, Research Paper
Geology of La Brea Tar Pits
Background
The La Brea Tar Pits, located near Hollywood, California, contain one of the worlds greatest troves of Pleistocene fossils. Over one million bones have been recovered to so far. Dating from 38,000BC to 8,000 BC, the bones represent more than 420 species of animal, including saber tooth tigers, dire wolves, and 140 species of plants.
Geologic History
Leaves and grass blew across the tar, forming a thin top layer identical to the surrounding ground. Unsuspecting animals grazed right out into the stuff. The tar quickly trapped them; their frantic struggles only engulfed them deeper and deeper. Their frenzied cries attracted carnivores, including the saber-toothed cat, which pounced on the helpless victims, only to find itself trapped by the same tar.
In all, researchers have identified more than 420 species of animals and about 140 species of plants.
Giant mammals ruled the Pleistocene. Imperial mammoths, largest of the elephant tribe, stood 4.5 meters tall and weighed around 6,800 kg. Ground sloths the size of a rhinoceros ambled from tree to tree. Huge camels and bison grazed on the plains. A lion the size of a grizzly bear.
Dire wolves were just that – dire: About the size of ponies, they hunted in packs. Over 1,600 of their skulls have been found here so far.
The tar asphalt is especially effective for saving fragile bird bones, and has yielded the largest collection of fossil bird bones ever: over 100,000 carefully excavated bones. The most impressive bird of the time was Merriam’s giant condor with its 4-meter wingspan. For comparison, today’s California condor has a wingspan of about 2.9 meters.
The two long, slashing fangs protruding from its upper jaw gave the saber-toothed cat its name. (The giant ground sloth had a special pair of bones wrapping its neck as protection from the long fangs.) Sometimes loosely called the saber-toothed tiger, these big predators actually belonged to a separate sub-family of the Felidae, called the Machairodontidae, now completely extinct.
Other exhibits in the museum include a two-thirds-scale moving model of a woolly mammoth realistic enough to send small children scurrying behind parents. The reconstructed skeleton of a mastodon with huge curling tusks impresses everyone. The massive skeleton of the American lion makes you glad you don’t have to worry about meeting one on the walk back out to the car.
Perhaps most affecting is the collection of 404 dire wolf skulls neatly arranged in rows filing one wall. Look at these remnants of pack hunters long enough and you start to get the chills.
It is more than a bit surreal that these tar pits are in the heart of downtown Los Angeles, right on the “Miracle Mile” and a short walk from the exclusive boutiques of Rodeo Drive. The tar pits still bubble and burp from gas seeping up. In some places the tar is seeping and pushing under protective chain link fences. Some kids (and grown-ups) go home with a fresh tar coating on their soles.
Tar pits form when crude oil seeps to the surface through fissures in the Earth’s crust; the light fraction of the oil evaporates, leaving behind the heavy tar, or asphalt, in sticky pools.
The bones occasionally found in the tar were first thought to be those of unlucky cattle.
Life in Los Angeles was somewhat cooler and moister 40,000 years ago than it is today, as we can tell by examining the plant fossils from La Brea. Many of the plants and animals found in La Brea are identical or almost identical with species that still live in the area — or that would be living in the area had Los Angeles not gotten in the way. Yet a number of the large animal species found at La Brea are no longer found in North America: native horses, camels, mammoths and mastodons, longhorned bison, and sabre-toothed cats.
In today’s ecosystems herbivores are much more abundant than carnivores. It is therefore curious that at La Brea about 90% of the mammal fossils found represent carnivores. Most of the bird fossils are also predators or scavengers, including vultures, condors, eagles, and giant, extinct, storklike birds known as teratorns. Why is this the case? If a pack of carnivorous mammals were to chase a lone prey animal into the tar pits, both predators and prey would become trapped. This would not have to be a frequent occurrence — an average of one major entrapment every ten years, over a period of 30,000 years, would be sufficient to account for the number of fossils found at La Brea. Scavenging animals, drawn to feed on trapped animals, would have a chance of getting trapped themselves. This would explain the preponderance of carnivores and scavengers.
Animals
In the heart of Los Angeles lies one of the world’s richest Ice Age fossil sites. Countless plants and animals were trapped in deposits of natural asphalt and preserved as fossils. These fossils now provide us with an incredibly complete picture of what life was like as the Ice Ages drew to a close between 10,000 and 40,000 years ago.
