Submitted to: PLoS One
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/17/2014
Publication Date: 4/9/2014
Citation: Holden, A.R., Koch, J.B., Griswold, T.L., Erwin, D.M., 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. Interpretive Summary: Fossil bees are rare in general, and finding nests of bees is extremely rare. A nest of a leafcutter bee (Megachile) was discovered in the La Brea Tar Pits in Los Angeles, California. Using a combination of nest structure and reconstructed environmental conditions during the time the fossil was preserved (radiocarbon dated to 23,000 to 40,000 years before present) the nest is identified as belonging to the leafcutter species Megachile gentilis, a species still found in the Los Angeles basin. Utilizing scanning techniques the presence of two individual cells with intact pupae were detected, one a male, the other female. This is the first fossil nest of this bee species. The nest was constructed from leaves of four differ plants. The types of leaves suggest a wooded or stream-side environment. Comparison of reconstructed distribution of Megachile gentilis based on climatic limitations at the time the fossil was preserved with current distribution suggests an expansion of this species.
Technical Abstract: The Rancho La Brea Tar Pits is the world’s richest and most important Late Pleistocene fossil locality and best renowned for numerous fossil mammals and birds excavated over the past century. Less researched are insects, even though these specimens frequently serve as the most valuable paleoenvironmental indicators due to their narrow climate restrictions and life cycles. Our goal was to examine fossil material that included insect-plant associations, and thus an even higher potential for significant paleoenvironmental data. Micro-CT scans of two exceptionally preserved leafcutter bee nest cells from the Rancho La Brea Tar Pits in Los Angeles, California reveal intact pupae dated between ~23,000 – 40,000 radiocarbon years BP. Here identified as best matched to Megachile (Litomegachile) gentilis Cresson (Hymenoptera: Megachilidae) based on environmental niche models as well as morphometrics, the nest cells (LACMRLP 388E) document rare preservation and life-stage. The result of complex plant-insect interactions, they offer new insights into the environment of the Late Pleistocene in southern California. The remarkable preservation of the nest cells suggests they were assembled and nested in the ground where they were excavated. The four different types of dicotyledonous leaves used to construct the cells were likely collected in close proximity to the nest and infer a wooded or riparian habitat with sufficient pollen sources for larval provisions. LACMRLP 388E is the first record of fossil Megachile Latreille cells with pupae. Consequently, it provides a pre-modern age location for a Nearctic group, whose phylogenetic relationships and biogeographic history remain poorly understood. Megachile gentilis appears to respond to climate change, as it has expanded its distribution across elevation gradients over time as estimated by habitat suitability comparisons between low and high elevations; it currently inhabits mesic habitats which occurred at a lower elevation during the Last Glacial Maximum ~21,000 years ago. Nevertheless, the broad ecological niche of M. gentilis appears to have remained stable.