Scientists who found an ancient insect enveloped in 16-million-year-old amber in the Dominican Republic say it is the first-ever fossil of a dirt ant (a group of ants) from the Caribbean.
The long-extinct species, named Basiceros enana, measures just over 5 millimetres in length, making it considerably smaller than its closest living ant relatives, which can grow up to 9 millimetres. Yet despite its diminutive size, the specimen – perfectly preserved in the fossilised tree resin – holds a wealth of intriguing information.
The researchers, who published their findings in Proceedings of the Royal Society B, say the discovery offers direct evidence that the enigmatic dirt ant group once inhabited the Caribbean islands before going locally extinct 23 to 5.3 million years ago.
Dirt ant: a hidden group
Dirt ants, or Basiceros, are a group of ants known for their incredible camouflage abilities. Their bodies are covered in specially adapted hairs that trap particles of soil and leaf litter, allowing them to blend seamlessly into their surroundings.
Modern-day species are extremely difficult to find, often going unnoticed by both scientists and predators. So, discovering one preserved in amber is both rare and exciting, says Gianpiero Fiorentino, co-author of the study and a PhD candidate at the New Jersey Institute of Technology's (NJIT) Barden Lab.
"It's like finding a diamond,” says Fiorentino. “This fossil is singularly distinct from all its modern relatives and reshapes the evolutionary history of Basiceros.”
Until now, all known Basiceros ants lived in Central and South America, from Costa Rica to southern Brazil. The unexpected Caribbean discovery has led researchers to reconsider the group’s evolutionary history. It suggests that these ants once lived on the Caribbean islands but eventually became locally extinct – likely during the Miocene epoch, between 23 and 5.3 million years ago.
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Inside the amber
To examine the fossil, scientists used advanced imaging techniques, including 3D reconstruction and micro-CT scanning.
“Because amber preserves entire organisms in three dimensions, we can extract a ton of data from even a tiny ant,” explains Phil Barden, senior author and associate professor of biology at NJIT. This allowed the scientists to analyse the ant's fine details, from the structure of its head and mandibles to the unique arrangement of hairs on its body.
One of the most fascinating findings is that the ant already possessed the distinctive camouflage features seen in today’s dirt ants. These include two types of hairs – long, brush-like ones that pick up debris and shorter ones that hold it close to the body. The team believes this indicates that their strategy of hiding in plain sight was already well-developed millions of years ago.
“What this shows is that playing dead and hiding pays off,” says Fiorentino. “Uncovering a unique fossil like this helps us understand how long organisms may have been employing this strategy, though the presence of these characteristics does not necessarily guarantee they behaved in this way.”
“Because amber preserves entire organisms in three dimensions, we can extract a ton of data from even a tiny ant."
The fossil also shows other features typical of modern dirt ants, such as a trapezoidal head and sharp, toothed mandibles – perfect for a predatory lifestyle.
Yet, despite their adaptations, the Caribbean dirt ants vanished, likely due to environmental changes or competition with other species.
“The presence of Basiceros in Dominican amber suggests ancient land bridges may have provided pathways for these ants to traverse from the mainland to the Caribbean,” Barden says. “This fossil is a piece of a larger puzzle that will help us understand why some groups of organisms undergo extinction and others stick it out for millions of years.”
“Over a third of ant genera have gone extinct on the island of modern-day Dominican Republic since the formation of Dominican amber," adds Fiorentino.
“Understanding what has driven this pattern of local extinction is crucial to mitigating modern human-driven extinction and protecting biodiversity.”
Find out more about the study: A fossil-informed pattern of body size increase and local extinction in Basiceros dirt ants (Hymenoptera: Formicidae)
Main image: Basiceros enana, a newly discovered extinct dirt ant species preserved in Dominican amber, formed from tree resin that fossilised millions of years ago. Credit: Gianpiero Fiorentino (NJIT)
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