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Phylogenetic definition. Hypsilophodon foxii, Edmontosaurus regalis, their most recent common ancestor, and all of its descendants (Norman, 2015). Unambiguous synapomorphies. For the topology recovered by parsimony, Clypeodonta has nine unambiguous synapomorphies: presence of a quadrate buttress or " hamular process " (63.1), quadrate with a lateral condyle that is larger than the medial condyle (69.2), mandibular articulation that is horizontal to dorsomedially inclined in caudal view (70.0 / 1), maxillary and dentary teeth with crowns that taper toward the root (127.1, 128.1), with the base of the crown defined by an everted lip which makes the crown slightly inset from the root (146.1, 147.1), presence of a primary ridge on labial side of maxillary teeth (139.1), and elongate centra of postaxial cervical vertebrae, with craniocaudal length more than twice the dorsoventral height (159.1). Within the Bayesian topology, Clypeodonta is characterized primarily by features of the teeth and jaws: presence of a diastema in the maxilla (16.1), equal lengths in the oral margin of the premaxilla and predentary (84.1), a coronoid process that extends more than one crown height dorsal to the tooth row (101.1), surangular with a small fenestra positioned dorsally on or near the dentary joint (111.1), surangular foramen rostral to the lateral lip of the glenoid (114.1), cheek teeth with asymmetrically distributed enamel (134.1), ridges running the full length of the crown on the labial side of maxillary teeth and the lingual side of dentary teeth (135.1), and a femoral head separated from the greater trochanter by a distinct constriction (292.1). Topology. In the parsimony tree (Figures 6, 7), Clypeodonta is the sister clade to Thescelosauridae. This node has a jackknife value of 15, but a relatively high Bremer support of 4. Hypsilophodon is recovered as the only non-iguanodontian clypeodontan. In the Bayesian topology (Figures 6, 8), Hypsilophodon is recovered within a large