Death of the dinosaurs: doom for the extinct reptiles arrived in spring

  • A new study suggests animals and dinosaurs were wiped out by a massive meteor at a time when they were just emerging from hibernation
  • Among the few survivors were paddlefish and sturgeon, which survive to this day
Reuters |

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An artistic reconstruction of a dinosaur trying to escape from the killer seiche wave at the Tanis site, which occurred after an asteroid hit Earth. Illustration: Reuters

On a spring day 66 million years ago, paddlefish and sturgeon swam in a river that meandered through a flourishing landscape populated by mighty dinosaurs and small mammals at North Dakota’s southwestern corner. Death came from above that day.

Scientists said on Wednesday well-preserved fish fossils unearthed at the site are providing a deeper understanding of one of the worst days in the history of life on Earth and shedding light on the global calamity triggered by an asteroid 7.5 miles (12km) wide striking Mexico’s Yucatan Peninsula.

The ensuing mass extinction erased about three-quarters of Earth’s species, including the dinosaurs at the end of the Cretaceous Period, paving the way for mammals – eventually including humans – to become dominant.

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The researchers determined that it was springtime at the fossil site called the Tanis deposit – and throughout the northern hemisphere, including the spot where the asteroid hit – based on sophisticated examinations of bones from three paddlefish and three sturgeon that died within about 30 minutes of the impact that occurred 2,200 miles (3500km) away.

They found evidence that a hail of glass pelted the site, finding small spherules – molten material blasted by the impact into space that crystallised before falling back to Earth – embedded in fish gills.

The Tanis fossils also indicated that a huge standing wave of water swept through after the impact, burying the local denizens alive. Among the dinosaurs living in the Tanis area was apex predator Tyrannosaurus rex.

“Every living thing in Tanis on that day saw nothing coming and was killed almost instantaneously,” said Melanie During, a palaeontology doctoral student at Uppsala University in Sweden and lead author of the research published in the journal Nature.

During points at an image of a Cretaceous Period paddlefish fossil. Photo: Reuters

During compared the fossils deposited at Tanis to “a car crash frozen in place.”

Multiple lines of evidence pointed to a springtime impact.

Annual growth rings in certain fish bones – resembling those in tree trunks – showed increased growth levels associated with springtime after reduced growth in leaner winter months. Chemical evidence from one of the paddlefish indicated that food availability was increasing as it does in springtime, but not at peak summer levels.

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Springtime marks a time of growth and reproduction for many organisms.

“This season is crucial for the survival of species,” said study co-author Sophie Sanchez, an Uppsala University senior lecturer in palaeohistology.

In the southern hemisphere, it was autumn at the time, Sanchez noted, a season when many creatures prepare for the deprivations of winter.

The fossil of a Cretaceous Period paddlefish from the Tanis site in what is now southwestern North Dakota, United States. Photo: Reuters

Dinosaurs – aside from their bird descendants – went extinct, as did major marine groups, including the carnivorous reptiles that dominated the seas. Among the survivors were paddlefish and sturgeon, which survive to this day.

The Tanis fossils helped the researchers better understand the events following the impact, which left a crater about 110 miles (180km) wide at a Yucatan site called Chicxulub.

The asteroid rocked the continental plate, generated earthquakes, sparked extensive wildfires, unleashed a massive shock wave in the air and seismic waves on the ground, and spawned massive standing waves called seiche waves – perhaps hundreds of yards tall – in water bodies.

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These waves, carrying immense amounts of sediment and debris, inundated the Tanis site within about 15 to 30 minutes after the impact, burying alive all the inhabitants, including the fish whose fossils were studied.

The peril did not end that day. A cloud of dust enrobed Earth, precipitating a climate catastrophe akin to a “nuclear winter” that blocked sunlight for perhaps years, condemning countless species to oblivion.

“Although most of the extinction unfolded during the aftermath of the impact, which lasted much longer, zero hour – the exact timing of the impact – determined the course of the mass extinction,” said study co-author Jeroen van der Lubbe, a geochemist and paleoclimatologist at Vrije Universiteit Amsterdam in the Netherlands.

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