Clues to the season of impact lingered in delicate fish fossils.
Spring is a time for budding flowers, tender green leaves and baby animals. But 66 million years ago, that gentle season instead brought mass death and carnage from Earth's catastrophic impact with a massive space rock.
Earth was forever changed after an enormous asteroid smashed into our planet at the end of the Cretaceous period (145 million to 66 million years ago), triggering a global extinction that wiped out 76% of life on Earth, including all nonavian dinosaurs, pterosaurs and most marine reptiles. Scientists recently pinpointed the season of the disaster and linked it to springtime in the Northern Hemisphere, after analyzing fossilized animals that died minutes after the impact.
They found the fossils at a site called Tanis, where a river once flowed through what is now North Dakota. After the asteroid struck near Mexico's Yucatán Peninsula, the shock sent powerful waves roaring upstream toward Tanis, sweeping up fish and forest creatures and burying them alive under layers of soil. When the water subsided, it left behind an astonishingly well preserved 3D snapshot of destruction, captured within 30 minutes after the asteroid struck, the researchers reported in a new study. Fossils of those filter-feeding fish also held clues about their seasonal growth cycles, hinting that spring had sprung when the fish died and the dinosaurs' reign abruptly ended.
(Image: SPL
The moment of mass, instantaneous death preserved in Tanis, with broken and splintered fish fossils wrapped around tree branches and strewn in all directions, "was like the worst car crash you've ever seen, frozen in place," said lead study author Melanie During, a doctoral candidate in the Department of Organismal Biology at Uppsala University in Sweden. It was also "the most spectacular deposit I've ever seen in my life," During said at a news conference on Feb. 22.
During excavated Cretaceous fish at Tanis in August 2017, spending two weeks digging out fossils of paddlefish and sturgeons. Fish skeletons — even after fossilizing — retain records of an animal's growth, which depends on seasonal food availability. By mapping these patterns in bone cell growth and density, the scientists hoped to identify which part of the growth cycle the Tanis fish had reached when they died, which could indicate what time of year it was.
The study authors scanned the fossils using synchrotron X-ray imaging, nondestructively imaging and reconstructing the fossils in 3D. They found tiny glass balls called spherules embedded in the fishes' gills; these small spheres fused from ultrahot sediments when the asteroid struck and ejected towering plumes of dirt from the impact crater. Particles flew into Earth's atmosphere and beyond and then rained back down on the planet as glassy beads.
Other researchers who studied Tanis' Cretaceous death pit calculated that impact spherules would have fallen between 15 and 30 minutes after the asteroid crashed into Earth. Because spherules were in the fishes' gills but had not been swallowed, the fish were likely buried alive immediately after inhaling the glassy beads — within 30 minutes after the asteroid impact, according to the new study.
FOR THE FULL ARTICLE: https://www.livescience.com/dinosaur-killing-asteroid-spring-impact
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