Asteroid Timing Erased the Dinosaurs?

Wrong place. Wrong time. Dinosaurs didn't have to die off, but the space impact that blasted the Earth some 66 million years ago arrived at a most inopportune moment for the long-time rulers of the planet, according to a report released Monday.

There's never a good time for an asteroid impact, of course, but debates have roiled among scholars for decades over whether volcanoes or a long-running decline in species may have played a bigger role in the demise of the dinosaurs. (Related: "Dinosaurs 101.")

Now a Biological Reviews journal report concludes that the asteroid or comet that created the Yucatan's Chicxulub crater was indeed the likely leading culprit. Other factors, most notably a vulnerable sub-population of big plant eaters, essentially left the dinosaurs ripe for the asteroid wipe-out.

"If the asteroid hit five million years later or earlier, the dinosaurs might still be around," says paleontologist Stephen Brusatte of the United Kingdom's University of Edinburgh, a member of the report panel that included experts from leading dinosaur museums and universities worldwide. "An impact would have been horrible for them, but they had survived dips and dives for more than 150 million years," he says.

At least six miles (ten kilometers) wide, the Chicxulub impact object left a crater some 110 to 180  miles (177 to 290 kilometers) wide and 12 miles (19 kilometers) deep beneath Mexico's Yucatan peninsula and the Gulf of Mexico.

Since Nobel Prize winning physicist Luis Alvarez and colleagues first suggested the asteroid explanation for the death of the dinosaurs in 1980, scholars have debated its role, over time coming to what the review authors are calling a consensus on its central place in the dinosaur doomsday.

"It's pretty remarkable to have paleontologists agree on anything," says paleontologist Richard Butler of the United Kingdom's University of Birmingham, a review author. "Most would now agree the impact played the largest role."

Birds are now thought to be the only dinosaur survivors of the space impact's destruction, which ended roughly 160 million years of animal kingdom domination by dinosaurs, including through other mass extinctions. (Related: "Siberian Discovery Suggest Almost All Dinosaurs Were Feathered.")

"What has really changed is that we now have a great deal of evidence from fossils, from dating and from geology, of what dinosaurs were doing," Butler says. "Were they all dwindling away before the impact and would have disappeared anyway? They weren't. The evidence is that the impact did it."

Weak Links

Dinosaurs disappeared during the era of the Tyrannosaurus Rex and Triceratops well known from museum exhibits, the Late Cretaceous. While the overall diversity of dinosaur species at this time wasn't broadly decreasing, the review finds that it was among one key group-big plant eaters such as the duck-billed dinosaurs that were the prey for the big carnivorous dinosaurs.

"There were just as many individual dinosaurs, but fewer species, and they looked more similar," Butler says, pointing to ecological analyses cited in the review. "This loss of diversity among these key species is the best mechanism we have for explaining why they were more prone to a collapse."

Were they all dwindling away before the impact and would have disappeared anyway? They weren't. The evidence is that the impact did it.

Essentially, the review suggests that the big plant eaters, the horned ceratops and duck-billed hadrosaurs, were keystone species in dinosaur ecology. Their species-wide similarity and lack of diversity pointed to dinosaurs having fewer places of refuge, no alternative prey and fewer adaptive characteristics to rely on, once the asteroid hit.

The impact plunged the Earth into a global firestorm followed by decades of "impact winter," triggering the loss of perhaps 75 percent of all species worldwide, including the non-avian dinosaurs.

"In any ecosystem when you remove links to key species, that community has problems," Butler says. An asteroid was just a really big problem that came at a bad time, he says.

Good Times, Bad Times

Dinosaurs had other troubles around 66 million years ago. A period of intense volcanism seems to have started several hundred thousand years before the impact. Climate temperatures swiftly cycled up and down, perhaps in response.

And a period of lowered sea level removed inland seas such as one that once covered the middle of North America and provided marshy homes for diverse groups of disconnected dinosaurs. Land bridges forming in the era may have connected formerly isolated populations, spurring a uniformity among species of large plant eaters.

The massive numbers of volcanoes preceding the Chicxulub impact came from the "Deccan Traps", a 200,000-square-mile (518,000-square-kilometer) mountainous region in central India. Princeton geophysicist Gerta Keller, who has long argued these eruptions played a bigger role in extinguishing the dinosaurs, disagrees with the asteroid "consensus" view in the Biological Reviews report.

"Compared with this volcanic catastrophe, the Chicxulub impact is a much lesser catastrophe," Keller says, by email. Each one of the eruptions would have had the climate impact of the asteroid, she suggests, adding that perhaps 30 to 100 eruptions took place. "Deccan volcanism is clearly the main catastrophe leading to the mass extinction."

However, Brusatte says that dinosaur fossils are still evident after the era of the Deccan eruptions, but not after the impact. A May Proceedings of the National Academy of Sciences journal study concluded that the impact caused a global winter lasting for decades, "a key contributory element in the extinctions of many biological clades, including the dinosaurs."

Continental Divide

Although dinosaur researchers now know of many more species ("about one new one a week is being reported," Brusatte says) than they did in 1980, their analysis still largely relies on North American fossils, which the review concedes is one weak point in the analysis.

"We need to find more fossils outside North America," Brusatte says. "But we do think we have enough information now to be confident that the asteroid impact was the primary factor in the extinction of dinosaurs."

"One question that always gets asked is what would have happened if the asteroid hadn't hit," Butler says. "Given how successful the dinosaurs were, able to return to normal after past dips, it's likely they would still be here. And we wouldn't."

Correction: The article originally misstated the era of the last nonavian dinosaurs; it was the late Cretaceous.

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