Puzzle Pieces : Paleontology: Fossils from freeway construction site reveal what San Diego County was like during the Ice Age.

Remains of a camel, a mastodon and other mammals unearthed during freeway construction in National City this week are helping scientists understand what San Diego County was like during the Ice Age.

“This site is a piece of the puzzle in figuring out the distribution of mammals that lived here,” Tom Demere, assistant curator of paleontology at the San Diego Natural History Museum, said Wednesday.

A week after the operator of an earth-grading machine discovered the first prehistoric bone, five paleontologists from the Natural History Museum were still eagerly uncovering fossils from the Pleistocene epoch, 10,000 to 120,000 years ago.

Among the bones dug up so far at the California 54 construction site near Reo Drive is about half the skeleton of an ancient horse, including five vertebrae, some ribs and hind limbs.

“I’m not going to leave until we find the skull--I’ll dig this mountain away,” museum paleontologist Richard Cerutti said.

Other bones being plastered for transport to the Natural History Museum include a tooth from another horse, part of the tusk of a mastodon, the jaw and limbs of a gopher, limbs and a fragment from the lower jaw of a camel, and the vertebrae of what may be a bison.

Excavators are working from dawn to dusk to uncover the remains. At night, security guards are posted to watch out for fossil thieves.

“There are people drooling to get their hands on this stuff,” Caltrans spokesman Jim Larson said.

Paleontologists have documented ancient bones at four different sites over the last week in a hilly area that was once a river flood plain. Excavators have yet to dig around the tusk remains of a mastodon that were found just a few feet from a nearby house.

“Wherever the earth is turned up and you find sedimentary rocks, there’s a good chance that fossils will be found there,” Demere said.

Until now, most of the Ice Age fossils found in the county have been remains of marine animals, Demere said. A few scattered bones from Ice Age mammals have been found inland--including a small discovery several years ago at a Chula Vista mortuary--but never has one site in the county’s coastal area produced so many mammal body parts from the Ice Age.

The find pales in comparison with the famed La Brea tar pits in Los Angeles, which include extensive remains of Ice Age mammals--mastodons, elephants, camels, ground sloths and saber-tooth cats. Other discoveries in Orange County have also been more extensive, Demere said.

The Natural History Museum has a $15,000 contract with Caltrans to look for fossil remains as Caltrans continues its work on California 54. The $12-million project will widen the highway to a six-lane freeway from Interstate 805 to about a mile east of Reo Drive.

In the past, many potential fossil beds have been pulverized by bulldozers and other earth-moving equipment. But cooperation between Caltrans and scientists in the last two years has led to several major finds in the county.

The National City find is the third major paleontological discovery in little more than a year during highway construction in the county. The other two finds were of fossils from much earlier periods--from 43 million to 47 million years ago.