Scientists have determined that the fish bones found from the day a 6-mile-wide asteroid hit Earth belonged to fish that died in the spring. This event marked the end of the reign of dinosaurs. The asteroid collision occurred over 66 million years ago near the Yucatán Peninsula of Mexico, leading to the mass extinction of three-quarters of Earth's plant and animal species. In 2019, palaeontologists discovered a mass graveyard in southwestern North Dakota containing creatures that died shortly after the impact. Although North Dakota was located approximately 2,000 miles away from where the meteor struck, the seismic waves caused by the impact, equivalent to a magnitude 10 or 11 earthquake, resulted in water being sloshed out of lakes and rivers, leading to the death of fish. Additionally, tektites, which are small glass beads propelled into the air by the impact, fell from the sky. Researchers spent years investigating the site, known as Tanis, located in the fossil-rich Hell Creek formation spanning four states. While an article in The New Yorker hailed Tanis as a treasure trove of fossil discoveries, the initial scientific paper about the site was more focused on the geological context. The new scientific findings have now allowed for insights into the cataclysm that were previously unattainable. "It's incredible that we can identify an event that occurred 66 million years ago down to a specific time of the year," remarked Stephen L Brusatte, a palaeontologist at the University of Edinburgh who was not part of the study. He described it as a top-notch detective story. Animals in the Northern Hemisphere, such as those waking from hibernation or giving birth, may have been more at risk of extinction. Meanwhile, animals in the Southern Hemisphere, preparing for autumn, may have been better protected from the sudden climate change. According to During, being able to hibernate could have increased an animal's chances of survival. Brusatte agreed that if one could isolate themselves in a burrow or seek shelter underwater, it could potentially help in understanding the extinction patterns and processes. During first learned about Tanis in 2017 during a talk by Jan Smit, an expert on dinosaur extinction at Vrije University in Amsterdam, while she was pursuing her master's degree. She was fascinated by the North Dakota fossil discoveries and immediately reached out to Smit to inquire about conducting isotopic analysis on the fish bones found. She contacted Robert DePalma, the paleontologist leading the investigation at Tanis. In August 2017, During traveled to North Dakota and spent ten days at Tanis excavating fossils of six fish - three sturgeon and three paddlefish. In the lab, researchers took thin slices of bone from the lower jaws of the paddlefish and the pectoral fin spines of the sturgeon. They observed alternating light and dark lines indicating seasonal growth patterns, similar to tree rings. The outermost part of the bones showed that the fish became more active and grew faster after winter. During suggested that the fish perished in April, ruling out summer based on the levels of carbon isotopes in the bones. Jeroen van der Lubbe, a paleo-climatologist at Vrije University and co-author of the Nature paper, also pointed to other evidence indicating that the fish died in the spring. Tektites were discovered in the fish's gills, but not in the digestive tract. "They were unable to swim any further," During explained. "They died right away." Another group of researchers, headed by DePalma, conducted a comparable analysis on fish fossils and published nearly identical findings in Scientific Reports last December. ©2022 The New York Times Company.