Scientists working in Laurel's Dinosaur Park on Wednesday excavated the largest dinosaur fossil found in Maryland in five years.
It's too early to say for sure what type of bone it was. "It's not a femur; maybe part of a femur head," said Smithsonian fossil preparator Steve Jabo, 50, who did most of the digging to free the fossil bone from the site's dense clay.
The importance of the discovery won't be known until after the fossil is cleaned and studied at the Smithsonian Institution's National Museum of Natural History, Jabo said.
But he said it could be a portion of a leg bone of a plant-eating sauropod, perhaps a relative of Astrodon johnstoni — the Maryland state dinosaur — whose teeth and other remains have been turning up in the area for 150 years.
A 6-foot-long, 239-pound Astrodon femur was discovered there in 1991, and a 2-foot specimen emerged in 2006.
They all date to the middle of the Cretaceous Era, about 112 million years ago, when the Interstate 95 corridor was a coastal bayou much like southern Louisiana today, with meandering rivers and oxbow lakes.
The latest discovery was poking out of the clay Sept. 10 when amateur paleontologist David Hacker spotted it while inspecting the site for new fossils exposed by heavy rains from remnants of Tropical Storm Lee.
Hacker, 52, is the lead volunteer at the park, and he's been collecting there for years. He checks the site regularly and cares for the fossils until they can be transferred to the Smithsonian.
Before the dig, the visible portion of his find was 8 to 10 inches wide and appeared to some to be the femoral condyle — the lower part of the thigh bone that forms the upper half of the knee joint.
"David and [amateur paleontologist] Mike Styer probed around it and couldn't determine how far [into the dirt] it is," said Peter Kranz, a Washington geologist who runs public programs at the park. "There could be very little left of it, or it could be the whole thing."
Rather than risk damaging the fragile bone, Kranz said, "they both came to the conclusion that the best thing is to have the Smithsonian technicians get it out."
"I think it's a dinosaur bone, for sure," Jabo said soon after he arrived at the site. "There was nothing quite that big that wouldn't be a dinosaur here."
During the hourlong dig, Jabo and fellow preparator Peter Kroehler, 56, carefully cleared away the sticky clay, ironstone pebbles and lignite — the charcoal-like remains of ancient trees — that encased the fossil.
They placed white plaster bandages over the exposed portions of the bone to protect it from breaking, and resumed digging with a trowel and scalpel.
At first, it appeared the bone extended deeper into the clay, raising hopes that the entire 4-to-6-foot shaft of a leg bone had been preserved. But after removing more clay and stone from around and beneath the bone, Jabo found there really wasn't much more to it beneath the exposed portion.
He finally asked Hacker to lift the fossilized bone off the clay pedestal his digging had left beneath it.
Free of the earth for the first time in 112 million years, the bone was about the size of a football, weighed what Jabo judged to be two or three pounds, and remains something of a puzzle.
"We'll bring it back to the lab, clean it up, prep it out," Jabo said. "We'll figure out what it is and report back to … everybody, catalog it and put it in the collection."
Heavy rain since late August eroded other fossils from the clay. On Sunday, Kranz found what appeared to be the end of a much smaller dinosaur femur, "probably from a therapod [a meat-eater] of some sort," he said.
There were also some bits of turtle shell, a broken claw, and what appeared to be a tail vertebra, he said.
The deposit, part of what geologists call the Arundel Formation, holds fossil remains of life forms from the Cretaceous Era.
It has yielded up the remains of dinosaurs, turtles crocodiles, small mammals, snails and mussels. Their bones and shells were washed into what was likely a coastal river, sank into the muck and eventually turned to stone.
The first dinosaur fossils found there emerged in 1859, when Philip Tyson, then Maryland's state agricultural chemist, found a large tooth. It was later linked to a large plant-eating dinosaur called Astrodon johnstoni, since designated the state dinosaur.
Prince George's County Archaeologist Donald Creveling has called the site "one of the most prolific sites for dinosaur and plant fossils east of the Mississippi River."
But because the remains were laid down in an environment where they could be torn apart by predators and currents, most fossil finds have been small and "disarticulated," rather than complete skeletons.
The largest surfaced in 1991, when amateur paleontologist Arnold Norden and his children found a 6-foot-long femur in the clay.
In 2009, the 7.5-acre Dinosaur Park in Muirkirk, near Laurel, opened to the public, on the site of a 19th-century iron mine and, later, clay pit where the General Shale Co. operated a brick factory.
The park, operated by the Prince George's County Department of Parks and Recreation, provides opportunities for scientists and the public to search for fossils and related traces of their environment. All finds go to the Smithsonian.
"Over the past two years, we have been receiving 3,500 visitors per year at our open houses," Creveling said. "It's been very popular. It's a great way to encourage children to become interested in science."
The Dinosaur Park is open to the public from noon to 4 p.m. on the first and third Saturday of every month. Call 301-627-7755.