For decades, scientists have gone back and forth about whether massive volcanic eruptions or an asteroid impact — or maybe both — caused a mass extinction that saw the demise of all nonbird dinosaurs about 66 million years ago.

Now, geologic evidence and data on dinosaur habitats, combined with climate and ecological simulations, suggest it wasn’t the volcanism. Instead, a decades-long cold winter triggered by the giant impact wiped out dinosaur habitats and made it impossible for the creatures to survive, researchers report June 29 in Proceedings of the National Academy of Sciences.

In a plot twist, volcanism at the Deccan Traps, in what is now India, may have actually ameliorated the negative effects of the long winter, warming the planet quicker than would have occurred otherwise and allowing mammals room to thrive, the researchers say.

“It’s a complete change in the narrative of Deccan volcanism … [which] may well have been the benevolent hero of the time,” says Alexander Farnsworth, a paleoclimatologist at the University of Bristol in England.

The new research adds to a growing body of evidence that suggests the impact, not the eruptions, caused the die-off, including a recent study suggesting the bulk of volcanic outgassing happened either too early or too late to have caused the mass extinction (SN: 1/16/20).

An estimated 75 percent of the planet’s plant and animal species disappeared in a relative blink of an eye during the extinction event at the end of the Cretaceous Period. Previous research has indicated that a giant asteroid impact, at Chicxulub in what’s now the Yucatán Peninsula in Mexico, released enough ash, dust and gases to block out the sun and dramatically cool the planet for an extended period of time (SN: 11/2/17), possibly causing the extinction.

But around the same time, Deccan Traps eruptions released massive amounts of climate-altering gases too, as well as hundreds of thousands of cubic kilometers of lava, although the timing is uncertain. Such intense volcanism and associated bursts of gases have triggered other mass extinctions, such as the “Great Dying” at the end of the Permian Period 252 million years ago (SN: 12/6/18). As a result, the Deccan Traps still hadn’t been ruled out as the main culprit of the dinos’ die-off.

Farnsworth, paleontologist Alfio Alessandro Chiarenza of University College London and their colleagues evaluated how different dinosaur habitats would be affected under various impact and volcanism scenarios. The team ran dozens of climate and ecological simulations and compared the simulations with geologic evidence for the amount and types of gases spewed out by the eruptions, as well as with evidence of the dust kicked up into the atmosphere from crust pulverized by the asteroid impact.

The researchers found that suitable habitat existed for the dinosaurs up until the end of the Cretaceous. After the impact, however, dust from the impact caused sunlight that reaches Earth to dim 10 to 20 percent, the simulations show, which aligns with previous research.

Ankylosaurus with asteroid impact in background
The asteroid impact 66 million years ago would have wiped out habitat for dinosaurs such as the ankylosaurus in this illustration. But volcanism, which has also been suggested as a cause for the mass extinction, might have actually helped ecosystems recover — though not quickly enough for dinos.Fabio Manucci

With just 10 percent solar dimming, the team found that there was “almost complete blotting out of the niches for dinosaurs,” Chiarenza says, as dramatically cooling global temperatures  killed off plants. “At 15 percent, you have the blue screen of death for dinosaurs.” No volcanism scenario showed such total habitat destruction, the team found.

As the impact-induced winter settled in, global temperatures reached –30° to –40° Celsius, Chiarenza says. Temperatures would not have recovered fully for at least 20 years, Farnsworth adds. If not for warming caused by carbon dioxide from the volcanic eruptions, it would have taken another 10 years for temperatures to bounce back to pre-impact averages, the simulations suggest. The volcano-induced warming, Chiarenza says, would have sped up animals’ and plants’ recovery.

Previous research has suggested that mammals in particular had a relatively rapid recovery after the impact, including a study that shows mammal diversity in Colorado’s Denver Basin doubled in just 100,000 years after the impact (SN: 10/24/19).

Examining the habitat suitability of various dinosaurs as it relates to the extinction event is new and interesting, says Tyler Lyson, curator of vertebrate paleontology at the Denver Museum of Nature and Science. But accurately simulating past climate is difficult, he says. So even though much of the evidence so far points to the impact as the mass extinction culprit, uncertainty still lingers.

It may well be that the Deccan Traps played the role of “a creator rather than a destroyer,” Lyson says. More high-resolution datasets from around the world — such as “terrestrial datasets with plants and pollen and vertebrates from the Southern Hemisphere” — would be helpful to parse out the Deccan Traps’ influence in both extinction, if there was any, and recovery.

If the storyline holds, notes Farnsworth, “unfortunately for the dinosaurs, [the volcanism] couldn’t offset enough of the cooling. But perhaps fortunately for us, it might have been enough to allow mammals to inherit and eventually dominate the Earth.”