Into the Pits : Ice Age Fossils Are Trophies of Annual La Brea Dig

The seekers went back into Pit 91 Wednesday, digging through the asphalt for treasures large and small.

Since 1969, scientists and volunteers have been removing the remains of extinct ground sloths, saber-toothed cats and many smaller Ice Age fossils from Pit 91, one of the still-oozing La Brea tar pits in Hancock Park.

The dig has been an annual summer event since 1984, the only excavation of its kind in the United States. Thousands of visitors are expected to peer into the 28-by-28-foot pit from a glassed-in viewing station at ground level. The dig, conducted by the George C. Page Museum of La Brea Discoveries, will continue Wednesdays through Sundays to Sept 3.

Shortly after 8 Wednesday morning, museum staff member Eric Scott and volunteers Michael Charters and Harold Christensen descended by ladder to the pit floor, 13 feet below ground level.

Wearing hard hats, rubber boots and their grubbiest jeans and T-shirts, the excavators began working with brushes and dental picks to expose the asphalt-soaked remains of plants and animals that were trapped in the pit as long as 40,000 years ago. Sometimes the asphalt peels off the bones. Sometimes the diggers need a chisel and hammer to chip it from the fossils.

Asphalt (not tar), which an observer once described as “the color of very good licorice and the consistency of very bad fudge,” is everywhere in Pit 91--on the fossils, on the boots, jeans and noses of the excavators, even perfuming the air.

“You get used to the smell,” said Christopher A. Shaw, a museum staff member who coordinates the summer dig. Shaw noted that Pit 91 was originally excavated in 1915, and is one of more than 100 sites in the Hancock Park area first explored by paleontologists almost 100 years ago.

On the whole, digging in Pit 91 is “wonderful,” he said. While the work itself is demanding, dirty and painstaking, the atmosphere is convivial.

As Shaw explained, the asphalt deposits that bubble to the surface throughout the area have served as a “self-baiting trap” for animals for millenia.

The fossilized remains of more than 550 species of plants and animals have been found in the La Brea tar pits, one of the richest deposits of Ice Age fossils in the world. Pigeons and other small animals continue to become victims of the tar.

‘Rocketed Through Crust’

Much to his embarrassment, senior excavator Scott was once trapped in the stuff when he tried to retrieve a traffic cone that someone had tossed onto the surface of Pit 61/67. The ground looked stable enough, but, Scott recalled, “I rocketed right through the thin crust of the otherwise gooey asphalt.” Scott sank almost to his knees. Worse was yet to come. “One man came by and said to his son, ‘See! That’s how the dumb animals got stuck,’ and then walked away!”

Scott was saved from premature fossilization by colleagues who tugged him out.

Volunteer excavators, who have included police officers, physicians, plumbers and rock musicians, must be at least 16 years old. Before they are allowed to dig, they must spend at least 96 hours working in the paleontology laboratory at the Page Museum, where they learn to identify and handle fossils.

In the pit, they learn such additional skills as how to determine the orientation of a half-buried bone so they can uncover it without damaging it and how to “pedestal” a fossil, exposing almost all of it on a tiny, supporting pedestal of earth.

Had Bias Toward Large

According to Shaw, one reason for the re-opening of Pit 91 in 1969 was the desire of scientists to correct what he termed “the collection’s bias toward the large.” Of the more than 1 million fossils removed from the tar pits during the early excavations, fewer than 1% were the remains of plants or animals “smaller than a rabbit,” Shaw said.

Today scientists believe microfossils can often be even more instructive than large specimens. For example, mice and other small animals usually don’t travel as far as large ones and may reveal more about the immediate environment.

Most of the fossilized insects, plants, mollusks and other small organisms unearthed in the original digs were thrown away. In some instances, the only microfossils that remain from the original excavations were those found decades later in dirt inadvertently left inside the brain cavities of saber-toothed cats and other large animal fossils. Fossilized plant fragments have also been found packed into pits in the teeth of some extinct bison and other large plant-eaters.

‘Change Our Technique’

“To get these small fossils, we had to change our technique from basic pick and shovel and wheel barrow to using nothing larger than trowel and brushes and dental tools to excavate,” Shaw said. Modern techniques have been developed for uncovering the tiniest microfossils. The small bones of birds, for instance, are put into donated baby food jars and cleaned ultrasonically. Now, instead of being dumped, the dirt dug from the pit is scrutinized under a microscope for insect parts, snails and other microfossils.

During the modern excavation of Pit 91, the ratio of tiny things being discovered to large ones is 500 to 1, Shaw said. The modern excavators have added more than 300 additional species of plants and animals--most of them small--to the 250 identified during the early digs.

Last year, 724 fossils were found during the summer dig, plus 24 buckets of asphalt that have yet to be examined for microscopic fossils. The finds included the wishbone of an extinct condor estimated to be 35,000 years old.