Brigid Lynch was working along a wooded creek in a mountainous part of the San Francisco Bay Area on a habitat restoration project when something that didn’t look like the rocks she’d been studying caught her eye. It turned out to be the fossilized tooth of a Pacific mastodon. Lynch, a geomorphologist at Balance Hydrologics, made the discovery earlier this year. She spotted the fossil while working on a project to protect the threatened California red-legged frog in a preserve in San Mateo County. The public land management agency announced the find this week, and that it has been donated to Stanford University.