Beneath Wilshire Boulevard and Hancock Park lie vast fossil beds, where the remains of hundreds of species are jumbled together. The deposit pictured here was discovered during construction of a parking lot at the Los Angeles County Museum of Art.
What is the single coolest thing about Los Angeles?
Hint: It’s not our sports teams or our beaches, Griffith Observatory or the Hollywood sign. It’s not any movie star or the fact that Los Angeles invented the hot rod, the lowrider, the French dip sandwich, and the Korean fusion pork belly taco.
No, what Los Angeles has that other cities don’t is a time machine. It’s accurate and reliable, and it lets us travel back in time to understand what the area was like during the Earth’s most recent glacial period—a time when temperatures were cooler and much of the planet was covered in ice. The La Brea Tar Pits have preserved millions of fossils in near perfect condition—so many and in such detail, we can identify species of plants, for instance, based on their preserved leaves, thorns, and seeds. From mammoths and saber-toothed cats all the way to tiny beetles and grass seeds, the La Brea Tar Pits have created an encyclopedia of past life. No city in the world matches Los Angeles for quality and quantity of fossils. The La Brea Tar Pits make up the single best Ice Age fossil site in the world, and it just happens to sit along Wilshire Boulevard.
Beneath Wilshire Boulevard and Hancock Park lie vast fossil beds, where the remains of hundreds of species are jumbled together. The deposit pictured here was discovered during construction of a parking lot at the Los Angeles County Museum of Art.
A clarification: some people think dinosaurs are preserved at La Brea—the time scale isn’t right for this. Dinosaurs (not including birds, who descended from dinosaurs) lived 66 to about 231 million years ago. Most La Brea fossils are only 10,000 to 50,000 years old, from the very end of a time known as the Pleistocene (ply-stuh-seen) epoch, which lasted from 11,700 to about 2.6 million years ago. This wasn’t one stable, uniform phase—it was comprised of a series of glacial-interglacial cycles. The Pleistocene and our current epoch, called the Holocene, together form the current ice age (for you trivia buffs, named the Quaternary Glaciation). Because ice sheets and glaciers exist today, we’re technically living in an ice age, it just happens to be a warmer, interglacial period.
This is a copy, called a cast, of a baby American mastodon skull that was excavated in 1914. American mastodons are rarely found in the tar pits; Columbian mammoths are more common. Mastodons are distant relatives to modern elephants, but mammoths are much closer on the family tree. In fact, today’s Asian elephants are more closely related to the extinct mammoths than they are to African elephants.
Just like today, the end of the Pleistocene was marked by dramatic climate change—the Earth was leaving a colder glacial period and entering a warmer interglacial period. By studying the fossil record, we gain insight into how the environment in Los Angeles reacted to ancient climate changes, and how it might react in the future.
Imagine that you have just stepped out of a time machine into Pleistocene Los Angeles. Rub your eyes, stretch your legs, and get ready for an adventure.
The first thing you notice is the air. How cool it feels on your face, and how much it smells like a pine forest. Things feel wetter, a bit more “mountainy” than present-day Los Angeles. It feels similar to today’s coastal Northern California around Monterey or Santa Cruz.
That Northern California feeling is due in large part to the trees: lots of iconic Monterey cypress, plus familiar Southern California oaks, juniper, and gray pine. In the distance, you notice dense trees lining the canyons of the Santa Monica Mountains. Their height gives them away: these are coastal redwoods, then and now the tallest trees in the world.
Between you and the trees sprout large clumps of green grass standing three to four feet tall. Nothing about today’s Southern California landscape prepared you for this: a bunchgrass prairie stretching across the lowland plain for miles in every direction, lush like Africa’s Serengeti in the rainy season.
This grassy plain is dotted with brown blobs, ambling on all fours and occasionally standing up to browse the tops of trees. Are they some kind of slow, tired bison? No. You recognize one as a lumbering ground sloth, ten feet long and weighing over a ton, bigger than any bear alive today. And look, over there! A Columbian mammoth, a relative of modern elephants, using its trunk to pull up bunchgrass one clump at a time. To your right is a herd of hundreds of bison. The adults are enormous—larger than modern bison—standing six to seven feet tall and weighing over 2,500 pounds. They’re bigger than the largest rodeo bull you’ve ever seen, bigger even than the cape buffalo of Africa.
The sun blinks out, and for a split second you think a plane has flown overhead. But here in the Pleistocene, it’s actually a massive condor-like bird, Teratornis, soaring above you with wings that stretch twelve feet wide. When it lands in a nearby oak, it stands nearly three feet tall—taller than a Great Dane.
Something is moving a short distance in front of you—a large animal crouched down, prowling through the bunchgrass. Then there’s another, and a third, and a fourth. It’s a pride of saber-toothed cats, known also as Smilodon. Each is roughly the size of an African lion, with two elongated dagger-like teeth that give the animal its common name. You’re no longer the top predator—in Pleistocene-era Los Angeles, you’re prey.
