Eyeless army ants chomped their way through Europe millions of years ago

Update (March 23, 2022): The journal Biology Letters retracted “An Eocene army ant” on March 22, 2023. According to the New Jersey Institite of Technology, “once published, other scientists in the field questioned the age of the specimen based on similar samples and upon further analysis, the researchers concluded that their findings were unfounded, and worked with Biology Letters to issue a retraction.”

The army ant (Dorylinae) is generally good at two things: traveling all around the world and having a ravenous appetite. Through their highly coordinated foraging, the ants can eat up to 500,000 prey animals in a single a day. Its nomadic lifestyle has taken the insects to most continents on Earth and there are currently about 270 army ant species living in the planet’s Eastern Hemisphere and roughly 150 species across North and South America.

Thanks to a rare fossil discovery, scientists are now exploring the first evidence that the predators once swarmed where they are not eating and scrurrying around today—Europe.

In a new paper published yesterday in the journal Biology Letters, researchers from New Jersey Institute of Technology (NJIT) and Colorado State University detail the discovery of the oldest army ant on record. The specimen was preserved in Baltic amber dates back to the Eocene Epoch, about 35 million years ago.

[Related: How many ants are there on Earth? Thousands of billions.]

The specimen is about three millimeters long (less than an inch), shows an animal without eyes, and is named Dissimulodorylus perseus (D. perseus), after the mythical Greek hero Perseus. The legend goes that Perseus defeated Medusa with limited use of sight.

The fossil is just the second fossilized evidence of an army ant species ever described and is the first army ant fossil recovered from the Eastern Hemisphere, according to the study.

The team says that this ant fossil is evidence of previously unknown army ant lineages that would have existed across Continental Europe before going extinct throughout the past 50 million years AGO.

Incidently, this huge find was hidden for nearly 100 years at Harvard University’s Museum of Comparative Zoology.

“The museum houses hundreds of drawers full of insect fossils, but I happened to come across a tiny specimen labeled as a common type of ant while gathering data for another project,” the paper’s lead author and NJIT PhD candidate Christine Sosiak said in a statement. “Once I put the ant under the microscope, I immediately realized the label was inaccurate. I thought, this is something really different.”

It’s likely that the amber encasing the fossil was excavated sometime near or before the 1930s.

“From everything we know about army ants living today, there’s no hint of such extinct diversity,” said Phillip Barden, assistant professor of biology at NJIT and senior author of the paper. “With this fossil now out of obscurity, we’ve gained a rare paleontological porthole into the history of these unique predators.”

The team used X-rays and CT-scans to analyze the fossil and determined that D. perseus as a close relative to eyeless species of army ants currently found in Africa and Southern Asia, named Dorylus.

[Related: Ants have teeth. Here’s how they keep them sharp.]

When this fossil was formed, Europe had a much hotter and wetter climate than it has today, which might have provided an ideal living environment for ancient army ants. Since the Eocene (over tens of millions of years), Europe has undergone several cooling cycles, which may have made the continent been inhospitable for the ancient ants.

They also found an enlarged antibiotic gland on the specimen that is typically found in other army ants, that helps them live underground. This gland suggests that this European army ant lineage was well suited for subterranean living.

According to Sosiak, it’s one factor that sets this fossil a rarity.

“This was an incredibly lucky find. Because this ant was probably subterranean like most army ants today, it was much less likely to come into contact with tree resin that forms such fossils,” said Sosiak. “We have a very small window into the history of life on our planet, and unusual fossils such as this provide fresh insight.”