Feb. 23 (UPI) -- The asteroid that struck the Earth during the dinosaur age likely hit during the springtime, findings published in the journal Nature on Wednesday suggest.
By studying the remains of fossilized fish from North Dakota's geological site Tanis, researchers were able to determine from the growth pattern of the fish bones that the animals probably died just as the weather was warming up.
Paleontologist Melanie During collected the samples during a visit to Tanis alongside her instructor, geologist Jan Smit, in 2017. Working with other researchers, they created high-resolution cross-section models of three paddlefish jawbones and three sturgeon pectoral fin spines.
From those, they could see lines demarcating periods of slower bone growth, indicating winter, when food is scarce, and faster bone growth, indicating the warmer months, when food is more abundant. The fish died just after a period of faster growth began, pointing to the springtime months in the northern hemisphere.
"I think their argument is convincing," Michael Newbrey, a biologist at Columbus State University in Georgia familiar with their method of study, told Nature. He added he would have liked to see a larger sample size.
The timing of the asteroid strike would have contributed to its deadliness for animals north of the equator. Those species would have emerged from hibernation by that point in the season, making them more vulnerable to the blast of initial heat that followed the hit.
To the south, animals already hunkered down in autumnal burrows would have fared better.
"To be able to fight that nuclear winter, you first had to survive the actual impact," During told The Guardian. "Anything in the southern hemisphere already sheltering had a much better chance of surviving."
However, the findings are controversial -- scientists are split on whether Tanis is an accurate snapshot of the moment an asteroid struck the Yucatán peninsula 66 million years ago, ending the Cretaceous period and with it, the dinosaurs.
According to a paper published by paleontologist Robert DePalma in 2019, the impact created 30-foot waves in a shallow sea that stretched across what would later become the southeastern United States. One wave surged up a river valley in what is now North Dakota, carrying mud, sand and organisms, then crashed and retreated to form the Tanis site.
If that's the case, DePalma's findings would mean that Tanis perfectly preserved one of the most consequential moments in Earth's 4.5 billion-year history.
But some are more skeptical, pointing out that the 2019 paper didn't include a detailed study of Tanis' geology. The remains at the site could be linked to a smaller, entirely separate event that could have taken place thousands of years before the asteroid hit.
"For a site of such potential importance, I'd really like to see a long-format paper that dives deep into the sedimentology and stratigraphy of the site, and supports it with lots of imagery and data," Thomas Tobin, a geologist at the University of Alabama in Tuscaloosa, told Nature.
During's findings largely aligned with a similar report from DePalma last year, who used the same method of mapping fish fossils to estimate that the asteroid hit in spring or early summer. DePalma told Nature that the studies "independently reinforce each other."