The SoCal Ice Age Fossil Treasure Trove You’ve Never Heard Of

California’s Ice Age feels close enough to touch. The sticky asphalt seeps of La Brea are still oozing away as they did in the days when they trapped dire wolves, and many of the Golden State’s residents – from coyotes to leafcutter bees – have been around since the days when sloths were trundling through the warm grasslands. But there’s more to California’s Pleistocene past than golden eagles overhead or the bubbling ponds off Wilshire Boulevard. In a small town between LA and San Diego, there’s a monument to one of the richest megafaunal boneyards found in the state.

If you don’t know the Western Science Center is there, you might speed right past it. The boxy building is set back from road on the outskirts of Hemet, where rolling hills and not skyscrapers are the tallest things around. But step into the dark halls of the museum and you’ll meet some of the locals from over 10,000 years ago. Xena the mammoth and Max the American mastodon stand tall in silhouette reconstructions that give you some idea of their bulk, Harlan’s ground sloth and other ancient residents filling out the menagerie. All of them represent animals excavated from an enormous dig that’s now underwater.

Xena, Max, Little Stevie, and all the rest came from the ground beneath Diamond Valley Lake. Paleontologists didn’t need scuba gear to get them. No, the dig was a rescue mission, with paleontologists racing the clock before 280 billion gallons of water were flooded into the reservoir.

At first, no one was expecting to find this osteological treasure trove. Back in 1991 the official impact statement for the Eastside Reservoir Project stated that the eventual site of Diamond Valley Lake had a “low to moderate potential to unearth vertebrate fossils” and that any such finds would probably be middling fragments.

Geologist Kathleen Springer, now of the U.S. Geological Survey but working at the San Bernardino County Museum at the time, knew that this couldn’t be right. Significant Ice Age fossils had previously been found on the surface in the area, Springer says, and given the plans to dig far deeper the construction crews would almost certainly find more bones. Springer even went so far as to offer the Metropolitan Water District the services of the museum to help handle the bones she knew they were going to find, but she didn’t hear any reply. Digging started in 1993.

It didn’t take long for bones to start turning up. Archaeologists who were onsite during excavations brought some big animal bones to paleontologist Eric Scott at the San Bernardino County Museum, Springer says. These were mastodon bones, and the archaeologists said that bones were popping out of the ground as soon as the earth started to be shifted by all the heavy equipment.

The Metropolitan Water District finally called in Springer and Scott to assess the site. “We started finding an enormous amount of material immediately,” Springer says, and by 1995 she and Scott were selected as the official paleontologists for the operation. What followed was years of harrowing work to extricate what could be saved from the boneyard.

“We were monitoring the largest excavation in the United States,” Springer recalls, in a project that moved 110 million cubic yards of dirt from 1993 to 2000. During that timeframe there were two-person paleontology teams at each end of the valley working 10 hour shifts for 20 hours a day, six days of the week. This put them in close quarters with the massive machinery rumbling around the valley. The paleontologists “would find a fossil, flag it off, and the giant equipment would have to go around them as it was excavated,” Springer says. “This went on every day for seven years.”

Despite having totally different goals, however, Springer says that eventually everyone worked well together. “We tried a lot of techniques and it involved a lot of communication with MWD higher ups, their foremen, and workers,” Springer says, but in time all the kinks worked out as the paleontologists went about their excavations, even in the dark. “At night, the project was lit up like some surreal world,” Springer says.

All that work yielded some fantastic fossils. The spot that yielded the skeleton of Little Stevie the American mastodon, Springer says, also contained the remains of bison, sloths, horses, and more. “It was excavated beautifully by our crew,” Springer says, the efforts eventually yielding radiocarbon dates, ancient pollen, and other hints that allowed the paleontologists to envision the spot as a big lake that once sat over sixty feet below the valley floor.

All told, over 100,000 specimens were excavated by Springer, Scott, and their colleagues, now resting in the Western Science Center. This makes it the second richest Ice Age collection in California after La Brea, says Western Science Center’s executive director Alton Dooley, and offers a counterpoint to the predator-dominated assemblage for the asphalt seeps.

Even though the Diamond Valley Lake species lived in the area less than 50,000 years ago, equivalent to the younger deposits at La Brea, the environment was very different. Diamond and Domenigoni Valleys seem to have been wetter during the Ice Age, Dooley says, with lakes growing and receding with the seasons and possibly rivers flowing nearby year round. Still, Dooley notes, there are some mysteries about what was going on here. The small mammals found in the Ice Age deposits are very similar to those living in the area today despite the more arid climate and this, Dooley says, “raises the possibility that the big mammals may have used the valley as a migration corridor, or only lived in the valley part of the year” when conditions were wet and lush.

That’s not the only puzzle presented by the Diamond Valley beasts. The Western Science Center’s collection holds about three times as many American mastodons as mammoths, which is the opposite of what’s found at La Brea. Not only that, but the California mastodons are strange. “We’re starting to see that the California mastodons have long, narrow teeth compared to mastodons from the east and Midwest,” Dooley says, and specimens like Max – a very large mastodon with comparatively small teeth – indicate that tooth size is not a good indicator for body size in these ancient elephants. Figuring out why this might be, Dooley says, may help paleontologists figure out how different mastodon populations were mixing and if tooth anatomy reflects aspects of ancient environments. (The Western Science Center is wrapping up a crowdfunding campaign that may help the museum do just that.)

There are many secrets yet to be drawn out from the Diamond Valley Lake fossils. More tantalizing, though frustrating, is that there is so much left to discover beneath the lake itself. All in all, Springer says, fifty San Bernardino County Museum staff actively worked the site and the bones before the water was poured in, but they were only working the spots the construction crew was digging up. There were plenty of spots that are likely rife with fossils that were never touched. “There are absolutely fossils beneath the lake,” Springer says, with each inch of water lost in California’s drought bringing them close to the surface again.