LOCUST GROVE, Ohio (AP) — During a day of digging along stream banks, Tom Johnson turned up dozens of trilobite pieces.
He squatted for hours, quarrying layers of limestone and shale that had formed hundreds of millions of years ago when Adams County — and most of the state, for that matter — lay under a warm, shallow ocean.
"Those are trilobite tracks," Johnson said, pointing to faint lines on one rock.
"That's a part of a tail," he said of a dark spot on another.
Trilobites were invertebrate marine animals that thrived for nearly 300 million years. The last of the 20,000 species died out 299 million years ago.
Johnson has been collecting, selling and donating trilobite fossils — some as small as his thumbnail, others as large as serving trays — for more than 30 years.
"The little ones I can sell on eBay for $10 or $20," he said.
But it's those big trilobites, such as Isotelus, Ohio's state fossil, that keep Johnson digging every day in two 11-acre lots.
His most-recent find went to Ohio State University's Orton Geological Museum in late March. The fossil is 14.5 inches long and sold for $14,000. The buyer, a retired chemist with his own collection, donated it to the museum.
"I almost fell over," said Dale Gnidovec, the museum's curator. "His girlfriend told him the specimen was too nice to be kept in their home, and he agreed that it should be in a museum."
Over time, new trilobite species appeared and old ones died out, Gnidovec said. Some were smooth and others spiny. Some were blind, and others had huge, compound eyes.
Isotelus, which was named state fossil in 1985, lived late in the Ordovician Period, about 448 million years ago.
"A horseshoe crab is probably the closest modern comparison," Gnidovec said.
Isotelus fossils are rare, said Joe Hannibal, curator of invertebrate paleontology at the Cleveland Museum of Natural History.
"Not only are those pieces more difficult to recognize, but it requires a certain amount of skill to put the pieces back together," he said.
Johnson, 61, began collecting fossils near his family's summer home in Canada when he was 4 years old. He said an elementary schoolteacher in Michigan introduced him to trilobites. By age 10, he was selling them.
Johnson said he decided to become a full-time fossil hunter in 1978. Since then, he has found thousands and has lent hundreds to museums, including the Smithsonian Institution.
His biggest find — a 16-inch-long Isotelus maximus — is in his own rock-and-mineral shop.
For a trilobite hunter, finding an Isotelus is like unearthing a Tyrannosaurus rex, Johnson said.
"It's like going out and looking for diamonds," he said.
Johnson said he always hopes to place the biggest, best specimens in museums to inspire other enthusiasts.
The one at the geological museum at Ohio State took several months to piece together.
And two museums are on a waiting list for his next big finds, Johnson said.
"I don't see an end to this unless I go blind," he said. "I haven't found the biggest one yet."
Information from: The Columbus Dispatch, http://www.dispatch.com