A researcher has accidentally discovered a rare fossil of a 35-million-year-old driver ant in a collection at Harvard University's Museum of Comparative Zoology. The amber-enclosed insect had been in the collection for nearly 100 years, but had previously been misclassified. New studies now show that it is the oldest known example of a driver ant and also the first to be discovered in Europe.
Driver ants are known to hunt in large groups and move their nest regularly. This also earned them the names army ants and army ants. These designations encompass several families of ants that share a migratory and predatory lifestyle and, unlike other ant species, reproduce via wingless queens. Today there are about 270 species in the Eastern Hemisphere and about 150 in North and South America. As voracious predatory insects, they play a major role in ecosystems in tropical regions. In Europe, on the other hand, they hardly ever occur. Fossils of driver ants are considered extremely rare: since most species live underground, it was unlikely that an individual would be struck by a drop of resin and thereby preserved in amber.
Accidental find in the museum
Christine Sosiak from the New Jersey Institute of Technology in Newark was all the more astonished when she discovered just such a specimen in the Museum of Comparative Zoology at Harvard University: "The museum houses hundreds of drawers full of insect fossils, but I happened to come across a tiny specimen known as a a common species of ant when I was collecting data for another project,” says the researcher. "When I put the ant under the microscope, I immediately realized that the labeling was inaccurate - I knew that was something else entirely."
Together with her colleagues, she examined the fossil, which was about three millimeters long, using X-ray and CT scan analyzes and found that it was actually a driver ant, closely related to the eyeless Dorylus driver ants that are found today in Africa and southern Asia occur. The research team named the newly discovered fossil species Dissimulodorylus perseus; "dissimulo" means to hide and refers to the fossil's identity hidden for decades, "perseus" is a reference to the Greek hero Perseus, who according to legend defeated Medusa by avoiding looking at her - a reference to the ant's lack of sight . Dissimulodorylus perseus is only the second fossil driver ant species ever described.
Ancient European representative
Enclosed is the ant in Baltic amber unearthed around 1930. "To learn now that it contained a rare driver ant is surprising enough, let alone proof that these ants were present in Europe," says Sosiak's colleague Phillip Barden. The fossil is dated from the Eocene 35 million years ago. "At the time the fossil was formed, Europe was hotter and wetter than it is today and may have provided an ideal habitat for the primeval driver ants," says Barden. "However, since the Eocene, Europe has undergone multiple cooling cycles over several million years that may have been inhospitable to these tropically adapted species."
Analysis of the body structure and specific characteristics of the newly described specimen indicated that it was most likely a worker and that the species lived underground. “The workers of the driver ants participate in swarms and hunt other insects and even vertebrates. Because these ants are blind, they use chemical communication to coordinate with each other and hunt down large prey,” explains Sosiak. "This worker ant may have strayed too far from her hunting companions and got caught in sticky tree sap that eventually solidified and trapped the ant as we see it today." D. perseus is now returning to the Museum of Comparative Zoology for further study preserved – now with the correct classification.
Source: Christine Sosiak (New Jersey Institute of Technology, Newark, New Jersey, USA) et al., Biology Letters, doi: 10.1098/rsbl.2022.0398