An Ice Age ecosystem in asphalt
The "tar" at La Brea is not tar. It is natural asphalt, or bitumen, a low-grade crude oil that has been seeping up from the Salt Lake Oil Field for tens of thousands of years. As the lighter petroleum fractions evaporate, they leave behind thick, sticky asphalt. Over millennia, this created dozens of pools that often became hidden by water, leaves, and dust. Most of the fossils recovered from these seeps date from 11,000 to 50,000 years ago, during the Late Pleistocene Epoch. The asphalt preserves bone, insect exoskeletons, and plant material with incredible fidelity; the bones are not petrified but are instead saturated with asphalt, which stains them a characteristic dark brown.
This natural trap gives scientists a record of a complete prehistoric ecosystem. While large mammals are the most famous finds, the pits also preserve microfossils that help reconstruct the past environment of the Los Angeles Basin. Preserved pollen grains, seeds, and wood fragments show the local flora. Recent studies of juniper seeds, for example, have identified the presence of the Rocky Mountain juniper (Juniperus scopulorum), a species that no longer lives in California, allowing researchers to track ancient climate shifts.
The predator trap
The most common feature of the La Brea fossil collection is the overwhelming number of predators. For every trapped large herbivore like a bison or camel, paleontologists have found roughly ten carnivores. This skewed ratio is the result of a "carnivore trap." A large, struggling herbivore would attract numerous predators who saw an easy meal, only to become mired in the asphalt themselves.
The most common large mammal found is the dire wolf (Aenocyon dirus), with remains from more than 4,000 individual animals recovered. The second most common is the saber-toothed cat (Smilodon fatalis), with over 2,000 individuals found. Other predators include American lions (Panthera atrox), coyotes (Canis latrans), and the short-faced bear (Arctodus simus). The most common large herbivore is the ancient bison (Bison antiquus), with fossils from at least 300 individuals. The only human remains ever found belong to the "La Brea Woman," dated to about 10,220 years ago. She was between 18 and 25 years old at the time of her death.
Science in action
Excavation is an ongoing process at La Brea. Since 1913, paleontologists have excavated more than 100 pits, but Pit 91 is still actively worked on by researchers during the summer. In 2006, a major new deposit was discovered during the construction of an underground parking garage for the adjacent Los Angeles County Museum of Art. This led to "Project 23," the recovery of 16 fossil deposits inside 23 large wooden boxes. This single project is expected to double the museum's existing collection of 3.5 million specimens.
One of the most significant finds from Project 23 is the nearly complete skeleton of a Columbian mammoth (Mammuthus columbi) nicknamed "Zed." The skeleton is about 80% complete and includes both of its 10-foot-long tusks, making it the most complete individual mammoth ever found at the site. Visitors to the museum can watch paleontologists work on fossils from Project 23 in the glass-enclosed Fossil Lab.