The meteorite that killed the dinosaurs 66 million years ago hit Earth in the spring of the northern hemisphere, as revealed by the study of the fish bones that died just an hour after that brutal impact.
This research, carried out by an international team and published Nature, it 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 asteroidwhich hit 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 reptiles marine, 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, an especially sensitive time for the numerous species of the northern hemisphere, when they reproduce and have developing young.
Ecosystems in the southern hemisphere, which were hit during the fall, appear to have recovered up to twice as fast as those in the northern hemisphere, the authors note.
Fossils of the spines of sturgeons and paddle fish
The study sheds light on the circumstances surrounding the diverse extinction of the different groups and the answers come from the fossils of the spines of sturgeons and paddle fish found at the paleontological site of Tanis (United States).
The impact shook the continental plate and caused huge standing waves in the water masses, which mobilized large volumes of sediments that engulfed the fish and buried them alive, while the impact spherules (glass beads of terrestrial rock) rained down from the Darling.
Fossils from the Tanis site in North Dakota were perfectly preserved, their spines showing almost no signs of geochemical alteration, filtered impact spherules still attached to their gills, and even some soft tissue 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 the University of Uppsala (Sweden). .
Rings reveal the season in which the extinction occurred
The recovered growth rings “captured the life histories of the fish and the late Cretaceous seasonality and, with it, the season in which the catastrophic extinction occurred,” said Jeroen van der Lubbe of the Free University 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 cell density and volumes were increasing, but had not yet peaked during the year of death, implying, according to Dennis Voetem of Uppsala University, that “growth it 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’s growth record confirms that the feeding season had not yet peaked. “The death occurred in the spring,” infers Melanie During, from Uppsala University.
Since the extinction must have started abruptly during the Northern Hemisphere spring, “we are beginning to understand that this event took place during particularly sensitive life stages of late Cretaceous organisms, including the start of reproductive cycles,” he adds.
And since fall in the southern hemisphere coincides with spring in the north, the 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.
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