An archaeologist says he has dug up primitive tools that suggest humans lived in North America 13,500 years earlier than the date commonly accepted by scientists.

Don Wyckoff, director of the Oklahoma Archaeological Survey, said Tuesday he is convinced his preliminary dating is accurate but that expensive testing is needed to prove the age of the tools unearthed along with bison bones at a site near Alva in September.Carbon-dating tests on the bones were inconclusive, but Wyckoff said the same test on snail shells found at the same level indicates an age of 25,000 years.

Based on current evidence, scientists have thought that ancient humans first lived in North America about 11,000 to 11,500 years ago.

Experts say carbon-testing on snail shells can be inaccurate because snails incorporate old carbon molecules in their shells.

"These are the kind of dates that often turn out to be inaccurately old," said Donald Grayson, professor of archaeology at the University of Washington.

Wyckoff acknowleges the problem but said he is confident that the discovery 30 miles west of Alva in northwest Oklahoma will prove to be the oldest evidence of human inhabitation of North America.

"I feel pretty comfortable about the whole thing. The geological context is good. I look at this location and I say it's got to be old," Wyckoff said.

The state agency he heads is affiliated with the University of Oklahoma and uses its students for research and field work.

Wyckoff reported on his initial findings last weekend at a meeting of the Plains Anthropological Conference in Wichita, Kan.

He said a resident came across bones in June 1986 while digging a pond, and called the archaeological survey.

A University of Kansas archaeologist, Larry Martin, who was visiting at the time, recognized the skull of an ancient bison.

"We really weren't thinking that the site had anything to do with people. We were just looking to get a look at what Oklahoma looked like in the Ice Age," Wyckoff said.

After more than a year of tedious sifting of dirt from the site, the archaeologists turned up two flakes identified as debris from resharpening of a stone tool.

They then tried radiocarbon dating on the bones, but when that didn't work, "out of desperation we submitted a group of aquatic snails that were present around the bison skull," Wyckoff said.

The test, completed in October 1987, showed they were 31,150 years old - give or take 700 years.

Wyckoff said he allows another 20 percent for possible carbon contamination to come up with the age of about 25,000 years.

"It would be very rare for a snail shell to be more than 20 percent contaminated," he said.

The National Geographic Society turned them down for funding to do the expensive accelerator dating, saying the few flakes were not evidence enough of a human site.

"So we just decided we'll get a bunch of volunteers, go back up and dig a few more squares. We did that in September, and recovered a good broken tool as well as part of another tool and more flakes," Wyckoff said.

Wyckoff said it costs $600 a sample for testing by accelerator dating, based on extracting specific proteins from tiny amounts of material.

Meanwhile, Wyckoff said he is awaiting the results, expected by week's end, of another dating test based on the presence of uranium thorium in the bison teeth.

The Southern Methodist University doctoral student conducting that test said that while his results aren't complete yet, "he felt like . . . it was going to be fairly compatible with our" age of 25,000 years, Wyckoff said.

If so, he said it would be the most important archaeological find in the last 60 years.