Senckenberg Research

Fossil Ants from the Messel Pit

Queen of the giant ant Titanomyrma simillimum from Messel, about 5 cm long.
Queen of the giant ant Titanomyrma simillimum from Messel, about 5 cm long.

 

 

Ants represent the most successful group of the social insects. They comprise over 14,000 species. Their total biomass is enormous - several times that of mammals in areas such as the Amazonian rainforests. Without doubt, ants are extremely important components of ecosystems. Some of their predecessors have been preserved in fossil form in the Messel Pit. They help researchers to answer various questions about the way in which biodiversity has developed.

Giant ants in the Messel Pit

The 47 million-year-old sediments of the fossilrich layers of the Messel Pit contain very many insect fossils, and ants are represented amongst them frequently. Especially common and conspicuous specimens belong to the extinct genus Titanomyrma. As the name suggests, these ants were very large, with the queens reaching up to 8 cm in length. The males were much smaller. Only two species from this group have been found at Messel. Analysis of the wing structure has allowed categorization of the males and females to one of these species respectively. The upper figure shows a queen of the smaller T. simillimum species, ‘only’ 5 cm in length. The giant ants, whose phylogenetic position still remains to be properly clarified, have been found in Eocene fossil deposits in Germany, England, eastern and western North America. They were probably very thermophilic (requiring high ambient temperatures), and became extinct at the end of the Eocene period. As winged sexual forms have been found, as is typical of ants, it is certain that they were social animals, and that worker ants existed as well. However, although hundreds of queens and kings that fell into the former Messel Lake and sank down after their nuptial flight during the Eocene have been fossilized, not one worker ant has ever been found. It is generally the case that winged individuals are found more often than ones without wings in former lakes, as the surface tension makes it more difficult for winged insects to escape from the surface of the water once they have fallen in.

Poneromorph ants and their evolutionary development

Queen of the poneromorph species Cyrtopone striata from Messel, about 1.5 cm long.
Queen of the poneromorph species
Cyrtopone striata from Messel, about 1.5 cm
long.

Ants of the so-called poneromorph subfamilies (Poneromorpha) are much smaller and less conspicuous than the giant ants. The females have an effective sting. There are comparatively few fossil specimens in the Messel Pit, but these cover an unusually wide range of species. As a recent study published together with the Moscow ant specialist Gennady M. Dlussky reveals, these fossils can provide clues as to how species diversity amongst ants developed. Amongst just 75 poneromorph Messel fossils that were investigated, 22 different species could be distinguished. Most of the poneromorph ants found in the Messel Pit belong to genera that have become extinct and can only be studied in fossil form. However, a few specimens belong to the genus Pachycondyla, which is still extant.

Through comparison with other fossilized ants, we can deduce that shifts have occurred in the degrees of dominance amongst the ant sub-groups of previous ecological communities during geologically recent times. For instance, the poneromorph ants also appear to have been very species-rich in the Chinese site of Fushun, which is somewhat older than the Messel Pit. However, the poneromorph fauna found in somewhat more recent Baltic amber has significantly fewer species: In an analysis of approximately 980 inclusions of poneromorph ants, only 16 different species could be distinguished. The same trend can be observed in several other, more recent finds. Generally, it seems that the poneromorph ants, represented by many different genera, dominated the ant communities of the early and middle Eocene. During the late Eocene, a shift took place amongst the poneromorph fauna. Many of the older Eocene genera became extinct, while a few new ones developed. Most of the latter have survived to the present day.

In 2005, the prominent ant researchers Edward Wilson and Bernd Hölldobler coined the term ‘ponerine paradox’. With this, they give a name to the phenomenon that poneromorph ants, representing a geologically ancient group amongst the ants, are globally successful even though they have distinctly primitive social structures. By way of explanation, they put forward the ‘dynastic succession hypothesis’. According to this, a major change took place in the composition of the ground litter as the flowering plants gained dominance over the gymnosperm plants during the Cretaceous period. It became much more complex, offering ants that used it as a habitat better opportunities to develop. Poneromorph ants form one of two large ant groups that, today, mainly inhabit ground litter. Of these two groups, the poneromorph ants are represented by fewer species. Wilson and Hölldobler developed a scenario to describe the historical development of these two groups. According to this, the species variety of the poneromorph ants was already being suppressed by the other litter-dwelling ant group, the so-called Myrmicinae, during the early Eocene. The findings of the investigation of the poneromorph ants found in the Messel Pit have enabled us to refine this scenario and explain the processes in greater detail. It could be shown that the poneromorph ants of the Middle Eocene definitely dominated over the Myrmicinae. They only began to lose ground in terms of ecological significance during the late Eocene, with the Myrmicinae sub-group taking over the leading role.

Fossil remains of weaver ants in Messel

Queen of the weaver ant Oecophylla longiceps, about 1.5 cm long.
Queen of the weaver ant Oecophylla
longiceps
, about 1.5 cm long.

Other ants that could be identified in Messel include weaver ants. These fossils are amongst the oldest known specimens for this group. A total of 13 fossil species have been described, almost all of them from European fossil sites and amber. The only exception to this is a find in Miocene deposits in Kenya. Today, there are two species of weaver ants. They can be found in tropical to subtropical regions of the Old World and in northern Australia.

 

Extant worker of the species Oecophylla longinoda
Extant worker of the species Oecophylla longinoda, on leaf nest in
South Africa, body length about 1 cm.

Formerly, weaver ants appear to have been represented by considerably more species than is the case today, and because of the warm climate across the globe, they were much more widely distributed. The present distribution range is relictual, but in today’s ecosystems their populations are very large in some cases. As predators that hunt many kinds of insect, they play important roles ecologically and in pest control. Modern weaver ants are distinctive in that they do not build one large nest, but inhabit many small nests that they weave together out of living leaves of trees. For weaving, they use the silk that the final larval stage usually uses to construct a cocoon. Instead of allowing them to do this, the weaver ant workers carry the larvae in their mandibles to the site of a new leaf nest. At the site, groups of workers use their mandibles to pull leaves closely together, others hold the larvae and move them back and forth, while the latter excrete their silk. Unfortunately, it is not possible to deduce from the morphology of the Messel Pit finds whether these ants also used a similar nestbuilding technique. However, we do know that this behaviour had evolved during the Miocene at the latest, as the remains of a leaf nest were found at the Kenyan site.

  

Author

 Dr. Sonja Wedmann

Dr. Sonja Wedmann is a biologist who has been on the scientific staff of the Senckenberg Research Institute and Natural History Museum since 2007. She is Coordinator of the Senckenberg Messel Pit Research Station and responsible for the annual excavations that are conducted there. Her field of research covers fossilized insects from various Tertiary sites and in Baltic amber. Her research focuses on the evolution of biodiversity and biogeography.

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