Surrounded by Hollywood sets, the La Brea tar pits may seem a bit out of place in the middle of Los Angeles. The famous location is known for turning up extraordinarily well-preserved specimens of dire wolves, saber-tooth cats and wooly mammoths—and tells a story arguably more fascinating than any we could recreate ourselves.
Exactly how long it took for the animals to sink down into the sticky tar after they first became trapped has been a mystery. Now, a new study is providing a suspected time span—as well as some fascinating suspicions about when the tar pits were at their most lethal. Interestingly, the La Brea tar pits are still an active hazard to this day—but the risks they pose are far less that what prehistoric animals faced.
“Working at the tar pits, at some point, you’re going to step in a tar seep. It’s almost a right of passage,” Anna Holden, a paleontomologist at the Natural History Museum of Los Angeles County in California, and the leader author of the new study, wrote for PLOS ONE.
The heavy oil fractions of asphaltum are seeping up from the Salt Lake Oil Field, which has been active for tens of thousands of years. It only takes four centimeters of tar to ensnare a large animal—so it is no wonder that throughout its existence, the tar pits have swallowed up countless animals—providing an epic fossil collection.
Interestingly, some of these fossils show distinct chew marks from insects, likely left on the bones after the animals died but before they were completely submerged in the tar. Holden and her team studied the life cycles of the insects in the trace marks to determine how long the animals were likely on the surface of the tar before they sank into the pits. The first step was determining what kinds of insects left the chew marks.
“The best way to do this is to have live insect colonies nibble on bones and compare the marks left by living beetles with the trace fossils,” Holden notes in her report. Researchers fed bones to colonies of different types of beetle larvae, concluding the majority of the tracks had been left by the larval stage of dermestid and tenebrionid beetles.
After studying thousands of fossils, Holden, and her team concluded the insect damage seemed to be confined to the lower leg and foot bones of juvenile herbivores. This area tends to be very spongy and vascular, providing the perfect feeding environment for larvae. Once the larvae hatched, they fed on the remains, including the bones. Given the life cycles of the two beetles identified, researchers concluded the carcasses must have remained on the surface of the tar for at least 17 to 20 weeks.
“It’s kind of surprising to find out what kinds of insects did this damage to the bones because these beetles are still crawling all over Southern California,” Blaire Van Valkenburgh, a paleobiologist at UCLA noted.
The presence of the two warm-weather beetles helps to confirm suspicions about when the tar pits were most likely to trap animals.
“The probability of entrapment likely varied with temperature in a predictable way,” Van Valkenburgh commented. “These bugs are associated with warmer climates, and when it’s warm, the tar is stickier and things are more easily trapped.”
Holden believes this is the beginning of many important discoveries that can be revealed by insects.
“The saber-tooth cats and mammoths tend to get a lot of attention,” Holden concluded. “But I think insect traces might turn out to be one of the best available tools for studying this ecosystem.”
Sources: Earth Magazine, La Brea Tar Pits & Museum