99-Million-Year-Old ‘Hell Ant’
Paleontologists have found trapped in a piece of Burmese amber a unique scene of a prehistoric ‘hell ant’ (subfamily Haidomyrmecinae) attacking a nymph of Caputoraptor elegans, an extinct cockroach relative. The ancient encounter presents some of the first direct evidence showing how the newly-identified hell ant species, Ceratomyrmex ellenbergeri, and other hell ants once used their killer features — snapping their bizarre, but deadly, scythe-like mandibles in a vertical motion to pin prey against their horn-like appendages. The study was published in the journal Current Biology.
“Fossilized behavior is exceedingly rare, predation especially so,” said lead author Dr. Phillip Barden, a researcher in the Department of Biological Sciences at the New Jersey Institute of Technology and the Division of Invertebrate Zoology at the American Museum of Natural History.
“As paleontologists, we speculate about the function of ancient adaptations using available evidence, but to see an extinct predator caught in the act of capturing its prey is invaluable.”
“This fossilized predation confirms our hypothesis for how hell ant mouthparts worked. The only way for prey to be captured in such an arrangement is for the ant mouthparts to move up and downward in a direction unlike that of all living ants and nearly all insects.”
The hell ant lineage, along with their striking predatory traits, are suspected to have vanished along with many other early ant groups during periods of ecological change around the Cretaceous-Paleogene extinction event 65 million years ago.
“Since the first hell ant was unearthed about a hundred years ago, it’s been a mystery as to why these extinct animals are so distinct from the ants we have today,” Dr. Barden said.
“This fossil reveals the mechanism behind what we might call an evolutionary experiment, and although we see numerous such experiments in the fossil record, we often don’t have a clear picture of the evolutionary pathway that led to them.”
Dr. Barden and colleagues suggest that adaptations for prey-capture likely explain the rich diversity of mandibles and horns observed in the 16 species of hell ants identified to date.
Some species with unarmed, elongate horns such as Ceratomyrmex ellenbergeri apparently grasped prey externally, while other hell ants such as the recently-discovered Linguamyrmex vladi were thought to have used a metal-reinforced horn on its head to impale prey — a trait potentially used to feed on the internal liquid (hemolymph) of insects.
“The earliest hell ant ancestors would have first gained the ability to move their mouthparts vertically,” Dr. Barden said.
“This, in turn, would functionally integrate the mouthparts and head in a way that was unique to this extinct lineage.”
“Integration is a powerful shaping force in evolutionary biology — when anatomical parts function together for the first time, this opens up new evolutionary trajectories as the two features evolve in concert,” he said.
“The consequences of this innovation in mouthpart movement with the hell ants are remarkable. While no modern ants have horns of any kind, some species of hell ant possess horns coated with serrated teeth, and others like Linguamyrmex vladi are suspected to have reinforced its horn with metal to prevent its own bite from impaling itself.”
To explore further, the researchers compared the head and mouthpart morphology of Ceratomyrmex ellenbergeri and several other hell ant species (such as head, horn and mandible size) with similar datasets of living and fossil ant species.
They also conducted a phylogenetic analysis to reconstruct evolutionary relationships among both Cretaceous and modern ants.
Their analyses confirmed that hell ants belong to one of the earliest branches of the ant evolutionary tree and are each other’s closest relatives.
Moreover, the relationship between mandible and head morphology is unique in hell ants compared to living lineages as a result of their specialized prey-capture behavior.
The analyses also demonstrated that elongated horns evolved twice in hell ants.
While the fossil has finally provided the scientists with firmer answers as to how this long-lost class of ant predators functioned and found success for nearly 20 million years, questions persist such as what led these and other lineages to go extinct while modern ants flourished into the ubiquitous insects we know today.
Phillip Barden et al. Specialized Predation Drives Aberrant Morphological Integration and Diversity in the Earliest Ants. Current Biology, published online August 6, 2020; doi: 10.1016/j.cub.2020.06.106