DINOSAURS were killed off by a meteor that hit the Earth 300,000 years after the one blamed for their extinction, a scientist has claimed.

Dr Gerta Keller, from Princeton University, New Jersey, insists the Chicxulub impact off the coast of Mexico 65 million years ago could not on its own have wiped out the dinosaurs.

Evidence of the crater left by the giant asteroid or comet has been found under the sea off the coast of Yucatan.

But according to Dr Keller, Chicxulub was only the warm-up for a much larger impact more than a quarter of a million years later. It was this meteor which left a tell-tale layer of extraterrestrial iridium in rocks around the Earth, not the earlier one, she says.

However, no-one has yet found the crater from the "final straw" impact which ended the age of reptiles in one of the largest ever mass extinctions.

"There is some evidence that it may have hit in India," said Dr Keller.

The crater, named Shiva by one expert, is estimated to measure 310 miles in diameter. However, there is little proof of its existence.

Dr Keller said marine microfossils in sediments drilled from the ocean floor showed that Chicxulub hit Earth 300,000 years before the mass extinction it was supposed to have caused.

The small marine animals that produced the microfossils escaped virtually unscathed.

The Chicxulub impact combined with the Deccan-flood basalt eruptions in India - a long period of intense volcanic activity - to nudge species towards the brink, said Dr Keller.

Vast amounts of greenhouse gas were pumped into the atmosphere by the Deccan volcanism over a period of more than a million years. By the time Chicxulub struck, land temperatures were 7-8C warmer than they had been 20,000 earlier.

Weakened by these events, species were finally killed off by the second impact.