Brigid Lynch was walking through a redwood forest in the hills of San Mateo County when she noticed something unusual, half submerged in the water flowing through a stream: a large, smooth object that didn’t look like any of the other rocks. “It really stood out,” he said. “I had no idea what it was. I thought maybe it was a mollusk or something, but that didn’t make sense since it wasn’t near the ocean.” He took some photos, went home, and entered them into Google photo search to solve the mystery. Lynch had made a remarkable discovery: He found a huge, surprisingly well-preserved molar from a Pacific mastodon, an extinct species of prehistoric elephant that roamed western North America during the Ice Age. These 9-foot-tall creatures, weighing up to 13,000 pounds, with long fangs and fur, were common in California and other Western states from more than 2.5 million years ago to about 11,000 years ago, along with saber-toothed tigers, camels and other long-vanished species. The tooth, a molar the size of a toaster, is one of the best-preserved and most intact specimens ever found in the Bay Area, experts say. “The tooth itself is in perfect condition. It’s the most complete mastodon molar in our area and the most spectacular mastodon molar I’ve ever seen,” said Wayne Thompson, a paleontologist and owner of Pacific Paleontology, a Santa Cruz County-based consulting firm. “I felt chills and chills run down my spine. I was very happy to see this discovery because it can teach us a lot about the creatures themselves as a species and about this area in particular. It’s another puzzle in the Ice Age saga that we can solve.” Lynch, a geomorphologist, made the discovery in March when the company she works for, Berkeley-based Balance Hydrologics, was conducting research as part of an effort to restore endangered red-legged frogs to the Midpeninsula Regional Open Space District. The district does not say exactly which of its preserves or which stream the molar was found in. “We really don’t want people digging in the creek for other fossils,” said Ryan McCauley, district spokesman. But he said it has inspired staff members at the agency, which owns a network of 26 reservations covering more than 70,000 acres in San Mateo and Santa Clara counties. “My imagination goes to Jurassic Park with big ferns and dinosaurs,” McCauley said. “But this isn’t that. This was only about 10,000 years ago, when there were mammoths and saber-toothed tigers in the Bay Area. It wasn’t that long ago. It’s mind-blowing.” A more than 11,000-year-old molar from a Pacific mastodon, an extinct species of prehistoric elephant, was found in March 2026 in a stream in San Mateo County on property owned by the Midpeninsula Regional Open Space District. (Credit: Haley Grunloh and the Midpeninsula Regional Open Space District) After Lynch realized he had discovered a mastodon molar, he asked his colleagues who were in the same forest if they could find it. They did it. They put it in a bucket and brought it back. The company called in experts from Stanford University, who also brought in Thompson. They confirmed that it did indeed come from a mastodon, and today the fossil is in the collection of the Doerr School of Sustainability at Stanford University. Researchers plan to use carbon dating and other techniques to determine its exact age, the mastodon’s diet and the local climatic conditions when it lived. “It’s pretty rare to come across a fossil like this,” said Christine Garcia, curator of the Doerr School’s specimen collection and collection manager. “Most of these finds come from construction projects. For Brigid to have the eye to see this and know it’s something special, it’s rare and unique.” Every few years a major fossil discovery emerges, he noted, such as in 2013, when workers rebuilding the dam at Calaveras Reservoir unearthed a trove of prehistoric fossils, including teeth from huge megalodon sharks, whale skeletons and hippopotamus skeletons from 15 to 20 million years ago during the Miocene epoch, when the East Bay was covered by a shallow, tropical-looking inland sea. Those fossils are now in the UC Berkeley Museum of Paleontology. Garcia said the mastodon tooth, which was in the forest for thousands of years before the Roman Empire or before the Egyptians built the pyramids, would not be displayed to the public. He is currently in a temperature-controlled room at Stanford, where he is slowly drying and being monitored, he said. The curators have also not put chemicals or other preservatives in it, which is common for preserving bones and fossils, for now, so it can be investigated first. “Because of how well preserved it is, I don’t think it was in the creek the entire time,” Garcia said. “It was probably washed down from somewhere more recently, like an upstream hillside. When we have storms, there will be large depressions of soil that can disappear immediately. That’s often a time when fossils become exposed.” Pacific mastodons are slightly different from mammoths. They had more robust body proportions and spent considerable time in forests, while mammoths were mostly grassland animals. Although the Ice Age (15,000 or 20,000 years ago) is a recent geological period, it was a very different world from today. Because temperatures were colder around the world and much of Earth’s water was trapped in ice, sea levels were about 300 to 400 feet lower than today. San Francisco Bay was a valley. And the edge of the Pacific Ocean was more than 20 miles further west than it is now. Mastodons and other animals could walk to the Farallon Islands, across a vast plain that now lies hundreds of feet below the ocean’s surface. People lived nearby and hunted mastodons, bison and other Ice Age animals. “The Bay Area was known as the Serengeti during the Ice Age, with plains, grasslands and all kinds of animals roaming around, from mastodons to camels to saber-toothed tigers,” Garcia said. “Humans overlapped with these animals. They didn’t overlap with dinosaurs. This is a different, more recent time period. But it’s particularly fascinating to think about what our area must have been like.” During the Ice Age, from 11,000 years ago to 2.5 million years ago, mastodons, mammoths, giant sloths, saber-toothed tigers, dire wolves and other wildlife roamed the Bay Area. (Credit: Mason Schratter and Midpeninsula Regional Open Space District) Mastodons are believed to have eaten many of the same trees, such as alders, that exist in the area today. Lynch said he sent photos of the huge tooth to many friends and family. “I’ve never encountered anything like this before,” he said. “I’m terrible at finding cool rocks in the field. This was really unique. I was sharing it with everyone in my life. Everyone was so excited. They texted their families. It really spread. It’s a once-in-a-lifetime thing.” A more than 11,000-year-old molar from a Pacific mastodon, an extinct species of prehistoric elephant, was found in March 2026 in a stream in San Mateo County on property owned by the Midpeninsula Regional Open Space District. (Photo: Midpeninsula Regional Open Space District)
