Evidence for the evolution of eusociality in stem ants and a systematic revision of Gerontoformica (Hymenoptera: Formicidae)

February 14, 2021

Brendon E Boudinot (1), Adrian Richter (1), Julian Katzke (2), Júlio C M Chaul (3), Roberto A Keller (2,4), Evan P Economo (2), Rolf Georg Beutel (1), Shûhei Yamamoto (5)
Coatings, February 2022. DOI: 10.1093/zoolinnean/zlab097


micro; computed tomography; palaeontology; phenotypic plasticity; sociobiology; taxonomy


It is generally assumed that Cretaceous stem ants were obligately eusocial, because of the presence of wingless adult females, yet the available evidence is ambiguous. Here, we report the syninclusion of a pupa and adult of a stem ant species from Mid-Cretaceous amber. As brood are immobile, the pupa was likely to have been transported by an adult. Therefore, the fossil substantiates the hypothesis that wingless females were cooperators, thus these were true ‘workers’. Re-examination of all described Cretaceous ant species reveals that winged–wingless diphenism – hence a variable dispersal capacity – may have been ancestral to the total clade of the ants, and that highly specialized worker-specific phenotypes evolved in parallel between the stem and crown groups. The soft-tissue preservation of the fossil is exceptional, demonstrating the possibility of analysing the development of the internal anatomy in stem ants. Based on the highest-resolution µ-CT scans of stem ants to date, we describe †Gerontoformica sternorhabda sp. nov., redescribe †G. gracilis, redefine the species group classification of †Gerontoformica, and provide a key to the species of the genus. Our work clarifies the species boundaries of †Gerontoformica and renders fossils relevant to the discussion of eusocial evolution in a way that has heretofore been intractable.

How Our Software Was Used

Dragonfly was used for the segmentation of micro-CT images.

Author Affiliation

(1) Institut für Zoologie und Evolutionsforschung, Friedrich-Schiller-Universität Jena, Erberstraße 1, 07743 Jena, Germany.
(2) Biodiversity and Biocomplexity Unit, Okinawa Institute of Science and Technology Graduate University, Onna, Japan.
(3) Pós-Graduação em Ecologia, Departamento de Biologia Geral, Universidade Federal do Viçosa, 36570-900, Viçosa, Minas Gerais, Brazil.
(4) Museu Nacional de História Natural e da Ciência & cE3c, Faculdade de Ciências da Universidade de Lisboa, 1749-016 Lisbon, Portugal.
(5)Hokkaido University Museum, Hokkaido University, Kita 8, Nishi 5, Kita-ku, Sapporo 060-0808, Japan.