A fossil snapshot of mass extinction | NOVA | PBS | NOVA

Sixty-six million years ago, the age of the dinosaurs came to an abrupt and cataclysmic end. On that fateful day, a multi-kilometer asteroid struck the Earth along the modern Mexican coast, digging a hole some thirty kilometers away and causing an explosion equivalent to about 10 billion bombs of Hiroshima. The fallout from the explosion, consisting of tiny particles of molten bedrock, has precipitated out of space, warming the global atmosphere to the temperature of a pizza oven. About 75 percent of plant and animal species, including non-avian dinosaurs, were cremated or died as a result of other causes, paving the way for the rise of birds, mammals and eventually we.

Today, Robert DePalma, a paleontologist at the Palm Beach Natural History Museum and a graduate student at the University of Kansas, claims to have unveiled an unprecedented time capsule of this apocalyptic event. With an international team of collaborators, DePalma describes the site of North Dakota, called Tanis, in an article published Monday in the newspaper PNAS.

In Tanis, according to the study, more than four meters of mud and sediment buried a dense entanglement of species. Among them are burnt tree trunks and hundreds of extraordinarily well-preserved fossil fish, forming a unique snapshot of the first minutes and hours of the disaster. "This document vividly describes in one place the devastation of this impact event," says David Kring, a geologist at Houston's Lunar and Planetary Institute, who was one of the critics of the newspaper.

While many experts recognize the potential importance of the site, some remain cautious, in part because the discovery of DePalma was first announced in a New Yorker article before the publication of the peer-reviewed document. the New Yorker colorful depicts a wealth of other amazing discoveries on the site, including dinosaur fossils, feathers, ant nests, mammalian burrows and even a pterosaur egg containing an intact embryo, none of which are mentioned in the PNAS paper. DePalma said The New York Times that the initial document was intended to establish the chronology of the site and that the publication of dinosaurs and other discoveries would be planned.

Apart from the dinosaurs, the evidence described in the document is certainly remarkable. Tanis is found in the Hell Creek Formation, one of the few places in North America with a succession of layers of rocks that overlap the last two million years of the Cretaceous and the transition to the Paleogene and the rise mammals. Hell Creek is famous for its finely preserved fossils, including leaves that seem freshly fallen. But the Tanis site has a unique cemetery.

At the time of the impact of the asteroid, Tanis was a sandy bar in a winding river that flowed in the west of the Inland Seaway, an immense inland sea that divided America from the North almost in two. In the scenario described in the document, the impact sent a pair of powerful waves of at least 10 meters high to the site, while molten bedrock glass beads, called tektites, were coming down. from the sky. Fragments of sea creatures have been swept away in Tanis mud, including ancient shark teeth and oceanic ammonite shells, which resemble today's nautilas.

In contrast, a mass of freshwater fish feeding on the bottom, including sturgeon and paddlefish six feet long, spawn chaos amidst a tangle of fossilized branches, head facing seaward against the mighty current of the torrent. The gills of more than half of the fish are clogged by tiny tektites that have fallen from the sky and then dipped into the water, suggesting that glassy millimeter-sized pearls or turbulence of the water wave salt could have killed the fish. Some of the larger tektites penetrate into multiple layers of sediment, suggesting that they have whipped waters still floating at the time of their fall; others were captured by amber sticky resin on fallen branches and trunks.

"Just the idea of ​​fish with impact particles stuck in the gills of 66 million years ago and trees with shock amber particles, it's so extraordinary that you make a double taken for sure, "Matthew Lamanna, vertebrate paleontologist at the Carnegie Museum of Natural History in Pittsburgh who was not involved in the research, said The New York Times. "With the caveat that what they're trying to show is really very hard to show, I think they've done a great job in defending their cause."

Tanis is definitely not the first site to capture the Cretaceous closure. But while preserving what appears to be an entire ecosystem trapped in the disaster, it clearly alludes to the far-reaching and instant devastation caused by the arrival of the asteroid.

"I certainly approached the question with skepticism, but to be honest, after reading the paper, I would have a hard time finding another explanation," Victoria Arbor, paleontology curator at the Royal BC Museum, who was not involved in the study, told Michael Greshko in National Geographic.

The evidence presented so far is essentially geological, but the results described in the article seem "very credible," says Steve Brusatte, paleontologist at the University of Edinburgh, who was not involved in the study. Given the "disconnection" between paper and New Yorker article, however, only time will tell what the site has to offer regarding the disappearance of dinosaurs, he says. "It's a very exciting discovery," says Brusatte, "but the site is still a mystery."

In Tanis, the mud layers containing the fossils are covered with a thin veneer of iridium-rich clay, an element abundant in meteorites, but relatively rare on Earth. This evidence and many others link the site directly to the impact of asteroids, but the question of the importance of the role played by the disaster in mass extinction will always fuel the debate. Other scientists have hypothesized that, due to prolonged volcanic activity or climatic fluctuations towards the end of the Cretaceous, the dinosaur world would have already been in a fragile state.

"We have a glimpse of what has happened catastrophically in this part of the world," said Kring. But, he adds, the evidence to date "does not really lend itself to a broader understanding of the massive extinction event, because killing some organisms in this part of the world would not have brought about extinction." World. Nevertheless, he paints a fascinating picture of what has happened in this part of the world. "