Paris. The reign of the dinosaurs on Earth ended with the impact of a meteorite about 65 million years ago, in the current Yucatan, a phenomenon that occurred during the boreal spring, according to a study published yesterday in Nature.
On a spring day, paddlefish and sturgeons swam in a river that wound through a flourishing landscape populated by mighty dinosaurs and small mammals in far southwestern North Dakota. That day death came from above.
Scientists noted yesterday that well-preserved fish fossils unearthed at the site provide a deeper understanding of one of the worst days in life on Earth.
The subsequent mass extinction wiped out some three-quarters of Earth’s species and paved the way for mammals – including humans – to become dominant.
The impact that created the Chicxulub crater caused catastrophes that led to dramatic climate change.
That mass extinction continues to baffle scientists, as it was one of the most selective in the history of life: all non-avian dinosaurs, pterosaurs, ammonites, and most marine reptiles disappeared, but mammals, birds, crocodiles and turtles.
The exact year in which this phenomenon occurred will never be known, but a team of scientists led by Melanie During, from the Faculty of Sciences of the Free University of Amsterdam, and Denis Voeten, from the University of Uppsala, Sweden, elaborated after years of research an hourly chronology of that great extinction, the fifth that the planet suffered.
In general, it is known that organisms that were exposed died almost immediately. So those who took refuge in caves or burrows because they were hibernating were much more likely to survive in the Paleogene.
The Tanis archaeological site in North Dakota is key to that theory. In that place are the fossil remains of numerous animals that were engulfed by a seiche, a kind of tidal wave, according to the study. This occurs in closed or semi-closed aquatic spaces, when an external event, such as the wind or an earthquake, rocks the waters.
The phenomenon occurred a few tens of minutes after the impact of the meteorite. The shock wave traveled almost 3,000 kilometers and raised gigantic waves in the Western Interior Sea, which at that time crossed present-day America from north to south.
The scientists studied the fossils of three sturgeons and three paddlefish from Tanis, using high-resolution X-ray tomographic analysis at the European Synchrotron Radiation Laboratory in Grenoble.
They first verified that the fish did indeed die during that seiche.
As the water rose and devastated everything in its path, a rain of vitrified sand fell from the sky, caused by the enormous heat of the impact.
The fish “were alive and feeding when the impact occurred, in the last minutes of the Cretaceous,” the report explains.
They died “between 15 and 30 minutes” after the meteorite hit, according to During. “The carbon isotope signal in the growth record of this unfortunate paddlefish confirms that the feeding season had not yet climaxed: death came in the spring.”
According to the scientists, the findings will help future research on the selectivity of the mass extinction: in the northern hemisphere, it was spring, and therefore the reproductive cycles of organisms were beginning, then stopping abruptly. In contrast, in the southern hemisphere it was autumn and many organisms were preparing for winter.
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