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Armoured dinosaurs are well known for forms that evolved specialized tail weapons: paired tail spikes in stegosaurs, and heavy tail clubs in advanced ankylosaurs 1 . Armoured dinosaurs from southern Gondwana are rare and enigmatic, but likely include the earliest branches of Ankylosauria 2-4 . Here, we describe a mostly complete, semiarticulated skeleton of a small (~2m) armoured dinosaur from the late Cretaceous of Magallanes in southernmost Chile, a region biogeographically related to West Antarctica 5 . Stegouros elengassen gen. et sp. nov. evolved a large tail weapon unlike any dinosaur: A flat, frond-like structure formed by 7 pairs of laterally projecting osteoderms encasing the distal half of the tail. Stegouros shows ankylosaurian cranial characters, but a largely primitive postcranial skeleton, with some stegosaur-like characters. Phylogenetic analyses placed Stegouros in

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