The oldest army ant ever discovered reveals the iconic predator once raided Europe

A rare 35-million-year-old fossil army ant, discovered in a 100-year-old museum collection, uncovers previously unknown European relatives of the infamously voracious insect.

Their nomadic lifestyle and ravenous raiding have taken army ants (Dorylinae) to most continents on Earth, but a rare fossil discovery is now offering the first evidence that the infamous predators once swarmed a land they are strikingly absent from today – Europe.

Researchers at New Jersey Institute of Technology and Colorado State University have reported the discovery of the oldest army ant on record, preserved in Baltic amber dating to the Eocene approximately 35 million years ago.

The eyeless specimen Dissimulodorylus perseus (D. perseus) — named after the mythical Greek hero Perseus, who famously defeated Medusa with the limited use of sight — marks just the second fossil army ant species ever described, and the first army ant fossil recovered from the Eastern Hemisphere.

Sized at roughly 3mm in length, researchers say the fossil brings to light previously unknown lineages that would have existed across continental Europe before undergoing extinction in the past 50 million years.

Remarkably, the fossil had been kept in obscurity for nearly 100 years in the Museum of Comparative Zoology at Harvard University, before being identified by the paper’s lead author and NJIT PhD candidate, Christine Sosiak.

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rare army ant

Sosiak said: “The museum houses hundreds of drawers full of insect fossils, but I happened to come across a tiny specimen labelled as a common type of ant while gathering data for another project. Once I put the ant under the microscope, I immediately realised the label was inaccurate … I thought, this is something really different.”

Phillip Barden, assistant professor of biology at NJIT and senior author of the paper, added: “This amber would have been excavated around or before the 1930sso to now learn it contained a rare army ant is surprising enough, much less one that demonstrates these ants roamed Europe.

“From everything we know about army ants living today, there’s no hint of such extinct diversity. With this fossil now out of obscurity, we’ve gained a rare palaeontological porthole into the history of these unique predators.”

Today, there are about 270 army ant species living in the eastern hemisphere, and roughly 150 across North and South America.

Based on X-ray and CT-scan analysis of the fossil, the NJIT team gathered phylogenetic and morphological data that places D. perseus as a close relative to eyeless species of army ants currently found in Africa and southern Asia, called Dorylus.

Barden said: “At the time the fossil formed, Europe was hotter and wetter than it is today and may have provided an ideal habitat for ancient army ants. Europe underwent several cooling cycles over tens of millions of years since the Eocene, however, which may have been inhospitable to these tropical-adapted species.”

lucky find

The team’s analysis further revealed that the specimin possessed an enlarged antibiotic gland, typically found in other army ants for sustaining life underground, suggesting the long-lost European lineage was similarly suited to subterranean living.

It’s a factor that Sosiak says makes this fossil, and other fossil army ants, a rarity. Only one definitive fossil, from around 16 million years ago, had been recorded until now, unearthed from the Caribbean.

Sosial said: “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. We have a very small window into the history of life on our planet, and unusual fossils such as this provide fresh insight.”

Army ants’ distinct combination of behaviour and traits is so unusual in the ant world, that it’s warranted its own name — army ant syndrome.

Barden says the syndrome is a case of convergent evolution that would have occurred twice — once in the Neotropics and once in the Afrotropics.

He stated: “The discovery is the first physical evidence of the army ant syndrome in the Eocene, establishing that hallmarks of these specialised predators were in place even before the ancestors of certain army ants like Dorylus.”

For now, the newly identified fossil joins just eight fossil species within the ant subfamily that army ants belong to, called Dorylinae – five from Dominican amber from 16 million years ago and three species known from Baltic amber from around 34 million years ago.

The research is published in Biology Letters.

Image: Researchers at New Jersey Institute of Technology and Colorado State University have reported the discovery of the oldest army ant on record, preserved in Baltic amber dating to the Eocene around 35 million years ago.

Credit: Sosiak et al. 2022, Museum of Comparative Zoology, Harvard University. ©President and Fellows of Harvard College.