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Newswise — Wherever there’s dirt there’s bound to be ants, but one particular group is so adept at blending in with the ground that they hold the name “dirt ant” (Basiceros) all to themselves.

Now, an ancient fossil has revealed these elusive ants have seen much more of the planet’s dirt throughout their history than scientists previously realized.

In in the Proceedings of the Royal Society B, a team led by New Jersey Institute of Technology (NJIT) biologists has reported the first fossil Basiceros dirt ant recovered from the Caribbean, preserved in 16-million-year-old amber from the Dominican Republic.

Researchers say the fossil adult worker ant — a newly discovered species notably smaller than its modern relatives, named Basiceros enana — now offers direct evidence that the cryptic ant group once inhabited the Caribbean islands before undergoing local extinction sometime during the Miocene epoch (23 to 5.3 million years ago).

“Dirt ants are rare finds in the wild. Finding one today is exciting given how well they stay hidden, but captured in amber, it's like finding a diamond,” said Gianpiero Fiorentino, corresponding author of the study and Ph.D. candidate at NJIT's Barden Lab. “This fossil is singularly distinct from all its modern relatives and reshapes the evolutionary history of Basiceros.”

Until now, Basiceros ants — best known for their ability to camouflage themselves in dirt using specialized particle-binding hairs on their bodies — had only been known to the neotropical rainforests stretching from Costa Rica to Southern Brazil. While the genus includes a total of nine living species today, the unexpected fossil discovery raises new questions about how the ant group reached their present-day habitats.

“Often lineages will have what appear to be fairly straightforward biogeographic histories. If you find a group of animals that only live in South America up to Costa Rica today, you really have no reason to expect that their early relatives lived in the Caribbean,” said Phil Barden, the paper's senior author and associate professor of biology at NJIT. “A fossil like this underscores how the distribution of living species can belie the complex evolutionary history of life on our planet.”

To explore this long-hidden chapter of Basiceros' history, the team applied advanced imaging and 3D reconstruction techniques at NJIT and Japan's Okinawa Institute of Science and Technology Graduate University to capture Basiceros enana in detail. “The use of Micro-CT scanning really amplified this study, enabling us to capture features that were virtually impossible to see otherwise,” Fiorentino explained.

They compared the specimen's physical characteristics with those of all known modern dirt ant species and conducted molecular dating analyses to trace its evolutionary lineage. “Because amber preserves entire organisms in three dimensions, we can extract a ton of data from even a tiny ant,” Barden said.
 

Above: Artistic rendition of Basiceros enana preserved in Dominican amber from CT-scan data. Specialized hairs used for picking up dirt can be seen highlighted in yellow covering the ants’ legs and head. Credit: Gianpiero Fiorentino (NJIT)

Measuring at 5.13 millimeters long, Basiceros enana is considerably smaller than its modern relatives which can reach nearly 9 millimeters in length, making the Caribbean species the smallest now known across the lineage.

“Our results show that the embiggening of these ants was relatively rapid,” explained Fiorentino. “They almost doubled in size in the span of 20 million years. Previous hypotheses suggested that these ants were ancestrally large and shrank over time, so this flips that on its head and really illustrates how important fossils can be to understanding the evolution of a lineage.”

However, Basiceros enana also suggests some of the same adaptations that make modern dirt ants nearly invisible to predators and prey in their environment (an ability known as crypsis) were already in place at least 16 million years ago.

These features include two layers of specialized hairs (or setae) for adhering soil and leaf litter particles against their bodies: longer erect “brush hairs” and shorter, appressed “holding hairs” that trap particles against its exoskeleton, or cuticle. 

“What this shows is that playing dead and hiding pays off,” said 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.”

The fossil ant also possesses other distinctive morphological characteristics like today’s dirt ants, including an upturned propodeal spine, a trapezoid-like head structure, as well as predatory features such as mandibles with 12 triangular teeth.

Despite these specialized adaptations, the ancient Caribbean dirt ants ultimately vanished from the region during significant ecological changes of the Miocene.

“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 said. “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.”

“Their extinction could have come down to a loss of available niches or interspecific competition. These ants are predators, and an overall trend that we see from the Caribbean is a loss of predator ant diversity,” said Fiorentino. “Over a third of ant genera have gone extinct on the island of modern-day Dominican Republic since the formation of Dominican amber.

“Understanding what has driven this pattern of local extinction is crucial to mitigating modern human-driven extinction and protecting biodiversity.”