East Bay Municipal Utility District Ranger Naturalist Greg Francek was patrolling familiar ground on the perimeter of the Camanche and Pardee reservoirs in July last year when he spotted what appeared to be a petrified tree buried partially in the ground.

"It caught my eye as unusual," said Francek, who has a background in paleontology and geology. "I stopped in my tracks and looked down at my feet. This is an area I patrol and spend a lot of time in. I had no idea that existed here." 

The discovery spurred an archaeological investigation that unearthed a treasure-trove of late Miocene-era fossils — two-tusked mastodon, four-tusked gomphothere, rhinoceros, camel, horse, bird, fish, tortoise and tapir — all of which roamed the area, now known as the Mokelumne River watershed near Valley Springs, 5 to 10 million years ago.

"I can't imagine this is going to happen again," Francek said. "The implications to science, it's very important. There may not be as many sites in California which are as significant as this."

Most significant about the find is the volume of the specimens and their diversity, buried and fossilized on or near the shores of the reservoirs and preserving an epoch of North American biological history rarely observed by scientists.

"It's a find once every 10 million years," joked Nelsy Rodriguez, spokeswoman for EBMUD.

The Lodi-based district owns the reservoirs and land surrounding them, comprising approximately 28,000 acres of protected area in the Mokelumne River watershed. The water serves as the primary drinking water source for 1.4 million people in the East Bay Area, Rodriguez said.

The discovery area includes small excavation sites within a geographic range of 10 linear miles, Rodriguez said. The district is not releasing the exact location of the digs in order to protect the fossils and discourage any hobbyist explorations on the protected land. The excavation does not disrupt the flow of water.

The discoveries are considered to represent species that lived long before the appearance of bipedal hominids — of which human beings are likely the most well-known ancestors. And though reconstructions of the flora and fauna composition of the region can be largely speculatory, the discoveries may assist in filling in some of the gaps.

The closest modern comparison could be like an African savanna, Francek said, long before the Sierra Nevada mountain range arose approximately 4 million years ago in the midst of tectonic shifts and cyclones of volcanic activity. 

"That's something we're trying to figure out,” he said. “Based on studies in the region, the landscape was changing from more forested to grasslands. There were probably river deltas and river and stream channels that attracted a lot of wildlife. That would be the situation we would have here because we have so many animal burials occuring.”

Treading over the top of this prehistoric savannah would have been the great beasts of mammalia, tens of millions of years following the mass extinction of dinosaurs and some reptiles 65 million years ago.

These days, outside of a water mandate, the reservoirs serve as a recreation site for thousands of people.

Still, the former titans of the land were buried, preserved and turned to stone underneath the ground.

The excavation of the land has been ongoing since Francek's discovery, involving the EBMUD as well as paleontologists and geologists from the Department of Geological and Environmental Sciences at California State University, Chico.

"Experts from Sierra College and Sierra Nevada University, California Geological Survey, Environmental Science Associates, California State University Sacramento, the Natural History Museum of Los Angeles and other national and international institutions also are participating in the ongoing recovery and fossil identification process,” a news release stated. “An official geochronology study to date the terrain more precisely is underway.”

The discoveries now count in the hundreds for individual specimens from a dozen prehistoric species, including the  massive mastodon, an elephant-like pachyderm known for its exaggerated and trademark tusks. A tooth of the creature found by the scientists is about as large as a human hand. 

There were also the fossils of a tapir (what may have appeared like a piglet, but which is actually more biologically like a rhino), a gigantic camel the size of a giraffe, a dog-sized Merychippus (or a three-toed horse), a squat-horned Miocene rhinoceros, ground-bound tortoises, and the gomphothere.

The gomphothere, which means "welded beast" in Greek, is an extinct creature between 6 to 8 feet tall at their shoulders. The removal of a 350-pound fossil required a backhoe, the EBMUD website said. 

“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. “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.”

The fossils are owned in perpetuity by EBMUD. Many are on display in Chico and others are available to scientists at the University of California Museum of Paleontology in Berkeley. 

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

Contact Giuseppe Ricapito at gricapito@uniondemocrat.net or (209) 588-4526.