The skeleton was found late last month beneath Robertson Ranch and is the second fossil of an ancient mammal to be discovered by construction crews digging in the area, The San Diego Union-Tribune reported.
The 400-acre ranch at El Camino Real and Cannon Road is being redeveloped for homes. Last June, construction crews found tusks and jawbones of a mastodon.
Both the giant ground sloth and the mastodon lived during the Pleistocene Epoch, also known as the Great Ice Age, which is believed to have lasted 1.8 million years and ended about 10,000 years ago.
The sloth skeleton has been turned over to scientists at the San Diego Natural History Museum, who believe it to be the most complete remains of that species ever discovered in the county, the Union-Tribune reported.
"We know we have part of a pelvis, part of a front leg and part of a back leg," paleontology curator Tom Demere told the newspaper. "There were two ribs we could see and part of a vertebra ... They'd been held together a couple of hundred thousand years by mudstone."