Paleontologists have found in a piece of Burmese amber a unique scene of a prehistoric ‘helmite’ (subfamily Haidomyrmecinae) attacking a nymph of Caputoraptor elegans, an extinct cockroach relative. The old encounter presents some of the first direct evidence showing how the newly identified helmet species, Ceratomyrmex ellenbergeri, and other half-ants once used their killer features – snapping their bizarre but deadly, shard-like mandibles in a vertical motion to pinch prey against their horny appendages. The study was published in the journal Current biology.
A worker of hell ant Ceratomyrmex ellenbergeri catching a nymph of Caputoraptor elegans stored in amber from Myanmar. Image Credit: New Jersey Institute of Technology / Chinese Academy of Sciences / University of Rennes.
“Fossilized behavior is very 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 Department 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 of value.”
“This fossil predation confirms our hypothesis for how hell mills worked. The only way to catch prey in such an arrangement is for the mill parties to move up and down in a direction other than that of all living ants and almost all insects. “
The hell-ant tribe, along with their striking predatory traits, are thought to have disappeared 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 helmet was discovered about a hundred years ago, it’s been a mystery why these extinct animals are so distinct from the ants we have today,” said Drs. Barden.
“This fossil reveals the mechanism behind what we might call an evolutionary experiment, and although we see several such experiments in the fossil record, we often have no clear picture of the evolutionary path that leads to them.”
Dr. Barden and colleagues suggest that adaptations for prey traps probably explain the rich variety of mandibles and horns observed in the 16 species of helmets identified so far.
Some species with unarmed, elongated horns like Ceratomyrmex ellenbergeri apparent prey external grip, while other helmets such as the recently discovered Linguamyrmex vladi he was thought to have used a metal-reinforced horn on his head to impale prey – a property that was potentially used to feed on the internal fluid (hemolymph) of insects.
“The first hell ant ancestors would first have won the opportunity to move their mills vertically,” said Drs. Barden.
“This in turn would integrate the mouthparts and head function in a way that was unique to this extinct strain.”
“Integration is a powerful formative force in evolutionary biology – when anatomical parts first function together, it opens up new evolutionary trajectories as the two functions evolve in concert,” he said.
“The consequences of this innovation in mouth-to-mouth movement with the half-ants are remarkable. Although modern ants do not have horns of any kind, some species of helium ants have horns covered with serrated teeth, and others, such as Linguamyrmex vladi, are suspected of reinforcing the horn with metal to prevent its own bite from provoking itself. “
To further explore, the researchers compared the morphology of head and mouth parts 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 between both Cretaceous and modern ants.
Their analyzes confirmed that helmets belong to one of the earliest branches of the ant evolutionary tree and are closely related to each other.
Moreover, the relationship between mandible and head morphology is unique in helmets compared to lifelines as a result of their specialized prey-capture behavior.
The analyzes also prove that elongated horns evolved twice into half ants.
While the fossil has finally given scientists more solid answers about how this long-lost class of ant predators functioned and succeeded for nearly 20 million years, questions remain about what caused these and other pedigrees to become extinct while modern ants flourished in the ubiquitous insects that what we know today.
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Phillip Barden and others. Specialized predation drives abrupt morphological integration and diversity in the early ants. Current biology, published online August 6, 2020; doi: 10.1016 / j.cub.2020.06.106
