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 in North Dakota's southwestern corner.
Death came from above that day.
Scientists said on Wednesday that 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 12 kilometres 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.
What happened right after impact
The researchers determined that it was springtime at the fossil site called the Tanis deposit — and throughout the northern hemisphere — based on sophisticated examinations of bones from three paddlefishes and three sturgeons that died within about 30 minutes of the impact that occurred 3,500 kilometres (2,200 miles) away.
They found evidence that a hail of glass pelted the site, after finding small spherules — molten material blasted by the impact into space that crystallized 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 paleontology doctoral student at Uppsala University in Sweden, and lead author of the research published in the journal Nature.
During compared the fossils deposited at Tanis to "a car crash frozen in place."
How scientists know it was a spring day
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 paddlefishes indicated that food availability was increasing as it does in springtime, but not at peak summer levels.
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 paleohistology.
In the southern hemisphere, it was autumn at the time, Sanchez noted, a season when many creatures prepare for the deprivations of winter.
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 paddlefishes and sturgeons, which survive to this day.
The Tanis fossils helped the researchers better understand the events following the impact, which left a crater about 180 kilometres (110 miles) wide at a Yucatan site called Chicxulub.
Most extinction caused by climate catastrophe that followed
The asteroid rocked the continental plate, generated earthquakes, sparked extensive wildfires, unleashed a massive shockwave 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.
These waves, carrying immense amounts of sediment and debris, inundated the Tanis site within approximately 15 to 30 minutes after the impact, burying alive all the inhabitants, including the fish whose fossils were studied.
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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.
