PLEISTOCENE PARK

The unearthing of a new species of prehistoric animal is usually a solitary task performed in splendid isolation in the middle of a remote desert several days from the nearest tourist mecca.

With one major exception.

The La Brea Tar Pits, in the heart of Los Angeles’ Miracle Mile district, have been the source of more than 4 million fossils, including more than 625 species. They represent one of the most densely packed accumulations of fossils in the world, according to paleontologist William Akershem of the Idaho Museum of Natural History, and they sit in the middle of one of the more expensive shopping districts in the country.

Two months of every year, paleontologist Christopher A. Shaw opens Pit 91, just beyond the rear door of the Los Angeles County Museum of Art, for viewing by the public.

Every day, from 10 until 5, Angelenos and tourists alike can observe the laborious, often painfully slow reading of the geological record as scientists and volunteers prize out the bones of 25,000-year-old animals, reconstructing life in the Los Angeles Basin as it existed in the Pleistocene period, long before the Chumash Indians first ventured into the region.

Thirteen feet below the surface, in a 28-square-foot pit, researchers carefully unearth 3-by-3-foot sections of asphalt only 6 inches thick. In eight weeks of hard, hot work, they will remove three, maybe four such sections, producing 800 to 1,100 individual fossils--enough to keep researchers at the George C. Page Museum busy cleaning and cataloging them for the other 10 months of the year.

All this occurs under the observation of tens of thousands of people.

The tar pits are not unique, but they are rare, Shaw said. Perhaps a dozen are known throughout the world. Three others are found associated with oil fields in California--in Carpinteria, Taft and McKittrick--and one in Texas. The rest are scattered across the globe.

Despite their name, the pits do not actually contain tar, which is a synthetic material formed only when petroleum is heated to high temperatures. They are actually filled with asphalt, which is the heaviest fraction of petroleum, left behind when the rest of the petroleum evaporates.

Beginning about 40,000 years ago, the soft asphalt oozing up from submerged oil fields collected in depressions in what is now Hancock Park. Unwary animals that put a foot into the pools found they could not pull it out. Others, seeing the trapped animals as a handy source of food, attacked them, only to be trapped themselves. Insects, snakes, plants, even trees, also fell in.

Over time, the bones collected in a massive jumble “like Pick-Up Sticks,” Shaw said. “Sometimes you will find 30 or 40 specimens that will stay jumbled until you find the key bone that allows them to unlock.” Virtually every scoop of asphalt yields a fossil of some sort.

It’s not only the quantity of the bones in the pit, but the quality of the preservation that is important, Shaw added. The asphalt saturates the bones so that water cannot leach out organic chemicals that are an integral part of the bone. Unlike normal fossils, which have been turned to stone, the La Brea bones are close to their natural state. In one case, the Page team has even been able to analyze DNA from a saber-toothed tiger.

Even very delicate bones, like the hollow bones of birds, which are normally crushed when they are fossilized, are preserved “almost perfectly,” he said.

But perhaps most important, he added, is the sheer diversity. The pits yielded not just one dire wolf, for example, but hundreds of them of different ages and sizes, most of them healthy, but some injured or sick. Multiply this abundance by the large number of species present, and researchers are able to put together a comprehensive picture of life in the late Pleistocene.

This diversity allows Shaw and his colleagues to draw shrewd inferences about life in the Pleistocene. Some saber-toothed tigers drawn from the pits, for example, show extensive damage from fighting or accidents that healed before the animals were trapped by the asphalt.

That finding indicates that the tigers were communal animals rather than loners, he said. Other members of the pack must have helped the injured beasts get food and protected them from other predators, or they could not have survived to heal.

Large bones from Pit 91 are cleaned up and taken directly to the museum for study, but most of the material dug from each square is stored in black five-gallon buckets. Over the course of the winter, solvents will be used to dissolve the asphalt in them, converting five gallons of diggings into about one gallon of dirt, bones and debris.

Volunteers pick carefully through the debris, looking for microfossils and other evidence, such as fecal pellets and intestinal contents. Those volunteers can also be observed as they perform their tedious tasks behind glass in the “fishbowl” inside the museum.

The paleontological record at La Brea ends abruptly about 11,000 years ago, perhaps as a result of climate change in the basin. Botanical evidence suggests that rainfall in the region dropped about 50% then, Shaw said, disrupting the vegetation that had hidden the tar from animals. Lessened vegetation may also have allowed periodic rains to wash more bones and debris away before it could sink into the asphalt.

As a consequence, there are few fossils in the pits that are younger than 11,000 years. One notable exception is a Chumash Indian woman dating from about 9,000 years ago, but researchers believe she was deliberately buried in the pits, not trapped there.

Researchers have also excavated the odd goat and sheep from about the late 1800s, Shaw said, but aside from the occasional pigeon that gets trapped now, “that’s about it.”

Researchers will be digging in Pit 91 from 10 a.m. to 5 p.m., Wednesday through Sunday, until Sept. 1.