Sunday, March 1, 2009 | Pat Sena is hard to locate among a swarm of construction workers digging a 25-foot hole for a new building in downtown San Diego. His orange faded t-shirt and hard hat and work boots do little to distinguish him from the rest of the workers on the site.

Spot him, though, among the bulldozer operators and grade-checkers, and you’ll spot the paleontologist of the hour in San Diego. Sena is the man whose discovery of an intact mammoth skull and tusk and foot bones 20 feet underground a few weeks ago — followed by a find Wednesday of the youngest whale skeleton in San Diego County — have given him yet more notches in his excavating belt, already full of notable discoveries in the ground beneath modern San Diego.

In a dozen years on the job, Sena has found fossilized whales, rabbits, a horse, clams and a previously undiscovered genus of cat in the ground of a Chula Vista site. He extracted the most complete skull of a manatee cousin in the county. He drops these superlatives into conversation shyly.

This site is not a National Geographic expedition or a purely academic exercise. This hole in the ground marks the future home of downtown’s next big building, the Thomas Jefferson School of Law. It is a place where mammoths once roamed and whales once swam. State law requires Sena or someone like him be stationed on site to make sure that construction equipment doesn’t tear through, miss or dismiss precious remnants of long-ago species buried among millions of years of earth.

“As far as paleontology goes, my job is like the front lines, the grunt work,” he says at the corner of Park Boulevard and Island Avenue on a recent morning. “Basic geology” is his self-effacing description of his expertise — “I collect information from rocks.”

Sena is a fervent scholar of dirt and rocks. He pieces together stories of ancient swamps and rivers and tides and sediment and erosion and deposition. A layer of dirt as thin as a piece of paper might mean a million years. He traces the effects wrought by those changing periods and topographies and reconstructs pictures in his mind of how the world looked and what animals lived right here, in this spot. It’s tough, to say the least, to imagine mammoths and other erstwhile inhabitants of this part of Earth a half-million years ago, all while standing in the shadows of gleaming high-rise condo and office towers. That is Sena’s job; it is his daily task to discover more of the evolutionary history of what is now San Diego County.

A paleontologist with the San Diego Natural History Museum, Sena is one of five the museum contracts out around the county. State law mandates these paleontologists be stationed onsite during excavation of certain construction jobs, to monitor the backhoes and bulldozers and watch carefully the overturning piles of dirt for anomalies and hints of fossils.

When he’s not excavating a fossil, Sena spends his time mapping the geology of the site. He carries an orange safety flag so workers don’t run him over. He keeps a stash of tools in the site: shovels, brushes, buckets, screens, plaster, burlap, picks and water. He notes patterns and determines the location of ocean currents. He picks at rocks and soil to study and learn their stories.

Even if he’s not watching directly, in his peripheral vision, Sena always observes the mixing dirt. When a backhoe or a bulldozer strikes bone, it often splinters — revealing a texture much like the inside of a Butterfinger chocolate bar, a white piece among the brown piles of dirt, Sena says. When he sees that, he knows that’s a place to dig. He redirects the bulldozers to another corner of the site so they can keep working while he figures out how to dig up the fossil.

In the case of this site, the law school has proven academically interested in the treasures in the ground beneath its proposed building. School staff collects shells to embed in the lobby of the new building; the school’s dean has researched and drawn connections between the historic fossil find and Thomas Jefferson’s affinity for prehistoric life.

It’s not always this way. Some developers gripe about the state rule, resenting Sena’s presence on the site and the $50 an hour they must pay for his services. They chalk the paleontological requirement up to another fee attached to development to make building here more difficult.

But Sena doesn’t see the construction workers in his midst as adversaries. Without a developer excavating a site, the contents of the ground might remain a mystery. “Without the bulldozers,” confirms the museum’s curator of paleontology, Tom Deméré, “we wouldn’t find the mammoth.” Sena thinks of a construction site as a giant grandfather clock, and says he’s well aware he’s the mouse hopping around inside, costing the developer $250 per minute he slows them down. “That’s a lot of stress,” he says.

But somehow he’s had only a few tense moments. He shrugs: “I’m a nice guy.” When they found the mammoth early last month, Sena says the work on the site was slowed only slightly. He typically gets the construction workers to help extract the fossil as quickly as possible.

