Thunder beast or swamp rhino? Fossils of prehistoric animal uncovered at SoCal construction site
Southern California work crews were doing some routine excavation recently when they dug up one very strange animal.
It could be a “thunder beast” or possibly a “swamp rhino.”
What’s clear, according to scientists, is that it was a massive mammal that inhabited what is now Oceanside around 40 million years ago.
The fossilized remains of the hoofed mammal were found at a construction site, where crews unearthed several limb bones, ribs and other fragments that likely belonged to a single animal, the city of Oceanside said in a statement Tuesday.
Joseph El Adli, consulting paleontologist, spoke with The Times on Wednesday about the find.
The fossil bones, the largest of which was more than 2 feet long, were found about 12 feet below the surface during routine excavation for an Oceanside water infrastructure project, said El Adli, whose environmental consulting firm was overseeing the construction site.
Workers were digging in a fossil-rich sedimentary rock unit known as the Santiago Formation.
El Adli said the fossils likely belonged to an animal from one of two extinct mammal families that roamed North America during the Uintan North American Land Mammal Age.
One possibility is that the animal is a Brontotheriidae, or “thunder beast.” It is a massive, hoofed mammal with a deep chest and pronounced hump at the shoulder.
It could also be an amynodon, or “swamp rhino,” a member of an extinct family of of large-bodied herbivores that are distantly related to rhinos. These semi-aquatic animals had canine teeth instead of a horn and barrel-shaped torsos.
El Adli discovered the first fossil fragment on Aug. 31.
The excavation process took longer than usual, about 10 days, since the fossils were encased in very hard mudstone under six feet of rock, he said.
Specimens were taken to a lab in Carlsbad for further study, and will eventually be transferred to a museum — perhaps the San Diego Natural History Museum or the Los Angeles County Natural History Museum.
El Adli said the area that crews were digging in is located on what is potentially an ancient floodplain. Oceanside is withholding the exact location where the fossils were found due to public safety concerns and to preserve the site’s integrity.
The rocks of the Santiago Formation have also produced early saber-toothed carnivores and primates, El Adli said.
Southern California has a rich fossil history that often comes to light during modern construction projects. In 2024 in San Pedro, construction workers found a massive deposit of marine fossils beneath a local high school. Construction of L.A.’s Metro D Line extension in 2017 led to the discovery of camel and possible mammoth fossils. And in 2009, a huge cache of ice age fossils was found in L.A.’s Miracle Mile under the parking lot of a former May Co. It included a nearly intact skeleton of a Columbian mammoth, dubbed Zed.
More fossils could still be uncovered in Oceanside as construction continues, El Adli said.
The paleontologist said he hoped to find teeth — or even a lower jaw — because it is key to further identifying the animal.
“It’s just sort of a hope and a prayer at this point, but that would be fantastic,” El Adli said.
El Adli said he was able to narrow it down to two candidates even without the skull because of the big limb bones, and the fact that the fossils came from a rock layer with few cow-sized animals.