# Prehistoric fossil discovery in Oceanside, California

> Live situation record from CLSTR: https://clstr.news/situations/prehistoric-fossil-discovery-in-oceanside-california
> Updated: 2026-09-26T11:58:09.000Z. Sources: 10. Developments: 2.

During a water infrastructure project in Oceanside, California, paleontologists discovered fossilized remains of a massive prehistoric mammal. The fossils, estimated to be between 40 and 45 million years old, were located approximately 12 to 15 feet below the surface within the Santiago Formation.

The excavation recovered several specimens, including ribs, limb bones, and a femur measuring between 2.5 and 3 feet in length. Researchers, including Joseph El Adli and Joseph L. Athly, suggest the remains may belong to the extinct mammal family Brontotheriidae, described as ‘thunder beasts,’ or the Amynodontidae, known as ‘swamp rhinos.’

The bones were extracted using plaster casts to prevent damage and have been transported to a laboratory for cleaning, reinforcement, and further study.

## Timeline

### 2026-09-26: California discovery of 40-million-year-old rare fossils

Rare 40-million-year-old fossils, potentially belonging to Eocene-era mammals like Brontotheres or Amynodonts, were discovered during infrastructure projects in Oceanside, California.

2 sources. https://clstr.news/cluster/california-discovery-of-40-million-year-old-rare-fossils

### 2026-09-23: Oceanside construction project unearths 40-million-year-old prehistoric fossils

Paleontologists discovered 40-to-45-million-year-old fossils of a massive prehistoric mammal, possibly a ‘thunder beast’ or ‘swamp rhino,’ during a construction project in Oceanside, California.

8 sources. https://clstr.news/cluster/prehistoric-fossils-discovered-during-oceanside-construction-project

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Cite as: Prehistoric fossil discovery in Oceanside, California. CLSTR, https://clstr.news/situations/prehistoric-fossil-discovery-in-oceanside-california