California crews digging 12 feet below an Oceanside infrastructure project uncovered limb bones, ribs and fragments that paleontologists say may belong to a ‘thunder beast’ up to 45 million years old

California crews uncover possible 45-million-year-old ‘thunder beast’ fossils in Oceanside (Image Credit: City of Oceanside)

A routine infrastructure project in Oceanside, California, has led to an unexpected discovery from deep beneath the ground: fossilised bones that may have belonged to a huge hoofed mammal that lived as long as 45 million years ago. According to the City of Oceanside, which announced the discovery, crews uncovered several limb bones, ribs and other fragments about 12 feet below the surface. Paleontologists believe the remains could belong to a prehistoric animal known as a “thunder beast”; another possibility is a creature known as a “swamp rhino”. Researchers are now studying the fossils to determine the animal’s exact identity.

Fossils emerge during routine construction

The discovery was made recently while crews were carrying out excavation for a city infrastructure project in Oceanside, about 40 miles north of San Diego. Workers digging roughly 12 feet below the surface came across several fossilised bones embedded in hardened sedimentary rock. The remains included multiple limb bones, ribs and numerous other bones and fragments. Their size and shape suggested to paleontologists that they belonged to a large hoofed mammal that lived in the region during the Eocene Epoch.The fossils were found in what the City of Oceanside describes as “Member C of the Santiago Formation,” a thick sequence of sedimentary rock in northwestern San Diego County. The formation dates to a period when the landscape of present-day Southern California was very different from what it is today. Rather than stopping construction entirely, the discovery prompted paleontologists to secure the area and carefully excavate the remains.

Seven days of careful excavation

Joseph El Adli, a paleontologist who was monitoring the construction site, recognised the importance of the discovery and led additional excavation work. According to the City of Oceanside, El Adli’s team spent seven days exposing the fossils from beneath several tons of hardened mudstone and sandstone. The bones were then placed inside a protective plaster field jacket so they could be transported safely to an off-site laboratory for further preparation.The fossils are still encased in rock, meaning researchers have not yet been able to examine every part of the specimens. That is also why the precise identity of the animal remains uncertain. The exact location of the discovery is not disclosed yet, citing public safety concerns and the need to protect the site.

Was it a ‘thunder beast’ or a swamp rhino?

Based on the size and shape of the recovered bones, El Adli believes the animal could belong to one of two groups of extinct mammals. One possibility is a Titanothere, commonly referred to as a “thunder beast”. These were large, hoofed mammals that lived in North America during the Eocene. They were members of the extinct brontothere group and could reach considerable sizes.The second possibility is an Amynodon, sometimes described as a “swamp rhino”. Despite the name, these animals were not modern rhinoceroses. They belonged to an extinct group of large-bodied mammals and are thought to have had adaptations associated with life around wetlands. At this stage, paleontologists have not confirmed which of the two animals the Oceanside fossils represent. Further preparation and examination of the bones should help narrow down their identity.

A glimpse into ancient Oceanside

The fossils provide a window into a period when the landscape around present-day Oceanside was home to very different animals and environments. Officials said the mammal lived on ancient floodplains roughly 40 million to 45 million years ago. The rocks surrounding the remains belong to the Eocene-aged Santiago Formation and fall within the Uintan North American Land Mammal Age. The find also adds to evidence that the region contains significant fossil deposits. According to the Los Angeles Times, other fossils have previously been recovered from the Santiago Formation, including remains of early saber-toothed carnivores and primates.Modern construction has also uncovered fossils elsewhere in Southern California, demonstrating how buried geological deposits can sometimes remain hidden until excavation reaches them.

What happens to the fossils next?

The recovered specimens are expected to undergo further preparation and scientific examination once researchers remove more of the surrounding rock. The additional work could reveal features that distinguish a thunder beast from a swamp rhino and provide more information about the animal’s size and biology. The researchers will also be able to determine whether the fragments belong to a single individual. The City of Oceanside said the specimens are ultimately expected to become part of a museum collection, where they can be preserved and made available to scientists and the public for further study.For now, the discovery remains a scientific mystery. What began as a routine excavation 12 feet beneath an Oceanside construction project has instead uncovered bones from an animal that may have walked across Southern California’s ancient floodplains tens of millions of years ago. Further examination of the fossils will determine whether the long-buried remains belonged to a “thunder beast”, a swamp rhino or another prehistoric mammal.