KENT CITY, Mich. — It all started with a very large femur.
When Kevin Busscher dipped the scoop of his excavator into the soft Michigan soil last month, he knew the thigh bone he plucked from the dirt was far too big to have belonged to a cow or a horse. And he knew the culvert he was replacing would have to wait.
“My first thought was, ‘woolly mammoth!’” said Mr. Busscher, who reported his find to the county officials overseeing the project, who relayed photos of the bones to scientists.
As it turned out, he had found the skeleton of a mastodon, an elephant-like beast that roamed North America during the last ice age. By the next morning, a team of university and museum researchers had assembled to extract the rest of the bones. When they pulled out the mastodon’s massive jaw, several bright white teeth were still in place.
That discovery, a few feet below the ground between a rural road and a hayfield, was the latest in a long tradition of construction workers becoming accidental paleontologists. Over the years, construction crews have stumbled upon horned dinosaurs in Colorado, horse bones dating back thousands of years in Nevada and a mammoth graveyard in South Dakota, turning job sites for new houses, backyard pools and government buildings into spontaneous science labs.
“As paleontologists, we wish that we could go out with this kind of heavy equipment and start cutting through, looking through hills and things like that, but we don’t really get to,” said Blaine Schubert, a professor at East Tennessee State University who oversees the Gray Fossil Site and Museum, which became a research area after workers on a highway project discovered a trove of bones in 2000.
There is a natural symbiosis between paleontology and construction, both professions where digging in the dirt is part of a day’s work. And because there are so many more construction workers than there are paleontologists, and because of the powerful machinery used for building, it makes sense that construction workers are often the first to uncover bones.
Joe Sertich, who until recently was the curator of dinosaurs at the Denver Museum of Nature & Science, said he routinely heard from people who thought they may have found notable fossils. Sometimes they were false alarms or relatively minor finds, like common fish fossils or bones so badly damaged by construction equipment that they held limited scientific value. But every now and then, construction workers stumbled into major discoveries.
He helped excavate thousands of ice age fossils, including mammoths, mastodons, camels, plants and insects, at the site of a reservoir expansion in Snowmass Village, Colo. And in the Denver suburbs, at two separate construction sites, one for a police and fire station and another for an assisted living facility, workers turned up horned dinosaur remains.
“I put together huge expeditions around the country to go out and spend eight weeks digging in remote field areas, looking for things like horned dinosaurs,” Dr. Sertich said. “And it turns out that some of those finds are sitting right in our own backyards.”
In Hot Springs, S.D., work on a housing development stopped abruptly in the 1970s when workers found the skeleton of a mammoth. When Jim I. Mead and other paleontologists went there and started digging, they found another skeleton, and then another. The site, Dr. Mead said, turned out to be a long-ago sinkhole pond where mammoth after mammoth drowned after finding itself unable to climb out. The housing developer agreed to stop building, and decades later, mammoths are still being uncovered there.
“We’re totally lucky,” said Dr. Mead, now the director of research at the Mammoth Site, which hosts tourists, school groups and scientists. “It’s just phenomenal that this person said, ‘I want this to be preserved.’”
There can sometimes be tension between science and construction. Unlike with human remains and Native American cultural artifacts, there is often no legal requirement in the United States to report paleontological finds on privately owned land, meaning that some animal bones end up being plowed over or sold to private collections instead of turned over for study. And given the tight deadlines facing many construction projects, calling in scientists can be seen as an expensive diversion from the task at hand.
Earlier this year in Utah, construction equipment damaged a set of rare dinosaur footprints on federal land, bringing criticism that paleontologists had not been more involved in supervising the site.
When Tennessee road workers found what became the Gray Fossil Site more than two decades ago, converting the place into a permanent research area required intervention from the governor and money to reroute the highway that was supposed to go there. In the years since, East Tennessee State University has started a paleontology program, thousands of visitors have stopped at the museum and scientists have unearthed bones dating back about five million years, including red pandas, rhinos, tapirs and alligators, providing a unique lens into prehistoric Appalachia.
“It’s telling us what these forests were like at this time, when we had no idea what they were like millions of years on either side,” said Dr. Schubert, who oversees the site, where excavations continue. He added, “It was a tremendously expensive endeavor to save this fossil site, and I don’t know if something like that would happen today.”
Scientists know in general terms where dinosaur bones or ice age remains are most likely to be discovered: in places where sediments or sedimentary rock strata of the right age are now close to the surface and may be exposed by natural erosion or construction work. Much of North America meets that description, though, and exactly where important new finds may lie hidden is largely a matter of chance.
When fossils do turn up, long-term excavations like the site in Tennessee are the exception. Often, scientists can complete their work in a few days or weeks if construction workers report a notable find. In California, which has stringent laws for alerting scientists about paleontological finds, construction crews and scientists have generally coexisted well. Peter Tateishi, the chief executive of Associated General Contractors of California, said construction workers were often able to continue building other parts of a structure when scientists had to be called in to evaluate a discovery.
“That can be a bit of a pain, but the laws are written in such a way that we can continue to keep schedules moving,” Mr. Tateishi said.
Dan Wagner, a construction inspector in the Denver area, was helping oversee the building of the police and fire station in Thornton, Colo., a few years ago when he found a hunk of bone where crews were drilling holes for concrete piers. The bone came from deep in the ground, suggesting that it was probably very old. He wondered, “‘Could this be a dinosaur, even?’”
When he dug some more and unearthed a much larger bone, the site managers halted work in that area. Over the next couple weeks, as work continued on other parts of the new building, Dr. Sertich and other paleontologists excavated a largely intact Torosaurus in a small, fenced-off section of the building site. Mr. Wagner said he would sometimes check on progress during his breaks, and would join in the digging when his work day ended.
“I’ve never been into dinosaurs before, but I was super-excited,” said Mr. Wagner, who got a tattoo of the Torosaurus and later took his children to see it displayed at a museum. “I’d go to bed wondering what the heck it was, and how many bones were going to be there.”
In Michigan, where the mastodon bones were found in August, there was never hesitation about giving experts access to the site, where crews had been clearing a long-neglected drain network needed to move water off farmland.
“You’re sitting in this spot on this earth that has had a creature that lived here that we have never seen and never will see,” said Ken Yonker, the Kent County drain commissioner, whose agency oversaw the construction project. “It’s almost like a gift.”
Mr. Busscher, who found that initial femur and who owns the construction company, let his employees spend the next day working in the dirt with the scientists. The owners of the land where the bones were found agreed to donate the bones to the Grand Rapids Public Museum, which is now conducting an extensive cleaning and drying process to prepare the bones for display.
The bones will join other mastodons at the museum, including one partial skeleton known as Smitty, whose bones were found at a housing construction site in Michigan in the 1980s.
Cory Redman, the museum’s science curator, who used a garden hose to gently clean dirt off the newly found mastodon’s bones, said it was not yet clear how that skeleton, which probably belonged to a juvenile, came to rest beside that country road north of Grand Rapids. He said researchers may look to see whether the bones, which were at least 11,000 years old and dated back to a time when glaciers covered portions of Michigan, showed any signs of having been butchered by humans.
Several days after the discovery, normalcy had mostly returned to the construction site. The excavator that unearthed the femur was still parked in the dirt; water had settled in the hole where the bones were found; and Mr. Busscher and his crew were hard at work beyond the “Road Closed” sign.
After all, they still had to finish clearing that drain.