A wrinkle in time: Finding the ice age in urban Los Angeles
Just beyond the traffic and palm trees of Wilshire Boulevard, hidden beneath the shadows of nondescript office buildings, lie the tar-slicked bones of many thousand long-dead creatures.
These unfortunates found themselves mired in the sticky bitumen of La Brea’s tar pits tens of thousands of years ago during the last glacial period. They have remained there ever since, providing scientists with an unrivaled window into the past, even as the city of Los Angeles materialized around them.
Dire wolves, saber-toothed cats, mastodons, and giant ground sloths all fell prey to the pits’ lethal sludge, along with countless lizards, rodents, birds, insects, and the accidental detritus of trees and plants.
“It’s a library of the ice age,” says Luis Chiappe, the Vice President of Research and Collections at the Natural History Museum, which manages the site. The tar pits contain the most complete record of this ancient world found anywhere on Earth.
“The specimens found here paint the best picture of ancient climate,“ Chiappe says, which is increasingly important for understanding the significance of modern changes.
“We like to call it the miracle of Miracle Mile,” Chiappe says.
And now, it’s a better time than ever to pay a visit. The Page Museum at the La Brea Tar Pits has reopened two shuttered excavation pits to the public that give a scientist’s-eye view of La Brea’s world-class fossil soup. They will also restart excavations in Pit 91, which has produced over a million specimens of ancient life since it opened in 1969.
::
When the first excavators swung their pickaxes into La Brea’s rocks more than 100 years ago, they found themselves in a desolate landscape populated only by a skeletal forest of oil derricks.
The riches of the Salt Lake Oilfield lie a few hundred feet below ground underneath the area bounded by Robertson and Vine, Wilshire and Beverly boulevards. Tar from the oilfield, also known as asphalt, leaked out through fissures in Earth’s crust, pooling at the surface where unsuspecting creatures strayed into its clutches. The tar then engulfed and preserved the bones of its victims.
In the beginning, excavators focused on the obvious stuff: mastodon femurs the size of two-by-fours, saber-tooth fangs as long as daggers, a fearsome multitude of dire wolf skulls.
These treasures are proudly displayed in the Observation Pit, which will open for daily tours this summer after decades of closure. A spiral staircase winds down the circular walls of the historic white brick building, depositing visitors face-to-face with a jumbled, chalky centerpiece of partially exposed tar.
It’s a morbid mélange, all bones and teeth and death, but it’s a unique perspective for visitors, who rarely have the opportunity to see these fossils as scientists do: breathtakingly real and cemented forever in the tar-stained surface of the Earth.
::
The original census based only on big bones suggested that many more carnivores perished in the pits than herbivores, although prey animals should have significantly outnumbered their predators. The tar probably formed a carnivore trap where one stranded animal might attract a dozen scavengers, but there was another possibility.
“We were getting a skewed perspective,” says John Harris, chief curator of the Page Museum. Lost in the black goo, innumerable small fossils went unnoticed.
So when scientists set to work on Pit 91 the first time, the idea was to document everything. Every chunk of wood, every insect. Suddenly, scientists began to piece together vivid snapshots of a long-lost place. Not only did they know which animals roamed nearby, they knew which bugs hovered around the festering carcasses, which trees provided shade, which bird songs filled the ancient air.
“We have found over 650 species so far,” Harris says. “It doesn’t get much better than that.”
Pit 91 opens for business once again this summer after an eight-year hiatus while museum staff picked through fossils excavated during the constructing of LACMA’s parking garage, which neighbors La Brea. (They expect more fossils to emerge from the nearby Metro extension.)
But the pit will close again in the fall, when temperatures drop too low for the tar to flow. In the meantime, visitors can peer over the shoulders of volunteers in orange shirts as they scoop spoonfuls of tar into black-caked buckets, the goo sheeting off their spades in thick, syrupy strands.
::
One of the best-preserved specimens ever found at La Brea offers insights disproportionate to its diminutive size about the conditions that prevailed in Southern California during the last ice age.
Curled tightly, as if in slumber, a young leaf-cutter bee plucked from the tar looks ready to emerge from its nest and embark on a full life of pollination and leaf-cutting. Anna Holden of the Natural History Museum dated the pupa and discovered it died between 23,000 and 40,000 years ago.
Leaf-cutter bees still live in North America today, so Holden set out to learn all she could about them.
“They have mandibles perfect for cutting out smooth little circles,” Holden gushes. “And they use their body as a ruler!”
