SCIENCE FILE : The Valley of Hidden Treasures : In building a reservoir near Hemet, workers discover some of the best Ice Age fossils west of the California deserts.

THE FOSSILS MINED FROM THE RICH La Brea Tar Pits have played the starring role in defining life in Southern California in the waning Ice Age. But now a contributing role is being cast at a most unlikely site: below onetime onion and wheat fields near Hemet in western Riverside County.

Preliminary excavations have uncovered some of the best Ice Age fossils to be discovered west of California’s deserts, scientists say. More important, the valley’s hidden treasures seem to be yielding an enlightening cross-sectional representation of Ice Age animal life in Southern California.

“This is a brand new and incredible suite of fossils,” said Kathleen Springer, a paleontologist at the San Bernardino County Museum. “We’re filling in data gaps to reflect what was going on in this part of Southern California that we had no idea about before.”

The La Brea Tar Pits have surrendered the most and some of the biggest and best-preserved pieces to the prehistoric puzzle of what life was like here at the close of the Pleistocene Epoch. But frustrated scientists have long acknowledged that despite the millions of asphalt-encased fossils recovered there--making it the single richest cache of fossils in the world--many critical pieces to help frame that puzzle remain to be found.

The La Brea Tar Pits did not freeze-frame the entire eco-picture for Southern California, but instead captured for posterity just the unlucky animals that met an untimely death in the 23-acre asphalt trap.

Now some of those missing pieces to help put La Brea’s discoveries in broader context are turning up below the pastoral Domenigoni Valley, south of Hemet and east of California 79.

For most people, the valley will eventually be recognized as the site of Southern California’s largest reservoir, a man-made lake in the earliest stages of construction that will eventually contain more water than Lake Havasu.

But as construction crews begin work, they have unearthed several hundred fossils.

The findings show that the area was home, 10,000 to 120,000 years ago, to lumbering 3,500-pound ground sloths and herds of 5,000-pound primitive elephants called mastodons, eight-foot-tall camels, 12-foot-tall, 10,000-pound mammoths, Western horses and long-horned bison. The only predators found so far have been a dire wolf, which traveled in packs and was larger and stronger than today’s wolf, and a North American lion, which was one-third larger than today’s African lion.

The valley, arid now, was hospitable to the vegetarians at the time because it would have been cooler and wetter. The San Bernardino County Museum was contracted by the landowner--the Metropolitan Water District of Southern California--to retrieve whatever fossils its earthmovers uncover after workers unwittingly unearthed what turned out to be part of the shoulder blade of an ancient mammoth.

“This is probably the most significant Pleistocene site in Southern California,” Springer said. “This was an area where no fossils had previously been found and is turning into an incredible assemblage of fossils that are contemporaneous to La Brea.”

Christopher Shaw, collections manager at the George C. Page Museum at the La Brea Tar Pits, also is excited by what is being uncovered near Hemet.

“It’s filling in more of the picture of what the entire Southern California area was like at the time, especially on this side of the mountains,” Shaw said.

“It’s a fascinating assemblage of animals, something quite different than what we have at La Brea,” he said. “What we have here [at La Brea] is an accumulation of an unnatural occurrence--animals trapped in asphalt--compared to a real-life ecosystem. Domenigoni is a more natural reflection of the ecology.”

Though most of the Domenigoni animals also were found at LaBrea, Eric Scott, another San Bernardino County Museum paleontologist, said the fossils might shed light on the movement of these animals around Southern California.

The fossils of bison teeth might reveal the age, within months, of the animal at the time it passed through the inland valley. When compared to the fossils of teeth found at La Brea, scientists might be able to tell in which direction the bison were migrating between birthing seasons.

“There’s an indication that bison migrated into this [Los Angeles] area from a drier environment, like the desert,” Shaw said. “Perhaps they came from the Domenigoni area on their migratory path to the tar pits--or could have walked out there after coming through here--assuming they didn’t get trapped. When we put all this together, we’ll have a more complete picture.”

The recovery of fossils near Hemet reflects some of the serendipity that sparks paleontological discoveries.

When the environmental impact report was prepared for construction of the two-mile wide, 4 1/2-mile-long reservoir, its authors doubted that significant fossils would be discovered. But Springer told the Metropolitan Water District to keep its eyes open because she suspected the sediment that has filled the valley from the neighboring hills and the San Jacinto Mountains might have preserved prehistoric life.

Sure enough, when the first excavations were dug about two years ago, fossils started turning up just a few feet beneath the surface--in addition to archeological findings reflecting that Native Americans thrived there as long as 8,000 years ago.

“Most of the work I’ve done [at other paleontology sites] is finding just scraps here and there,” Scott said. “Here the stuff just started tumbling out of the ground. I felt like God was saying to us, ‘OK, you guys have worked hard enough, so here’s a nice find for you.’ ”

Adding fortune to the Domenigoni site is the presence of layers of volcanic ash, which will assist the scientists in accurately dating their finds.

But Scott and Springer, along with a handful of technicians, know that they are going to work quickly, as giant earthmovers dig foundations for two dams scheduled for completion by 1999, when the reservoir will be slowly filled.

Because there is no funding for such speculative digs, the fossils might not ever have been discovered had the reservoir not been planned.

“So far,” Scott said, “every day we’re out there, we’re coming back [to the museum] with more fossils.”