(CNN)Black gooey methane bubbles pop on the surface of the Lake Pit outside the La Brea Museum in Los Angeles. It's the only warning of the sticky, heavy asphalt on the bottom of the pit. The asphalt seeps have been there for thousands of years, stemming from a nearby underground large petroleum reservoir called Salt Lake Oil Field.
Preserving the unique history of the La Brea Tar Pits
Thousands of years ago, the liquid asphalt trapped unsuspecting mammoths, horses, giant ground sloths, camels and bison that roamed the area, thirsty for a drink. Only 1.5 inches of the asphalt was required to trap a creature as massive as a mammoth or bison.
Predatory carnivores saw the stuck animals as the perfect way to get a meal. Dire wolves, saber-toothed cats and lions would creep up on their trapped prey but they too would become stuck in the liquid asphalt.
It was a slow death by starvation. They decayed on the surface and became buried by the asphalt and sediment over time.
But the unique nature of the La Brea Tar Pits is that they preserved an entire ecosystem between 10,000 to 50,000 years ago, containing massive mammoth tusks and giant sloth bones alongside acorns and microscopic plant and insect fossils. More than 100 species of birds and a number of other species were first described after being found at La Brea.
"It's one of the most important paleontological sites in the world that records an entire ecosystem going through time," said Emily Lindsey, assistant curator. "We can see how it changed in response to major climatic events over the last 50,000 years. And we've been able to look how different species who went extinct, like large predators, as well as the few who didn't go extinct, such as coyotes and mountain lions."
Lindsey said there are about a dozen known fossilized tar pits in the world: the western part of the Americas, the Caribbean, Trinidad, Cuba, Venezuela, southern Ecuador and northern Peru, and some less well studied in Asia. But only a few of them formed in the way that La Brea did, capturing the rich, diverse nature of the landscape like a time capsule.
The site has been excavated since 1906 and more than 3.5 million specimens have been recovered, representing hundreds of species and many that are extinct today. Excavations remain ongoing.
Specimens fill the on-site museum, including the three most common animals found in the pits: dire wolves, saber-toothed cats and coyotes.
The asphalt seeps preserve fossils so well that scientists can extract molecular data and determine the age of the specimens as well as when they died. But studying the specimens in the museum's lab requires damaging part of the bone or plant materials to obtain samples.
The bones tell a story. A timber wolf that adapted to survive after a traumatic amputation. A mammoth that likely died fighting another male during the competitive mating season. Animals and insects that were sick or had arthritis or survived as others went extinct around them when the climate changed.
In order to preserve the size and shape of each bone, plaster casts and photos captured the full physical details. But given that new fossils can essentially be discovered on site every day, the fossils were stacking up in boxes and on shelves. In 2006, construction work for a new parking garage uncovered 16 new fossil deposits, including a nearly complete skeleton of an adult mammoth nicknamed Zed that had 10-foot-long tusks. Work is continuing on the fossils found in new deposits, but the museum estimates it will double their amount of specimens when completed.