Most importantly, however, they live in cooler, wetter conditions similar to those found at higher elevations today. This corroborates other evidence from La Brea’s archive that Los Angeles looked very different during the last glacial period than it does today.
However, even in a cooler climate, scientists say summers must have been balmy. How do they know? Because beetles had time to nibble away at the bones of animals lost to the tar, something they can only accomplish at temperatures above 65 degrees Fahrenheit, Holden says.
(Scientists also used this bone damage to estimate how quickly skeletons sank into the pits. They discovered it probably took months.)
But Holden said she is most excited about Clyde, a new camel specimen whose remains contain delicate insects never before found at La Brea.
“Clear ant wings, damselflies, not just hardy beetles,” Holden says of his companions. She is sure Clyde’s entourage will further transform scientists’ understanding of the climate and ecosystems of La Brea’s past.
“It’s the kind of thing that makes you wish you could live two or three lives,” she says, just to have the chance to see it all.
::
Inside the Page Museum, hundreds of dire wolf skulls have been pinned to the wall inside a colossal shadow box. The display covers an entire side of the building, backlit by carnivorous orange.
“I came to work here because I saw this wall and I thought, ‘that’s a lot of fossils,’” says Robin O’Keefe, a paleontologist at Marshall University. He realized he could attempt something here that scientists rarely have the opportunity to do: he could study population-level questions in the dire wolf species, which is well represented in La Brea’s collection. More than 4,000 skulls have been excavated to date.
Instead of looking at a single fossil, he took 80 skulls and analyzed how their shapes changed over time, between the snapshots represented in La Brea’s various tar deposits.
He and his colleagues found that the animals responded directly to climate change; they exhibited signs of stress during cold periods, including stunted growth and brittle teeth. They also tackled smaller prey.
And then humans arrived sometime between 15,000 and 12,000 years ago. For a few millennia, everything seemed fine, but when the next major cold period struck, many species took a swan dive into extinction.
“This place is an amazing natural laboratory,” O’Keefe says. Because of the diversity and number of specimens, he says, “we can look into the role of humans and get beyond correlation to causality.” Or at least, he hopes so. This will be the subject of his next research grant to continue working at La Brea.
If he succeeds in disentangling the competing roles of human invasion and climate change in eradicating North American megafauna, he would help lay to rest a long-running debate among paleontologists.
O’Keefe suspects humans bear at least some of the blame because they simply usurped the role of top predator when they arrived, pointy sticks and all.
“People tend to think there’s natural ecosystems and then there are humans,” he says. But are we really so separate?
“When you walk out of this museum, you enter another ecosystem,” O’Keefe says.
::
They call it “the stacks” — a cavernous repository filled with row upon row of gunmetal cabinets. This hidden realm holds a century’s worth of work, and Aisling Farrell is its benevolent overlord. She’s well suited to the task because, for her, organizing “comes naturally,” she says with a faint Irish accent.
That’s a good thing, since La Brea’s collection contains more than 5 million specimens.
“I’ve been on other digs,” she says, “where you can walk for days and find nothing.” When you finally stumble on a fragment of a bone, she says, it’s exhilarating. But here at La Brea, it’s a different story.
“Stuff is everywhere,” she says. “Even my generation is not going to get through this material.” (She’s not that old.) “It’s important to preserve this heritage for the future.”
In fact, the only reason La Brea’s tar pits don’t sit beneath an apartment complex today is due to the foresight of the Hancock family, who donated the land to the county after recognizing its tremendous scientific value, Farrell said.
That decision laid the foundation for more than 700 scientific papers, and researchers have hardly scratched the surface of Farrell’s collection.
Many of the most recent papers used techniques that didn’t exist just a few decades ago.
For instance, a tray of a dozen dainty saber-tooth femurs sits out on a workbench. Small holes have been bored in each one and the missing material sent off to a lab at UC Irvine for detailed chemical analysis. The results will tell the scientists not only how long ago the cat prowled La Brea, but what it ate.
In the future, Farrell says, scientists might extract intact DNA from well-preserved bones — something they have so far pursued unsuccessfully — or they might apply big data strategies to wrestle the overwhelming amount of information contained in the collection into manageable chunks.
“We have the richest site in the world,” Farrell says, her seahorse earrings dancing beside her face, “and it’s accessible to everybody.”
Right here, in the middle of the vast human ecosystem of Los Angeles.
For all things science, follow me @ScienceJulia
