SANTA CRUZ, Calif. — During the last ice age, known as the Pleistocene, Santa Cruz County was a wild place populated with ancient humans and larger-than-life creatures, or megafauna, such as mastodons and mammoths. But not until a group of adventurous kids from the Tara Redwood School in Soquel brought in a mysterious fossil to the Santa Cruz Museum of Natural History did anyone in the scientific community know that massive ground sloths once roamed the region.

“We learned about this discovery in the late spring of last year around the same time as the mastodon frenzy in the community,” said Executive Director of the Santa Cruz Museum of Natural History Felicia Van Stolk, referencing the mastodon tooth found in Aptos. “Our paleontology adviser Wayne Thompson was immediately so excited by this specimen, and when he gets excited about something, you know that it’s going to be good.”

According to Van Stolk, a group of kindergarteners and first-graders from Tara Redwood School were playing outside one fateful day on a field trip in the Santa Cruz Mountains when they stumbled upon the fossilized bone.

“One of the students pulled up this great big stick, except that it wasn’t a stick,” said Van Stolk. “There was this great debate among the kids about it. The kids then brought it to their classroom and were cleaning it off with paint brushes, pretending to be paleontologists.”

The bone remained in the classroom for a short time before the family of one of the young citizen scientists thought to bring it into the Museum of Natural History to get to the bottom of the strange fossil’s identity. Thompson and Van Stolk had their suspicions about what the log-like fossil could be from and when, but they had to do a bit of sleuthing before they knew for certain that it was part of an ancient sloth.

“We tapped into our resources and connections and had it confirmed it to be this very rare specimen,” said Van Stolk. “We were just over the moon.”

Thompson, principal paleontologist at Pacific Paleontology, has been steeped in the world of ancient creatures since he was a boy as his family were the original owners of the long-closed “Lost World” amusement park in Scotts Valley that had life-size replicas of dinosaurs. Thompson was also manning the front desk at the Museum of Natural History as a teenager when a community member brought in a mastodon skull that still resides near the main entrance of the museum.

At the museum Friday, Thompson told the Sentinel that the bone has been identified as the left radius bone of a Jefferson’s ground sloth, named after the Founding Father Thomas Jefferson, and is the first of its kind ever to be found in the county. However, the exact age of the specimen is still being determined.

“That data hasn’t yet come back to us from the testing facility,” said Thompson. “There are a number of ways to date fossils and the first one that we are trying relies on the presence of the biomolecule collagen, which is a protein. If there’s enough collagen in the bone, we can calculate its age up to about 50,000 years. If it’s older than that, we’ll have to use other methods using elements like uranium or thorium.”

Thompson explained that Jefferson’s ground sloths lived between about 300,000 years ago to about 11,700 years ago and remnants of their existence are rarely found in California.

“So far in my research, I’ve found that this is the biggest radius (bone) that’s been recorded,” said Thompson. “It is from an adult male. We know that because the ends of the bone are fused on, which means it had reached full maturity.”

Jefferson’s ground sloths lived near creeks and rivers and under canopy forests and were about as big as an ox and generally weighed more than a ton. When the massive, slow-moving herbivores weren’t camping out in caves, they would be roaming around, sometimes standing on their hind legs, using their three long claws to strip the leaves from branches of trees such as spruce and alderwood.

“Whereas the mammoths were more out in the grasslands and were grazers, these sloths were browsers,” said Thompson. “They would be about 6 feet tall on all fours and they’d be a good 8 to 9 feet tall when they stood up.”

Thompson said that the sloth fossil adds one more piece to the still incomplete puzzle of ancient Santa Cruz, which would have had a coastline that was miles out from its current location.

“It’s because of all the ice,” said Thompson. “Ice age Santa Cruz would have been bigger and we have found mammoth fossils in the bay.”

Thompson mentioned that he was stoked about the specimen not only because of its rarity and the fact that it was the first of its kind in the county, but also because it was discovered by a group of young kids. For those who are inspired by the find and want to venture out on a fossil-finding mission of their own, Thompson has some advice.

“The golden rule there is to know before you go,” said Thompson. “Do some background research and know the regulations of where you’re going. If you are on public land, it is illegal to take this kind of artifact unless it is in immediate danger of being destroyed. Always document with photographs and try to pin where it is and report any finds to a museum. Most of all, be excited. There is so much more that is out there yet to be discovered.”

The Jefferson’s ground sloth’s fossilized arm bone is on display with the museum’s current exhibition “The Art of Nature,” which runs until May 26 and features scientific illustrations from 40 local artists and includes depictions of extinct megafauna such as the Jefferson’s ground sloth and others.

For information, visit santacruzmuseum.org.