The Day Nearly All the Dinosaurs Died Was in Spring
The asteroid, which killed almost all the dinosaurs, crashed into the Earth in the spring. That’s the conclusion of an international team of researchers examining thin sections of bones, high-resolution synchrotron X-ray scans and carbon isotope recordings of fish that died less than 60 minutes after the asteroid struck. The team’s findings were published two ago in Nature.
Researchers at Uppsala University in Sweden, Vrije University in Amsterdam, Brussels-Vrije University, and the European Synchrotron Radiation Facility (ESRF) in France set out to search for fossilized spoonfish and sturgeon at the Tanis excavation site in North Dakota, United States. Direct survivors of the Chicxulub meteorite collision, these fish also symbolize the last day of the dinosaurs. The collision that took place shook the continental plate, causing massive standing waves to form in the water bodies. These giant mounds of sediment in motion surrounding the fish, along with impact globules falling from the sky within an hour of impact, caused the fish to be buried alive in the ground.
The fossil fish found in the sediments where the Tanis event took place have been preserved intact until today. The bones of the fish show almost no signs of geochemical change. Available to anyone who wants to do research, data from synchrotron X-rays show that the floating collision globules are still stuck in the fish’s gills. Even soft tissues are preserved!
Selected fish bones were studied while reconstructing the last seasonal situation in the Cretaceous period. “These bones, like trees, have recorded seasonal growth,” says Sophie Sanchez of Uppsala University and ESRF.
“The growth rings obtained record the fish’s life history as well as the last seasonal situation during the Cretaceous,” says senior author Jeroen van der Lubbe of Vrije University in Amsterdam. “Therefore, it shows in what season the disaster occurred.”
The distribution, shapes and sizes of bone cells, which are known to fluctuate with the seasons, also provide additional findings. “Bone cell density and volumes can be tracked over multiple years in all the fish studied,” says Dennis Voeten of Uppsala University. “They appear to be on the rise, but not yet peaking in the year of their death.”
By subjecting one of the spoonfish under study to stable carbon isotope analysis, the annual feeding pattern of the fish was revealed. The availability of zooplankton, the preferred prey of fish, fluctuates seasonally, peaking between spring and summer.
Suzan Verdegaal Warmerdam of Vrije University in Amsterdam explains: “This temporary increase in digested zooplankton enriched the predator’s skeleton with the heavier 13C carbon isotope compared to the lighter 12C carbon isotope.” Melanie During, who works at Uppsala University and Vrije University in Amsterdam and is the lead author of the paper, said: “This carbon isotope signal seen in the growth records of the unfortunate spoonfish confirms that the feeding season has not yet peaked; death came in the spring,” he says.
This mass extinction, which occurred in the late Cretaceous, represents one of the most selective extinctions in the history of life. In this event, non-avian dinosaurs, pterosaurs, ammonites and most marine reptiles disappeared; mammals, birds, crocodiles and turtles survived. Now that we know that the extinction event began abruptly during the northern hemisphere spring season, we are beginning to understand that it took place during a sensitive life stage (including the beginning of their reproductive cycle) of the last Late Cretaceous creatures. Since the autumn season in the southern hemisphere coincides with the spring season in the northern hemisphere, the creatures preparing for the winter in the southern hemisphere may have managed to avoid disaster.
During concludes, “This pivotal finding will help us unravel why most dinosaurs disappeared while birds and early mammals escaped extinction.”