Remarkable 40 Million Year Old Discovery in Oceanside

Crews working on a City of Oceanside infrastructure project made a remarkable discovery this month: the fossilized remains of a large, hoofed mammal that walked the ancient floodplains of Oceanside, some 40 to 45 million years ago.

These significant fossil finds, which included several limb bones, ribs, and numerous other bones and fragments, were uncovered approximately 12 feet below the surface during routine excavation for a city infrastructure project.

Joseph El Adli, Ph.D., principal paleontologist, made the discovery on site during routine monitoring and immediately secured the area to begin additional excavation. His team worked over a period of seven days to expose the fossil bones from beneath several tons of hardened mudstone and sandstone before encasing them in a plaster field jacket for safe transport to an off-site laboratory for eventual preparation.

The bones were recovered from Member C of the Santiago Formation, a thick sequence of sedimentary rocks exposed across parts of northwestern San Diego County. The formation dates to the Eocene Epoch and has previously yielded fossils from that time period within the city, as well as elsewhere in the region. Based on the known age of the surrounding rock and the types of organisms found within the unit, paleontologists estimate that the fossils are between 40 and 45 million years old, falling within the Uintan North American Land Mammal Age.

Based on the size and shape of the bones uncovered during the excavation, Dr. El Adli believes they likely belonged to an animal from one of two extinct mammal families that once roamed North America, both of which belong within the order Perissodactyla, the odd-toed hoofed mammals.

The first possibility is a member of the Brontotheriidae, sometimes called titanotheres or "thunder beasts." These were massive, hoofed mammals with a deep chest, pronounced hump at the shoulder, and, in many species, a distinctive Y-shaped bony horn on the nose. The species of brontothere known from the Santiago Formation was roughly the size of a modern black rhino and possessed small horns on its saddle-shaped skull. They were land-dwelling browsers, feeding on soft leaves and vegetation in forested habitats.

The second possibility is that the discovery represents the remains of an amynodon, historically referred to as a "swamp rhino," amember ofan extinct family of large-bodied herbivores distantly related to rhinos. Amynodon had hornless faces and squat, hippopotamus-like bodies. They had low, barrel-shaped torsos, short sturdy legs, a broad flat skull, and tusk-like canine teeth rather than a horn. Some amynodons were semi-aquatic, spending much of their time wallowing in rivers, ponds, and lakes and grazing on coarse aquatic and streamside vegetation, while others were more lightly built and lived on land.

The next steps for the fossils will involve removal from their plaster jacket followed by careful removal of the hard mudstones that surround each bone. During this process, the bones will be hardened and stabilized using museum-grade glues and resins to ensure long-term preservation. Further examination of the bones once they have been fully removed from the rock that encases them should allow paleontologists to narrow down the precise identity of the Oceanside fossil discovery. The specimens are expected to ultimately join a museum collection, where they will be preserved and made accessible to the public and to other scientists for further study.

As part of its standard practice on major infrastructure work and in compliance with California state environmental laws, the city maintains both paleontological and cultural resource monitors on site throughout construction, a measure that allowed this discovery to be identified, protected, and properly handled when it was uncovered.

The City of Oceanside is currently withholding the exact location of the discovery to protect both public safety and the integrity of the site. Active construction zones carry hazards and are vulnerable to unauthorized digging, which can damage or destroy fossils before they can be properly studied.