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The reign of the dinosaurs ended in spring

23 February 2022
An international team of researchers led by VU Amsterdam has discovered that the aftermath of the Chicxulub meteorite impact which killed nearly all of the dinosaurs, struck Earth during springtime. They arrived at this conclusion after examining thin sections of the bones from fossil fish, using high-resolution synchrotron X-ray scans and carbon isotope records. These fish died less than 1 hour after the asteroid impacted.

The team presents its findings in this week’s issue of the journal Nature. The unique Tanis locality in North Dakota (United States) preserves a fossilised ecosystem that includes paddlefishes and sturgeons which died as a direct consequence of the Chicxulub meteorite impact. “The impact triggered fast-moving surface waves across the continental plate that caused massive standing waves in water bodies. These mobilised enormous volumes of sediment that engulfed fishes and buried them alive while impact spherules rained down from the sky, less than an hour after impact”, says Jan Smit, emeritus professor of VU Amsterdam, who was also part of the team led by Robert DePalma of the Palm Beach Museum of Natural History, that presented the Tanis site in 2019.

Pristinely preserved bones
Fossil fishes in the Tanis event deposit were pristinely preserved, with their bones showing almost no signs of geochemical alteration. Even soft tissues have been preserved. Selected fish bones were studied for the reconstruction of latest Cretaceous seasonality. These bones registered seasonal growth very much like trees do. Such growth patterns were especially visible in the scanning data.

“The retrieved growth rings not only captured the life histories of the fishes but also recorded the latest Cretaceous seasonality and thus the season in which the catastrophic extinction occurred”,  states senior author Jeroen van der Lubbe of VU. An additional line of evidence was provided by the distribution, shapes, and sizes of the bone cells, which are known to fluctuate with the seasons as well.

Unfortunate paddlefishes
One of the studied paddlefishes was subjected to stable carbon isotope analysis to reveal its annual feeding pattern. The availability of zooplankton, its prey of choice, oscillated seasonally and peaked between spring and summer. This temporary increase of ingested zooplankton enriched the skeleton of its predator with the heavier 13C carbon isotope relative to the lighter 12C carbon isotope. “The carbon isotope signal across the growth record of this unfortunate paddlefish confirms that the feeding season had not yet climaxed – death came in spring”, infers Melanie During from VU Amsterdam and Uppsala University, and lead author of the publication.

The end-Cretaceous mass extinction represents one of the most selective extinctions in the history of life that saw the demise of all non-avian dinosaurs, pterosaurs, ammonites, and most marine reptiles while mammals, birds, crocodiles, and turtles survived. Because we now know that the extinction must have abruptly started during northern-hemisphere spring, which coincides with southern-hemisphere autumn, we start to understand that this event took place during particularly sensitive life stages of Latest Cretaceous organisms, including the onset of reproduction cycles in the Northern Hemisphere and hibernation in the Southern Hemisphere. “This crucial finding will help to uncover why most of the dinosaurs died out while birds and early mammals managed to evade extinction”, concludes Melanie During.

Master thesis
This publication is largely based on Melanie During’s Master thesis, which won the Escher Price in 2018  for best Earth Sciences thesis of the Netherlands (Escherprijs - KNGMG). The thesis was supervised by Jeroen van der Lubbe and Jan Smit.

The image accompanying this article was created by Joschua Knüppe.