La Brea Tar Pits: Ice Age Fossils in the Heart of Los Angeles

The La Brea Tar Pits are a group of natural asphalt seeps in Hancock Park, Los Angeles, that have trapped and preserved animals and plants for around 50,000 years. Known scientifically as Rancho La Brea, the site has yielded millions of Ice Age fossils, including sabre-toothed cats, dire wolves and mammoths. Excavated since the early twentieth century, it is one of the richest fossil sites of its age anywhere in the world and a rare working dig in the middle of a major city.

How the asphalt seeps formed

Beneath this part of Los Angeles lies the Salt Lake Oil Field. Crude oil migrates upwards through cracks and porous sediments, and at the surface its lighter components evaporate, leaving thick, sticky asphalt. The seeps have been active for tens of thousands of years and are still active today.

On warm days the asphalt softens. A thin layer could be covered with water, dust or leaves, disguising the danger. An animal that stepped into it could become stuck after only a few centimetres. Over time the bodies sank or were buried, and the asphalt soaked into the bones, protecting them from decay.

A predator trap

The fossils at La Brea show an unusual pattern: predators and scavengers vastly outnumber plant-eaters. Palaeontologists explain this as a “predator trap”. A single trapped horse or bison would attract wolves, big cats and vultures, many of which were caught in turn. This is why so many carnivores are preserved.

The remains include:

• Predators: dire wolves (more than 4,000 individuals), Smilodon sabre-toothed cats (more than 2,000), coyotes, the American lion and the giant short-faced bear.
• Plant-eaters: Columbian mammoths, American mastodons, ancient bison, horses, camels and ground sloths.
• Birds: thousands of birds, especially eagles, hawks, vultures and the extinct teratorns, huge condor-like birds.
• Small life: insects, snails, rodents, reptiles, seeds, pollen and wood, which reveal the climate and plant life of Ice Age Los Angeles.

Most of the fossils date from roughly 50,000 to 11,000 years ago, during the last glacial period. The region was then cooler and moister than today, with woodlands of pine, oak and juniper alongside open grassland.

What the fossils reveal

Because so many individuals of the same species are preserved, La Brea allows scientists to study whole populations rather than single skeletons. Researchers have used the collections to examine growth, injuries, disease and changes in body size over thousands of years. Studies of broken teeth in dire wolves and sabre-toothed cats, for example, found much higher rates of breakage than in living carnivores, which some palaeontologists interpret as a sign of intense competition for food, with predators eating carcasses more completely, bones included.

Chemical analyses of bones and teeth help reconstruct what animals ate and where they lived. Results suggest that Smilodon and the American lion tended to hunt prey from more wooded areas, while dire wolves fed more on grazing animals of open country. Plant remains and insects record how vegetation shifted as the climate warmed at the end of the Ice Age.

The fossils also have a distinctive appearance. Asphalt stains the bones a deep brown or black, and specialists must clean them with solvents before study. Tiny fossils, such as seeds, beetle wing cases and the bones of small lizards, are recovered by washing large volumes of asphalt-soaked sediment, a slow process that continues in the museum’s laboratory today.

Indigenous use and early records

Long before excavations, the asphalt was a valuable resource. The Tongva people of the Los Angeles basin and their neighbours, including the Chumash along the coast, used asphalt from seeps in the region to waterproof baskets, seal plank canoes and fix tools.

The first written description came from the Spanish expedition of Gaspar de Portolá. On 3 August 1769 the Franciscan friar Juan Crespí recorded in his diary that the party had seen many large marshes of a substance like pitch, bubbling up from the ground. In 1828 the area became part of a Mexican land grant, Rancho La Brea, named after the asphalt.

After California became part of the United States in 1848, settlers in the growing town of Los Angeles continued to use the asphalt. It was dug out, heated and spread on flat roofs to keep out the winter rains, and it was also used to pave streets. The seeps were therefore known chiefly as a commercial resource rather than a scientific one, and for decades nobody realised what lay beneath the surface.

