Scientists studying the bones of fish that perished when a 6-mile-wide asteroid hit Earth have concluded that the creatures died in spring. This discovery was made by Melanie During, a graduate student at Uppsala University in Sweden and lead author of a paper published in the journal Nature. The asteroid strike, which occurred just over 66 million years ago off the Yucatán Peninsula of Mexico, marked the end of the reign of dinosaurs in the Cretaceous period. While the mass extinction that followed wiped out three-quarters of Earth's plant and animal species, finding fossils directly linked to the meteor impact has been challenging. However, in 2019, palaeontologists discovered a mass graveyard in southwestern North Dakota containing creatures that died shortly after the collision. Even though North Dakota was far from the meteor impact, the seismic waves caused an earthquake-like event that displaced water from lakes and rivers, resulting in the death of fish. Tektites, 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. The New Yorker described Tanis as a treasure trove of fossil discoveries, while the initial scientific paper focused more on the geological aspects of the site. The new scientific findings from the fossils now offer valuable insights into the previously incomprehensible cataclysm. Stephen L Brusatte, a palaeontologist at the University of Edinburgh who was not involved in the research, expressed amazement at the ability to pinpoint a specific event that occurred 66 million years ago to a particular time of the year. He described it as a detective story of the highest calibre. Animals in the Northern Hemisphere, some emerging from hibernation or giving birth to young, may have been more vulnerable to extinction if the event occurred in spring. On the other hand, animals in the Southern Hemisphere, hunkering down in autumn, may have been more sheltered from the sudden change in climate. According to During, if an organism could hibernate, it would increase its chances of survival. Brusatte agreed that if one could hide in a burrow or seek shelter underwater, it could provide assistance. He believes there is potential in using this method to better understand the extinction patterns and processes. During first learned about Tanis in 2017 from Jan Smit, an expert on dinosaur extinction at Vrije University in Amsterdam. She was captivated by Smit's description of the North Dakota fossil discoveries and immediately reached out to him about conducting isotopic analysis on the fish bones found at the site. 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 lower jaws of the paddlefish and from the pectoral fin spines of the sturgeon. The scientists observed alternating light and dark lines in the bone, which reflected seasonal changes in growth rate, similar to tree rings. The outer part of the bones showed that the fish became more active and grew faster after winter ended. During suggested that the fish likely perished in the spring based on the fluctuation in carbon isotopes in their bones, indicating lower plankton levels than during summer. Jeroen van der Lubbe, a paleo-climatologist at Vrije University, added that this evidence, along with the presence of tektites in the fish's gills but not in the digestive tract, supports the theory that the fish died in the spring. "They were unable to continue swimming," During stated. "They died right away." Another group of scientists, 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.