A study of fish that died en masse during the first hours of the extinction reveals the season in which the asteroid impacted
66 million years ago, an asteroid about 12 kilometers long crashed into what is now the Yucatan Peninsula in Mexico. The overwhelming impact, equivalent to 10,000 million atomic bombs like the one in Hiroshima, caused the disappearance of 75% of existing life, including non-avian dinosaurs. Until now, the studies that have analyzed this crucial event in the history of the Earth have focused on millennial time scales, without being able to specify at what time of the year it happened. But European researchers may have spun finer. Analysis of the fish that died en masse during the first hour after the impact suggests that the end of the Mesozoic era occurred in the spring.
The study appears published this Wednesday in the magazine ‘Nature’.
[Dinosaurs already suffered from colds with cough, fever, and sneezing]Researchers from the Universities of Uppsala (Sweden) and the Free University of Amsterdam and Brussels studied the remains of sturgeons and paddlefish found in a deposit from the Late Cretaceous in the town of Tanis, in North Dakota (USA). These fish were direct victims of the meteorite. The impact shook the continental plate. Tens of minutes later, huge waves in rivers and lakes called seiches moved huge volumes of sediment that engulfed extinct fish, bivalves, and mollusks. They were buried alive.
Instant death
The Tanis fishes were preserved pristinely, even with soft tissues, and their spines showed almost no signs of geochemical alteration. Brilliant X-rays from the European Synchrotron Radiation Laboratory (ESRF) in Grenoble, France, confirmed that the impact spheres were still trapped in their gills, but not anywhere further down the digestive system, suggesting near-instantaneous death.
Much like tree rings, fish spines retain growth records unchanged from embryonic development to death. A new layer grows each year on the outside of the bone. These rings “reflect not only the life history of the fish but also the last seasonality of the Cretaceous and thus the season in which the catastrophic extinction occurred,” says Jeroen van der Lubbe of the Free University of Amsterdam.
That information added to an additional line of evidence provided by the distribution, shapes, and sizes of bone cells, which also fluctuate with the seasons, revealed that the fish died in the spring. In addition, one of the paddlefish studied underwent stable carbon isotope analysis to show its annual feeding pattern. The availability of zooplankton, their preferred prey, fluctuated seasonally, peaking between spring and summer. Before it died, this unfortunate individual had not yet reached the climax of the feeding season.
[They discover in the Pyrenees a dinosaur as big as an articulated bus]The advantage of the southern autumn
During the late Cretaceous age, the climate of present-day North Dakota had four seasons that have been documented in tree-ring records recovered from other sites. In winter, temperatures ranged between 4ºC and 6ºC, while in summer the average was 19ºC.
The time of the catastrophic impact would have coincided with a particularly sensitive stage for the many species in the northern hemisphere that reproduce and have offspring during the spring. Animals with longer incubation times, such as pterosaurs and most dinosaurs, were possibly more vulnerable to sudden environmental changes.
However, ecosystems in the southern hemisphere, which were hit during the austral fall, recovered up to twice as fast as those in the north. For example, the underground shelter possibly contributed to the survival of the cynodont, which belongs to the group that gave rise to mammals. Similarly, large-scale wildfires across the southern hemisphere may have been prevented by hibernating mammals that were already sheltered in burrows before the austral winter.
Other modes of seasonal dormancy or torpor, practiced today by various mammals and certain amphibians, birds, and crocodilians might have facilitated greater survival underground. “Our results will help to discover why most dinosaurs became extinct while birds and early mammals managed to evade extinction,” concludes Melanie During from Uppsala University.