On a dry, chilly morning in the Southern Colorado grasslands, Bruce Schumacher led a group of AmeriCorps volunteers across the narrow, shallow Purgatoire River to an out-of-place bump in the landscape.

He pushed aside loose, reddish dirt with his hands, revealing a bone the size of a microwave oven — the limb of a sauropod dinosaur, said Schumacher, a paleontologist. The workers were surprised that the fossil was just sitting out on the plains, unguarded. Two of them joked about coming back at night to pull it out of the ground.

A two-hour ride down a dirt road, far from cellphone service or any other signs of human life, Picketwire Canyon is a dinosaur lover’s dream, largely because of hundreds of hubcap-size theropod and sauropod footprints pressed into a nearby layer of limestone, which abuts the river.

Schumacher, 48, who lives in La Junta, Colo., is one of two field paleontologists employed by the U.S. Forest Service. They are responsible for protecting and promoting dinosaur remnants scattered across the agency’s 193 million acres, primarily in the Rocky Mountain states.

The fossil site to which Schumacher led the volunteers is just one of many in the Morrison Formation, layers of rock lining 100-foot-tall mesas alongside the canyon. The site was discovered months beforehand, but because there were too many dinosaur bones and too few people to excavate them, it was reburied, for protection against thieves and the elements. For months, these bones lay undisturbed in the grasslands like buried treasure.

The two stewards of the Forest Service’s dinosaur bones are far outnumbered by the approximately 350 archaeologists employed to manage the traces of human life though the dinosaurs were around far longer than humans have been.

“It all goes back to the laws and the homocentricity of those laws,” Schumacher said of the disparity.

So to get dinosaur bones from Picketwire Canyon to museums and scientists, Schumacher has developed a creative strategy. Twice a year, for a week at a time, these bones and footprints are uncovered by a group of about two dozen volunteers, many in their 70s and 80s, whom Schumacher has been training for the last 15 years.

Because of the program’s popularity, he no longer advertises it. Most of the volunteers have devotedly returned for every project and become a highly skilled team.

During their first week, in 2001, volunteers searched along the lower edges of the canyon walls that frame the valley for the bluish-white fossils that stand out among the brown and gray rocks and bright yellow grasses — “developing their bone goggles,” Schumacher calls it.

By the second-to-last day, they’d found nothing. So one volunteer took from his knapsack some petrified wood he had brought back to show his dejected partners.

“That’s not wood,” Schumacher recalled telling the volunteer, who led the team back to where he had found the fossil.

There, over the next several seasons, the crew spent a week or two every year uncovering a Camarasaurus that they named Woody. In all, they pulled out about 15 percent of the skeleton, now on display in the Sternberg Museum of Natural History at Fort Hays State University in Kansas.

Most of the volunteers are amateurs interested in paleontology, like a retired meat cutter, a retired secretary of an oil and gas company and a retired aerospace engineer..

Bigger bones often require several weeks of tedious work, spread out over seasons, as the volunteers delicately coax them out of clay or mudstone.

Theropod tracks in the riverbed follow, and sometimes overlap, those of sauropods. For this reason, some at the site speculated that the tracks might be a game trail: theropods stalking their sauropod prey.

“This is where the big boy started getting in trouble,” said Sonny Fernandez, 77, resting by sauropod tracks that get deeper with each step and appear to be angling erratically left. “All those young ones seen him coming, said, ‘Hey, fat boy.’ ”