California workers made an unexpected discovery while working on an infrastructure project in northwestern San Diego County. Buried 12 feet deep in the Santiago Formation, they uncovered fossilized bones of a large, hoofed mammal. Paleontologists estimate the fossils are between 40 and 45 million years old, placing them in the Eocene epoch—a period when mammals were rapidly evolving into larger forms. The find adds to the growing body of evidence from the same geological period in the region, which has already yielded similar fossils. Among the uncovered bones are several ribs, a femur, and other limb bones and fragments. These could belong to a brontothere, commonly called a "thunder beast," or an amynodont, a hornless, hippo-like mammal sometimes referred to as a "swamp rhino" due to its tusk-like canine teeth. Both species are members of the order Perissodactyla, which includes modern animals like horses, rhinos, and tapirs—mammals with an odd number of toes on each hoof. The Santiago Formation, where the fossils were found, is known for preserving remnants of ancient ecosystems from the Eocene. The fossils were exposed after a week of careful excavation, during which several tons of mudstone were removed. Once exposed, the bones were carefully encased in a plaster field jacket to ensure safe transport to a climate-controlled laboratory for further study. There, the fossils will be cleaned, stabilized with museum-grade adhesives and resins, and examined over the coming weeks. Researchers hope this process will help them determine the exact species to which the Oceanside fossils belong. Workers are continuing to search for more remains, including a skull, which could provide further clues about the animal’s identity. To protect the site and ensure the integrity of the discovery, several blocks of land remain closed, and the location is not being disclosed publicly. Similar fossils of brontotheres and amynodonts have been found across North America, including in Oregon’s Hancock Mammal Quarry. Some of these ancient mammals are even on display at the John Day Fossil Beds National Monument visitor center in Kimberly.