The fossils were discovered this month approximately 12 feet below ground while excavation was underway on an Oceanside city infrastructure project. The exact location is intentionally being withheld by city officials because the site remains an active construction zone and unauthorized digging could endanger visitors and damage additional fossils that may remain underground.
The discovery was made by principal paleontologist Dr. Joseph El Adli, who was monitoring excavation at the project site. According to reporting by The San Diego Union-Tribune, El Adli had been called to the project because its regular archaeological monitor was out sick, and he spotted an unusual object protruding from the wall of the excavation within minutes of arriving.
What initially appeared to be an isolated find quickly became considerably more significant. El Adli and his team spent seven days excavating several tons of hardened mudstone and sandstone, ultimately recovering multiple limb bones, ribs and numerous additional bones and fragments that appear to represent a single large animal.
The largest specimen is approximately 2.5 to 3 feet long and appears to be a femur. Its enormous size helped paleontologists narrow the animal's identity to two possibilities: a member of the Brontotheriidae family, commonly called brontotheres, titanotheres or "thunder beasts," or an amynodont, an extinct relative of modern rhinoceroses sometimes described as a "swamp rhino."
Both belonged to Perissodactyla, the mammalian order that today includes horses, tapirs and rhinoceroses, but neither creature would have looked quite like anything walking around San Diego County today. The brontothere species previously identified from the same geological formation was roughly the size of a modern black rhinoceros, with a deep chest, pronounced shoulders and small bony horns on its unusually shaped skull.
Brontotheres were terrestrial herbivores that browsed on leaves and other soft vegetation in forested environments. Although their nickname translates roughly to "thunder beast," they disappeared millions of years before humans arrived and were themselves part of a remarkable radiation of mammals that followed the extinction of non-avian dinosaurs.
The other candidate is no less strange. Amynodonts were distant relatives of rhinoceroses with hornless faces, broad skulls, barrel-shaped bodies, short sturdy legs and enlarged canine teeth resembling tusks, and some species appear to have adopted a semi-aquatic lifestyle that has led paleontologists to compare their appearance and behavior with hippopotamuses.
The Oceanside animal lived during the Eocene Epoch, long after the dinosaurs disappeared but tens of millions of years before the Ice Age. At that time, what is now coastal Southern California was dramatically different from today's dry Mediterranean landscape, supporting lush vegetation and an assortment of mammals that would appear almost alien to modern observers.
The remains were embedded in Member C of the Santiago Formation, a sedimentary rock formation exposed throughout portions of northwestern San Diego County. The formation has long been known as a rich source of Eocene fossils, with remains of turtles, crocodilians, snakes, lizards, birds and an extraordinary variety of mammals, including primates, rodents, tapirs, rhinoceros relatives and brontotheres previously documented within its layers.
Indeed, Oceanside has produced a surprising number of important fossils over the years. A cat-like saber-toothed predator known as Diegoaelurus, which lived approximately 42 million years ago, was discovered during road construction near College Boulevard, while a roughly 120,000-year-old extinct capybara was uncovered during construction of an Oceanside shopping center in 1994.
More recent Ice Age discoveries have also emerged from beneath the city. Construction at the Quarry Creek development near State Route 78 uncovered fossils of mammoths, horses, turtles and a prehistoric bison estimated to be approximately 50,000 to 200,000 years old, underscoring how multiple chapters of prehistoric San Diego County remain preserved beneath the modern landscape.
The newly discovered animal apparently did not die and immediately become buried. Based on the scattered and weathered condition of the remains, El Adli believes the carcass may have remained exposed long enough to dry out and potentially attract predators or scavengers before a flood eventually buried it beneath sediment, beginning the extraordinarily long process that transformed its bones into fossils.
Researchers have not yet found the crucial piece of evidence that could definitively identify the creature: its skull. Several large blocks of fossil-bearing rock remain enclosed in protective plaster jackets, however, meaning additional skeletal material could emerge as technicians carefully prepare the specimens in the laboratory.
The fossils will now undergo a painstaking preparation process in which hardened mudstone is gradually removed from around the specimens. The fossil material will then be stabilized with museum-grade adhesives and resins before researchers conduct a more detailed anatomical examination that could determine precisely what kind of animal was buried beneath Oceanside for more than 40 million years.
Ultimately, the collection is expected to be deposited in a museum where it can be preserved permanently and made available to researchers and potentially the public. El Adli, who is also a research associate at the San Diego Natural History Museum, has more than 16 years of geological and paleontological field, laboratory and research experience.
The fact that the fossils were recognized at all was not simply a matter of luck. Oceanside requires paleontological and cultural-resource monitoring during major infrastructure work in accordance with applicable environmental requirements, particularly in geologic formations known to contain fossils, allowing excavations to be stopped and scientifically significant material salvaged rather than inadvertently destroyed.
Previous environmental studies in Oceanside illustrate just how fossil-rich the Santiago Formation can be. A paleontological records search conducted for the city's Fire Station 8 project identified 78 previously recorded Santiago Formation fossil localities within just one mile of that project site and characterized the formation as having high paleontological sensitivity.
For now, whether Oceanside's newest prehistoric resident was a thunder beast or swamp rhino remains unresolved. Once the plaster jackets are opened and millions of years of hardened sediment are removed, paleontologists may finally be able to put a name to the enormous animal whose remains survived beneath the city from a time when San Diego County looked almost nothing like the place it is today.
Originally published on September 23, 2026.
