Asteroid impact 66 mln yrs ago 'last straw' for dinosaurs

Scientists have determined that an asteroid or comet impact which occurred exactly 66,038,000 years ago - the most precise date yet for the cataclysmic event - indeed dealt dinosaurs their final death blow.

The demise of the dinosaurs is the world's ultimate whodunit. Researchers have not been able to exactly determine whether it  was a comet or asteroid impact, volcanic eruptions or climate change that wiped out the giant creatures.

In an attempt to resolve the issue, scientists have now determined the most precise dates yet for the well-known asteroid or comet impact which they believe resulted in extinction of the dinosaurs.

The dates are so close, the researchers say, that they now believe the comet or asteroid, if not wholly responsible for the global extinction, at least dealt the dinosaurs their death blow.

"The impact was clearly the final straw that pushed Earth past the tipping point," said researcher Paul Renne, BGC director and University of California Berkeley professor in residence of Earth and planetary science.

"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," Renne said in a statement.

The revised dates clear up lingering confusion over whether the impact actually occurred before or after the extinction, which was characterised by the almost overnight disappearance from the fossil record of land-based dinosaurs and many ocean creatures.

The new date for the impact – 66,038,000 years ago – is the same within error limits as the date of the extinction, said Renne, making the events simultaneous, according to the study published in the journal Science.

The extinction of the dinosaurs was first linked to a comet or asteroid impact in 1980. A 110-mile-wide crater in the Caribbean off the Yucatan coast of Mexico is presumed to be the result of that impact.

Called Chicxulub, the crater is thought to have been excavated by an object six miles across that threw into the atmosphere debris still found around the globe as glassy spheres or tektites, shocked quartz and a layer of iridium-enriched dust.