The dinosaurs were not in decline before the asteroid that wiped them out.

Sixty-six million years ago, the most iconic mass extinction in Earth's history occurred: that of the dinosaurs . While some researchers argue that their disappearance was a gradual process, others argue that it occurred suddenly after an asteroid impact. A team of researchers, led by geologist Andrew G. Flynn of New Mexico State University (USA), with Spanish participation, aims to shed light on this event, the debate over which has divided the scientific community for decades.

The research, published today in the journal Science , suggests strong evidence that dinosaurs remained diverse, active, and well-adapted until the end of the Cretaceous, just before the impact that ended their era. Based on a new geochronology of the Naashoibito Member, a fossil-rich rock unit within the Kirtland Formation in northern New Mexico, the analysis places its deposits between 66.4 and 66.0 million years ago, indicating that the fossil remains found there correspond to some of the last dinosaurs to have inhabited North America.

According to the authors, including Spanish researcher Jorge García Girón (University of León), the data on diversity and activity reinforce the idea that their disappearance was not a gradual process, but the result of a sudden and global event. This is how Flynn, lead author of the research, explains: “For years, it was thought that the western United States had a homogeneous fauna, a kind of uniform community of dinosaurs . But that was only because we didn't have good rock records from that time.” This geologist, an expert in paleoclimatology, continues: “Now, with two well-dated sites (Hell Creek and Kirtland), we see that they are very distinct from each other. They have different dinosaurs, which tells us that there were separate ecological regions in which different species thrived.”

Dinosaurs, the researcher says, constantly interacted with their environment, like any other group of animals. And it is this factor that explains the ecological differentiation and variety of species: “Our models show that the southern fauna preferred high temperatures. It was a warmer region, and that seems to be the main discriminating factor. Latitude plays a role, but it is linked to the climate.”

However, the New Mexico fossils are missing an important element: the layer that marks the asteroid's impact, the so-called K–Pg boundary, with its characteristic iridium deposit. "We're missing the ash and debris layer," Flynn explains. "We know our fossils date to the last 300,000 years of the Cretaceous, which, in geologic time, is close to the impact. Everything indicates that the dinosaurs were thriving until the event that wiped them out occurred."

Although Flynn acknowledges that this record is still limited, he is hopeful that new sites will shed light on this period of prehistory: “Our site in New Mexico is only the second in North America with well-controlled ages. But with each new site, especially outside the United States, such as in Asia, South America, or Europe, we will be able to reconstruct a more accurate picture of what dinosaur diversity was like just before their disappearance.”

According to Thomas Richard Holtz, an American paleontologist unrelated to the new research, it provides a solid foundation for the extinction debate. “The article takes several lines of evidence, primarily geological dating and a census of terrestrial vertebrates from the end of the age of dinosaurs in North America, and combines them to show that diversity did not decline toward the end of their reign,” he explains.

Holtz points out that the fossil record is, by nature, patchy. “Not every organism or species fossilizes, and not every place on the map will have fossils forming at the same time. So there are always questions about what the true diversity of a group was in the past.” But, he explains, this new study helps clarify an important question: the age of the Naashoibito Member. For years, there was debate about whether those deposits—with remains of Alamosaurus , a colossal herbivorous dinosaur—were older than the Hell Creek deposits in Montana, Wyoming, and the Dakotas. “The new study combines different types of geological data to show that it is, in fact, a Late Cretaceous deposit, equivalent in time to Hell Creek. In other words, there were dinosaurs that were part of the Naashoibito community and would have witnessed the effects of the final impact.”

Paleontologist Penélope Cruzado Caballero highlights a key advance: “What's interesting about this work is its multidisciplinary approach,” she explains. “They've not only analyzed the fossils and the environments in which they lived, but they've also incorporated very precise geochronological data. This allows us to place the remains on the brink of extinction.”

However, the researcher—an expert in the paleobiology of ornithopod dinosaurs and also unrelated to the new study—urges caution. “The article itself acknowledges: the results are based on a specific region of the world, relatively close to the impact area. The conclusions, therefore, should be taken with caution,” she warns. “In Spain, for example, in the Aragonese and Catalan Pyrenees, we have sites with similar dating, but with less faunal diversity because we still need to do fieldwork.”

Abrupt end or gradual decline?

Although he remains cautious, he recognizes a trend: “Saying it was black and white is difficult. Everything has nuances. But it's true that the latest studies are increasingly pointing to an immediate catastrophic impact. Although many years of research will still be needed to fully understand what happened.” In his opinion, the major task ahead is expanding the scope of research. “The work of Flynn and his team is extraordinary, but we need more studies like this in other regions of the world. If we can replicate this research elsewhere, we'll be able to compare results. We need a lot of comparability of data to better understand the context of what happened.”

Elena Cuesta, a specialist in the paleobiology of theropod dinosaurs, shares a similar opinion: “The article itself acknowledges it: we're still only talking about North America. We don't know what was happening in the rest of the planet,” she points out. “The United States has a great funding infrastructure and a very strong paleontological tradition. That's why iconic species like Tyrannosaurus rex and Triceratops are studied much more. But there are other regions with enormous potential to contrast or qualify these conclusions.” The specialist emphasizes that studies being conducted in countries like Argentina, where the last dinosaurs of Patagonia are being analyzed, could help balance the scales of a science that, according to her, has historically been very focused on the Northern Hemisphere: “These data could offer a view of the global south.”

Gerta Keller, a professor in the Department of Earth Sciences at Princeton University (USA), has expressed her disagreement with the study's conclusions. "That the disappearance of the dinosaurs was abruptly triggered by an asteroid impact at the end of the Cretaceous was a theory first proposed in 1980 by Nobel Prize winner Luis Álvarez and his son Walter with great enthusiasm. Since then, the theory has been repeated with little or no change, and most people believe it. The article is of poor quality because it omits a lot of information," she comments.

Keller advocates an alternative explanation, based on climate change and volcanic eruptions in the Deccan Mountains of India. According to the researcher, this led to the long-term decline of the dinosaurs for 300,000 to 400,000 years. She also argues that the dating of the impact of the large asteroid has been misinterpreted: "The true age of the Chicxulub crater revealed that it crashed in Yucatán 200,000 years before the mass extinction of the dinosaurs 66 million years ago." She emphasizes: "It was a simple incident in our universe, while life continued as usual."

According to Cuesta, the most valuable aspect of this work is its demonstration that science is alive. “Each new fossil, each new dating can change what we thought we knew. Quite robust hypotheses can suddenly be dismantled years later, giving rise to new ones. We must continue exploring, excavating, and funding science. Only then will we have a deeper understanding of this period in Earth's history.”