepkoski," Muller said. "We are the first to reconstruct diversity from the final version of Sepkoski's Compendium, and the first to use the 2004 geochronology time scale. In a sense, our work has verified the new time scale."

Muller and Rohde have been working on this study for nearly two years, and first discovered the 62 million year biodiversity cycle in November, 2003. They spent the next year trying to either knock it down or explain it. Despite examining 14 possible geophysical and astronomical causes of the cycles, no clear explanation emerged. Muller and Rohde each has his own favorite guess.

Muller suspects there is an astrophysical driving mechanism behind the 62 million year periodicity.

"Comets could be perturbed from the Oort cloud by the periodic passage of the solar system through molecular clouds, Galactic arms, or some other structure with strong gravitational influence," Muller said. "But there is no evidence even suggesting that such a structure exists."

Rohde prefers a geophysical driver, possibly massive volcanic eruptions triggered by the rise of plumes to the earth's surface. Plumes are upwellings of hot material from near the earth's core that some scientists believe have the potential to reoccur on a periodic basis.


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Contact: Lynn Yarris
lcyarris@lbl.gov
510-486-5375
DOE/Lawrence Berkeley National Laboratory
14-Mar-2005


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