In downtown Los Angeles is one of the most fantastic fossil sites in the whole world—a place that has given us an unparalleled glimpse of a small corner of ice age earth. Rancho La Brea, frequently referred to as the La Brea tar pits, has yielded around 1 million bones since excavations began there in 1908. The site is actually above an oil field, and oil has been seeping to the surface through fractured rocks for 38,000 years. When the oil reaches the surface, the more volatile chemicals evaporate, leaving a heavy, thick tar (asphalt).
For millennia, Amerindians used the tar from the asphalt pools for waterproofing shelters and canoes as well as for glue. It was even considered valuable enough to be traded. A Franciscan friar, Juan Crespi, makes the first written mention of the asphalt deposits during his expedition with Gaspar de Portola (the first Spanish governor of the Californias) in 1769—1770. Later, the site was part of an 1,800-hectare Mexican land grant given to Antonio Jose Rocha in 1828. It then found its way into the hands of the Hancock family, and Captain George Allen Hancock donated the 23 acres of Hancock Park to Los Angeles County in 1924.
The oil beneath the asphalt deposits is itself a fossil, the oily, organic remnants of the tiny organisms that make up marine plankton. Between 5 and 25 million years ago, this part of California was actually a shallow sea, and these single-celled organisms died and sank to the bottom, where they became part of a thick layer of sediment. As the climate changed and the continents moved around, the sea disappeared, and the dead plankton, entombed under tonnes of overlying sediment, were slowly converted into an oil and gas deposit by the pressure and heat. Fractured rocks above this oil field provided a path to the surface, through which the crude oil seeped, accumulating in numerous pools that dotted what is today known as Hancock Park. Over thousands of years, some of these seeps ended, while new ones began, but all the while, they were a trap for a myriad of species of animals. Sometimes a seep produced a tar pit that was deep enough to trap really large animals. How animals became trapped in the tar isn't known for sure, but it is thought that water, leaves, and dust accumulated on the tar pits and animals were deceived into wading in to bathe or drink. This was the last mistake the animal made, as the sticky tar snared its legs and made escape impossible. The commotion caused by struggling animals and the smell of dead animals that had already perished in the sticky goo attracted the attention of predators and scavengers. Not only are the number and diversity of the fossils from La Brea unprecedented, but it is the only fossil assemblage on earth where predators outnumber prey. This is because a large animal, like a mastodon, struggling in the tar attracted numerous predators and scavengers, all of which were keen to get their teeth and claws into the doomed beast, and in trying to do so, some of them also met their end in the tar. It is even possible that prey and predators became trapped during a chase that ended badly for all parties concerned. This might seem unlikely, but a major entrapment like this only needed to happen once every 10 years over a 30,000-year period to account for all the bones in the asphalt deposits. The dead bodies would sink into the tar, and as the seep stopped, the volatile elements of the oil continued to evaporate, leaving hard, asphalt-impregnated clay and sand, and the bones.
Even before the paleontological importance of this site was recognized, ranchers took notice of the bones protruding from the asphalt deposits but mistakenly believed them to be the remains of cattle and pronghorn that had wandered into the sticky tar. To date, more than 660 species of plant and animal have been found in the asphalt deposits, all of which got trapped in the tar between 8,000 and 38,000 years ago.
Rancho La Brea Asphalt Deposits—This is an example of how an animal met its end in the asphalt deposits. The bison is attracted to the seep to drink the water that has pooled on the asphalt. It becomes trapped in the sticky asphalt and dies. Its bones sink into the asphalt, and after thousands of years, humans find and excavate them. (Phil Miller)
The discoveries include 59 species of mammal and 135 species of bird. Some of the animals recovered from the pits are extinct—for example, the saber tooth cat Smilodon fatalis, ground sloths, mastodons, and mammoths—while others are still around today, for example, pronghorn (Anti-locapra americana), elk (Cervus Canadensis), coyote (Canis latrans), and bobcat (Lynx rufus). The asphalt deposits have yielded the remains of more than 1,600 dire wolves and around 1,200 saber tooth cats. Apart from being stained brown after several millennia entombed in tar, the asphalt deposit bones are brilliantly preserved, and in some cases, tar has seeped into the cranial cavity of the skull to produce an endocast of the brain.
The bones of the large extinct animals discovered in the asphalt deposits have always attracted the most attention, especially the perfectly preserved skulls of the saber tooth cat, but relatively recently, scientists realized that alongside these bones were fossil seeds, pollen, insects and mollusks, and the bones of fish, amphibians, small birds, and rodents. These microfossils allow paleontologists to build up a very detailed picture of the habitat and climate in Los Angeles during the final part of the last ice age.
Interestingly, of all the bones recovered from Rancho La Brea, only one human skeleton has been found: a 1.5-m-tall woman in her mid-twenties, who appears to have suffered a blow to the head.
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