Dinosaurs would not have gone extinct without a catastrophic asteroid impact, researchers said, disputing the idea that the animals were already in decline.
About 66 million years ago, during the late Cretaceous period, a massive space rock crashed into Earth, triggering a mass extinction that wiped out all dinosaurs except birds. However, some experts say dinosaurs were already in decline.
Now researchers say the dating of a rock formation in New Mexico casts doubt on that idea, suggesting dinosaurs were thriving until the fateful impact.
Dr Andrew Flynn, first author of the research at New Mexico State University, said: “I think, based on our new study, it shows that, at least in North America, they were not on the verge of extinction. »
In the journal Science, Flynn and colleagues report how they dated a rock unit called the Naashoibito Member in the San Juan Basin using two methods.
The first was to analyze the ratio of two isotopes of argon in crystals found in the rock, thus providing a maximum age for its formation. The second involved analyzing the alignment of magnetic particles in the rock-forming material – a characteristic that reflects the direction of the Earth’s magnetic field at the time of its formation.
Flynn said: “The extinction event essentially occurs directly in the middle of a fairly short period of polarity. [where Earth’s magnetic poles are] overturned.”
Taken together, the results suggest that the section of the Naashoibito Member where the youngest dinosaur fossils were found formed, at most, about 350,000 years before the mass extinction. “These are the very last dinosaurs from southern North America,” Flynn said.
The team says the results suggest that dinosaurs from this period were more diverse than previously thought. “There is no uniform dinosaur fauna in North America that really makes them vulnerable to extinction,” Flynn said.
Indeed, while there are some species common to northern and southern North America, including large predators such as the T rex, there are also marked differences, which researchers believe reflect climate variations.
Professor Steve Brusatte, co-author of the study at the University of Edinburgh, said: “In the north there were lots of horned triceratops and duck-billed dinosaurs like Edmontosaurus. But in the south there were duck-bills with elaborate crests and, most striking of all, there were huge, long-necked sauropods.”
He noted that one sauropod, Alamosaurus, was nearly 30 meters (100 feet) long and weighed more than a Boeing 737.
“There’s no sign that these dinosaurs were in trouble, or that anything unusual was happening to them, or that they were in long-term decline,” Brusatte said.
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Flynn said the perception that overall dinosaur diversity was declining before the asteroid impact could be because there were fewer exposed rocks, and therefore fossils, dating to the late Cretaceous than early.
“It would seem, to our knowledge, that there is no reason why they would have disappeared, with the exception of [the] asteroid impact,” he said.
Professor Michael Benton, from the University of Bristol, welcomed the study. The paleontologist, who was not involved in the work, said: “The new evidence for these late-surviving dinosaurs in New Mexico is very interesting and shows in at least one locality that the faunas were diverse.” »
But Benton noted that the article only focused on one location and did not represent the complexity of dinosaur species at the time in North America or around the world.
“As the authors also show in the paper, in general, dinosaurs in the last 6 million years of the Cretaceous were less diverse, declining from 43 species previously to 30 species in western North America,” he said.
“We suggest that there is evidence of an overall decline of dinosaurs towards the end of the Cretaceous, with individual faunas rich where climates were favourable.”
