The celestial object that hit Earth 66 million years ago, ending the Age of Dinosaurs, arrived on our planet in the spring of the Northern Hemisphere, a new study reveals. The timing of the impact probably further magnified its destructive effect on species and ecosystems, affecting them at a delicate stage of their development.
The finding, which comes from careful detective work coordinated by European scientists, has just appeared in the scientific journal Nature, one of the most important in the world. “We are the first to conclusively demonstrate that the impact occurred in the spring, although work published since 1990 has already tried to make this estimate,” he told sheet Dutch paleontologist Melanie During, a doctoral student at Uppsala University in Sweden. During coordinated the new study, also signed by scientists from other European institutions.
The bolide that caused the mass extinction formed the Chicxulub crater on the Yucatan Peninsula (Mexico). The rocks in the region were scarred by the tsunamis that the impact caused, but the problem is that there are no fossils of animals killed directly by the blow in these layers – or, at least, such fossils have not been found so far. “This makes a study like ours not possible with this material,” explains During.
Therefore, the solution was to analyze layers of rock corresponding to the time of the impact that are located in the US state of North Dakota, almost 3,000 km from the Yucatan. The distance may seem unreasonable, but it was already known that the fossiliferous site of Tanis, discovered in the region, harbored a treasure: fish whose gills contained impact spherules, that is, tiny spheres produced when the meteorite crashed into Earth.
Therefore, the unlucky fish could serve as time capsules, as they carried geological imprints from the moment of the collision (15 minutes to 30 minutes after the disaster, to be exact, scientists calculate). During says he learned all this in 2017, when he listened to a lecture by professor emeritus Jan Smit, from the Free University of Amsterdam, about the North Dakota site. “And yes, the lecture also took place in the spring of that year,” he jokes.
Talking to Smit and her mentor at the time, Jeroen van der Lubbe, she managed to travel to the US in the same year and began the study of fish, related to today’s sturgeons (the same ones whose eggs are the famous caviar). The animals were quickly buried when a phenomenon called “seiche” (basically a tsunami in a confined body of water, with oscillating waves) hit the region shortly after the impact.
“I collected the jaw bones of paddlefish and the pectoral fin spines of sturgeons. I chose these bones because I found that they grow very similarly to trees, adding a new layer each year. So we were curious to know if it would be possible to reconstruct the season in which they died,” she says.
On the fly: Analysis of the microscopic structure of the bone growth lines showed that the fish died just as a new growth line was starting to form. This coincides with the Northern Hemisphere spring, when food availability increases again after the lean cow phase of winter.
Another important clue is the chemical composition of bones. The fish probably fed on small crustaceans, and this is reflected in a periodic change in the presence of an isotope (variant) of the chemical element carbon in their bones, which is related to diet. And this variation follows growth cycles, as expected.
“They managed to combine several different techniques very well, in a relatively simple way”, says Brazilian paleontologist Rafael Delcourt, a postdoctoral researcher at USP in Ribeirão Preto who commented on the study at the request of the sheet.
Another good idea, according to him, was to study fossils of fish that still have living relatives in the region and whose biology is well known. “This gives some predictability to understand how they grew, used resources and deposited minerals in their skeleton.”
During and his colleagues argue that the spring impact would have been especially harsh for Northern Hemisphere species because the season would have coincided with the breeding season, when all resources are devoted to raising and caring for young. Thus, many terrestrial species, in addition to dealing with the immediate effects of the catastrophe, would still lose the next generation, even if they managed to survive.
In the Southern Hemisphere, where it was autumn, some animals could have already prepared to hibernate, staying more protected in burrows (in the case of primitive mammals at the time). In fact, there are some indications that the southern hemisphere recovered first from the tragedy. “Most of the research on the subject was carried out in the Northern Hemisphere. I think the work is a stimulus for us to better understand the phenomenon in the Southern Hemisphere, and it gives many ideas that can be applied, including in Brazilian fossils”, says Delcourt.
