Paleontologists expected to find fossils at the 28-acre site as it was prepared for the construction project, but they said they had no idea they would find so many.
Nearly 1,500 specimens representing 35 different species were unearthed, including a giant cat that was the ancestor of the saber-toothed tiger, ground sloths the size of a modern-day grizzly bear, two types of camels and more than 1,200 bones from small rodents.
"We found fossils here that haven't been found before. The go back probably 1.4 million years. So they'll rewrite the geological history of this area," said Rick Greenwood with Southern California Edison.
Workers also uncovered signs of plant life that indicate marsh reeds and sycamore, pine and birth trees once grew in the area that is now dry and sparsely vegetated.
The bones are about 1 million years older than those found in the famous La Brea Tar Pits in Los Angeles.
All of the unearthed fossils have been dated to the Irvingtoninan period.
The fossils go on display at the Western Science Center in Hemet in 2011.
The Associated Press contributed to this report.
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