There may come a time when humanity can celebrate, or mourn, World Asteroid Day, marking the exact anniversary of the impact that killed the non-avian dinosaurs and made space for us. We're not there yet, but we're identifying the time of year at which the impact occurred with ever-increasing precision – the asteroid struck during springtime, according to a new study.

Last year, paleontologists revealed many pieces of evidence, all pointing to the fact that it was spring or summer in the northern hemisphere when a space rock created the Chicxulub crater and unleashed the sixth mass extinction. Now, a separate team have made finer measurements of one of the indicators used in that research to conclude the event was in the earlier part of that range, a result they published in Nature. 

The work made use of the Tanis deposit in North Dakota, perhaps the most astonishing fossil site ever found. Tanis includes the skeletons of many animals buried by a standing wave (seiche) of water caused by seismic waves set off by the impact, followed by a rain of ejecta thrown up from the crater thousands of kilometers away.

Last year's paper revealed the presence of recently hatched fish at Tanis, along with older fish whose bones indicated they died during a growth season. Ancestors of fish that today summer in fresh water and winter in salty conditions were found in a freshwater environment.

Dr Sophie Sanchez of Uppsala University and co authors focused on the bones of Tanis' paddlefishes and sturgeons. These fish died with spherules from the impact stuck in their gills but not their digestive systems, indicating they did not survive that deadly day. “These bones registered seasonal growth very much like trees do,” Sanchez said in a statement, with alternating light and dark layers depending on the growing season.

Besides color, the rings differ by cell density and volume. "These were on the rise, but had not peaked during the year of death,” said co-author Dr Dennis Voeten. Carbon isotopes offer clues to the diet at the time a ring was laid down, which also varies with the season.

 The consistency of the findings across species adds to the authors' confidence in their conclusions.

First author, PhD student Melanie During, explained this means “the feeding season had not yet climaxed – death came in spring.”

Before the discovery of Tanis, the one effort to determine the impact's season suggested it occurred in June, consistent with last year's paper, but contradicted by this one.

The team hope to use this knowledge to understand the differences between the three quarters of species that died out in the multi-year winter that followed the impact, and the quarter – including surprising examples like frogs – that survived.

“This crucial finding will help to uncover why most of the dinosaurs died out while birds and early mammals managed to evade extinction,” concluded During.

It is thought an event such as Chicxulub would be more devastating during growing season than at a time when many plants were dormant, or at least in preparation. If so, this would lead to the prediction a larger proportion of southern hemisphere species should have survived the catastrophe to repopulate the world – possibly including our ancestors. This is something that is still being investigated, but is supported by at least one study that found Gondwanan ecosystems recovered twice as fast as those in the north.