A creature from the late Cretaceous period smashed sales records on Tuesday in an auction that also included works by Picasso, Pollock and Monet, leaving auction watchers wondering which anonymous buyer now owned a multimillion-dollar Tyrannosaurus rex.

The T. rex skeleton, nicknamed Stan, closed the 20th Century Evening Sale, nearly quadrupling its high estimate of $8 million to bring in $31.8 million, with fees. In the 20-minute bidding war that ended with buyers on the telephone in London and New York, the price rocketed up from a start of $3 million, with the final bid ultimately taken in London by James Hyslop, head of the auction house’s Scientific Instruments, Globes and Natural History department. The buyer has not been identified.

It’s rare that paleontologists find Tyrannosaur fossils as complete as Stan, according to Mr. Hyslop, and even rarer that such skeletons appear on the market. The last time a comparable specimen came to auction was in 1997, when a T. rex named Sue sold for $8.36 million — or nearly $13.5 million today, given the rate of inflation — to the Field Museum of Natural History in Chicago.

“I’ll never forget the moment I came face to face with him for the first time,” in Colorado, Mr. Hyslop said of Stan. “He looked even larger and more ferocious than I’d imagined.”

Christie’s New York, facing the pandemic’s unpredictable art market, found a way to break with auction tradition. For starters, it chose to stage the auction in early October, avoiding its usual November date that this year would have followed the presidential election; the company hopes to mount another large sale before the December holidays.

Christie’s has also retrofitted its typical in-person experience for digital audiences by livestreaming the sale with commentary from market experts, reducing its famously thick catalogs to miniature versions and overhauling the staging of its auction with help from an outside production company, Gradient Experiential.

“These are not temporary changes,” Adrien Meyer, Christie’s chairman of global private sales, said in an interview before the auction. “When we go beyond our traditional values, it’s because there is a feeling that clients will respond.”

That effort included featuring the dinosaur fossil among Impressionist and Modern paintings.

The decision to put Stan at the forefront of the marketing campaign seems partly responsible for attracting interest in the larger sale. Last month, Christie’s redesigned its public galleries so that the dinosaur could peer onto Rockefeller Center from the auctioneer’s flagship store, where it will remain on view through Oct. 21. The fossil, named after the amateur paleontologist Stan Sacrison who discovered the remains in 1987, has spent recent years at the Black Hills Institute of Geological Research in South Dakota for study.

Standing 13 feet high and 40 feet long, Stan casts an imposing shadow. Over the last two decades, researchers have theorized that punctures in Stan’s skull and fused neck vertebrae demonstrate that this Tyrannosaur was a warrior, one likely to have survived attacks from his own species. Scientists also estimate that the dinosaur would have weighed nearly 8 tons when it was alive, more than twice the weight of a modern African elephant.

“I thought it would go for a lot more money,” said Mark Norell, a paleontologist at the American Museum of Natural History, explaining that the sale’s lack of reproduction rights for three-dimensional models and related online merchandise was a deal breaker for many public institutions. “We would never purchase something unless we owned the rights.”

Some members of the scientific community have also argued that dinosaur fossils like Stan should not be sold to private collectors, explaining that such competition has increasingly priced museums out of the running. The booming market for fossils has even attracted Hollywood stars, including Nicolas Cage and Leonardo DiCaprio.

“It’s an astounding amount of money,” Darla Zelenitsky, associate professor of dinosaur paleobiology at the University of Calgary, said of Stan’s final figure. “I think it would make it tough for museums to buy fossils, especially at a price peg like that.”

For now, Stan’s buyer has remained anonymous, although members of the scientific community speculate that the collector is from the Middle East. “If you noticed, it was sold on the London desk,” said Mr. Norell. “That usually means it’s Middle Eastern money, and I know for a fact that there was Middle Eastern interest in the fossil.”

Last month, the Society of Vertebrate Paleontology published a letter urging Christie’s to restrict the sale to public institutions committed to curating specimens for the public good, writing, “Scientifically important vertebrate fossils are part of our collective natural heritage and deserve to be held in public trust.”

But the sale went ahead as planned, with Stan bringing energy to an otherwise muted auction. Of the 59 works, four were withdrawn before the auction began; nine failed to sell.

Nevertheless, Christie’s reported having 280,000 viewers for the evening sale, more than triple the viewership record it set in July. The auction house also set two other sales records, in addition to Stan. One was $7.3 million for a painting by the German artist Emil Nolde, and another was $28.7 million for a work on paper by the French artist Paul Cézanne.

“After several months of a new Covid-19 environment,” Mr. Meyer, Christie’s chairman, said, “our clients are still in a buying mood.”