She Went Looking for Frogs and Found a 10,000-Year-Old Mastodon Tooth

Imagine wading through a quiet California creek, looking for ways to improve habitat for frogs, when something strange catches your eye.

It looks like a rock.

A very unusual rock.

Then you discover that what you are looking at is actually a massive fossilized tooth from an Ice Age animal that disappeared thousands of years ago.

That is exactly what happened recently in California’s San Mateo County, where a geomorphologist discovered an exceptionally well-preserved molar from a Pacific mastodon (Mammut pacificus). The Midpeninsula Regional Open Space District announced the discovery on September 9, 2026.

It Started With a Frog Habitat Survey

The person who found the fossil was Brigid Lynch, a geomorphologist with Balance Hydrologics.

She was not searching for fossils.

Lynch was conducting fieldwork in a creek within a Midpeninsula Regional Open Space District preserve. The project was designed to improve habitat for the California red-legged frog, a threatened species native to California.

While examining rocks in the creek bed, Lynch noticed something that did not quite belong.

Its shape was unusual enough to catch her attention, but at first she assumed it was simply an oddly shaped rock.

She took a photograph and continued her work.

Only later did the true significance of the object become clear: paleontologists identified it as an intact molar from an adult Pacific mastodon.

For someone who spends plenty of time studying landscapes and geology, it was still an extraordinary discovery.

And once you see the fossil, it is easy to understand why.

This Was No Ordinary Tooth

The mastodon molar measures about 9 inches, or 23 centimeters, long.

But its size is only part of what makes the discovery remarkable.

The fossil is unusually complete.

Both the crown and the roots of the tooth remain preserved, including structures that are often damaged or lost over thousands of years.

Paleontologist Wayne Thompson of Pacific Paleontology described it as the most complete mastodon molar he had seen from the area and praised its exceptional condition.

That level of preservation could make the specimen particularly useful to scientists studying the animals that lived in what is now the San Francisco Bay Area during the Ice Age.

And the fossil may still have much more information to reveal.

What Exactly Was a Pacific Mastodon?

At first glance, a mastodon might look like a mammoth or an oversized modern elephant.

They were related, but they were not the same animal.

The fossil belongs to the Pacific mastodon, scientifically known as Mammut pacificus.

The species was recognized relatively recently as distinct from the better-known American mastodon (Mammut americanum). Researchers have identified differences including narrower teeth, thicker leg bones and variations in their tusks.

Pacific mastodons lived across parts of what are now California, Oregon, the northern Rocky Mountain region and Mexico.

They were enormous herbivores, but their diet was also different from that of many mammoths.

Mammoths were more strongly associated with grazing on grasses, while mastodons were browsers.

They likely used their trunks to pull leaves, soft shoots and fruit from trees and other woody vegetation.

Their teeth reflect that lifestyle.

Instead of the flatter grinding surfaces associated with grass-eating mammoths, mastodon molars had large, prominent cusps that were useful for crushing tougher vegetation.

That distinctive shape is also what makes a mastodon tooth look so strange when it suddenly appears among ordinary rocks in a creek.

An Ice Age Animal in a Familiar California Landscape

Mastodons lived during the Pleistocene, a geological epoch that stretched from roughly 2.5 million years ago until about 11,700 years ago.

The landscape around today's San Francisco Bay Area would have looked very different in some ways, yet surprisingly familiar in others.

Mastodons shared the region with animals that have since disappeared, including giant ground sloths, western camels and Columbian mammoths.

At the same time, ancestors or relatives of animals still found in California today also occupied the landscape.

That makes discoveries like this tooth especially fascinating.

They are reminders that Ice Age animals were not living in some completely alien world. They once walked through landscapes that eventually became the forests, valleys and creeks people recognize today.

How Old Is the Tooth?

The discovery has widely been described as a roughly 10,000-year-old mastodon fossil, but researchers still want to determine its precise age.

The Midpeninsula Regional Open Space District donated the molar to Stanford University's Doerr School of Sustainability, where it can be preserved and studied.

Researchers plan to use dating analysis to better establish when the animal lived.

They also intend to create a 3D scan of the specimen. A replica can then be produced for education and public outreach while the original fossil remains available for scientific research.

Researchers may also be able to learn more about the mastodon's diet and the environment in which it lived.

A single tooth, in other words, can contain far more information than its appearance suggests.

The Strangest Part of the Discovery

Perhaps the most interesting part of this story is how completely accidental the discovery was.

Nobody went into the creek looking for mastodons.

The team was there because of a living animal: the California red-legged frog.

A project intended to protect wildlife today unexpectedly uncovered evidence of an animal that vanished thousands of years ago.

There is something almost poetic about that coincidence.

While scientists were working to improve the habitat of a threatened species, they discovered the remains of another species that disappeared from the landscape long ago.

Sometimes a Rock Isn't Just a Rock

Most of us would probably have walked straight past it.

Sitting in a creek surrounded by stones, mud and flowing water, the fossil apparently looked unusual but not immediately prehistoric.

Yet that strangely shaped “rock” turned out to be part of an animal that once shared California with mammoths and giant ground sloths.

Now the tooth is no longer sitting unnoticed in a creek bed.

It is part of Stanford's scientific collection, where researchers can study it for clues about the Pacific mastodon, its environment and the Ice Age ecosystem that once existed in the region.

A frog habitat survey began the story.

A strange rock caught someone's attention.

And thousands of years after the animal disappeared, one remarkably preserved tooth brought a Pacific mastodon back into the spotlight.