courtesy of Robert DePalma
By Colin Barras
A fossil site in North Dakota records a stunningly detailed picture of the devastation minutes after an asteroid slammed into Earth about 66 million years ago, a group of paleontologists argue in a paper due out this week. Geologists have theorized that the impact, near what is now the town of Chicxulub on Mexico’s Yucatán Peninsula, played a role in the mass extinction at the end of the Cretaceous period, when all the dinosaurs (except for birds) and much other life on Earth vanished.
If the team, led by Robert DePalma, a graduate student in paleontology at the University of Kansas in Lawrence, is correct, it has uncovered a record of apocalyptic destruction 3000 kilometers from Chicxulub. At the site, called Tanis, the researchers say they have discovered the chaotic debris left when tsunamilike waves surged up a river valley. Trapped in the debris is a jumbled mess of fossils, including freshwater sturgeon that apparently choked to death on glassy particles raining out of the sky from the fireball lofted by the impact.
“That’s the first ever 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 deposit may also provide some of the strongest evidence yet that nonbird dinosaurs were still thriving on impact day.
“Outcrops like [this] are the reasons many of us are drawn to geology,” says David Kring, a geologist at the Lunar and Planetary Institute in Houston, Texas, who wasn’t a member of the research team. “Those few meters of rock record the wrath of the Chicxulub impact and the devastation it caused.” But not everyone has fully embraced the find, perhaps in part because it was first announced to the world last week in an article in The New Yorker. The paper, in the Proceedings of the National Academy of Sciences (PNAS), does not include all the scientific claims mentioned in The New Yorker story, including that numerous dinosaurs, as well as fish were buried at the site.
“I hope this is all legit—I’m just not 100% convinced yet,” says Thomas Tobin, a geologist at the University of Alabama in Tuscaloosa. Tobin says the PNAS paper is densely packed with detail from paleontology, sedimentology, geochemistry, and more. “No one is an expert on all of those subjects,” he says, so it’s going to take a few months for the research community to digest the findings and evaluate whether they support such extraordinary conclusions.
courtesy of Robert DePalma
In the early 1980s, the discovery of a clay layer rich in iridium, an element found in meteorites, at the very end of the rock record of the Cretaceous at sites around the world led researchers to link an asteroid to the End Cretaceous mass extinction. A wealth of other evidence has 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 killing is captured in forensic detail in the 1.3-meter-thick Tanis deposit, which it says formed in just a few hours, beginning perhaps 13 minutes after impact. While fish fossils are normally deposited horizontally, at Tanis, fish carcasses and tree trunks are preserved haphazardly, some in near vertical orientations, suggesting they were caught up in a large volume of mud and sand that was dumped nearly instantaneously. The mud and sand are dotted with glassy spherules—many caught in the gills of the fish—isotopically dated to 65.8 million years ago. They presumably formed from droplets of molten rock launched into the atmosphere at the impact site, which cooled and solidified as they plummeted back to Earth. A 2-centimeter-thick layer rich in telltale iridium caps the deposit.
Tanis at the time was located on a river that may have drained into the shallow sea covering much of what is now the eastern and southern United States. DePalma’s team argues that as seismic waves from the distant impact reached Tanis minutes later, the shaking generated 10-meter waves that surged from the sea up the river valley, dumping sediment and both marine and freshwater organisms there. Such waves are called seiches: The 2011 Tohoku earthquake near Japan triggered 1.5-meter-tall seiches in Norwegian fjords 8000 kilometers away.
DePalma and his colleagues have been working at Tanis since 2012. “Robert has been meticulous, borderline archaeological in his excavation approach,” says Manning, who has been working at Tanis from the beginning.
But others question DePalma’s interpretations. “Capturing the event in that much detail is pretty remarkable,” concedes Blair Schoene, a geologist at Princeton University, but he says the site does not definitively prove that the impact event was the exclusive trigger of the mass extinction. Schoene and some others believe environmental turmoil caused by large-scale volcanic activity in what is now central India may have taken a toll even the before impact.
courtesy of Robert DePalma
Other geologists say they can’t shake a sense of suspicion about DePalma himself, who along with his Ph.D. work is also a curator at the Palm Beach Museum of Natural History in Wellington, Florida. His reputation suffered when, in 2015, he and his colleagues described a new genus of dinosaur named Dakotaraptor, found in a site close to Tanis. Others later pointed out that the reconstructed skeleton includes a bone that really belonged to a turtle; DePalma and his colleagues issued a correction.
DePalma may also flout some norms of paleontology, according to The New Yorker, by retaining rights to control his specimens even after they have been incorporated into university and museum collections. He reportedly helps fund his fieldwork by selling replicas of his finds to private collectors. “His line between commercial and academic work is not as clean as it is for other people,” says one 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 points out that all fossils described in the PNAS paper have been deposited in recognized collections and are available for other researchers to study. “It saddens me that folks are so quick to knock a study,” he says. “That some competitors have cast Robert in a negative light is unfortunate and unfair,” says another co-author, 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 it was accepted at PNAS. He says the reviewers for the higher-profile journal made requests that were unreasonable for a paper that simply outlines the discovery and initial analysis of Tanis. “After a while, we decided it wasn’t a good route to go down,” he says. The paper cleared peer review at PNAS within about 4 months.
Several more papers on Tanis are now in preparation, Manning says, and he expects they will describe the dinosaur fossils that are mentioned in The New Yorker article. Its author, Douglas Preston, who learned of the find from DePalma in 2013, writes that DePalma’s team found dinosaur bones caught up in the 1.3-meter-thick deposit, some so high in the sequence that DePalma suspects the carcasses were floating in the roiling water. Such a conclusion might provide the best evidence yet that at least some dinosaurs were alive to witness the asteroid impact. But just one dinosaur bone is discussed in the PNAS study—and it is mentioned in a supplement document rather than in the paper itself. That “disconnect” bothers Steve Brusatte, a paleontologist at the University of Edinburgh. “I just hope this hasn’t been over-sensationalized.”
Until a few years ago, some researchers had suspected the last dinosaurs vanished thousands of years before the catastrophe. If Tanis is all it is claimed to be, that debate—and many others about this momentous day in earth history—may be over.
Source: Science Mag