Voracious Insect's Previously Unknown European Relatives Revealed by Rare 35-Million-Year-Old Fossil Army Ant

Army ants (Dorylinae) have spread to most continents due to their nomadic lifestyle and ravenous raiding, but a rare fossil discovery provides the first evidence that the infamous predators once swarmed a land they are now noticeably absent from.

The oldest army ant ever discovered reveals an iconic predator

(Photo : Prabir Kashyap/Unsplash)

The finding of the oldest army ant on record, preserved in Baltic amber dating back to the Eocene (35 million years ago), was reported in the journal Biology Letters by researchers from the New Jersey Institute of Technology and Colorado State University.

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

The ant fossil, which is about 3 millimeters long, researchers say reveals previously unknown army ant lineages that would have existed across Continental Europe before extinction in the last 50 million years.

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

"The museum has hundreds of drawers full of insect fossils, but while gathering data for another project, I came across a tiny specimen labeled as a common type of ant," Sosiak explained.

"Once I looked at the ant under the microscope, I realized the label was incorrect. I thought, this is something completely different," he added, as per ScienceDaily.

A Paleontological Window into the Evolution of a Rare Predator

There are approximately 270 army ant species in the Eastern Hemisphere, and approximately 150 in North and South America, as per Benzinga.

The NJIT team gathered phylogenetic and morphological data from the fossil-based on X-ray and CT-scan analysis.

Perseus is a close relative of the eyeless army ant Dorylus, which is currently found in Africa and Southern Asia.

According to Phillip Barden, assistant professor of biology at NJIT and senior author of the paper, Europe was hotter and wetter at the time the fossil formed, making it an ideal habitat for ancient army ants.

Europe has experienced several cooling cycles over tens of millions of years since the Eocene, which may have rendered these tropical-adapted species inhospitable.

The ant also had an enlarged antibiotic gland, which is typically found in other army ants for sustaining life underground, implying that the long-lost European army ant lineage was similarly suited to subterranean living.

It's a factor, according to Sosiak, that makes this fossil, as well as other fossil army ants, rare. Until now, only one definitive fossil had been discovered, in the Caribbean.

According to Sosiak, the anatomical characteristics of D perseus, such as its sharply pointed mandibles and lack of eyes, help categorize the specimen as a worker ant in its colony, which would be involved in carrying its queen's larvae and raiding for food with soldier ants when it was alive.

Army ants' unique combination of behaviors and traits is so unusual in the ant world that it has its name: army ant syndrome.

Unlike other ant lineages, Army ants have wingless queens capable of laying millions of eggs per day.

Their nomadic colonies temporarily occupy nests in the shape of bivouacs, sometimes involving millions of ants stretching for 100m.

Carnivores are best known for their highly coordinated foraging, which can involve consuming up to 500,000 prey items per day.

According to Barden, army ant syndrome is a result of convergent evolution that occurred twice: once in the Neotropics and once in the Afrotropics.

According to Barden, the discovery is the first physical evidence of the army ant syndrome in the Eocene, establishing that the hallmarks of these specialized predators existed even before the ancestors of certain army ants like Dorylus.