Oldest army ant discovered in Baltic amber dating back 35 million years

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Biologists from the New Jersey Institute of Technology and Colorado State University have identified a fossil of an army ant trapped in a 35-million-year-old block of Baltic amber. It is now the oldest known army ant fossil and the first evidence that this insect, now relatively common in Africa and South America, once roamed the European continent.

The piece of amber in question had been stored at Harvard University since 1930. But while examining several ant fossils embedded in amber, insect biologist Christine Sosiak realized that one of them appeared to have been misidentified as a specimen of the gender Platythyrea — a common genus of tropical and subtropical regions. ” Once I put the ant under the microscope, I quickly realized the label was inaccurate. I thought it was something really different “says the expert.

Further examination revealed that this ant bore many similarities to modern army ants. Legion ants (Doryline), which include about 270 species, are today the main predators of the tropics. These ants form huge colonies, which can contain several million individuals; their nests are occupied only temporarily, as they move constantly throughout their lives. Though blind, they are known to hunt their prey in the form of particularly aggressive swarms.

The first army ant fossil discovered in the Eastern Hemisphere

X-ray and CT scans revealed that Sosiak and his colleagues were in the presence of a previously undescribed specimen. The phylogenetic and morphological data collected place this ant as a close relative of the eyeless nomadic army ant species found today in Africa and southern Asia, called Sleep it. The specimen has been named Dissimulodorylus perseus — in reference to the mythical Greek hero Perseus, who defeated Medusa without looking at her; the term Dissimulodorylus comes from the Latin I dissemblewhich means to hide or conceal, because the true identity of the insect has eluded scientists for about 80 years.

(a) Photomicrograph of a side profile view of D. perseus. (Bottom) Micro-CT reconstruction of D. perseus, in lateral (b) and front (c) view. Scale bars: 0.5mm. © Museum of Comparative Zoology/Harvard University/President and Fellows of Harvard College

This is only the second species of fossilized military ant discovered so far: the first, a species known as Neivamyrmex ectope, was found in 16-million-year-old amber in the Dominican Republic. It is also the first military ant fossil discovered in the Eastern Hemisphere; thus highlights previously unknown lineages, which would have existed in continental Europe before going extinct in the last 50 million years.

About 3 millimeters long, the insect had no eyes, but had very sharp mandibles. According to the researchers it was probably a worker within her colony, who would have participated in the transport of her queen’s larvae and food raids. This ant also had a well-developed antibiotic gland, essential for underground life and typical of other modern army ants. The presence of this gland therefore suggests that this extinct line of European army ants was also adapted to life underground.

Christine Sosiak and her colleagues were therefore very lucky: since this ant was probably underground, it was much less likely to come into contact with the tree resin that forms such fossils, explains the specialist.

The oldest physical evidence of military ant syndrome

Being blind, army ants use chemical communication, via pheromones, to stay coordinated with one another and successfully bring down large prey items. ” This worker may have strayed too far from her hunting companions and ended up in the sticky resin of the tree, which eventually solidified and encased the ant as we see it today. “, supposes the biologist.

Unlike other ant lineages, army ants have wingless queens, whose only job is to lay (millions of eggs per day), while their nomadic colonies temporarily occupy the nests between each movement: millions of ants a Sometimes they can extend over 100 meters! Their impressive food forays can lead them to consume up to 500,000 prey per day.

To designate all these particular traits and behaviors, specialists speak of “Legionnaire Ant Syndrome”. Previous research suggests that this syndrome evolved twice in the middle Cenozoic: once in the Neotropics and once in the Afrotropics. This Eocene fossil is the first physical evidence of army ant syndrome, showing that the characteristics of these predators were present 35 million years ago, even before the ancestors of the ants Sleep it appear, points out Phillip Barden, assistant professor of biology at NJIT and lead author of the study describing the finding.

Currently, there are about 270 army ant species living in the Eastern Hemisphere and about 150 in the American continent. While unexpected, the discovery of this military ant in the Baltic is “logical” for researchers: ” During the Eocene, Europe was warmer and wetter overall than it is today, creating a vast expanse of suitable habitat across Eurasia “, they write. But throughout the second half of the Cenozoic, Europe underwent several cooling cycles, undoubtedly difficult for these species adapted to tropical climates, which would explain their current absence from the continent.

Source: C. Sosiak et al., Biology Letters

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