Today’s ants grab their prey horizontally with their biting tools – but in the Cretaceous there were apparently species that snapped vertically, an amber fossil now documents. The fossil resin preserved the attack by a representative of the mysterious hell ant on a prey animal. The find illustrates how the insect presses the victim with its scythe-like lower jaw vertically against its horn-like extension on the head and thereby pegs it down.
In the course of the development history of some animal groups, evolution has experimented a lot: especially in the case of insects, it has produced an enormous variety of forms and adaptations, some of which still exist today and others have disappeared again. This also applies to the case of the ants: there was once a “model” that is now extinct and that raised some eyebrows among biologists. “Since the first hell ant fossil was discovered about a hundred years ago, it has been a mystery why these extinct animals were so different from today’s ants,” says Phillip Barden of the New Jersey Institute of Technology in Newark. The meanwhile 16 known representatives of the hell ants (Haidomyrmecinae) from amber have unusual lower jaws and horn structures on the head that are no longer found in any modern ant species.
A frozen scene from the Cretaceous Period
It has already been suggested that these ants used their bizarre structures vertically to snatch or pierce prey. The new amber fossil now shows this mechanism in action. “Petrified behavior is extremely rare. As paleontologists, we speculate about the function of ancient adaptations based on the evidence available, but seeing how an extinct predator managed to capture its prey is invaluable, ”says Barden.
The amber fossil comes from Myranmar and is dated to an age of 99 million years. In the Cretaceous period there was a forest at the site of the discovery, the sap of which has already transformed some living beings into greetings from prehistoric times. The detailed examination of the insects trapped in the newly discovered amber now revealed that it is a specimen of a previously unknown species of infernal ants that holds a victim with its head structures that the researchers identify as a young of Caputoraptor elegans – an extinct relative of the Cockroaches. Bards and his colleagues gave the name to the new species of hell ant Ceratomyrmex ellenbergeri.
Snapped vertically
“This find confirms our hypothesis about how the jaw structures of hell ants functioned. The only way the prey can get into this position is for the head structure’s capture mechanism to be vertical, ”says Barden. “This system differs from that of all ants today and almost all insects,” says the scientist.
Accordingly, the horn-like nose on the head of Ceratomyrmex ellenbergeri served the insect as a counterpart for the sharp lower jaw, which moved upwards when it snapped shut. Barden assumes that adaptations to specific prey animals explain the different mandibular and horn structures found in the 16 species of hell ants identified so far.
To explore further, the researchers compared the morphology of the head and mouthparts of Ceratomyrmex and other hell ant species with the characteristics of living and fossil species. The team then classified the infernal ants in a phylogenetic tree based on their characteristics in order to reconstruct the evolutionary relationships between Cretaceous and modern ants. The team’s analyzes confirmed that hell ants belong to one of the earliest branches of the ant evolution tree. The analyzes also make it clear that elongated horns in the hell ants evidently developed in parallel twice.
Extinct with the dinosaurs
From the evidence so far it is clear that the representatives of the hell ants existed successfully for a long time until they then disappeared together with other early ant groups as part of the mass extinction 65 million years ago. Thus, the question arises as to why the ancestors of our today’s ants of all things survived. In this context, Bardens and his colleagues are now hoping for new fossil discoveries that can shed light on how extinction processes affect certain groups of living things differently.
“Over 99 percent of all species that have ever lived are extinct,” says Barden. “Now that our planet is experiencing its sixth mass extinction, it is important that we also understand the extinct diversity in order to assess what causes certain lineages to persist while others become extinct. I think fossil insects are a reminder that even something as ubiquitous and familiar as ant species have become extinct in the course of evolution, ”says the scientist.
Source: New Jersey Institute of Technology, technical article: Current Biology, doi: 10.1016 / j.cub.2020.06.106