The Cretaceous–Palaeogene mass extinction around 66 million years ago triggered the Chicxulub asteroid impact. The event caused highly selective extinction that killed almost 76% of species, including dinosaurs, pterosaurs, ammonites, rudists, and most marine reptiles.
Studies have reported the timing of the impact and its aftermath. However, the season of the effect remains unconstrained.
A new study reported that the asteroid that killed dinosaurs struck Earth during springtime. Scientists came to this conclusion after examing thin sections, high-resolution synchrotron X-ray scans, and carbon isotope records of the bones of fishes that died less than 60 minutes after the asteroid were impacted.
Scientists turned to the unique Tanis locality in North Dakota (United States) to find fossilized paddlefishes and sturgeons. These fossils were the direct casualties of the so-called Chicxulub meteorite impact.
The Chicxulub impact event was an ~100 million megaton blast that rocked the continental plate and caused massive standing waves in water bodies. These mobilized 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.
Fossil fishes in the Tanis event deposit were pristinely preserved, with their bones showing almost no signs of geochemical alteration. The synchrotron X-ray data, available for anyone to explore, confirm that filtered-out impact spherules are still stuck in their gills. Even soft tissues have been preserved!
Sophie Sanchez of Uppsala University and the ESRF said, “Selected fish bones were studied to reconstruct the latest Cretaceous seasonality. These bones registered seasonal growth very much as trees do.”
Senior author Jeroen van der Lubbe of the V.U. in Amsterdam said, “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.”
Dennis Voeten of Uppsala University said, “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. Bone cell density and volumes can be traced over multiple years in all studied fishes. These were on the rise but had not yet peaked during the year of death.”
“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.”
Suzan Verdegaal-Warmerdam of the VU Amsterdam said, “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.”
“Because we now know that the extinction must have abruptly started during northern-hemisphere spring, we start to understand that this event took place during susceptible life stages of Latest Cretaceous organisms, including the onset of reproduction cycles. And because southern-hemisphere autumn coincides with spring in the Northern Hemisphere, the preparation for winter may have just protected organisms in the Southern Hemisphere.”
Melanie During from Uppsala University and the VU Amsterdam and lead author of the publication, said, “This crucial finding will help to uncover why most of the dinosaurs died out while birds and early mammals managed to evade extinction.”
- During, M.A.D., Smit, J., Voeten, D.F.A.E., et al. The Mesozoic terminated in boreal spring. Nature (2022). DOI: 10.1038/s41586-022-04446-1