Evidence is accumulating that it wasn't an asteroid that did the beasts in, but volcanoes – the first real challenge the extinction theory has met in three decades.
A combination of studies on dinosaur fossils, magnetic signatures in rocks and the timing of the disappearance of different species suggest it was volcanoes, not an asteroid, that caused the dinosaurs' extinction.
"We're discovering ... amazingly large flows, amazingly short time scales and amazing volcanic (eruptions)," said Vincent Courtillot of the University of Paris, who is is presenting new evidence for the volcano theory this week at the American Geophysical Union conference here.
For the last 30 years, the prevailing theory has been that an asteroid, around six miles across, hit the Yucatan peninsula 65 million years ago, throwing debris into the atmosphere, blocking the sun and chilling the planet to the point that nearly half of all species went extinct.
Physicist Luis Alvarez of the University of California, Berkeley, first presented the asteroid impact hypothesis in 1980. It was based on an extensive layer of iridium, which is associated with impacts, that could be found in many places across the globe in the same geologic time sequence. A decade later, the Chicxulub crater was discovered on the Yucatan peninsula, adding weight to the idea that an impact killed off the dinosaurs.
The idea that Indian volcanoes, known as the Deccan Traps, might have contributed to the mass extinction is not new. But scientists at the AGU meeting think the eruptions could be the sole cause of the die-offs, and that the asteroid had little or no effect on life at all.
"If there had been no impact, we think there would have been a massive extinction anyway," Courtillot said.
Courtillot has studied the magnetic signatures of the Indian volcanic deposits that lined up with the Earth's magnetic field as they cooled. Because the orientation of the magnetic field has changed over time, lava that cooled at different times have different signatures.
The more than 2-mile thick pile of Deccan Traps deposits has several major pulses that occurred over the course of several decades each, almost certainly less than 100 years. And the entire sequence erupted in less than 10,000 years, rather than the million years or more that has been suggested.
All told, this would have put 10 times more climate-changing emissions into the atmosphere than the asteroid impact.
Also supporting the volcanic theory is fossil evidence from Texas and Mexico that most of the species extinctions coincided with the final pulse of eruptions, not with the asteroid impact, which may have occurred approximately 300,000 years earlier, according to Gerta Keller of Princeton University.
"There is essentially no extinction associated with the impact," Keller said.
Evidence that dinosaurs survived in India right up to the final volcanic onslaught further bolsters the case.
But it will take a lot of evidence to convince the bulk of the scientific community that the asteroid theory is wrong.
"There was volcanism at the time. There's always volcanism, but that impact is so significant that you can't ignore it," said Rick Firestone of Lawrence Berkeley National Laboratory who studies the link between impacts and extinctions. "The only question is, were there other things that happened as result of it."
- Wired Science's complete coverage of AGU 2008
- Asteroid Belt Collision Likely Led to Dinosaurs' Death
- Scientists Can't Explain Why Dinosaurs Ruled the Earth
- The Rise of the Dinosaurs was Slow Going
Image: Deccan Traps volcanic deposits. Gerta Keller/Princeton University