The La Brea Tar Pits, or Rancho La Brea, are a famous group of tar pits located in Hancock Park in the heart of downtown Los Angeles, California. This location contains one of the richest and best preserved assemblages of fossilized Pleistocene life in the world. The plant and animal remains found represent hundreds of species that populated southern California from almost 40,000 to 10,000 years ago. Rancho La Brea provides a wealth of information on life during the most recent ice age.
Tar pits form in places where crude oil seeps to the surface of the earth through cracks in the bedrock. A portion of the oil evaporates, leaving thick, viscous pools of asphalt behind. Local natives used this resource for thousands of years to waterproof baskets and canoes. As Caucasian settlers moved into the area, the tar was collected and used for roofing in the nearby town of Pueblo de Nuestra Senora la Reina de los Angeles.
A family named Hancock owned and worked a 4,400-acre ranch that encompassed the tar pits in the late 1800s and continued to mine the pits for tar to use as a sealant. Occasional bones unearthed were assumed to be those of cattle that strayed, got stuck in the tar, and perished. Eventually paleontologists heard about the bones and performed the first scientific excavation in 1901. Work intensified as the number of finds increased. From 1913 to 1915, digging at over 100 pits produced more than 1 million bones. In 1924, businessman G. Allan Hancock donated the 23-acre plot containing most of the tar pits to Los Angeles County. Early excavators tended to keep only the large, complete bones for museum display. When digging resumed in 1969, more care was taken in cleaning and analyzing over 40,000 additional specimens retrieved.
Tallying and cataloging the bone and plant remains provided insight into the activity around the tar pits so long ago. Wood, plant, and pollen remnants illustrate a cooler and wetter environment than exists today. The park is best known for the large mammals found in the pits, including mammoths, camels, sabertooth cats, dire wolves, and giant ground sloths, but many other species have been revealed. Since excavations at Rancho La Brea began, evidence of more than 650 species has been recovered: 231 vertebrates, 159 plants, and 234 invertebrates. More than 95% of the mammal bones found belong to only seven species. Four were carnivores: the dire wolf, the sabertooth cat, the coyote, and the North American lion, and three were herbivores: the ground sloth, the bison, and the western horse. The most common bones found are those of the dire wolves, with over 2,000 sabertooth cat specimens ranking second. Of the mammals found, 80% were predators, and 60% of the birds were birds of prey. These percentages reflect the exact opposite of what you would expect to find in the wild.
In a stable ecosystem, herbivores always outnumber carnivores by about 10 to 1. The leading theory to explain this discrepancy is called entrapment. Asphalt is very sticky, especially when it is warm. Only one inch of this tar can hold a horse or cow until it dies of starvation or exposure. In dry weather, dust can blow across the surface, making the ground appear solid. A trapped animal thrashing around trying to escape would attract the attention of many predators. In attacking the prey, some of the attackers would also get stuck, providing additional food for other carnivores. This cycle repeated for 30,000 years to create the accumulation seen at Rancho La Brea today. Although the collection seems to represent a huge number of animals, it averages out to one herbivore being trapped every decade, and several scavengers attempting to retrieve the meat and joining their prey.
Although thousands of individual animals have been found in La Brea, only one human skeleton has been discovered. The skull and partial skeleton of a Native American woman was recovered in 1914. She was in her early 20s when she appears to have been killed by a blow to the head. Her bones have been dated to about 9,000 years ago.
The La Brea Tar Pits provide a treasure trove of information about life during the late Pleistocene. While studying the bone collection reveals information about ancient food chains, the sophisticated chemical tests available today reveal a surprising amount of detail about how animals became trapped in the tar pits and what happened to them in their final hours of life. The Page Museum in Hancock Park opened in 1977 to house the heritage of over 1 million specimens recovered from the tar. The museum features more than 30 exhibits of reconstructed skeletons and robotic sculptures, a glass-walled working laboratory where visitors can watch the curators work, hands-on displays, films, and many wall murals. Rancho La Brea sheds light on a fascinating time period when glaciers last covered much of the hemisphere.
Jill M. Church
See also Dating Techniques; Extinction and Evolution; Fossil Record; Fossils, Interpretations of; Museums; Paleontology
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Stock, C., & Harris, J. M. (1992). Rancho La Brea: A record of Pleistocene life in California. Los Angeles: Los Angeles County Museum.