On a spring day 66 million years ago, as flowers bloomed and baby birds hatched in what is now North Dakota, a ball of fire streaked across the sky and wiped out nearly three-quarters of life on Earth. So says a new high-resolution study of fossilized fish bones, which pinpoints the season of the Cretaceous-Paleogene mass extinction to the Northern Hemisphere spring.
“It’s amazing that this is an event that happened 66 million years ago, and we’re talking about the season,” says Kenneth Lacovara, a paleontologist at Rowan University who wasn’t involved with the paper. “It’s a remarkable degree of resolution.”
When it struck Mexico’s Yucatán Peninsula, the 10-kilometer asteroid tossed debris into the atmosphere and literally shook Earth. Gases and rock filled the air, kicking off a nuclear winter and leading to the extinction of 75% of Earth’s species, including all nonavian dinosaurs.
Some creatures died on the day the asteroid struck. At a site called Tanis in present-day North Dakota (more than 3500 kilometers away from where the asteroid hit), a tsunamilike wave called a seiche sloshed out of a river and swept up all life in its path, depositing sediment, trees, and animal carcasses in a jumbled heap. Now, paleontologists are analyzing the pile of ancient bones frozen among Cretaceous tree trunks and stacks of rock. “[The site] looked incredibly violent, like a car crash,” says Melanie During, a paleontologist at Uppsala University and lead author of the new paper, who visited Tanis in 2017.
During wasn’t part of the discovery of Tanis, which generated controversy after an article in The New Yorker revealed details about the find that were not included in the first research paper, released several days later in the Proceedings of the National Academy of Sciences (PNAS). Since then, the team that discovered Tanis has released more data in subsequent papers and at scientific meetings. “It’s a really remarkable site that captures this short window of time immediately after impact,” says James Witts, a paleontologist at the University of Bristol who has visited Tanis.
The PNAS paper did describe tiny glass particles trapped in the gills of fossilized fishes. These spherules formed from rocks that were flung into the air by the impact, crystallized, and rained down 15 to 30 minutes after the asteroid hit. Their presence in the fishes’ gills suggests the animals died soon after the catastrophe, Lacovara says.
During was “flabbergasted” when she learned about the fishes in 2017. She reached out to the researchers who discovered the site, and they invited her to North Dakota to excavate six acipenseriform fish, filter feeders closely related to modern sturgeons. She brought six of the fossils back to Europe, where she and her colleagues used powerful x-rays from a synchrotron to image the fish bones. They scrutinized thin layers of bone cells in the fish’s fins, whose thickness marks the arrival of different seasons. Layers thickened each spring, grew robustly in summer, then tapered off in fall and winter, During explains.
The researchers also measured the isotopic carbon incorporated into the fish’s bones. In warmer months, the fish ate photosynthesizing zooplankton, which are rich in carbon-13. Spring layers of the fin bones were more enriched with the isotope, During explains. Both growth patterns and isotope data suggest all six fish perished in the spring, the researchers report today in Nature.
That finding fits with previous research published by members of the Tanis discovery team. Growth layers and carbon-oxygen isotope ratios in other fish fossils suggested the fish died in “late spring or summer,” they wrote in Scientific Reports in December 2021. During says her team’s synchrotron scanning shows the growth layers in more detail—down to the cellular level—letting scientists pinpoint the season more precisely. The new paper presents “good evidence” that the fish died at the start of their growing season, soon after the asteroid hit, Lacovara says.
Kay Behrensmeyer, a vertebrate paleontologist at the Smithsonian Institution’s National Museum of Natural History, agrees. “They’ve set a really good standard” for dating and analyzing fossils from Tanis, she says. She notes that this level of analysis requires rapid, catastrophic burial, which is relatively rare in the fossil record. “There aren’t a lot of sites quite like this,” Witts adds.
Although there’s never a good time for an asteroid to hit Earth, During and her team posit that a springtime impact would have been the worst-case scenario for animals in the Northern Hemisphere. They would have been spending their time outside and reproducing, whereas animals preparing to hibernate in the Southern Hemisphere, then in its fall, might have had a better shot at surviving.
“Of course the extinction is more than just the impact, and nobody knows how long the nuclear winter that followed lasted,” During says. “But if you didn’t survive the first blow, then you weren’t around to fight the cold.”