Ankylosauria was independently defined as a stem-based taxon by Carpenter (1997) and Sereno (1998) using textual variants of the same definition with the same genera as specifiers (Stegosaurus, Ankylosaurus). The active definition is a first-order revision of these initial definitions that uses species as specifiers (Stegosaurus stenops, Ankylosaurus magniventris). As in Sereno’s (1998) taxonomy, Ankylosauria here is party to a node-stem triplet (with Eurypoda and Stegosauria).