Hunters find the ancient tusks clustered on sandbars near the Arctic Ocean, carried there by spring melt waters flowing from the Siberian tundra. A pair of them, dried, polished and elegantly mounted for a trophy room or home museum, can weigh 400 pounds and cost as much as $75,000.

Perhaps a bargain. Fossil ivory, from woolly mammoths that died between 20,000 and 5,000 years ago, is the only raw ivory legally available since uncontrolled killing of African elephants led to an international ban on sales of modern ivory 15 years ago.

At the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), which ended yesterday in Bangkok, at least one prominent delegate suggested that mammoth ivory might permanently supply the ivory market instead of elephant ivory.

This may not be as Solomonic a solution as it seems. The substitution of mammoth ivory for elephant ivory presents an unusual example of how conservation of one limited resource could lead to the uncontrolled, and perhaps catastrophic, exploitation of another.

"From a paleontologist's perspective, [taking mammoth tusks] means the destruction of specimens," said Ross McPhee, curator of vertebrate zoology at New York's American Museum of Natural History. "People take the tusks and never tell anybody where they found them. And it's not like we'll ever see them again."

Still, for the foreseeable future, it appears that mammoth ivory will continue to be the only game in town. CITES delegates in Bangkok refused to grant Namibia an annual quota for legal sales of elephant ivory and also won an agreement from African nations to crack down on sales of poached ivory.

The only potential opening for elephant ivory is a pending authorization for four southern African countries to sell 50 tons from existing inventory to Japan in a one-time auction, but only after implementation of an international warning system to monitor illegal elephant killings.

Ginette Hemley, vice president of the World Wildlife Fund, said that the network -- despite difficulties funding it -- should be completed in 2005, and that the sale will probably occur the following year.

The monitors will be able to tell if the auction prompts poachers to kill elephants and then "launder" the ivory at the auction. Resource economist Erwin H. Bulte of Tilburg University in the Netherlands said that this is unlikely but acknowledged that researchers need the data that monitors can provide.

"I think [controlled sales] is the way to go for the short and medium term," said Bulte, lead author of an article on elephant conservation in today's issue of the journal Science, but he noted that at least two of his co-authors "don't agree."

Ivory has been used for thousands of years to make such things as crude Ice Age tools, exquisite carvings and jewelry. For centuries, the biggest consumers have been Asian countries, especially China and Japan.

McPhee said exploitation of mammoth ivory began with the colonization of Russian Siberia in the 1600s. Mammoths, elephant-like behemoths standing 11 feet tall, went extinct about 5,000 years ago after several millenniums of decline, probably caused by climate change or hunting.

"There's been a huge trade," said McPhee, a mammal specialist who has worked in the Siberian Arctic. "The Chinese cleaned up their stuff early, but Asiatic people got the message right away. It was immediately clear that mammoth ivory was as valuable as any item up there."

In 1987, Pierre Pare, president of Calgary-based Fossils Canada Ltd., bought seven tons of mammoth ivory from the Soviet Union to get tusks for museum restoration. Two years later the convention -- created by international treaty to control trade in rare animals and plants -- banned sales of elephant ivory.

"There was a massive slaughter in Africa during the 1980s, and at its peak 100 tons of ivory were going to Hong Kong and Japan every year," Hemley said. "The elephant population dropped from 1.2 million animals to 500,000 or 600,000 by the end of the decade."

Thanks to his timely purchase, Pare has prospered, opening a line of jewelry to go with restorations and bulk sales of ivory tusks and chunks to whoever asks.

"It's not a huge business, but it's been growing since the ban," said Pare, who said he sells between $700,000 and $1 million in ivory each year. "It's a good alternative to modern ivory, and it's not going to endanger any species."

This was the point that Esmond Bradley Martin of the East African Wildlife Society made in Bangkok, telling the Reuters news agency that opponents of the African ivory trade "are trying to encourage" sales of mammoth ivory "as a substitute for elephant ivory."

Mammoth ivory, often fossilized with minerals that can give it a spectacular range of colors, has its own market with its own range of products. Pare sells small pieces of jewelry -- charms, pins and earrings -- starting at less than $100, while a pair of double-twist tusks, fully restored, can cost $75,000.