A comet or asteroid impact has long been considered a possibility, but others theories have focused on volcanic eruptions or climate change.
Now scientists at the University of California, Berkeley, with British and Dutch colleagues, have determined the most precise dates yet for both the dinosaur extinction around 66 million years ago and for the well-known cosmic impact that occurred around the same time when a 6-mile wide object slammed into the Caribbean off the Yucatan coast.
The dates are so close, the researchers say, they now believe the comet or asteroid, if not wholly responsible for the global extinction, at least dealt the dinosaurs their final death blow.
"The impact was clearly the final straw that pushed Earth past the tipping point," Berkeley earth and planetary science Professor Paul Renne said. "We have shown that these events are synchronous to within a gnat's eyebrow, and therefore the impact clearly played a major role in extinctions, but it probably wasn't just the impact."
The new date for the impact -- 66,038,000 years ago -- is the same within error limits as the date of the extinction, Renne said, suggesting the events were simultaneous.
He cautions this doesn't mean the impact was the sole cause of the extinction.
Dramatic climate variations over the previous million years, possibly made more extreme by volcanic eruptions, probably brought many creatures to the brink of extinction, ready to be kicked over the edge by the asteroid or comet impact, he said.