New fossils may capture the minutes after the dinosaur-killing asteroid impact

About 66 million years ago, a giant asteroid smashed into Earth off the coast of what’s now Mexico. Less than an hour later, a riverbed 3,000 kilometers away sloshed violently back and forth, swiftly burying fish, plants and other organisms in the sediment, a study finds. Evidence of those surges, as well as tiny traces of the impact itself, appear to be preserved in a meter-thick layer of rock in southwestern North Dakota.

Set off by the impact, an immense earthquake — equivalent to a magnitude 10 or even 11.5 — sent seismic waves pulsing through Earth’s crust, triggering the sloshing, researchers argue online April 1 in the Proceedings of the National Academy of Sciences. If true, the scenario would add a new kill mechanism to the mass extinction event that marks the boundary between the Cretaceous and Paleogene Periods, often called the K-Pg. At least 75 percent of species, including all nonbird dinosaurs, died out (SN: 2/4/17, p. 16).

The site, found in the Hell Creek Formation and dubbed Tanis, represents a unique snapshot of what happened on land in the immediate aftermath of the impact, says paleontologist Robert DePalma of the University of Kansas in Lawrence.

“It’s a critical moment in time,” he says. “We have a high-resolution image of the first couple of hours after the impact. That level of detail is not really known elsewhere.”

Despite the potential uniqueness of the find, the publication of the PNAS paper was overshadowed by a profile of DePalma published online March 29 in the New Yorker, just a few days before the study’s planned release. The profile included tantalizing hints of fossilized dinosaurs and pterosaurs and even rarely preserved feathers that the researchers say they found at Tanis. If so, the site could hold the answer to a much larger question: Was it really the asteroid strike that killed dinosaurs, or were they already dying out? The new paper doesn’t discuss those fossils, and paleontologists have expressed skepticism and frustration over how to evaluate the claims.

Moment in time

The paper in PNAS does provide evidence that the fossil site opens a window on a key time in Earth’s history. At Tanis, a river once drained eastward from a vast inland sea. Sandy deposits reveal where the meandering river carved a deep channel into the rock. Above that channel lies an unusual rock sequence that DePalma and his colleagues call the “event deposit.” That 1.3-meter-thick layer has two distinct sublayers. The bottom layer has large pebbles at its base and finer-grained sediment toward the top, ending in fine silt. Overlying that is another layer that starts with large sand grains and then gets finer toward the surface. This pattern, and the direction of water flow preserved by the grains, point to massive inundation, DePalma says.

The deposit also contains tiny glass spheres, remnants of vaporized rock cast into the atmosphere from the impact that then rained back down potentially thousands of kilometers away. Fossils are also abundant in the deposit, particularly bits of logs and groups of fish skeletons. The fish, the researchers suggest, may have died en masse after becoming rapidly buried by mud displaced during the inundation. Some of the fish’s gills contain the tiny spherules, possibly snagged from the water just before death.

Above the event deposit is a thin layer of volcanic ash-turned-clay that also is found in other parts of the central United States. This layer contains impact spherules and dates to the K-Pg, helping connect the Tanis site to the extinction event.

Because a vast shallow sea covered much of the ancient central United States at one time, the team first suspected that the back-and-forth sloshing indicated that a giant tsunami had swept northward from the Gulf of Mexico in the wake of the impact. But it’s not clear how large the sea was or if it still existed at the time of the impact. And later calculations by DePalma’s team suggested that such a tsunami would have taken at least 18 hours to travel from the Gulf to Tanis.

However, the spherules found embedded in the event deposit indicate that the powerful wave action must have occurred nearly instantly after the impact, DePalma says. Even a swift tsunami wouldn’t have been that fast. Instead, the team suggests, strong seismic waves might have shaken up a local body of water, such as a river or lake, producing the deposits.

Plausible past

The team convincingly argues that the whole sequence of events took only a few hours, says Paul Olsen, a paleontologist and geologist at Columbia University’s Lamont-Doherty Earth Observatory in Palisades, N.Y. Fish consumed the spherules that rained into the water, became entombed in sediment displaced during the sloshing of the water, and were then covered by a second layer of sediment bearing iridium, an element found in asteroids. “I think they’ve got that nailed. It’s hard to imagine how that would occur any other way,” Olsen says.

