Detectives have a mammoth task trying to trace ancient fossils worth £15,000 which been stolen from storage in west Oxfordshire.

Between 1989 and 1999, Dr Kate Scott and Dr Christine Buckingham uncovered more than 1,500 bones, tusks and teeth at gravel pits in Stanton Harcourt, near Witney.

The finds dated back 200,000 years and have shed new light on the Ice Age climate in Britain and the evolution of mammoths.

But thieves broke into the building where the items were being stored and stole a 7ft mammoth tusk and other items.

Dr Scott, a research fellow at St Cross College, Oxford, said today that she believed the fossils were stolen to order for a collector and could have been taken out of the country before being sold in Germany or elsewhere in Europe.

She said: "It's so disappointing that this theft has taken place, because it's so difficult to get these materials out of the ground - they're often very frail.

"With the mammoth tusk, we were able to retrieve the entire tusk. It's beautifully preserved and there were plans for the entire collection to become part of the university's museums collection and go on display at the Museum of Natural History in Oxford.

"This isn't a case of kids breaking in - these thieves were experts, who knew what they were looking for. The mammoth tusk could be out of the country by now.

"I have looked on the Internet and there are hundreds of fossils for sale there - some of them fetch very high prices.

"Someone might know something about this theft and, if they do, we would urge them to contact the police."

Dr Scott said the remains of the mammoth and other animals were so important as archaeological remains, because they demonstrated a warm spell - a previous episode of global warming - during the Ice Age 200,000 years ago.

She and her colleagues believe the animal found at Stanton Harcourt was not the well-known woolly kind, but a variation called the steppe mammoth, which inhabited grassland areas.

She added that the tusk had previously been displayed at the Museum of Natural History in Oxford.

Police said the fossils were stolen between October 21 last year and February 1, after thieves broke into the Stanton Harcourt store, but Dr Scott thinks the items were taken in December or last month, after thieves forced open the back door.

Pc Claire Sadler, of Witney police, said: "The stolen fossils are 200,000 years old and include a mammoth tusk and teeth, bison limb bones and reindeer antlers. The mammoth tusk is curved and over 6ft long.

"These are incredibly rare fossils and I would urge anyone who has been offered these items for sale to contact me immediately."

Anyone with information should call Pc Sadler on 08458 505505 or Crimestoppers on 0800 555111.