Dinosaurs would not have gone extinct if not for a catastrophic asteroid impact, researchers say, casting doubt on the idea that the animals were already in decline.
About 66 million years ago, at the end of the Cretaceous period, a huge space rock crashed into the Earth, causing a mass extinction event that wiped out all dinosaurs except birds. However, some experts argue dinosaurs were already in decline.
Now researchers are talking about dating the rock formation in New Mexico casts doubt on this idea, suggesting that dinosaurs were thriving before the fateful impact.
Dr Andrew Flynn, first author of the study at New Mexico State University, said: “I think based on our new research that shows that, at least in North America, they were not heading towards extinction.”
Article in Science magazineFlynn 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 and determine the maximum age of its formation. The second involved analyzing the alignment of magnetic particles within the material forming the rock, a feature that reflects the direction of the Earth's magnetic field at the time it was deposited.
Flynn said: “The extinction event actually occurs directly in the middle of a fairly short period of polarity. [where Earth’s magnetic poles are] inverted”.
Taken together, the results indicate 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 in southern North America,” Flynn said.
The team says the results show that dinosaurs from this period were more diverse than previously thought. “There is no uniform dinosaur fauna in North America that would make them truly prone to extinction,” Flynn said.
Indeed, while there were some species in northern and southern North America, including large predators such as Tyrannosaurus rex, there were also notable differences – which the researchers believe reflects differences in climate.
Professor Steve Brusatte, co-author of the study at the University of Edinburgh, said: “The north contained many horned triceratops and standard duck-billed dinosaurs such as Edmontosaurus. But the south contained intricately crested platypuses and, most strikingly, 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 is no indication that there were any problems with these dinosaurs, or that anything unusual was happening to them, or that they were in any long-term decline,” Brusatte said.
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Flynn said the idea that overall dinosaur diversity was declining before the asteroid impact may have been a result of there being less exposed rock, and therefore fossils, dating from the end of the Cretaceous than earlier in the era.
“There seems to be no reason, as far as we can tell, why they should have gone extinct, except [the] asteroid impact,” he said.
Professor Michael Benton from the University of Bristol welcomed the research. A paleontologist not involved in the work said: “The new evidence for these very late-surviving dinosaurs in New Mexico is very interesting and shows, at least in one location, that the fauna was diverse.”
But Benton noted that the paper only looked at one location and did not reflect the complexity of dinosaur species at the time in North America or around the world.
“As the authors also show in the paper, overall dinosaurs of the last 6 million years of the Cretaceous were less diverse, declining from 43 species to 30 species in western North America,” he said.
“We suggest there is evidence for a general decline in dinosaur numbers towards the end of the Cretaceous, with isolated rich faunas where the climate was favourable.”
