The origin and early evolution of many major dinosaur groups are poorly known because specimens are rare. One of these groups, the Ankylosauria, or armour-plated dinosaurs, is best known from well-preserved specimens from the Upper Cretaceous period of Asia and North America. Here we describe a well-preserved skull of an earlier, Late Jurassic ankylosaur, which will be important in clarifying the early history of this group. The specimen, Gargoyleosaurus parkpini gen. et sp. nov., was collected from the Upper Jurassic Morrison Formation of Wyoming, USA. Despite its geological age, the skull shows features seen in Late Cretaceous ankylosaurs, including fusion of bone armour to the surface of the skull and mandible and closure of two skull openings, the antorbital and upper temporal fenestrae. The new taxon also has characters common to the two ankylosaur families, the Ankylosauridae and Nodosauridae1,2,3,4, supporting the proposal that the Ankylosauria originated from a single ancestor2,3,4. Nevertheless, specialized characters place Gargoyleosaurus as the most primitive, or basal, member of the Ankylosauridae.