Science Gazette

The dinosaurs’ fate was determined in the spring

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The asteroid that wiped off almost all dinosaurs hit Earth in the springtime. After analyzing the remains of fish that died shortly after the meteorite hit the Earth, an international team of scientists from the Vrije Universiteit (VU) Amsterdam (The Netherlands), Uppsala University (Sweden), Vrije Universiteit Brussel (Belgium), and the ESRF, the European Synchrotron (France) determined when the meteorite hit the Earth. Their findings were just published in the journal Nature.

The so-called Chicxulub meteorite slammed into the Earth some 66 million years ago, in what is now Mexico’s Yucatán peninsula, signaling the end of the Cretaceous epoch and the extinction of dinosaurs. Scientists are still puzzled by this mass extinction since it was one of the most selective in the history of life: non-avian dinosaurs, pterosaurs, ammonites, and most marine reptiles perished, but mammals, birds, crocodiles, and turtles survived.

A collaboration of researchers from the Vrije Universiteit, Uppsala University, and the ESRF have now thrown light on the conditions behind the various extinctions. The answers were discovered in the bones of fish that perished shortly after the meteorite impacted.

When the meteorite collided with Earth, it shook the continental plate and created massive waves in rivers and lakes. Impact spherules (glass beads of Earth rock) poured down from the sky less than an hour after impact, engulfing and burying fish alive. Today, the Tanis event deposit in North Dakota (USA) maintains a fossilized environment that contains paddlefish and sturgeons, both of which were direct victims of the event.

The fossil fishes were remarkably well preserved, with essentially no indications of geochemical change in their bones. Melanie During, main author of the study and researcher from Uppsala University and the VU Amsterdam, visited onsite to unearth the valuable specimens: “It was evident to us that we needed to investigate these bones to acquire vital information about the time of the impact,” she adds.

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With a partial fish specimen and representative portions of the bones, the researchers went to the ESRF, a particle accelerator that generates the world’s brightest x-rays, and performed high-resolution synchrotron X-ray tomography.

The ESRF is the ideal instrument for studying these types of materials, and the facility has established unique palaeontology expertise over the previous two decades. “We discovered that the bones showed seasonal development, much as trees do, sprouting a new layer on the exterior of the bone every year,” says Sophie Sanchez of Uppsala University, a visiting scientist at the ESRF.

“The recovered growth rings not only preserved the fishes’ life histories, but also the final Cretaceous seasonality, and hence the season in which the terrible extinction happened,” says senior author Jeroen van der Lubbe of the VU in Amsterdam.

The X-ray scans also revealed the bone cells’ distribution, forms, and sizes, which are known to change with the seasons. “Bone cell density and volumes in all analyzed fishes may be tracked across numerous years, indicating whether it was spring, summer, autumn, or winter. Both cell density and volume were increasing but had not yet peaked during the year of death, suggesting that growth suddenly halted in the spring “Dennis Voeten, an Uppsala University researcher, agrees.

In addition to synchrotron radiation research, the researchers used carbon isotope analysis to determine a fish’s yearly dietary cycle. The availability of zooplankton, its preferred food, fluctuated throughout the year, peaking in the summer. The heavier 13C carbon isotope was concentrated in the skeleton of the fish as a result of the brief increase in ingested zooplankton compared to the lighter 12C carbon isotope. “The carbon isotope signal throughout this sad paddlefish’s development record reveals that the feeding season had not yet climaxed — death occurred in April,” says During.

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The results will enable future study into the mass extinction’s selectivity: it was spring in the Northern Hemisphere, which meant that species’ reproductive cycles were beginning, only to be suddenly terminated. In the meanwhile, fall had arrived in the Southern Hemisphere, and many species were presumably preparing for the coming winter. In general, it is generally established that creatures exposed to the substance perished very instantly. As a result, people hibernating in caves or burrows had a far better chance of surviving into the Paleogene. “Our findings will assist to explain why most dinosaurs went out while birds and early mammals escaped extinction,” During concludes.

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