The hypogaeic ant subfamilies Leptanillinae and Martialinae likely form the sister group to the remainder of the extant Formicidae. In order to increase the knowledge of anatomy and functional morphology of these unusual and phylogenetically crucial ants, we document and describe in detail the cranium of a leptanilline, Protanilla lini Terayama, 2009. The mandibular articulation of the species differs greatly from that of other ants studied so far, and clearly represents a derived condition. We propose a mode of movement for the specialized mandibles that involves variable rotation and sophisticated locking mechanisms. While a wide opening gape and a unique articulation are characteristics of the mandibular movement of P. lini, the observed condition differs from the trap-jaw mechanisms occurring in other groups of ants, and we cannot, at present, confirm such a functional configuration. Protanilla lini displays hardly any plesiomorphies relative to the poneroformicine ants, with the possible exception of the absence of the torular apodeme. Instead, the species is characterized by a suite of apomorphic features related to its hypogaeic and specialized predatory lifestyle. This includes the loss of eyes and optic neuropils, a pronouncedly prognathous head, and the derived mandibular articulation. The present study is an additional stepping-stone on our way to reconstructing the cephalic ground plan of ants and will contribute to our understanding of ant evolution.Open access, licensed under CC BY 4.0. © 2021 The Author(s).DOI: 10.25849/myrmecol.news_031:085 Pages: 85-114 Volume: 31 Year: 2021 Journal: Myrmecol. News