New evidence reveals when the asteroid that killed the dinosaurs fell

The meteorite that makes 10 millions of years ago wiped out the dinosaurs, it hit the Earth in the spring of the northern hemisphere, as revealed by the study of the fish bones that died just one hour after that brutal impact.

This research, carried out by an international team and published by Nature, helps explain the pattern of extinctions that followed and adds to the understanding of that pivotal moment in Earth’s history.

Massive and selective extinction… in spring

The Chicxulub asteroid, which hit the what is now the Yucatan Peninsula, Mexico, put an end to the Mesozoic era and caused a mass extinction event, but also a selective one, since it wiped out non-avian dinosaurs, pterosaurs, ammonites and most marine reptiles, while mammals , birds, crocodiles and turtles survived.

Although it was known when and where the impact occurred, until now the season of the year was unknown and this new study suggests that it was in spring, a particularly sensitive time for the many species in the northern hemisphere, when they reproduce and have developing young.

The fossils from the Tanis site, in North Dakota, were perfectly preserved, their spines had almost no signs of geochemical alteration, the filtered impact spherules were still attached to their gills and even some soft tissues were preserved.

Through various tests, the scientists discovered that the fish spines registered a seasonal growth very similar to that of the trees, adding a new layer each year on the outside of the bone, explained Sophie Sánchez, from Uppsala University (Sweden).

Rings reveal the season in which the extinction occurred

The recovered growth rings “captured the life histories of the fish and the last seasonality of the Cretaceous and, with it, the season in which the catastrophic extinction occurred”, said Jeroen van der Lubbe of the University Free City of Amsterdam.

The distribution, shape and size of bone cells, which are also known to fluctuate with the seasons, provide an additional line of evidence.

In this case, both the density and the volumes of the cells were increasing, but had not yet peaked during the year of death, implying, according to Dennis Voetem of Uppsala University, that “growth stopped abruptly in the spring.”

One of the paddlefish studied was subjected to stable carbon isotope analysis to reveal its annual feeding pattern. The availability of zooplankton, their preferred prey, fluctuated seasonally, peaking between spring and summer.

“Death occurred in spring”

The carbon isotope signal in the fish growth record confirms that the feeding season had not yet peaked. “The death occurred in spring,” infers Melanie During, from Uppsala University.

Since the extinction must have started abruptly during the northern hemisphere spring, “we began to understand that this event had place during especially sensitive vital stages of the organisms of the later Cretaceous, including the beginning of the reproductive cycles”, he adds.

And since autumn in the southern hemisphere coincides with spring in the north, preparation for winter may have protected organisms in the southern hemisphere.

“This crucial finding -he emphasizes- will help to reveal why most of the dinosaurs died, while the Birds and the first mammals managed to avoid extinction.

With information from DW.