Scientist Searching for Threatened Frogs Stumbles Upon an Oddly Shaped Rock—Then Discovers What It Really Is

Brigid Lynch was surveying habitat for a threatened California frog when something in a San Mateo County creek caught her eye. What looked like an oddly shaped rock turned out to be an exceptionally preserved mastodon tooth.

Brigid Lynch went into the field with a very different kind of discovery in mind.

The geomorphologist with Balance Hydrologics was conducting surveys for a Midpeninsula Regional Open Space District habitat enhancement project designed to support the threatened California red-legged frog. During the work, she spotted something unusual in a creek bed in a Midpen preserve in San Mateo County.

At first glance, it appeared to be nothing more than an oddly shaped rock.

But paleontologists later determined that the object was an intact adult molar belonging to an ancient Pacific mastodon—an animal that once roamed landscapes that are now part of the western United States.

For Lynch, the unexpected find was something special.

“As a geologist, I've gone out and seen stuff in the field, but definitely nothing as exciting as this,” she said, according to the Midpen press release.

And the more closely experts examined the tooth, the more remarkable it became.

Wayne Thompson, a paleontologist and owner of Pacific Paleontology, said the fossil was in extraordinary condition.

“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,” Thompson said, according to the press release.

He added, “It's another puzzle in the Ice Age saga that we can put into place.”

The discovery is especially intriguing because the fossil belonged to a Pacific mastodon, the newest mastodon species to be recognized, according to Midpen.

The species differed from the better-known American mastodon in several ways, including narrower teeth, thicker leg bones and differences in its tusks. Researchers believe Pacific mastodons ranged across what is now California and Oregon, as well as the northern Rocky Mountain region of the western United States and parts of Mexico.

These enormous herbivores were once part of the ecosystems of the Pleistocene epoch, a period stretching from about 2.5 million years ago until their extinction around 11,700 years ago, according to the press release.

And they lived in a world that may sound both ancient and strangely familiar.

Unlike mammoths, which fed on grasses, mastodons were browsers. They used their massive trunks to strip leaves, soft shoots and fruits from high-growing, generally woody plants.

The landscapes they inhabited also supported wildlife that still exists today. Midpen says mountain lions and bobcats may have shared the environment with now-extinct saber-toothed cats and dire wolves, while tule elk, coyotes and American badgers likely moved through valleys inhabited by mastodons and enormous Jefferson's ground sloths.

In other words, the tooth Lynch noticed in a modern creek offers a glimpse of a landscape that existed thousands of years ago—and of an animal that disappeared from it long before the present day.

The fossil now has a new role of its own.

Midpen donated the tooth to Stanford University's Doerr School of Sustainability to support ongoing scientific research. Researchers plan to use carbon dating to determine its exact age, while the Stanford team will create a 3D scan and printed copy for Midpen to use for public outreach and education.

For Midpen staff, there is something particularly striking about the fact that this ancient discovery emerged during a modern effort to protect wildlife.

Biologist Matt Sharp Chaney said the fossil shows that the presence of mastodons “wasn't that long ago, in geologic time.”

“That mastodon were out here in the same landscape that we’re in today,” Chaney said, “really drives home how important the work that we do is to keep species from winking out forever.”

And that gives the unexpected discovery a remarkable connection between past and present.

Lynch went into the creek to help understand and improve habitat for a threatened species living today. Instead, she came across a surviving piece of evidence from a species that vanished thousands of years ago.

A strange rock in a California creek had turned out to be much more than it first appeared—a small, remarkably preserved piece of a world that is gone, but not entirely forgotten.

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