Fossil Fish Reveal the Season of the Dinosaur-Killing Asteroid Impact

Fossil fish caught in impact-driven water surges suggest the asteroid that ended the dinosaurs struck Earth during Northern Hemisphere spring.

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About 66 million years ago, the asteroid impact linked to the end-Cretaceous mass extinction sent enormous disturbances through coastal waterways far from the impact site. At Tanis in what is now North Dakota, fish and other remains were rapidly buried in sediments moved by an impact-driven seiche, preserving an unusually detailed snapshot of that day. Researchers examined growth patterns in the bones of sturgeons and paddlefishes, along with carbon isotope changes that record seasonal feeding. The fish were still in an earlier part of their yearly growth and feeding cycle when they died, pointing to spring in the Northern Hemisphere. That corresponds to autumn in the Southern Hemisphere. The result gives scientists a rare seasonal timestamp for an event roughly 66 million years old. The researchers also proposed that seasonal differences in reproduction, feeding, dormancy, and other life cycles may have affected which organisms were more vulnerable to the extinction. That broader survival effect remains an interpretation rather than something the fossil fish can prove by themselves. The Tanis deposit is especially valuable because the water disturbance arrived within tens of minutes of the impact and trapped organisms and impact debris together, allowing researchers to connect the fossil evidence to the same catastrophic event.

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