VALLEY SPRINGS, Calaveras County (CBS SF) — Fossils dating back some five to ten million years have been found in the Mokelumne River watershed in the Sierra Nevada foothills in what’s being called one of the most significant fossil discoveries in California history, researchers announced Tuesday.

California State University, Chico and the East Bay Municipal Utility District (EBMUD) said the trove of fossils was found in areas along the Mokelumne River near the town of Valley Springs. The watershed provides drinking water to EBMUD’s 1.4 million customers.

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The initial discovery of fossils was made in June 2020 by EBMUD Ranger/Naturalist Greg Francek, who came upon a petrified tree that was partially buried in sediment. Following the discovery, he began to look more closely around the area.

“So I looked around the area further and I discovered a second tree, and then a third, and so on,” says Francek in a virtual tour of the site on the EBMUD web page. “After finding dozens of trees, I began to realize that what I was looking at was the petrified remains of a forest.”

Weeks later, Francek found the first fossilized bone in the area, which led to the discovery of an assortment of animal fossils from the Miocene Epoch.

“During the third week of a more organized survey, I located the first vertebrate fossils,” says Francek on the virtual tour. “What I didn’t comprehend at the time was the amazing fact that I was looking at the bones of great beasts that had roamed this landscape millions of years ago.”

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Gomphothere fossil recovery in Mokelumne River watershed. (EBMUD)

EBMUD along with paleontology and geology experts from CSU Chico established excavation sites over several miles of EBMUD land. Scientists recovered samples from hundreds of specimens from about a dozen prehistoric species, including a two-tusked mastodon, a four-tusked gomphothere, rhinoceros, camel, horse, bird, fish, tortoise and tapir. Some of the specimens have yet to be identified.

The sites are now secured and protected under the United States Paleontology Resources Preservation Act and the Municipal Utility District Act. The sites are also part of the field research program at CSU Chico.

“This historic discovery has revealed that the Mokelumne Watershed plays a much deeper role in our understanding of the natural history of North America,” said EBMUD Board President Doug Linney in a prepared statement. “It is simply wondrous that these fossils will help fill gaps in our understanding of the formation of the region and planet.”

An official geochronology study to date the terrain more precisely is underway.

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“This new find is highly significant for both the sheer volume and diversity of the fossils,” said CSU Chico Department of Geological and Environmental Sciences professor Dr. Russell Shapiro, also a lead scientist on this discovery. “This was a profound juncture in time when land animals evolved as forestland shifted to grassland. The partnership with EBMUD allows our students – the next generation of field scientists – an invaluable, first-hand experience from the discovery site to the preparation and the protection of these amazing fossils.”