La Brea Tar Pits (Part 1)

La Brea Tar Pits in Los Angeles, CA is the only actively excavated urban Ice Age fossil site in the world. It is part of Hancock Park which lies 1,000 feet above the Salt Lake Oil Field just to the north. La Brea is Spanish for “the tar” or “the pitch” but the eponymous substance is actually asphalt, the lowest grade of crude oil. A number of plants and animals got stuck in the sticky asphalt where their remains were preserved. Excavations at La Brea have recovered more than 3.5 million specimens representing over 600 species, most of which are stored at the George C. Page Museum of La Brea Discoveries.

During the Miocene (23.03-5.33 million years ago) the tectonic plates around Southern California ceased subduction and formed a transform boundary off the coast now known as the San Andreas Fault. Rotation of the tectonic plates stretched the Los Angeles region and created a deep ocean basin where the remains of marine organisms were deposited. Rivers then transported sediments from the Santa Monica Mountains that buried the organic matter and the added pressure converted it into oil over time. Eventually, the oil made its way upward through cracks in the overlying rock to form shallow pools at the surface where lighter elements like kerosene evaporated until only asphalt was left. Over the last 50,000 years, millions of plants and animals became trapped and died in the viscous asphalt which preserved their remains. Around 12,700 years ago, 70% of large North American mammals died off in a megafaunal extinction event that paleontologists speculate was driven by large human-caused fires introduced to areas already experiencing increasing temperatures, shifting habitats, and a megadrought that lasted for centuries. The remains recovered from La Brea Tar Pits have greatly enhanced available knowledge on extinct Ice Age animals of Southern California such as dire wolves, saber-toothed cats, Columbian mammoths, American mastodons, giant ground sloths, yesterday’s camels, and ancient bison.

Indigenous peoples used the La Brea asphalt as glue or caulk but the earliest historical record of the site occurred during the 1769-1770 expedition of Gaspar de Portolá y Rovira, the first Spanish Governor of the Californias. The expedition was documented by Franciscan friar Juan Crespi who described the asphalt as "springs of pitch" in a diary entry from 1769. The site was later included as part of a 4,400-acre Mexican land grant called Rancho La Brea that was given to Antonio Jose Rocha in 1828. Members of the surrounding pueblo used the asphalt to seal their roofs and were allowed to collect as much as they needed. In 1870, Major Henry Hancock bought part of Rancho La Brea from Jose Jorge Rocha and began operating an asphalt mine at the Lake Pit now located in front of the museum. Over time, rain and groundwater accumulated above the asphalt to form the small lake. The bubbles that float to the surface are methane and hydrogen sulfide produced by the natural breakdown of hydrocarbon molecules in crude oil. Although methane is an odorless gas, hydrogen sulfide exudes a strong odor reminiscent of rotten eggs. Bones recovered from Rancho La Brea were originally thought to belong to domesticated livestock and other animals of the region until geologist William Denton identified a fossil from the Lake Pit as a saber-toothed cat in 1875. That year, the Boston Society of Natural History published an article by Denton making the first mentions of the Rancho La Brea fossils.

Henry Hancock’s son, George Allen Hancock, unearthed more fossils while drilling for oil on the family land in 1901. Over about four years, hundreds of dire wolf, saber-toothed cat, and ground sloth fossils were collected during this process by geologist W.W. Orcutt and fellow scientist F.M. Anderson. In 1905, Anderson sent news of the finds to John Campbell Merriam at the University of California, Berkeley, raising the profile of the site. In 1907, Los Angeles High School zoology teacher James Z. Gilbert started bringing students to excavate fossils. The following year, Merriam made the Rancho La Brea fossils famous when he published the article, "Death Trap of the Ages." Gilbert continued student excavations with funding from the Southern California Academy of Sciences and the Los Angeles County Board of Supervisors and directed a large excavation at the Academy Pit in 1910. Specimens obtained through these efforts would contribute greatly to the fossil vertebrate collections of the Los Angeles Museum of History, Science and Art (later renamed the Natural History Museum of Los Angeles County). After Merriam secured funding, larger-scale excavations were conducted by the University of California, Berkeley between 1912-1913, yielding thousands of specimens. 

