The discovery of what appears to be the fossil footprint of a 7-foot-tall prehistoric bird in the Puget Sound basin has excited paleontologists.

If confirmed, it would probably be the most significant fossil find made in Western Washington, the only footprint of its kind and the first evidence large animals roamed what was once a prehistoric swamp.

Until the sandstone footprint and its site are examined in late May, however, experts say they can't confirm the find's authenticity.

"Based on the photos I have seen, I expect it is a genuine avian footprint," said Allison Anders, an expert on a 7-foot-tall flightless bird called Diatryma.

"When I saw the photographs I was absolutely dumbfounded," said Anders, who works at the Museum of Natural History in New York. "I've studied these birds so long, and then to see something look exactly like you expect is exciting."

J. Wyatt Durham and John Savage, both paleontology professors at the University of California at Berkeley, have also said it appears it could be the footprint of Diatryma.

No footprint of the extinct bird has ever been found, and although fossil bones have been found from New Mexico to the Canadian Arctic, none have been seen west of Wyoming.

The fossil was discovered on a hike by John Patterson of Maple Valley, a former Boeing employee who studied geology in college and is now a self-employed inventor.

Patterson said if the footprint can be proved to be authentic he wants it displayed in a museum in this state. He said he spoke up about his find because he fears state officials may seize it shortly and possibly fine him for removing the fossil from state land.

Patterson, a wood carver by hobby, chiseled the 85-pound sandstone piece free of a larger slab when it appeared threatened by a rising stream, and backpacked it away.

The 13-by-13-inch track is striking when seen, easily summoning up a vision of the weight and size of a 350-pound bird with a huge head and parrot beak.

Patterson said he doesn't intend to keep his find and wants no money for it.

Scientists are voicing caution, however, because until more study is done there is always the possibility the footprint is a natural depression or even a hoax.

"Seeing is believing," said Anders, who has only seen photographs.

Also cautious is John Rensberger, a paleontologist at the University of Washington's Burke Museum, who has examined the sandstone slab.

At first he told Patterson it was not a real fossil, in part because there was no other animal record here. But with other scientists suggesting Diatryma, he plans now to examine the site with an open mind.

"Footprints are very hard to verify unless you find more than one because they are just depressions," explained Rensberger. "There are a lot of natural depressions in rocks, and when you have that many there are always a few which look like footprints."

Patterson visited the slab several times before breaking out the footprint and believes many people have passed by the mossy slab without noticing the print. At one point, a young girl was standing on top of it without noticing what was underneath.

Judging from surrounding sandstone, the specimen apparently is about 50 million years old.

That would place it in the Eocene period after the age of dinosaurs, a time when Western Washington was just emerging from the sea. The ocean shoreline was roughly where Interstate 5 is now and the landscape was a tropical swamp, which produced coal beds that today extend from Bellingham to Kelso.

While plant fossils have been found, land-animal fossils have been almost non-existent. A partial tooth found near Mount Rainier is about all that has been discovered, said Rensberger.

To find a footprint - which are much rarer in the fossil record than bones - would be extraordinary.

"Rocks of that age have never produced animal fossils before," Rensberger said. "If it is a track, there should be another one" in the same formation, and that would be the best proof the track is genuine.

Anders said Diatryma fits because it inhabited coastal plains and swampy areas similar to Western Washington at that time. Even more convincing to him is the shape of the talon depressions at the front of the foot.

Paleontologists had theorized for years that Diatryma was a fleet-footed carnivore with sharp nails to catch prey.

But in 1988 after several years of study, Anders concluded the bird was more likely a slow-walking plant-eater with nails that were blunter and closer to hooves.

That fits Patterson's specimen.

The footprint also fits the expected size for Diatryma, and a deeper depression midway along the track mimics what would be expected if the bird rocked forward on its forefoot, as scientists expect after examining skeletal remains.

Anders is applying for state permits to investigate and possibly excavate the site, while Patterson is expecting a visit soon from state officials demanding the fossil be placed in their custody.

"It's got to go back to the state because, of course, they own it," Patterson agreed. "I just don't want it to end up in the basement of the Burke Museum. There's never been a Diatryma track found, and if this is proven the people of the state of Washington should get to see it."