A microCT scan of an Ice Age leafcutter bee inside the nest.
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From Holden et al., 2014.
A microCT scan of an Ice Age leafcutter bee inside the nest.

Tar Pits Bees Connect California’s Past to the Present

The La Brea asphalt seeps are practically synonymous with megafauna. Sculptures of American lions and scrapping Smilodon draw visitors into the on-site Page Museum, well-stocked with Ice Age celebrities that have been reconstituted from the mind-boggling number of bones found beneath Los Angeles. Even more bones are kept in rows upon rows of bins in the collections – perhaps the greatest fossil dataset in the world – but it would be a mistake to think that La Brea is all about sabercats and mammoths. The story of prehistoric California was like, and how the world has changed as the last Ice Age slipped away, is kept by a diversity of meeker creatures, including a pair of unborn leafcutter bees that may be the most intricate fossils ever to be pulled from La Brea.

In 1970, from a La Brea dig called Pit 91, excavators found a tiny little nub of plant material. Anywhere else this pill-shaped fossil might have been treated as an uninteresting bit of ancient scrap, but La Brea had made a policy to collect and catalog every scintilla of fossil material they found, down to beetle wings and plant pieces. The fossil was saved, curated, and was later found to be part of a prehistoric bee nest, but it was otherwise forgotten. The insects of La Brea haven’t received anywhere near the same scientific attention as the site’s large beasts.

That lack of interest from other researchers created an opportunity for entomologist Anna Holden, who wanted to know just what the Page Museum had in their insect stockpile. “I knew that I would find treasures going through the insect collections,” Holden says, and when she opened a small fossil labeled LACMRLP 388E, she immediately knew she had something special. “I got to this snap cap and said ‘Oh my god, these are leafcutters.'”

The fossil itself wasn’t an adult, petrified bee, but rather a leafcutter bee nest. The distinctive way these bees use plant material to make capsule-like enclosures for their young gave them away. No one had found evidence of these bees at La Brea before.

Leafcutter bees aren’t as well known as their honey- and bumble- relatives, but they’re still around. “They’re everywhere and people just don’t know about them,” Holden says. They’re nonsocial pollinators than zip around dusting their bodies with pollen, which the La Brea leafcutters undoubtedly did as mammoths, sloths, and camels trod around southern California around 40,000 to 35,000 years ago.

But the fossil is more than just a shell. Holden took an X-ray of the prehistoric nest to see if anything might be in side. I can’t repeat her exclamation upon seeing the X-ray here – a joyously-delivered expletive common to scientific discovery – but she was elated to see little blobs indicating that there were leafcutter pupae entombed inside their fragile nest. The next step, with the help of paleontologist Justin Hall, was to CT-scan and visualize the Ice Age bees.

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A comparison of a modern leafcutter bee pupa with one of the Ice Age pupae from La Brea. From Holden et al., 2014.

“When I saw the CT reconstructions, I would just play them over and over again,” Holden says. “I just couldn’t believe how well-preserved these pupae were.” The two developing bees were so intricately fossilized, in fact, that Holden and her colleagues were able to identify them as Megachile gentilis – a leafcutter bee that still lives in the American northwest and southwestern Canada.

How could something so delicate become preserved in a place that had totally disarticulated and dissembled countless mammal skeletons? Holden pored over diagrams and field notes to retrace where exactly the nest had been found. The possibility that the bee nest had somehow been washed to its location from somewhere else had to be ruled out. The finely-detailed data collecting standard at La Brea became essential. “I’m very grateful that people were very responsible in taking all that information,” Holden says.

The upshot of all those collection details is that leafcutters really did live at La Brea. Rather than tumbling into a tar pit, Holden says, this nest was made in the ground – just as with living leafcutters – and oil seeped into the cells to effectively embalm the nest and pupae inside. And while such a nest might seem fragile, the fact that leafcutters are ground-nesters means that the bees have to make strong surroundings for their offspring. “I have an understanding now that these nests are much sturdier than they appear,” Holden says. That solid construction and a matter of circumstance saved these bees for the fossil record. “If it wasn’t for this fossilization circumstance, Holden says, “we wouldn’t have these specimens.”

Now, with those specimens in hand, Holden and other Ice Age ecologists can get a finer understanding of what the end-Pleistocene world was like. From what’s known of living Megachile gentilis, Holden says, the presence of this leafcutter species at La Brea indicates that Ice Age Los Angeles was a moister environment with woody habitat near streams. The Pacific Northwest habitats where the bee lives now are a rough proxy for Los Angeles 23,000 years ago, giving us another line of evidence for how changing climate has dramatically altered ecology.

And the pupae may yield even more information about the lost world of the Ice Age. As CT scans get better, paleontologists can see and study small fossils in ever-greater detail. Holden and colleagues might be able to scan the nest again to look for pollen that the nesting bee placed in each cell with the pupae. “Bees are often very specific about the kind of pollen they use,” Holden says, so Ice Age pollen would add even more flourishes to the ecological picture these bees are helping to create.

This fossil is “the gift that keeps on giving,” Holden says. “The yield of paleoecological information is so rich. We learn from the leaves, the bees themselves, even where the nest was found, on the ground at Pit 91.” Some people may still prefer mastodons or short-faced bears, but, given the simple beauty of the bee’s nest and all it can teach us, Holden says “This fossil is my favorite one.”


Holden, A., Koch, J., Griswold, T., Erwin, D., Hall, J. 2014. Leafcutter bee nests and pupae from the Rancho La Brea Tar Pits of Southern California: Implications for understanding the paleoenvironment of the Late Pleistocene. PLoS ONE. 9,4: e94724. doi:10.1371/journal.pone.0094724