The horse fossil found in California. © Southern California Edison
The 1.4 million-year-old fossils have been described as the most comprehensive collection of fossils of its era in Southern California, more than a million years older than those at the well-known La Brea Tar Pits.
They were discovered by paleontologists working for Southern California Edison (SCE), one of the largest electric companies in the US, at the site of a proposed sub-station in Riverside County.
As part of its work to prepare the land for the substation near San Timoteo Canyon, SCE had staff and contract paleontologists and biologists carefully monitor grading work at the 28-acre site.
Because of the soft sediments in the soil, SCE expected to find some fossils, but nothing close to the size and scope that was discovered.
"This is an incredible find," said Rick Greenwood, SCE's director of corporate environment, health and safety.
"These fossils rewrite the history of this area," he said, adding that it would allow scientists and students to learn more about the important era in which the fossils originated.
Part of the horse fossil showing bite marks from a sabre-toothed cat. © Southern California Edison
The fossil collection contains specimens estimated to date back 1.4 million years to a period known as the Irvingtonian North American Land Mammal Age.
The collection is about 1.2 million years older than those from the better known Late Pleistocene era, such as fossils at the La Brea Tar Pits.
The collection is estimated to contain more than 1450 specimens, including about 250 large vertebrate fossils (bigger than rabbits) and about 1220 smaller vertebrate fossils (rabbit size and smaller). More than 27 different kinds of fossils have been identified.
Some of the fossils discovered are extremely rare. Some are notable for their relative completeness. Among those are a small, sabre-toothed cat known as Smilodon Gracilis, which is 1 million years older than its descendant, a larger sabre-toothed cat, Smilodon Fatalis, at the La Brea Tar Pits.
Among the largest pieces in the collection are two ground sloths, at least two types of camels, a llama, horse, deer and two cats. Plant fossils include birch, pine, sycamore and oak trees.
In the early part of the excavation process last autumn, paleontologists discovered the fossil of one of the ground sloths, a finding that drew attention in the region and the paleontology community.
The entire collection, known as the El Casco Fauna and Flora, is scheduled to be transferred this autumn to the Western Science Centre in Hemet. The fossils are expected to be on display at the museum sometime next year.
Supervisor Marion Ashley said: "These fossils provide tremendous insight into our past. Hidden for eons, they now offer scientists and our community a permanent collection for research and education," he said.