The asteroid which struck the earth, killing nearly all dinosaurs, hit during the spring, researchers have concluded.
Using extremely powerful X-rays and carbon isotope records of the bones of fish that died less than an hour after the asteroid hit, experts were able to work out when the asteroid might have struck the earth.
The team suggested that their findings could help to explain why some animals managed to survive the impact, while others did not.
Experts searched parts of North Dakota in the United States to find fossilised paddlefishes and sturgeons that were killed as the asteroid hit the earth.
The shock of the impact produced huge standing waves of water, which moved sediment and buried the fish alive.
Researchers found that the fossilised fish were pristinely preserved, their bones showed almost no signs of chemical alteration and their soft tissues remained intact.
Looking at how the bones of the fish were growing helped experts to work out the time of year at which the fish died.
One of the paddlefishes also underwent carbon isotope analysis which showed its annual feeding pattern.
It liked to eat zooplankton, which was most available during the spring and summer.
Melanie During, lead author from Uppsala University, said: “The carbon isotope signal across the growth record of this unfortunate paddlefish confirms that the feeding season had not yet climaxed – death came in spring.”
The team said their findings could help to explain why some animals, including birds, crocodiles and turtles, survived the asteroid impact 66 million years ago, while others were not so lucky.
“This crucial finding will help to uncover why most of the dinosaurs died out while birds and early mammals managed to evade extinction,” explained Ms During.
The study is published in the journal Nature, and included researchers from Uppsala University in Sweden, Vrije Universiteit in Amsterdam, Vrije Universiteit in Brussels and the European Synchrotron Radiation Facility in France.