The most iconic mass extinction in Earth’s history undoubtedly occurred up to 66 million years ago. It marked the end of an era like the Cretaceous, and with it, the disappearance of dinosaurs that were not birds. But what was that extinction really like? This is the big question that experts have asked themselves and that is already beginning to shed light.
For decades the scientific community has debated whether dinosaurs were already in decline before they abruptly went extinct or whether they were wiped out while they were still thriving. This is where the new study published in the journal has had an impact Science in which the Spanish researcher Jorge García-Girón from the University of León participates, who sheds light on this debate.
Simply put, the research refutes the idea of a prolonged decline and suggests that dinosaurs were diverse and divided into distinct ecological regions just before the asteroid impact.
The fossils of the south. Much of the uncertainty about this issue comes from a bias in the fossil record. The only well-dated faunas that span the extinction boundary come from northern North America (in the famous Hell Creek Formation). This made it impossible to know whether the extinction pattern observed there was a global or local phenomenon.
In this case, the research team focused on a fossil-rich unit much further south, in the San Juan basin of New Mexico, known as the Naashoibito Member. The age of this formation has been a matter of controversy for years and was often considered much older.
But now by applying geochronology techniques with Argon dating and magnetostratiography, the study has finally achieved precise dating. The results are conclusive: the Naashoibito Member dates back to the latest Cretaceous, which corresponds to up to 66 million years.
This means that the fossils found there, which include a variety of species, preserve some of the last known non-avian dinosaurs. They lived at most 340,000 years before the asteroid impact and were contemporaries of the Hell Creek fauna.
Separated by weather. This finding is crucial because, for the first time, it allows us to compare two different faunas from the same end of the Cretaceous. And the result refutes the idea that we had all about decline in our minds.
And the study not only dates the fossils, but also uses powerful ecological models to analyze the diversity of terrestrial vertebrates throughout North America. The results show that, far from forming a homogeneous and cosmopolitan fauna, the dinosaurs maintained high diversity and clear endemism until the end.
In other words, it can be said that the dinosaurs were “strong” and divided into distinct regional assemblages. In this case, the study identifies two clear bioprovinces in the north and south that remained stable during the late Cretaceous.
What separated these faunas? The analysis suggests that the main factor was temperature. More than a simple geographic division, different dinosaur communities were adapted to different climates. For example, the data propose that warmer southern regions may have been more tolerable for sauropods, while colder, more temperate northern regions were more suitable for hadrosaurines.
The conclusion. The sum of the evidence points directly to the fact that non-avian dinosaurs were abruptly annihilated at the end of the Cretaceous. They were not in a decline as was thought, so they did not have this factor on top of them that would already condemn them to extinction if the disastrous event on Earth had passed.
Instead, it has been seen that its ecosystem was diverse and biogeographically compartmentalized. Extinction in this way was sudden and, as the later fossil record demonstrates, was followed almost immediately by the rapid diversification and rise of mammals.
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