Fate of the dinosaurs sealed in spring

Date:
February 23, 2022
Source:
European Synchrotron Radiation Facility
Summary:
The asteroid that killed nearly all dinosaurs struck Earth during
springtime. An international team of scientists have determined
when the meteorite crashed onto the Earth, after analyzing the
remains of fishes that died just after the impact.



FULL STORY ==========================================================================
The asteroid that killed nearly all dinosaurs struck Earth during
springtime.

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), have determined when the meteorite crashed onto the Earth, after analysing the remains of fishes that died just after the impact. Their
results are published in the journal Naturetoday.


========================================================================== Around 66 million years ago, the so-called Chicxulub meteorite crashed
into the Earth, in what today is the Yucata'n peninsula in Mexico,
marking the demise of dinosaurs and end of the Cretaceous period. This
mass extinction still puzzles scientists today, as it was one of the most selective in the history of life: all non-avian dinosaurs, pterosaurs, ammonites, and most marine reptiles disappeared, whilst mammals, birds, crocodiles, and turtles survived.

A team of scientists from the Vrije Universiteit, Uppsala University,
and the ESRF have now shed light on the circumstances surrounding the
diverse extinction across the different groups. The answers came from
the bones of fishes that died moments after the meteorite struck.

When the meteorite impacted Earth, it rocked the continental plate and
caused huge waves in water bodies, such as rivers and lakes. These moved enormous volumes of sediment that engulfed fish and buried them alive,
while impact spherules (glass beads of Earth rock) rained down from the
sky, less than an hour after impact. Today, the event deposit of Tanis
in North Dakota (United States) preserves a fossilised ecosystem that
includes paddlefishes and sturgeons, which were direct casualties of
the event.

The fossil fishes were exceptionally preserved, with their bones showing
almost no signs of geochemical alteration. Melanie During, researcher
from Uppsala University and the VU Amsterdam and lead author of the publication, went onsite to excavate the precious specimens: "It was
obvious to us that we needed to analyse these bones to get valuable
information about the moment of the impact," she explains.

The team came to the ESRF, a particle accelerator that produces the
world's brightest x-rays, with a partial fish specimen and representative sections of the bones and carried out high-resolution synchrotron X-ray tomography.

The ESRF is the perfect tool to research this kind of samples and the
facility has developed unique expertise in palaeontology over the last
two decades.

"Thanks to the ESRF's data, we found that the bones registered seasonal
growth, very much like trees do, growing a new layer every year on the
outside of the bone," explains Sophie Sanchez of Uppsala University,
and visiting scientist at the ESRF.

"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 the VU in Amsterdam.

The X-ray scans also showed the distribution, shapes, and sizes of the
bone cells, which are known to fluctuate with the seasons as well. "In
all studied fishes, bone cell density and volumes can be traced over
multiple years and they indicate whether it was spring, summer, autumn,
or winter. We saw that both cell density and volumes were on the rise
but had not yet peaked during the year of death, which implies that
growth abruptly stopped spring" says Dennis Voeten, researcher at
Uppsala University.

In parallel to synchrotron radiation studies, the team carried out carbon isotope analysis to reveal the annual feeding pattern of a fish. The availability of zooplankton, its prey of choice, oscillated seasonally and peaked in summer. This temporary increase of ingested zooplankton enriched
the skeleton of the fish 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," asserts During.

The findings will aid future research into the selectivity of the mass extinction: in the Northern Hemisphere, it was spring and therefore
the reproduction cycles of organisms were starting, only to be abruptly stopped.

Meanwhile, it was autumn in the Southern Hemisphere, where many organisms
were likely preparing for winter. In general, it is well understood
that organisms who were exposed died virtually immediately. So those
sheltering in caves or burrows because they were hibernating were far
more likely to survive into the Paleogene. "Our results will help to
uncover why most of the dinosaurs died out while birds and early mammals managed to evade extinction," concludes During.

========================================================================== Story Source: Materials provided by
European_Synchrotron_Radiation_Facility. Note: Content may be edited
for style and length.


========================================================================== Journal Reference:
1. 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 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220223111233.htm

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