A fossil site can capture the impact of dinosaur death, but this is just the beginning of the story | Science

This can be considered one of the worst days in the history of life on Earth. Sixty-six million years ago, a huge asteroid crashed into what is today the Yucatán peninsula in Mexico, causing world devastation and the world's fifth mass extinction. . Non-avian dinosaurs, pterosaurs and cousins ​​of spiral-shelled squid called ammonites have completely disappeared. Even surviving groups, such as mammals and lizards, have suffered dramatic deaths as a result. The people who perished and who have survived have set the stage for the next 66 million years, including our own origin, 300,000 years ago.

The impact of Chicxulub has been a catastrophic transition to a new world. The distinct layer of rock she left behind, enriched with an element called iridium often found in asteroids and meteorites, marks the end of the Cretaceous and the beginning of the Paleogene, known to experts as the K / Pg limit. This line in the stone is also the marker of the end of the dinosaur era and the beginning of the age of mammals, a change that has been intensely debated and studied for decades. Now, a fossil site in North Dakota is stirring a new stir, documenting the last minutes and the last hours of dinosaur rule.

The fossil assemblage, nicknamed Tanis after the Egyptian city of real life referenced in The adventurers of the lost archhas been described for the first time in an article by New Yorker. Rummaged and studied by Robert DePalma, a graduate student of the University of Kansas, and a team of international collaborators, the site contains glassy spherules made of materials believed to come from impact, thousands of miles away . Also embedded in rock and debris, the New Yorker reported, are delicately preserved fossil fish, marine organisms away from the nearest sea, ancient plants, prehistoric mammals and, perhaps most significantly, dinosaur bones, eggs and even feathers .

Many paleontologists quickly raised an eyebrow at the results presented in the New YorkerHowever, especially because some of the claims of the article are not mentioned in a scientific article on the site. This research, published by DePalma and colleagues, was published on Monday in the Proceedings of the National Academy of Sciences. The only dinosaur fossil mentioned in the document is an altered hip fragment, but the study nevertheless gives rise to agitation, which gives an idea of ​​the extreme effects caused by the impact of the asteroid.

"Unfortunately, many interesting aspects of this study only appear in the New Yorker article, not in the scientific paper, "says Kirk Johnson, director of the Smithsonian National Museum of Natural History. "It's a sloppy way of doing science and it leaves many questions unanswered. At the present time, interesting data is presented in the document, while other elements of history that could be data are, for the moment, only rumors. "

As for the document itself, the details are part of a more general picture of what happened 66 million years ago in the west of the country. 39 North America, on the sidelines of an endangered seaway that was retreating from the continent at the time. According to DePalma and his colleagues, seismic waves emanating from the asteroid impact reached the Tanis region within minutes. The disturbance disrupted local water bodies in a phenomenon called squid – similar to water flowing into a bathtub – throwing fish and other organisms around the wave. "As far as we can know," says DePalma in an email, "the majority of articulated carcasses come from animals that were killed when they were encapsulated by muddy sediment or very little time previously, as part of the same wave of violent flooding. Event."

In addition to articulated fish fossils whose scales are still in place, the site contains shell fragments from sea molluscs called ammonites. DePalma and his colleagues suspect that their presence is a sign that part of the Inner Westerly Interior Route of the Inland Seaway, hitherto unknown, has supplied water that has torn the ground and buried the site of Tanis.

Sites demarcating the K / Pg boundary have been discovered around the world and vertebrate fossils located within or within the boundary have also been discovered before. According to DePalma, part of what distinguishes the Tanis site is that it is the first known example of articulated carcasses, probably killed as a direct consequence of the impact, associated with the boundary.

Despite the controversy over how the site's allegations were heard by the media before the peer-reviewed scientific paper was available, outside experts noted that Tanis really seemed to be an exceptional place. "This is not the only site that holds fossils at the K / Pg limit, but it seems to be the most sensational ever discovered," says Shaena Montanari, paleontologist and researcher in science and technology policy at the K-Pg. AAAS. The fossil preservation of fish in particular is unusual. "I've flipped through the fossil images included in the supplement and they look absolutely amazing," says Montanari. Some of these fish have impact debris preserved in their gills, small pebbles of natural glass, perhaps aspirated by the water while the particles landed in the former North Dakota shortly after the impact.

