Bizarre tail weaponry in a transitional ankylosaur from subantarctic Chile

Abstract

Armoured dinosaurs are well known for their evolution of specialized tail weapons— paired tail spikes in stegosaurs and heavy tail clubs in advanced ankylosaurs1 . Armoured dinosaurs from southern Gondwana are rare and enigmatic, but probably include the earliest branches of Ankylosauria2–4 . Here we describe a mostly complete, semi-articulated skeleton of a small (approximately 2 m) armoured dinosaur from the late Cretaceous period of Magallanes in southernmost Chile, a region that is biogeographically related to West Antarctica5 . Stegouros elengassen gen. et sp. nov. evolved a large tail weapon unlike any dinosaur: a fat, frond-like structure formed by seven pairs of laterally projecting osteoderms encasing the distal half of the tail. Stegouros shows ankylosaurian cranial characters, but a largely ancestral postcranial skeleton, with some stegosaur-like characters. Phylogenetic analyses placed Stegouros in Ankylosauria; specifcally, it is related to Kunbarrasaurus from Australia6 and Antarctopelta from Antarctica7 , forming a clade of Gondwanan ankylosaurs that split earliest from all other ankylosaurs. The large osteoderms and specialized tail vertebrae in Antarctopelta suggest that it had a tail weapon similar to Stegouros. We propose a new clade, the Parankylosauria, to include the frst ancestor of Stegouros— but not Ankylosaurus—and all descendants of that ancestor.