The dinosaurs were probably heading for extinction even before an asteroid strike wiped them out 65 million years ago, argue New Zealand scientists.

Dr Chris Hollis, a palaeontologist at the Institute of Geological and Nuclear Sciences in Lower Hutt and his team, have uncovered evidence of significant global climate change even before the meteor strike.

"An unknown number of species may have been in sharp decline when the asteroid struck, and the impact winter probably finished them off quite quickly," Hollis said. "There's no scientific agreement on what caused this climatic instability, but it's quite likely that current studies are over-estimating the effect of the asteroid impact."

By studying fossils and sediments at six New Zealand sites, the team found a centimetre-thick layer of meteorite dust formed precisely at the time of major environmental change 65 million years ago. They also found abrupt changes in microscopic plants and animal fossils in marine sediments.

This supports the idea that the main effect of the asteroid was to throw up a global dust cloud that blocked out the Sun for months to years. But the cool climate that prevailed in New Zealand for millions of years after the strike might not be, as some had supposed, evidence of a prolonged 'impact winter'.

"Instead, it may represent a return to normality following unusual warming at the end of the Mesozoic," Hollis said. At around the time of the impact, toward the end of the Mesozoic Era (from 63 million to 230 million years ago), the planet's climate was changing rapidly with a period of long-term cooling.

But the scientists believe there had been unusually warm conditions just before the impact. "The warming may have allowed a final flourishing of some species that were already on the path to extinction," he said.

The reappearance of several survivor species after the impact shows that, even though the effects were global, the survival rate of species in New Zealand was higher than in the northern hemisphere.

Because New Zealand was about 1,500 km closer to Antarctica at the time, the local flora and fauna were probably adapted to cold and darkness and therefore better able to withstand an impact winter.

The institute earlier said its study of New Zealand evidence suggests that the destruction of forests as a result of the impact winter was largely confined to the American continent, within a radius of several thousand kilometres from the impact site on the Yucatan Peninsula, off the coast of what is today Mexico.

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