Specialized Predation Drives Aberrant Morphological Integration and Diversity in the Earliest Ants

Extinct haidomyrmecine "hell ants" are among the earliest ants known [1, 2]. These eusocial Cretaceous taxa diverged from extant lineages prior to the most recent common ancestor of all living ants [3] and possessed bizarre scythe-like mouthparts along with a striking array of horn-like cephalic projections [4-6]. Despite the morphological breadth of the fifteen thousand known extant ant species, phenotypic syndromes found in the Cretaceous are without parallel and the evolutionary drivers of extinct diversity are unknown. Here, we provide a mechanistic explanation for aberrant hell ant morphology through phylogenetic reconstruction and comparative methods, as well as a newly reported specimen. We report a remarkable instance of fossilized predation that provides direct evidence for the function of dorsoventrally expanded mandibles and elaborate horns. Our findings confirm the hypothesis that hell ants captured other arthropods between mandible and horn in a manner that could only be achieved by articulating their mouthparts in an axial plane perpendicular to that of modern ants. We demonstrate that the head capsule and mandibles of haidomyrmecines are uniquely integrated as a consequence of this predatory mode and covary across species while finding no evidence of such modular integration in extant ant groups. We suggest that hell ant cephalic integration-analogous to the vertebrate skull-triggered a pathway for an ancient adaptive radiation and expansion into morphospace unoccupied by any living taxon.