From asphalt mine to fossil site

In 1860 the ranch passed to Henry Hancock, a lawyer and surveyor, whose family mined the asphalt and sold it for paving and roofing. Workers often found bones, but they were assumed to belong to cattle or other animals that had recently become stuck.

In 1875 a visiting geologist, William Denton, described a large canine tooth from the site given to him by Hancock; it later proved to belong to a sabre-toothed cat. Wider recognition came in 1901, when the oil geologist William Warren Orcutt realised the bones were from extinct Ice Age animals.

Large-scale scientific excavation began in 1913, when Henry Hancock’s son, G. Allan Hancock, granted the Los Angeles County Museum of History, Science and Art exclusive rights to dig for two years. Between 1913 and 1915 the museum excavated more than 90 pits and recovered several hundred thousand bones. In 1924 Hancock donated the land to Los Angeles County, on condition that the fossils be preserved and displayed. It became Hancock Park.

La Brea Woman

In 1914 excavators found part of the skeleton of a young woman. Known as La Brea Woman, she remains the only human found at the site. Radiocarbon dating suggests she lived roughly 10,000 years ago, making her one of the oldest known human remains in the Los Angeles area. Her remains are treated with care in recognition of their significance to Native communities.

Modern excavations and the museum

Excavation resumed on a smaller scale in later decades. Pit 91 was reopened in 1969 and dug for many years with modern methods, recording the exact position of each specimen and collecting tiny fossils that earlier excavators had ignored.

In 1977 the George C. Page Museum opened on the site to display the fossils and allow visitors to watch scientists cleaning bones in a glass-walled laboratory. It is now known as the La Brea Tar Pits museum and is part of the Natural History Museums of Los Angeles County.

In 2006, construction of an underground car park for the neighbouring Los Angeles County Museum of Art uncovered 16 new fossil deposits. Rather than excavate them on the spot, workers enclosed them in 23 large wooden boxes and moved them to the park. This work, called Project 23, included a nearly complete Columbian mammoth nicknamed “Zed”.

The Project 23 boxes have been excavated gradually over the following years, in full view of visitors. They have yielded many thousands of new specimens, including fossils of animals rarely found elsewhere at the site, and they show that the fossil-rich ground of Hancock Park extends well beyond the historic pits.

Myths and facts

• Myth: The pits contain dinosaurs. All the fossils are from the last Ice Age, not the age of dinosaurs.
• Myth: Animals sank like stones into deep pools of tar. Most were trapped in shallow asphalt only a few centimetres deep and died of exhaustion, starvation or predators. Their bodies were buried gradually.
• Myth: The Lake Pit is a natural lake. The large pond with mammoth sculptures fills a pit left by nineteenth-century asphalt mining.
• Fact: The seeps are still active. Asphalt and methane bubbles continue to rise in the park, and fresh seeps sometimes appear in nearby buildings and lawns.

By the numbers

Timeline

• c. 50,000 years ago: The oldest known fossils at the site are preserved.
• c. 10,000 years ago: La Brea Woman lives and dies in the area.
• 3 August 1769: The Portolá expedition records the asphalt seeps.
• 1828: The Rancho La Brea land grant is made.
• 1860: Henry Hancock acquires the rancho and mines its asphalt.
• 1901: William Warren Orcutt recognises the fossils as extinct Ice Age animals.
• 1913 to 1915: The Los Angeles County museum excavates more than 90 pits.
• 1924: G. Allan Hancock donates the land to Los Angeles County.
• 1977: The George C. Page Museum opens.
• 2006: Project 23 recovers 16 new fossil deposits.

Why it matters

The La Brea Tar Pits give an unusually complete picture of an Ice Age ecosystem, from mammoths and sabre-toothed cats to beetles and pollen. Because the fossils span tens of thousands of years, they allow scientists to track how animals and plants responded to climate change and to the arrival of humans, evidence central to understanding the Quaternary megafauna extinction. And they do so in the middle of one of the world’s largest cities.