The asteroid which killed nearly all dinosaurs struck the Earth during the spring, researchers have concluded.
In a new study, experts used extremely powerful X-rays and carbon isotope records of the bones of fish that died less than an hour after the asteroid hit.
These carbon isotopes act as a tracer for how carbon atoms have undergone transformations over the years.
The team said their findings may help explain why some animals managed to survive the impact while others died.
To reach their conclusions, experts searched parts of North Dakota in the US to find fossilised paddlefishes and sturgeons that were killed when the asteroid made impact.
The shock of impact caused huge standing waves of water which moved sediment, engulfing fish and burying them alive.
The study, published in the journal Nature, included researchers from Uppsala University in Sweden, Vrije Universiteit in Amsterdam, Vrije Universiteit in Brussels and the European Synchrotron Radiation Facility in France.
They found that the fossil fish were pristinely preserved, with their bones showing almost no signs of chemical alteration, and soft tissues still intact.
A close look at how the bones of the fish were growing helped experts work out the season in which they died.
One of the paddlefishes also underwent carbon isotope analysis to reveal its annual feeding pattern.
The availability of zooplankton, which it liked to eat, was at its peak between 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 may help explain why some animals, including birds, crocodiles and turtles, survived the asteroid impact 66 million years ago.
The southern hemisphere autumn coincides with spring in the northern hemisphere, meaning preparation for winter may have protected animals in the southern hemisphere.
“This crucial finding will help to uncover why most of the dinosaurs died out while birds and early mammals managed to evade extinction,” said Ms During.
