A geomorphologist surveying a San Mateo County creek in California this summer found what she initially thought was an oddly shaped rock, which paleontologists have since confirmed is an intact adult Pacific mastodon molar dating back at least 10,000 years. The find, announced by the Midpeninsula Regional Open Space District, is being hailed by experts as exceptionally rare and could deepen understanding of the Pacific mastodon, a species only formally recognised in 2019 that remains poorly understood.

Brigid Lynch, a geomorphologist with Balance Hydrologics, spotted the tooth in June while conducting field surveys for a project protecting the threatened California red-legged frog. Wayne Thompson of Pacific Paleontology described it as the most complete and spectacular mastodon molar he had ever seen from the region. Pacific mastodons, elephant-like browsers distinct from grass-eating mammoths, roamed the West Coast during the Pleistocene epoch until their extinction roughly 11,700 years ago. The tooth was donated in July to Stanford University's Doerr School of Sustainability, where researchers plan to carbon-date it and create a 3D-printed replica for public education.