Ants are a hugely successful family of eusocial insects with over 14,000 modern species described. They are known from every continent except Antarctica and show a wide range of ecologies. Whilst many of us are familiar with their highly organised social structures and castes, there still remain a lot of public misconceptions about how their societies function.

The evolutionary history of ants is equally as impressive, with roughly as many fossil ant species known as there are of dinosaurs! Since their appearance in the Cretaceous, several early lineages of ants (stem ants) have gone extinct. In this episode, we’re joined by Dr Christine Sosiak of the Okinawa Institute of Science and Technology as we explore what some of these stem ants were like and ask how the different groups of ants fared over geological time.

Many different animals are social to varying degrees. Ants are ‘eusocial’ animals which is the highest level of sociality. Within an ant colony, millions of individuals all work towards a common good. Colonies can be divided into physical castes, with the queen(s), workers, soldiers, drones each playing a defined role in the division of labour. The queen (centre) of a colony is just one role, not the ruler of the colony.

There are a large number of different kinds of nests and not all are located in the ground. Whilst a typical ant colony will occupy a single nest, some species can possess multiple. An ant taken from one such nest and introduced to another, will be accepted leading to the idea of ‘supercolonies’ that can span entire continents. Image: The carton nest of Crematogaster castanea.

Whether as prey, predators or symbionts, ants have a huge influence on ecosystems. Many species depend on the presence of ants, either as a food source or as protection and numerous fascinating strategies to benefit from the presence of ants have evolved across the evolutionary tree. Not this ant though. Image: regular normal ant, not an ant-mimicking jumping spider.

Ants possess many different ecologies (defined as all the interactions with the physical environment and other organisms, but essentially ‘ways of life’) and functional morphologies adapted to that purpose. Whether thy are predatory (P) or omnivorous (O), the different ecologies (coloured ants above) appear all over the evolutionary tree. This means that different groups of ants will evolve similar solutions to occupy a given ecology/habitat; if the space is there, they will adapt to fill it. Image: Sosiak and Barden 2021.

Stem Ants

In cladistics (the science of classifying organisms), it is useful to be able to differentiate the crown and stem members of a group. The crown group is the branch of a tree that contains all the living members and traces back to their last shared common ancestor. So you could say “every living species came from this point on the tree”. Conversely, the stem group are those early evolutionary trajectories whose lineage diverged before that point and ultimately went extinct. (see diagram below). You can have crown ant that went extinct, but you can’t have a modern stem ant. Therefore, stem ants can only be found in the fossil record.

A phylogenetic reconstruction and comparative morphospace plot of stem (orange) and crown (blue) ants. This diagram shows how the stem ants saw relative success early in ant evolution with a greater number of species (diversity) showing a greater difference in form (disparity). It is only relatively recently that crown ants showed an increase in diversity and disparity. Image: Sosiak et al., 2024.

The chronomorphospace and generic longevity of ants. A: The total range of body shapes at a given time slice. Each slice shows the modern ‘average’ in grey. This shows that even the stem ants (orange) were not too dissimilar in overall shape to crown ants (blue). Even looking at the Cretaceous as a whole (B), there are no ants that really stray too far from the modern ant shape. C: The longevity of different ant genera from the fossil record and modern. The stem ants (orange) are known to have existed for around 20 million years as a minimum. This is roughly in line with other modern and fossil ants. Image: Sosiak et al., 2024.

Haidomyrmecines or ‘hell ants’ are stem ants that possessed highly specialised mandibles that were uniquely articulated in a vertical plane. Haidomyrmex had a patch of sensory hairs on what we might call its forehead (see below) that could have functioned as a trigger for its ‘L-shaped’ mandibles to rapidly snap shut, capturing prey. This is a strategy employed by modern trap-jaw ants, though they, like the majority of insects, have horizontally-oriented mandibles. Image Christine Sosiak.

Reconstruction of Haidomyrmex scimitarus by Franz Anthony (permission granted).

Ceratomyrmex is a hell ant from 99 Ma (Cenomanian, Late Cretaceous) Myanmar amber that had incredibly elongate mandibles and a large horn in the centre of its head (see below). The horn was adorned with sensory hairs on its lower surface that, similar to Haidomyrmex, could have trigged the trap-jaw mechanism. However, given that the mandibles were so long, they extended well beyond the head of the ant and so the horn itself was used as the opposing force. Image: Christine Sosiak.

Reconstruction of Ceratomyrmex ellenbergeri by Franz Anthony (permission granted).

Linguamyrmex vladi, also from Myanmar amber, had a wider paddle-shaped horn against which the mandibles struck. Their two mandibles were kept tight together and operated as one likely to puncture prey against the horn. It’s been suggested that as the two mandibles lay adjacent, a channel was naturally formed along which haemolymph (akin to insect blood) could have flowed. This presumed method of feeding and a liquid diet led to the species epithet vladi in honour of Vlad the Impaler/Dracula. Whilst this might sound like an unnecessarily gruesome way to feed, there is precedent given that some modern species are known to feed on haemolymph of their own larvae.

Reconstruction depicting life habits of Linguamyrmex by John Paul Timonera.

Dr Christine Sosiak.

Tags: Ants, arthropod, Eusociality, Hell ant, insect, Stem ant