The earliest fossil ant ever discovered was a terrifying creature with specialized scythe-like jaws that it presumably used to impale its prey . The "hell ant" wasn't the earliest ant – just the earliest one we have found in fossil form – but its degree of specialization at that stage of myrmecological evolution was a surprise, we learned last week.
Separately, a paper published Wednesday in the Proceedings of the Royal Society B shows another unexpected trajectory in ant evolution: the "dirt ant" began small, and grew larger. The thinking had been that its development went the other way. Anyway, after growing larger, the ant went extinct in the Caribbean, though survives elsewhere to this day, including Honduras.
Giant snakes in world's largest prehistoric etchings may have marked territories
Earliest tadpole discovered from 170 million years ago – and it was a Jurassic monster
Archaeologists exploring prehistoric cave find earliest evidence of ritual in Israel
Come hell ant or high water
Not all ants are carnivorous, but the "hell ant" species discovered in Brazil from 113 million years ago, the mid-Cretaceous, wasn't using that mandible to spike fleeing ferns. It was discovered in the cellars of the Museum of Zoology of the University of São Paulo in Brazil, where it lived with the collection from the insect-rich Crato fossil formation.
Its identity was revealed by a methodical study of what exactly the museum had stashed away.
Some ant species extant today are frightening enough – like the fast-spreading fire ant, which has reached Israel too. Nobody likes them. But the accoutrements of the hell ant were other level. The ancient myrmex and the implications of its existence 113 million years ago were described Thursday in Current Biology by Anderson Lepeco and colleagues.
Happily for humankind, hell ants are truly extinct. They apparently only existed during the Cretaceous, not before and not after as far as we know. They died with the dinosaurs, though before doing so they reached most major land masses of their time.
Their intimidating name is a function of their startling predatory adaptations. Until now, the hell ants – a group characterized by extraordinary facial cutlery – were thought to have lived from 100 to 79 million years ago in Canada, France and Burma (i.e., North America and Eurasia). Now we have this much earlier specialist living in northeastern Brazil.
Note that the newly reported specimen is the earliest ant found in the fossil record. There are specimens in amber that may be older. Separate "molecular clock" analysis of ant genetics done at Harvard 20 years ago concluded that ants emerged as much as 168 to 140 million years ago, but diversified in the Cretaceous.
"To date, molecular estimates (which are also dependent on fossil information) recover an age of about 127 million years for extant ant lineages," Lepeco told Haaretz by email. "However, the earliest fossil ants known before our finding date to 100 million years ago. The existence of an ant species with such specialized morphology during the Lower Cretaceous reveals that ants, especially the extinct lineages, diversified earlier than we thought."
In and of itself, the hell ant's extreme features strongly indicate that it was not a stem ant – meaning an ancestor from which today's ant families arose. It wasn't even a particularly early one. What we can say is that with jaws like that, it likely exhibited unique hunting behaviors, the team says. They're not speculating. One specimen in urine-colored amber from about 99 million years ago shows a hell ant clutching a cockroach with its jaws – which, Alex Fox of the Smithsonian points out, swung upwards, unlike that of any other type of ant. Hell ant jaws closed vertically, while other ants' jaws close horizontally, he explains. They also featured horn/s on their foreheads.
"We were shocked by the characteristics of its feeding apparatus," Lepeco says. Unlike modern ants with sideways-moving mandibles, this species possessed mandibles that run forward, parallel to the head. They also had facial projections behind their eyes, the researchers explain. This was an extreme form that would have arisen from a simpler form, they surmise.
Before their demise, hell ants were obviously widely distributed across the globe early on, and must have crossed Cretaceous landmasses repeatedly, the authors say.
The existence of an ant species with such specialized morphology during the Lower Cretaceous reveals that ants, especially the extinct lineages, diversified earlier than we thought.
Anderson Lepeco
So what have we? Ants apparently go back farther than the evidence we have in hand shows. The new one, the oldest fossil ant, is already a specialist – and a scary one at that. Much about it we can't say, except that you wouldn't want to encounter one in the dark. It is true that we could squish it easily enough, but if it bit first, that probably hurt.
Why did an insect that fearsome, that widely distributed, go extinct?
That comet again? Lots of insect lineages were lost in the great Cretaceous-Paleogene extinction event 65 million years ago that killed off most of the dinosaurs, paleoentomologists have recognized.
"When I encountered this extraordinary specimen, we immediately recognized its significance, not only as a new species but as potentially the definitive evidence of ants in the Crato Formation," says Lepeco. "This finding highlights the importance of thorough examination of existing collections – private or in museums."
