Fossil Site May Capture the Dinosaur-Killing Impact, but It’s Only the Beginning of the Story

The Tanis site in North Dakota contains evidence of the asteroid impact that killed off the dinosaurs

Fish Fossil
A partially exposed, 65-million-year-old fish from the Tanis deposit in North Dakota. Robert DePalma / University of Kansas

It may be considered one of the worst days in the history of life on Earth. Sixty-six million years ago, an immense asteroid smacked into what is now the Yucatán Peninsula of Mexico, triggering global devastation and the world’s fifth mass extinction. The non-avian dinosaurs, pterosaurs and coil-shelled squid cousins called ammonites disappeared completely. Even groups that survived, like mammals and lizards, suffered dramatic die-offs in the aftermath. Who perished, and who survived, set the stage for the next 66 million years—including our own origin 300,000 years ago.

The Chicxulub impact was a catastrophic transition into a new world. The distinctive rock layer it left behind, spiked with an element called iridium often found in asteroids and meteorites, marks the end of the Cretaceous period and the beginning of the Paleogene, known by experts as the K/Pg boundary. This line in the stone is also the marker for the end of the Age of Dinosaurs and the beginning of the Age of Mammals, a shift that has been intensely debated and studied for decades. Now a fossil site in North Dakota is causing a new stir, said to document the last minutes and hours of the dinosaurian reign.

The fossil assemblage, nicknamed Tanis after the real-life ancient Egyptian city referenced in Raiders of the Lost Ark, was first described in an article the New Yorker. Excavated and studied by University of Kansas graduate student Robert DePalma and a team of international collaborators, the site contains glassy spherules of material believed to have come from the impact event, thousands of miles away. Also embedded in the rock and debris, the New Yorker reported, are delicately preserved fossil fish, marine organisms far from the nearest sea, ancient plants, prehistoric mammals, and, perhaps most significantly, dinosaur bones, eggs and even feathers.

Many paleontologists were quick to raise an eyebrow at the findings presented in the New Yorker, however, particularly because some of the claims in the article are not mentioned in a scientific paper about the site. That research, published by DePalma and colleagues, was released Monday in the Proceedings of the National Academy of Science. The only dinosaur fossil mentioned in the paper is a weathered hip fragment, but the study is nevertheless causing a stir as a window into the extreme effects caused by the asteroid impact.

Fossil Site May Capture the Dinosaur-Killing Impact, but It’s Only the Beginning of the Story
Mass of articulated fish from the Tanis inundation surge deposit. Robert DePalma / University of Kansas

“Unfortunately, many interesting aspects of this study appear only in the New Yorker article and not in the scientific paper,” says Kirk Johnson, director of the Smithsonian’s National Museum of Natural History. “This is a sloppy way to conduct science and it leaves open many questions. At the present moment, interesting data are presented in the paper while other elements of the story that could be data are, for the moment, only rumors.”

As for the paper itself, the details are part of a broader picture of what transpired 66 million years ago in western North America, along the margins of a vanishing seaway that was draining off the continent at the time. According to DePalma and colleagues, seismic waves emanating from the asteroid impact reached the Tanis area within minutes. The disturbance sloshed local bodies of water in a phenomenon called a seiche—similar to water flowing back and forth in a bathtub—tossing fish and other organisms around in the wave. “As far as we can tell,” DePalma says in an email, “the majority of the articulated carcasses are from animals that were either killed when they were encapsulated by the muddy sediment, or very shortly prior as part of the same violent inundation surge event.”

In addition to articulated fish fossils with their scales still in place, the site contains shell fragments from seagoing mollusks called ammonites. DePalma and colleagues suspect that their presence is a sign that a previously unrecognized pocket of the Western Interior Seaway provided the water that ripped over the land and buried the Tanis site.

Sites demarcating the K/Pg boundary have been found all over the world, and vertebrate fossils at or within the boundary have also been discovered before. Part of what makes the Tanis site stand out, DePalma says, is that “this is the first known example of articulated carcasses, likely killed as a direct result of the impact, associated with the boundary.”

Despite the controversy over how claims of the site hit mass media before the peer-reviewed science paper was available, outside experts note that Tanis truly does seem to be an exceptional spot. “This isn’t the only site that preserves fossils at the K/Pg boundary, but it seems this might be the most sensational one ever discovered,” says Shaena Montanari, a paleontologist and AAAS science and technology policy fellow. The fossil preservation of the fish in particular stands out as unusual. “I thumbed through the pictures of the fossils included in the supplement and they look absolutely incredible,” Montanari says. Some of these fish have debris from the impact preserved in their gills, little pebbles of natural glass, perhaps sucked up from the water as the particles landed in ancient North Dakota shortly after the impact.

