Brigid Lynch, a geologist, was conducting a field survey for a frog habitat near a San Mateo County, Calif., creek on a morning in March when something caught her eye.
“I looked down and was kind of like, ‘That’s really weird looking,’” she recalled. “‘What is that?’”
She alerted her colleague, Jeneya Fertel, and they both stared at it.
“I was like, ‘Do you think we should touch it?’” Ms. Lynch recalled. She thought it might be a living thing, so she said to Mr. Fertel, “You pick it up.”
They assessed that it was a little bigger than a toaster oven and weighed maybe three or four pounds. They put it back, and forgot about it.
Ms. Lynch continued her field work and the next day noticed the oddity again while going through her photos.
She ran her photo through a reverse-image search on Google and was shocked by what she had found.
“This is absolutely a Pacific mastodon molar,” she recalled thinking.
The Midpeninsula Regional Open Space District, a public agency known as Midpen that has preserved more than 70,000 acres of open space in the Bay Area, announced the discovery on Wednesday.
After leaving the molar in the creek, Ms. Lynch contacted colleagues at the environmental consulting firm Balance Hydrologics to properly retrieve the fossil.
Wayne Thompson, a paleontologist since 1976 and owner of Pacific Paleontology, confirmed the find as a Pacific mastodon molar and suggested ways to relocate it.
“I helped them store it in a way that would not deteriorate the molar because once molars are removed from a watery environment, they start decaying rapidly,” he said.
Staff members at Midpen were able to keep the molar stored in a bucket of creek water.
In its news release, Midpen said it donated the fossilized tooth to the Doerr School of Sustainability at Stanford University, where researchers plan to determine its age through carbon dating and to create a 3-D scan of it.
Mr. Thompson said the Pacific mastodon is distinct from American mastodons because of their narrower teeth and thicker leg bones.
In 2019, researchers published a study with the U.S. Geological Survey naming the Pacific mastodon (Mammut pacificus) as a newly recognized species of mastodon.
Pacific mastodons lived during the Pleistocene Epoch, or the Ice Age, which began roughly 2.5 million years ago. They disappeared about 11,700 years ago.
Mr. Thompson said the shape of a Pacific mastodon’s molar is distinct because the creatures were herbivores that used their cone-shaped teeth to browse trees and grind leaves and small twigs.
“So you can think of a proboscidean, whether it’s an elephant or a mammoth or a mastodon, as having teeth that are on conveyor belts,” he said. “And they come in from the back of the jaw and they migrate inside the jaw up front and then they erupt and then they continue on the conveyor belt moving forward until they’re ejected.”
Mr. Thompson believes this particular Pacific mastodon lived to be around 35 or 40 years old, and that the molar may have only “recently” detached from the jawbone.
Mr. Thompson called the find “a really unique discovery” and said that the remains of Pacific mastodons are rare in the area and are more commonly found in Southern California.
“This mastodon discovery really gives us a lot of information in terms of where these creatures were living,” he said.
Georgia Gee contributed research Jackeline Luna contributed reporting.
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