Eric Prokopi, of Gainesville, in the five-thousand-square-foot fossil workshop that he built in his back yard. Photograph by Richard Barnes

Natural history goes to auction five or six times a year in America, and one Sunday last May a big sale took place in Chelsea, at the onetime home of the Dia Center for the Arts. The bidding, organized by a company called Heritage Auctions, began with two amethyst geodes that, when paired, resembled the ears of an alert rabbit. Then came meteorites, petrified wood, and elephant tusks; centipedes, scorpions, and spiders preserved in amber; rare quartzes, crystals, and fossils. The fossils ranged from small Eocene swimmers imprinted on rock to the remains of late-Cretaceous dinosaurs. That day, the articulated toe and claw of a Moroccan dinosaur sold for sixty-three hundred dollars. A tyrannosaur tooth—ten and a half inches from root to spike—went for nearly forty thousand.

Along one wall, behind ropes, loomed the skeleton of a Tarbosaurus bataar. T. bataar, as it is known, was a Tyrannosaurus rex cousin that lived some seventy million years ago, in what is now the Gobi Desert of southern Mongolia. Eight feet tall and twenty-four feet long, the specimen had been mounted in a predatory running position, with its arms out and its jaws open, as if determined to eat Lot No. 49220—a cast Komodo dragon, crouching ten yards away, on blue velvet.

After a German sea-lily fossil sold to a live bidder, for forty thousand dollars, Greg Rohan, Heritage’s president, who had been standing near the lectern, handed the auctioneer a note. The auctioneer announced, “The sale of this next lot will be contingent upon a satisfactory resolution of a court proceeding.” He was talking about the dinosaur, which he called the auction’s “signature item.” Largely intact dinosaur skeletons are not easily found, and this specimen had been advertised as seventy-five per cent complete. “It can fit in all rooms ten feet high,” the auctioneer added. “So it’s also a great decorative piece.”

As the bidding opened, at eight hundred and seventy-five thousand dollars, Robert Painter, an attorney from Houston, stood up, a BlackBerry in his hand. Painter is six feet three and forty-two, with dark hair, rimless eyeglasses, and a deep voice. “I hate to interrupt this,” he told the room. “But I have the judge on the phone.” The previous day, Carlos Cortez—a state district judge in Dallas, where Heritage has its headquarters—had signed a temporary restraining order forbidding the company to auction the T. bataar, on the ground that the dinosaur was believed to have been stolen from Mongolia. The judge, defied, was not pleased.

The auction had come to the attention of the Mongolian government the preceding Friday, after Bolortsetseg Minjin, a Mongolian paleontologist who lives in New York, saw a television report about the auction and suspected that the dinosaur had been taken from her country. Bolor, as she is called, discovered that the online auction catalogue listed the item’s provenance as “Central Asia”—a vague term often considered code for Mongolia and China, both of which forbid the commercial export of fossils found within their borders. Other catalogue items, such as the tyrannosaur tooth, openly referred to the Nemegt Formation, a fossil-rich expanse of sandstone and mudstone in the Mongolian Gobi.

Bolor e-mailed Oyungerel Tsedevdamba, an aide to President Tsakhia Elbegdorj. Mongolian dinosaur fossils had appeared on the black market for years, but there had been few, if any, organized efforts to stop their sale. Even though the Mongolians had only two days to intervene in the auction, they decided to try. Bolor enlisted two of the world’s top experts in Mongolian dinosaurs—Mark Norell, the head of the American Museum of Natural History’s paleontology division, and Phil Currie, of the University of Alberta—who wrote open letters of protest. Norell argued, “These specimens are the patrimony of the Mongolian people and should be in a museum in Mongolia.” The letters were distributed to reporters. Online, paleontologists, geologists, students, and Mongolians signed a petition against the auction, adding comments: “Fossils belong in museums where EVERYONE can see and learn from them, not in some rich, fat douchebag’s mansion or in some Wall Street office”; “Mongolian fossils are spectacular . . . selling them as mantelpieces is akin to using the Mona Lisa as a placemat.”

Bolor wrote to Heritage: Where had the T. bataar come from? Did it possess provenance papers? Heritage’s attorney replied, “Although we appreciate your concerns . . . it is our conclusion that no impropriety exists.” He added, “Mongolia won its independence in 1921 and this specimen is quite a bit older than that.”

The Mongolian government lawyered up, retaining Painter, the Houston attorney. He had experience representing Western mining interests in Mongolia, whose vast, untapped reserves of copper, gold, and coal are at the center of an international scramble. Like most lawyers, he had never handled a case involving a dinosaur, but he drafted the restraining order, got Judge Cortez to sign it, and boarded a plane for New York.

As Painter interrupted the auction, his BlackBerry aloft, the auctioneer eyed him but never broke patter. He called for a bid of nine hundred thousand dollars.

Rohan, Heritage’s president, met Painter in the aisle, and for five seconds they squared off in a quiet little dance, four arms waving. A security guard stepped in. Painter repeated that he had Judge Cortez on the phone. “O.K.—well, you need to walk,” the guard said, escorting Painter to the rear of the auction floor. Outside, on the sidewalk, a small, pro-Mongolia protest had formed, with banners reading “National Heritage Is Not for Sale” and “Return Our Stolen Treasure.”

An attorney for Heritage approached Painter, who handed him his BlackBerry. While the attorney was having an awkward discussion with Judge Cortez, the dinosaur sold to an anonymous phone bidder, for nearly a million dollars.

Heritage brokered the T. bataar on behalf of a thirty-seven-year-old bone hunter named Eric Prokopi, who lives in Florida, a great state for fossils. For roughly the first half of the past fifty million years, the region lay beneath a warm, shallow sea. As land repeatedly surfaced and receded, the remains of marine creatures got mixed up with those of terrestrials, forming one big Ice Age graveyard: sea cows, prehistoric sharks, spike-tailed armadillos the size of refrigerators.

Shark teeth attract kids to fossil hunting because they’re so easy to find. Sharks shed thousands of teeth per year, and have been doing so for eons. The teeth, exposed by erosion and tides, can be as big as a human hand. The largest look like the arrowheads of giants, and can sell for thousands of dollars.

Prokopi, who grew up outside Tampa, is the son of a music teacher and a homemaker. He found his first shark tooth as a small boy, in the late seventies, at nearby Venice Beach. By age ten, he had a diving license. His mother, a competitive swimmer, accompanied him on river expeditions. As he explored underwater, holding a rope, she rode in a canoe, tugging the line if she saw an alligator.

Through fossil clubs and field trips to quarries, Prokopi got to know older hunters who spent their lives beachcombing or standing chest deep in muck, searching for bone. Paleontology books explained what he’d found and taught him what to look for next. When he was in high school, fossils began to take over the family’s house, and around 1990 he started selling them, making eight hundred dollars at his first trade show, in Lakeland. At such events, he bartered with other hunters, who often brought entire trailers filled with specimens. Some fossils were still sheathed in “field jackets”—the lumpy white plaster encasements that excavators apply at dig sites, for safe transport, making the artifacts look like misshapen mummies.