The ant who evolved backward
Meanwhile, over in the Caribbean, there once had been dirt ants who were masters of camouflaging themselves in leaf litter. We didn't know about them because they went extinct on the islands, but survive elsewhere from Honduras to southern Brazil, Gianpiero Fiorentino of the New Jersey Institute of Technology explains with colleagues.
We know about them now because a Caribbean dirt ant, species Basiceros enana sp. nov., has been found in 16-million-year-old amber on the island of Dominica. It changes what we thought about the trajectory of dirt ant evolution.
It was very much the surprise to find that the ancient dirt ant was very small, and all extant ones are bigger. If one evolves to hide in dirt, one evolves to be smaller, no? Elephants are notoriously bad at camouflage.
Fossils can tell us about strange evolutionary trajectories that went extinct, like the hell ant. Here, the fossil tells us a great deal about a creature living today. In the case of the dirt ant, the fossil teaches that they went from small (2-5 millimeters body length) to big (more than 6-7 millimeters).
One species, Basiceros singularis, can reach up to 8.4 millimeters in length – which is about four times the size of the ancient ant.
Why are they called dirt ants? "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," Fiorentino explains. "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," he adds.
They blend into the environment by camouflaging themselves with the help of tiny "hairs" on their exterior that attract dirt and leaf particles, which they wear like a coat.
Whatever their size, dirt ants went extinct on Dominica. So did many other ant species. How rare is it, where it survives? We are not sure. They live the crypto life.
In any case, the fossil one has no coat but similarities with another recently revealed dirt ant species, B. browni. Browni was collected from and close to a rotten log. Wondrously, worker ants collected inside the log lived in the altogether, lacking the thick layer of dirt-and-leaf camouflage observed in older ants living outside.
Dirt ants are rare finds in the wild. Finding one today is exciting given how well they stay hidden
Gianpiero Fiorentino
Possibly, the authors suggest, the new extinct ant was nesting in a rotten log or at the base of the tree, so it didn't have a coat. "The tree resin could have captured a younger worker of this species," the authors say.
Asked what being a younger ant might mean – because ants, having finished their metamorphosis (egg, larva, pupa, ant) do not grow any more – Fiorentino explains: "When we refer to this ant being young, we do not make that statement in reference to its size but to its biology. Dirt ants cover themselves with dirt as a defense mechanism. Throughout their lives, they will accumulate more and more dirt to the point that they are almost unrecognizable. In this way, you can usually tell the age of a dirt ant depending on how covered in dirt it is," he says. "This particular fossil is completely dirt-free, thus we believe it is likely a younger ant that has yet to cover itself in dirt."
An interesting technique. As for its dimensions, this possibly young ant was 0.512 centimeters long, while modern ones can be twice that size. "You can really see this evolutionary trend toward larger body sizes," Fiorentino sums up.
Wasp in the closet
So, ants have been around for a long time. Who did they descend from, when and where?
"The ants currently comprise the family Formicidae, the single one within the superfamily Formicoidea. This superfamily is sister-group to the Apoidea, which comprises bees, cockroach wasps, digger wasps and their relatives. Due to the behavior of extant species and the morpho-functional traits of living and fossil wasps, we can infer that the ancestors of ants were wasp-like and likely built nests to rear their brood," Lepeco says. Wouldn't you know that ants, the carnivorous juggernauts of the insect world, descend from something like a wasp.
"Given the widespread carnivorous habit in Apoidea and many lineages of ants, we can also attest that the ancestors of ants were also carnivorous. This is further supported by hell ants preserved in amber attacking other insects," he adds.
Where did this come about? "The exact locality where ants first diversified is still under debate. The earliest fossil ants are known from South America, which was part of the Gondwana supercontinent during the Cretaceous. However, slightly younger ants are known from France and Myanmar. While the first was part of Laurasia, in the northern hemisphere, the latter had dubious relationships with past landmasses," he explains. "Altogether, with biogeographic reconstructions based on molecular data, South America has been strongly indicated as a relevant place for the diversification of ants."
So they evolved fast, and extant dirt ants lurk well and scientists don't see them. The dirt ants they do see are ones that got lost or caught in traps, the team explains. As far as we know, there are nine species of dirt ant extant today. But maybe there are more, secretly looking up at us from the dirt with their beady little eyes and we can't see them because we aren't looking back properly.
Ruth Schuster