By Jonathan Amos

Science reporter, BBC News, San Francisco



Eight tusks dating to some 35,000 years ago all show signs of having being peppered with meteorite fragments.

The ancient remains come from Alaska, but researchers also have a Siberian bison skull with the same pockmarks.

The scientists released details of the discovery at a meeting of the American Geophysical Union in San Francisco, US.

They painted a picture of a calamitous event over North America that may have severely knocked back the populations of some species.

Blast direction

"We think that there was probably an impact which exploded in the air that sent these particles flying into the animals," said Richard Firestone from the Lawrence Berkeley National Laboratory.

"In the case of the bison, we know that it survived the impact because there's new bone growthlarge surfaces such as tusks; and we decided it was worth a shot to go look for them."

Allen West began the hunt at a mammoth tusk sale in his home state of Arizona.

He immediately found one tusk with the tell-tale pockmarks and asked the trading company if he could look through its entire collection. He sorted literally thousands of items.

"There are many things that can cause spots, such as algae, and there were a few of those; but I was only interested in the ones that were magnetic," he recalled. "It was just a tiny magnet on a string, but very strong. It would swing over and stick firmly to these little dots."

The search turned up a further seven ivory specimens of interest, together with the bison skull.

Further