Unassuming Rock Turned Out to Be Rare Mastodon Molar in California Preserve

Researchers working in a nature preserve near San Francisco have identified a remarkably intact fossilized molar from an adult Pacific mastodon, a species that disappeared at the end of the last ice age. Initially dismissed as an oddly shaped stone by geologists conducting fieldwork, the specimen was recognized as a significant paleontological find and subsequently submitted for carbon dating at Stanford University.

The discovery was made this summer by Brigid Lynch, a geomorphologist with Balance Hydrologics, while she was surveying a creek bed in San Mateo County. Lynch’s team was engaged in a habitat restoration project aimed at supporting the threatened California red-legged frog within the Midpeninsula Regional Open Space preserve. She described the find as unlike anything she had encountered in her professional career.

Wayne Thompson, CEO of Pacific Paleontology, conducted a preliminary analysis and expressed astonishment at the specimen’s preservation. He noted that it is the most complete mastodon molar ever found in the region and the finest example he has ever examined, stating that the condition of the tooth gave him “chills and shivers up and down my spine.”

Dating back at least 10,000 years, the fossil provides insight into the Pacific mastodon (Mammut pacificus), which went extinct around the time the Americas were being populated by early humans. First classified as a distinct species in 2019, the Pacific mastodon was closely related to the American mastodon but inhabited areas further west, including parts of present-day Mexico, California, Oregon, and the northern Rocky Mountains. Fossil evidence indicates slight physical differences between the two, such as a narrower third molar and thicker hind-leg bones in the Pacific variety.

In July, the preserve donated the fossil to Stanford University’s Doerr School of Sustainability to facilitate precise dating. Additionally, researchers plan to create a 3D replica of the molar for public education and outreach purposes. Thompson characterized the discovery as another critical piece of the Ice Age historical puzzle.