Fossil Site May Capture the Dinosaur-Killing Impact, but It’s Only the Beginning of the Story
Tiny spherules thought to have been ejected from the Chicxulub impact and deposited at the Tanis site in North Dakota. Robert DePalma / University of Kansas

Much of what makes Tanis exciting, according to University of New Mexico postdoctoral fellow James Witts, is that it offers a range of geologic clues about what happened after the impact. “This study convincingly links evidence from impact ejecta, sedimentology and geochemistry with well-dated physical remains of animals and plants that appear to have been alive right at the time of the impact event.” It could be a snapshot of life not thousands or hundreds of years before, but during the cataclysm that shook the Earth.

How Tanis was created is also something of a novelty. Geologists have studied disturbances that the Chicxulub impact caused at other sites, but these spots represent what happened in the ancient ocean and not on land. If DePalma and colleagues are correct, then seiche waves washing over terrestrial environments is another effect of the impact that hasn’t been examined before, depositing the remains of sea creatures where they otherwise had no business.

A number of additional mysteries remain about the site as well. The marine fossils, for example, might not have come from a nearby remnant of sea but could have been fossils when the asteroid struck, ripped up by the seismic and seiche waves that buried Tanis. “It has to remain an open question as to whether the ammonites were reworked out of rocks that would have essentially been the bedrock at Tanis, or [if] they come from a population that lived in a reduced seaway to the east of Tanis that we have no record of because of later erosion,” Witts says.

Other geologic details of the site also merit further investigation. “It seems like the geochemical data are scant and in some cases being stretched a bit to make interpretations,” Montanari says, “although this is not a new thing for paleontology.” These data points can be used to measure when and how quickly the Tanis site formed, critical details when attempting to determine what the site actually records. Montanari says that additional data points and analysis would strengthen the case that Tanis represents a very short window of the last Cretaceous moments. “We need to be sure we are developing rigorous hypotheses and then testing them with the available evidence rather than trying to craft a scenario that fits exactly what is uncovered,” Montanari says.

Fossil Site May Capture the Dinosaur-Killing Impact, but It’s Only the Beginning of the Story
Robert DePalma points to the K/Pg boundary impact fallout layer. Robert DePalma / University of Kansas

University of California, Berkeley paleontologist Pat Holroyd says that the estimations of when and how quickly the Tanis site formed are based on models without consideration of other possible interpretations. “I don't think there is any way to conclusively determine the exact amount of time represented in the site,” she says, “but it would have been useful to see how they estimated it.”

The details of what the site actually looks like, and how the layers were deposited, is not clear from what was published in the paper, Holroyd says. Such data is needed to compare Tanis to other K/Pg sites around the world. “Higher resolution images of the entire section would be of interest to many people as a resource for comparison to other types of deposits thought to be produced by seismic waves,” Holroyd says.

For now, Tanis is a localized phenomenon. It’s relevance to other sites in North America, and around the globe, awaits further study. “Seismic shaking from the impact could potentially have caused surges in other pockets far from the impact site, affecting that tapestry of microecologies as well,” DePalma says.

The site is also unique in that it appears to capture a small moment of geologic time. “It’s very tricky interpreting any rock outcrop as recording and preserving events operating on such a short timescale,” Witts says. The study does seem to show a rapid, violent event, but the details of the site will undoubtedly be further investigated and tested to see if the extraordinary claims hold up to scrutiny.

Witts hopes that the paper will help spur further discussion and analysis of other K/Pg sites around the globe. While geology is often thought of in terms of slow, gradual change, sometimes rapid transformation occurs. “I think Tanis reminds us geologists that sometimes it looks like the depositional stars align, and remarkable events could leave a signature preserved in the rock and fossil record,” he says.

Ultimately Tanis will be another part of a much broader story. The extinction at the end of the Cretaceous was a global event that played out over the course of days, weeks, months and years. Despite the fact that the site has been heralded as recording “the day the dinosaurs died,” there’s no way to know when the very last non-avian dinosaur went extinct. The last terrible lizard likely fell long after the events recorded at Tanis, likely in another part of the world.

DePalma says there is more to come from the Tanis site, and the mismatch between the claims made in the New Yorker article and the PNAS paper comes down to “triage” of what papers get priority. “We are already working on multiple follow-up papers and will be fully examining and reporting on everything found thus far,” he says.

The discussion about what Tanis means is only just beginning. “I’m sure paleontologists will be eager to see this material and do additional studies on Tanis,” Montanari says. “I can’t wait to see the rest of what’s to come.”