By Paul Rincon
BBC News Online science staff
Scientists have cast doubt on the well-established theory that a single, massive asteroid strike killed off the dinosaurs 65 million years ago.
The controversy over what killed the dinosaurs may run and run
New data suggests the Chicxulub crater in Mexico, supposedly created by the collision, predates the extinction of the dinosaurs by about 300,000 years.
The authors say this impact did not wipe out the creatures, rather two or more collisions could have been responsible.
The report is published in Proceedings of the National Academy of Sciences.
An international group of scientists led by Professor Gerta Keller, of Princeton University, US, looked at a continuous sequence of rock - a core - drilled out of the Chicxulub structure.
They analysed rock from this core using five separate indicators of age, including fossil planktonic organisms and patterns of reversals in the Earth's magnetic field.
The results suggest the 180-km wide crater was punched into the Earth 300,000 years before the dinosaurs disappeared from the face of the planet.
At numerous sites around the world, a clay layer separates rocks laid down in the Cretaceous Period from those deposited in the Tertiary and is known as the K-T boundary.
It marks the point in time when the dinosaurs died out and was first linked to the Chicxulub crater in 1991.
Professor Keller and her team contend their findings prove the Chicxulub impact did not by itself trigger the extinction of the great beasts.
Instead, they believe a cooling of the global climate shortly followed by a period of greenhouse warming placed enormous stress on the dinosaurs.
This warming could have been kicked off by carbon dioxide released by a massive eruption of lava seen today in the Deccan traps of India.
The Chicxulub impact occurred during this warming period and, although the environmental effects were severe, it did not cause the extinction of the dinosaurs.
The team believes a second impact, 300,000 years after the Chicxulub collision, finished off the creatures.
"When the K-T boundary impact finally came, it hit an already stressed community. To use a cliche, it was the straw that broke the camel's back. Almost anything could have wiped them out at that point," Professor Keller told BBC News Online.
The structure of the sea bed beneath the Indian Ocean suggests this second impact could have been there, Professor Keller added.
Understandably, the team's conclusions have met with strong opposition.
Over a million cubic km of lava erupted on to surface
Event occurred over several hundred thousand years
"It appears to contradict many other lines of evidence that seem rather unambiguously to indicate that the [Chicxulub] crater formed at the K-T boundary," said Dr David Kring of the University of Arizona, US.
Professor Alan Hildebrand of the University of Calgary, Canada, told BBC News Online: "[This theory] has survived every test. The asteroid that made Chicxulub acted alone."
Professor Hildebrand and Dr Kring were authors of the 1991 paper proposing Chicxulub as the site of the K-T boundary asteroid strike.
Strong supporting evidence comes from molten material laid down at the K-T boundary in rocks from Haiti which is similar to deposits from the Chicxulub crater.
In addition, debris thrown out by this collision gets thicker the closer you get to Chicxulub like a trail pointing to the impact site.
And Dr Joanna Morgan, of Imperial College, London, UK, told BBC News Online: "An impact the size of Chicxulub occurs on Earth about every 100m years.
"That two such impacts occurred within 300,000 years and both hit the Earth at almost exactly the same place is statistically unlikely.
"Not impossible, but very, very unlikely," said the researcher who is also investigating Chicxulub core material.
K-T BOUNDARY MARKED BY MASS EXTINCTIONS INCLUDING DINOS
Numbers of planktonic creatures suddenly fall off after K-T impact - 70% of species, including dinosaurs, disappear from fossil record
K-T boundary sediments contain high levels of element iridium - which is common in extraterrestrial material that falls on planet
Combination of environmental factors brought on by impacts and volcanism may have pushed dinosaurs to global extinction