To prevent the collections from becoming scattered, George Allan Hancock granted the County of Los Angeles the exclusive right to excavate at Rancho La Brea for two years. From 1913-1915, excavations of 96 quarries or “pits” collectively produced over 750,000 plant and animal specimens. At least 50 of the digs proved completely unproductive while the majority of the fossil deposits were uncovered at only about a dozen pits. In 1914, one pit yielded the partial skeleton of a woman who was 17-25 years old at the time of death and whose remains date to around 8,301-8,271 BCE. Most of the research on the fossils recovered from the pits was performed by California Institute of Technology professor Chester Stock who became one of the most famous North American paleontologists for his publications on the Rancho La Brea discoveries.

George Allan Hancock donated 23 acres of his family ranch to the County of Los Angeles in 1924 for the creation of a public park to preserve the tar pits and fossils. Five fenced areas across Hancock Park exhibit the remains of the most prolific excavation sites including Pits 3, 4, 9, 10, 13, 61, 67, and 91, as well as the Lake Pit which contains Pits 17, 50, and the Academy Pit. In 1964, Rancho La Brea was designated a National Natural Landmark. In 1969, the Rancho La Brea Project resumed excavation in Pit 91 to include collection of smaller specimens which had previously been overlooked or ignored in favor of larger, more charismatic plants and animals. In 1977, the George C. Page Museum (later the George C. Page Museum of La Brea Discoveries) opened on the grounds of Hancock Park in order for visitors to view Rancho La Brea fossils on site rather than seven miles away at the Natural History Museum of Los Angeles County. In 2019, the museum and tar pits were collectively renamed La Brea Tar Pits to highlight that park and museum are one destination.

The nearby Los Angeles County Museum of Art discovered 16 fossil deposits including an almost complete adult mammoth skeleton when it started digging for a new underground parking garage in 2006. Large wooden boxes were constructed around the deposits and moved to their current location at La Brea Tar Pits where excavation commenced. A total of 23 crates and 327 buckets of fossil material were recovered during the construction.

Excavation of Pit 91 began in 1915 and has continued each summer since 1969. The pit is 15 feet deep and has produced fossils ranging from 14,000-44,000 years old. A diversity of fossils has been recovered from Pit 91, including 73 saber-toothed cats, 56 dire wolves, 16 coyotes, 13 western horses, 12 ancient bison, six Harlan’s ground sloths, six giant jaguars, four short-faced bears, two yesterday’s camels, and one American mastodon. Fossils of extant species such as rainbow trout and Monterey cypress tree indicate that freshwater flowed through the local environment in the past and that winters were likely colder and wetter. The blue flag on the top right marks a Harlan’s ground sloth pelvis, the orange flag on the lower left marks a dire wolf shoulder blade, the red flag left of center marks a western horse leg bone, and the green flag below it marks a saber-toothed cat shoulder blade.

This block of asphalt and fossils was extracted from Pit 81 in 1914 and transported seven miles away to the Los Angeles Museum of History, Science and Art in Exposition Park. It demonstrates the sheer density of specimens that paleontologists have worked with at La Brea Tar Pits.

This is a reproduction of the fossils within the Pit 81 asphalt block as they might appear if the matrix were clear. Fossils of about 140 species of birds, over 60 mammals, 13 snakes, seven lizards, six amphibians, and three fish have been discovered at La Brea Tar Pits.

One museum exhibit highlights the proboscidean family tree, outlining the ancestry of mammoths, mastodons, elephants, and their relatives over 55 million years of evolution. More than 150 species are assigned to the order Proboscidea, with the earliest known member being a rabbit-sized mammal called Eritherium azzouzorum, whose remains were discovered in Morocco. Proboscideans first arose in Africa and later expanded into Asia, Europe, and the Americas.

Moeritherium is a primitive genus of proboscidean that lived about 37-30 million years ago during the Late Eocene and Early Oligocene. The genus was first described in 1901 by Charles William Andrews, who named it Moeritherium, Greek for "beast from Lake Moeris." The genus is part of an extinct side branch of proboscideans not believed to be directly ancestral to mammoths, mastodons, and elephants. It grew to about 2.3 feet high at the shoulder and weighed about 518 pounds. Although Moeritherium did not have an elephant-like trunk, it likely had a long snout and displayed large second incisor teeth which represented an early form of tusks. The animal was semi-aquatic and probably spent a great deal of time in freshwater similar to modern hippos. Several nearly complete skeletons of Moeritherium have been discovered around North and West Africa.

