Just a few years ago, the theory behind this gloomy history of mollusks was unproven. Now, thanks to Linda Ivany, Kyger Lohmann, and William Patterson, it's fact. The three scientists used nature's record to learn about the behavior of the climate long ago.
Tree rings are nature's way of documenting climate changes. The team simply applied that same principle to shellfish.
"We proved that winter temperatures caused the extinction. Existing records weren't able to resolve the changes 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 to the other side of the boundary," said Patterson, assistant professor of earth sciences at Syracuse University.
During a period known as the Eocene/Oligocene boundary, 90 percent of tiny, shell-bearing sea creatures were wiped out around the globe. In the largest mass extinction since the dinosaurs disappeared, these mollusks were replaced almost overnight by an entirely new species.
The incident was a mystery until Ivany, Lohmann, and Patterson figured out a way to unravel it.
The team analyzed the chemical composition of fish fossils. The 'earstones,' or otoliths inside the fish held evidence of temperature changes in their watery home. By putting their heads together, the scientists were able to analyze microsamples of the fossils, which were a fraction of a size of a grain of salt.
"We found that, while the summer temperatures remained the same, winter temperatures dropped 4 degrees Celsius," said Ivany, a visiting assistant professor of earth sciences at Syracuse University. "Paleontologists had always suspected that temperature changes had something to do with the extinction, but we couldn't prove it. Mean annual temperature records for the Gulf Coast that exist for that time period don't show any change. But they don't tell you about seasonal variations in temperature."
What lies ahead for the team is the task of strengthening their proof. Ivany plans to continue her work by sharing the team's techniques with Belgian scientists and her colleagues in Antarctica. The goal is to better understand extinction and evolution.
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