Nearly 34 million years ago, shellfish the world over perished in the largest mass extinction since the one that claimed the dinosaurs. Along the U.S. Gulf Coast, for example, almost 90 percent of the mollusks were wiped out--to be replaced later by new species of their ilk. The cause of this fatal event, however, has remained somewhat of a mystery. Until now. The results of a study published this week in the journal Nature suggest that a colder climate was the culprit.

Researchers had suspected that temperature changes might have played a role in the extinction that took place at what geologists call the Eocene/Oligocene boundary, but they didn't have any proof. "Mean annual temperature records for the Gulf Coast that exist for that time period don't show any change," notes co-author Linda C. Ivany of Syracuse University, "but they don't tell you about seasonal variations in temperature."

In order to investigate those variations, Ivany and her colleagues turned their attention to fossilized ear stones, known as otoliths, from fish that survived the mass extinction. These tiny calcium carbonate bodies form part of the hearing and balance system in most bony fish. New material accumulates on an otolith in a series of growth rings, accruing relatively more of a particular form of oxygen when the water is cooler. Scientists can thus read the rings to determine the seasonal temperature conditions that existed during the fish's lifetime. In so doing, the team found that whereas summer temperatures remained the same, winter temperatures dropped by as much as four degrees Celsius.

"This is the first time anyone has looked at seasonality as a variable for an extinction event across a geological time boundary," says team member William Patterson. "Existing records weren't able to resolve the change because the records are based on summertime growth. The fish survived the drop in winter temperatures and left a permanent record, while the mollusks didn't make it." Ivany has plans to investigate the seasonal temperature variations that existed in the North Sea and on Antarctica's Seymour Island during that same period.