Scientists have concluded that the fish bones examined belonged to creatures that died in spring when a massive asteroid collided with Earth. The extinction event marked the end of the reign of dinosaurs and occurred just over 66 million years ago off the Yucatán Peninsula of Mexico. Despite the mass extinction that followed, it has been difficult to find fossils directly linked to the meteor impact. In 2019, palaeontologists discovered a mass graveyard of creatures that died shortly after the collision in southwestern North Dakota. Even though North Dakota was located approximately 2,000 miles away from where the meteor struck, the seismic waves caused an earthquake-like event with a magnitude of 10 or 11 which resulted in water being displaced from lakes and rivers, leading to the death of fish. Tektites, which are small glass beads ejected into the atmosphere by the impact, fell from the sky. Scientists spent years studying the site, called Tanis, which is part of 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 provided limited details, focusing on the geological context. Thanks to new scientific findings, the fossils now offer valuable insights into the catastrophic event that was previously difficult to interpret. "It is incredible that we are able to identify an event that occurred 66 million years ago, pinpointing it to a specific time of the year," stated 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, including those emerging from hibernation or giving birth, may have been more at risk of extinction. On the other hand, animals in the Southern Hemisphere, preparing for autumn, may have been better protected from the sudden climate change. "Being able to hibernate could have increased your chances of survival," During mentioned during a telephone news conference organized by Nature. Brusatte agreed that if one were able to isolate themselves in a burrow or seek shelter underwater, it could be beneficial. He believed that this could aid 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, where she was studying for her master's degree. She was fascinated by the North Dakota fossil discoveries and immediately reached out to Smit about conducting isotopic analysis on the fish bones. She contacted Robert DePalma, the palaeontologist leading the investigation at Tanis. She traveled to North Dakota in August 2017 and spent 10 days at the site, 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. These slices showed alternating light and dark lines that indicated seasonal changes in growth rate, similar to tree rings. The outermost part of the bones revealed that the fish became more active and grew faster after winter. During suggested that the fish likely perished in April, rather than summer, based on the fluctuations in carbon isotopes in their bones, which indicated lower levels of plankton in the water. This evidence, along with the presence of tektites in the fish's gills but not their digestive tract, supports the conclusion that the fish died in the spring. Jeroen van der Lubbe, a paleo-climatologist and co-author of the Nature paper, noted that there are multiple lines of evidence pointing to this conclusion. "They were unable to continue swimming," During stated. "They died right away." DePalma and another group of scientists conducted a separate analysis on fish fossils and reached nearly identical findings, as reported in Scientific Reports last December. ©2022 The New York Times Company.