Perhaps Sena’s construction worker getup helps. He wears none of the pieces a dinosaur-loving child might assemble for a paleontologist Halloween costume — cargo pockets, or a khaki vest or an Indiana Jones-style hat. His outfit might well serve as disguise in a tense moment, a uniform to remind a developer, Hey, let’s be on the same team here. Sena is a bald, brawny evolution enthusiast, an expert in the land before time who will fight for science’s chance to learn more about what used to live here. But ultimately, in his hard hat and vest, he blends in.

“We don’t plan to find these things,” he says. “But if you have a situation where you find something, you need to triage that resource. There’s no price on these fossils.”

Sena finds fossils more frequently than you’ve heard about. Even though this recent mammoth, the whale, and the mastodon find in Carlsbad in 2007 made big headlines, the paleontologists usually work in secrecy, Sena says. Time is money on these projects; it takes long enough to pull the bone out of the ground without adding time to explain to pesky reporters what is going on.

Before a developer can move an inch of dirt, the company must compile an approved study of the project’s environmental impacts, ranging from the imposition of construction machinery on animal habitats to the traffic and pollution effects of a particular design. The developer must also review what scientists know about the site. The site is given a score for its paleontological resource potential: high, moderate, low, or zero. In the case of this site, the score was high, as the museum expected to find scallops and oyster fossils.

It got much more than that. Sena found the mammoth tusk, skull and bones on his first day on the site, Feb. 4. The fossils were between two layers where shells were found, in a layer of river bed sediment, Sena says. He figures the layers signify periods of global warming, cooling, and warming again — in the warming periods, this spot was ocean floor. While there was solid ground here in the cooling period, the mammoths walked on it.

On a recent morning, he waves a visitor over to a big find. The dirt has some white flaky pieces on top. In Sena’s palm is a piece of bone or tooth — maybe from a camel jaw, he guesses, drawing on the knowledge that a team found a camel jaw under the nearby condo building Metrome. “It may be something as bizarre as a porcupine,” he says.

He marks the find with a bucket to keep it safe until he can get the bulldozer to dig it up and take it to the museum for study. He later confirms that the pieces were horse teeth and some horse bones.

Like many inquisitive children, a 6-year-old Sena dug herpetology, attempting to solve the pressing natural mysteries of frogs and tadpoles on his family’s farm in Colorado. His dad was a geophysicist with the U.S. Geological Survey, conducting nuclear tests and completing underground studies. The elder Sena explained everything about geology, and Sena started digging up fossils as a child. He once found a tropical shark fossil in the ground, a stark anachronism in the middle of a frozen Colorado landscape.

“It’s one of those things that really makes you think,” he says.

Sena says he has always had a fascination with life and evolution, an affinity he traces back from childhood through his days as a commercial fisherman in Alaska and forward. He served as a corpsman in the first Gulf War, an enlisted member of the U.S. Marines who was trained in first aid and basic medical treatment. In the service, he traveled to Africa and saw elephants, the kin of which he now extracts from the ground. When he left the military in 1992, he went to college to become a physician’s assistant, the civilian cousin to his enlisted work.

But he soon switched tack. “My true passion was paleontology, the history of the life of Earth,” he says.

He became certified in paleontology at Anza-Borrego State Park, volunteering with the park’s chief paleontologist, George Thomas Jefferson. That his mentor’s name is the same as the site where he found this mammoth is not lost on Sena.

He began work as a paleontologist with the museum in 1997. Applied paleontologists, the ones out on construction sites, earn between about $30,000 and $50,000 a year, Deméré says.

A single dad, Sena lives with his almost-13-year-old daughter and brings her on digs with him sometimes. She’s interested in paleontology, he says, but wants to be an artist.

Sena works on construction sites about 90 percent of the time. The rest he spends in the museum, preparing fossils and assembling the pieces of what he and others have found out in the field. He demonstrably loves his work. “I’ll see environments where I think I’m going to find something, and I usually do,” he says. “I just have weird luck.”

On Wednesday, a few days after finding the horse teeth, Sena found what he believed to be a baleen whale skeleton — the lower jaw, most of the ribs, the vertebra and some other bones. Sena spotted the first rib about 15 feet below the spot where he first extracted the mammoth bones. The whale is a significant find, the first marine mammal ever found from the Pleistocene Epoch time period in San Diego County.

It’s another of Sena’s major discoveries.

“He can find fossils like nobody’s business,” Deméré says. “Pat really has a gift.”

Please contact Kelly Bennett directly at kelly.bennett@voiceofsandiego.org with your thoughts, ideas, personal stories or tips for future installments in the People at Work series. Or set the tone of the debate with a letter to the editor.

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