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.
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