A glimpse into Cretaceous ant cephalic anatomy – the head of †Gerontoformica from Burmese amber (Hymenoptera, Formicidae)

Cretaceous amber from Myanmar has revealed a hitherto undreamt-of wealth of information about the evolution of extinct and extant insect lineages. In Formicidae, a host of stem group fossils has been discovered in recent years, many of them with morphological features unlike any recent species. Here, we report the finding of an exceptionally well-preserved worker specimen of †Gerontoformica in ~99 myo amber, belonging to †Sphecomyrminae, a group considered as generalized. The fossil contains exceptionally preserved soft tissue, allowing us to present the first three-dimensional reconstruction of the internal anatomy of a Cretaceous insect based on µCT scanning microscopy. In the head this includes a nearly complete muscle set, the digestive tract, several glands, and the brain, which has highly preserved optic neuropils, mushroom bodies, and antennal glomeruli. Through comparison with the literature on recent ants as well as newly generated µCT data of apoid and vespoid outgroup taxa, we analyze the morphological groundplan of the ant head. We present a hypothesis for the evolution of the modern shovel-like (“triangular”) ant mandible from the plesiomorphic bidentate condition of †Gerontoformica and provide an anatomical specification of the “geniculate” condition for ant antennae based on pedicel morphology. Total clade Formicidae retain well developed paraglossae while crown Formicidae are characterized by surprisingly elaborate sclerotization of the prepharynx. Our study highlights the potential of µCT scans for resolving fossil anatomy and the value of anatomical details for understanding evolutionary transitions in ants.