According to postdoctoral fellow James Witts of the University of New Mexico, Tanis is primarily passionate about the fact that it offers a range of geological clues about what happened after the # 39; impact. "This study convincingly combines the evidence of impact ejections, sedimentology and geochemistry with well-dated physical remains of animals and plants that appeared to be alive at the time of the study." impact. "It could be a snapshot of life, not thousands or hundreds. years ago, but during the cataclysm that shook the Earth.

The creation of Tanis is also a novelty. Geologists have studied the disturbances caused by the impact of Chicxulub on other sites, but these spots represent what happened in the ancient ocean and not on Earth. If DePalma and his colleagues are right, the cuttlefish waves sweeping the terrestrial environments is another effect of the impact that has not been examined before, depositing the remains of sea creatures where they would not have had business. .

A number of additional mysteries also remain on the site. Marine fossils, for example, might not come from a nearby sea remnant, but could be fossils when the asteroid struck, torn apart by the seismic and cuttle waves that buried Tanis. "The question of whether the ammonites were reworked from rocks that would have been essentially the bedrock of Tanis must remain an open question. [if] they come from a population that lived in a reduced sea lane east of Tanis and that we have no trace of subsequent erosion, "says Witts.

Other geological details of the site also deserve further investigation. "It seems that geochemical data are scarce and sometimes stretched a little to allow interpretations," says Montanari, "although this is not new to paleontology." These data points can be used to measure when and with what speed The Tanis site is formed, critical details to try to determine what the site actually records. Montanari says that additional data points and analysis would strengthen the fact that Tanis represents a very short window of the last Cretaceous moments. "We need to be sure that we develop rigorous assumptions and then test them with available evidence rather than trying to create a scenario that exactly matches what is discovered," Montanari said.

Pat Holroyd, a palaeontologist at the University of California at Berkeley, explains that estimates of the date and rate of formation of the Tanis site are based on models without consideration of alternative interpretations. "I do not think it's possible to conclusively determine the exact time shown on the site," she says, "but it would have been helpful to see how they had estimated it."

The details of what the site really looks like, and how the layers were deposited, are unclear from what has been published in the newspaper, says Holroyd. Such data is needed to compare Tanis with other K / Pg sites around the world. "The high-resolution images of the whole section would interest many people as a resource for comparison with other types of deposits thought to be produced by seismic waves," says Holroyd.

For the moment, Tanis is a localized phenomenon. Its relevance to other sites in North America and around the world deserves further study. "Seismic shocks caused by the impact could potentially have caused outbreaks in other pockets far from the site of impact, also affecting this mosaic of microecologies," says DePalma.

The site is also unique in that it seems to capture a little geological moment. "It's very difficult to interpret a rock outcrop as a recording and preservation of events operating over such a short period of time," says Witts. The study appears to reveal a violent and rapid event, but the details of the site will undoubtedly be further investigated and tested to determine whether the extraordinary requests withstand scrutiny.

Witts hopes the paper will help stimulate discussion and analysis of other K / Pg sites around the world. While geology is often thought of in terms of slow and gradual changes, sometimes rapid transformations occur. "I think Tanis reminds geologists that sometimes it seems like the stars of the repositories line up, and that some remarkable events could leave a preserved signature in the rock and fossil record," he says.

In the end, Tanis will be another part of a much larger story. The extinction at the end of the Cretaceous was a global event that took place over days, weeks, months and years. Although the site was announced as recording "dinosaur death day", there is no way to know when the last non-avian dinosaur died out. The last terrible lizard probably fell long after the events recorded in Tanis, probably in another part of the world.

DePalma says the Tanis site still needs to provide more information and that the imbalance between the statements made in the New Yorker article and the PNAS paper comes down to "triage" of what papers take precedence. "We are already working on several follow-up documents and will review and report on everything that has been found so far," he said.

The discussion of what Tanis means is just beginning. "I'm sure paleontologists will be eager to see this material and do further studies on Tanis," says Montanari. "I can not wait to see the rest of what will happen."