Dirt ants were once masters of disguise in the Caribbean

An exceptionally well-preserved ant fossil encased in 16-million-year-old Dominican amber is offering scientists surprising new insights into the evolutionary journey of a rare group known as dirt ants, or Basiceros.

The fossilized ant – a previously unknown species named Basiceros enana – is the first of its kind ever uncovered in the Caribbean. The discovery suggests that these elusive ants once inhabited the islands before vanishing during the Miocene epoch, which spanned from roughly 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, the study’s lead author and a PhD candidate at the New Jersey Institute of Technology (NJIT). “This fossil is singularly distinct from all its modern relatives and reshapes the evolutionary history of Basiceros.”

Discovery of a dirt ant species

Previously, all known members of this ant genus were limited to neotropical rainforests stretching from Costa Rica down to Southern Brazil. Currently, nine living Basiceros species are documented.

The discovery of B. enana, however, raises new questions about how these ants once dispersed and colonized parts of the Caribbean.

“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, senior author of the study 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.”

Tracing the dirt ant’s ancestry

To analyze the ancient ant, the team employed cutting-edge imaging methods, including Micro-CT scanning at NJIT and the Okinawa Institute of Science and Technology Graduate University. These techniques enabled them to reconstruct the fossil in 3D and examine intricate anatomical details otherwise invisible.

“The use of Micro-CT scanning really amplified this study, enabling us to capture features that were virtually impossible to see otherwise,” Fiorentino explained.

The researchers compared these structural features to all known living dirt ant species and used molecular dating methods to trace the species’ ancestry. Because amber preserves creatures in three dimensions, the team was able to collect extensive information from the small specimen.

Significance of the fossil

The fossil ant, measuring just 5.13 millimeters in length, is significantly smaller than its modern descendants, which can grow to nearly 9 millimeters. As a result, B. enana now holds the distinction of being the smallest member of its genus ever recorded.

“Our results show that the embiggening of these ants was relatively rapid,” Fiorentino said. “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.”

Ancient camouflage already in play

Yet despite its smaller size, B. enana already displayed some of the key adaptations that help modern dirt ants remain nearly invisible in their environment – a form of camouflage known as crypsis.

These include two layers of specialized hairs designed to trap dirt and debris: long brush-like setae that lift particles and shorter, tighter hairs that hold them close to the body’s surface.

“What this shows is that playing dead and hiding pays off,” Fiorentino said. “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 also shares other recognizable features with today’s dirt ants, such as curved propodeal spines, a distinctive trapezoidal head, and sharp mandibles lined with a dozen triangular teeth – traits suggesting a predatory lifestyle.

A piece of a larger puzzle

Despite these survival advantages, Basiceros ants eventually disappeared from the Caribbean, likely due to dramatic environmental shifts during the Miocene. The team suggests that ancient land bridges might have allowed the ants to migrate from mainland South America to the islands.

“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.”

Loss of ants in the Caribbean

Fiorentino noted that the dirt ants’ 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.”

The study is published in the journal Proceedings of the Royal Society B.

Image Credit: Gianpiero Fiorentino (NJIT)

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