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Ancient WorldsAncient Worlds

New Fossils Might Capture the Moment of Mass Extinction That Wiped Out the Dinosaurs

Fossils uncovered in North Dakota appear to preserve the catastrophic fallout of the asteroid impact that wiped out the dinosaurs–but experts remain cautious.

ByEvan Hadingham, Katherine J. WuNOVA NextNOVA Next
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Kylie Ruble and Robert DePalma excavate fossils from the Tanis site. Image courtesy of Robert DePalma, University of Kansas

Sixty-six million years ago, the dinosaur age came to an abrupt and cataclysmic end. On that fateful day, a miles-wide asteroid slammed into Earth along the coast of modern Mexico, punching a hole some 20 miles deep and triggering an explosion equivalent to about 10 billion Hiroshima bombs. Fallout from the blast, consisting of tiny particles of melted bedrock raining down from space, heated the global atmosphere to the temperature of a pizza oven. About 75 percent of plant and animal species, including the nonavian dinosaurs, were incinerated or perished from other causes in the aftermath, clearing the way for the rise of birds, mammals and, ultimately, us.

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

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At Tanis, according to the study, over four feet of mud and sediment buried a dense tangle of species. Among them are scorched tree trunks and hundreds of extraordinarily well-preserved fossil fish, forming a unique snapshot of the first minutes and hours of the catastrophe. “This paper captures in one place, vividly, the devastation of this impact event,” says David Kring, a geologist at Houston’s Lunar and Planetary Institute, who was one of the paper’s reviewers.

While many experts acknowledge the potential importance of the site, some remain cautious, due in part to the fact that DePalma’s discovery was first announced in a New Yorker article before publication of the peer-reviewed paper. The New Yorker colorfully describes a wealth of other astonishing finds at the site, including dinosaur fossils, feathers, ant nests, mammal burrows, and even a pterosaur egg containing an intact embryo, none of which are mentioned in the PNAS paper. DePalma told The New York Times that the initial paper was intended to establish the site’s chronology, and that publication of the dinosaurs and other finds would be forthcoming.

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A mass of fossilized fish from the Tanis site in North Dakota. Image courtesy of Robert DePalma, University of Kansas

Dinosaurs aside, the evidence described in the paper is certainly remarkable. Tanis lies within the Hell Creek Formation, one of the few locations in North America with a sequence of rock layers that straddles the final two million years of the Cretaceous period and the transition to the Paleogene and the rise of mammals. Hell Creek is famous for its exquisitely preserved fossils, including leaves that look freshly fallen. But the Tanis site holds a graveyard unlike anything unearthed before.

At the time of the asteroid impact, Tanis was a sandy bar in a meandering river that flowed into the Western Interior Seaway, a huge inland sea that split North America nearly in two. In the scenario outlined in the paper, the impact sent a pair of powerful waves at least 30 feet high surging over the site, while glassy beads of molten bedrock, called tektites, hailed down from the sky. Fragments of seafaring creatures were swept into the Tanis mud, including the teeth of ancient sharks and the shells of ocean-dwelling ammonites, which resemble today’s nautili.

In startling contrast, a mass of freshwater bottom-feeding fishes, including sturgeon and six-foot-long paddlefish, are strewn in chaos among a logjam of fossilized branches, their heads pointed seawards against the powerful flow of the incoming torrent. The gills of over half the fish are clogged with tiny tektites that had fallen from the sky and then sunk through the water, suggesting that the glassy, millimeter-sized beads—or the turbulence of the saltwater surge—might have killed the fish. Some of the bigger tektites penetrate multiple layers of sediment, hinting that they pelted still-sloshing waters as they fell; others were captured by sticky amber tree resin on downed branches and trunks.

“Just the idea of fish with impact particles stuck in their gills from 66 million years ago, and trees with amber with impact particles, it’s so extraordinary that you do a double take for sure,” Matthew Lamanna, a vertebrate paleontologist at the Carnegie Museum of Natural History in Pittsburgh who was not involved with the research, told The New York Times. “With the caveat that what they’re trying to show is really, really hard to show, I think they’ve done an excellent job of making that case.”

Tanis is certainly not the first site to capture the close of the Cretaceous. But in preserving what appears to be an entire ecosystem caught up in the catastrophe, it hints strongly at the wide-reaching and instantaneous devastation spurred by the asteroid’s arrival.

“I certainly was approaching it from a place of skepticism, but to be honest, having read through the paper, I’d be personally hard-pressed to come up with an alternate explanation,” Victoria Arbour, paleontology curator at the Royal B.C. Museum, who was not involved with the study, told Michael Greshko at National Geographic.

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A microscope slide containing tektites from the Tanis site. Image courtesy of Robert DePalma, University of Kansas

The evidence presented so far is primarily geological, but the findings described in the paper seem “very credible,” says Steve Brusatte, a paleontologist at the University of Edinburgh who was not involved in the study. Given the “disconnect” between the paper and the New Yorker article, however, only time will tell what the site has to offer with regards to the demise of the dinosaurs, he says. “This is a very exciting discovery,” Brusatte says, “but there’s still a real mystery about the site.”

At Tanis, the mud layers containing the fossils are capped by a thin clay veneer rich with iridium, an element abundant in meteorites, but relatively rare on Earth. This and many other lines of evidence link the site directly to the asteroid impact, but the question of how pivotal a role the catastrophe played in the mass extinction will still stoke debate. Other scientists have theorized that, due to prolonged volcanic activity or climate fluctuations toward the end of the Cretaceous, the world of the dinosaurs might already have been in a fragile state.

“We get a snapshot of what happened, catastrophically, in this part of the world,” Kring says. But, he adds, the evidence so far “doesn't really lend itself to a broader understanding of the mass extinction event, because killing a few organisms in this part of the world wouldn’t have caused the global extinction. Nonetheless, it paints a fascinating picture of what happened in this part of the world.”

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