Skeptics greet fossils claiming to impact asteroids with dinosaur killing | Science


Robert DePalma

By Colin Barras

A fossil site in North Dakota records an incredibly detailed picture of the devastation minutes after an asteroid struck Earth about 66 million years ago, a group of paleontologists argue in a document due to be released this week. Geologists theorized that the impact, near the present-day city of Chicxulub, on the Yucatan Peninsula, Mexico, played a role in mass extinction at the end of the Cretaceous period, when all dinosaurs (except birds) and other life forms on Earth. disappeared.

If the team, led by Robert DePalma, a graduate student in paleontology at the University of Kansas in Lawrence, is correct, discovered a record of apocalyptic destruction 3000 miles from Chicxulub. At the site, called Tanis, the researchers say they have discovered the chaotic wreckage left when tsunamilike waves have risen up the river valley. Stuck in the rubble there is a confused confusion of fossils, including the freshwater sturgeon that apparently dies suffocated by vitreous particles falling from the sky by the fireball thrown by the impact.

"This is the first evidence of the interaction between life on the last day of the Cretaceous and the impact event," says team member Phillip Manning, a paleontologist at the University of Manchester in the United Kingdom. The depot may also provide some of the strongest evidence that non-pupal dinosaurs were still thriving on impact day.

"Outcrops like [this] are the reasons why many of us are attracted to geology, "says David Kring, a geologist at the Lunar and Planetary Institute in Houston, Texas, who was not a member of the research team. "These few feet of rock record the wrath of Chicxulub's impact and the devastation it has caused." But not everyone fully embraced the discovery, perhaps in part because it was announced to the world last week in an article in The New Yorker. The paper, in the Annals of the National Academy of Sciences (PNAS) does not include all the scientific claims mentioned in The New Yorker history, including that numerous dinosaurs as well as fish were buried on the spot.

"I hope it's all legitimate – I'm still not 100% convinced," says Thomas Tobin, a geologist at the University of Alabama at Tuscaloosa. Tobin says that the PNAS paper is densely packed with details of paleontology, sedimentology, geochemistry and more. "No one is an expert on all of these issues," he says, so it will take months for the research community to digest the results and assess if they support these extraordinary conclusions.

Robert DePalma

In the early 1980s, the discovery of a layer of iridium-rich clay, an element found in meteorites, at the end of the rocky Cretaceous record at sites around the world led researchers to link an asteroid to Cretaceous mass extinction. A wealth of other evidence persuaded most researchers that the impact played some role in the extinctions. But no one has found direct evidence of its lethal effects.

DePalma's team says the death was captured in forensic details at the 1.3-meter-thick Tanis depot, which she said was formed in just a few hours, starting perhaps 13 minutes after impact. While fish fossils are usually deposited horizontally in Tanis, fish carcasses and tree trunks are preserved at random, some in almost vertical orientations, suggesting that they were caught in a large volume of mud and sand that was poured almost instantly. The mud and sand are dotted with glassy beads-many of them stuck in the gills of the fish-dated isotopically 65.8 million years ago. Presumably they formed from molten rock droplets thrown into the atmosphere at the impact site, which cooled and solidified as they fell back to Earth. A layer of 2 centimeters thick rich in iridium denouncing the deposit.

Tanis at the time was located on a river that may have drained into the shallow sea covering much of what is today the east and south of the United States. DePalma's team argues that when the distant impact seismic waves reached Tanis minutes later, the tremor generated 10-meter waves that rose from the sea to the river valley, dumping sediment and freshwater and marine organisms there. These waves are called seiches: the Tohoku earthquake in 2011 near Japan triggered 1.5-meter-high fights in the Norwegian fjords 8,000 kilometers away.

DePalma and his colleagues have been working at Tanis since 2012. "Robert has been meticulous, archaeological boundary in his digging approach," says Manning, who works at Tanis from the start.

But others question DePalma's interpretations. "Capturing the event in such detail is quite remarkable," admits Blair Schoene, a geologist at Princeton University, but he says the site does not definitively prove that the impact event was the sole trigger of mass extinction. Schoene and some others believe that the environmental turbulence caused by volcanic activity on a large scale in what is now central India may have affected the previous impact.

Robert DePalma

Other geologists say they can not shake the suspicion about DePalma himself, who along with his Ph.D. The work is also curator of the Natural History Museum of Palm Beach in Wellington, Florida. His reputation suffered when, in 2015, he and his colleagues described a new genus of dinosaur called Dakotaraptor, found on a website near Tanis. Others later pointed out that the reconstructed skeleton includes a bone that actually belonged to a turtle; DePalma and his colleagues issued a correction.

DePalma may also disregard some norms of paleontology, according to The New Yorker, maintaining the rights to control their specimens even after they have been incorporated into collections of universities and museums. He supposedly helps finance his fieldwork by selling replicas of his discoveries to private collectors. "His line between commercial and academic work is not as clean as other people," says a geologist who asked not to be named. DePalma did not respond to an email request for an interview.

Manning declined to describe the details of DePalma's approach, but pointed out that all the fossils described in PNAS have been deposited in recognized collections and are available for other researchers to study. "It saddens me that people are so quick to overturn a study," he says. "That some competitors have pitched Robert in a negative light is unfortunate and unfair," says another coauthor, Mark Richards, a geophysicist at the University of California, Berkeley.

Manning confirms rumors that the study was initially submitted to a journal with a higher impact factor before being accepted in PNAS. He says the reviewers of the most influential periodical made unreasonable requests for an article that simply described Tanis's initial discovery and analysis. "After a while, we decided it was not a good way down," he says. The newspaper cleaned the peer review PNAS within about 4 months.

Several other works on Tanis are now in preparation, says Manning, and he hopes they will describe the dinosaur fossils mentioned in The New Yorker article. Its author, Douglas Preston, who learned of DePalma's discovery in 2013, wrote that the DePalma team found dinosaur bones trapped in the 1.3-meter-thick deposit, some so tall in the sequence that DePalma suspects that the carcasses were floating in water . Such a conclusion could provide the best evidence, however, that at least some dinosaurs were alive to witness the impact of the asteroid. But only one dinosaur bone is discussed in the PNAS study – and it is mentioned in a supplementary document, not in the document itself. This "disconnect" bothers Steve Brusatte, a paleontologist at the University of Edinburgh. "I just hope this was not super-sensational."

Until a few years ago, some researchers suspected that the last dinosaurs had disappeared thousands of years before the catastrophe. If Tanis is all that is claimed, this debate – and many others about this memorable moment in Earth's history – may be over.


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