Washington state has no laws governing finds of this type. And Jack Horner of the Museum of the Rockies and Montana State University said that is true anywhere in the United States.
"Americans like their private land," said Horner, one of the nation's most famous paleontologists. Americans don't like to pass laws putting restrictions on owners of private land, even to protect history, he said.
The landowner decided to donate the tusk to the Burke Museum of Natural History and Culture at the University of Washington, just as Horner hoped would happen.
It's a relatively rare find and should be preserved for educational reasons, so children will know mammoth elephants once lived in Seattle, he said.
"A lot of times, people think these things are worth a lot of money," Horner said. Their true value is educational, not what someone can sell a tusk for on eBay, he said.
As paleontologists and graduate students began carefully digging away the dirt around the tusk on Thursday afternoon, Julie Stein, executive director of the museum, said AMLI Residential has been wonderful to work with.
Scott Koppelman, senior vice president of AMLI Residential, said that after contractors found the fossil buried about 25-to-30 feet below street level, the company turned quickly to the Burke museum for assistance.
Company officials' first response when they heard of the find was to think of the community benefit, Koppelman said.
"The excavation will cause us some construction delay. But the scientific and educational benefits of this discovery clearly outweigh the costs and delay. This is an exciting discovery for our local Northwest history," Koppelman said.
Mammoth elephants lived all over the United States and Europe in ancient times, but finding a tusk or any part of those animals is rare, Horner and other experts said.
"We don't find them every year or even every five years," he said. In most cases, artifacts found at construction sites are destroyed by a big machine before anyone even notices them, Horner said.
Allyson Brooks, the Washington state preservation officer, said the situation would be different if construction workers had found human remains or other items or archaeological value because Washington state has laws for those situations.
With paleontological finds, the landowner can do whatever he or she wants - sell it, destroy it, donate it or ignore it, Brooks said.
In 2004, Washington state halted construction on a section of a major bridge project, on which $58 million had already been spent, at the request of the Lower Elwha Klallam Tribe after remains of an ancient Indian village and burial ground were discovered.
Discoveries of animal remains from the Ice Age are less common than human remains in western Washington. Preservation of bone and tusks depends on the environmental conditions, such as the water table, the acidity of the soil and how deeply the object was buried, Brooks said.
"A lot of time, teeth preserve better than other bones," she said, likening tusks to teeth. Teeth and tusks are what she and the scientists she works with consider "biological rock," Brooks said.
The last big find of an ancient animal of this sort in western Washington happened in 1977, when a Mastodon tusk was found near Sequim, Wash., on the Olympic Peninsula.
Mammoths and Mastodons are related and probably roamed the Earth around the same time. Both were very large and hairy. Mammoths and modern-day elephants are members of the same biological family.
Scientists at the Burke believe this tusk came from a Columbian mammoth, which is the Washington state fossil. The tusk, which could be as large as 8 feet long, is expected to be the largest and most intact mammoth tusk ever found in the Seattle area.
Gov. Jay Inslee did not wish to comment on the tusk discovery, but a member of his team had something to say.
"This has turned every adult in Seattle back into a 10-year-old," said Jaime Smith, the governor's spokeswoman.