Extinct ant sister-group illuminates eusociality origin and post-K/Pg persistence. Aculeata

Ants are an ecologically dominant lineage of eusocial organisms and are influential in many terrestrial environments4. Because all extant ants are eusocial, the primary means for understanding transitions leading to and following the origin of eusociality is through reconstruction of functional morphological transformations from fossil evidence. Here, we report a lineage of wasp-ant intermediates from Mid-Cretaceous amber that does not display the hallmark modifications for eusociality observed in other ants. Through the combination of genomic and phenotypic data spanning the total diversity of stinging wasps, the Aculeata, we find that the origin of eusociality during the Early Cretaceous was probably preceded by adaptations for walking and predation. These functional novelties would have allowed ants to more efficiently exploit the expanding brown and green food webs as solitary-foraging ground-hunting predators during the rise of angiosperm crown groups. Subsequent adaptations may have improved locomotory efficiency, antennal dexterity, olfaction, mandibular grip, and other functions. We observe that a vast radiation of stem ants was extirpated by the end Cretaceous, and hypothesize that increased social integration of crown ants, as indicated by morphology, made the difference between survival and extinction across the K/Pg boundary.