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 Ankylosauria^2,3,4. 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 flat, 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; specifically, 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 first ancestor of Stegouros—but not Ankylosaurus—and all descendants of that ancestor.

装甲恐龙以进化出专门的尾部武器而闻名——剑龙类中的成对尾刺和高级甲龙类中的重尾棒¹。来自冈瓦纳南部的装甲恐龙是稀有而神秘的，但可能包括甲龙科^2,3,4最早的分支。在这里，我们描述了智利最南部麦哲伦晚期白垩纪时期的一种小型（约 2 m）装甲恐龙的大部分完整的半铰接骨骼，该地区与西南极洲⁵生物地理相关。剑齿虎将军 等人。十一月 进化出一种不同于任何恐龙的大型尾巴武器：一个扁平的叶状结构，由七对横向突出的骨皮包裹着尾巴的远端。剑龙展示了甲龙类的颅骨特征，但主要是祖先的颅后骨骼，带有一些类似剑龙的特征。系统发育分析将剑龙归入甲龙类；具体而言，它与来自澳大利亚^6的昆巴拉龙和来自南极洲⁷的南极洲有关，形成了最早从所有其他甲龙中分裂出来的冈瓦纳甲龙进化枝。南极洲的大型骨皮和专门的尾椎骨暗示它有一个类似于Stegouros的尾部武器。我们提出了一个新的进化枝，Parankylosauria，包括剑龙的第一个祖先——但不包括甲龙——以及该祖先的所有后代。