VALLEY SPRINGS, Calif. (CBS Sacramento) – A trove of 5- to 10-million-year-old fossils of prehistoric species was discovered in the Mokelumne River watershed in Calaveras County, the East Bay Municipal Utility District announced on Tuesday.

Among the fossils were samples of species including a two-tusked mastodon, a four-tusked gomphothere, rhinoceros, camel, horse, fish, bird, tortoise, and tapir. The EBMUD said others are still to be identified.

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“A discovery like this makes us realize that there is so much more to learn. In terms of our geography is really a drop in the bucket compared to what the world is,” said Nelsy Rodriguez with EBMUD.

EBMUD Ranger Greg Francek made the discovery last July while on patrol in the 28,000 acres of protected land in the foothills, which sits near the town of Valley Springs and is a primary drinking water source for more than a million East Bay residents, according to the EBMUD. Francek reportedly discovered the fossils after identifying petrified trees.

Francek reached out to scientists at Chico State to be a second set of eyes on the collection of bones. That gave way to a large-scale excavation project to help fill the gap of what life was like before humans moved in.

“Some of the first discoveries were large proboscideans, which are essentially ancestral elephants such as the gomphothere, parts of mastodons, we found tortoise remains, horses,” Francek said.

“The diverse collection of fossil flora and fauna reveals so much about the dynamic natural history of the land we work with every day. This discovery and the resulting scientific study add to the understanding of North American biotic evolution and climate change that has occurred over millions of years,” Francek said.

The fossil recovery and high concentration of fossils point to the area once being “made up of multiple river channels with an abundant and diverse grassland and forested ecosystem,” the EBMUD said. The EBMUD said new research also suggests the Sierra Nevada “began to rise tens of millions of years ago, and these fossils represent a significant bookmark in that story millions of years in the making.”

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“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. “It is simply wondrous that these fossils will help fill gaps in our understanding of the formation of the region and planet.”

The EBMUD said it established excavation sites over several miles of EBMUD land with experts in paleontology and geology from Chico State University. Additionally, recovery efforts remain ongoing and experts from Sacramento State, Sierra College, Sierra Nevada University, the California Geological Survey, Environmental Science Associates, and the Natural History Museum of Los Angeles are assisting, the EBMUD said.

Scientists are hoping to make even more jaw-dropping discoveries in the months ahead, looking to shed more light on what used to call the foothills home.

“I think we’re going to find some very, very important things as we go through this study,” Francek said.

Officials are also working on an official geochronology study to date the terrain more precisely.

“This new find is highly significant for both the sheer volume and diversity of the fossils,” said Dr. Russell Shapiro, Professor of the CSU Chico Department of Geological and Environmental Sciences and 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.”

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Experts say there aren’t too many fossil deposits found in California, so this find is significant. But if you’re hoping to check it out in person, you won’t have much luck. The public is not allowed at the dig site.