A field survey in San Mateo County, California, has uncovered the fossilized tooth of a Pacific mastodon, a prehistoric mammal that roamed North America during the Ice Age. The fossilized molar, found in a nature preserve near San Francisco, was initially mistaken for an oddly shaped rock by geologists. However, it was soon identified as a rare and well-preserved fossil from the last Ice Age and was sent to Stanford University for carbon dating. The molar was discovered by Brigid Lynch, a geomorphologist with Balance Hydrologics, during a routine survey of a creek bed this summer. Lynch was part of a team working on a project to support the threatened California red-legged frog within the preserve. The fossil was an unexpected find, dating back at least 10,000 years, a time when the Pacific mastodon is believed to have gone extinct. Wayne Thompson, a paleontologist and CEO of Pacific Paleontology, described the molar as the most complete and visually striking one he had ever seen, calling the discovery "spectacular." The Pacific mastodon, first identified as a distinct species in 2019, is closely related to the more widely known American mastodon. Both species lived during the Pleistocene epoch, which spanned from about 2.6 million to 11,700 years ago. However, the Pacific mastodon is thought to have inhabited regions farther west, including parts of modern-day California, Oregon, and the northern Rocky Mountains. Fossil evidence suggests differences between the two species, such as a narrower third molar and variations in bone structure. The discovery of this molar adds another piece to the puzzle of Ice Age life in North America. Midpen has donated the fossil to Stanford University's Doerr School of Sustainability, where researchers will create a 3D copy for public education and outreach. This effort aims to share the story of the Pacific mastodon and its place in the region's ancient history with a wider audience.