Clearer picture emerging of dinosaurs’ last days

Neither a giant asteroid nor a gradual die out can take full blame for dinosaurs’ demise.

Rather, the culprit may be both, two new studies suggest.

Tens of millions of years before the asteroid delivered its killer blow some 66 million years ago, the number of dinosaur species had already begun to drop, researchers report online April 18 in the Proceedings of the National Academy of Sciences. But not all dino groups were in decline, including some maniraptoran dinosaurs, a different group of researchers suggests online April 21 in Current Biology.

At first glance, the two studies seem to conflict, but “they can coexist,” says paleontologist Michael Benton, who coauthored the PNAS paper. Both studies add to what has become an increasingly intricate picture of dinosaurs’ final days.

“Things are a wee bit more complicated than we used to think,” says Benton, of the University of Bristol in England.

In the 1960s and ‘70s, scientists generally believed that dinosaurs petered out after a long, gradual decline. That view took a U-turn in 1980, when researchers proposed that, instead, an asteroid impact might have suddenly triggered the extinction. “The flip-flop was quite extreme,” Benton says of the changed thinking. “Dinosaurs went from long-term decline to instant death.”

What actually happened, he says, is probably more nuanced. Benton and colleagues analyzed the number of dinosaur species emerging and going extinct over a huge timescale: roughly 175 million years. Around 40 million to 50 million years before the mass extinction, dinosaurs started losing species faster than they were gaining new ones, the researchers found. This loss in diversity could have made it harder for dinosaurs to bounce back from the asteroid’s catastrophic impact.

“This doesn’t in any way attack the importance of the impact,” Benton says. But across the board, he says, dinosaur species numbers were dwindling. At least two groups, however, seemed to buck the trend. Hadrosaurs (duck-billed dinosaurs) and ceratopsids (the group that includes Triceratops) were booming up until the end, the team found.

According to the Current Biology analysis, toothed maniraptorans (small birdlike relatives of velociraptors) were thriving, too. A detailed examination of more than 3,000 of these dinosaurs’ teeth suggests that these dinos’ ecosystem was pretty stable millions of years before the extinction, says study coauthor Derek Larson, a paleontologist at the Philip J. Currie Dinosaur Museum in Alberta and the University of Toronto.

Larson and colleagues looked for variations in the teeth’s dimensions, and the size of tooth serrations. Then they determined how much that variation changed over time. Big changes could be a hint that these dinos were on the decline, Larson says. But instead, “things basically stayed the same through the last 18 million years of the Cretaceous,” he says.

Toothed maniraptorans “seemed to be doing just fine right up until the extinction,” says University of Oxford paleobiologist Roger Benson, who was not involved in either study.

Larson’s team wondered why the toothed, meat-eating maniraptorans went extinct after the impact while their relatives — the beaked ancestors of modern birds — didn’t. The answer could be dietary, the researchers propose. They analyzed the diets of modern birds to try and figure out what an ancestral bird might have eaten. It probably relied on seeds, Larson says, a hardy food source that could have lasted for decades.

Seeds might have sustained ancient birds through a “nuclear winter,” the debris-darkened skies that could have blotted out the sun following an asteroid impact. When hoards of plants and animal species died out, and dinosaurs ran out of food, he says, “the only resource that would have been reliable and available would have been seeds.”