Scientists have determined that the fish bones found were from creatures that perished during the asteroid collision that ended the reign of dinosaurs in the spring. The impact occurred over 66 million years ago near the Yucatán Peninsula of Mexico, marking the end of the Cretaceous period. While most species were wiped out in the mass extinction that followed, it has been difficult to find fossils directly linked to the meteor impact. However, a discovery in 2019 in southwestern North Dakota revealed a mass graveyard of creatures that died shortly after the collision. Even though North Dakota was located around 2,000 miles away from where the meteor struck, the seismic waves caused a massive earthquake-like event, with a magnitude of 10 or 11, that resulted in water being displaced from lakes and rivers, leading to the death of fish. Small glass beads called tektites were also ejected into the air by the impact and fell from the sky. Researchers spent years investigating the site, known as Tanis, situated within the fossil-rich Hell Creek formation spanning four states. While an article in The New Yorker portrayed Tanis as a treasure trove of fossil discoveries, the initial scientific paper focused more on the geological aspects. The newly obtained scientific findings from the fossils now offer valuable insights into the catastrophic event that was previously difficult to interpret. "It is remarkable that we are able to identify an event that occurred 66 million years ago to a specific time of year," stated Stephen L Brusatte, a palaeontologist at the University of Edinburgh, who was not part of the study. He described it as a high-caliber detective story. Animals in the Northern Hemisphere, such as those emerging from hibernation or giving birth, may have been more susceptible to extinction during this event. Conversely, animals in the Southern Hemisphere, entering autumn, may have been better protected from the climate change. During a news conference organized by Nature, it was mentioned that hibernation could have increased an organism's chances of survival. Brusatte agreed that if one could isolate themselves in a burrow or take shelter underwater, it could be beneficial. He believes that this has the potential to help in understanding the extinction patterns and processes. During first learned about Tanis in 2017 from a talk by Jan Smit, an expert on dinosaur extinction at Vrije University in Amsterdam. She found his description of the North Dakota fossil finds intriguing and immediately reached out to him about conducting isotopic analysis on the fish bones. She contacted Robert DePalma, the paleontologist leading the study of Tanis. She traveled to North Dakota in August 2017 and spent 10 days at Tanis excavating fossils of six fish - three sturgeon and three paddlefish. In the lab, thin slices of bone were taken from the fish's lower jaws and pectoral fin spines. The scientists observed light and dark lines in the bones that indicated seasonal changes in growth, resembling tree rings. The outermost part of the bones showed that the fish became more active and grew faster after winter. During speculated that the fish likely perished in April rather than summer based on the levels of carbon isotopes in their bones, which indicated a lower plankton abundance in the water. Jeroen van der Lubbe, a paleo-climatologist at Vrije University and one of the authors of the Nature paper, mentioned that this evidence, along with the discovery of tektites in the fish gills but not in the digestive tract, supported the theory that the fish died in the spring. "They were unable to continue swimming," During stated. "They died right away." A separate group of researchers, led by DePalma, conducted a comparable analysis on fish fossils and reached nearly identical findings, as reported in Scientific Reports last December. ©2022 The New York Times Company.