The bones of fish that perished when a massive asteroid collided with Earth have led scientists to conclude that the event took place in the spring, marking the end of the reign of dinosaurs. The impact, which occurred over 66 million years ago off the Yucatán Peninsula of Mexico, ended the Cretaceous period and resulted in a mass extinction of three-quarters of Earth's plant and animal species. While it has been difficult to find fossils directly linked to the meteor strike, in 2019, palaeontologists discovered a mass graveyard in southwestern North Dakota containing creatures that died shortly after the impact. Despite being approximately 2,000 miles away from the meteor impact, North Dakota experienced seismic waves equivalent to a magnitude 10 or 11 earthquake, causing water from lakes and rivers to overflow and kill fish. Tektites, small glass beads propelled into the air by the impact, fell from the sky. Researchers spent years studying 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 focused more on the geological aspects of the site. The new scientific findings now offer valuable insights into the previously incomprehensible cataclysm. Stephen L Brusatte, a paleontologist at the University of Edinburgh who was not involved in the research, expressed amazement at the ability to pinpoint an event that occurred 66 million years ago to a particular time of year. He described it as a detective story of the highest caliber. 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 when organisms are not likely to be in hibernation. On the other hand, animals in the Southern Hemisphere hunkering down in autumn may have been more sheltered from the sudden change in climate. During a telephone news conference arranged by Nature, it was suggested that hibernation could increase an animal's chances of survival during such an event. Brusatte agreed that if one could isolate themselves in a burrow or seek shelter underwater, it could be beneficial. He sees potential in using this approach to better understand the patterns and processes of extinction. During first learned about Tanis in 2017 from a talk by Jan Smit, an expert on dinosaur extinction at Vrije University in Amsterdam, during her master's degree studies. She was captivated by the fossil finds in North Dakota and immediately reached out to Smit about conducting isotopic analysis on the fish bones found there. She contacted Robert DePalma, the palaeontologist leading the study of Tanis. She traveled to North Dakota in August 2017 and spent 10 days at Tanis digging up fossils of six fish - three sturgeon and three paddlefish. In the lab, researchers carefully extracted thin slices of bone from the paddlefish's lower jaws and the sturgeon's pectoral fin spines. They observed distinct light and dark lines in the bones, which indicated seasonal changes in growth rates, similar to tree rings. The outermost sections of the bones showed that the fish became more active and grew faster after winter. During expressed his belief that the fish likely perished in April rather than summer based on the fluctuation of carbon isotopes in their bones, indicating lower levels of plankton in the water. This evidence, along with the discovery of tektites in the fish's gills but not in their digestive tract, suggests that the fish died in the spring. Jeroen van der Lubbe, a paleo-climatologist at Vrije University and one of the authors of the Nature paper, also noted that these findings support the conclusion that the fish died in the spring. "They were unable to continue swimming," During explained. "They died right away." Another group of scientists, led by DePalma, conducted a separate analysis on fish fossils and reached nearly identical conclusions, as reported in the journal Scientific Reports in December of last year. ©2022 The New York Times Company.