A western horse is scavenged by saber-toothed cats, while dire wolves and a coyote impatiently wait their turn. This image is from a larger mural by Mark Hallett on display at the La Brea Tar Pits.
The name La Brea comes from an 1828 Mexican land grant. The 4,500-acre expanse was called Rancho La Brea (the tar ranch). The black substance oozing out of the ground here technically isn’t tar—which is a human-made material—it’s asphalt, a heavy crude oil that comes from petroleum. Asphalt can be gooey and almost liquid when warm, but when it cools, it hardens like a rock.
Deep underground, below the surface streams and pools, sits the Salt Lake Oil Field. The petroleum in this hidden oil field comes from tiny algae, called diatoms, that lived millions of years ago when the LA Basin was beneath the sea. Over time, sediments piled on top of the dead diatoms, causing intense pressure and heat, which converted their organic matter into crude oil. When crude oil bubbles up through cracks in the Earth’s surface—which happens a lot in this part of earthquake-prone Los Angeles—some of the thinner part evaporates. What’s left behind is sticky black asphalt.
The Gabrieleño/Tongva community and their neighbors to the north and west, the Chumash, knew about this tar and found it useful. They used it to attach arrowheads to shafts and hooks to fishing line. Filling black walnut shells with the tar created a kind of dice for games. They also applied it as waterproofing on all kinds of objects, including their sea-going canoes. European settlers followed their example and used it to waterproof roofs. In the 1860s and 1870s, Angelenos mined it for building roads. The big Lake Pit that faces Wilshire Boulevard in front of the La Brea Tar Pits is the remains of an early mine.
It wasn’t until 1875 that a scientist realized the bones in the tar weren’t all from modern sheep and cattle, but from ancient creatures trapped long ago. Still, the value of the deposits remained largely unknown until 1901, when petroleum geologist William W. Orcutt started excavating the pits and quickly amassed a collection of Ice Age animal bones. The secret was out, and scientific excavations have gone on there ever since.
Workers excavate fossils from the tar pits during one of the first major excavations (1913–1915). In the background are oil derricks pulling up black gold from the Salt Lake Oil Field. The city has changed dramatically, but the tar pits remain.
Entrapment: How the Tar Pits KillA visit to the La Brea Tar Pits might make you think that animals became stuck in deep pools of asphalt. The pits you see today, however, are the result of more than one hundred years of scientific and commercial excavations. Thousands of years ago, the sticky asphalt trap was likely a small seep, not much more than a shallow puddle of ooze.
In winter, the asphalt was cold and hard. Even the heaviest animals could walk across safely most of the time. But in the warmth of summer, the asphalt warmed and softened. The path an animal walked without a problem a few weeks earlier—or maybe even earlier that morning—became a death trap. The asphalt was probably camouflaged beneath wind-blown dust and drying leaves; passing wildlife wouldn’t recognize anything unusual until their feet sank into unstable ground. Just a few inches could be enough to trap a big mammal long enough for it to die of dehydration or exhaustion, or be killed by a passing predator. Sometimes, that predator would get caught too—causing an entrapment pileup.
This story is told in the fossils found today. The most common large animal species found in the tar pits are predators and scavengers. A trapped bison or mammoth could attract dire wolves, saber-toothed cats, coyotes, eagles, and vultures. If some of these latecomers also got trapped, there’d be a weeks-long buffet of carrion (dead animal meat) in various stages of decomposition. The pileup would likely continue until the bones sank deeper into the pit or the weather cooled and the asphalt hardened again. A prolonged entrapment event like this only had to happen about once a decade to yield the number of large animal fossils found today.
Scientists have recovered millions of fossil specimens from the tar pits, making it the richest Ice Age fossil site in the world. In all, they’ve recorded evidence of over six hundred plant and animal species. Besides the well-known mammoths, dire wolves, and ground sloths, they’ve found horses and bison, tapirs and peccaries, and camels and llamas—animals whose relatives can be found elsewhere in the world but are long gone from Los Angeles. Not everything here is extinct though—from coyotes to lizards to oak trees, most species found in the tar pits still live here today.
La Brea fossils are special because the tar provides a “glue” that captures an organism’s finer details. Fossils are usually made when plants or animals die and sink into silt or mud that eventually turns to stone, preserving only the bones. And many fossil beds elsewhere in the world contain only large creatures, because heavy animals or rough conditions destroyed smaller, more delicate ones. But because the La Brea Tar Pits capture organisms of all sizes and preserve a variety of materials (bone, cellulose, chitin), we get a much richer mix of fossils than usual. In fact, the pits can preserve just about everything—researchers can even study pollen and insects, specimens known as microfossils. It’s usually not great to be a hoarder, but when it comes to studying fossils, all scientists agree: the more we have, the better!
These fossil bones were excavated from the tar pits between 1913 and 1915. They were stored in the basement of the Natural History Museum before they were moved to their new home at the La Brea Tar Pit Museum.
