Late on this misty morning in Dinosaur Park, Steve Jabo of the Smithsonian Institution kneels on an embankment and begins scraping around a piece of prehistory with his thumb and forefinger. He carefully pushes dirt and debris clear of a just-larger-than-fist-size fossil, roughly triangular, poking out of the ground.

There’s no doubt it’s a fossilized dinosaur bone, Jabo says. What type of bone, and from what species of dinosaur, is unclear. But the fossil’s rounded dome suggests the end of an arm or leg bone. It’s easy to imagine a five-foot femur stuck deep into the moist earth.

The heavy rain of late summer exposed the fossil, the largest found here since 2006.

Park volunteer Dave Hacker spied it on Sept. 10, a few days after the rain stopped. That’s the best time for prospecting. “The fossils are here,” says Hacker, 52, who lives in Silver Spring. “We find bits and pieces all the time.”

That Sunday, Hacker picked up a small vertebra and a piece of a claw. He was ready to head home when he noticed something bigger.

He called Peter Kranz, a paleontologist who spearheaded the effort to preserve this 40-acre patch of land — some of the richest fossil grounds east of the Mississippi.

Kranz then called the Smithsonian, whose National Museum of Natural History houses the fossil that was pulled from the park in 2006. That was a tibia from a theropod, the two-legged carnivore typified by Tyrannosaurus rex.

But Hacker is convinced that this fossil is not a theropod. As he watches Jabo work a widening circle with a trowel, Hacker guesses it’s from a long-necked, heavy-bodied plant-eater known as a sauropod. Sauropod fossils are the most common type found in the area.

In 1858, Maryland’s state geologist, Philip Thomas Tyson, recovered two sauropod teeth near here. The species was later dubbed Astrodon johnstoni, and in 1998 it became the Maryland state dinosaur. The behemoths grew to 60 feet in length, 10 feet or taller at the shoulder, with long, swinging necks good for plucking leaves from treetops.

Kranz rediscovered these fossil grounds in the late 1980s. Since then, the patch has yielded hundreds of teeth, claws and other bone fragments from dinosaurs, crocodiles, turtles and sharks. They all thrived in the warm swamplands — similar to Louisiana today — that dominated this area in the Cretaceous period about 110 million years ago.

In 2009, the Maryland-National Capital Park and Planning Commission declared the area Dinosaur Park. Real estate developer Jackson Shaw donated seven acres, sparing it from becoming another in the company’s strip of large warehouses.

Jabo sets down his trowel and directs Hacker’s daughter, Laura, another park volunteer, to lay eight-inch strips of wet plaster cloth on top of the fossil.

“We don’t want it to survive millions of years and then destroy it,” Hacker says.

Plastering complete, Jabo goes back to his gentle digging. Another inch of the fossil appears. If this really is a big bone, the excavation could take days.

But after another half an hour, the job is complete. It’s not the rounded top of a femur after all — or if it is, the rest of the leg bone is long gone. The piece comes out as a four-pound hunk that tapers to a sharp-edged bottom.

Is it from an Astrodon ? Jabo shrugs. He’ll take it to the museum, clean it and call in other experts to help identify it.

“They aren’t dug up with nametags attached,” Kranz says a short time later. “It’s a good bone. It’d just be nice if it were attached to the rest of the skeleton.”