Workers building a substation in California have discovered 1,500 bone fragments from about 1.4 million years ago.
The fossil haul includes remains from an ancestor of the sabre-toothed tiger, large ground sloths, deer, horses, camels and numerous small rodents.
Plant matter found at the site in the arid San Timoteo Canyon, 85 miles (137km) south-east of Los Angeles, showed it was once much greener.
The bones will go on display next year.
The find is a million years older than the famous haul from the tar pits at Rancho La Brea in Los Angeles, said Rick Greenwood, a microbiologist and also director of corporate environment health and safety for Southern California Edison.
"If you step back, this is just a huge find," he said. "Everyone talks about the La Brea Tar Pits, but I think this is going to be much larger in terms of its scientific value to the research community."
The number of skeletons found at the site may be explained by a marsh or lake bed that trapped animals looking for water, leaving them victim to predators, palaeontologists think.
Tom Demere, a San Diego Museum of Natural History palaeontologist, said the find was not directly comparable to La Brea, as it comprised different species from another era.
But he said it would be valuable.
"We have a fuzzy view of what this time period was like in terms of mammal evolution," Mr Demere said. "A discovery like this - when they're all found together and in a whole range of sizes - could really be an important contribution."