The fossils excavated from the La Brea Tar Pits can tell us a lot about entire ecosystems—something that most fossil beds can’t do. There’s just one problem. All the critters that got trapped inside the tar ended up floating around like ingredients in a big bowl of soup. There’s no rhyme or reason to the placement of the animals within the deposits, so a paleontologist digging up fossils today might find a 35,000-year-old mastodon tusk alongside a 10,000-year-old coyote skull. To figure out which plants and animals lived together at the same time, scientists use a procedure called carbon-14 dating.
Carbon-14 is a type of chemical found in all living plants and animals. When an organism dies, carbon-14 decays at a very predictable rate. By measuring how much carbon-14 is left inside a fossil, paleontologists can estimate how long ago it died. This trick allows researchers to make educated guesses about the communities of animals that lived and died together. And by using it on the tiniest fossils, they can learn what the climate was like thousands of years ago.
While fossils from big animals might be awe inspiring, those animals tend to need a lot of space, so their remains don’t usually give us much detail about the environment in smaller geographic areas. Insects, rodents, plants, small birds, and reptiles usually stick to a small territory, so they can tell us a lot more about local environmental conditions. As a result, beetles can reveal more about ancient climate patterns than, say, mammoths can.
Starting around 50 million years ago—long before the asphalt seeps at La Brea began trapping animals—most of the world’s wildlife communities, including the ecosystem right here in Los Angeles, began looking something like modern African ones. There were a few tiny mammals and a few giants, but most were small or medium-sized. That arrangement lasted for millions of years, right up until around 50,000 years ago when the planet began to lose its big mammals. The exact timing of these megafaunal extinctions varies around the globe, but in North and South America they occurred between approximately 15,000 and 11,000 years ago. In a relatively short period of time, dire wolves, short-faced bears, four species of big cats including Smilodon and the American lion, mammoths, sloths, camels, horses, bison, the teratorn, several species of eagles, and other big species all went extinct. The fauna of Los Angeles started changing—and it happened fast.
The jury is still out on what caused this rapid extinction, but an increasing number of scientists think it was a one-two punch of changing climate and the arrival of humans. The world was going through a period of severe climate change at that time, not unlike the one we’re experiencing today. The planet was coming out of a glacial period. The atmosphere was warming, and melting ice made the world’s oceans rise. Plus, humans had crossed into the Americas from eastern Asia and were spreading out, occupying new territories and increasing their populations.
This beautifully preserved shrew jaw, roughly half the size of a penny, is an example of a microfossil that helps scientists understand changes in climate and biodiversity through time.
Before the continent’s megafauna began to die off, the world was colder and drier than it is now. Ice sheets covered huge parts of North America, and much of the rest, including Southern California, was covered with savannahs (wide-open grasslands dotted with trees) rather than with forests.
The ice receded as the planet warmed, and the once-endless savannahs were carved up by expanding forests. Many of the region’s big mammals needed the wide-open spaces to survive—closely clustered trees are not optimal if you’re a camel.
The Pleistocene era was defined by constant shifts between warm and cold, wet and dry. The landscape bounced back and forth while forests shrunk back then spread out over and over again. As forests grew thicker, groups of savannah animals became isolated as their habitats shrank and became disconnected. These are the same problems plaguing wildlife today: habitat loss and reduced habitat connectivity.
The planet teetered back and forth between these two extremes like a seesaw many times during the Pleistocene, but it was only toward the end that big animals became unable to survive the isolation created by encroaching forests. Why was the most recent transition from glacial to interglacial and savannah to forest so different than the previous transitions? Scientists believe human activity exerted pressure on wildlife populations already impacted and stressed by climate change.
It’s tempting to imagine small bands of hunters with bows and arrows slaughtering hordes of wild animals into extinction, but that’s not what’s necessary to lose a group of animals. People, other predators, diseases, and accidents just have to cause more deaths each year than there are births. Given enough time, the population will eventually vanish.
By the time our species began interacting with the megafauna of ancient Los Angeles, the animals were already suffering from the effects of fragmented, smaller populations. The massive beasts of the time were at a tipping point and an efficient new predator coming onto the landscape—that’s us—may simply have been the straw that broke the ancient camel’s back.
The Rancho La Brea story is one of change. Because the asphalt seeps were actively trapping animals before and during the megafaunal extinction, they offer a unique glimpse at how ancient life was affected by changing climate and the arrival of humans. The tar pit fossils teach us that the dramatic changes faced by wildlife in Los Angeles during these last few hundred years of rapid urbanization are part of sweeping environmental changes that began long ago.
Comparing animals that did or didn’t survive the Pleistocene-era extinction helps us understand what makes a species better at surviving changes to the environment. The fossils at La Brea allow us to peer far into our future, showing us what Los Angeles could look like in one hundred, one thousand, or even ten thousand years. And if we’re fast enough, and clever enough, we just might be able to use the lessons of the past to create a better, more sustainable, more comfortable future. Not just for us, but for all the plants and animals with whom we share this planet.