Phiomia is an extinct proboscidean genus that lived in North Africa about 37-30 million years ago during the Late Eocene and Early Oligocene. The genus was first described in 1902 by Charles William Andrews and Hugh John Llewellyn Beadnell, who named it Phiomia, Greek for “animal of Faiyum,” in reference to the region of Egypt where the first fossil was discovered. P. serridens reached about 4.41 feet at the shoulder while P. major could grow larger. Phiomiais considered the most elephant-like of the early proboscideans, having a very short trunk with a pair of upper tusks which may have been used for defense or scraping bark off trees and a pair of lower shovel-like tusks likely used for gathering food.

Gomphotherium is an extinct proboscidean genus that lived about 23-5 million years ago from the Late Oligocene/Early Miocene until the Early Pliocene. The first species of the genus was described in 1817 by Jean Léopold Nicolas Frédéric “Georges” Cuvier but the genus was named in 1837 by Karl Hermann Konrad Burmeister, who called it Gomphotherium, Greek for "welded beast." It had pairs of upper and lower tusks, although only the upper tusks are visible in this photo. Gomphotherium likely originated in Africa and later spread to Europe, Asia, and North America. Fourteen species within the genus are considered valid and the largest, G. steinheimense, is estimated to have reached 10.4 feet tall and weighed 7.4 short tons.

Amebelodon is an extinct proboscidean genus that lived in North America about 15-5 million years ago from the Middle Miocene until the Early Pliocene. The genus was first described in 1927 by Erwin Hinckley Barbour, who named it Amebelodon, Greek for “shovel tusk,” in reference to its narrow, elongated, flattened lower tusks which were likely used for scraping bark from trees and shoveling during feeding. In addition to the shovel-like lower tusks, the animal had upper tusks like modern elephants, grew to be about equivalent in size, and probably also had a long, flexible trunk.

Deinotherium is an extinct proboscidean genus that lived about 25-2 million years ago from the Late Oligocene until the Early Pleistocene. The genus was first described in 1829 by Johann Jakob Kaup, who named it Deinotherium, Greek for “terrible beast.” The range of the animal extended from East Africa to Europe and Asia. It had a longer, more flexible neck than modern elephants as well as a robust, muscular trunk, longer, slender limbs adapted for long-distance travel, and lower tusks that grew downwards and curved back, likely to serve some function in feeding. The largest discovered Deinotherium measured about 13.2 feet tall at the shoulder and weighed about 14.6 short tons.

Lyuba is a nearly complete one-month-old female woolly mammoth (Mammuthus primigenius) discovered frozen by Siberian reindeer herders along a riverbank on the Yamal Peninsula of Russia in 2007. Lyuba is Russian for “love” and this animal died from asphyxiation about 42,000 years ago after becoming trapped in a mudhole. As Lyuba laid in the mud after her death, most of her hair to fell out and fungi growing on her skin formed pockmarks on her head and body. Over time, blue crystals of vivianite formed on her skin. Within her stomach and intestines, scientists identified milk from her mother, pollen, algae, and mammoth fecal matter. Woolly mammoths lived in cold, dry regions so many of their remains became well-preserved, which has allowed scientists to learn more about them than any other species of extinct proboscidean. 

On the left is Moeritherium which lived in North Africa about 37-30 million years ago. In the center is the woolly mammoth (Mammuthus primigenius) which lived in Europe, Siberia, and North America about 700,000-3,900 years ago. On the right is the Columbian mammoth (Mammuthus columbi) which lived in North and Central America about 1.6 million-10,000 years ago.

On the left is the American mastodon (Mammut Americanum) which lived in North America about 2 million-10,000 years ago. In the center is the pygmy mammoth (Mammuthus exilis) which lived on the Channel Islands of California about 100,000-10,000 years ago. On the right is the African savannah elephant (Loxodonta africana) which has lived in West, Central, and South Africa since about 2 million years ago.