Fossilized 10,000-year-old mastodon tooth found in San Mateo County, California

Brigid Lynch came across what she took to be a peculiar rock while working at the preserve. The Midpeninsula regional open space district (Midpen) said in a Wednesday news release that paleontologists later confirmed the object was not a stone but an adult Pacific mastodon molar.

"The tooth itself is in immaculate condition. It's the most complete mastodon molar from our area, and the most spectacular mastodon molar that I've ever seen. I had chills and shivers up and down my spine," paleontologist Wayne Thompson said in a statement. "It's another puzzle in the Ice Age saga that we can put into place."

The Pacific mastodon, a relative of modern-day elephants, was most prominent in the American west, including what is now California, Oregon, the northern Rocky Mountain region and parts of Mexico. The herbivore dates back about 2.5m years in North America and went extinct more than 10,000 years ago, a result of the climate crisis's effects on its habitat coupled with overhunting by humans.

Midpen said the fossilized tooth has been donated to Stanford University, where researchers at the school of sustainability will use carbon dating to determine the molar's exact age.

"Fossils are our only opportunity to see into past biodiversity," said Christine Garcia, a curator and manager of the geoscience specimen collection at Stanford. "We can study fossils to better understand Earth's past, past wildlife and the ecosystem they were a part of, and to help us make better predictions and better decisions about how to help our ecosystems and biodiversity thrive into the future."

Several ancient mastodon fossils have surfaced in recent years. In 2024, a New York state resident found a mastodon jaw under the surface of their lawn. A year earlier, a woman stumbled across a foot-long ancient mastodon tooth sticking out of the sand on a beach near California's central coast.