The death of the dinosaurs when a giant asteroid impacted the Earth around 66 millions years ago may have been a classic case of being in the wrong place at the wrong time, a study suggests.

When the asteroid struck, many dinosaur species were already teetering on the edge of extinction, hammered by global volcanic activity, temperature changes and sea level changes, the study authors say.

A reduction of dinosaur diversity among plant-eating dinosaurs that were prey for other dinosaurs weakened their food chain and made all species vulnerable.

If the asteroid had struck at a different time -- earlier or later by just a few million years -- they might have survived, says Steve Brusatte of Edinburgh University in Scotland, calling the timing of the impact "colossal bad luck."

"It was a perfect storm of events that occurred when dinosaurs were at their most vulnerable," he says.

The impact of a 6-mile-wide asteroid in what is now Mexico would have caused earthquakes, tsunamis, wildfires and environmental shifts.

All of those factors would have contributed to a collapse of the food chain and the extinction of one dinosaur species after another, researchers from the United Kingdom, the United States and Canada suggest in their study published in Biological Reviews.

They might have survived if the asteroid impact had come earlier, when dinosaur species were more diverse and the food chain more robust, or if it had come later when new, more adaptable species might have evolved, they say.

"Not only did a giant asteroid strike, but it happened at the worst possible time, when their ecosystems were vulnerable," Brusatte says.

The researchers based their conclusions on hundreds of dinosaur fossils gathered over the last decade that yielded a clear picture of changes in their diversity around the time of the asteroid impact.

Although the evidence shows a drop in the diversity of dinosaurs prior to the impact, such changes had occurred before without resulting in global extinctions, Richard Butler of the University of Birmingham in Britain points out.

"Although our research suggests that dinosaur communities were particularly vulnerable at the time the asteroid hit, there is nothing to suggest that dinosaurs were doomed to extinction," he says.

The asteroid, which did in fact seal their doom, had a part in creating the modern world, he notes, as mammals only began to widely populate the world after the dinosaurs had exited the picture.

"Without that asteroid, the dinosaurs would probably still be here, and we very probably would not."

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