Ranging in size from giant mammoth skeletons to microscopic plant remains, the fossils include one celled organisms; pollen, seeds, leaves and wood; clam and snail shells; insects and spiders; fish; frogs, salamanders, and toads; snakes, lizards and turtles; birds; and mammals – in all, more than 565 species.
Many of the large animals in the Rancho La Brea fossil deposits are now extinct. While many of the plant and small animal species uncovered in Rancho La Brea are still found in southern California, some plants and animals no longer live here due to changes in the climate and changes due to human development.
History
Layers of gravel, sand, and clay were laid down by streams. These layers cover much older marine sedimentary rocks rich in petroleum oil. Movements in the earth’s crust have folded and cracked the older rocks. Heat and pressure force the oil up slowly through the cracks and onto the surface of the ground. Shallow pools of sticky asphalt formed in low-lying areas such as stream beds. Oil continues to seep up to the surface as it has for hundreds of thousands of years.
Contrary to popular belief, the “tar” that trapped the animals is not tar at all, but asphalt.
The Tar Pits
Summer’s heat dried the streams and warmed the semi-solid asphalt to a gooey liquid. It was during the summer that the plants and animals became trapped. During the winter, cooler temperatures harden the asphalt. Winter and spring rains fill streams with sand and silt that bury the remains of plants and animals trapped in the shallow asphalt pools. The following summer, new pools of asphalt form at the surface. Over tens of thousands of years, this cycle produced the cone-shaped bone masses found at Rancho La Brea Fossil deposits range in size from a few feet to over 30 feet (ten meters) deep.
Very few fossils of nocturnal animals have been found at Rancho La Brea . Why? Just as the asphalt hardens during the cool winter months, so it also hardens on cool nights.
Often the shallow pools of asphalt were hidden by a surface layer of fallen leaves and dust, and occasionally an unwary animal wandered or was chased into the pool and became stuck. Here American lions chase a bison calf. If the lions chase the calf over a hidden asphalt deposit, they might become trapped.
Fossils
Fossils are evidence of past life. They are formed in many ways. After a plant or animal dies, its soft tissues usually decay or are eaten. Most fossils are formed when the hard parts are protected from decay by natural burial soon after the organism dies. For this reason, most fossils are found in sediments that were laid down by water – either ancient rivers, lakes, or oceans.
The Rancho La Brea fossils were preserved by a unique combination of sedimentation and asphalt impregnation. First the bones were saturated with asphalt, which inhibited decay. Preservation of the fossils took place after the saturated bones had been buried beneath water-borne sediment. Unlike most fossils, the fossils from Rancho La Brea are unchanged, original material.
This exposed deposit contains bones of many different types of animals. After all the bones from this deposit were carefully excavated, they were cleaned and taken to the Museum for study. Some excavated bones are reassembled into skeletons for display.
Rancholabrean Stage
The Rancholabrean stage was a major division of Pleistocene time and deposits in North America (the Pleistocene epoch began about 1,600,000 years ago and ended about 10,000 years ago). The Rancholabrean stage follows the Irvingtonian and was named for the La Brea Tar Pits, Los Angeles, which are noted for their abundant remains of fossil animals. The Rancholabrean apparently includes the span of time covered by the Illinoian glacial stage, the following Sangamon interglacial stage, and the Wisconsin glacial stage, the last major glacial episode to affect North America.
The Rancholabrean fauna includes many modern forms, such as skunks, bats, bison, rodents, beavers (including a giant form), bear, antelope, and deer, as well as extinct animals: mammoths, mastodons and sabre-toothed cats. Farther south, tapirs, sloths, peccaries, and armadillos were found. Rancholabrean faunas are widely represented across the United States; many of the fossils found are in an excellent state of preservation.
At the end of Rancholabrean times the mammal fauna of North America underwent a drastic reduction in numbers and diversity. This reduction may have been connected with environmental changes. It has been suggested that the arrival of humans in North America was responsible for the widespread extinction. It seems unlikely, however, that humans were present in sufficient numbers or had the technology to cause the extinction of all the animals in question, but they may have dealt the final blow to forms already greatly reduced and vulnerable to his predation.
References
The George C. Page Museum of La Brea Discoveries
5801 Wilshire Boulevard, Los Angeles, CA 90036, online at: http://www.tarpits.org/
The Discovery Channel, online at: http://www.discovery.com/news/features/tarpits/tarpits.html
Encyclopedia Britannica Rancholabrean Stage, online at: http://www.britannica.com/
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