This is apparent from an analysis of the remains of fish that died within an hour of the impact.

About 66 million years ago, a large space rock hit the Earth. The impact and its consequences killed countless species, the most famous examples of which are without a doubt the dinosaurs. Thanks to intensive research in the crater as well as in other places on Earth – which, for example, still testify to the impact through fossil victims – we have now obtained a fairly good picture of the impact and its consequences. But there are still many loose ends. For example, for a long time it was unclear in which season the asteroid impact took place. One new research creates more clarity and reveals that it must have been in the spring.

Fishing

The researchers reached that conclusion after studying fish remains found in the so-called Tanis Deposit in North Dakota. The fish died between 15 and 30 minutes after the asteroid hit and are direct victims of the impact. “Some of their bones grew similar to trees and added a new layer every year on the outside,” explains researcher Melanie During. These ‘annual rings’ follow an annually recurring pattern, characterized by periods in which the fish bones alternately expand relatively hard (in summer) and relatively slowly (in winter). “We also knew we could use their bones to reconstruct the Cretaceous seasonality, including the impact season itself.” And the growth rings paint a clear picture: the fish died in the spring.

bone cells

In their study, the researchers also looked at the individual bone cells of these fish. It was already known that their density, shape and size vary through the seasons. “We were able to determine the density and volume of bone cells over the years in all the fish studied,” said researcher Dennis Voeten. “And we quickly saw that these fish registered seasonality in this way and all died in the same season,” says During. Namely in the spring. “We saw that bone cell density and volumes increased during the year these fish died, but had not yet peaked, suggesting that the growth of these fish came to an abrupt halt in the spring,” Voeten said.

isotopes

The conclusion that the fish died in the spring is further supported by isotope research. Isotopes are atoms of the same chemical element with the same number of protons, but a different number of neutrons in the atomic nucleus. In this study, scientists focused on two isotopes of carbon: C13 and C12. The relationship between them
is influenced by what the fish eat and is also recorded in the growing skeleton. The bones reveal that the ratio of C13 to C12 is not stable, but fluctuates throughout the year. This is because the diet of the fish changes with the seasons. For example, there is much more zooplankton – the favorite meal of these fish – available in the summer than in the winter. And we see that in the bones. “This temporary increase in zooplankton consumption enriches the skeleton of the fish with the heavier carbon isotope C13,” said researcher Suzan Verdegaal-Warmerdam. But an analysis of the bones shows that shortly before the fish died, the carbon isotope had not yet reached its summer peak. “The feeding season hadn’t peaked yet: death came in the spring.”

Here you can see the fossilized remains of a fish found in the Tanis deposit. Pieces of debris – known as tektites – created by the impact have been found in the gills of the unfortunate fish. “Melted rock thrown into space by the violent impact had already started to rain like a hail of glass and stone,” explains During. “These spheres, called tektites, that crystallize in space have a distinctive round shape and often contain the lightest elements in the center – something that could not happen on Earth, where gravity would lead the lightest elements to the top. The hail of tektites struck the water and began to clog the gills of the fish.” At the same time, the fish were forcibly brought together by a haul or seiche and buried alive by sediments. “A seiche occurs in confined waters, which are rocked by continental shock waves. The shock wave moves very quickly through the earth’s crust and causes huge waves in the overlying waters (lakes, rivers); very similar to a swimming pool during an earthquake.” Escape was impossible for these fish. “Every living thing in Tanis on that day saw nothing and was killed almost immediately.” Image: Melanie During.

Better understanding

And so there are several indications that things went horribly wrong for these fish – and many other animals – in the spring. It is a valuable addition to what we already knew about the impact, says During. For by accurately identifying when the impact occurred down to the season, we can truly understand the mass extinction that followed the impact. For it is well known that there were many species that perished. But there were also many species that managed to survive, and some of them have quite a few similarities with the species that became extinct. It begs the question of why some species disappeared and others did not. “Extinction is selective in several ways, both in groups of extinction (the flightless dinosaurs and pterosaurs, but not the birds and mammals), and by hemisphere,” says During. By determining the season in which the impact occurred, we can better understand that. “Since spring in the northern hemisphere coincides with autumn in the southern hemisphere, the conditions when the asteroid hit were very different in both hemispheres. For many organisms, spring is the most important season for growth and reproduction after the harsher winter months. In the southern hemisphere, many organisms were just preparing for winter: plants dropped their leaves and many animals sought shelter for hibernation. The season of the impact will probably have had a profound effect on the selectivity of the mass extinction,” thinks During. This is further supported by those finds that suggest that organisms in the southern hemisphere recovered much faster than in the northern hemisphere. “The recovery phase in the southern hemisphere doesn’t seem to last half as long as that of the northern hemisphere, something that the impact season may have contributed to.”

Other study
The research is in line with a study published in December, which concludes – also based on finds in the Tanis deposit – that the impact occurred in the spring. While this study was published earlier, During said it was conducted later. “Our work is the preceding work and is in no way based on the data or conclusions of DePalma et al. (the research team that concluded in December that the asteroid struck in the spring, ed.).” Although the main conclusion of both studies is similar, there are also some differences. “First, our publication was able to achieve higher seasonal resolution through advanced visualization techniques that can represent the microstructure of fish bones in 3D. This allowed us to extract independent indicators of growth, for example through fluctuating bone cell distributions and sizes, and limit the catastrophic asteroid impact to one season: spring. Second, the current publication provides all the primary data needed to understand the underlying research and extend it in future studies. We offer our complete set of isotope measurements and even the scan data (high resolution) of the examined fish fossils are made available for anyone to explore in any research application of their choice. Third, we have identified a number of concrete directions for future research into the selectivity of the late Cretaceous mass extinction, with the aim of providing valuable leads for further studies on the subject.”

Now that it is clear that the impact occurred in the spring, the researchers hope to gain a better understanding of the selective nature of the mass extinction in the future. “Our results will help reveal why most dinosaurs became extinct, while birds and early mammals didn’t,” says During. And who knows, in the future researchers will also be able to determine the moment of impact even more accurately. “With the fossils found in Tanis, we’ve reached the highest resolution at the moment,” says During. “But, for example, if we manage to find shells (that died directly from the impact) in such deposits, we may be able to get a daily resolution.” In that scenario, it might even be possible to determine whether the impact occurred in the morning, afternoon or evening.