Jessica Whiteside, a geochemist at the University of Southampton in England, agrees that the sedimentary evidence supports the idea that the impact produced violent sloshing. And the possibility that a massive quake provoked those waves is plausible, she adds. As the authors say in the study, the 2011 magnitude 9.2 Tohoku earthquake in Japan is known to have triggered a 1.5-meter-high waves in a Norwegian fjord some 8,000 kilometers away.

“But it’s not the only plausible sequence of events that could have happened,” Whiteside says. And there may be no way to know for sure if the scenario is the right one, or exact timing of the seismic waves’ arrival, because there are so many unknowns about the lay of the land 66 million years ago.

“That said, I find it really exciting work,” Whiteside says.

Antoine Bercovici, a paleobotanist and sedimentologist at the Smithsonian Institution in Washington, D.C., has worked in North Dakota for years, studying the K-Pg boundary within the Hell Creek Formation. He agrees that the sedimentary evidence described in the study is compelling. “It’s extraordinary that [the sediments of] this oxbow lake were able to record in a detailed way the moment of the impact,” he says. “It would be hard to find another place like that.”

The fish fossils, he adds “are pretty amazing,” and their preservation is exceptional. Yet, he says, he’s a bit skeptical that the haphazard orientation of the fossils definitively represents a snapshot of mass death right as the waves impacted the animals, spherules still held in their mouths. “It’s a bit dramatic and hard to verify,” he says.

Dino drama

But the drama of a mass fish grave is minimal compared with the reaction to some of the other fossils DePalma says that he has found at Tanis, as described in the New Yorker profile: a mammal burrow, dinosaur feathers, a ceratopsian hip bone with a skin impression (the ceratopsian fossil is briefly mentioned, though not officially described or pictured in detail, in the supplemental material accompanying the PNAS paper).

A general dearth of dinosaur fossils dating to just before the impact has led some scientists to speculate that the animals were already vanishing before the asteroid hit. So the discovery of dino fossils at Tanis could help prove the impact was the culprit after all.

That wouldn’t, in itself, be all that surprising. Most paleontologists already think that the strike killed the dinosaurs. Still, such extraordinary claims for what Tanis holds require extraordinary evidence, says Thomas Holtz, a paleontologist at the University of Maryland in College Park. “I think a lot of the reaction to this stems from the particulars of how the announcement came out,” Holtz says, referring to the online publication of the New Yorker article a few days ahead of the PNAS study. “Perhaps [those claims] are indeed accurate, but if so [the researchers] should either have been included in the initial paper or they should not be reporting it so far in advance if they aren’t ready to back up their claims with data.”

DePalma and colleagues’ data on those tantalizing dinosaur fossils remain unpublished, for now. The PNAS paper “is not a paper about dinosaurs,” DePalma says, adding that he didn’t want the information about the dinosaur fossils to appear in the New Yorker, either. “Of course we’d seek to keep that under wraps; we want to publish it later,” he says.

Whether or not Tanis contains dinosaur fossils, the site is still interesting, Bercovici says. “Tanis is the first unequivocal site of this kind,” in that it represents the first minutes of impact, he says. Furthermore, he adds, although dinosaurs tend to take center stage in the popular imagination, “the K-Pg extinction is not a nonavian dinosaur story. It involved the demise of entire and complex ecosystems.”

And as interesting as Tanis is, he adds, scientists studying the K-Pg extinction have many lingering questions — who died, who survived, was the extinction gradual or abrupt, was the recovery fast or slow — that extend far beyond that one snapshot in time. After all, the majority of the dying happened later, over the ensuing decades.

“The immediate effect of the impact certainly killed many plants and animals,” he adds. But what really placed particular pressures on different ecosystems, and ultimately led to mass extinctions, were the longer-term effects on the climate: dramatic cooling and warming, the acid rain, the emission of soot and the darkening of the sky, he says.

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Editor’s note: This story was updated April 8, 2019, to correct how many species died in the K-Pg extinction, and where glass spheres were found in the fish fossils. At least 75 percent of all species perished, not just land creatures, and the spheres were in the fish’s gills, not stomachs.