(CNN) — Imagine a calm spring day 66 million years ago in what is now North Dakota. Perhaps a Triceratops was basking in the sun, while freshwater paddlefish, mouths open, searched for plankton in the river.
Seconds later, a 10-meter-high wall of water advances from the east and then crystal spheres begin to rain down from the sky, some of them still ablaze as they fall into the river.
These could have been the last moments of the age of the dinosaurs, which came to a cataclysmic end when a city-sized asteroid hit the shallow ocean off Mexico’s Yucatan Peninsula, wiping out three-quarters of all species. from the earth.
An artist’s impression of the Tanis River site in North Dakota, moments after the asteroid impact that doomed the dinosaurs to extinction 66 million years ago.
According to a new study of fossil fish found in the Tanis fossil bedin North Dakota, who perished as a result of the devastating impact, the asteroid collided in the spring.
The moment of impact, at least in the northern hemisphere, occurred at a particularly sensitive stage in the life cycle of many plants and animals.
This probably made what was already a disastrous event more catastrophic, said Melanie During, a doctoral student at Uppsala University, Sweden, lead author of a new study published this Wednesday in the academic journal Nature.
Melanie During excavating a paddlefish at the Tanis, North Dakota, site.
“I think spring puts a large group of Late Cretaceous biota (plant and animal life) at a very vulnerable point because they were out foraging, tending their young and trying to accumulate resources after the harsh winter,” he said. a news conference.
By contrast, the researchers said ecosystems in the southern hemisphere, which were in autumn when the asteroid collided with Earth, appear to have recovered almost twice as fast as those in the northern hemisphere.
A “shock that froze”
How did the researchers pinpoint the station where the asteroid hit?
Although 3,000 kilometers from the impact crater, the bones of paddlefish and sturgeon preserved in the rock at the Tanis site, in the Hell Creek Formationprovide a unique record of what was perhaps the most important event in the history of life on our planet.
The fish, up to a meter long, died in shocking fashion immediately after the asteroid hit, being buried alive by displaced sediments as a huge body of water was displaced upriver by the asteroid impact. Think of the ripples of water that occur when you drop a stone into a pond, but on a much larger scale.
Unlike tsunamis, which can take hours to reach land after a tidal wave, these moving bodies of water, known as seiche, arose immediately after the massive asteroid crashed into the sea.
A paddlefish from the Tanis fossil site.
The researchers are confident that the fish died within an hour of the asteroid’s impact, and not as a result of the massive wildfires or nuclear winter that ensued in the days and months that followed. This is because “impact spherules” – small bits of molten rock blasted into space from the crater, where they crystallized into a glass-like material – were found lodged in the gills of the fish.
“These impact spherules were ejected into space, and some of them may have circled the Moon and then rained down on Earth again,” During said.
“This deposit literally looks like a frozen shock. It’s the most violent thing I’ve seen so far, preserved in pristine condition,” he said of the fossil bed.
In addition, the fish were found just below a layer of rock known as the iridium anomaly, which is rich in a dense element common to asteroids and rare on Earth. This feature was the first revealed to geologists by an asteroid impact more than three decades ago.
geological “snapshot”
Like tree rings, fossilized fish skeletons preserve a diary of the animals’ growth, from their development as embryos to their untimely demise. Analysis of thin sheets of bone, as well as the distribution, shape and size of bone cells, which also fluctuate with the seasons, suggests that they died in the spring.
The team of researchers also examined the chemical signatures of different carbon isotopes in one of the unlucky paddlefish, and the ratio between the different isotope variants revealed how the availability of their favorite food, plankton, had affected its skeleton.
Isotopic records suggested that the annual growth of the fish, which would coincide with the peak availability of its prey in summer, had not yet been reached.
Section of fossilized paddlefish bones prepared for analysis at Vrije University, Brussels.
Other larger study from the same site, published last year, also pointed to a similar time frame in spring, while a much older study from 1991 on fossil leaves had suggested that it occurred in June.
During said he believed the asteroid impact likely occurred in April, but more research is needed for a definitive answer.
Alfio Alessandro Chiarenza, a postdoctoral researcher and paleontologist at the Universidade de Vigo, Spain, who was not involved in the study, said this type of research was very valuable to paleontologists.
“The Tanis site may offer some of the most important insights into understanding this mass extinction: since we paleontologists have to deal with very imprecise temporal resolution, having the opportunity to analyze an image like this of a geological event can further enhance our understanding of this pivotal event in (the) history of our planet.”