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Oct. 2 (Bloomberg) — Samson is big, bony, really old and possibly female.

He’s also not cheap.

The 40-foot-long, 66-million-year-old Tyrannosaurus rex bears a price estimate of $6 million and forms the centerpiece of Bonhams & Butterfields 42-lot “Natural History” auction.

It takes place tomorrow at the Venetian hotel and casino on the Las Vegas strip. The entire sale is estimated to fetch $8 million.

“Because of its stature, it certainly belongs in some sort of institution,” said Thomas E. Lindgren, co-director of Bonhams natural-history department, of Samson. The skeleton is the third-most-complete T. rex ever excavated, Bonhams says.

The sale was assembled after Lindgren received a call from the anonymous seller three months ago. Samson was discovered on a South Dakota ranch in 1987 and has changed hands since then.

Samson is the first major T. rex specimen up for grabs since Sotheby’s 1997 auction of a T. rex skeleton dubbed Sue and similar in size. Chicago’s Field Museum acquired Sue for a record $8.36 million, which has remained the top price for a dinosaur, or any fossil, according to Lindgren. Sue is considered the largest and most complete T. rex skeleton ever excavated.

Despite the masculine moniker, Samson is probably female, based on her robust size and the calcium build-up in her bones that occurs when female dinosaurs prepare to lay eggs, explained Lindgren.

“In retrospect, the owner probably should have used Delilah,” said Lindgren.

Crowd Pleasers

Major dinosaur specimens are usually collected by natural- history museums in the U.S., Europe and Asia because they boost attendance. When the Field Museum unveiled Sue in 2000, lines formed around the block.

The dinosaur collection of New York’s American Museum of Natural History is “by far the most popular attraction here,” said Mark Norell, chairman of the paleontology division. “It’s incredibly popular, and doesn’t seem to be slowing down.”

Private collectors generally seek out smaller items, such as a dinosaur arm or tooth, which can be displayed in the home and cost in the low thousands. Bonhams’s sale includes the lower jawbone of a Triceratops, with a top estimate of $10,000. A 19- inch-long Triceratops vertebra is estimated at $1,500.

High-profile collectors include Sheikh Saud Al-Thani of Qatar, Nathan Myhrvold, the retired Microsoft technology officer, and actors Nicholas Cage and Leonardo DiCaprio, according to dealers.

‘Jurassic Park’

The market for dinosaurs was moribund until the 1970s. “The whole dinosaur thing had gone out of vogue,” said Peter Larson, president of the South Dakota-based Black Hills Institute for Geological Research, a for-profit supplier of fossils and replicas. Larson, who led the team that excavated Sue, is consulting with Bonhams on the sale.

Scientific discoveries in the 1970s spurred curiosity. “There was more interest in dinosaurs. Museums wanted to rejuvenate exhibitions and needed to increase attendance,” Larson said.

Michael Crichton’s bestselling 1990 “Jurassic Park,’ and Steven Spielberg’s 1993 Oscar-winning film also boosted popularity. Prices rose steadily, although the recession has tempered museum buying, Larson said, and he expects more attention from private bidders.

‘‘I’m seeing limited interest from the scientific community,” he said. “Not so much because of the scientific importance but because of the short time to put together the financing.”