By Raetta Holdman

DENVER (CBS4) – “I think we found a triceratops.”

That was the assessment from scientists at the Denver Museum of Nature and Science in August when they inspected bones found at a construction site in Thornton.

(credit: CBS)

The city was expanding its first response services in a growing part of the city. Construction had been underway for weeks when Dan Wagner of Terracon arrived on the site for soil inspection.

On Aug. 25 he spotted a bone that led to more bones and a call to the museum.

Dr. Joe Sertich is the Curator of Dinosaurs at the museum and believed almost immediately it was the bones of triceratops.

But once he got down into the dirt and started finding pieces of the frill, or the shield around the skull, he started to believe the fossils actually belonged to a torosaurus, a cousin of the triceratops.

Dr. Joe Sertich explains the clues to CBS4’s Stan Bush that confirmed the discovery was a Torosaurus. (credit: CBS)

For Sertich, initially calling the discovery a triceratops was just common sense.

“The most common dinosaur that comes out of the American West is triceratops, more than 1,000 have been discovered,” he said.

But those pieces of frill just didn’t feel right.

“On the triceratops, it’s a solid frill. It’s fairly short for these guys so when we started to reveal some of the frill, more of the shield as we went through the prep process we started to realize more things that weren’t triceratops,” Sertich said.

While he had suspicions on the dig site, he wasn’t ready to commit.

“It’s hot, there’s a lot of dirt, we don’t want to clean them too much. So we really need to get them back to the lab. We needed these few months to really see what this frill looked like.”

Thirteen weeks after the discovery, Sertich walked into a lab in the museum to see how work was progressing on revealing the skull.

“I came in here and the preparators had cleaned off this part of the shield, the frill behind the head and I don’t think the preparators realized what this part was. This was a complete window opening in the frill,” he said.

Outlining the frill or shield of the Torosaurus (credit: CBS)

“I was blown away. I was so excited. I had hints this was not a triceratops but this completely nailed it.”

And that opening is the definitive difference between a triceratops and a torosaurus.

(credit: CBS)

More of us may know about the triceratops. The Colorado Rocky’s mascot is one, Dinger, named after a bone found during the construction of Coors Field.

While thousands of triceratops have already been found around the world, there are only about half dozen known torosauruses.

“This adds a really important single individual to a really rare dinosaur,” Sertich explained. “This is probably the most complete skull of a torosaurus.”

A Torosaurus drawning, by artist Sergey Krasovskiy (credit: CBS)

And Sertich agrees it’s likely some of those triceratops skulls may actually be torosauruses.

“If you were to only find the face, the brow horns forward, you would call it a triceratops everytime.”

(credit: CBS)

For Sertich and the other paleontologists, learning they had a torosaurus in their hands, not a triceratops, emphasizes the importance of taking a close look at every discovery.

“That’s how science works. We’re always revising what we know. It really highlights the importance of going out and digging these things. If we ignored every triceratops that came out we wouldn’t make discoveries like this.”

It’s a discovery Sertich calls important and one that could rewrite what we know about how the dinosaurs lived.

REPORTER’S NOTEBOOK: Triceratops Assignment Leads To ‘Out-Of-Body Experience’

Raetta Holdman is a veteran newscast producer. She’s been with CBS4 for more than 25 years, coordinating events — large and small — from the control room. Contact her by clicking here.