Some species of fish that survived worldwide extinctions about 34 million years ago left behind tiny fossils that suggest that cool winters caused the die-off.
Otoliths—literally, ear stones—are calcium carbonate structures in fishes’ ears. These structures grow throughout the fishes’ lives and lay down layers resembling tree rings. Because fish are cold-blooded, the ratio of two oxygen isotopes—oxygen-18 to oxygen-16—trapped in a layer directly reflects the temperature of the water in which the fish lived at that time. The colder the temperature, the higher the ratio.
The researchers, led by Linda C. Ivany, a paleobiologist at Syracuse (N.Y.) University, took microscopic samples from the fossil otoliths of two types of bottom-dwelling eels. Such fish were common in the Gulf of Mexico both before and after the massive die-offs. Although the eels survived the extinctions at the end of the Eocene, about 90 percent of their neighboring mollusk species didn’t.
Each tiny sample of otolith-derived calcium carbonate represents as little as 2 weeks of growth in the eel’s 2-to-3-year life, Ivany notes. This enabled the scientists to measure the ancient, seasonal water temperatures where the eels lived.
The oxygen-isotope ratios in the otoliths didn’t show a significant difference between the summer temperatures before and after the extinctions. Winter temperatures after the die-offs, however, consistently were about 4°Celsius colder than they were earlier, Ivany and her team report in the Oct. 19 Nature.
The larger seasonal swings in temperature probably had a significant effect on organisms in the Gulf of Mexico and elsewhere, Ivany says. She contends the cooler winters may have been the primary cause of the widespread extinctions at the time.
Low-temperature tolerance is often critical, especially for marine organisms such as mollusks, says Thor A. Hansen, a paleontologist at Western Washington University in Bellingham who’s studying the mollusk extinctions in the gulf.
By examining otoliths, researchers can resolve seasonal temperature variations across much shorter periods than geologists typically distinguish, Hansen notes. “This is cool,” he says. “It’s like getting a weather report from 35 million years ago.”