While searching for rare agate, Larry Walker happens upon a smattering of fossilized dinosaur claws on a plateau near Green River. Should he alert experts or uncover the secret graveyard himself? For three years, he illegally chips away at the rock, risking detection only when cash to continue his dig runs low.

Each transaction was simple and discreet.

Larry Walker prowled the parking lots at the Tucson Gem and Mineral Show in the late 1990s, approaching rockhounds as if scalping tickets. "Interested in a dinosaur claw?"

He didn't brag or talk up customers despite the fact that he was selling the bones of a new dinosaur, one scientists would later describe as a missing link in one family's transition from meat eaters to plant chompers.

Walker had a cover story for the curious, although few asked questions. Needing cash to continue secretly unearthing the beast, Walker lied when he had to, telling prospective buyers the bones he mined were 100 million-year-old Utahraptor fossils from a private ranch near Blanding.

Walker actually stumbled upon the bones while rummaging for rare agate in the desert near Green River. When he first happened on the plateau-top site, fossilized claws littered the ground. Digging into the soft mudstone, he found more bones.

He spent the next three years mining these bones, eventually recognizing that differences in the neck vertebrae suggested they couldn't belong to the Utahraptor.

Walker also realized that excavating fossils from federal land broke the law, yet his infatuation with the find compelled him to continue his clandestine search.

In a makeshift lab in his Moab home, Walker sifted through remains of dozens of dinosaurs for enough bones to create his own composite skeleton. He viewed the "extras," especially the vicious-looking claws, as a way to help pay the bills.

His enterprise wasn't unique.

Determined buyers have little trouble finding illegal fossils amid the din of gem and fossil expositions in cities such as Tucson and Denver, where dealers crowd ballrooms and exhibition spaces and also pile hotel rooms full of geodes and gems.

"The black market is fairly visible," said James Kirkland, Utah's state paleontologist.

An illegal fossil trade thrives partly because Utah and surrounding states are "like Egypt with mummies," Kirkland said.

Layers of rock on undeveloped land erode to bring new fossils to light each year, creating a haven for paleontologists. But the terrain also proves an irresistible draw for fossil thieves.

In the time it takes paleontologists to properly excavate one bone, thieves can plunder a dozen.

"There's simply not enough eyes and ears to see every inch of land all the time," said Laura Williams of the U.S. Bureau of Land Management.

Fossil thieves working the badlands - like Walker - often are self-educated, developing their passion through books and on the Internet, said Gib Wilson, an FBI agent working to stem the flow of bones from Utah.

"Dinosaurs get in the blood," he said.

Some bones sold at the expositions are smuggled from China, a fossil-rich nation with poor paleontological resources protection. Fossils unlawfully removed from other countries may be sold legally in the United States. But the U.S. government actively pursues anyone selling illicitly obtained indigenous bones.

The FBI and the BLM took notice when bones Walker sold under the table for $100 started appearing among dealers' offerings for $600 or more.

By 2001, his claws became hot sellers. Because Walker sold no goods in the open, most of the profits eluded him.

Investigators weren't the only ones interested in his bones.

As claws disappeared into private hands, paleontologists hungry for career-making finds also took note. Their expert eyes began to suspect the supposed Utahraptor bones might be from an unknown therizinosaur.

Kirkland, before taking over as Utah's state paleontologist, helped find North America's first therizinosaur a few years earlier in New Mexico. Until then, all discoveries of these bizarre, clawed dinosaurs had occurred in China.

Volunteers from Kirkland's Grand Junction, Colo., lab suspiciously noted the bones' appearance didn't match their story.

Dealers spun tales that these bones came from the Morrison formation on a private ranch near Blanding. The light color of the claws looked more like fossils found in the Cedar Mountain formation, named for a geological feature that snakes through southern Utah like a stone river.

With no Cedar Mountain outcrops anywhere near Blanding, the bones had to be coming from elsewhere.

The conflicting facts about the black market bones intrigued Kirkland.

He would become the second person irresistibly drawn to this fossilized beast.

glavine@sltrib.com