It wasn’t your typical construction site find.

An unmarked utility or underground storage tank? Sure, that’s not surprising. But dinosaur fossils aren’t exactly par for the course, Saunders Construction project manager Aaron Pollick said.

“When they first found it, just the nature of what they found, I think there was a lot of excitement,” said Pollick, who was not on site at the time but has been involved since. “Not necessarily knowing what the exact next step was going to be was a challenge. Certainly, it’s not a common occurrence for something like this to be found.”

An operator was scooping dirt Aug. 25 when an on-site geotechnical engineer — someone who reviews soil conditions — told him to stop. The engineer noticed that there was something inconsistent with the surrounding claystone and sand. Brushing off the dirt and cleaning the area with water, the crews knew what they were seeing wasn’t normal.

But Saunders Construction doesn’t have an action plan in place for when crews stumble across a 66-million-year-old fossil. The crews quarantined the area, which will eventually be a weight room at the upcoming Fire and Police Substation at East 132nd Avenue and Quebec Street, and made sure construction equipment stayed clear, Pollick said. A biologist contracted by the city of Thornton recommended a local paleontologist, who stopped by Sunday to confirm that they were looking at dinosaur bones.

On Monday, Denver Museum of Nature & Science dinosaur curator Joe Sertich quickly determined that, more specifically, it was a triceratops. The museum started excavating Wednesday, Pollick said.

Volunteers from Saunders Construction, the museum and Thornton police and fire are helping with the excavation. Pollick is among them.

“What are the chances that you ever have this opportunity?” he said. “There’s lots of excitement — that you get to go do it, play around and be a kid.”

So far, both brow horns, part of the skull that surrounds the brain, parts of the snout, parts of the frill (the shield behind its head), the lower jaw beak, parts of the neck, vertebrae and lots of ribs have been found, Sertich said. While talking on the phone, more of the frill was uncovered.

The legs and arms are missing at the moment. Sertich hoped they were still around, especially since this is one of the most complete Cretaceous-period fossils found on the Front Range, he said.

“Really, even the fact that they noticed this is a fossil is a miracle,” he said of the construction workers. “They’re really the heroes of a dig like this.”

Triceratops were some of the most common dinosaurs in the American West during the time period, Sertich said. The Denver-area dinosaurs are smaller than those found in Montana, South Dakota, North Dakota and Wyoming, he said.

Scientists don’t have a definitive answer as to why. But Sertich said the Thornton triceratops is likely the size of a rhino, while those in the Midwest are closer to a mid-sized elephant.

At the time when our rhino-sized triceratops was roaming, the Rocky Mountains would have just been coming up and volcanoes would have graced the horizon. It would have been a subtropical savanna climate that was warm and humid with a lush landscape, he said.

The Thornton triceratops, likely a young adult, died on a dry land surface, falling apart over time in a way similar to a cow lying in a field. Animals may have scattered its parts. It sat there for a while until it was quickly covered by sand, likely from a river changing course or a flood, which kept the little and big bones together.

Some of the fossils will be brought to the museum Friday. People visiting the museum over the weekend will be able to see them through a window. Although it depends on what else the crews find, Sertich said the museum will likely finish by the middle of next week.

“We’re kind of in the middle of what I would consider the best part of the day,” Sertich said of the crews uncovering fossils. “It’s kind of like Christmas. We’re just opening all the presents right now.”

Paul Murphy, the paleontologist with Paleo Solutions who first confirmed the fossils were dinosaur bones, said there are many factors involved with fossil preservation, making it actually quite difficult.

The bones may be weathered by elements or scavenged. They need to be buried in sediment. And once in there, all sorts of chemical processes occur. The Denver area has a lot of plant fossils because the soil is acidic, but that makes it difficult to preserve bones.

“It’s not common in Denver to have a fossil that would actually require excavation,” he said.